The silicon and oxygen isotope compositions of Precambrian cherts: A record of oceanic paleo-temperatures?

摘要

Oxygen and silicon isotopes in cherts have been extensively used for the reconstruction of seawater temperatures during the Precambrian. During the past decade, the advance of in situ analysis of Si isotopes has enhanced the interest on cherts as a paleo-environmental proxy. The coupled O and Si isotope composition variations show secular and correlated trends that have been interpreted as a progressive cooling of the ocean. 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. In this case, the secular oxygen and silicon isotope variation are considered as reflecting a mixing between seawater and hydrothermal sources. A key point in this discussion deals with the origin of cherts: sedimentary, hydrothermal or chemically silicified? Therefore, several petrographical and geochemical criteria are proposed to recognize the pristine sedimentary origin of a chert. Namely they are: (1) the occurrence of microquartz, (2) a ~(18)O-rich bulk oxygen isotopic composition, (3) the occurrence of large δ~(18)O ranges at a micrometer scale, (4) variation of trace element compositions coupled with δ~(30)Si, (5) the occurrence of large ranges of δ~(30)Si in pure microquartz. These criteria should be regarded as guides to the identification of pristine diagenetic cherts in order to better constrain seawater paleo-temperature reconstructions by taking into account the effect of diagenesis. This article will review the different interpretations about O and Si isotope variation and propose a model of formation based on ancient and modern chert studies.