TY - JOUR
T1 - The signature of devolatisation
T2 - Extraneous 40Ar systematics in high-pressure metamorphic rocks
AU - Smye, Andrew J.
AU - Warren, Clare J.
AU - Bickle, Mike J.
N1 - Funding Information:
A.J.S. acknowledges support of a Jackson Postdoctoral Fellowship. C.J.W. acknowledges funding from the Natural Environment Research Council (NE/E0114038/1 and NE/H016279/1). We are grateful to David Shuster for his editorial handling of the manuscript. Formal reviews by Jan Wijbrans, Ethan Baxter and an anonymous reviewer served to greatly improve the clarity of the manuscript. This work has benefitted from fruitful discussion with Simon Kelley, Chris Hawkesworth and Marian Holness.
PY - 2013/7/5
Y1 - 2013/7/5
N2 - The validity of using the 40Ar/39Ar system for thermochronology relies on the assumption that the source mineral is surrounded by a grain boundary reservoir defined by an effective 40Ar concentration of zero. However, the presence of extraneous 40Ar (Are) in metamorphic rocks shows that this assumption is invalid for a significant number of cases. Are is common in micas that have equilibrated under (ultra-)high pressure ((U)HP) conditions: metasediments from six Phanerozoic (U)HP terranes yield apparent 40Ar/39Ar phengite ages ≲50% in excess of the age of peak (U)HP conditions, whereas cogenetic mafic eclogites yield ages up to ~700% older despite lower K2O concentrations. A model is developed that calculates Are age fractions as a function of variable mica-fluid K D, bulk K2O and porosity under closed system conditions. Measured Are concentrations in mafic eclogites are reproduced only when porosities are ≲10 - 4 volume fraction, showing that mafic protoliths operate as closed systems to advective solute transport during subduction. Porosities in eclogite-facies metapelites are ≲10 - 2, reflecting loss of significant volumes of lattice-bound H2O relative to mafic rocks during subduction. Retention of locally-generated 40Ar in mafic eclogites shows that the oceanic crust is an efficient vehicle for volatile transport to the mantle.
AB - The validity of using the 40Ar/39Ar system for thermochronology relies on the assumption that the source mineral is surrounded by a grain boundary reservoir defined by an effective 40Ar concentration of zero. However, the presence of extraneous 40Ar (Are) in metamorphic rocks shows that this assumption is invalid for a significant number of cases. Are is common in micas that have equilibrated under (ultra-)high pressure ((U)HP) conditions: metasediments from six Phanerozoic (U)HP terranes yield apparent 40Ar/39Ar phengite ages ≲50% in excess of the age of peak (U)HP conditions, whereas cogenetic mafic eclogites yield ages up to ~700% older despite lower K2O concentrations. A model is developed that calculates Are age fractions as a function of variable mica-fluid K D, bulk K2O and porosity under closed system conditions. Measured Are concentrations in mafic eclogites are reproduced only when porosities are ≲10 - 4 volume fraction, showing that mafic protoliths operate as closed systems to advective solute transport during subduction. Porosities in eclogite-facies metapelites are ≲10 - 2, reflecting loss of significant volumes of lattice-bound H2O relative to mafic rocks during subduction. Retention of locally-generated 40Ar in mafic eclogites shows that the oceanic crust is an efficient vehicle for volatile transport to the mantle.
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U2 - 10.1016/j.gca.2013.03.018
DO - 10.1016/j.gca.2013.03.018
M3 - Article
AN - SCOPUS:84876976703
SN - 0016-7037
VL - 113
SP - 94
EP - 112
JO - Geochimica et Cosmochimica Acta
JF - Geochimica et Cosmochimica Acta
ER -