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Deciphering fluid inclusions in high-grade rocks

Hits: 1190
Year:
2014
Type of Publication:
Article
Keywords:
Fluid inclusions
Authors:
van den Kerkhof, Alfons; Kronz, Andreas; Simon, Klaus
Journal:
Geoscience Frontiers
Volume:
5
Number:
5
Pages:
683 - 695
ISSN:
1674-9871
BibTex:
Note:
\{SPECIAL\} ISSUE: The role of fluids in the lower crust and upper mantle: A tribute to Jacques Touret
Abstract:
Abstract The study of fluid inclusions in high-grade rocks is especially challenging as the host minerals have been normally subjected to deformation, recrystallization and fluid-rock interaction so that primary inclusions, formed at the peak of metamorphism are rare. The larger part of the fluid inclusions found in metamorphic minerals is typically modified during uplift. These late processes may strongly disguise the characteristics of the “original” peak metamorphic fluid. A detailed microstructural analysis of the host minerals, notably quartz, is therefore indispensable for a proper interpretation of fluid inclusions. Cathodoluminescence (CL) techniques combined with trace element analysis of quartz (EPMA, LA-ICPMS) have shown to be very helpful in deciphering the rock-fluid evolution. Whereas high-grade metamorphic quartz may have relatively high contents of trace elements like Ti and Al, low-temperature re-equilibrated quartz typically shows reduced trace element concentrations. The resulting microstructures in \{CL\} can be basically distinguished in diffusion patterns (along microfractures and grain boundaries), and secondary quartz formed by dissolution-reprecipitation. Most of these textures are formed during retrograde fluid-controlled processes between ca. 220 and 500 °C, i.e. the range of semi-brittle deformation (greenschist-facies) and can be correlated with the fluid inclusions. In this way modified and re-trapped fluids can be identified, even when there are no optical features observed under the microscope.

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