2010 Annual Science Report
University of Wisconsin Reporting | SEP 2009 – AUG 2010
Project 3D: Stable Isotope and Mineralogical Studies of Banded Iron Formations: O & Si Isotopes by SIMS
The oxygen isotope ratio of modern seawater is 0‰ (δ18O, VSMOW) and the oceans are thought to partly balance high δ18O crustal rocks relative to a primary mantle δ18O value of 5.5‰. Isotope ratios of O and Si from cherts up to 3.5 Ga have been controversially interpreted to reflect either a hot Archean ocean of 50-70°C or a low δ18O Archean ocean of -10 to -13‰. These interpretations assume that cherts record the primary seawater δ18O. We have conducted in situ SIMS analysis of oxygen and silicon isotope ratios in cherts from banded iron formations (BIFs) at Isua, Greenland (3.8 Ga); Hamersley, Western Australia (2.5 Ga); Transvaal, South Africa (2.5 Ga), and Biwabik, Minnesota, USA (1.9 Ga). Correlated values of δ18O and δ30Si are used to test assumptions about the degree to which these isotope ratios record ocean compositions, or exchange during diagenesis or metamorphism. Silicon isotopes may also have the potential to distinguish between continental (δ30Si > -0.4‰) and hydrothermal sources of Si in BIF cherts.
Oxide facies BIFs are essentially composed of equal parts quartz and iron oxides. Magnetite and hematite are the dominant Fe-oxides and the paragenesis of these minerals is important to understanding the fluid and thermal history of unmetamorphosed or low-temperature (sub-greenschist facies) BIFs. We identified silician magnetite overgrowths by BSE-SEM and used three diffraction techniques to verify that silicon is structural in magnetite. Silician magnetite overgrowths may form in reducing alkaline conditions during BIF diagenesis and metamorphism. We have observed silician magnetite in several low-temperature BIFs from 2.6 to 1.9 Ga and hypothesize that the former presence of organic matter may be required to attain the low oxygen fugacity necessary to stabilize silicon in magnetite. Silician magnetite is thus proposed as a novel biosignature.
Stable Isotope and Mineralogical Studies of Banded Iron Formations: O and Si isotopes by SIMS
We used the UW-Madison CAMECA ims-1280 ion microprobe to develop a silicon isotope standard for quartz. The spot-to-spot precision in δ30Si is ±0.2‰ (2SD, n = 14) for WiscSIMS quartz standard UWQ-1, which is more homogeneous than grains of NBS-28 quartz (±0.3, 2SD, n = 26, also called African Sand and NIST RM 8546).
In samples of quartz from BIF, values of δ30Si show a range of ~5‰, from -3.7‰ to +1.2‰ and extend to the lowest values reported for the Precambrian (Fig. 1). The average δ30Si for Isua is -1.6‰ while for Hamersley, Transvaal, and Biwabik, the average δ30Si value is identical and 1‰ higher at -0.6‰ (Heck et al., 2010). The range in δ30Si values is 4.5‰ for Isua, 3.5‰ for Hamersley, 2.7‰ for Biwabik, and 2.6‰ for Transvaal. The lowest δ30Si value for Isua (-3.7‰) is 1‰ lower, and the range in δ30Si values is 1‰ greater than for Hamersley, Transvaal and Biwabik. Values of δ18O range from +7.9‰ to +31.4‰, including the highest reported δ18O values in BIF quartz (Heck et al., 2010; Huberty et al., 2010). We interpret the narrow range in δ18O for Transvaal and Hamersley BIFs to reflect homogenization of quartz, thus these samples are not in equilibrium with seawater and cannot be used for oxygen isotope thermometry.
Individual amphibolite-facies samples from Isua are homogeneous in δ18O at cm-scale, but show similar variability in δ30Si as BIFs that have not experienced high temperature metamorphism. The wide range of δ30Si at Isua suggests that silicon isotopes are not homogenized at the cm-scale by amphibolite facies metamorphism, as is observed for δ18O values. Most of our δ30Si values fall below the mantle value of -0.4‰ and agree with the range of hydrothermal silica (δ30Si = -0.4 to -3.0‰), while δ30Si values greater than -0.4‰ may suggest a continental source. Thus our data show a mixture of both sources for Si in BIF quartz.
We have studied in detail the mineralogy of magnetite from the Dales Gorge Member of the Brockman Iron Formation, Hamersley Group, Western Australia. Figure 2 shows a BSE image where contrast has been adjusted to show only magnetite (Huberty et al., 2010a). Quartz, carbonate, and stilpnomelane are present but appear black in this image. Internal magnetite lamellae (white/ light grey) contain numerous μm to sub-μm inclusions of carbonate and quartz whereas silician magnetite overgrowths (darker grey) are devoid of mineral inclusions. In situ micro-X-ray diffraction using a 40 μm beam shows that the lattice parameter for silician magnetite is 8.388(2) to 8.392(2) Å (Huberty et al., 2010b), lower than the reference value for pure magnetite, 8.397 Å, indicating that silicon is present in magnetite as solid solution. Similarly, high resolution bright field TEM images and select area diffraction patterns confirm that silicon is in solid solution in magnetite. Silician magnetite is the dominant Fe-oxide in the Dales Gorge BIF. We hypothesize that the former presence of organic matter may be required to attain the low oxygen fugacity necessary to stabilize silicon in magnetite and propose that silician magnetite is a biosignature.
PROJECT INVESTIGATORS:John Valley
PROJECT MEMBERS:Noriko Kita
RELATED OBJECTIVES:Objective 4.1
Earth's early biosphere.
Biosignatures to be sought in Solar System materials