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Boron isotope record of peak metamorphic ultrahigh-pressure and retrograde fluid-rock interaction in white mica (Lago di Cignana, Western Alps)

Journal article
Authors R. Halama
Matthias Konrad-Schmolke
J. C. M. De Hoog
Published in Contributions to Mineralogy and Petrology
Volume 175
Issue 3
Pages 19
ISSN 0010-7999
Publication year 2020
Published at Department of Earth Sciences
Pages 19
Language en
Links dx.doi.org/10.1007/s00410-020-1661-...
Keywords Boron isotopes, White mica, Subduction, Metamorphism, Fluid-rock, interaction, Devolatilization, zermatt-saas zone, subduction-zone, trace-elements, tauern window, metasomatic tourmaline, metasedimentary rocks, oxygen-isotope, slab-mantle, fractionation, lithium, Geochemistry & Geophysics, Mineralogy
Subject categories Earth and Related Environmental Sciences

Abstract

This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subduction-related metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluid-rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 mu g/g B) occur in peak metamorphic phengite from a garnet-phengite quartzite. The B isotopic composition is variable (delta B-11 = - 10.3 to - 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid-rock interaction during peak metamorphic conditions. This interpretation is supported by fluid-rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in delta B-11 and [B]. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive delta B-11 values (delta B-11 = 20 +/- 5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 mu g/g and delta B-11 values that cluster around 0% (delta B-11 = - 5.0 to + 3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid-rock interaction with low-to-moderate (< 3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones.

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