Crowther, S A

Gilmour, J D

Schönbächler, M

Theis, K J

English

Allegan (H5 OC) yields an I-Xe age of 4565.8 My for pyroxenes suggesting a closure temperature higher than 800?875 oC. Dhofar 125 (acapulcoite) yields an I-Xe age of 4557.9 My, consistent with related lodranites and older than Pd-Ag age of 4549 My

The University of Manchester - Institutional Repository

COMBINED PALLADIUM-SILVER AND IODINE-XENON ISOTOPE SYSTEMATICS FOR ALLEGAN (H5) AND DHOFAR 125 (ACAPULCOITE). K. J. Theis1, M. Schönbächler2, S. A. Crowther1 and J. D. Gil-mour1. 1SEAES, Isotope Group, The University of Manchester, Williamson Building, Oxford Road, Manchester, M13 9PL. ka-ren.theis@manchester.ac.uk. 2IGP, ETH Zürich, Clausiusstrasse 25, 8092 Zürich, Switzerland.  Introduction: Short-lived chronometers are ideal for dating processes that occurred early during solar system formation be-cause they record different events depending on the closure tem-peratures of the specific host minerals. Short-lived chronometers have a high time resolution (~1 My) and thus provide precise ages. However, such age determinations are relative and must be tied to an absolute timescale. This is done by measuring the unknown sample relative to a sample with a well constrained absolute age. Such a sample is currently missing for the Pd-Ag system [1,2] re-stricting the precision that can be placed on the absolute ages. We have analysed two samples, Allegan, an H5 ordinary chondrite and Dhofar 125, an acapulcoite, for both I-Xe and Pd-Ag in order to cross calibrate the two chronometers. Acapulcoites cooled quickly (100 ± 40 K/My [3-5]) such that the distinct closing temperatures (T) should not result in resolvable age differences. Analytical Techniques: Pd-Ag: the Ag isotope compositions were measured on a Nu Plasma MC-ICP-MS relative to the NIST SRM978a Ag standard. Palladium and Ag concentrations were obtained using isotope dilution. I-Xe: samples were irradiated and measured on RELAX [6]. Corrections were applied for fission and trapped Xe before the ages were calculated relative to Shallowater (4562.3 ± 0.4 My [7]).  Results and discussion: Allegan (H5) yielded an I-Xe age of 4565.8 ± 2 My for pyroxenes. This just overlaps with the Hf-W age of 4562.4 ± 0.9 My for H5 chondrites, which reflects thermal metamorphism [8]. However, the Allegan I-Xe age is more con-sistent with the H4 chondrite Hf-W age (4566.3 ± 0.7 My), which is considered to reflect the robustness of high-Ca pyroxene to thermal metamorphism and dates chondrule formation [8]. Based on this, the I-Xe closure T must be higher than the 800 - 875 ± 75 oC estimated for Hf-W in high-Ca pyroxenes [8]. The closure T for Pd-Ag is lower because the scatter of the Allegan Pd-Ag data indicates a disturbed system. Modeling suggests that Ag from metal was lost to silicates accompanied by stable isotope fraction-ation. Further I-Xe data from troilites and metal will show whether this is because Ag is mostly hosted in metal or sulphides, which re-crystallize at lower T. Dhofar 125 yielded an I-Xe age 4557.9 ± 0.8 My for pyroxene, consistent with the I-Xe ages of related lodranites [9]. The Pd-Ag data appears to define an isochron cor-responding to an age of 4549 (+11.2/-8.7) My, which just overlaps within error with the I-Xe ages. Modeling indicates that the last reset involved either loss of volatile Ag or addition of metal.  References: [1] Schönbächler M. et al. 2008. GCA, 72:5330-5341. [2] Horan M. F. et al. 2012. EPSL, 351-352:215-222. [3] Pellas P. et al. 1997. GCA, 61:3477-3501. [4] McCoy T.J. et al. 1997. GCA, 61:639-650. [5] Rubin A.E. 2007. GCA, 71:2383-2401. [6] Gilmour J.D. et al. 1994. Rev. Sci. Instrum., 65:617-625. [7] Gilmour J.D. et al. 2009. MAPS, 44:573-579. [8] Kleine T. et al. 2008. EPSL, 270:106-118. [9] Crowther S. A. et al. 2009. MAPS, 44: 1151-1159. 5210.pdf76th Annual Meteoritical Society Meeting (2013)

Combined Palladium-Silver and Iodine-Xenon Isotope Systematics for Allegan (H5) and Dhofar 125 (Acapulcoite)

Abstract

Similar works

Full text

Available Versions

The University of Manchester - Institutional Repository