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Isotopic and Geochemical Investigations of Meteorites

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Isotopic and Geochemical Investigations of Meteorites FINAL REPORT FOR NASA GRANT #NAG510425 200 1-2005 ISOTOPIC AND GEOCHEMICAL INVESTIGATIONS OF METEORITES PI: RICHARD J. WALKER Department of Geology University of Maryland College Park, MD 208742 Results of Work: 2001-2005. our previous work, the attainment of low, reproducible Re chemical processing blanks FINAL REPORT permitted us to make precise Re-Os isotopic measurements for the highest-Ni, lowest-Re Overview of work. IIAB and IIIAB endmembers. The primary goals of our research The Re-Os isochron ages for the over the past four years are to constrain the complete suites of IIAB and IIIAB irons are timing of certain early planetary 4530f50 Ma and 45 17+32 Ma, respectively, accretioddifferentiation events, and to and are similar to previously reported Re-Os constrain the proportions and provenance of ages for the lower-Ni endmembers of these materials involved in these processes. This two groups (e.g. Shen et al., 1996; Smoliar work was achieved via the analysis and et al., 1996; Chen et al., 2002). The results interpretation of long- and short-lived isotope for both systems are consistent with initial systems, and the study of certain trace crystallization of both asteroidal cores elements. Our research targeted these goals within several tens of Ma of the origin of the primarily via the application of the I8'Re- solar system. The results for the most i870s, 190Pt-'860s,~&Tc-~~Ru and ?'c-%u highly-evolved irons in both groups are isotopic systems, and the consistent with, but do not require, complete determinatiordmodeling of abundances of the crystallization during the same time frame as highly siderophile elements (HSE; including the initiation of crystallization. Re, Os, Ir, Ru, Pd, Pt, and maybe Tc). The The first high-precision Pt-Os specific events we examined include the isochrons for IIAB (Fig. 1) and IIIAB irons segregation and crystallization histones of were produced. The Pt-Os isochron ages for asteroidal cores, the accretion and the IIAB and IIIAB irons, calculated using metamorphic histories of chondrites and the current best estimate of the h for '%Pt chondrite components, and the accretionary (Begemann et ai., 2001), are 4323 k 80 Ma and differentiation histories of Mars and the and 4325 k 26 Ma respectively. The Re-Os Moon. The results of the proposed research and Pt-Os ages do not overlap within the have: I) provided us with a better uncertainties. The younger apparent ages understanding of the metal segregation and recorded by the Pt-Os system may reflect crystallization histories/pathways in iron much later system closure of the most meteorite parent bodies, 2) provided improved evolved irons from each group, either as a constraints on the behavior of HSE during result of Pt diffusion or late-stage nebular condensation through chrondrite crystallization. A more likely possibility is formation and subsequent alteration, 3) that the decay constant used in the age provided important constraints on the HSE calculations is incorrect. The slope from the budgets of lunar and Martian mantles, and 4) Pt-Os isochron was combined with the age examined the magnitude of early solar system from the Re-Os isochron for the IIIAB irons isotopic heterogeneities in the transition to calculate a revised value of 1.4 I5 x 1 O-'' element Mo. The results/progress of six major a-l for the I9'Pt decay constant (Cook et ai., studies advanced during this hnding cycle are in review). reviewed as follows. The trace element and isotopic compositions of these suites reflect the 1. 'x7Re-'"70s& 'ui'Pt-'x60sIsotopic and HSE crystal-liquid fractionation processes Abundance Studies of Iron Meteorites. involved, and time of crystallization of the In order to examine the asteroidal cores these suites sample. As crystallization histories of two major previously noted for iron meteorite groups asteroidal cores. the '87Re-'870s and lwPt- and experimental systems. solid metal-liquid I86 Os systematics of 13 IIAB and 18 IIIAB metal bulk distribution coefficients (D's) for iron meteorites were analyzed. In contrast to both IIAB and IIlAB systems show 2 Os>Re>>Pt>l, during the initial stages of The short-lived 98T~-98R~(tx between 4 core crystallization (e.g. Pernicka and and -10 Ma), *Tc-~~Ru(tx = 0.21 Ma) Wasson, 1987; Jones and Drake, 1983; chronometers were studied because they Morgan et al., 1995). The latter stages of may yield important information on the crystallization for each core are generally relative chronology of processes that characterized by D's for Re>Os>>Pt. The affected metal phases in the early solar relative partitioning behaviors of these system. These include volatility-related elements are much more complex in the processes, such as condensation and IIIAB core relative to the IIAB core. evaporation, metal-silicate segregation in asteroids, early core crystallization and aqueous alteration. Furthermore, the proof of extant *Tc in the solar system would also constrain the time interval between production of the nuclide and injection of s- -10 process isotopes into the protosolar cloud. Previous studies of the Tc-Ru systems 0.126 reported positive deviations for 98R~in iron 0.125 meteorites relative to a terrestrial standard, a small positive deviation for *Ru in 0.124 chondritic metal, but normal compositions for calcium-aluminum-rich inclusions 0.123 (CAIs) from chondrites. Our Ru isotopic data for group IIAB 0.122 irons (Negrillos, Bennett County, Coahuila, 0.121 Filomena, Old Woman) and IIIAB irons (Casas Grandes, Costilla Peak), and the 0.120 chondrites Allende (CV3) and Allegan (H5) indicate that the abundances of 98R~and 0.119 " '... " '' '.. ''". 99 Ru in these meteorites overlap with the 0.0 0.2 0.4 0.6 0.8 1.0 terrestrial values at the k0.8 and kO.3 E levels (parts in 1O,OOO), respectively (Figs. 2-3). Lack of heterogeneities of 98R~and 99 Ru among the iron meteorites is not Fig. 1. 1wPt-1860sisochron for group IIAB surprising in light of a recent experimental iron meteorites (Cook et al., in review). study that shows that crystal- liquid Open triangles-Cape York suite. Solid fractionation of metal changes the Tc/Ru triangles-low Ni. Triangles with crosses- (and Tc/Mo) ratio by substantially less than high Ni. A is the per mil deviation from the a factor of 2 (Lazar et al., in review). Using IlAB isochron. these constraints and solar system initial abundances of *Tc derived from This work was largely the result of astrophysical models, evolution models for the Masters Thesis work of D.L. Cook (now the *Tc-*Ru chronometer can produce a Ph.D. candidate at the University of barely resolvable deviations in *Ru (0.3 E) Chicago) and was recently published (Cook only if these fractionation events occurred et al., 2004). It was done in collaboration during collapse of the molecular cloud that with M.F. Horan at the Carnegie Institution formed our solar system (<lo ka, Becker of Washington, Department of Terrestrial and Walker, 2003). Absence of 98 Ru Magnetism, and J.T. Wasson of UCLA. deviations indicates 98T~/%R~,,< 2 x IO". Ratios of the stable isotopes of Ru also 2. Analytical work on the "Tc-"~Ru, 99 Tc- overlap within error with the terrestrial Y!J Ru chronometers. values at better than 1 E unit (Becker and 3 individual analyses. Uncertainties are 20, (in-run). 3. Scale of isotopic heterogeneity between inner solar system bodies. Recent Mo isotopic studies on meteorites reported evidence for differences in isotopic compositions for whole rocks of some primitive (Yin et al., 2002; Dauphas et al., 2002a, b) and differentiated meteorites (Qi-Lu and Masuda, 2000; Dauphas et al., 2002a), relative to terrestrial materials. Relative enrichments of r- and p-process isotopes of up to 3-4 E units (E unit = parts in NAB Iron 1 04) over s-process dominated isotopes are - the most prominent features. Certain types 0 -1 of presolar grains show enrichments in s- .-E process isotopes (Nicolussi et al., 1998). If I, the reported variability in r- and p-process 4 isotope enrichments reflects the true isotopic E characteristics of the whole rocks, the .--0 u, -1 implications are quite profound. It would suggest the presence of large scale Mo w 0 8ennetlCo 0 Coaiwta isotopic heterogeneity within the solar 8 Old Woman accretion disk with likely collateral effects for other elements. Because similar effects are not apparent from our Ru isotope work, nor from Zr isotopic studies of meteorites Fig. 2. Ru isotopic composition of IIAB iron (Schonbachler et al., 2002), we examined meteorites. Data are means of multiple the Mo isotopic composition of iron analyses. Uncertainties are 20,. Grey area meteorites, chondrites and CAls using indicates *20, uncertainty of multiple (n = negative thermal ionization mass 4-6) standard runs. spectrometry. 2[ I I1 With the exception of '"Mo, the Mo isotopic composition of the bulk meteorites and CAIs we analyzed overlap with terrestrial Mo within *I E unit, when normalized to 92M0P8M0(Figs. 4-5). 'ooMo - is the only Mo isotope outside the mass range used for normalization, and hence, may yield the least precise results. Our -UMD Aldrich Ru I results on IIAB iron meteorites and the w . A Nlends(CV3) 1 . v Allegan (HS) Allende whole rock show no evidence for -2 1 enrichments in p- or r-process isotopes, relative to pure s-process 96~~.In contrast, two CAIs from Allende show patterns that bear a similarity to the whole rock patterns Fig. 3. Ru isotopic composition of the for Allende reported by Yin et al. (2002) chondrites Allende and Allegan. Data are and Dauphas et al. (2002a), and patterns of Allende leachates and residues (Dauphas et could reflect heterogeneity of Allende on the al., 2002b).
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