Preliminary Results from the AIDP-2 and AIDP-3 Drill Cores Hint at Systematic Mo Enrichments in the ~2.65 Ga Roy Hill Shale

Abstract:

In order to better understand the timing of the earliest oxygenation of Earth’s surface environment, we are pursuing a multi-proxy investigation of paleoredox conditions in diamond drill cores through sedimentary rocks of the Archean Fortescue & Hamersley Groups. These cores were recovered in 2012 by the Agouron Institute from the Pilbara Craton of Western Australia. The AIDP-2 core samples a stratigraphic succession of carbonate and sulfidic, organic-rich shale in the Carawine Dolomite and Jeerinah Formation representing a shallow near-shore depositional setting. Core AIDP-3 samples a transition from BIF in the Marra Mamba Formation to organic-rich shales in the underlying Jeerinah Formation representing a deeper offshore depositional setting. We have analyzed 322 black shale samples from the Roy Hill Member of the Jeerinah Formation deposited just before the transition from the Fortescue to Hamersley Group. Roy Hill black shale units are mostly pyritic in AIDP-3, but are less so in AIDP-2. The Roy Hill Member of AIDP-3 extends from 2.629 Ga to2.676 Ga and contains the 2.632Ga Jeerinah impact layer, whereas the Roy Hill member of AIDP-2 is slightly older, lying beneath the Jeerinah impact layer, and has been dated to 2.636 Ga to >2.643 Ga [1].

Our initial findings reveal that Mo concentrations range between 0.7 and 7 ppm in the Roy Hill black shale member of AIDP-2 and AIDP-3. Corresponding Mo/Al ratios range between 1-9×10^-5 ppm/ppm, indicating slight Mo enrichment relative to average continental crust. These results are consistent with a previous study by Scott et al. [2], which suggested little or no Mo enrichment. However, the higher resolution sampling in this study allows us to clearly resolve the Mo/Al depth profiles in these late Archean cores. These data suggest that the variations we see are not due to analytical scatter or sample variability, but instead represent real variations in Mo scavenged into these sediments.

Ongoing work is focused on obtaining additional complementary datasets including Fe-speciation, TOC, and traditional and non-traditional isotopes. These data will provide the additional constraints needed to understand the origin and significance of Mo enrichments in these Archean sediments.

[1] Rasmussen & Fletcher (2010) Geology, 38: 299-302

[2] Scott et al. (2011) Geology, 39:119-122.