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| Titel |
Excess 40Ar as a tracer for metamorphic porosity |
| VerfasserIn |
A. J. Smye, C. J. Warren, M. J. Bickle |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250069596
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| Zusammenfassung |
| Argon decoupled from its parent K (40ArE) is a ubiquitous feature of metamorphic rocks,
specifically those which have formed under high pressure conditions. The factors controlling
its presence are not clearly understood. Traditionally, 40ArE is interpreted as detrimental to
geochronological studies, where the aim is to measure meaningful 40Ar/39Ar cooling ages. A
global compilation of high-pressure (HP) muscovite and phengite 40Ar/39Ar data shows that
ages typically lie between 1—200% in excess of the age of peak metamorphism as
constrained by U—Pb, Lu—Hf or Sm—Nd geochronology. Intriguingly, mafic lithologies
consistently yield older ages than co-genetic pelitic lithologies. Concentrations of 40ArE
commonly vary on sub-millimetric lengthscales both between and within individual grains.
These data confirm that the grain boundary network does not operate as an infinite
reservoir to mica—fluid Ar exchange, as required for Dodson closure-temperature
behaviour.
A physical model has been built for the accumulation of 40ArE under closed-system
conditions. The model characterises the control of variable mica—fluid KD (10-3—10-5),
whole-rock mica abundance, porosity (10-6—10-1 vol. fraction) and protolith history on
muscovite 40ArE concentration. Furthermore, the model provides a means of estimating
time-averaged metamorphic porosities from inversion of mica 40ArE concentrations. Porosity
estimates based on recently published single-grain HP 40Ar/39Ar data from Oman and the
Eastern Alps lie between 10-6 and 10-4 volume fraction, consistent with independent
constraints on the mid—lower crustal background porosities. Calculated porosities for mafic
rocks are 1—2 orders of magnitude smaller than pelitic lithologies, which is in agreement
with the relative difference in devolatisation histories. Phase equilibria calculations show that
a typical MORB loses ~25% of mineral-bound H2O, between the greenschist and eclogite
facies, compared to ~47% in the pelitic system. Model results require that the transition
between open and closed system behaviour occurs at higher temperature and pressure
conditions in pelites than mafic eclogites. This is supported by critical dihedral angle
experiments.
The model results show that the accumulation of 40ArE may therefore be used a tracer for
the time-averaged effects of transient crustal porosity. Furthermore, the results show that
the build-up 40ArE is controlled by the presence or absence of a connected grain
boundary network rather than merely the presence or absence of a fluid phase in the
rock. |
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