B-Be Systematics in Subduction-Related Metamorphic Rocks: Characterization of the Subducted Component

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The mobility of B and Be in H2O-rich fluids and felsic silicate liquids produced during metamorphism of subducted oceanic slab and sediments has been investigated through analysis of subduction-zone metamorphic rocks of the Catalina Schist, California. In metasedimentary rocks, B/Be and the range in B/Be decrease with increasing metamorphic grade (mean = 72, std. dev. = 41 for lowest-grade lawsonite-albite fades rocks; mean = 21, std. dev. = 11 for higher-grade greenschist and epidote-amphibolite facies equivalents). This decrease to more uniformly low B/Be may be attributed to the preferential removal of B in H2O-rich fluids produced by devolatilization reactions over the approximate temperature interval of 350–600°C. Metamafic rocks do not show pronounced decrease in B/Be with increasing metamorphic grade; however, all metamafic samples have B/Be< 30, lower than values for many altered seafloor basalts. In amphibolite-grade exposures, felsic leucosomes and pegmatites reflecting partial melting have low B/Be similar to their metasedimentary and metamafic hosts, which presumably experienced prior reduction in B/Be during lower temperature devolatilization.

This evidence for B and Be mobility during high-P/T metamorphism complements studies of B-Be systematics in arc volcanic rocks in further characterizing mechanisms by which slab-derived elements can be added to the source regions of arc lavas. Before subducted mafic and sedimentary rocks reach Wadati-Benioff zone depths beneath arcs ( 80–150 km), the B/ Be of these rocks is likely to have decreased to

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Geochimica et Cosmochimica Acta, v. 57, issue 10, p. 2227-2237