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

• Main Authors: Peter B. Kelemen A, Stanley R. Hart A, Stefan Bernstein B
• Other Authors: The Pennsylvania State University CiteSeerX Archives
• Format: Text
• Language: English
• Published: 1998
• Subjects: Greenland
• Online Access: http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.520.2218; http://www.ldeo.columbia.edu/files/uploaded/file/1998 Kelemen et al EPSL.pdf

Mantle peridotite xenoliths from Archean cratons generally have high molar Mg=(Mg C Fe), or Mg#. The best known suites, from the Kapvaal and Siberian cratons, have high modal orthopyroxene (Opx). These high Opx compositions are probably not residues of partial melting. Less well known cratonic xenolith suites from Greenland and North America include high Mg # peridotites with much lower modal Opx. Such low Opx compositions could be residual from high degrees of polybaric, decompression melting, ending in the spinel lherzolite stability field at pressures of 30 to 20 kbar. This paper presents additional evidence that the great majority of both spinel- and garnet-bearing xenoliths are also residues of polybaric melting that ended at pressures 30 kbar. Where xenoliths record equilibration pressures> 30 kbar, this must result from tectonic transport of peridotites to greater depth after melting. Proposed mechanisms for producing the high Mg#, high Opx compositions include metamorphic differentiation of high pressure residues, mixtures of residual peridotites and high pressure igneous cumulates 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#, high Opx cratonic mantle compositions. The observed correlation is probably not produced by partial melting, metamorphic differentiation, or formation of igneous cumulates. It can be produced by reaction between SiO2-rich liquids (e.g., small degree melts of