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Stable isotopic and trace element evidence for restricted fluid migration in 2 GPa eclogites
Author(s) -
GETTY S. R.,
SELVERSTONE J.
Publication year - 1994
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.1994.tb00057.x
Subject(s) - eclogite , geology , omphacite , metamorphism , trace element , geochemistry , metamorphic rock , mineral , dolomite , mineralogy , subduction , tectonics , chemistry , paleontology , organic chemistry
Mineral stable isotopic and trace element studies in 2 GPa banded eclogites of the Tauern Window, eastern Alps, record mm‐ to cm‐scale heterogeneities that reflect compositional variations in the accompanying metamorphic fluids. A close correlation between dolomite mode and dolomite δ 18 O is consistent with equilibrium partitioning among coexisting minerals and fluids. Small variations in dolomite δ 13 C values correspond with δ 18 O variations, but an overall decrease in dolomite δ 13 C by c . 1%o across a 12‐cm sample is a relict feature that pre‐dates eclogite equilibration. Garnet, omphacite, and clinozoisite rims show little systematic mineral‐mineral partitioning behaviour for Ti, V, Cr, Y, Sr, or Zr; major elements, however, are well equilibrated among these same minerals. Despite the apparent lack of mineral‐mineral trace element equilibration, most of the trace elements vary systematically with water activity calculated in each layer. Trace element behaviour during the eclogite metamorphism thus appears to have been controlled largely by mineral‐fluid interactions along grain boundaries. Shallow structural levels in other subduction complexes ( c . 10‐45 km) typically exhibit fracture‐controlled permeability and extensive metasomatism, but there is no field or geochemical evidence for extensive fluid advection during high‐pressure metamorphism in the Tauern eclogites. Because most dewatering and devolatilization during tectonic burial occurs prior to eclogite conditions, the volumetric fluid/rock ratio in eclogites should generally be low. Low fluid/rock ratios, coupled with the possible non‐wetting nature of the fluids, permits the production and preservation of fine‐scale chemical heterogeneities in deeply subducted eclogites and associated fluids. However, the eventual breakdown at greater depth of volatile‐bearing dolomite, phengite, clinozoisite, zoisite, or amphibole could lead to renewed fracture‐controlled fluid release from the subducted rocks to regions appropriate for arc magma generation.