Peridotite weathering is the missing ingredient of Earth’s continental crust composition

Published by: Beinlich, A., Austrheim, H., Mavromatis, V., Grguric, B., Putnis, Ch. V., Putnis, A.

Textural and chemical changes during ultramafic rock alteration. Back-scattered electron (BSE) images and element distribution maps of a serpentinised peridotite, b serpentinite, c talc-carbonate altered serpentinite, and d laterite show that the reaction sequence is accompanied by progressive partitioning of Si and Ni into weathering resistant phases. a, b show samples from the Feragen Ultramafic Body (Norway), c, d show samples from the Six Mile Well-Goliath Complex, SGC (Australia). For the discussion of different alteration environments we distinguish between weathering that happens exclusively at the surface (e.g. laterite formation) and hydrothermal alteration (e.g. serpentinisation, carbonation) at depth and at higher temperature. Awr = awaruite, Br = brucite, Chr = chromite, Gth = goethite, Hem = hematite, Mag = magnetite, Mgs = magnesite, Ol = olivine, Qtz = quartz, Srp = serpentine, Tlc = talc

The chemical composition of the continental crust cannot be adequately explained by current
models for its formation, because it is too rich in Ni and Cr compared to that which can be
generated by any of the proposed mechanisms. Estimates of the crust composition are
derived from average sediment, while crustal growth is ascribed to amalgamation of
differentiated magmatic rocks at continental margins. Here we show that chemical
weathering of Ni- and Cr-rich, undifferentiated ultramafic rock equivalent to ~1.3 wt% of
today’s continental crust compensates for low Ni and Cr in formation models of the
continental crust. Ultramafic rock weathering produces a residual that is enriched in Ni and
also silica. In the light of potentially large volumes of ultramafic rock and high atmospheric
CO2 concentrations during the Archean, chemical weathering must therefore have played a
major role in forming compositionally evolved components of the early Earth’s crust.

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Published Feb. 26, 2018 10:51 AM - Last modified Feb. 26, 2018 10:51 AM