Fast assimilation of serpentinized mantle by basaltic magma

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.

💡 Research Summary

The paper revisits the conventional view that mid‑ocean‑ridge basalts (MORB) and ocean‑island basalts (OIB) are derived solely from a depleted MORB mantle (DMM) source or a variably enriched plume mantle. Instead, the authors demonstrate that interaction between asthenospheric tholeiitic melt and hydrated lithospheric peridotite (serpentinite) can substantially modify the melt chemistry, producing the chlorine‑rich, radiogenic‑Sr, and transition‑metal‑enriched signatures observed in many oceanic lavas and high‑Mg cumulates.

To quantify the process, high‑pressure (≈1 GPa) and high‑temperature (≈1300 °C) piston‑cylinder experiments were performed with a natural tholeiitic basalt melt and a natural serpentinite rock. The reaction proceeds through three distinct stages. (1) The serpentine minerals break down, releasing pore fluids and forming a Cr‑rich spinel‑bearing harzburgite. This harzburgite contains residual fluid that remains trapped in the crystal lattice. (2) The newly formed harzburgite partially melts; silica‑oxygen networks destabilise, generating interstitial hydrous melts that are enriched in MgO, Cr and Ni. (3) The hydrous melt assimilates the Cr‑rich spinel‑bearing harzburgite/dunite, producing a hybrid melt with markedly higher MgO, Cr and Ni than the starting basalt and with the potential to precipitate chromite layers (chromitite).

A key kinetic control identified is silica diffusion in the melt. The authors show that the overall assimilation rate scales with the diffusion coefficient of SiO₂ in the hydrous basaltic melt; faster Si diffusion accelerates the breakdown of the peridotite framework and promotes Cr‑spinel growth. Consequently, melt‑to‑rock ratios of order 1–5 are sufficient for rapid assimilation, consistent with field observations of high‑Mg, high‑Cr cumulates adjacent to basaltic flows.

Geochemically, the hybrid melts retain a MORB‑like trace‑element pattern but acquire elevated MgO (up to ~15 wt %), Cr (up to ~0.5 wt %), and Ni (up to ~0.3 wt %). They also inherit the high Cl and radiogenic ^87Sr/^86Sr signatures of the serpentinized lithosphere, explaining the anomalous compositions of many oceanic lavas that cannot be reconciled with a purely depleted mantle source.

The authors argue that many MORB and high‑Mg, Cr‑rich orthopyroxene cumulates previously interpreted as direct mantle melts may instead be the product of basaltic melt modification by serpentinized lithospheric peridotite. This mechanism provides a unified explanation for the coexistence of depleted MORB trace‑element signatures with enriched incompatible‑element and volatile signatures, and it offers a plausible pathway for the formation of chromitite layers in oceanic crust.

Overall, the study supplies experimental evidence that the rate‑limiting step in serpentinized peridotite assimilation is silica diffusion in the melt, and it quantifies the conditions under which basaltic melts can be chemically transformed by hydrated lithospheric mantle. This insight refines our understanding of mantle melting dynamics, crustal construction at mid‑ocean ridges, and the genesis of chemically diverse oceanic basalts.