Abstract
The most abundant terrestrial lavas, mid-ocean ridge and ocean island basalt (MORB and OIB), are commonly considered to be derived from a depleted MORB-mantle component and more specific, variably enriched mantle plume sources. However, findings of oceanic lavas and mafic cumulates issued from melts, enriched in chlorine and having a radiogenic Sr ratio, can be attributed to an interaction between the asthenosphere-derived melts and lithospheric peridotite variably hydrated due to penetration of hydrothermal water down to and below Moho level. To constrain mechanisms and rates responsible for the interaction, we report results of experiments of reaction between serpentinite and tholeiitic basaltic melt. Results show that the reaction proceeds via a multi-stage mechanism: (i) transformation of serpentinite into Cr-rich spinel-bearing harzburgite containing pore fluid, (ii) partial melting and dissolution of the harzburgite assemblage with formation of interstitial hydrous melts, and (iii) final assimilation of the Cr-rich spinel-bearing harzburgite/dunite and formation of hybrid basaltic melts with high MgO and elevated Cr and Ni contents. Assimilation of serpentinite by basaltic melt may occur under elevated melt/rock ratios and may lead to chromitite formation. Our experiments provide evidence that MORB and high-Mg-Cr orthopyroxene-rich cumulates depleted in incompatible elements can be produced from common mid-ocean ridge basaltic melts modified by reaction with hydrated lithospheric peridotite. We established that the rate of assimilation of serpentinized peridotite is controlled by silica diffusion in the reacting hydrous basaltic melt.
Highlights
Mid-ocean ridge basalt (MORB) and ocean island basalts (OIBs) are considered as products of decompression melting of several asthenospheric sources characterized by contrasted isotopic signatures (e.g., depleted MORBmantle component (DMM), HIMU, EM-1, and EM-2; e.g., Zindler and Hart, 1986)
Our experiments provide evidence that MORB and high-Mg-Cr orthopyroxene-rich cumulates depleted in incompatible elements can be produced from common mid-ocean ridge basaltic melts modified by reaction with hydrated lithospheric peridotite
Mid-ocean ridge basalt (MORB) and ocean island basalts (OIBs) are considered as products of decompression melting of several asthenospheric sources characterized by contrasted isotopic signatures (e.g., DMM, HIMU, EM-1, and EM-2; e.g., Zindler and Hart, 1986)
Summary
Mid-ocean ridge basalt (MORB) and ocean island basalts (OIBs) are considered as products of decompression melting of several asthenospheric sources characterized by contrasted isotopic signatures (e.g., DMM, HIMU, EM-1, and EM-2; e.g., Zindler and Hart, 1986). Hydrated peridotites, in particular, upper mantle serpentinites are highMg rocks variously enriched in refractory elements (Cr, Ni) and fluid-mobile elements (e.g., halogens, H, B, O, He, Ar, As, S, Sb, Sr, and Pb) due to interaction between peridotite and seawaterderived low-to-moderate-temperature hydrothermal fluids (e.g., Guillot et al, 2001; Früh-Green et al, 2004; Bonifacie et al, 2008; Deschamps et al, 2010; Evans et al, 2013; Guillot and Hattori, 2013; Kendrick et al, 2013) These rocks crop out preferentially below the mantle-crust transition zone (Evans et al, 2013) which is locally exposed to the seafloor at mid ocean ridges (e.g., Bach et al, 2004) and may be sampled as upper mantle xenoliths by lavas of such oceanic islands as Canary Islands (Neumann et al, 2015) at possible reaction depths of up to 1.0 GPa pressure. Our new data contribute to constrain the rate, mechanism and compositional impact of the assimilation of the serpentinized lithospheric mantle by the basaltic magma
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