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Project Progress

*Mass-independent isotope effects in sedimentary rocks by ion microprobe analysis.

We and our NAI collaborators at UCLA have reported on a new high-resolution technique using the UCLA Cameca ims1270 ion microprobe in multicollection mode to investigate mass-independent (Δ³³S) sulfur isotope variability in a variety of Precambrian sediments including the oldest known rocks of sedimentary origin (Mojzsis et al. (2003) Geochim. Cosmochim. Acta: 67(9), 1635-1658). We have now moved beyond the feasibility stage and are actively engaged in measuring hundreds of samples spanning the “Great Oxygenation Event” between ~2.47 and 2.33 Ga that irreversibly oxidized the surface zone of the planet.

*Discrimination between preservation of original igneous zircon populations and inherited grains.

We described a method whereby the chemistry of a complex metamorphic rock coupled with U/Th ratios in zircon from the same rock permits direct assessment of zircon “inheritance” vs. true igneous ages. The technique has been used to resolve outstanding problems in the genesis of lower crustal rocks that stabilize the continents (Mojzsis et al. (2003) J. Geol. 111(4), 407-425) as well as overcome the long-standing problem of interpreting primary “igneous” ages from overprinted “metamorphic” ages in the oldest rocks. This new methodology is being extended to samples of described pre-3.7 Ga rocks worldwide (Acasta gneisses, Canada; Manfred Complex, Australia; etc.)