Manganese-chromium formation intervals for ... - Wiley Online Library

Meieoriiics & Planetary Science 36, 91 1-938 (2001) Available online at http://www.uark.edu/meteor

Manganese-chromium formation intervals for chondrules from the Bishunpur and Chainpur meteorites L. NYQUIST'*, D. LINDSTROM', D. MITTLEFEHLDTI, C-Y. SHIH2, H. WIESMANN2, S. WENTWORTH2 AND R. MARTINEZ2 'Mail Code SN2, NASA Johnson Space Center, Houston, Texas 77058, USA 2Mail Code C-23, Lockheed-Martin Space Mission Systems and Services Company, 2400 NASA Road I , Houston, Texas 77058, USA *Correspondence author's e-mail address: 1aurence.e.nyquist 1 @jsc.nasa.gov

(Received 2000 May 18; accepted in revised form 2001 April 6)

Abstract-Whole-chondrule Mn-Cr isochrons are presented for chondrules separated from the Chainpur (LL3.4) and Bishunpur (LL3.1) meteorites. The chondrules were initially surveyed by instrumental neutron activation analysis. LL-chondrite-normalized Mn/Cr, Mn/Fe, and Sc/Fe served to identify chondrules with unusually high or low MdCr ratios, and to correlate the abundances of other elements to Sc, the most refractory element measured. A subset of chondrules from each chondrite was chosen for analysis by a scanning electron microscope equipped with an energy dispersive x-ray spectrometer prior to high-precision Cr-isotopic analyses. 53CrPCr correlates with 55Md52Cr to give initial (53Mn/55Mn)1= (9.4 2 1.7) x 10-6 for Chainpur chondrules and (53Mn/55Mn)1= (9.5 ? 3.1) x 10-6 for Bishunpur chondrules. The corresponding chondrule formation intervals are, respectively, AtLEW = -1 0 ? 1 Ma for Chainpur and -10 k 2 Ma for Bishunpur relative to the time of igneous crystallization of the Lewis Cliff (LEW) 86010 angrite. Because MdSc correlates positively with MdCr for both the Chainpur and Bishunpur chondrules, indicating dependence of the MnlCr ratio on the relative volatility of the elements, we identify the event dated by the isochrons as volatility-driven elemental fractionation for chondrule precursors in the solar nebula. Thus, our data suggest that the precursors to LL chondrules condensed from the nebula 5.8 ? 2.7 Ma after the time when initial (53Mn/55Mn)1= (2.8 ? 0.3) x 10-5 for calcium-aluminum-rich inclusions (CAIs), our preferred value, determined from data for (a) mineral separates of type B Allende CAI BR1, (b) spinels from Efremovka CAI E38, and (c) bulk chondrites. Mn-Cr formation intervals for meteorites are presented relative to average I(Mn) = (53MdssMn)ch = 9.46 x 10-6 for chondrules. MdCr ratios for radiogenic growth of 53Cr in the solar nebula and later reservoirs are calculated relative to average (I(Mn), c(53Cr)1)= ((9.46 5 0.08) x 10-6, -0.23 k 0.08) for chondrules. Inferred values of Mn/Cr lie within expected ranges. Thus, it appears that evolution of the Cr-isotopic composition can be traced from condensation of CAIs via condensation of the ferromagnesian precursors of chondrules to basalt generation on differentiated asteroids. Measured values of ~(53Cr)for individual chondrules exhibit the entire range of values that has been observed as initial c(53Cr) values for samples from various planetary objects, and which has been attributed to radial heterogeneity in initial 53MnPMn in the early solar system. Estimated 55Md52Cr = 0.42 rt 0.05 for the bulk Earth, combined with c(53Cr) = 0 for the Earth, plots very close to the chondrule isochrons, so that the Earth appears to have the Mn-Cr systematics of a refractory chondrule. Thus, the Earth apparently formed from material that had been depleted in Mn relative to Cr contemporaneously with condensation of chondrule precursors. If, as seems likely, the Earth's core formed after complete decay of 53Mn, there must have been little differential partitioning of Mn and Cr at that time.

91 1 PvelMde preprint MS#4372

0Meteoritical Society, 2001. Printed in USA.

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INTRODUCTION The identification in meteorites of the decay products of now-extinct radionuclides has contributed greatly to understanding the timescale for formation of the solar system (cf., Wasserburg, 1985; Cameron, 1985, 1993, 1995, 2001; Podosek and Cassen, 1994). Here, we seek to apply the decay of now-extinct 53Mn to 53Cr (half-life t1/2 = 3.7 Ma, 1 Ma = 106 years) to events in the formation of chondrules extracted from the Chainpur (LL3.4) and Bishunpur (LL3.1) chondrites. In the model ofnebular evolution described by Cameron (1995, 2001), for example, the events we seek to date occur in Stages 3 and 4, during which planetesimal and planet accumulations take place. Cameron's estimate of the duration of Stage 3 is 12 Ma, and of Stage 4 is -107years for stellar nebulae in general, but 1 2 Ma for the solar nebula. Alternatively, in the model described by Shu et al. (1996), chondrule formation occurs in the fourth stage, after the outflowing stellar wind has reversed the infall of gas and dust to the central star and revealed it and its surroundings as optical and near-infrared objects. The total time to form a 1 solar mass star was estimated by Shu et al. (1996) to be -3 Ma. 53Mn is thus well suited to dating events in the solar nebula. Birck and Allegre (1985) presented the first evidence for live 53Mn in the early solar system from Cr-isotopic measurements for calcium-aluminum-rich inclusions (CAIs) from the Allende meteorite. Cameron (200 1, and earlier papers) argues that the solar system's initial complement of 53Mn probably was formed in a Type I1 supernova explosion of a massive 0 star in a molecular cloud "grandparent" of the solar system. He estimates this may have occurred -25 Ma before the birth of the solar system itself (Cameron, 1993, 1995). Recently, Goswami et al. (1 997) considered the possibility of production of shortlived nuclides in the early solar system by solar energetic particles (SEP). They concluded that such production was unlikely with the possible exception of 53Mn. A "low" initial 53Mn abundance, 53MnI55Mn = 7 x 1 0 4 could be explained by an SEP flux from an early active Sun enhanced by a few hundred-fold compared to the present-day flux. A similar enhancement also could explain the observed excess of spallogenic 21Ne in solar flare irradiated olivine grains from CM chondrites (Hohenberg et al., 1990), lending some support to this view. However, the higher, conventionally accepted, value of 53MnPMn = 4.4 x 10-5 (Birck and Allegre, 1985), would require a correspondingly greater enhancement by about six-fold in the early T-Tauri phase irradiation, making local production less favored. Mn-Cr analyses of "nebular components" preserved in meteorites may yield important astrophysical and cosmochemical information as well as chronological information. For example, Cassen and Woolum (1997) interpreted the apparent variation in the initial 53CrPCr ratio of various solar system objects with radial distance from the Sun (Lugmair et al., 1996) as due to volatility-controlled

fractionation of Mn and Cr from an initially uniform nebula. If so, 53CrPCr measurements may be useful to constrain models of nebular thermal evolution as well as temporal evolution. Chondrules are most often interpreted as objects formed in the solar nebula, and which can be related to processes occurring there. In the Cameron (1995, 2001) model, for example, formation of the solar nebula begins with the collapse of a "core" of material in the parent molecular cloud, accompanied by formation of giant gaseous protoplanets. As temperatures in the nebula rise, most of the gas in the protoplanets is dissipated, leaving dust grains to fall to the planetary centers, ultimately forming the interiors of the planets. Dust grains in other portions of the nebula probably collected first into dustballs, were flash-heated by an as yet uncertain mechanism, and ultimately became chondrules or CAIs in primitive meteorites. In the model of Shu et al. (1996) dustballs are entrained into an outflowing "x-wind" at a distance of -0.06 AU from the Sun, are carried up and out of the shade of the nebular disk where they can be radiatively heated and melted by the Sun, and sprayed back into the nebula to be incorporated into the planetesimals forming at -2.5 AU (i.e., into meteorite parent bodies). Our own observations do not address the physical processes of chondrule formation, but do, we believe, place constraints on the chronology of some of the processes of chondrule formation. Grossman and Wasson (1 982) concluded that chondrules from the Chainpur (LL3.4) meteorite formed via melting of random mixtures of grains comprising a limited number of nebular components. Kurat (1988) developed a "unified" model of chondrule and meteorite formation that qualitatively fits many of the observations to be presented in the following. The most important nebular processes leading to chondrule formation identified in his model are: (1) partial to total evaporation of presolar matter; (2) recondensation beginning with olivine and followed by other phases; (3) aggregation of the condensates to millimeter-sized objects; (4) partial or total compaction of early aggregates by condensation into the aggregate's pore space; (5) a second heating event leading to sintering and partial melting of the chondrules and mild vapor fractionation (chondrule formation); (6) vapor-solid exchange reactions introducing Fe+2 and other elements into the silicates and forming FeS via S-metasomatism. We suggest that consequences of steps (2) to (5) are recognizable in the data presented here. Chronological studies of CAIs and chondrules based on decay of short-lived 26AI (t1/2 = 0.73 Ma) suggest that chondrules postdate CAIs by several million years (cf., Hutcheon et al., 1989; Hutcheon and Hutchison, 1989; Zinner and Gopel, 1992; Swindle et al., 1996; Kita et al., 2000). Cameron (1 995) considered the first ten million years of the solar nebula and addressed the CAI-to-chondrule time difference. His Stage 1 of nebular evolution comprised the collapse of the parent molecular cloud and lasted -1 05 years.

Manganese-chromium formation intervals for chondrules from the Bishunpur and Chainpur meteorites Stage 2 was an FU Orionis phase in which material was simultaneously accreting to the Sun and being dissipated from the system, and lasted -5 x 104 years. Stage 3 was the classical T-Tauri phase during which the Sun was undergoing its terminal accumulation, and lasted -1-2 Ma. The last Stage, 4, was the residual static nebula that is analogous to observed circumstellar disks. From the observations of Strom et al. (1993), Cameron (1 995) estimated that this stage lasted -3-30 Ma, or on the order of 1 to 8 half-lives of 53Mn. Not only does 53Mn have a favorable half-life for dating events in the nebula, but both Mn and Cr are minor rather than trace elements, so their abundances are sufficiently high that high-precision Cr-isotopic analyses are possible for individual chondrules. Additionally, Mn is one of the moderately volatile elements whose abundance in chondrites is diagnostic of (pressure, P; temperature, I") conditions in the nebular regions where the various classes of chondrites formed (cf., Palme et a[., 1988). The 53Mn-53Cr method of dating solar system events is analogous to the I-Xe method pioneered by Reynolds (1960), and based on the decay of 1291 to 129Xe (t1/2 = 15.7 Ma). Lee et al. (1976) developed the AI-Mg method based on the decay of26A1 to 26Mg ( t 1 =~ 0.73 Ma). Carlson and Lugmair (2000) have summarized data from these and other short-lived chronometers. Recently, the possibility that I-Xe formation intervals may not measure real differences in the formation times of primitive meteorites has received considerable attention (cf., Swindle et al., 1996). Many such concerns were derived from the "astrophysical limit" of I 1 Ma for the lifetime of the nebula (cf., Crabb et al., 1982). Podosek and Cassen (1994) concluded that a longer nebular history of -1 07 years was more probable, however, and Strom et al. (1 993) inferred from astronomical observations that circumstellar disks associated with solar-type stars survive as optically thick structures for variable times ranging from