SIMS analyses of silicon and oxygen isotope ratios for quartz from Archean and Paleoproterozoic banded iron formations

Details SIMS analyses of silicon and oxygen isotope ratios for quartz from Archean and Paleoproterozoic banded iron formations SIMS analyses of silicon and oxygen isotope ratios for quartz from Archean and Paleoproterozoic banded iron formations

Oct 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011gecoa..75.5879h&link_type=abstract

Geochimica et Cosmochimica Acta, Volume 75, Issue 20, p. 5879-5891.

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Scientific paper

Banded iron formations (BIFs) are chemical marine sediments dominantly composed of alternating iron-rich (oxide, carbonate, sulfide) and silicon-rich (chert, jasper) layers. Isotope ratios of iron, carbon, and sulfur in BIF iron-bearing minerals are biosignatures that reflect microbial cycling for these elements in BIFs. While much attention has focused on iron, banded iron formations are equally banded silica formations. Thus, silicon isotope ratios for quartz can provide insight on the sources and cycling of silicon in BIFs. BIFs are banded by definition, and microlaminae, or sub-mm banding, are characteristic of many BIFs. In situ microanalysis including secondary ion mass spectrometry is well-suited for analyzing such small features. In this study we used a CAMECA IMS-1280 ion microprobe to obtain highly accurate (±0.3‰) and spatially resolved (˜10 μm spot size) analyses of silicon and oxygen isotope ratios for quartz from several well known BIFs: Isua, southwest Greenland (˜3.8 Ga); Hamersley Group, Western Australia (˜2.5 Ga); Transvaal Group, South Africa (˜2.5 Ga); and Biwabik Iron Formation, Minnesota, USA (˜1.9 Ga). Values of δ18O range from +7.9‰ to +27.5‰ and include the highest reported δ18O values for BIF quartz. Values of δ30Si have a range of ˜5‰ from -3.7‰ to +1.2‰ and extend to the lowest δ30Si values for Precambrian cherts. Isua BIF samples are homogeneous in δ18O to ±0.3‰ at mm- to cm-scale, but are heterogeneous in δ30Si up to 3‰, similar to the range in δ30Si found in BIFs that have not experienced high temperature metamorphism (up to 300 °C). Values of δ30Si for quartz are homogeneous to ±0.3‰ in individual sub-mm laminae, but vary by up to 3‰ between multiple laminae over mm-to-cm of vertical banding. The scale of exchange for Si in quartz in BIFs is thus limited to the size of microlaminae, or less than ˜1 mm. We interpret differences in δ30Si between microlaminae as preserved from primary deposition. Silicon in BIF quartz is mostly of marine hydrothermal origin (δ30Si < -0.5‰) but silicon from continental weathering (δ30Si ˜ 1‰) was an important source as early as 3.8 Ga.

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