SILICON ISOTOPE COMPOSITION OF ARCHEAN CHERTS FROM >3.7 GA TO 2.7 GA DETERMINED BY SECONDARY ION MASS SPECTROMETRY (SIMS)

Precambrian chert commonly forms an essential component of (1) banded iron formation (BIF), a banded, silica- and iron-rich chemical sedimentary rock capable of recording ocean chemistry and (2) silicified rocks, which do not necessarily record ocean chemistry as they result from replacement of pre-existing rocks. The compiled silicon isotope (δ³⁰Si) record of chert shows a gradual rise to more positive δ³⁰Si values through time possibly as the result of gradual cooling of the oceans, or mixing of continental and hydrothermal inputs. To better understand the Si-isotope record, we have studied samples of chert from bands in BIF and in silicified rocks from three different localities (>3.7 Ga Isua greenstone belt, SW Greenland; 3 Ga Buhwa greenstone belt, Zimbabwe; 2.7 Ga Abitibi greenstone belt, Canada). Silicon-isotope data were collected using high-spatial resolution secondary ion mass spectrometry (SIMS) on individual quartz crystals of silicified rocks, hydrothermal (Algoma-type) BIF, and continental (Superior-type) BIF. Comparable to previous studies, silicified rocks possess δ³⁰Si values close to zero per mil, reflecting the δ³⁰Si isotope composition of the igneous host rocks. Algoma-type BIF preserve a distinct decrease in δ³⁰Si values between >3.7 Ga and 3 Ga, with a return to less negative values at 2.7 Ga. Superior-type BIF show a distinct shift from negative to positive δ³⁰Si values between 3 Ga and 1.9 Ga. We conclude that silicified rocks, Algoma BIF, and Superior BIF, preserve individual δ³⁰Si trends, which suggests that separation of rock types could provide insight into the mechanisms behind broad trends within the complete δ³⁰Si archive.