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Anomalous unradiogenic 87 Sr// 86 Sr ratios in ultrahigh‐pressure crustal carbonates – evidence for fluid infiltration during deep subduction?
Author(s) -
Romer R. L.,
Wawrzenitz N.,
Oberhänsli R.
Publication year - 2003
Publication title -
terra nova
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.353
H-Index - 89
eISSN - 1365-3121
pISSN - 0954-4879
DOI - 10.1046/j.1365-3121.2003.00503.x
Subject(s) - geology , radiogenic nuclide , metamorphic rock , geochemistry , subduction , dolomite , calcite , mineralogy , isotopes of strontium , isotopic signature , strontium , mantle (geology) , isotope , seismology , tectonics , physics , quantum mechanics , nuclear physics
Marbles from Changpu (Dabie Shan, eastern China), subducted to 4.4 GPa, have 87 Sr/ 86 Sr values < 0.7040. These low 87 Sr/ 86 Sr values, which would imply a sedimentation age > 2 Ga if considered as primary signature, reflect fluid–rock interaction with a fluid from a low‐ 87 Sr/ 86 Sr source. The introduction of low‐ 87 Sr/ 86 Sr was paralleled by introduction of Mg and loss of Si, K and Na in such a way that carbonates from the purest marbles have the least evolved Sr isotopic composition. Introduction of Mg is also indicated by the distribution of calcite and dolomite. Calcite forms inclusions in garnet, whereas dolomite is restricted to the matrix. These chemical changes, inferred from the mineralogy, in combination with textural evidence require a mobile metamorphic fluid. P – T – X constraints for fluid generation and for permeability increase related to mineral reactions and phase transitions suggest that the marbles acquired their anomalous Sr‐isotopic composition during subduction below 60 km. The marbles with the least radiogenic Sr isotopic composition demonstrate that crustal rocks may lose their isotopic fingerprint during deep subduction.