V31A-2102
Reappraisal of fluid and sediment contributions to Lesser Antilles magmas
The Lesser Antilles arc has long been noted for its pronounced along-arc gradient in radiogenic isotope ratios which have been used to infer a southward increase in the relative sediment contribution. However, estimates of the extent of this contribution have varied and there has been debate as to what extent some of this signal may instead originate via shallow-level crustal contamination. The Lesser Antilles was the subject of several early U-series studies but these did not reach consensus on the processes and time scales involved, most probably as a result of some of these difficulties. We present new U-Th disequilibrium data for mafic lavas from the Lesser Antilles arc. These are combined with published data in an internally consistent model that quantitatively estimates the amount of sediment and fluid added to the source for the Lesser Antilles arc and infers the rate of magma transfer through the mantle wedge using trace elements and Nd-Th-Pb isotopes. The lavas from the northern segment are characterized by 230Th deficits, Th isotope ratios similar to MORB ((230Th/232Th) ~1.25) and high 143Nd/144Nd, low 207Pb/204Pb. Lavas from the central to southern segments show 230Th excesses and deficits, a greater range of Th isotope composition and low 143Nd/144Nd, high 207Pb/204Pb. Mixing calculations suggest ~0.25% sediment contribution in the north and ~1-2 % contribution in the central to southern segments. Further calculations infer 0.5-2% fluid addition for the northern and southern segments. Combined with the Th isotope data this allows 25-90 ka for slab to surface transport of U.
V31A-2103
Rifting process of the Izu-Ogasawara-Mariana arc-backarc system inferred from active source seismic studies
The Izu-Ogasawara-Mariana (IBM) arc-backarc system has continued the crustal growth through crustal thickening by magmatic activities and crustal thinning by backarc opening. Tatsumi et al (2008) proposed petrological crustal growth model started from basaltic magmas rising from the slab, and showed the consistency with the seismic velocity model. Although crustal growth by the crustal thickening are modeled, crustal structural change by the backarc opening are not still unknown yet. The Shikoku Basin and Parece Vela Basin were formed by the backarc opening during approximately 15-30 Ma. Since 6 Ma, the Mariana Trough has opened and the stage already moved to spreading process from rifting process. In the northern Izu-Ogasawara arc, the Sumisu rift is in the initial rifting stage. Therefore, understanding of the crustal change by the backarc opening from rifting to spreading is indispensable to know the crustal growth of whole Izu-Ogasawara-Mariana island arc. Japan Agency for Marine-Earth Science and Technology (JAMSTEC) has carried out seismic studies using a multichannel reflection survey system and ocean bottom seismographs (OBSs) around the IBM arc since 2003 (Takahashi et al., 2007; Kodaira et al., 2007; Takahashi et al., 2008; Kodaira et al., 2008). We already obtained eight P-wave velocity models across the IBM arc and these structures record the crustal structural change during the backarc opening process from the rifting stage to the spreading stage. As the results, we identified characteristics of the crustal structural change accompanied with backarc opening as follows. (1) Beneath the initial rifting stage without normal faults, for example, in the northern tip of the Mariana Trough, crustal thickening are identified. (2) Beneath the initial rifting stage with normal faults, for example, in the Sumisu Rift, the crustal thickness is almost similar to that beneath the volcanic front. Although an existence of the crust-mantle transition layer with velocity of 7.5-7.7 km/s below the Moho are similar to that beneath the volcanic front, the thickening of a 6.5-6.8 km/s-layer and the thinning of the middle crust with velocity of 6.0- 6.5 km/s are distinctive. (3) Beneath the advanced rifting stage, for example, in the Ogasawara Trough and the Nishinoshima Trough, the crustal thickness is approximately 10-15 km, thicker than that of typical oceanic crust and thinner than that of the arc crust. In addition, the velocity of the lower crust is ~7.5 km/s , and commonly faster than that beneath the volcanic arc. (4) The lower crust with high velocity of ~7.5 km/s beneath the advanced rifting region has a Vp/Vs ratio of ~1.8. This suggests that the origin of the high velocity lower crust is not serpentinized mantle but the mafic crustal materials.
V31A-2104
Silicic Magmas in the Izu-Bonin Oceanic Arc and Implications for Crustal Evolution
The Izu-Bonin-Mariana volcanic arc (IBM arc) has included abundant rhyolite from its early stage (Eocene) to the present. Geochemically, three types of Quaternary rhyolites exist in the Izu-Bonin arc front, and they are closely related to volcano type and crustal structure. The dominantly basaltic islands of the volcanic front produce small volumes of rhyolites that we will call R1. The submarine calderas of the volcanic front erupt mostly rhyolite that we will call R2. Seamounts, knolls, and pillow ridges in the backarc extensional zone are mostly basaltic but also contain rhyolites that we will call R3. The thickest total crust, and the thickest intermediate composition middle crust, occurs predominantly below the dominantly basaltic volcanoes, while the intermediate composition middle crust tends to be thinner beneath the submarine calderas. R1 rhyolites may be derived from Quaternary andesitic sources whereas R2 and R3 rhyolites may be derived from Oligocene ones. The difference in CaO/Al2O3 between R1 versus R2 and R3 rhyolites, which values are higher in R1 than those of R2 and R3, can be attributed to the difference between Quaternary and Oligocene andesite sources, respectively. LREE-depleted patterns of Quaternary andesites and flat to slightly LREE- enriched patterns of Oligocene andesites (turbidites) are almost parallel to R1 and R2, respectively. Partial melts of Quaternary andesite will have low Zr/Y values such as in R1, whereas melts of Oligocene andesites will have the higher Zr/Y values of R2. Lavas from basalt-dominant volcanoes and those from rhyolite- dominant volcanoes have significantly different Sr-Nd-Pb isotopes and the latter is lower in all three isotopes as is the Oligocene arc. Why and how crustal sources differ systematically and alternately along and behind the Izu-Bonin arc? Locally developed hot regions within the mantle wedge (hot fingers) would be responsible for the production of large basaltic volcanoes. If these mantle sources remain stationary for millions of years, then basalt-dominant volcanoes eventually will overlie thicker crust. Remelting of middle crust to form rhyolite magmas takes place beneath both basaltic and rhyolitic volcanoes (R1 and R2 rhyolite, respectively). However, basalt volcanoes consume new middle crust to produce rhyolite magma whereas rhyolite volcanoes consume old Oligocene middle crust. Moreover, rhyolite volcanoes have no mantle roots beneath the crust. Instead, dikes from basalt volcanoes provide the heat source to partially melt the crust.
V31A-2105
Did Tholeiitic Basalts Erupt First After Initiation of Subduction in the IBM System?
Manned submersible Shinkai 6500 diving in 2006 and 2008 (cruises YK06-12 and YK08-08 of the R/V Yokosuka) in the Mariana fore-arc southeast of Guam discovered extensive outcroppings of tholeiitic pillow basalt and diabase. Such rocks appear to be the most abundant lithology in this region between 6500 and 2000 m depth. Similar basalts were found during Shinkai 6500 diving in the Bonin forearc (Debari et al., 1999), dredging and diving on the Hahajima Seamount (Ishwatari et al., 2006.), and drilling at DSDP sites 458 and 459 in the central Mariana forearc. In both the 2006-2008 dive area and at site 458, these basalts underlie boninites. Based on their position in the fore-arc, and their unique petrographic and geochemical characteristics, we have given these lavas the name "fore-arc basalts" (FAB). The FAB differ petrographically from the back-arc basin and mid-ocean ridge basalts in that phenocrysts are rare in FAB, and typically are only iddingsitized euhedral olivine. Pillow lava interiors mostlyconsist of quench-textured intergrowths of acicular to skeletal plagioclase less than a few tenths of a millimeter long and finer acicular augite with granular Fe-Ti oxides. Some samples lack vesicles but others have as much as 20 percent vesicles. FAB diabases consist of intergrown acicular to lath-shaped plagioclase partially surrounded by anhedral to subhedral augite and Fe-Ti oxides. FAB are characterized by MORB-like REE and high field strength (HFS) element concentrations. Their large-ion lithophile (LIL) element concentrations range from MORB-like to somewhat LREE-enriched. Some LIL enrichment resulted from alteration. However, the most LIL-enriched samples of FAB are those at the base of DSDP site 458, which are fresh glasses that were analyzed by LA- ICPMS. Ti/V ratios are lower in FAB than in MORB and most back-arc basin lavas but are similar to Ti/V in arc basalts. The low Ti/V ratios in subduction-related lavas have been attributed to oxidation of mantle sources, resulting in greater incompatibility of oxidized V (Shervais 1982), and the low Ti/V in FAB also suggest a similarly oxidized source. We infer that FAB's form during subduction initiation, when asthenosphere rises to fill the space created by the sinking Pacific plate. This asthenosphere melts by adiabatic decompression, in the presence of an oxidizing fluid derived from the subsiding slab. This further implies that the FAB are the oldest subduction-related lavas of the IBM system, and that boninites erupted later and with a significantly greater mass contribution from the subducting plate. The origin of the FAB soon will be further constrained with 40Ar/39Ar dating of whole rocks and U-Pb dating of baddelyite.
V31A-2106
Earliest Izu-Bonin arc volcanism found on the submarine Bonin Ridge
By extensive sampling of the Izu-Bonin forearc, we have found evidence of MORB-like volcanism above the new slab at the very initiation of volcanism. Bonin Ridge is an unusually prominent forearc massif in the Izu- Bonin arc that exposes early arc volcanic rocks on islands of Chichijima, Hahajima, and smaller islands. In 2007 we sampled along the entire length of the Bonin Ridge between the Bonin Islands and Izu-Bonin Trench to establish the nature and timing of the earliest stages of Izu-Bonin arc and to attempt to recover material from the deepest sections of the arc crust. Dredge stations on the deeper part of the landward slope (as deep as 6420m) of the Izu-Bonin Trench recovered pillow basalts as well as gabbroic rocks and peridotite. This is the first recovery of these types of rocks (i.e., upper mantle and lower crust) from the Izu-Bonin forearc other than at serpentine seamounts. Preliminary data imply that these basalts are MORB-like (i.e., with no slab signature). Interestingly, these lavas have lower Ti/V (14-16) which distinguishes them from subducting Pacific MORB (26-32) and Philippine Sea MORB (17-25) which are the potential basement components of the arc. Pb isotopes of the forearc MORB show that like other Izu-Bonin arc magmas they are derived from a mantle source with Indian Ocean characteristics. Chemically and petrographically they have similar characteristics to tholeiites from the Mariana forearc that are considered to predate boninitic volcanism in that region. This strongly implies that MORB-like tholeiitic magmatism was associated with forearc spreading along the length of the Izu-Bonin- Mariana arc. Like the adjacent boninitic magmatism, it can be linked to an Indian Ocean-type source. Low concentrations of incompatible elements and low trace element ratios such as Nb/Yb imply that these lavas were from depleted mantle and/or large degree of partial melting compared to typical Philippine Sea MORB. Dredge sampling at small bathymetric highs east of the Bonin Islands mainly recovered aphyric or ol-cpx basalt lava, which have not been recognized on the Bonin Islands nearby. This result implies that these forearc highs are remnant volcanic edifices. These basalts show weak slab signatures and could represent the arc prior to boninitic volcanism and immediately after MORB-like basalts. Boninite from the Bonin Islands are characterized by high Δ7/4Pb and low 143Nd/144Nd relative to local MORB sources and the MORB-like basalt recovered in the forearc. The Bonin Islands are also distinct from the 44 Ma volcanics from the Hahajima Islands and the Bonin Ridge escarpment (Ishizuka et al., 2006). So potentially the geochemical and isotopic characteristics of the arc may have evolved by 1) an initial decompressional melting without significant slab flux producing MORB-like basalt at the onset of forearc spreading, 2) melting of subducted pelagic sediment and extremely depleted mantle followed during a limited period (48-45 Ma), 3) subsequent melting with more extensive hydrous fluid input generated the tholeiitic and calcalkaline magma after 44 Ma. Dating results on the forearc basalts will be presented at the conference, which for the first time will enable the magmatic evolution of the early stages of arc initiation to be traced.
V31A-2107
Chemical Evolution of the Izu-Bonin Arc Recorded by Chlorine Stable Isotopes
δ37Cl values were determined for Izu-Bonin arc magmas erupted 0-44 Ma in order to better understand the time-dependent processing of volatiles in subduction zones. Pristine ash-sized particles (glass, pumice, scoria, and rock fragments) were handpicked from tephra drilled at ODP Site 782. δ37Cl values for these particles span a large range from -4.9 to +1.1‰ (error < ±0.4‰) vs. SMOC (Standard Mean Ocean Chloride, defined as 0‰). Different components from individual samples (pumices separated from amphibole or from scoria) were analyzed independently. In both samples, the pumice separate had a lower Cl isotope ratio (by 2.3-2.7‰) than the coexisting phase. This may be due to crystal fractionation enriching 37Cl in the crystallizing phases or to degassing of a phase enriched in 37Cl. The temporal data overlap with previously reported δ37Cl values of –2.6 to +0.4‰ for bulk ash and –5.4 to –0.1‰ for volcanic gases from the Quaternary Izu-Bonin-Mariana volcanic front (Barnes et al., in press). This range is considered to reflect the diversity of the trench input. However, the temporal data indicate a time-progressive evolution, as Eocene and Oligocene magmas generally have lighter δ37Cl (-2.7±1.4 ‰, n= 9) than the Neogene magmas (-0.5±0.9‰; n=8). We speculate whether this may be due to a trench input that became isotopically heavier with time, which might reflect the lack of an isotopically positive serpentinite component during the early history of the arc. This hypothesis can be tested by analyzing material from the earliest Izu-Bonin-Mariana arc (~45-50 Ma) preserved in the forearc. Additional work is also needed to assess potential artifacts from alteration or crustal differentiation, as well as the extent of crustal differentiation, such as crystallization or degassing, on Cl isotope fractionation.
V31A-2108
Nature of the crust-mantle transition layer during the crustal growth along the Izu-Bonin island arc deduced from the seismic amplitude modeling
The Izu-Bonin island arc is a typical oceanic island arc formed by subduction of the Pacific plate beneath the Philippine Sea plate and the location that the continental middle crust is produced (e.g., Suyehiro et al., 1996). From the seismic velocity structure, the Izu-Bonin island arc beneath the volcanic front has the 7.2-7.6 km/s layer (crust-mantle transition layer) underlying the lower crust (Kodaira et al., 2007). The crust-mantle transition layer is considered as the composition formed by the interaction between the crust and uppermost mantle during the crustal growth (e.g., Tatsumi et al., 2008). However, since this velocity structure beneath the volcanic front along this arc is calculated by the tomography method, the nature of the crust-mantle transition layer and uppermost mantle and the depth of the Moho are unknown. To understand the nature of this transition layer and the depth of the Moho along this arc, it is also important to know the seismic reflectivity at the top and bottom of this transition layer. In this study, we clarify the distribution of the seismic reflectivity at the top and bottom of the crust-mantle transition layer beneath the volcanic front along this arc using the velocity contrast values at these reflectors estimated by the amplitude modeling of wide-angle data. In 2004 and 2005, seismic refraction/reflection surveys using ocean bottom seismographs (OBSs) and controlled sources were conducted beneath the volcanic front along the Izu-Bonin island arc from Sagami Bay to Kaitoku Seamount (Kodaira et al., 2007). In record sections of several OBSs, not only the first arrival phases but also later phases reflected from interfaces in the crust and uppermost mantle are visible. These later phases can be considered as the reflected from the top and bottom of this transition layer. The velocity contrast values at the top and bottom of this transition layer were estimated from the comparison of the observed and synthetic wave forms computed by a finite difference wave propagation program code (Larsen and Grieger, 1998). Along this island arc, the top of this transition layer has a velocity contrast value of about 0.4 km/s from Kurose hall to Kaitoku Seamount, except for the area ranging from Nii-jima to Kurose hall (0.25 km/s). On the other hand, the velocity contrast value at the bottom of this transition layer is large (>0.4 km/s) between Nii-jima and Kurose hall, and small (0.2 km/s) from Kurose hall southward. Especially, this value at the bottom of this layer ranging from Kayo Seamount to Nishi-no-shima is almost 0 km/s. From these results of this study and the petrological study (e.g., Tatsumi et al., 2008), this transition layer beneath the volcanic front along this arc is a mixture of the mafic residues and olivine cumulates formed during the crustal growth. In addition, the no velocity contrast value at the bottom of this transition layer implies that this transition layer highly consists of olivine cumulates, so that the area from Kayo Seamount to Nishi-no-shima may show the infant stage during the crustal growth. The Moho along the volcanic front in the oceanic island arc probably has a complex signature, as suggested by the crust-mantle transition layer composed of mixture of mafic residues and olivine cumulates.
V31A-2109
The crustal characteristics at syn- and/or post-rifting in eastern Shikoku basin by seismic reflection survey
Imaging of the arc-backarc transition zone is important in relation to the backarc opening process. Shikoku Basin locates between the Kyushu-Palau Ridge and the Izu-Ogasawara Arc, which is an important area to reveal the opening evolution of the backarc basins as a part of the growth process of the Philippine Sea. The Shikoku Basin was in the backarc rifting and spreading stage during about 30-15 Ma (e.g. Okino et al., 1994). High P-wave velocity lower crust is identified in arc-backarc transition zone by refraction survey using OBSs (Takahashi et al., 2007). Japan Agency for Marine-Earth Science and Technology (JAMSTEC) carried out multi-channel seismic reflection (MCS) survey using 12,000 cu.in. air gun and 5 km streamer with 204 ch hydrophones in the Izu-Ogasawara region since 2004. We extracted and mapped the crustal characteristics from poststack and prestack depth migrated profiles. According to obtained profiles, the deformation structure with share component is recognized in arc-backarc transition zone, which located eastern side of Shikoku Basin from Zenisu Ridge to about 500 km south. The maximum width of this deformation zone is about 100 km. The relative displacement of horizon is little; however, it is strongly deformed from upper crust beneath seafloor. This deformation zone indicates the post- rifting activity in east side of Shikoku Basin. On the other hand, some knolls are indicated along the en- echelon arrangement from Izu-Ogasawara arc. Ishizuka et al. (2003) reported post-rifting volcanism with Miocene age in en-echelon arrangement. A part of these knolls are estimated to penetrate at syn-rifting and/or post-rifting stage in backarc opening. By comparing the both side of arc-backarc transition zone, we elucidate syn- and post-rifting effect in Shikoku Basin. We also carried out high density MCS surveys in Shikoku Basin in order to IODP proposal site for reconstruction of magmatic processes since Oligocene in rear arc. In this survey, we use new tuned airgun array with total capacity of 7,800 cu. in. for high resolution imaging. Preliminary result shows that the proposed site is covered with thick sediments, and acoustic basement is seen at depth of 1.5-2 km (1.5-2 sec in two way traveltime) from sea bottom, a part of which is discontinuous. Many clear reflectors can be observed within sediments, some of which corresponding to those identified in previous MCS lines.
V31A-2110
Mantle Heterogeneity in the Southern Mariana Trough Indicated Through B and Sr Isotopic Systematics.
We have studied glasses from the Southernmost Mariana Trough (13.4-14.3° N and 144-144.6° E) recovered during the Cook 7 expedition (2001). Due to their differing proximities to the volcanic front of the Mariana island arc (33 to 106 km), these samples are critical for evaluating across arc elemental and isotope transport and possible linkages between back-arc spreading ridges and the adjacent volcanic arc chains. Petrographic examination shows all of the samples to be unaltered dark colored volcanic glasses with rare crystals of olivine, pyroxene and plagioclase. Their Mg #s range between 33 and 60, with most Mg# >50. Although samples were recovered from water depths of 3.2 to 4.2 km, there is lack of major element and Cl/K concentrations that indicate any seawater alterations (e.g. LOI and Na contents). The lack of seawater signature is also shown by low (0.70275 to 0.70302) 87Sr/86Sr, which is depleted MORB range. The glasses also have low H2O (~1 wt%). This lack of secondary glass alteration allows study of the behavior of volatile and low- temperature alteration sensitive tracers (e.g. B, Cl and B and Sr isotopes). There is no correlation between depth of recovery and B, Cl or d11B systematics, indicating that the degree of vesiculation did not control the d11B. The B content of the glasses is low (generally MORB- like) and the Cl contents show progressive increase with distance from the volcanic front and range between 150 and 1300 ppm. The d11B ratios vary widely and are between +5.3 to -5.6 per mil. As expected, the d11B values become progressively heavier closer to the volcanic front, where they are indistinguishable from the Mariana arc lavas. Interestingly, the heaviest d11B samples are also associated with elevated ratios of Ba/La and U/Nb, and to some extent also with elevated Th/Yb. In arcs, U and Ba are fluid mobile, while Th is a sediment melt sensitive element. From the preliminary dataset it appears that the samples from the S. Mariana Trough are transitional between typical island arc and back-arc magmas. Our new data confirms the proposed highly variable mantle sources along the strike of the Izu-Bonin-Mariana back-arc basalts (Pearce et al., 2005) and stress the fact that distance from the volcanic front may be an important constraint to consider when evaluating high resolution tomographic imaging and modeling mantle flow in the southern Marianas.
V31A-2111
Fluid Source-based Modeling of Melt Initiation within the Subduction Zone Mantle Wedge: Implications for Geochemical Trends in Arc Lavas
The GyPSM-S (Geodynamic and Petrological Synthesis Model for Subduction) scheme couples a petrological model (pHMELTS) with a 2D thermal and variable viscosity flow model (ConMan), to describe and compare fundamental processes occurring within subduction zones. Here we supplement basic GyPSM-S models with a more sophisticated treatment of trace element partitioning in the fluid phase and of melt transport regimes to investigate the influences of slab fluid source lithology and fluid transport mechanisms on melt geochemistry, the implications of mantle source depletion related to fluid fluxing, and potential melt migration processes. Changing model parameters indicate that slab age and slab dip are the primary controls on slab-adjacent low-viscosity channel (LVC) shape and thickness, due to changes in the fluid release patterns. Slab age and convergence velocity, which contribute to the slab thermal structure, are significant for the locations of dehydration reactions within the different lithological layers of the slab. The fluid source lithology determines the fluid flux and the fluid-mobile trace element input to the wedge. This study focuses on two cases that represent extremes within our model set, an old slab with a low rate of convergence and and a relatively young slab with a higher rate of convergence. Results are compared to actual geochemical datasets for the Izu-Bonin intra-oceanic subduction system and the Central Costa Rican part of the Central American arc. We find that there is a progression of geochemical characteristics described in studies of cross-arc and along-arc lavas that can be duplicated assuming (i) limited fluid-rock interaction within the mantle wedge and (ii) that melt migration preserves the spatial distinction among melts initiated in different areas of the wedge. Specifically, volcanic front lavas have significant contributions from shallower slab fluid sources, and rear-arc lavas have significant contributions from deeper slab fluid sources. Evidence for limited fluid-rock interaction could imply either a rapid fluid transport mechanism or a fluid-dominated trace element budget within the LVC. Although we do not include a back-arc in these models, interpretations of the results lead to several potential mechanisms to explain hydrous inputs to back-arc source regions.
V31A-2112
Petrologic Evolution of Palau; A Nascent Island Arc
Initiation of subduction in intra-oceanic settings requires relative differences in plate thickness and density (i.e., age difference); a major zone of weakness separating the plates, e.g., a fracture zone; and a change in relative vectors of movement to cause convergence. These factors help explain the origin of the southern- most part of the > 2500 km long Kyushu - Palau Ridge (KPR). Palau Islands, at 7 deg 30 ' N, are the only significant emergent feature on KPR. Small islands are mainly uplifted Pliocene and younger reef carbonate. Large islands are mainly volcanic comprising rare boninite; major basalt, basaltic andesite and andesite; and minor dacite. Polymict breccia is abundant; sills, flows, dikes are common; pillows are rare. The same rock types, as well as high-Mg basalt, were dredged from the Palau Trench. Volcanism on Palau began in late Eocene and ended by early Miocene. Rocks are low-K primitive island arc- tholeiite series. None are MORB. REE and HFSE require a depleted mantle source. Zr* and Ti* suggest that melts interacted with OL-PX rocks of upper mantle or deep crust cumulates. Moderate enrichment of LILE and LREE indicate influx of "dehydration fluid." Ce/Ce* and Eu/Eu* show no evidence for subducted sediments or recycling of arc- derived clastics yet there is no accreted sedimentary prism. This paradox may be owing to lack of arc or terrigenous clastics on seafloor formed in open ocean "sterile" equatorial latitudes. Plate reconstructions and paleomagnetic data suggest that the "arc" probably formed on the trace of a transform fault that has migrated northward and rotated clockwise up to 90 deg. since Oligocene time. Episodes of transtension allowed upwelling of relatively fertile hot mantle into depleted mantle and sheared, altered, rocks of the transform. Episodes of transpression may have initiated subduction of seafloor having a thin cover of pelagic sediments (calcareous and radiolarian ooze, chert, chalk, limestone) deposited far from terrigenous sediment sources. The Palau "arc" is aseismic; we have no insight to depth of the subducted seafloor. Rocks similar to Palau series on Guam, and adjacent slopes of the Mariana Trench may have been part of this nascent convergence system.
V31A-2113
Source Components and Intensive Parameters of Magma Genesis in the CentAm and North IBM Arcs: Analyses using Arc Basalt Simulator Version 2 Model
We have developed a mass balance calculation model for basalt magma genesis in subduction zones. The Arc Basalt Simulator version 2 (ABS2) model includes: 1) calculation of slab fluid composition based on PerpleX (ver.7) metamorphic mineralogy for altered oceanic crust (AOC) and sediment (SED) along the slab P-T trajectory derived by the latest geodynamic model; 2) zone refining chemical modification of the slab-derived fluid by interaction with the mantle peridotite, using mineralogy calculated by PerpleX for low-T hydrous peridotite and for high-T peridotite with nominally anhydrous minerals; and 3) fluid-fluxed, open system melting of the high-T mantle peridotite based on pMELTS mineralogy and Katz et al. (2003)s hydrous melting parameterization. Application of the ABS2 model to the northern Izu arc (N-Izu) and the Central America arcs (CentAm) highlights differences between the two arc systems. N-Izu basalts are best fit by a mixture of 10% sediment (SED) and 90% altered oceanic crust (AOC), with fluid dehydration at 4GPa/850°C beneath the volcanic front (VF) and 5GPa/900°C rear arc beneath the rear-arc (RA). Mantle melting conditions for N-Izu lavas are F=24% with 2-5% fluid at 1.0-2.5GPa/1250-1400°C (VF) and 6-20%F with 1-5% fluid at 2.5 GPa/1000-1200°C (RA). Guatemala-Nicaraguan basalts require two AOC components: Cocos-Nazca Spreading Center (CNS) and Northern Galapagos domain (NGD) with increasing HIMU component in that order. Guatemala VF basalt (Izarco) requires CNS-AOC, SED, and PM with SED:AOC = 25:75, dehydrated at 5GPa/870°C, and mantle melting at 15%F/2.7GPa/1050°C with 4% fluid. Costa Rica VF basalt (Arenal: strongest HIMU signature) requires NGD-AOC, SED, and PM. Slab dehydration conditions are SED:AOC = 30:70 at 5.7GPa/990°C. Melting conditions are 13%F/2.8GPa/1100°C with 4% fluid. Slab dehydration temperature is generally higher beneath CentAm VF (830-900°C) than N-Izu VF (780-860°C), reflecting the much younger age of the CentAm slab. Mantle melting degree is high for N-Izu VF (F=25%) but low for CentAm VF (F=14- 16%) although fluid flux is similar (3-5%). Mantle melting temperature is high for N-Izu VF (1250- 1400°C), lower for CentAm VF and N-Izu RA (1000-1200°C), as reflected in the low degree of melting in the latter cases. These results are consistent with a colder slab and a hotter mantle beneath N-Izu. The slab SED:AOC contributions are different between arcs perhaps reflecting difference in the slab temperature of the subduction kinematics.
V31A-2114
Geochemical diversity of chromian spinel bearing alkaline Matsue Basalt in the Shimane Prefecture, southwest Japan
Matsue city located in Japan Sea coast side of southwest Japan arc. There are many alkaline basic- intermediat lavas that we called gMatsue Basaltsh in Matsue city area. The activity of the Matsue Basalt is about 10 Ma ago. Matsue basalt basically consist of 6 basic lavas and some itfs pyrocrastic deposite (sandstone and tuff). The pourpose of this study is clarifing geochemical characteristics of Matsue basalt. We here report the Bulk chemical composition, Mineral assemblege and mineral chemistry of Matsue Basalt. This is first report of systematic chemical study about Matsue Basalt. And we found chromian spinels in the many rock samples. Acording to our result of bulk rock chemical compositions, Matsue Basalt are divided into 6 groups, which are 3 basalt groups (Chausuyama, Hanamagari and Agenogi groupes), 2 basaltic andesite groups (Tsuda and Rakuzan groups) and 1 andesite group (Toukodai groupe). However it was clarified that it was a chemical same feature with Tsuda and Rakuzan groups among Matsue Basalt. That is Matsue Basalt is chemically consists of 5 groups. Chausuyama and Agenogi basalts show particularly with respect to their low FeO*/MgO ratios and high Cr content. And these two basalts have chromian spinels in and around olivine grains. Chromian spinels from Chausuyama and Agenogi basalts have Cr# (Cr/(Cr+Al) atomic ratio) of 0.16- 0.40, and olivines from these basalts have Fo of 88-76. Arai (1987, 1994) proposed that Fo-Cr# relations depend on the tectonic setting, due to differing melting conditions (pressure, temprature and water vaper conditions). That is Chausuyama and Agenogi basalts of Matsue Basalt derived from the relativery depleated lerzolitic mantle.
V31A-2115
Seismological evidence for thermally-controlled dehydration reactions in the oceanic crust of the Pacific slab
When the oceanic plate subducts at the trench, the hydrated oceanic crust carries water into the earth in the form of hydrous minerals. Hydrous minerals in the oceanic crust become unstable with increasing pressure and temperature, and dehydration reactions occur accompanied by the release of water to the surroundings. Tsuji et al. (2008) estimated detailed seismic-velocity structure around the Pacific slab in the central part of northeastern Japan by double-difference tomography and detected a remarkable low-velocity zone with a thickness of ~10 km, which corresponds to hydrated oceanic crust, at the uppermost part of the slab. The zone gradually disappears at depths of 70-90 km, suggesting the occurrence of intensive dehydration reactions there. This study extends the analysis by Tsuji et al. (2008) to a wider area from Hokkaido to Kanto to investigate whether or not the depth extent of the hydrated oceanic crust in the Pacific slab varies along the arc. Obtained results show a striking along-arc variation in the depth extent of the hydrated oceanic crust. Although the low-velocity oceanic crust is generally distributed down to depths of 70-90 km in Hokkaido and Tohoku, there are two exceptional regions where it is imaged down to deeper depths: 100-140 km in Kanto and 90-110 km in Hokkaido corner (Fig.1). Such preservations of the low-velocity oceanic crust to deeper depths can reflect the delay of dehydration reactions of hydrous minerals, which in turn suggests lower- temperature conditions in the slab. The lateral extent of the low-velocity oceanic crust preserved to deeper depths beneath Kanto corresponds well to that of a region where the Pacific slab is in contact with the overriding Philippine Sea slab. The contact of the Philippine Sea slab with the Pacific slab can hinder effective heat transfer to the Pacific slab from hot mantle wedge, and therefore, the Pacific slab can retain lower temperatures than normal subduction environments. On the other hand, the deeper preservation of the low-velocity oceanic crust beneath Hokkaido corner may be related to low-heat transfer because of accumulated materials above the slab by the fore-arc sliver. Interestingly, the upper-plane seismic belt (Kita et al., 2006; Hasegawa et al, 2007), a belt-like concentration of seismicity at depths of 70-90 km in the upper plane of the double seismic zone, is also shifted to deeper depths only beneath the two regions. These observations suggest that the preservation of the hydrated oceanic crust to deeper depths and deepening of the upper-plane seismic belt in the two regions can be caused by the delay of dehydration reactions resulting from lower-temperature conditions in the Pacific slab.
V31A-2116
Frequency Dependent Shear Wave Splitting Beneath Japan and Implications for the Mantle Wedge
Despite its importance for our understanding of physical processes associated with subduction, the geometry of mantle flow in subduction zones remains poorly understood. Shear wave splitting is a valuable tool used to characterize the geometry and strength of seismic anisotropy. In turn, this knowledge can help us learn about mantle flow and deformation. A complete characterization of shear wave splitting is particularly important for Japan, which overlies several subduction zones with complex slab morphologies; previous studies indicate that the upper mantle beneath Japan exhibits complicated anisotropy that manifests itself in complex splitting patterns. In order to add to our knowledge of anisotropy and mantle flow beneath Japan, we analyzed direct S waves from local earthquakes originating in the subducting slabs for evidence of shear wave splitting. Data was obtained from fifty-four broadband seismic stations in Japan's F-net array. In order to look for any dependence of splitting parameters on frequency that may indicate the presence of complex anisotropy, we carried out our splitting analysis in two different frequency bands (0.02-0.125 Hz and 0.125- 0.5 Hz). Our measurements indicate that shear wave splitting due to upper mantle anisotropy beneath Japan is highly complex and exhibits both dramatic spatial variations and a strong dependence on frequency. We identified several regions where local events exhibited trench parallel fast directions close to the trench with a flip to trench perpendicular farther away from the trench. This is consistent with the presence of B-type olivine fabric in the shallow corner of the mantle wedge and subduction parallel flow in the deeper mantle wedge. Delay times for all regions besides Tohoku ranged from 0.85 to 2.2 sec with a regional average of ~1.4 sec at low frequencies; in contrast, the average delay time using the higher frequency bandpass was ~0.6 sec. Although the data set is limited, we identified average delay times from the Tohoku region of ~0.3 sec using our higher frequency bandpass; these are somewhat larger than the delay times of ~0.1 sec observed by Nakajima & Hasegawa (2004) at much higher frequencies, but much smaller than local delay times elsewhere in Japan.
V31A-2117
Boron and trace element systematics of Quaternary basaltic rocks from Kyushu, Southwestern Japan
Volcanism in Kyushu is associated with subduction of young (26-15 Ma) Philippine Sea Plate (PSP) in the north and old (60-40 Ma) PSP in the south. The Kyusyu-Palau Ridge subducts nearly at right angles to central Kyushu, marking the boundary between these two segments. Prior work has shown that the subducted PSP contributes to the volcanic front however it does not reach the backarc region. We have analyzed boron (B), fluid mobile trace elements and high field strength elements in Quaternary volcanic rocks from Kyushu to investigate the influence of contrasting PSP segments on the mantle composition. Since B and other fluid-mobile elements are concentrated in slab-derived fluids, we can use these data to estimate the subducted contribution to the subarc mantle composition. Results show that along-arc variations of B ratios are not regular. Lavas from Aso and Kirishima volcanoes (central volcanic front) show the highest B/Nb (3.0-3.7), B/Be (5.4-18.6) and B/La (0.5-1.9). B/Nb, B/Be and B/La ratios of the southern Kyushu basaltic rocks (Sakurajima and Kaimon volcanoes) are higher than those of the northern Kyushu basaltic rocks (Kuju, Yufu, Oninomi). Other fluid-mobile and LIL element ratios (i.e, Cs/Th, Li/Yb, Ba/La) show much less along-arc variation than B. Basalts from these three regions show distinct trends on a plot of Ba/Zr vs. B/Zr: the Kirishima suite shows the highest slope array, while Aso, and the southern Kyushu volcanoes follow an intermediate trend, Yufu and Oninomi in northern Kyushu have the lowest B/Zr and the highest Ba/Zr. These observations are compatible with distinct contributions from the old and young segments of PSP. In northern Kyushu young, hot oceanic slab is probably strongly devolatilized before it reaches the volcanic front. In contrast, slower dehydration of the older and cooler PSP segment likely occurs beneath southern Kyushu Higher B/Nb, B/Zr, B/Be and B/La ratios observed in Kirishima basalts may reflect the subduction of a seamount chain which is the extension of Kyushu-Palau Ridge.
V31A-2118
Origin of amagmatic hydrothermal solutions inferred from TL, FT and K-Ar dating, and fluid inclusion data: A case study in the southern part of Kii Peninsula, SW Japan
Although there is no known evidence of volcanism during the Pliocene nor the Quaternary in the Kii
Peninsula, it has long been recognized to host several hot springs with discharge temperatures greater than
60°C. In addition, numerous small-scale vein-type metal deposits are distributed around the southern
part of the peninsula, with a heat source thought to be the Middle Miocene acidic magmatism associated with
Kumano Acidic Rocks. The results of the TL (Thermoluminescence), FT (Fission Track) and K-Ar dating of
altered rocks from these hot spring areas showed that the vein-type ore deposits and their surrounding
altered rocks experienced high temperature hydrothermal alteration related to acidic magmatism in the
Middle Miocene, whereas relatively low temperature alteration has occurred since the Pliocene in the Hongu
and Totsukawa hot spring areas [Hanamuro et al., 2008]. Chemical and isotope data were obtained for
fluid inclusions trapped in hydrothermal minerals in the peripheral parts of the high-temperature hot springs
and in vein-type ore deposits. The hot spring inclusions indicate temperatures reached ~100°C
with salinities of about 2 wt % (NaCl equiv.). In contrast, the inclusions in the vein-type deposits are
characterized by high temperature fluids (>260°C) with high salinity (>5 wt %). The
3He/4He ratios of the hot spring inclusions have relatively high values, generally in agreement with
those of the present-day hydrothermal fluids, indicating a significant contribution by deep source gases (i.e.,
mantle helium). These results suggest that the amagmatic hydrothermal system related to high-temperature
hot springs in the southern Kii Peninsula have formed since the Pliocene and were caused by high
temperature fluids with a lower crust provenance, presumably supplied from the subducting slab of the
Philippine Sea Plate (PHS) [Umeda et al., 2006]. After a hiatus of about 4 Ma, the PHS resumed
subduction beneath the SW Japan Arc at around 6 Ma [Kamata and Kodama, 1994]. This indicates that
the present-day plate system for the SW Japan arc formed at around 6 Ma and continues to the present day.
Although we lack definitive information on the age of hydrothermal alteration, it seems reasonable to infer
that the amagmatic hydrothermal activity since about 6 Ma in the southern Kii Peninsula has occurred in a
manner synchronous with the present-day plate system of the SW Japan Arc.
Reference
Hanamuro et al. (2008): Japanese Magazine of Mineralogical and Petrological Sciences, 37, 27-38 (in
Japanese with English Abstract).
Kamata and Kodama (1994): Journal of Geophysical Research, 233, 69-81.
Umeda et al. (2006): Journal of Volcanology and Geothermal Research, 149, 47-61.
V31A-2119
Wakurayama dacite as a possible adakites from the Matsue city, inner zone of Southwest Japan
The petrological and geochemical characteristics of the Wakurayama dacite, Matsue city, Southwest Japan have been examined. The activity of the Wakurayama Dacite is about 5 Ma ago. In this study, petrological details of Wakurayama dacite became clear. We classified this dacite by the color of the surface of the rock and texture under the microscope. And bulk chemical compositions of rocks ware determined by using of XRF method. Wakurayama dacite is basically divided into three groups which are Red, Olive and Gray groups by their color. Those groups can be also distinguished compared with the amount of the minerals and with bulk rock chemical compositions. These groups having flowed in order of Gray group, Olive group and Red group became clear by their stratigraphy. In addition, the important geochemical feature is that tholeiitic rock series and calc-alkaline rock series are coexistent in one volcano. That is Gray group (FeO*/MgO ratio: 2.55 in average), Olive group (FeO*/MgO ratio: 6.42 in average) and Red group (FeO*/MgO ratio: 2.51 in average) are tholeiite, calk-alkaline and tholeiitic rock series respectively. And almost all rocks of Wakurayama dacite show high in Al (18.96 wt% in average), low in Mg (1.36 wt% in average), high in Sr (636 ppm in average) and low in Y (10 ppm in average). Above chemical features of wakurayama dacite is very similar with adakites. That is Wakurayama dacite magma may derived from the subducted materials like oceanic sediment.
V31A-2120
Generation of rear-arc magmas induced by influx of slab-derived supercritical liquids beneath Rishiri Volcano, Kurile arc
Subduction zone is one of the most important sites of generation of the Earthfs magmatism. Although there is a consensus that water-rich materials released from the subducting slab play a fundamental role in the generation of arc magmas, debate has continued about the details of subduction-zone processes, such as the nature and chemical compositions of slab-derived materials, processes and time-scales of material transport from the slab to the source mantle, the relative importance of decompression melting to fluid-fluxed melting, and so on. To assess these issues, filed-based constraints, supported by high-quality geochemical data, have been accumulated. However, detailed studies on rear-arc lavas are still scarce, partly because the number of rear-arc volcanoes is limited owing to lower magma production rates in the rear-arc side than along the volcanic front. Because rear-arc lavas represent products of one end-member of arc magmatic processes, they have the potential to provide important information for general understanding of subduction- zone processes. In this study, we investigated magma generation processes for alkali basalt lavas from Rishiri Volcano, located towards the rear of the Kurile arc system (the depth to the subducting slab is 300 km), using major and trace elements and Sr, Nd, Pb and Th isotopic data. The Numaura and the Araragiyama lava flows, investigated in this study, show a significant variation in TiO2 contents despite a limited variation in SiO2 content. The TiO2 contents correlate positively with 143Nd/144Nd and negatively with 87Sr/86Sr, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb. The compositional variations of the lavas cannot be explained by magma chamber processes, such as fractional crystallization, crustal assimilation and magma mixing, and they are suggested to have formed principally during magma generation. The variation of the TiO2 contents essentially reflects a variation of the degree of partial melting of the source mantle, and it is inferred that the melting degree correlated positively with amounts of slab-derived materials influxed into the melting region. The melting appears to have occurred progressively under isothermal and isobaric conditions, as slab-derived materials were continuously supplied. The geochemical variations in the lavas can be explained by mixing of depleted mid-ocean ridge basalt source mantle with slab-derived materials consisting of an altered oceanic crust component and a sediment component, suggesting only single-stage transport of slab materials to the source mantle. The slab-derived materials are likely to have contained not only Sr, Ba, Pb and U, but also significant amounts of Nd and Th that are not highly soluble in aqueous fluids. The materials are thus suggested to have been supercritical liquids, and it is suggested that magma generation occurred at depths greater than that at which supercritical liquids were decomposed into aqueous fluid and silicate melt components. The lava samples show 238U-230Th disequilibrium with 230Th excess; this 230Th enrichment resulted primarily from the high-Th nature of the slab-derived materials.
V31A-2121
Zig-Zag Thermal-Chemical 3-D Instabilities in the Mantle Wedge: Numerical Study
To understand the plume initiation and propagation it is important to understand whether small-scale convection is occurring under the back-arc in the Low Viscosity Wedge(LVW) and its implication on the island-arc volcanism. Honda et al. [Honda and Saito, 2003; Honda, et al., 2007]) already deployed small- scale convection in the Low Viscosity Wedge (LVW) above a subducting slab with kinematically imposed velocity boundary condition. They have suggested that a roll (finger)-like pattern of hot and cold anomalies emerges in the mantle wedge above the subducting slab. Here, we perform three-dimensional coupled petrological-thermomechanical numerical simulations of intraoceanic one-sided subduction with spontaneously bending retreating slab characterized by weak hydrated upper interface by using multigrid approach combined with characteristics-based marker-in-cell method with conservative finite difference schemes[Gerya and Yuen, 2003a], to investigate the 3D instabilities above the slab and lateral variation along the arc. Our results show that water released from subducting slab through dehydration reactions may lower the viscosity of the mantle. It allows the existence of wave-like small-scale convection in the LVW, which is shown as roll-like structure in 2D petrological-thermomechanical numerical experiments [Gorczyk et al., 2006] using in-situ rock properties computed on the basis of Gibbs free energy minimization. However, in our 3D cases, the rolls aligning with the arc mainly occur earlier , while zig-zag small-scale thermal-chemical instabilities may episodically form above the slab at later stages, which is different from the aligning finger-like pattern in purely thermal models (Honda et al,2003;2007). Also in contrast to thermal convection chemically buoyant hydrated plumes rising from the slab in our models are actually colder then the mantle wedge [Gerya and Yuen 2003b] which also strongly modify both the convection pattern and the seismic structure in the wedge. The zig-zag wave-like 3D instabilities have another implication on the Quaternary volcanism in the Northeast Japan. References Gerya, T. V.,Yuen D. A. (2003a), Physics of The Earth and Planetary Interiors, 140, 293-318. Gerya, T.V., Yuen, D.A. (2003b), Earth and Planet Sci. Lett., 212, 47-62. Gorczyk, W., et al. (2006), Geochem. Geophys. Geosyst., 7, doi:10.1029/2005GC001075. Honda, S., Saito M. (2003), Earth and Planetary Science Letters, 216, 703-715. Honda, S., et al. (2007), Island Arc, 16(2), 214-223.
V31A-2122
Petrology and geochronology of eclogitic metasediments from the north Qilian Mountains, western China: constraints for the geodynamic evolution of early Paleozoic subduction- accretion complexs
In this contribution, we present new Petrological and U-Pb SHRIMP dating data for metasedimentary rocks from the North Qilian (NQL) orogenic belt, in which typical early-Paleozoic ophiolitic sequences and HP/LT metamorphic rocks have been previously recognized. The studied metasedimentary rocks contain eclogitic facies assemblages reflecting PT conditions of 450-520 °C and 19-23 kbar, consistent with those of adjacent eclogites. Geochemical data indicate that the precursor rocks of the metasedimentary rocks were immature sediments formed in continental margin or island arc environments. Zircons from three eclogitic metasediment samples were separated for U-Pb SHRIMP datings. CL images from three samples revealed the presence of a heterogeneous zircon population containing euhedral grains and fractured fragments. The U¨CPb data of two metasedimentary rocks from the Baijingsi, eastern segment of NQL HP/LT metamorphic belt, demonstrate input from sources with ca. 475Ma, 509Ma, 539Ma, 711-760Ma, 1500-1800Ma and >1.8 Ga zircon ages with prevailing age concentrations in about ca.500 Ma and around ca.1800 Ma, suggesting that they derived from the mixture of Proterozoic continent and early Plaeozoic island arc (?) materials. No evidence for recrystallization zircon during the Early Paleozoic high-pressure metamorphism was recognized and gave a maximum depositional age of ca. 475Ma. One metasedimentary rock sample from the Xiangzigou, about 70 km west of the Baijingsi, gave a metamorphic overgrowth age of 495 Ma, which is identical to the age of 489 Ma obtained from adjacent eclogite within error, representing eclogitic metamorphic time. Predominant Proterozoic detrital zircons and the absence of early Paleozoic detrital zircons suggest the protolith of this rock probably derived from Precambrian continental margin. These data implies that subduction erosion of the active continental margin occurred, as these sediments, formed in the upper plate (fore-arc basin or accretionary prism), were transported in the subduction zone to 60-70 km depth prior to their exhumation. In addition, these data also suggest that Early Paleozoic subduction may be multistage or diachronous in the NQL orogenic belt.
V31A-2123
Does the Depth to Slab Control Volatile Contents? New Insights from Glasses Erupted at the Fonualei Spreading Center (NE Lau Basin), a Very Arc-like Back-arc
The role of water and other volatiles in generating arc and back-arc volcanism is fundamental; however, the relationship between physical subduction parameters and volatile recycling remains poorly understood. Here, we present new volatile data from the Fonualei Spreading Centre (FSC), a back-arc environment in the NE Lau Basin where the depth to the slab increases from 125 km in the south (similar to typical arc settings) to 210 km in the north. The southernmost tip of the FSC lies within 20 km of the adjacent Tofua Arc, thus offering an ideal case study of arc-related magmatism. The FSC terminates in the north at the Mangatolu Triple Junction (MTJ), situated ~140 km to the south of the edge of the Australian Plate where a tear in the plate allows the influx from less depleted mantle1. Prior work2 shows that the major and trace element compositions of the FSC are relatively depleted and similar to that of the adjacent Tofua arc, whereas the MTJ samples are derived from a less depleted mantle and trend towards Lau back-arc basin basalt compositions. This geochemical contrast is also found in volatile contents reported here. CO2 and S contents are higher in the northern MTJ samples (up to 110 ppm and 800 ppm, respectively) as compared to the southern FSC samples (less than 10 and 40 ppm). To some extent, this difference can be attributed to enhanced degassing in the FSC samples since they are erupted at shallower depths. Indeed, S contents correlate broadly with CO2 contents; however, degassing cannot solely account for the sharp discrepancy in sulfur contents. The less depleted, more reduced nature of the MTJ mantle has likely influenced the S concentrations. The H2O contents of glasses cover an intermediate back-arc range (1 to 1.5 wt%) and show the characteristic negative correlation with TiO2, indicating that flux melting processes prevail3. A slab signature can be clearly recognized: B/Yb and Ba/Yb both show a positive correlation with H2O/Yb and with each other. The FSC glasses generally have higher H2O/Yb than the MTJ samples, consistent with the notion that the FSC has a more arc-like character2. These data suggest that the melting process is influenced by a slab component containing elevated H2O, B, Ba and Cl which, at the scale of the FSC-MTJ system, shows a stronger arc signature at shallower depth. However, within the FSC despite a range of depth to the slab of 125 km to 170km, no correlation can be seen between slab tracers and depth to the slab, which suggests that either the dehydration of the downgoing plate is a continuous process down to a depth of at least 170km, or that melt generation within the wedge is also influenced by regional mantle dynamics. 1 Turner and Hawkesworth, 1998 2 Keller et al., 2008 3 Kelley et al., 2006
V31A-2124
The Influence of Volatiles on Trace Element Systematics of Back-arc Basin Magmas and Sources
Water is a central component in back-arc basin processes, but the relationship between water and trace element indicators of melting processes or slab fluids is largely unknown. Here, we present new SIMS measurements of magmatic volatiles (H2O, CO2, S, Cl, F) and new LA-ICP-MS trace element data in primitive basaltic glasses (>7 wt.% MgO) from the Manus, Woodlark, and Lau basins in the western Pacific. These back-arc basin basalts (BABB) generally indicate vapor saturation or oversaturation with H2O- CO2 relative to the pressure of collection. The major element constituents of slab-derived fluids (H2O, Na, K, and Cl) are remarkably similar between these basins, compared to the greater diversity observed in arc settings. Water correlates with decreasing concentrations of elements sensitive to melting processes (e.g., TiO2), and with increasing concentrations of fluid-mobile trace elements (e.g., Ba/La) that likely derive from the slab. The H2O/Ce ratios of these BABB also span a wide range (136- 3000) from normal MORB (150-250) to high, arc-like ratios. Although arc melts also indicate elevated F/Nd (27-60) relative to MORB (20-24), the F/Nd ratios of BABB are equal to or lower than normal MORB (12-25), possibly caused by progressive dehydration of F-rich to F-poor slab phases at different depths. These new data also show the first high H2O concentrations in the Central Lau Spreading Center (CLSC; up to 1.2 wt.%), indicating a previously unknown slab influence. The trace element composition of these wet CLSC basalts suggests the presence of a distinct slab-derived component relative to water-rich basalts from the Eastern Lau Spreading Center (ELSC), which is closer to the Tonga arc. Wet basalts from the ELSC have higher Ba/La (>19) relative to those from the CLSC (~7), while CLSC basalts indicate higher La/Sm (~2) than the ELSC (0.8). These trace and volatile element variations show clear regional differences that may relate to the compositions of subducted inputs, dehydration reactions in the subducting plate, or fluid pathways through the mantle wedge.
V31A-2125
High-Ca Boninites From the Northern Tonga Region: Involvement of Four Independent Components During Petrogenesis, and Retention of Monosulfide Solid Solution in the Source
Samples of fresh glassy high-Ca boninites and associated lavas from the northern Tonga region were dredged during the 2004 NoToVe cruise by the RV Southern Surveyor. We present the major element, trace element, and low-level PGE and Au concentrations of samples from Volcano A of the arc front, and from the Fonualei Rift and Mangatolu Triple Junction of the backarc Lau Basin. Results from geochemical forward modelling indicate that the boninites were formed by an aggregate of approximately a total of 16% partial melting of depleted MORB mantle. The petrogenesis of the three suites can be successfully modelled as the products of various combinations of mixing and melting of four individual components; an extremely depleted mantle wedge, a fluid derived from the subducting oceanic crust, a fluid derived from the subducting pelagic sediments, and OIB material derived from the Samoan plume leaking into the Lau Basin asthenosphere. The LILE and PGE distributions of the samples can be independently modelled in the same 2-stage melting scheme; an 11% partial melt of depleted MORB mantle is first removed from the mantle wedge during backarc spreading, followed by the addition of slab-derived fluids to the restite, which is then melted to <5% to produce the lavas. Fluxes of the slab-derived fluids introduce S and LILE into the mantle wedge. Systematic spatial variations in lava chemistry occur in that there is a progressive increase in OIB signature towards the north. In order to account for the elevated Nb concentrations relative to other HFSE, additional melts of OIB must also added to the source of the Fonualei Rift and Mangatolu Triple Junction lavas. The elevated PPGE and Au concentrations (Pd ~20 ppb) and the depleted IPGE concentrations (Ir ~0.1 ppb, Rh ~0.5 ppb) suggest that mss was the only major sulfide phase retained in the restite during the second stage of melting. In light of the high Pd concentrations observed, we conclude that sulfide liquid cannot have been present in significant amounts in the source mantle of all three suites.
V31A-2126
Andesites/Dacites of the Oceanic Narcondam Volcano, Andaman Sea: Modification of Tholeiitic Arc Basalts by Crustal Contamination and Amphibole-Dominated Fractionation
The active Barren Island volcano and its 140 km distant northern neighbor, the Pleistocene Narcondam volcano, are the only two subaerially exposed arc volcanoes, which rise from the 1000-2300 m deep seafloor of the Andaman Sea, that result of the subduction of the Indian plate beneath the Burma plate. Lavas of Barren Island volcano range from basalt to andesite while lavas from Narcondam volcano range from andesite to silicic andesite/dacite. Similarities in the geochemistry of both lava suites include strong and comparable depletion in Nb and Ta (K2O/Nb ~0.7; Ba/Nb 130-250); low, MORB-like Nb/Zr (0.01- 0.03); and nearly constant U/Th (0.15-0.22). These characteristics suggest a genetic link between both magma suites. Distinct geochemical differences, however, include isotopic ratios which for Barren Island are: 87Sr/86Sr ~0.7039-0.7041, 143Nd/144Nd 0.51285-0.51296, and δ18O plagioclase 5.81-5.89, and for Narcondam are: 87Sr/86Sr 0.7049-0.7053, 143Nd/144Nd ~0.51270, and δ18O plagioclase 6.78-7.44. Other geochemical parameters (e.g. Sr/Y, Th/La, U/La, Ba/La) of Narcondam lavas positively correlate with increasing SiO2 but are anchored at the mafic end within compositions observed at Barren Island volcano. Narcondam magmas evolved through a multi-stage evolution characterized by fractional crystallization, contamination, and magma mixing. Prior to eruption, the latest event was marked by mixing of a silicic lava with a Barren Island-type basaltic magma that lowered the 87Sr/86Sr from values of 0.7053-0.7054 as observed in single plagioclase and amphibole phenocrysts to values of bulk rock and caused juxtaposition of mineral populations. The generally more-incompatible trace element enriched silicic Narcondam magmas are best explained by amphibole-dominated fractionation of a Barren Island-type basalt; being consistent with an increase of Sr/Y with increased silica of samples containing abundant amphibole. The shift in isotopic values from Barren Island to Narcondam samples is likely caused by crustal contamination in the upper plate and involved remains of extended continental crust and/or sediments from the fan associated with the Irrawaddy Delta at the Myanmar continental margin consistent with more proximal position of Narcondam to the continental margin.
V31A-2127
Geochemical Constraints on the Evolution of the Woodlark Rift
Late Miocene-Pliocene eclogites exhumed in the D'Entrecasteaux Islands of southeastern Papua New Guinea were apparently developed in a subduction system prior to being rapidly exhumed during rifting associated with westward propagation of the Woodlark Basin spreading center. The protolith of these mafic eclogites and their host gneisses has remained enigmatic, as has the relationship of magmatism to core complex formation and HP-UHP metamorphism. Whole-rock Hf, Nd, and Pb isotopic compositions were measured via MC-ICP-MS for twenty-six samples encompassing a variety of lithologies within the Woodlark Rift including mafic eclogites, their felsic to intermediate host gneisses, and upper-plate felsic to mafic intrusive and extrusive rocks in order to begin to constrain the origin of these rocks. The Pb isotopic compositions of all these samples exhibit little variation (206Pb/204Pb= 18.115 - 18.702; 207Pb/204Pb= 15.469 - 15.604; 208Pb/204Pb= 38.291 - 38.605) and lie within the field of overlap between Pacific MORB and marine sediments on terrestrial reservoir diagrams. Results are broadly similar to previously reported Pb isotopic compositions from alkaline volcanic rocks in southeastern PNG. The intrusive and extrusive upper plate rocks analyzed in this study exhibit juvenile Hf and Nd signatures (εHf = 7.5 - 11.3; εNd = 2.3 - 6.7; twelve samples), as did the mafic HP-UHP eclogite (εHf = 7.3 - 11.9; εNd = 2.8 - 6.2; four samples). Ten felsic to intermediate gneisses displayed a broader range of Hf and Nd compositions (εHf = -0.2 - 10.0; εNd = 0.8 - 6.4), and in some cases exhibit decoupling from the Hf-Nd terrestrial array possibly due to the influence of inherited Hf in zircon. In-situ U-Pb zircon ion probe analyses on HP-UHP host gneisses exhibiting anomalously low whole-rock Hf signatures revealed a population of 90-100 Ma zircon cores, but no evidence of an Archean Australian crustal protolith. Preliminary LA-MC-ICP-MS zircon Hf analysis from two samples of host gneiss yielded results consistent with an inherited component no older than the Mesozoic (εHf= 3.3 - 10.9). REE compositions of the felsic-intermediate gneisses and mafic eclogites were normalized to post Archean average shale (PAAS) and primitive mantle compositions respectively yielding flat patterns. The major element compositions of the mafic eclogites are variable, but are similar to previously reported compositions of basalt from the Papuan Ultramafic Belt. Felsic to intermediate host gneisses exhibited a broad range in major element compositions, which is likely a reflection of their metasedimentary origin. The U-Pb zircon results and whole-rock isotopic compositions support the hypothesis that a Mesozoic succession of arc-derived volcaniclastic sediments intercalated with basaltic dikes was subducted and remained metastable at depth until exhumation during the Pliocene.