Many mantle peridotite samples are too rich in Si02 (in the form of orthopyroxene) and have ratios of light to heavy rare earth elements that are too high to be consistent with an origin as the residuum of partial melting of the primitive mantle. Trace element studies of melt/rock reaction zones in the Trinity peridotite provide evidence for reaction of the mantle lithosphere with ascending melts, which dissolved calcium-pyroxene and precipitated orthopyroxene as magma mass decreased. This process can account for the observed major and trace element compositions of lithospheric mantle samples, and may accordingly be prevalent in the upper mantle.

Formation of harzburgite by pervasive melt/rock reaction in the upper mantle

Relative depletion of niobium in some arc magmas and the continental crust : partitioning of K, Nb, La, and Ce during melt/rock reaction in the upper mantle

The Luobusa ophiolite in the Indus—ladling Zjingbo suture of southern Tibet hosts the largest known chromite deposit in China. The podiform chromitites occur in a well-preserved mantle sequence consisting of harzburgite with abundant lenses of dunite. The harzburgites have relatively uniform bulk-rock compositions with mg-numbers [100Mg/(Mg + Fe) ] ranging from 89 to 91 and show flat, unfractionated, chondrite-normaliied platinum group element (PGE) patterns. Their accessory chromite varies widely in cr-number [100Cr/(Cr + Al)] (1866). These rocks are essentially residua left after extraction of mid-ocean ridge basalt (MORB)-type magmas. The podiform chromitites display nodular, massive, disseminated and banded textures and typically have dunite envelopes that grade into the surrounding harzburgite and diopsidic harzburgite with increasing pyroxene contents. They consist of relatively uniform chromite with high cv-numbers (74-82), have strongly fractionated, chondrite-normalized PGE patterns with enrichment in Os, Ir and Ru relative to Rh, Pt and Pt, and are believed to have formed from a boninitic magma produced by a second stage of melting. Dunites contain accessory chromite intermediate in composition between those of harzburgite and chromitite and are believed to be the products of reaction between new boninitic magmas and old MORB-type peridotites. The melt-rock reaction removed pyroxene from the peridotites and precipitated olivine, forming dunite envelopes around the chromitite pods. The melts thus became more boninitic in composition and chromite saturated, leading to precipitation of chromite alone. The interplay of melt—rock interaction, chromite fractionation and magma mixing should lead to many fluctuations in melt composition, producing both massive and disseminated chromitites and phase layering within individual podiform bodies observed in the Luobusa ophiolite.

Podiform Chromitites in the Luobusa Ophiolite (Southern Tibet): Implications for Melt-Rock Interaction and Chromite Segregation in the Upper Mantle

Orogenic, ophiolitic, and abyssal peridotites represent subcontinental, suboceanic, and subarc mantle rocks that were exhumed to the surface in various tectonic settings. These rocks provide coverage of vast regions of the Earth's upper mantle that are sparsely sampled by mantle xenoliths. They notably allow the observation of a wide range of lithospheric mantle compositions, including cratonic roots and subduction mantle wedges (high-pressure orogenic garnet lherzolites), variably rejuvenated subcontinental lithosphere (lower-pressure orogenic spinel and plagioclase lherzolites), and newly accreted oceanic lithosphere (ophiolitic mantle and abyssal peridotites). It is shown here that most of geochemical variability recorded by these mantle rocks is attributable to melt processes associated with partial melting and asthenosphere–lithosphere interactions. Rather than remnants of pristine mantle, the fertile orogenic lherzolites are now widely considered as former refractory lithospheric mantle refertilized by upwelling partial melt.

J. Vernieres

Papers

Modelling of Trace Element Transfer between Mantle Melt and Heterogranular Peridotite Matrix