Glacial diamictite research
Bill McDonough, Roberta Rudnick, Yongshen Liu, and me at the contact between Neoproterozoic Nantuo diamictite and cap carbonate in the Three Gorges region of China
Nic Beukes, his students, and me examining drill core of the Paleoproterozoic Duitschland diamictite outside of Johannesburg, South Africa
Glacial diamictites in their natural habitat. Top - Paleoproterozoic Bruce Formation, Huronian Supergroup, Ontario. Bottom - Neoproterozoic Chuos Formation, Otavi Group, Namibia
Elemental ratios for diamictite samples that differentiate between mafic and felsic source material, plotted versus diamcitite age. The dramatic drop in Ni/Lu with decreasing age and the changes in Th/Sc and Eu/Eu (i.e. europium anomaly) are all consistent with the continental crust becoming more felsic with time. "R&G UCC" is the average composition of the upper continental crust today (from Rudnick and Gao, 2003). These results make it clear that the upper continental crust at 3.0 Ga (and before) was fundamentally different from what we have today. (Figure from Gaschnig et al., 2016)
Glacial diamictites are the lithified (rock) equivalent of sediments deposited by ice sheets during periods of glaciation. The goal of this NSF-funded research on diamictites is to develop a new model for the average composition of the Earth’s upper continental crust (UCC) and determine the extent to which the composition of the UCC has changed through time. The approach is to use glacial diamictites of different ages as proxies for the average UCC. Diamictites derived from continental ice sheets should reflect a broad integrated provenance and should also be derived largely from physical weathering processes, reducing elemental loss associated with chemical weathering.
This work has been conducted with Roberta Rudnick (UCSB), Bill McDonough (U of Maryland), Jay Kaufman (U of Maryland), Zhaochu Hu (China U of Geosciences-Wuhan), and Shan Gao (also CUG-Wuhan but sadly passed away this year). Our diamictites samples have been collected from numerous localities on four modern continents and come from the Paleozoic (~0.3 Ga), Neoproterozoic (~0.73 to 0.57 Ga), Paleoproterozoic (~2.4 to 2.2 Ga), and Neoarchean (~2.9 Ga). The heart of the project is high quality trace element characterization. Other data sets that we have collected or plan to collect are: Li, O, Cr, Mo, and U stable isotopes; Nd, Pb, and Os isotopes; and detrital zircon U-Pb ages and Hf isotopes.
We have observed major differences in composition between the Mesoarchean and Paleoproterozoic diamictites and the younger ones, most notably higher levels of transition metals such as Ni, Cr, Co, V, and Sc in the older diamictites (see lower left). This is consistent with evidence for a more mafic Archean UCC with large amounts of greenstone and komatiite. We also see sharp depletion in Mo and other redox sensitive metals in the post-Paleoproterozoic samples. Change in behavior of redox sensitive metals is due to their conversion to more soluble oxidized states due to the rise of atmospheric and oceanic oxygen. This is consistent with the inverse trend seen in black shales through time, which reflects gradual accumulation of Mo in ocean basins due to oxidative loss from the continents. Some of the publicatons from this research are listed below:
Chen, K., Rudnick, R.L., Gao, S., Walker, R.J., Gaschnig, R.M., Puchtel, I.S., Tang, M., Hu, Z. (2016) Platinum-group element abundances and Re-Os isotopic systematics of the upper continental crust: evidence from glacial diamictites: Geochimica et Cosmochimca Acta, v. 191, p. 1-16.
Gaschnig, R. M., Rudnick, R. L., McDonough, W. F., Kaufman, A. J., Hu, Z., and Gao, S. (2014) Onset of oxidative weathering of continents recorded in the geochemistry of ancient glacial diamictites: Earth and Planetary Science Letters, v. 408, p. 87-99. PDF
Gaschnig, R.M., Rudnick, R.L., and McDonough, W.F. (2015) Determination of Ga, Ge, Mo, Ag, Cd, In, Sn, Sb, W, Tl, and Bi in USGS whole-rock reference materials by standard addition ICP-MS: Geostandards and Geoanalytical Research, v. 39, p. 371-379.
Gaschnig, R.M., Rudnick, R.L., McDonough, W.F., Kaufman, A.J., Valley, J.W., Hu, Z., Gao, S., and Beck, M.L. (2016) Compositional evolution of the upper continental crust, as constrained by ancient glacial diamictites: Geochimica et Cosmochimica Acta, v. 186, p. 316-343. PDF
Li, S., Gaschnig, R.M., and Rudnick, R.L. (2016) Origin of the chemical weathering signature in ancient glacial diamictite and the weathering signature of the upper continental crust: Geochimica et Cosmochimca Acta, v. 176, p. 96-117. PDF
Tang, M., Rudnick, R.L., McDonough, W.F., Gaschnig, R.M., and Huang, Y. (2015) Europium anomalies constrain the mass of recycled lower continental crust: Geology, v. 43, p. 703-706. PDF