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

Document Type

Article

Publication Date

5-1993

Keywords

B/BE mobility, Metamorphic rock, Catalina Schist (California)

Digital Object Identifier (DOI)

https://doi.org/10.1016/0016-7037(93)90565-E

Abstract

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 <30. Thus, highly fractionated, slab-derived hydrous fluids may be necessary to generate the high-B/ Be signatures observed in many arcs (B/Be of up to ~200). The B-Be data, together with previously presented stable isotope data for the Catalina Schist, demonstrate that subduction-zone metamorphic processes are capable of homogenizing presubduction variability in the concentrations of particularly “fluid-mobile” elements in rocks and may, through mixing, produce fluids which trend toward uniform trace element and isotopic compositions. These homogeneous fluids could infiltrate parts of the mantle wedge and contribute to the characteristic trace element and isotopic signatures of arc-magma source regions. In hotter subduction zones ( e.g., involving subduction of young, hot oceanic lithosphere ), silicate melts derived from previously devolatilized sedimentary and mafic rocks may contribute relatively low-B/Be signatures to arc source regions. Thus, significant variations, among arcs, in the ranges of B/Be observed in front-rank volcanoes (e.g., for the Bismarck arc, B/Be = 20–190; for the Cascades arc, B/ Be < 5) may be related in part to varying thermal structure, which could govern both the B/Be of hydrous fluids and the relative proportions of hydrous fluid and silicate melt derived from the subducted slab and sediments.

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Citation / Publisher Attribution

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

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