Silica enrichment in the continental upper mantle via melt/rock reaction

Mantle peridotite xenoliths from Archean cratons generally have high molar Mg/(Mg + Fe), or Mg#. The best known suites, from the Kapvaal and Siberian cratons, have high modal orthopyroxene (Opx). These high Opx compositions a re probably not residues of partial melting. Less well known cratonic xenol ith suites from Greenland and Noah America include high Mg# peridotites wit h much lower modal Opx. Such low Opx compositions could be residual from hi gh degrees of polybaric, decompression melting, ending in the spinel Iherzo lite stability field at pressures of 30 to 20 kbar. This paper presents add itional evidence that the great majority of both spinel- and garnet-bearing xenoliths are also residues of polybaric melting that ended at pressures l ess than or equal to 30 kbar. Where xenoliths record equilibration pressure s > 30 kbar, this must result from tectonic transport of peridotites to gre ater depth after melting. Proposed mechanisms for producing the high Mg#, h igh Opx compositions include metamorphic differentiation of high pressure r esidues, mixtures of residual peridotites and high pressure igneous cumulat es from ultramafic magmas, and addition of SiO2 to low Opx peridotites via melt/rock reaction. This paper focuses on a positive correlation between Ni contents of olivine and modal proportions of Opx in mantle xenoliths, and uses this correlation to constrain the processes that produced high Mg#, hi gh Opx cratonic mantle compositions. The observed correlation is probably n ot produced by partial melting, metamorphic differentiation, or formation o f igneous cumulates. It can be produced by reaction between SiO2-rich liqui ds (e.g., small degree melts of subducted eclogite) and previously depleted , low Opx peridotites. We propose a two step process. First, high Mg#, low Opx peridotites were created by large degrees of polybaric melting ending a t pressures < 30 kbar Later, these depleted residues were enriched in Opx b y interaction with SiO2-rich melts generated mainly by partial melting of e clogitic basalt and sediment in a subduction zone. Magmas modified by such a process could have formed a major component of the continental crust. Thu s, this hypothesis provides a genetic Link between cratonic upper mantle an d continental crust. (C) 1998 Elsevier Science B.V. All rights reserved.