Lunar and Planetary Science XXXIX (2008) 2504.pdf

I-Xe AGES AND THE THERMAL HISTORY OF THE IAB . O. Pravdivtseva1, A. Meshik, M. Petaev2 and C. M. Hohenberg1 and, 1McDonnell Center for the Space Sciences, Physics Department, Washington University, CB1105, One Brookings Drive, Saint Louis, MO 63117, USA. [email protected], 2Department of & Planetary Sciences, Harvard University, 20 Oxford St., Hoffman Lab 208, Cambridge, MA 02138, USA

Introduction: The IAB have an average-sized (~ 20 µg) grain took about 5 seconds. complicated textures and chemical-mineralogical fea- Xenon isotopic composition was measured in each tures that indicate a complex history for the parent grain by ion counting mass-spectrometry [3]. “Hot body. These meteorites commonly contain abundant blanks”, typically ~ 2×10-15 cm3 STP 132Xe, were silicate inclusions embedded in the metal phase. In an measured by similar laser power levels directed for 5 attempt to develop the least complicated formation seconds into the empty holes in the grain holder stub of scenario, Benedix et al. [1] explored a hypothesis in the laser cell and were always completely negligible. which the differentiation process on the is Concentrations of 132Xe ranged from 35.6×10-12 interrupted by a catastrophic impact. The subsequent to 357.8×10-12 cm3 STP/g, with average value of gravitational reassembly mixes together lithologies 184.5×10-12 cm3 STP/g, in agreement with the previ- from different formation regions of the parent body. ously reported data for (IAB) silicates and Although this hypothesis does not completely explain etched Pitts (IAB) silicates [4]. Radiogenic 129*Xe re- all properties of these meteorites, it does explain the mained loosely correlated with the Mg/(Fe+Mg) ratio, emplacement of solid silicates within molten metal as and as we previously found also consistent with [4]. well as slow cooling rates (< 700 K/Ma) of the reas- Final experimental points fill a triangular area on sembled debris. three isotope plot (a) and represent the total xenon re- To explore if the I-Xe system in IAB silicates can leased from each of the Toluca grains. The grains were provide information on the complex history of the IAB sorted according their Mg/(Mg+Fe) ratios, with 9 parent body, we studied the USNM 931 specimen of grains having content higher than 10% (marked the Toluca meteorite [1]. The specimen still contains by their numbers on plot a) being eliminated. remnants of three troilite inclusions, with one of them We considered the suite of 10 grains with the including abundant silicate grains of different habits. highest Mg/(Mg+Fe) ratios, which gradually decreased Our previous work showed that the 129Xe in chlorapa- by 8%, to be the Mg-rich subset (solid points, plot b). tite, pyroxene, and perryite grains from this nodule was Using a similar 8% increase in Mg/(Mg+Fe) ratios, dominated by radiogenic 129Xe (129*Xe from 129I decay) starting from the lowest value, we selected 6 grains [2]. Pyroxenes also exhibite a clear correlation between (open points, plot b) for the low-Mg subset. Different the Mg/(Fe+Mg) ratio and 129*Xe concentration, being formation times could then yield different isochrons for highest in the high-Mg grains. these two groups but, unlike stepwise heating, total To investigate this correlation, we separated 63 melting does not allow to identify the uncorrelated pyroxene grains from the same troilite nodule of 128*Xe. Toluca. The concentrations of Fe, Mg, Si, and Al were If the iodine-xenon system in a suite of grains determined by the EDAX which also revealed small would have closed at the same time, then the Xe re- amounts of Ni and Cr in some grains (< 1% atom.). To leased from these grains would form a pseudo isochron monitor possible troilite intergrowths S concentrations on a three isotope I-Xe plot. Additions of uncorrelated were also measured. Although the Mg-rich pyroxene 128*Xe (varying from grain-to-grain) would then move grains were more abundant, for our study we selected experimental points to the right. However, since most 32 grains which exhibite a continuous trend of 128*Xe (> 80%) is associated with 129*Xe, the scattering Mg/(Fe+Mg) ratios. of points to the right of the isochron will be slight and The selected grains, along with the Shallowater its slope, perhaps determined with less precision, enstatite irradiation standards, were weighed, sealed should not be seriously affected. Based on different under vacuum in fused-quartz ampoules and irradiated computational procedures, the I-Xe system in high-Mg in the pool area of the University of Missouri Research pyroxenes closed 0.3 – 4.9 Ma after Shallowater, but Reactor in a continuously rotating water-flooded cap- most probably 2.6 Ma after Shallowater (free fit corre- sule. The grains were then placed in the extraction sys- lation lines). With the absolute age of Shallowater de- tem of the mass-spectrometer and melted individually termined to be 4562.3 ± 0.3 Ma [5], this sets the clo- with a focused Nd-YAG (1.06 µm) CW laser. The laser sure time of the I-Xe system in high-Mg pyroxenes at power was increased until melting by a pair of water- 4559.7 Ma, in agreement with the previously reported cooled glan-thompson polarizers with continuously range of I-Xe ages for silicates from IAB iron meteor- variable orientations). Total degassing and melting of ites [4, 6]. Lunar and Planetary Science XXXIX (2008) 2504.pdf

The Toluca graphite-troilite nodules are almost 6 certainly formed by liquid immiscibility in the still Toluca molten Fe-Ni-S-C system, trapping solid pyroxene (individual pyroxene grains) grains. The troilite shows twinning and/or undulatory 5 extinction from plastic deformation, but it apparently 4 did not experience shock melting [7]. It led to the con- 10 21 12 clusion that some metallic melts formed by the time of 3 44 41 45 catastrophic breakup of the IAB parent body could 61 have remain intact [1]. Then, the I-Xe age of high-Mg 2 14

Toluca pyroxenes, separated from one of such graph- 55 1 ite-troilite nodules, most probably reflects the time of OC the metal/silicate mixing after the disruption and reas- a sembly of the IAB parent body about 7 Ma after the 0 formation of CAI’s [8]. 6 0123456 The age difference between the high- and low-Mg 5 isochrons (10.8 - 6.1 Ma) forms a basis for cooling rate calculations. The difference in melting temperatures of 4 high-Mg pyroxenes 411 K between the high- and low-Mg pyroxenes (1830 Xe K and 1419 K, respectively [9]) can be used as a first 3 order estimate of the difference in the Xe closure tem- 132 ∆T = 6.1 Ma peratures. Alternatively, difference in tammann tem- Xe/ 2 129 peratures for enstatite and ferrosilite of 214 K gives a low-Mg pyroxenes 1 lower limit of the difference in closure temperatures OC between the high- and low-Mg pyroxenes. Then a cool- b 0 ing rate for the temperature range of Xe closure in Toluca pyroxenes can be calculated in a straight for- 6 ward manner. It is ~ 40 ± 20 K/Ma, in a good agree- best-fit isochrons ment with the metallographic values reported for 7 IAB 5

meteorites [10]. 4 I-Xe system in Toluca silicates seems to survive high-Mg pyroxenes catastrophic impact and breakup of IAB parent body. It 3 supports a suggestion that accretion and melting of the ∆T = 10.8 Ma IAB parent occurred within a few Ma after the initial 2 formation of solid objects in the Solar System [6]. As a low-Mg pyroxenes 1 result of subsequent cooling at about 40 ± 20 K/Ma, I- OC Xe system in the high-Mg pyroxenes in Toluca nodules c closed at about 4559.7 Ma, while in low-Mg pyroxenes 0 at 4553.6 – 4548.9 Ma. 0123456 128Xe/132Xe Supported by NASA grant #NNG06GE84G. References: [1] Benedix G. K et al. (2000) Mete- oritics & Planetary Science 35, 1127– 141. [2] Pravdivtseva O. V. et al. (1998) LPS XXIX, 1818. [3] Hohenberg C. M. (1980) Rev. Sci. Instrum. 51, 1075 – 1082. [4] Niemeyer S. (1979) GCA 43, 843–860. [5] Gilmour J. D. et al. (2006) & Planetary Science. [6] Bogard D. D. et al. (2005) Meteoritics & Planetary Science 40, 207 – 224. [7] Buchwald V. F. (1975) Handbook of iron meteorites. Univ. California Press, Berkley. 1418 pp. [8] Amelin Y. et al. (2002) Science 297, 1678-1683. [9] Lodders K., Fegley B. Jr. (1998) Oxford University Press, ISBN 0-19-511694-1. [10] Herpfer M. A. et al. (1994) GCA 58, 1353 – 1365.