A petrographic and isotopic criterion of the state of preservation of Precambrian cherts based on the characterization of the quartz veins

Abstract

The coupled O and Si isotope variations of Precambrian cherts show secular and correlated variations that have been interpreted as a progressive cooling of the oceans on Earth. However, this reconstruction has been challenged because cherts can have various origins (hydrothermal, sedimentary, volcanic silicification) and their isotopic compositions might have been reset by metamorphic fluid circulation. Even pristine samples show evidence of fluid circulation by the presence of various quartz veins. Here, we have determined the effect of fluid circulation on the isotopic composition of the microquartz chert matrix by studying the mineralogical relationships between quartz veins and microquartz in two distinctive chert samples. These samples show different quartz habit and texture in their quartz veins, which imply various fluid system conditions. Therefore the presence of quartz spherulites in the quartz veins indicates a more moderate fluid circulation than the presence of columnar quartz. These observations were also confirmed by the oxygen isotope compositions of both microquartz matrix chert and quartz veins. We have demonstrated that in some conditions, fluid circulations do not modify the oxygen isotope composition of the chert matrix and thus the microquartz can still record paleo-environmental conditions. The petrography of quartz vein associated with in situ oxygen isotope composition provides one criterion to characterize the state of preservation of the microquartz.