Os isotopic compositions of MORBs from the ultra-slow spreading Southwest Indian Ridge: Constraints on the assimilation and fractional crystallization (AFC) processes

To investigate the origin of heterogeneous water distribution in the suboceanic lithospheric mantle, we performed a detailed petrological and geochemical analysis of abyssal peridotites collected from two localities (53°E and 63.5°E) on the Southwest Indian Ridge. These serpentinized peridotites display primary olivine‐orthopyroxene‐clinopyroxene‐spinel assemblage and record equilibrium temperature of 1150–1200°C and around 1000°C for the 53°E and 63.5°E locations, respectively. The rocks were thus equilibrated in the spinel stability field prior to their exhumation to the seafloor. Our FTIR analyses show variable water contents in orthopyroxene ranging from 24 to 262 wt ppm H2O. Orthopyroxene in the 63.5°E peridotites is characterized by homogeneous and high water content (>200 ppm), whereas orthopyroxene in the 53°E peridotites displays a wider range of water contents (24–246 ppm). We first demonstrate that differences in equilibrium conditions (i.e., pressure and temperature) and mineral chemistry and serpentinization cannot explain the water content variations. Melting modeling show that a fractional melting in both garnet and spinel stability fields is needed to explain the MREE and HREE concentrations in clinopyroxene. Enrichment in LREE, high water contents, and high H2O/Ce, however, require a postmelting rehydration event such as metasomatism. Based on petrographic evidence and investigation of chemical heterogeneities at segment/dredge scale, we suggest that this event involves a metasomatic agent enriched in water and incompatible elements. We infer that the small‐scale heterogeneities in water and trace elements may result either from successive infiltration of various amounts of melt/fluids as described in the formation of oceanic core complex or from spatial heterogeneities of melt infiltration as observed in peridotite massifs.

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“…The gray fields represent SWIR 53°E basalts from Yang et al . [] and basalts east of Melville FZ (60°E–70°E) from Meyzen et al . [].…”

Section: Discussionmentioning

confidence: 99%

Origins of water content variations in the suboceanic upper mantle: Insight from Southwest Indian Ridge abyssal peridotites

Li¹,

Soustelle

Zhang

et al. 2017

Geochem Geophys Geosyst

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“…The dashed line with crossed symbols is the calculated isochron of 103.7 with an initial 187 Os/ 188 Os of 0.133. Global modern MORBs (EPR, mid-Atlantic ridge and Indian ridge) data are from Gannoun et al 21, Escrig et al 45 and Yang et al 46.…”

Section: Figurementioning

confidence: 99%

Interactions of the Greater Ontong Java mantle plume component with the Osbourn Trough

Zhang

2016

Sci Rep

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The Ontong Java-Manihiki-Hikurangi plateau (OJMHP) is considered to have originated from a starting mantle plume, and have been rifted apart by two spreading ridges. However, the ages of these spreading ridges and their possible interactions with the presumed mantle plume are unclear. The Manihiki-Hikurangi plateau has been rifted apart by the Osbourn Trough which formed the southwestern Pacific crust to the east of the Tonga-Kermadec trench. Here we report Pb-Hf-Os isotopes of the basaltic crust (Site U1365 of IODP Expedition 329) formed by the Osbourn Trough. Linear regression of Re-Os isotopes results in an age of 103.7 ± 2.3 Ma for Site U1365 basalts, indicating that the Manihiki-Hikurangi plateau was rifted apart by the Osbourn Trough with a spreading rate of ~190 mm/yr. The superfast spreading rate supports the Osbourn as an abandoned segment of the early Pacific spreading ridge, which initially overlapped with the giant starting plume. Moreover, the Pb-Hf isotopes of some of Site U1365 basalts show distinct differences from those of the Pacific mid-ocean ridge basalts, while they are similar to the basalts of the Ontong Java and Manihiki plateaus. We suggest that the OJMHP mantle plume components has been involved by the Osbourn spreading center.

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“…Thicker lithospheric mantle leads to a shorter melting column and lower degrees of partial melting (Bown and White, 1994), which thus explain the unique chemical compositions of MORBs from ultraslow-spreading ridges (e.g., White et al, 2001). However, only very few studies concern how lithospheric thickness and the structure of ultra-slow spreading ridges control the magmatic evolution of MORBs (e.g., Yang et al, 2013).…”

Section: Introductionmentioning

confidence: 99%

Chalcophile elemental compositions of MORBs from the ultraslow-spreading Southwest Indian Ridge and controls of lithospheric structure on S-saturated differentiation

et al. 2014

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Os isotopic compositions of MORBs from the ultra-slow spreading Southwest Indian Ridge: Constraints on the assimilation and fractional crystallization (AFC) processes

Cited by 27 publications

References 60 publications

Origins of water content variations in the suboceanic upper mantle: Insight from Southwest Indian Ridge abyssal peridotites

Origins of water content variations in the suboceanic upper mantle: Insight from Southwest Indian Ridge abyssal peridotites

Interactions of the Greater Ontong Java mantle plume component with the Osbourn Trough

Chalcophile elemental compositions of MORBs from the ultraslow-spreading Southwest Indian Ridge and controls of lithospheric structure on S-saturated differentiation

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