T12A-01 10:20h
Segmentation in Oman Ophiolite and Fast Spreading Ridges Tectonics
New, fine scale mapping in the large NW-SE ridge segment formerly identified in the Oman ophiolite (Nicolas and Boudier, 1995) has revealed that this structure is composed of smaller nestled segments, each being centered on the small mantle diapirs already mapped. The contacts with adjacent lithosphere and the tips of these segments have been mapped in detail. Their nature and structure depend on the difference in age between the two lithospheres. When the difference in age is in the range of 1 Myr, strike slip shear zones, 1-2 km wide, are developed in the mantle of the new segment. When this difference drops to ~0.5 Myr, the shear zones are small and diffuse but, in the mantle wedge at the tip of the segment which penetrates the older lithosphere, spectacular deformations are observed. The mantle and lower crust of the older lithosphere near the Moho are shoveled vertically and kilometer-sized folds develop in the gabbro unit. In contrast, the lid is not affected, suggesting that, at present day fast spreading ridges, similar major tectonic structures, seen in Oman thanks to deep sections, may also be present. Contacts and tips of new segments are invaded by mafic dikes and sills issued from the segment magmatic activity and trapped against these colder boundaries. An important contribution to this magmatism results from massive seawater penetration down to the Moho, possibly favored by the segment tectonic activity. Inside crystallizing magma chamber, the hydrous reaction (Koepke et al.,2004) generates orthopyroxene gabbros which are interlayered with the olivine gabbros. Outside the magma chamber, it generates, by hydrous anatexis, copious melts which mix and react with the indigenous segment melts and crystallize as pargasitic clinopyroxene gabbros and plagiogranites. It is suggested that their magmatic signature should be looked for in present day ridges. Nicolas, A. and Boudier, F., 1995, J.G.R., 100, 6179-6197. Koepke, J., Feig, S.T., Snow, J., Freise, M., 2004. Contrib. Mineral. Petrol. 146, 414-432
T12A-02 10:35h
Hydrous partial melting in the lower crust of the Oman ophiolite
Series of water-saturated melting experiments have been performed on natural gabbros between 900° and 1000°C, at crustal pressure up to 200MPa (Koepke et al., 2004), that put new constraints on the composition of melt and residual crystals at increasing temperature and melt fraction produced. In the gabbro section of the Oman ophiolite, the development of high-T secondary parageneses is ubiquist, represented by orthopyroxene+pargasite rims within contact between olivine and plagioclase, while clinopyroxene is replaced by pargasite. The reference to the experimental results, and isotopic tracing (Bosch et al., 2004) lead to interpret these reactions as representing initiation of hydrous partial melting by fluids circulating at grain boundaries. The inferred mechanism allowing supercritical water to penetrate the deep gabbro section is a strong anisotropy of thermal compression inducing microcracking in the cooling lower gabbros (Nicolas et al., 2003). In the Oman gabbros, another important petrologic feature is the local occurrence, in the deeper section, of large amounts of orthopyroxene bearing gabbros either interlayered with olivine gabbros or intrusive as pegmatitic patches, in association with wehrlites, or mixed with pargasitic gabbros. The corresponding upper levels are rich in dioritic or trondjemitic dikes. These occurrences are restricted to localized areas that coincide with tips and segments limits as deduced from the detailed mapping along the NW-SE paleospreading axis. The origin of these parageneses as products of hydrous melting of the gabbros, at various melt fraction, is explored by reference to the experimental data. Koepke, J., Feig, S.T., Snow, J., Freise, M., 2004. Contrib. Mineral. Petrol. 146, 414-432. Bosch, D.et al., 2004, J. Petrology, 45, 1181-1208. Nicolas, A., Mainprice, D., Boudier, F., 2003, J. Geophys. Res. 108 (B8) 2371.
T12A-03 10:50h
Magmatic Segmentation at the Northern and Southern East Pacific Rise: Magma Supply Versus Magma Plumbing
Competing models for what controls the intensity of ridge crest processes are often at odds on the scale of magmatic segmentation of the mantle and crust. One popular and long held view is the magma supply hypothesis proposed by Macdonald and others, wherein magma is centrally injected at mantle depths into a ridge segment bounded by long-lived tectonic discontinuities and ridge parallel flow of magma over distances of 10's of kilometers gives rise to along-axis variations in ridge crest processes. In this view, the scale of mantle melting at depths of 30 to 60 km is thought to govern the scale of magmatic segmentation of the mid-ocean ridge. Axial variations in volcanic, tectonic and hydrothermal activity are attributed to waxing and waning episodes of melting events. Where magma supply is high, greater amounts of volcanic and hydrothermal activity are predicted. Where the ridge is `starved' of magma, tectonic activity predominates. Recent studies at the East Pacific Rise (EPR) Integrated Study Site (ISS), however, bring up several challenges to the magma supply hypothesis and an alternative model which we refer to as magma plumbing. The two essential features of the magma plumbing hypothesis are the scale of along-axis segmentation of shallow mantle upwelling and the vertical (mis)alignment of the mantle and crustal magmatic systems. In this view, magma injection from the mantle to the crust occurs at more-or-less equally spaced intervals within a ridge segment bounded by tectonic discontinuities and the cross-axis offset between a center of mantle upwelling and an axial volcano regulates the intensity of ridge crest processes. When the mantle and crustal plumbing systems are aligned, the ridge crest is more volcanically and hydrothermally active. Conversely, when the mantle and crustal plumbing systems are misaligned by many kilometers, the intensity of extrusive volcanic and hydrothermal activity is less, while tectonic activity is increased. As a consequence, even at constant magma supply (as determined by crustal thickness) the intensity of seafloor processes can be fundamentally different because of magma plumbing. We will discuss these two competing models for magmatic segmentation in the context of results from geological, geophysical and petrological studies along the northern and southern EPR.
T12A-04 11:05h
MORB Composition in Intra-transform Spreading Centers: A Key Test of Models of Mantle Flow and Melt Transport
Models of mantle flow, melt generation and melt transport predict the composition and volume of melts delivered to the base of the crust in a mid-ocean ridge system. Many models can be tuned to match normal crustal thickness and composition, but they differ in predicting how the system behaves when it is perturbed by a transform fault offset. However, this "transform edge effect" or the pattern of along-axis delivery of melt to the crust within a ridge segment can be obscured by along-axis transport of magma within the crust in dikes or long-lived, continuous magma chambers. The advantage of sampling intra-transform spreading centers is that they provide well-defined locations of upwelling and crustal formation in a perturbed part of the system that does not allow along-axis transport of melt in the crust from other parts of the system. The composition of basalts from these settings can thus provide a critical test of mantle flow and melting models. Basalts from both the Garrett and Siqueiros intra-transform settings on the East Pacific Rise (EPR) have primitive composition with very depleted trace element contents, low ratios of very incompatible to moderately incompatible elements, and (230Th/238U) activity ratios ranging from slightly higher to lower than 1 suggesting that the first incompatible-element-rich melts are missing and only the magma from subsequent, shallower melting of an already depleted mantle reaches the intra-transform spreading centers. We have constructed 3-D models of passive mantle flow and melt driven by dynamic pressure gradients and buoyancy using the specific geometry of these two transform settings to predict the resulting composition. We compare the measured trace element and isotopic composition of both the intra-transform and normal segment lavas to those calculated using our model. To minimize the influence of a heterogeneous source, we chose samples from the 9-10 N EPR and Siqueiros with very similar isotopic composition. In contrast, to evaluate the effect of mantle heterogeneities we selected basalts from the Garrett and 13-23 S EPR, where the mantle probably contains heterogeneities that melt at greater depth. This passive flow model is very successful in reproducing the composition of lavas from both the transform fault and the normal segment. Some other melt migration models can be rejected, as they fail to correctly predict the compositional variations.
T12A-05 11:20h
Mantle Heterogeneity and Melting Along a Regional Axial Depth Gradient: Th-U Disequilibria Along the Southeast Indian Ridge
Lateral variation in mantle temperature is generally considered to be the main cause for observed global correlations between regionally averaged mid-ocean ridge basalt (MORB) chemistry and ridge axis depth (Klein and Langmuir, 1987). Axial depth should be shallower above hotter mantle because the underlying mantle crosses its solidus at greater depth compared to cooler regions, and leads to greater crustal production. One expectation of this global model is that melting beneath ridges should involve progressively more (deep) garnet peridotite as ridge depth shallows. A negative correlation between ($^{230}$Th/$^{238}$U) and axial depth in a global dataset (largely dominated by eastern Pacific and Atlantic MORB) generally supports this notion, because garnet is known to fractionate Th from U during melting (Bourdon et al., 1996). The extent to which such global variations reflect only variations in melting conditions of passively upwelling mantle versus other conditions related to mantle heterogeneity or actively fed melting anomalies is presently unknown. To further address this question on the regional scale, we have measured Th-U isotopes on 11 basaltic glasses collected along the Southeast Indian Ridge (SEIR) from $\sim$90$\deg$ E to $\sim$117$\deg$ E. This $\sim$2600 km section of ridge is characterized by a west to east gradient in axial depth from $\sim$2300 m to $>$4500 m, similar to that of the global ridge system away from the influence of hotspots. However, unlike the global ($^{230}$Th/$^{238}$U)-axial depth correlation which uses regionally averaged data for ridge segments from a range of spreading rates, the SEIR is spreading at a nearly constant rate of 70-75 mm/yr and is devoid of large transform offsets. These first 11 SEIR glasses were selected to span the geographic range and to be representative of the elemental and isotopic compositions. Th and U concentrations range from 130-940 ppb and 55-267 ppb, respectively, with Th/U values ranging from 2.36 to 3.77. All samples show $^{230}$Th excesses, with ($^{230}$Th/$^{238}$U) ranging from 1.01 to 1.24. There is no correlation between ($^{230}$Th/$^{238}$U) and axial depth, in contrast to the simple prediction from the global scale model. On a ($^{238}$U/$^{232}$Th)-($^{230}$Th/$^{232}$Th) equiline diagram the SEIR data form 3 geographical, non-collinear groupings: three basalts from the westernmost portion of our study area have the highest ($^{230}$Th/$^{232}$Th) and ($^{238}$U/$^{232}$Th), the easternmost basalt has significantly lower values, and basalts from a central region (101$\deg$E to 114$\deg$E) have intermediate values. The 7 central region basalts form a well-correlated positively sloping ($^{230}$Th/$^{232}$Th) vs. ($^{238}$U/$^{232}$Th) array which is shallower than the equiline and extends from 7% to 24% $^{230}$Th-excess. Overall, ($^{230}$Th/$^{232}$Th) shows a strong negative correlation with axial depth and, along with Th/U, correlates well with other isotopic tracers such as $^{3}$He/$^{4}$He and $^{208}$Pb/$^{206}$Pb, which vary systematically along axis. In contrast, ($^{230}$Th/$^{238}$U) shows no systematic variations with these isotopic parameters or axial depth. Unlike the global dataset, the Th-U disequilibria in SEIR MORB suggests that, at a regional scale, melting in the Indian Ocean mantle is primarily controlled by variations in mantle composition.
T12A-06 11:35h
The Influence of Ridge Geometry on Crustal Accretion: Insights From U-series Disequilibria on the SW Indian Ridge
Recent work has proposed that crustal accretion on ultra-slow spreading plate boundaries, both magmatic and amagmatic, is largely a function of ridge geometry, melt production, and the ensuing change in the lithospheric thermal regime. We present U-series data from the ultra-slow spreading SW Indian Ridge in an attempt to unravel some of the complexities embedded in the issues surrounding spreading geometry and crustal accretion. Initial 238U-230Th-226Ra disequilibria measurements on 12 dredged glasses from between 10-25$\deg$ E on the SW Indian Ridge show 238U and 230Th disequilibria. We interpret the 238U excesses (230Th/238U $<$ 1) and 230Th excesses (230Th/238U $>$ 1) to be primary based on measured 226Ra/230Th disequilibria, which indicate the lavas are young ($<$ 8 ka). The suite as a whole exhibits a large range in 230Th/232Th and 238U/232Th, covering nearly the entire extent of variability seen in the global U-series MORB database. Glasses from the oblique segment show the most enriched compositions (230Th/232Th = 0.8 and 238U/232Th = 0.76) of any MORB measured but have minor 230Th excesses ($<$9%) or small 238U excesses (3.5%). In contrast, 230Th excesses in lavas from the orthogonal spreading segment vary from 2 - 29% and are elevated relative to values on the oblique spreading segment. To a first order, this may suggest strikingly different melting regimes beneath each spreading segment. Contrary to most existing MORB U-series data, the majority of the oblique segment lavas do not maintain a negative correlation between 230Th/238U and depth, while the orthogonal segment lavas do. Our U-series data indicate that the change in ridge geometry from the orthogonal spreading segment to the oblique spreading segment directly affects the lithospheric thermal regime (e.g. thickening of the conductive cooling lid), which therein produces changes in the melting column, depth of melting, melt production, and melt transport. We will address these melting parameters as well as constrain the effective source composition (e.g. garnet peridotite versus garnet pyroxenite).
T12A-07 11:50h
Lena Trough Basalts: Low degree garnet melting signatures
Lena Trough in the Arctic Ocean is an oblique amagmatic rift separating the continents of North America and Europe. It is the connection between Gakkel Ridge to the North and the rest of the global mid-ocean ridge system to the South. Basalts were recovered from Lena Trough on 4 cruises: Polarstern ARK XV-2, ARK XVII-2, USCGC Healy 0102, and Polarstern ARK XX-2. Basalts were only dredged from the extremities of Lena Trough however, as the central valley of Lena Trough is entirely composed of peridotite. The Northern Lena Trough basalts erupt along structures that are clearly allied with neighboring Gakkel Ridge. Their geochemical characteristics thus closely resemble those of the magmatically robust Western Volcanic Zone of the Gakkel Ridge. Rocks from the southern Lena Trough, however, are quite different. These rocks are moderately to highly enriched MORB, but show different geochemical characteristics than either MORB from Gakkel Ridge or MORB from the rest of the mid-ocean ridge system. They are slightly evolved (Mg# 65) alkali basalts with 4% Na2O (Na8.0 of 3.5), 1.5% K2O (K/Ti 0.54) and are LREE enriched with average (La/Sm)N of 1.6, and (Sm/Yb)N of 2.41. Their low HREE budget agrees with the presence of garnet in the melting source, which is consistent with results to date from Lena Trough Peridotites(1). One distinguishing feature is their vesicularity: Whereas mid-ocean ridge basalts are generally not highly vesicular, reflecting the great depth of their eruption, Gakkel Ridge basalts occasionally are, despite their great depth. Basalts from the southern Lena Trough are all highly vesicular (up to 25%), despite water depths of over 3000m. This suggests either very high volatile contents or significant subsidence post-eruption. Coherent garnet melting indicators in Lena basalt and peridotite thus seem to suggest a relatively hot thermal regime in Lena Trough juxtaposed against a cold thick lithosphere. Alternatively, deep melting may be occurring within or entraining ancient continental mantle 1) Hellebrand, E. and Snow, J., 2003. Deep melting underneath the highly oblique-spreading Lena Trough (Arctic Ocean). Earth and Planetary Science Letters, 216:283-299.
T12A-08 12:05h
Reconstruction of the Flanks of the Mid-Atlantic Ridge, $28\deg$ to $29\deg$ N: Implications for Evolution of Young Oceanic Lithosphere at Slow-Spreading Centers
We reconstruct the flanks of the Mid-Atlantic Ridge between $28\deg$ and $29\deg$ N from 1 to 10 Ma at intervals of 1 Myr for the purpose of investigating evolution of young oceanic lithosphere morphology and its variation through time using an innovative method that combines seafloor subsidence correction with interpolated isochrons and rotation poles. Reconstruction results are consistent with formation of abyssal hills every 1 to 2 Myr in 2-3 Myr old lithosphere at the outer edge of the ridge mountains as a result of transition from dynamic regime near the axis to isostatic regime of the flanks. The oblique passage of structures formed at the axial valley walls through this transition zone may play a role in the development of inside corner high bathymetry. Asymmetric juxtaposition of abyssal hill morphology in reconstruction is indicative of independent formation and evolution of the morphotectonic fabric on opposing flanks. The two major factors affecting asymmetric ridge flank morphology are found to be sense of axial offset and fluctuation of magmatic activity at the segment scale. Sense of axial offset determines the relative distribution of inside and outside corner bathymetry on the flanks. Enhanced magma supply and associated segment propagation may contribute to half-spreading rate asymmetry, accretion of thicker crust, and formation of larger abyssal hills on the faster-spreading flank. Consistent alignment in reconstruction of the base of the steep walls bounding inside corner bathymetry confirms that they mark the boundaries between segments on the flanks and that the deep sediment filled basins, typically identified as discordant zones, are attributable to outside corner bathymetry.