Latisha Brengman , Christopher M. Fedo , Martin J. Whitehouse, Neil Banerjee, Iffat Jabeen

Silica-rich Precambrian rocks often preserve geochemical information and microfossil remnants from the early biosphere. Because these rocks are such critical geochemical and paleontological archives, we need robust tools to identify the chemical and physical conditions under which siliceous Precambrian rocks form, and determine how such information links to the specific depositional environment. Here, we investigate a series of sub- to greenschist facies Si-rich Archean rocks from the ~2.7 Ga Abitibi Greenstone Belt, Canada that represent chemical sedimentary rocks and rocks formed via silica-addition through the process of silicification. We report data for major and trace element geochemistry, multi-crystal silicon and oxygen isotopes of quartz using isotope ratio mass spectrometry, and texture-specific silicon isotope values measured using secondary ion mass spectrometry on chemical sedimentary rocks, their silicified equivalents, and silicified volcanic rocks. We find that in such a well-preserved terrane where we can utilize petrographic textures and geochemical attributes to establish rock origin, we can also distinguish silicon isotope signatures of rocks that form via chemical sedimentation, from those that form via silicification. Though chemical sedimentary rocks display a wide range of silicon isotopes values, similar to modern, low-temperature quartz precipitates, silicified volcanic rocks often possess near igneous signatures, close to values measured for modern oceanic crust. We also find that we can explain some of the silicon isotope heterogeneity measured using high-spatial resolution techniques, by using targeted, texture-specific single crystal analysis of quartz within rocks with a well-established geologic context. Overall, we interpret that texturally linked micro-scale silicon isotope heterogeneity likely results from changing chemical and physical conditions during the precipitation of quartz within the sample.