Multistage asthenospheric melt/rock reaction in the ultraslow eastern SWIR mantle

Very small amounts of melt are produced during mantle upwelling beneath the ultraslow spreading South West Indian Ridge. Sectors of this Oceanic Ridge are characterized by nearly amagmatic spreading with rare limited eruptions of basalts spotting a mantle-derived serpentinitic crust. A large peridotite dataset was recovered during the Smoothseafloor French expedition leaded by D. Sauter and M. Cannat in 2005 (Sauter et al., 2013). Mantle-derived rocks show a significant modal variability from the sample to the dredge scale with frequent occurrences of millimetric to centimetric spinel-bearing pyroxenitic veins. Mantle residua record a multistage reactional history between small amount of transient melts and variably depleted mantle parcels. Incomplete mineral replacements are widespread showing that both pyroxenes are repeatedly dissolved and recrystallized leaving poekilitic pyroxene and spinel textures. Reacting conditions are modelled assuming an incremental open-system melting model under variable critical porosity/F ratios (Seyler et al., 2011; Brunelli et al., 2014). Incoming melts result to be generated by low degrees of melting in the garnet field then reacting with the rock under near-batch conditions, i.e. at low rates of melt extraction with respect to the actual rock porosity. As a consequence Na (and LREE) countertrends with melting indicators as mineral Cr# and concentration of the moderately incompatible elements (HREE, HFSE). This results in rotation of the REE patterns around a pivot element instead of showing progressive depletion as expected after suboceanic mantle decompression. Brunelli D., Paganelli E. & Seyler, M. 2014. Percolation of enriched melts during incremental open-system melting in the spinel field: A REE approach to abyssal peridotites from the Southwest Indian Ridge. Geoch. et Cosmoch. Acta, 127, 190–203. doi:10.1016/j.gca.2013.11.040. Sauter D., Cannat M., Searle R. 2013. Continuous exhumation of mantle-derived rocks at the Southwest Indian Ridge for 11 million years. Nature Geosci., 6(4), 1–7. doi:10.1038/ngeo1771. Seyler M., Brunelli D., Toplis M. J. & Mével C. (2011). Multiscale chemical heterogeneities beneath the eastern Southwest Indian Ridge (52°E-68°E): Trace element compositions of along-axis dredged peridotites. Geochem. Geophys. Geosyst., 12, Q0AC15. doi:10.1029/2011gc003585

Spreading Rate-Dependent Variations in Crystallization Along the Global Mid-Ocean Ridge System

We investigate crustal accretion at mid-ocean ridges by combining crystallization pressures calculated from major element contents in mid-ocean ridge basalt (MORB) glasses and vapor-saturation pressures from melt inclusions and MORB glasses. Specifically, we use established major element barometers and pressures estimated from 192 fractional crystallization trends to calculate crystallization pressures from \u3e9000 MORB glasses across the global range of mid-ocean ridge spreading rates. Additionally, we estimate vapor-saturation pressures from \u3e400 MORB glasses from PETDB and \u3e400 olivine-hosted melt inclusions compiled from five ridges with variable spreading rates. Both major element and vapor-saturation pressures increase and become more variable with decreasing spreading rate. Vapor saturation pressures indicate that crystallization occurs in the lower crust and upper mantle at all ridges, even when a melt lens is present. We suggest that the broad peaks in major element crystallization pressures at all spreading rates reflects significant crystallization of on and off-axis magmas along the base of a sloping lithosphere. Combining our observations with ridge thermal models we show that crystallization occurs over a range of pressures at all ridges, but it is enhanced at thermal/rheologic boundaries, such as the melt lens and the base of the lithosphere. Finally, we suggest that the remarkable similarity in the maximum vapor-saturation pressures (∼3 kbars) recorded in melt inclusions from a wide range of spreading rates reflects a relatively uniform CO2 content of 50–85 ppm for the depleted upper mantle feeding the global mid-ocean ridge system

Technical advances in near real time seafloor monitoring implemented for the momar-d project

Peer Reviewe

Serpentinization, Carbonation, and Metasomatism of Ultramafic Sequences in the Northern Apennine Ophiolite (NW Italy)

Fluid-rock interaction in ultramafic rocks considerably affects the chemical and isotopic composition of the oceanic lithosphere. We present a geochemical and petrological study of serpentinites and ophicalcites of the Northern Apennine ophiolite, Italy. This ophiolite sequence represents fragments of Jurassic oceanic lithosphere that have been denuded by low angle detachment faults, exposing peridotites on the ocean floor and triggering hydrothermal alteration. Seawater circulation is documented by (Jurassic) seawater-like 87Sr/86Sr values and δ13C values of 1.1–3.0‰ in carbonate veins of the ophicalcites. Bulk rock ophicalcites have low 87Sr/86Sr values of 0.70489–0.70599, elevated SiO2 contents, and talc druses filling calcite veins that record Si-metasomatism. In contrast, underlying serpentinites have 87Sr/86Sr values above Jurassic seawater values. Bulk rock δD and δ18O values of ophicalcites and serpentinites suggest interaction with an evolved seawater-derived and/or magmatic fluid. These chemical signatures result from a complex history of serpentinization, carbonation, and metasomatism. Multiphase water-rock interaction includes infiltration of basement-derived fluids during initial mantle upwelling within an opening ocean basin, followed by localized high-temperature fluid infiltration, extensive seawater circulation resulting in carbonation, and oxidation near the seawater-exposed surface, and finally, fluid-rock interaction with overlying mafic lithologies leading to Si-metasomatism. The studied sequence represents an excellent example of the evolution from serpentinite to ophicalcite during continuous uplift and exposure of ultramafic rocks on the seafloor and documents the complex hydrothermal evolution of ultramafic rocks associated with this process. The extensive chemical transformation of mantle peridotites likely has an impact on geochemical cycles and subduction zone processes

Tectonic structure, evolution, and the nature of oceanic core complexes and their detachment fault zones (13°20′N and 13°30′N, Mid Atlantic Ridge)

Microbathymetry data, in situ observations, and sampling along the 138200N and 138200N oceanic core complexes (OCCs) reveal mechanisms of detachment fault denudation at the seafloor, links between tectonic extension and mass wasting, and expose the nature of corrugations, ubiquitous at OCCs. In the initial stages of detachment faulting and high-angle fault, scarps show extensive mass wasting that reduces their slope. Flexural rotation further lowers scarp slope, hinders mass wasting, resulting in morphologically complex chaotic terrain between the breakaway and the denuded corrugated surface. Extension and drag along the fault plane uplifts a wedge of hangingwall material (apron). The detachment surface emerges along a continuous moat that sheds rocks and covers it with unconsolidated rubble, while local slumping emplaces rubble ridges overlying corrugations. The detachment fault zone is a set of anostomosed slip planes, elongated in the alongextension direction. Slip planes bind fault rock bodies defining the corrugations observed in microbathymetry and sonar. Fault planes with extension-parallel stria are exposed along corrugation flanks, where the rubble cover is shed. Detachment fault rocks are primarily basalt fault breccia at 138200N OCC, and gabbro and peridotite at 138300N, demonstrating that brittle strain localization in shallow lithosphere form corrugations, regardless of lithologies in the detachment zone. Finally, faulting and volcanism dismember the 138300N OCC, with widespread present and past hydrothermal activity (Semenov fields), while the Irinovskoe hydrothermal field at the 138200N core complex suggests a magmatic source within the footwall. These results confirm the ubiquitous relationship between hydrothermal activity and oceanic detachment formation and evolution

Pervasive melt percolation reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge, 15° 20′N : ODP Hole 1274A

Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 153 (2007): 303-319, doi:10.1007/s00410-006-0148-6.ODP Leg 209 Site 1274 mantle peridotites are highly refractory in terms of lack of residual clinopyroxene, olivine Mg# (up to 0.92) and spinel Cr# (~0.5), suggesting high degree of partial melting (>20%). Detailed studies of their microstructures show that they have extensively reacted with a pervading intergranular melt prior to cooling in the lithosphere, leading to crystallization of olivine, clinopyroxene and spinel at the expense of orthopyroxene. The least reacted harzburgites are too rich in orthopyroxene to be simple residues of low-pressure (spinel field) partial melting. Cu-rich sulfides that precipitated with the clinopyroxenes indicate that the intergranular melt was generated by no more than 12% melting of a MORB mantle or by more extensive melting of a clinopyroxene-rich lithology. Rare olivine-rich lherzolitic domains, characterized by relics of coarse clinopyroxenes intergrown with magmatic sulfides, support the second interpretation. Further, coarse and intergranular clinopyroxenes are highly depleted in REE, Zr and Ti. A two-stage partial melting/melt-rock reaction history is proposed, in which initial mantle underwent depletion and refertilization after an earlier high pressure (garnet field) melting event before upwelling and remelting beneath the present-day ridge. The ultra-depleted compositions were acquired through melt re-equilibration with residual harzburgites.Funding for this research was provided by Centre National de la Recherche Scientifique-Institut National des Sciences de l’Univers (Programme Dynamique et Evolution de la Terre Interne)

Mineralogical and geochemical features of sulfide chimneys from the 49°39′E hydrothermal field on the Southwest Indian Ridge and their geological inferences

© The Author(s), 2011. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Chinese Science Bulletin 56 (2011): 2828-2838, doi:10.1007/s11434-011-4619-4.During January–May in 2007, the Chinese research cruise DY115-19 discovered an active hydrothermal field at 49°39′E/37°47′S on the ultraslow spreading Southwest Indian Ridge (SWIR). This was also the first active hydrothermal field found along an ultraslow-spreading ridge. We analyzed mineralogical, textural and geochemical compositions of the sulfide chimneys obtained from the 49°39′E field. Chimney samples show a concentric mineral zone around the fluid channel. The mineral assemblages of the interiors consist mainly of chalcopyrite, with pyrite and sphalerite as minor constitunets. In the intermediate portion, pyrite becomes the dominant mineral, with chalcopyrite and sphalerite as minor constitunets. For the outer wall, the majority of minerals are pyrite and sphalerite, with few chalcopyrite. Towards the outer margin of the chimney wall, the mineral grains become small and irregular in shape gradually, while minerals within interstices are abundant. These features are similar to those chimney edifices found on the East Pacific Rise and Mid-Atlantic Ridge. The average contents of Cu, Fe and Zn in our chimney samples were 2.83 wt%, 45.6 wt% and 3.28 wt%, respectively. The average Au and Ag contents were up to 2.0 ppm and 70.2 ppm respectively, higher than the massive sulfides from most hydrothermal fields along mid-ocean ridge. The rare earth elements geochemistry of the sulfide chimneys show a pattern distinctive from the sulfides recovered from typical hydrothermal fields along sediment-starved mid-ocean ridge, with the enrichment of light rare earth elements but the weak, mostly negative, Eu anomaly. This is attributed to the distinct mineralization environment or fluid compositions in this area.This work was supported by the China Ocean Mineral Resources Research and Development Association Program (DY115- 02-1-01) and the State Oceanic Administration Youth Science Fund (2010318)

Numerical modelling of Non-Transform Discontinuity geometry: Implication for ridge structure, volcano-tectonic fabric development and hydrothermal activity at segment ends.

Ocean ridge discontinuities partition and offset spreading centres at a range of scales. Large scale discontinuities (10's–100's km) are synonymous with first-order transform faults, which have well defined linear fault zone valleys. In contrast, Non-Transform Discontinuities (NTDs) are diffuse, smaller scale offsets (0 to b20 km), characterised by central basins or topographic highs. The geometry of NTD offsets can be categorised by the sense of offset, either right-stepping or left-stepping, and by the relative positions of the segment tips. The segment tip configurations include under-lapping, over-lapping or simple across-axis jumps or stepping in the ridge axis. In this study finite difference software is used to model segment geometry at a slow-spreading ridge under a normal tensile-stress within a homogeneous and isotropic medium. Along- and across-axis segment separations were varied incrementally for left- and right-stepping senses. The results show that the ratio of along-axis to across-axis segment tip separation is a dominant control of stress field rotation within an NTD. Features which most clearly show rotation within an NTD include basins and tectonically controlled constructional ridges. The obliquity of these features along with measurements of the surrounding fault fabrics are used as a way of observing and determining stress rotations within NTDs along the Central Indian Ridge (CIR). These rotations were used to obtain segment geometries from models where the central tensor showed an equivalentrotation. The results show that geometry has a profound effect on stress field rotation under which large- and small-scale volcanotectonic fabrics form. In addition, a shortfall of the predicted model tip relative to interpreted positions, along with morphology and observation of the ridge fabrics at the terminations to some segments, suggests the existence of a zone, broadly analogous to theprocess zone observed in fracture mechanics, which we call a damage zone. Given the criteria for the promotion of hydrothermal circulation, this damage zone would have a greater potential for hosting hydrothermal activity.<br/

Analysis of satellite gravity and bathymetry data over Ninety-East Ridge: Variation in the compensation mechanism and implication for emplacement process

International audienceWe investigate the mode of compensation, emplacement history and deep density structure of the Ninety-East Ridge (Indian Ocean) using spectral analyses and forward modeling of satellite gravity and bathymetry data. We find that the northern (0–10°N) and the southern (20–30°S) parts of the ridge are flexurally compensated with an effective elastic thickness >15 km, whereas the central part (0–20°S) is locally compensated. Furthermore, we find that for a part of central block (10–20°S, over Osborn Knoll) the compensation depth is unreasonably very high (30–40 km). Therefore we favor a model with subsurface loading and interpret this to be due to underplating of mafic material at the base of the crust, a hypothesis that is supported by seismic results and direct modeling of gravity data along some profiles. These results suggest that the northern and southern parts of Ninety-East Ridge were emplaced off to a ridge axis compared to the central one, which might have been emplaced on or near a spreading center. Locally compensated large topography, thick underplated crust in the central part (near Osborn Knoll), might result from an interaction of a hot spot with the extinct Wharton spreading ridge