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| Titel |
40Ar/39Ar geochronology by stepwise crushing |
| VerfasserIn |
Jan Wijbrans, Huaning Qiu, Rongguo Hu, Fraukje Brouwer |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250054642
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| Zusammenfassung |
| Crustal fluid flow occurs at all levels of the continental crust. Fluid flow is assumed to be
episodic, resulting from protracted events in the crust such as diagenesis, ore genesis,
prograde metamorphism and hydrothermal circulation following magma emplacement, and
commonly leads to the deposition of secondary minerals in veins in the rock. Vein minerals
contain fluid inclusions that were encapsulated by the mineral during first growth, and
during later stages by crack-seal mechanisms. Primary fluid inclusion composition
reflects the composition of the fluids attending diagenesis and the early stages of
metamorphism, commonly aqueous fluids with varying levels of salinity. Most vein
minerals contain secondary fluid inclusions incorporated after original mineral
growth. Such secondary inclusions are introduced when cracks develop that are
subsequently sealed by deposition of minerals. Secondary fluid inclusions have been
used with success to decipher quite intricately changing physical conditions during
metamorphism.
We have developed a method for 40Ar/39Ar analysis of fluid inclusions by stepwise
crushing and we have applied this method to diagenitic quartz veins, ore deposits and
metamorphic veins. Although from fluid inclusion petrology it is known that fluid inclusion
patterns in most minerals are mixed, from our work on veins in many different settings we
note that argon release shows common features. In crushing experiment, the reservoir that
first contributes to fluid release quite consistently shows the highest amounts of excess 40Ar.
This reservoir is gradually exhausted, to be followed by argon from other reservoirs in the
mineral. The parameters that are most easily monitored are apparent age (40Ar/39Ar
ratio), Ca and Cl derived isotopes of argon such as 37Ar and 38Ar, and air argon
monitored by the 36Ar content. We have developed a first order model by which we can
explain the fluid inclusion content of vein minerals as a three reservoir system: excess
argon – radiogenic argon and air-argon. We use calcium derived 37Ar, chlorine
derived 38Ar and potassium derived 39Ar to obtain additional information on the
chemistry of the postulated reservoirs. In favorable cases age information on the
formation of such inclusions can be obtained, because radiogenic argon formed
from the in situ decay of 40K in solution in the fluid inclusions is an end-member
of our mixing model. The diffusion rate of argon in the mineral hosting the fluid
inclusion determines the closure temperature associated with the argon age obtained. |
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