Detrital,metamorphic and metasomatic tourmaline in high-pressure metasediments from Syros (Greece): intra-grain boron isotope patterns determined by secondary-ion mass spectrometry |
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Authors: | Horst R Marschall Rainer Altherr Angelika Kalt Thomas Ludwig |
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Institution: | 1.Mineralogisches Institut,Universit?t Heidelberg,Heidelberg,Germany;2.Department of Earth Sciences,University of Bristol,Bristol,UK;3.Institut de Géologie,Université de Neuchatel,Neuchatel,Switzerland |
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Abstract: | The boron isotopic composition of zoned tourmaline in two metasediments from the island of Syros, determined by secondary-ion
mass spectrometry (SIMS), reflects the sedimentary and metamorphic record of the rocks. Tourmaline from a silicate-bearing
marble contains small (≤20 μm) detrital cores with highly variable δ
11B values (−10.7 to +3.6‰), pointing to a heterogeneous protolith derived from multiple sources. The sedimentary B isotopic
record survived the entire metamorphic cycle with peak temperatures of ∼500°C. Prograde to peak metamorphic rims are homogeneous
and similar among all analysed grains (δ
11B ≈ +0.9‰). The varying δ
11B values of detrital cores in the siliceous marble demonstrate that in situ B isotope analysis of tourmaline by SIMS is a
potentially powerful tool for provenance studies not only in sediments but also in metasediments. A meta-tuffitic blueschist
bears abundant tourmaline with dravitic cores of detrital or authigenic origin (δ
11B ≈ −3.3‰), and prograde to peak metamorphic overgrowth zones (−1.6‰). Fe-rich rims, formed during influx of B-bearing fluids
under retrograde conditions, show strongly increasing δ
11B values (up to +7.7‰) towards the margins of the grains. The δ
11B values of metamorphic tourmaline from Syros, formed in mixed terrigenous–marine sediments, reflect the B signal blended
from these two different sources, and was probably not altered by dehydration during subduction. |
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Keywords: | Boron isotopes Tourmaline High-pressure Metasediment Subduction zone |
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