V33C-2226
Primitive Subduction Zone Magmatism at Mt. Shasta, California: Geochemical and Petrologic Characteristics of Hydrous Mantle Derived Melts
A primitive magnesian andesite (PMA) is present north of Mt. Shasta volcano (Anderson, 1974). This PMA outcrops as a lava flow that emanates from a pair of vents in the saddle between Whaleback and Deer Mtn. Detailed sampling of these lavas and the associated vents reveal significant compositional variability. The vent material and the flow are strongly compositionally zoned. One distal end of the PMA flow has 57.4 wt% SiO2 and 10.1 wt% MgO making it the most primitive PMA yet found in the Cascades. This sample contains olivine phenocrysts with Fo94 cores, which have previously been identified in the samples from one of the vents. These phenocrysts contain Cr-rich spinels indicative of a magmatic origin and are euhedral and normally zoned with the outermost rim compositions matching those of the groundmass olivines. Samples of the middle of the flow, 8.71 wt% MgO and 57.3 wt% SiO2 contain normally zoned, euhedral olivine phenocrysts with Fo90 cores. Again the olivine phenocrysts are clearly magmatic in origin, containing magmatic Cr-rich spinels and having uninterrupted normal zoning patterns with the outermost rims matching those of the groundmass olivine. Complexly reacted clinopyroxene and orthopyroxene are rarely found in the PMA lava flow and suggest that there was negligible contamination by any kind of silic-rich end member (dacite, rhyolites, granite, etc). Comparing the compositional variation in the PMA flow to that present in the PMA cinder cone, it is apparent that small amounts of contamination by a more SiO2-rich component are locally present in the cinder cone samples. However, the samples of the cinder cone contain only a small proportion of this silicic component and it has little influence on the overall major element chemistry. Os concentrations determined for the PMA, a Shasta dacite, and a sample of Trinity peridotite are 14, 9, and 3897 ppt, respectively, with measured 187Os/188Os of 0.1674, 0.1829, and 0.12434. Only 0.1 wt% addition of the peridotite to the dacite will lower its Os isotopic composition and raise its Os content to values similar to the PMA. However, this very small amount of peridotite has negligible effect on the major element content of the mixture. Hence, contamination with basement peridotite cannot simultaneously explain the high-Mg character and Os isotope characteristics of the andesite. Thus, the geochemical and petrologic evidence preclude the derivation of PMA magma from being mixtures of other Mt. Shasta lavas and ultramafic materials. Therefore, the PMA lavas can provide clues to the geochemical signature, HFSE and isotopic, of subduction zone fluids and primitive mantle melts. Overall, the difference in major elemental compositions of the flow and vent PMA samples is so little that the experimental results from previous studies of the Grove et al. (2003), on PMA sample 85-41c of Mt. Shasta, are applicable to the conditions of hydrous mantle melting that produced the PMA magma, and the PMA flow samples provide the most uncontaminated look at the primitive inputs into Mt. Shasta.
V33C-2227
Source Components and Mass Transfer in the Aleutian Arc from Hf, Nd and Pb Isotopes
Hafnium, Nd and Pb isotopic and trace element data from the eastern Aleutian arc, including new samples from dredged back arc volcanoes, are used to evaluate the sources of Hf and other high field strength elements (HFSE) in island arc lavas. A relatively simple subduction system and on-going efforts to accurately characterize the subducting sediment make the Aleutian arc an ideal natural laboratory to test models of mass transfer from the subducting plate through the sub arc mantle and back to the surface. Aleutian lavas sampled from the arc front have less radiogenic Nd and Hf isotope ratios than estimates of the local mantle wedge, consistent with the addition of small amounts of subducting sediment to the melt source. In general εNd and εHf increase westward, apparently due to decreasing subduction rates and sediment flux to the trench. In detail, Nd and Hf isotopes are well correlated west of Seguam Island but are decoupled to the east, where εHf decreases continuously and εNd is weakly correlated with the sediment flux. Correlations between sediment flux, Nd and Hf isotopes indicate that sediment-derived Nd and Hf are incorporated into the source of Aleutian magmas. This implies that Hf (+ other HFSE's?) is not conserved in the subducted sediment. An eastward increase in the terrigenous component of subducting sediments, which likely include a greater proportion of detrital zircon, may be the source of relatively unradiogenic Hf in eastern Aleutian lavas. This suggests that Hf, derived in part from detrital zircon, may be transferred from the subducting plate to the sub arc mantle. Dredged lavas from submerged, back-arc volcanoes located between Umnak Island and the Islands of Four Mountains display stronger isotopic diversity (e.g.,εHf= 11.6-19.9, εNd= 5.8-9.4, and 206Pb/204Pb= 18.4-19.0) than nearby emergent and arc front volcanoes. This suggests that the small seafloor volcanoes effectively sample isotopic end-members, which likely originate from incomplete mixtures of the subducted component and a MORB-like depleted mantle. Crustal magma plumbing systems that feed the much larger emergent, arc-front volcanoes are more likely to pool and homogenize isotopically diverse mantle derived melts, obscuring information about processes beneath the Moho. These observations demonstrate that magma sources beneath the Moho in the Aleutians are isotopically diverse, and that melting and melt-transport processes are capable of preserving this diversity during their ascent to crustal depths. This may imply that 'cold plumes' or diapiric assent are more likely melt transport mechanisms than porous flow.
V33C-2228
Trace Element Geochemistry including the HFSE in Magnetites of Calc-Alkaline Plutons: the Tanzawa Complex of the Izu-Bonin-Mariana Arc and the Ladakh Batholith Complex, NW Himalaya
In this study we attempt to contribute to the understanding of a prominent feature, namely the Nb-Ta depletion, in arc magmatic trace element geochemistry. Traditionally, this depletion is explained by residual mantle-wedge phases with Nb and Ta affinities, such as titaniferous ilmenite, rutile or titanite, or by an amphibole. Here we propose a mechanism long advocated to explain the calc-alkaline trend (Bowen vs. Fenner) in MgO-FeO (total Fe)-(Na2O + K2O) ternary diagram by early crystallization and separation of magnetite in subduction zone magmas associated with high oxygen fugacity environments. In support of our hypothesis we provide high-precision multiple trace element data, including the high field strength elements (HFSE), in separated magnetites and mafic mineral phases from mafic magmatic enclaves associated with tonalite suites of two different magmatic arcs â€" the Tanzawa Complex of the Izu-Tanzawa Collision Zone in Japan, and from the Ladakh Batholith Complex of NW Himalayas. The Tanzawa Complex is composed of diverse rock suites with SiO2 varying from 43 â€" 75 percent, ranging from hornblende gabbro through tonalite to leuco-tonalite. The geochemical characteristics of low K- tholeiites, enrichment of large ion lithophile elements (LILE), and depletion of HFSE in rocks of this plutonic complex are similar to those observed in the volcanic rocks of the IBM arc. The Ladakh batholith Complex is one of the granitic belts exposed north of the Indus-Tsangpo suture zone in Ladakh, representing calc-alkaline plutonism related to the subduction of the Neotethys floor in Late Cretaceous. This batholith comprises predominantly I-type granites with whole rock delta delta 18O values of 5.7-7.4 per mil, without major contribution from continental crustal material. We analyzed 22 trace elements by ICP-MS, including Nb-Ta-Hf-Zr, in separated magnetites from five gabbros of the Tanzawa tonalite-gabbro complex and from three tonalitic gabbros of the Ladakh batholith. In NMORB normalized plots the trace element patterns of all the magnetites analyzed show enrichment in Nb, Ta, Pb, Sr (5X NMORB) and Zr (2X NMORB) with characteristically high Nb/Ta and Zr/Hf ratios. In contrast, the patterns show anomalously low La, Ce, Pr, Nd, Sm and Hf concentrations (less than 0.1 NMORB). It is noteworthy that in the normalized trace element plot, all the magnetites show high Nb/Ta ratios in contrast with high Ta/Nb ratios observed in typical arc magmas. These data support our hypothesis that magmatic crystallization of Fe-Ti oxides under high oxygen fugacity conditions during initial crystallization and formation of the Izu-Bonin and Ladakh-type arc batholiths may be the primary cause of depletion of the HFSE in later magmatic differentiates of less mafic and more felsic granitic arc rocks.
V33C-2229
Magnetite fractionation of "chalcophile" elements
Submarine-quenched glasses provide the clearest record of magmatic evolution available, especially with respect to potentially volatile elements and compounds. A comprehensive suite of arc- and backarc glasses recovered during several research voyages in the SW Pacific are being analysed by a number of research teams in an effort to document genesis and fractionation processes involved in the evolution of subduction- related magma systems. As part of these studies, a suite of basalt to dacite composition glasses recovered from the arc-like Pacmanus Site in the Manus Backarc Basin have been examined in detail. Sun et al. (2004) found trends of increasing enrichments of Re, Cu, and Au with SiO2 in the range from 50 to 60 wt% SiO2. From this stage of magmatic evolution, marked depletions in all these elements are observed in glasses of higher SiO2 contents. The reasons for this trend have been unclear, although a mechanism involving the cessation of olivine and initiation of magnetite saturation was proposed by Sun et al. Our analysis of a larger suite of glasses from the Manus Basin has now confirmed these trends, and found the same behaviour for Zn and Mn. Similarly, analysis of a suite of basalt to dacite glasses recovered from the arc-like Valu Fa Ridge in the Lau Backarc Basin has produced trends for these elements very similar to those of the Pacmanus location. LA-ICP-MS analysis of microphenocrysts in these glasses has shown that magnetite is highly enriched in Re relative to the coexisting melt (Kd up to 7.8 for Re). Magnetite/melt partition coefficients for Re, Mn and Yb can be used to precisely model the liquid line of descent during magnetite (plus clinopyroxene and plagioclase) fractionation using Re vs. Mn/Yb. The variation of Pt abundances with SiO2 for both these suites contrasts with other nominally chalcophile trace elements in showing rapid early depletion from the most Mg-rich basalt through to more SiO2-rich compositions, with no maximum at 60 wt% SiO2. Silver on the other hand is essentially constant at 0.02 to 0.05ppm throughout the basalt to dacite compositional range. Several other compositional features of the Pacmanus and Valu Fa suites are also noteworthy. For example while S has an abundance maximum at 60wt% SiO2 (similar to Fe and Re), and then diminishes in concentration in more silica-rich magmas, Se is initially depleted at 60wt% SiO2 but then increases in abundance as SiO2 increases further. We suggest these features result from sulfide saturation coincidentally with (or shortly after) magnetite saturation. While Se is initially partitioned into the immiscible sulfide phase, moderate incompatibility remains between the overall fractionating assemblage of plagioclase-pyroxene- magnetite and minor sulfide. Pt on the other hand is likely being removed as a native element/alloy. Sun, W, Arculus, RJ, Kamenetsky, VS and Binns, RA, 2004, Release of gold-bearing fluids in convergent margin magmas prompted by magnetite crystallisation. Nature, v. 431, p. 975-978.
V33C-2230
Geochemical Behavior of Selenium in Igneous Systems
Selenium (Se) is generally assumed to behave much like sulfur (S) in igneous systems. However, it is unclear how valid this assumption is considering that so little is known about the geochemical behaviour of Se. Constraining the range in Se and S concentrations of mantle-derived magmas is important to studies of: core segregation; the composition of late-accreted material; collisional erosion models; processes of mantle melting in various tectonic environments; and recycling of lithospheric components into the mantle, to mention only a few. Previous estimates of the S/Se of primitive and depleted mantle assume that S-Se are similar to Zr-Hf and Nb-Ta in their geochemical coherence, and that S/Se of the Earth's mantle is chondritic (Palme and O"Neill 2003 and references therein). Due to the low abundances of Se in mantle-derived rocks and the lack of calibration materials for routine analysis (i.e. NIST 612), this assumption remains unchallenged. Using a combination of SHRIMP, electron probe, and LA-ICP-MS techniques the concentration of Se in NIST 612 and BCR-2G was obtained to permit rapid in situ LA-ICP-MS analysis of Se concentrations in volcanic glasses. We have obtained Se, S, major and comprehensive trace element data for volcanic glasses from a global range of tectonic settings (mid-ocean ridges, ocean island, island arc and back-arc basins) to improve understanding of the behaviour of Se during igneous processes (e.g. partial melting, fractional crystallisation, and volatile phase separation). Analysis of a range of mid-ocean ridge glasses shows that Se behaves as an incompatible element, but is decoupled from S as the S/Se extends to values well in excess of the chondritic estimate (i.e. 2528; Palme and O"Neill 2003). During back-arc basin and island-arc magmatism, the abundance, systematics and S/Se are diverse. For example, at a specific MgO content, the absolute abundance of Se varies with depletion of the mantle source. Within back-arc suites derived from relatively fertile mantle sources (e.g., Coriolis Troughs, New Hebrides), Se is negatively correlated with MgO abundances in the range from basalt to dacitic compositions. In contrast, Se abundances in back-arc suites derived from relatively depleted sources (e.g., Manus and Lau Basin) show less incompatible behaviour. In detail, these systematics are complex and in the case of the Pacmanus site in the Manus Basin and Valu Fa Ridge of the Lau Basin, may be related to timing of sulfide and magnetite saturation and fractionation. Palme, H. and O"Neill, H.St.C. (2003) Cosmochemical Estimates of Mantle Composition, Treatise on Geochemistry, Vol. 2: The Mantle and Core. Ed. R.W. Carlson, Elsevier, pp.1-38.
V33C-2231
Slab and Mantle controls of Nb and Ta: Evidence from the central Mexican Volcanic Belt
Arc magmas are hybrids of mantle, crust and slab components, yet the exact mixing proportions of these components are difficult to define. Of key significance to this controversy are the elements Nb and Ta which are difficult to fractionate, highly incompatible during mantle melting and fractional crystallization, and yet prominently depleted in arc magmas. It remains unclear whether slab or mantle processes control the variability of Nb, Ta and Nb/Ta ratios that is observed in arc magmas worldwide. In order to address this question, we obtained new Nb and Ta data from primitive basalts and andesites from the central Mexican Volcanic Belt (MVB). Samples are from the Quaternary Volcano Popocatepetl (=Popo) and three young (<4700 years), compositionally zoned monogenetic volcanoes (Chichinautzin, Guespalapa and Suchiooc) located ~55 km west of Popo at the arc volcanic front. The monogenetic basalt to basaltic andesites display a wide range in Nb (3.5-35.7 ppm) and Ta at high Nb/Ta ratios (15.9-19.5). In contrast, the high-Mg andesites and dacites of Popo have lower Nb/Ta ratios (12.3-16.6) at low Nb (4-9 ppm) and Ta abundances. The broad range in Nb and Ta can be generated by an increasing extent of melting (up to ~10%) of a single mantle source. This will produce shallow, positive trends in the Nb vs Nb/Ta space and such trends are followed by the magmas of the individual monogenetic volcanoes. However, mantle melting cannot account for the variable Nb/Ta at Nb and Ta abundances seen at Popo. We suggest that beneath Popo, Nb- and Ta- bearing slab components are added to a depleted subarc mantle. Furthermore, our data suggest that Nb- and Ta-rich slab melts may also enrich the more fertile subarc mantle, and become part of the observed melting fractionation trends.
V33C-2232
Nb/Ta - Zr/Hf Fractionations during Subduction: Implications for the'Missing' Nb.
Key differences between the chemical composition of terrestrial materials and those of meteorites have led to the suggestion that a 'hidden' high Nb/Ta reservoir exists in the Earth's mantle. In order to test this hypothesis we must identify the processes that can create such a reservoir. Here we report the first high precision HFSE data on products of the subduction processes thought to fractionate Nb from Ta: boninites (hydrous melting), adakites (slab melting), OIBs (Koolau, Walvis: plume with recycled oceanic crust), as well as kimberlites and lamproites. We developed a new method for the high precision determination of Nb, Ta, Zr, Hf concentrations based on a modified version of standard addition. All analyses were performed on a single collector ICPMS (ELEMENT 1), using Y and Yb as internal standards to correct for instrumental drift during the unspiked -spiked sample sequence. Concentrations are calculated using a York- type regression that accounts for all measured and propagated errors. Long-term reproducibility (multiple dissolutions and multiple spike solutions) for the standards BHVO-1, BIR-1 AGV-1 and BCR-1 are better than 0.8% (1s) for Nb/Ta and Zr/Hf ratios. The advantages of this method compared to previous methods are fast throughput, no column chemistry and low blanks. The Koolau and Walvis Ridge lavas have subchondritic Nb/Ta for a given Zr/Hf, overlapping other OIB suites and show no evidence for a recycled, high Nb/Ta reservoir in their source. OIB, considered as a group, have relatively constant Nb/Ta (15-16) but more variable Zr/Hf (35-50). In contrast, boninites (Chichi Jima) have significantly subchondritic Nb/Ta (4-12) at near constant Zr/Hf (35), while adakites (South Andes) extend to near chondritic Nb/Ta (13-19) at more variable Zr/Hf (30-40). The adakites showing the least evidence for crustal contamination have the highest Nb/Ta. The arc lavas cross the OIB trend at near right angle on a Nb/Ta vs. Zr/Hf plot having larger Nb/Ta fractionations at a given Zr/Hf than OIB. Our data suggests that during fluid flux melting of the mantle wedge (i.e. boninites), Ta is preferentially mobilized from the slab to the wedge, creating (by inference) a high Nb/Ta subducting slab. In contrast, during slab melting (i.e. adakites) the residual slab should develop subchondritic Nb/Ta. Our data suggests that only slab dehydration and not melting may increase the Nb/Ta of a subducting slab. In turn, the adakite data suggests that melting of the oceanic crust may generate high Nb/Ta melts. Our HFSE data from G2 kimberlites and lamproites extends to highly superchondritic Nb/Ta (15-27) for a low Zr/Hf (35-40), essentially overlapping the adakites. These kimberlites and lamproites require a long-term melt-enriched source in the subcontinental mantle. It is conceivable that the subcontinental mantle has high Nb/Ta ratios (and high Nb- Ta concentrations) produced by melts from subducting slabs, over multiple subduction events over the earth's history and represents a non-convecting (thereby 'hidden' from our sampling ability) high Nb/Ta reservoir.
V33C-2233
Noble Gas and Stable Isotope Systematics of Volcanic Gases and Hot Springs in the Aegean Arc, Greece
We present new data of stable isotopes (O, H) and noble gas isotopes (He, Ne) from hydrothermal sites associated with recent volcanism along the largely flooded, active Aegean continental volcanic arc (Greece) in order to quantify the different roles of the major reservoirs of origin of the fluids (i.e. atmosphere, mantle, seawater and crust). Low-temperature hydrothermal activity in the Aegean is widespread but mostly limited to coastal and shallow submarine environments. Much of the latent magmatic heat is dissipated into the large mass of seawater covering it. Older volcanic centers appear dominated by reservoir mixing with meteoric and seawater, while the more recently active centers show a greater excess mantle noble gas component and stable isotope shifts. Between those two endmember scenarios, active shallow hydrothermal systems tend toward greater stable isotope shifts and a lesser mantle helium component, while sites with either deep faults or suspected deep magmatic reservoirs show greater excess noble gases but lesser stable isotope shifts. Transient compositional deviations appear largely dependent on seismic activity. Excess helium isotope signatures demonstrate a similar mantle source composition for all sites, with an R/RA of approximately 9.3. Tritiogenic helium is negligible for sites with a mantle helium component greater than 10%. Observed compositional differences between sites are therefore attributed to shallow processes of hydrothermal activity, transient tectonic disruptions, and admixture of other reservoir components.
V33C-2234
Subduction- vs- Intraplate-Type Melt Migration in the Alboran Lithospheric Mantle: Insights From the Tallante Xenoliths (Betic Cordillera, SE Spain)
The Alboran Sea region has been affected since the late Oligocene by widespread eruption of tholeiitic to calc-alkaline magmas followed by Late Neogene alkaline basalts. These magmatic episodes are related to Neogene lithospheric extension beneath the Alboran domain, as a consequence of slab roll-back. According to recent models, subduction of oceanic lithosphere caused continental-edge delamination of subcontinental lithosphere, associated with upwelling of plume-type mantle sources. The Alboran lithospheric mantle thus constitutes a unique setting to investigate the effects of subduction- and intraplate-type metasomatism. Here we present a microstructural and geochemical study of mantle xenoliths from the Cabezo Tallante Late Neogene alkaline volcanic center (SE Spain). These xenoliths record multiple episodes of reactive porous melt percolation, and melt entrapment, tracking their progressive extension-related uplift from P > 20 Kb to 7-10 Kb. This is documented by i) crystallization of undeformed olivine replacing pyroxene porphyroclasts, and unstrained opx overgrowing undeformed olivine and pyroxene porphyroclasts, in porphyroclastic spinel peridotites, ii) development of annealed equigranular structure, likely enhanced by heating during melt percolation, iii) crystallization of interstitial (plag±ol±opx) aggregates between mantle minerals in porphyroclastic and equigranular xenoliths. Cpx in equigranular peridotites have smooth trace element spectra characterized by slight LREE depletion; computed equilibrium liquids have a tholeiitic-transitional affinity. Diffuse melt percolation was followed by intrusion of melts with distinct chemical affinity. The first event is documented by the intrusion of cm-sized gabbronoritic veins, showing a fine-grained opx reaction rim against the host peridotite. Similar gabbronoritic lithotypes were previously documented and ascribed to slab-derived melts. A quite remarkable textural feature in these veins is the occurrence of small but diffuse Cl-rich apatite crystals, mostly included in plagioclase. Cpx in the host peridotite, partly corroded by gabbronoritic apophyses, are enriched in Th,U, L-M-REE, relative to cpx in both sp-and plag-peridotites, whereas they preserve low Nb and Ta contents. Plag and opx in the vein are also LREE-enriched. Textural and chemical features indicate that parental melts to the gabbronoritic veins were Si-saturated, enriched in LILE and volatile (Cl) components and depleted in Nb,Ta, consistent with subduction-related metasomatic melts. The latest magmatic event recorded in the Tallante xenoliths is documented by the intrusion of cm- thick dikelets of Ti-rich amphibole pyroxenites, with clear alkaline affinity. Cpx and amph in the pyroxenites display similar convex-upward LREE-MREE spectra, and amphibole has high Nb,Ta concentrations. Pyroxenite intrusion provide the opportunity to observe the local effects of alkaline metasomatism. Cpx in the reaction zone close to pyroxenites are enriched in Ti and L-M-REE, approaching the composition of pyroxenite cpx. Extension-related uplift of the Tallante xenoliths was thus accompanied by interaction with melts of different sources tracking the transition from subduction- to intraplate-type magmatism in the Alboran domain.
V33C-2235
HFSE-REE fractionation in two groups of Sulu eclogites: protolith or process control?
Significant HFSE-REE fractionation occurs in subduction zone. However, our understanding on the causes is mainly built upon compositions of arc lavas, which represent the end products of subduction zone processes. Additional constraints are derived from comparing compositions of eclogites and their protoliths. However, the focus has been on eclogites of an oceanic affinity. Here, we show distinct HFSE-REE fractionation patterns from two groups of eclogites of a continental affinity from the Sulu ultra-high pressure metamorphic terrane, China. Having high iron (16.7-20.9%) and TiO2 (3-4%) with low MgO (6.03-7.01%) contents, the high-Fe-Ti eclogites are enriched in Ti but depleted in Nb-Ta-Zr-Hf. Although their low SiO2 contents (38.2-42.8%) are attributed to metamorphic modification, the decoupling between Ti and other HFSE can be modeled as inheriting from gabbroic protoliths crystallized from melts compositionally similar to the subunits 4 and 6 eclogites in the CCSD core. Similarly, the compositions of a subgroup of the Sulu high- Al eclogites characterized by Nb-Ta-Zr-Hf depletion also largely reflect protolith control for the resemblance to the Talkeetna arc gabbronorites. However, another subgroup of the Sulu high-Al eclogites shows unusual HFSE enrichment with Ti/Eu, Zr/Sm, and Nb/La ratios over two times of the chondritic values. Their major oxide and HREE contents are comparable to that of the Talkeetna gabbronorites (~9% MgO). Therefore, the HFSE enrichment is attributed to interacting with high-pressure fluids. The occurrence of interstitial zircon and cluster of small rutile grains (<50 um) in garnet and omphacite along the periphery of annealed fractures is also consistent with HFSE precipitation from the fluids. The role of high-pressure fluids is strengthened by the occurrence of zoisite in the Nb-Ta-Zr-Hf depleted but LILE-LREE enriched high-Al eclogites. Evidently, the HFSE-REE fractionation in subducted continental lithosphere could be protolith and process controls but might exert insignificant effects on the apparent HFSE-REE imbalance in the solid Earth (Rudnick et al., 1999), considering the small volume of such eclogites and limited mobility of most elements in high-pressure fluids.
V33C-2236
The Hf-Nd Systematics of Rutile-Bearing Eclogites From Koidu, Sierra Leone
Xenoliths from the Cretaceous Koidu kimberlite complex, Sierra Leone, West Africa, provide a rare opportunity to investigate the origin of rutile-bearing eclogites with variable, but superchondritic Nb/Ta, Nb/La and Ti/Zr ratios (Rudnick et al, 2000). Previous studies of the trace element and δ18O values of mineral separates and reconstructed whole rock compositions of two suites (high and low MgO) of eclogites lead to two inferred origins: as cumulates (high MgO eclogites, Barth et al., 2002) or residues of altered Archean oceanic crust (low MgO eclogites, Barth et al., 2001). We present the first Hf-Nd isotope data on clinopyroxenes (cpx) and garnets (gt) from Koidu eclogites. The reconstructed whole rock (cpx+gt) Hf isotopic compositions are heterogeneous, ranging in εHf from 0.2 to + 41 at the time of kimberlite eruption (85 Ma). Nd isotopic compositions are equally variable (εNd = 0.1 to +37), placing all eclogites at drastically more radiogenic values than terrestrial basalts. However, a significant part of the Hf- budget can reside in the rutile, which will most likely have a less radiogenic hafnium isotopic composition compared to the silicates. One can construct a limiting case by using mass balance arguments to calculate the hypothetical hafnium isotopic composition of the rutile. The bulk rock hafnium isotopic composition was constrained to be 4Ga old subducted oceanic crust. Even with a high modal abundance of rutile and a high Hf concentration in rutile, the calculated isotopic compositions are unrealistic and calculated rutile ages far exceed the age of the Universe. Using a younger age for the eclogite (2 or 3 Ga) does not significantly affect the calculated result. These first-order approximations show that the addition of rutile does not change the whole rock isotopic composition significantly from the radiogenic Hf-Nd compositions based on cpx and garnet alone. Our findings do not support geochemical models that predict 143Nd/144Nd and 176Hf/177Hf compositions of aged subducted oceanic crust to be less radiogenic in hafnium isotopic composition than the OIB array. Our measured compositions for the Koidu eclogites do, however, overlap with those of the Roberts Victor eclogites; the type locality for subducted Archean oceanic crust (Jacob et al., 2004). This result supports the conclusion that the isotopic composition of these eclogites represent small-scale heterogeneities in the Earth's mantle exaggerated by radiogenic in-growth following isolation in the subcratonic lithosphere. Eclogite cannot be treated as a homogeneous reservoir or end- member composition for global geochemical modeling.
V33C-2237
Petrologic, Geochemical and Isotopic Study of 3.1Ga Peridotite-Chromitite Suite from the Western Dharwar Craton, India: Evidence for Recycling of Oceanic Crust in the Mesoarchean
Sill-like ultramafic intrusions with massive chromitite bodies are common in Archean greenstone belts such as in the Zimbabwe craton and in the Singhbhum and Dharwar cratons of the Indian shield. In the western Dharwar craton deformed massive chromitites are hosted within dissected peridotitic rocks of the Nuggihulli schist belt and part of early to mid-Archean greenstone belts. The linear ultramafic belts are often associated with gabbroic rocks containing Ti-V-bearing magnetite bands and surrounded by the tonalite-trondhjemite- granodioritic (TTG) suites of rocks. Detailed electron microprobe study reveals high Cr-numbers (0.72- 0.87) and moderate Mg-numbers (0.47-0.59) for chromite and very high Fo content (~ Fo97) for interstitial olivine in massive chromitites from the seams. Our study suggests that the original igneous compositions of chromite grains are preserved in some of the massive chromitites, whereas accessory chromites in serpentinized peridotites are extensively altered to ferritchromit. The primitive composition of chromites along with high Fo content of olivine suggest parental melts produced by high degrees of partial melting of the source mantle. The tectonic discrimination plots plus parental melt calculations indicate derivation from a high-Mg komatiitic magma having a liquid Al2O3 ~ 9.64 wt% and liquid (FeO/MgO)wt% ~ 0.2 which is similar to the compositions recorded for komatiitic rocks within the schist belt. A Pb-isotope study of fourteen samples from this suite including peridotite, chromitite, gabbro and ultramafic-mafic schist rocks define an isochron yielding an age of 3159±180 Ma which is similar to the 207Pb/206Pb zircon age of ~ 3.1 Ga of the surrounding TTG-suite (Bidyananda et al. 2003). Ten samples from this suite have strongly scattering Sm-Nd model ages that range from 2.34 to 3.96 Ga (average 3.1 Ga) with respect to a depleted mantle source. The average initial Nd ratio, calculated at the presumed emplacement age of 3.1 Ga, is 0.50880±0.000171 with a corresponding average εNd value of +3.7 (range from -0.63 to +10.75). For these rocks this is consistent with an origin from a long-term depleted source region. The peridotitic rocks have nearly flat REE patterns with slight LREE enrichment whereas the mafic suites from the schist belts are variably enriched in LREE relative to HREE. Most of the samples display positive HFSE (Ta, Nb, Zr, Hf, Ti)-anomalies with negative Th-anomalies. The HFSE character together with positive εNd values strongly suggest derivation of the parental komatiitic magma from a long term depleted mantle source that was enriched by recycled oceanic crust due to early subduction processes. Our study suggests suprasubduction processes were important during early crustal evolution of the Earth. Reference: Bidyananda M., Deomurari M. P., Goswami J. N. (2003) Current Science, v.85, pp.1482-1485.
V33C-2238
Traces of Recycled Fe-Ti Gabbros in the Sources of Ferropicrites
Ferropicrites are sub-alkaline or mildly alkaline primitive magmatic rocks (MgO = 12-18 wt. %) characterized by exceptionally high FeOtot contents (> 13 wt. %) compared to other picrites. They are found in continental flood basalt (CFB) provinces as relatively thin basal lava flows (e.g., Paraná-Etendeka) or, in some cases, as dikes with ambiguous age relationships (e.g., Karoo). Ferropicrites typically are nearly uncontaminated and thus provide geochemical information on the asthenospheric mantle. The combination of unusually high Fe-contents and primitive olivines (up to Fo88) in near-primary ferropicrites is indicative of high mantle potential temperatures (~1600 °C) relative to mid-ocean ridges thus bearing evidence for mantle plume sources in LIPs. High (Sm/Yb)CN ratios (3-7) and low Al2O3 contents (~10 wt. %) indicate residual garnet in the mantle source of ferropicrites and high Ni contents and melting experiments suggest that the source was dominated by olivine-free pyroxenite. Such a garnet pyroxenite source has been recognized in many hotspots and has been ascribed to melt-olivine reactions in ascending eclogite-bearing garnet peridotite. Most of the purported pyroxenite-derived picrites have lower FeOtot contents (10-12 wt. %) compared to ferropicrites, however, which suggests that the generation of ferropicritic liquids requires specific melting conditions or exceptionally Fe-rich pyroxenite composition, or both. The characteristic positive V anomaly of ferropicrites is extremely rare in continental and oceanic basalts but common in cumulate Fe-Ti gabbros. In order to explain the geochemical differences between ferropicrites and common picrites we have modeled the behaviour of REE and V during melting of mantle pyroxenite with eclogite component representing distinct parts of subducted oceanic crust. Firstly, we modeled partial melting of 'basaltic' and 'Fe-Ti gabbroic' eclogites assuming 50% modal batch melting with clinopyroxene/garnet ratio of 9:1. Partial melting of secondary pyroxenite (eclogite/peridotite = 1:2) was modeled assuming modal batch melting with source modes corresponding to 2.5 GPa and 5.0 GPa conditions. The model parameters were adopted directly from experimental studies and the compositional data for the different components represent averages or typical values. Our results suggest that ferropicrites with high (V/Lu)PMN ratios (~1.5-2.5) contain traces of subducted Fe-Ti gabbros, whereas the geochemistry of many common picrites [(V/Lu)PMN ≤ 1.2] can be explained by partial melting of mantle pyroxenite including a MORB-type eclogite component. Some ferropicrites with relatively lower (V/Lu)PMN ratios may have been generated from ferrobasalt-bearing mantle sources, as low-degree melts from common pyroxenite sources, or, in fact, may not represent primitive ferropicritic melts at all. Interestingly, some mildly alkaline Fe-rich picrites from the rejuvenated parts of the Hawaii volcanic chain have anomalous high (V/Lu)PMN and may indicate the presence of subducted Fe-Ti gabbros in the Hawaiian plume.
V33C-2239
Titanium, tantalum, and niobium (TITAN) enrichment in high 3He/4He ocean island basalts.
Ocean island basalts (OIB) with high 3He/4He ratios are thought to be derived from an ancient, deep mantle reservoir. Global OIBs with high 3He/4He ratios are found to exhibit positive Ti, Ta, and Nb (TITAN) anomalies (on a Primitive Mantle normalized basis) relative to elements of similar compatibility in peridotite. High 3He/4He lavas also exhibit 187Os/188Os ratios (greater than 0.135) that are more radiogenic than putative Primitive Mantle values. Thus, TITAN-enrichment and radiogenic Os-isotopes are signatures associated with the high 3He/4He mantle. However, TITAN enrichment (and radiogenic Os-isotopes) are not unique to the high 3He/4He mantle: plots of 3He/4He versus TITAN-anomalies (or 187Os/188Os) reveal a "wedge- shaped" pattern. For example, Cape Verde lavas exhibit positive Nb anomalies, yet they have low 3He/4He. Similarly, lavas with low 3He/4He ratios can exhibit radiogenic Os-isotopes (e.g., Pitcairn, Mangaia,Tubuaii, etc.). Radiogenic 187Os/188Os and TITAN-enrichment in high 3He/4He OIBs indicate that the high 3He/4He OIB reservoir in the mantle hosts a non-primitive (non-chondritic) component. However, the origin of this non- primitive component, and how it became it associated with high 3He/4He ratios, remains unknown. One possible model is that recycled, refractory, rutile-bearing oceanic crust could contribute radiogenic Os- isotopes and TITAN-enrichment to the high 3He/4He reservoir in the mantle. Alternatively, extant partitioning data suggests that melts in equilibrium with Ca-perovskite could host positive TITAN anomalies; we cannot rule out the possibility that trace element partitioning between deep mantle melts and Ca-perovskite may be responsible for the TITAN enrichment in high 3He/4He lavas. These two models are highly dependant on uncertain partitioning data. Understanding the origin of the TITAN enrichment and radiogenic Os-isotopes in high 3He/4He lavas will be important for constraining the generation and evolution of the high 3He/4He reservoir in the mantle.