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2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 100-11
Presentation Time: 11:05 AM

GEOCHEMICAL AND SR, ND, AND OS ISOTOPIC SIGNATURES OF TARGET ROCKS AND IMPACTITES FROM THE ZHAMANSHIN CRATER, KAZAKHSTAN


SCHULZ, Toni1, SACKL, Florian1 and KOEBERL, Christian2, (1)Department of Lithospheric Research, University of Vienna, Althanstrasse 14, Vienna, 1090, Austria, (2)Department of Lithospheric Research, University of Vienna, Althanstrasse 14, A-1090 Vienna, Austria, also of the Natural History Museum, Burgring 7, A-1010 Vienna, Austria, toni.schulz@univie.ac.at

The 1 Ma, 14-km-diameter Zhamanshin impact structure, situated in a semi-arid region of Kazakhstan, has a heterogeneous suite of target rocks, including (i) basement Silurian quartzites, (ii) Devonian shales and ultramafic dikes, (iii) Carboniferous volcano-sedimentary rocks of andesitic composition, (iv) Cretaceous sandstones, shales, marls, and limestones, as well as (iv) Cenozoic clays. The center of the crater is filled with lake sediments and loess. There are a variety of impact glasses (zhamanshinites) and tektite-like objects (irghizites), subclassified into Si-poor and Si-rich zhamanshinites and irghizites. Several drill core and surface target rocks as well as impactite samples were analyzed in this study for their geochemical composition, including highly siderophile element (HSE) contents, as well as Sr, Nd, and Os isotopic compositions to study the relationship between target rocks and impactites and to search for possible meteoritic components within the impactites.

Major and trace element data agree with derivation of impactites from the known suite of target rocks. Rare earth element abundances and patterns and Sr and Nd isotopic compositions support a connection between Cenozoic clays and irghizites and Si-rich zhamanshinites, whereas Si-poor zhamanshinites are related to basic and ultramafic target rocks. Rhenium/Os ratios in all impactites are consistently lower than in most target rocks, possibly reflecting selective Re-loss during impact. Chromium, Co, Ni, and HSE concentrations and the respective elemental distribution patterns of most target rocks and zhamanshinites mirror upper continental crust. In agreement with earlier work by other researchers, most irghizites have significantly higher Cr, Co, and Ni concentrations than target rocks and zhamanshinites. Concentrations of HSE in one of the analyzed irghizites exceeds those of the average target by two orders of magnitude, resulting in a less fractionated HSE pattern and near-chondritic interelement ratios; this sample also has a 187Os/188Os ratio that is significantly less radiogenic compared to most target rocks (~0.31 vs. 0.66 to 5.22, respectively). Excluding any ultramafic contaminant in this sample and interpreted in terms of a meteoritic admixture, this represents about 0.02% of a CI chondritic component

Session No. 100

T173. Impact Cratering on Earth and in the Solar System
Monday, 2 November 2015: 8:00 AM-12:00 PM
Room 344 (Baltimore Convention Center)
Geological Society of America Abstracts with Programs. Vol. 47, No. 7, p.282
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