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Abstract

Abstract

Debated among scientists for decades, observations of compositional differences between different crystallographic faces within a same growth zone, called ‘sector zoning’, represents a major challenge for geochemistry and in particular for isotope geochemistry. Nowadays, convincing evidence for the existence of sector zoning has been reported in the literature. However, no consensus on the mechanisms triggering sector zoning has been reached. Understanding how and why sector zoning occurs is essential because the presence of sector zoning could skew the isotopic characterization of a carbonate mineral depending on the area being sampled. This would result in an ambiguous interpretation of the data.

The emphasis of this study, which is part of the Qatar Carbonates and Carbon Storage Research Center (QCCSRC), is therefore placed on deciphering the different processes leading to isotopic sector zoning in calcite, with the help of a new tool: the clumped isotopes (i.e. isotopologues).

Large calcite crystals with clearly visible growth zones have been collected from fracture infills in Oman. The minerals growth zones and sectors are recognized via cathodoluminescence microscopy and the trace element content of the different zones in several sectors is estimated via electron microprobe. These different sectors are then sampled by a micro-drill following single growth zones and analyzed for their oxygen and carbon isotopic compositions. The approach highlighted above allows for recognition of the presence of sector zoning, but does not constrain the type of mechanism(s) leading to the observed isotopic fractionation.

To gain further insight, we turn to clumped isotopes (multiply substituted isotopologues) which are molecules with distinct chemical and physical properties. This should theoretically lead to distinctive fractionations. Clumped isotopes have been used recently to measure the temperature of precipitation of carbonate minerals. If each of the crystalline phases of a given crystal were precipitated under thermodynamic equilibrium, one would expect a constant clumped isotope value between sectors. However, we intend to use clumped isotopes as a discriminative geochemical tool to check if differences in the Δ values between sectors exist. If this was the case, isotopologues could provide new insights on sector zoning and would give an extra dimension to the quest for the different factors resulting in these types of isotopic zonations.

Combining compositional, stable isotopes and clumped isotopes information could ultimately help to reveal the secrets of sector zoning.

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/content/papers/10.5339/qfarf.2010.EEP21
2010-12-13
2019-10-20
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References

  1. MacDonald 2010. I.A. MacDonald. , Using clumped isotopes to help understand isotopic sector zoning in calcite. , QFARF Proceedings , 2010 , EEP21 .
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