Other
Scientific paper
Jul 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993metic..28r.414n&link_type=abstract
Meteoritics, vol. 28, no. 3, volume 28, page 414
Other
3
Angra Dos Reis, Angrites, Eucrites, Extinct Nuclides, Initial 87Sr/86Sr, Meteorites, Differentiated, Radiogenic Ages
Scientific paper
We summarize Mn-Cr, Rb-Sr, and Sm-Nd investigations of LEW 86010 (LEW) and Angra dos Reis (ADOR) and present a synthesis, including data for other isotopic systems. Samarium-neodymium investigation of LEW 86010 showed some surficial terrestrial contamination and/or Antarctic weathering products that could be removed by leaching in 2N HCl. A conventional Sm-Nd isochron anchored by data for pyroxene and phosphate, containing ~97% of the REE, yields an age of 4.538 +/- 0.018 Ga (2 sigma) using the Williamson [1] regression; the error limit increases to +/-0.033 Ga for the York [2] regression. Including data for leached plagioclase and olivine doubles the range in Sm/Nd, lowers the age to 4.532 Ga, but increases the uncertainty to +/-0.040 Ga. The isochron age agrees with Sm-Nd ages of ~4.55-4.56 Ga previously reported for ADOR [3,4] and LEW [5]. Manganese-chromium isotopic studies show excess ^53Cr from extinct ^53Mn* (t(sub)1/2 = 3.7 Ma) in the olivine of both LEW [6,7] and ADOR [7] corresponding to initial ^53Mn*/^56Mn = 1.3-1.4 x 10^-6. Angrite olivine closed to Cr isotopic homogenization ~18 Ma after Allende inclusions formed with ^53Mn*/^55Mn = 4.4 +/- 1.1 x 10^-5 [8]. The Mn-Cr formation interval is ~2x longer than the ~8-Ma difference between the single stage Pb-Pb ages of Allende and the angrites [5,9,10]. Angrite PbPb ages of 4.551 +/- 0.004 [9] to 4.5578 +/- 0.0005 [5] have been interpreted as limiting the time between major volatile element loss from the angrite parent body (APB) and crystallization of the angrites to <=2 Ma [5]. The Mn-Cr and Sm-Nd ages both date element partitioning among crystallizing mineral phases, the end point for the Mn-Cr formation age, and the starting point for the Sm-Nd age. Their sum should equal the age of Allende, and is 4.556 +/- 0.018 Ga using our preferred Sm-Nd age (4.538 Ga). This agrees adequately with Allende Pb-Pb ages of 4.559 +/- 0.015 Ga [9] to 4.566 +/- 0.002 Ga [10]. A ^146Sm-^142Nd isochron gives initial ^146Sm/^144Sm = 0.0076 +/- 0.0009 for LEW, corresponding to solar system initial (^146Sm/^144Sm)(sub)o = ~0.0080 to ~0.0086, for LEW crystallization 8 and 18 Ma after Allende respectively. The latter value is especially consistent with "high" values of ^146Sm/^144Sm for the eucrite Ibitira (0.0090 +/- 0.0010 [11]) and a silicate inclusion from the Caddo IAB iron (0.0099 +/- 0.0021 [12]). Strontium-87/strontium-86 measurements for angrite minerals with low Rb/Sr give I^87(sub)Sr = 0.698972 +/- 8 and 0.698970 +/- 18 for LEW and ADOR respectively, relative to ^87Sr/^86Sr = 0.71025 for NBS987, in agreement with Lugmair and Galer [5]. I^87(sub)Sr for the angrites is thus ~0.00011 higher than ALL, measured for Allende inclusions [13,14], corresponding to ~9 Ma of growth in a solar nebula with a CI chondrite value of ^87Rb/^86Sr = 0.91, or 5 Ma in a nebula with ^87Rb/^86Sr = 1.51, as in the solar photosphere [15]. The interval of ~18 Ma between formation of Allende and closure of the Mn-Cr system in angrites implies an average Rb/Sr ratio ~2-3x lower than the nebular value, probably reflecting an episode of volatile loss from the APB during this time. A "best average" for four eucrite clasts analyzed in our laboratory gives I^87(sub)Sr (4.558 Ga) = 0.699002 +/- 16, corresponding to growth of radiogenic ^87Sr* for 2.4 +/- 1.4 Ma in a solar nebula with a CI Rb/Sr ratio, or 1.4 +/- 0.9 Ma in a nebula with the solar photospheric Rb/Sr ratio. References: [1] Williamson J. H. (1968) Canadian J. Phys., 46, 1845-1847. [2] York D. (1966) Canadian J. Phys., 44, 1079-1086. [3] Lugmair G. W. and Marti K. (1977) EPSL, 35, 273-284. [4] Jacobsen S. B. and Wasserburg G. J. (1984) EPSL, 67, 137-150. [5] Lugmair G. W. and Galer S. J. G. (1992) GCA, 56, 1673-1694. [6] Nyquist L. E. et al. (1991) LPS XXII, 989-990, and unpublished data. [7] Lugmair G. W. et al. (1992) LPS XXIII, 823-824. [8] Birck J.-L. and Allegre C. J. (1988) Nature, 331, 579-584. [9] Chen J. H. and Wasserburg G. J. (1981) EPSL, 52, 1-15. [10] Gopel C. et al. (1991) Meteoritics, 26, 338. [11] Prinzhofer et al. (1989) Astrophys. J., 344, L81-L84. [12] Stewart B. W. (1993) LPS XXIV, 1359-1360. [13] Gray et al. (1973) Icarus, 20, 213-239. [14] Podosek F. A. et al. (1991) GCA, 55, 1083-1110. [15] Anders E.and Grevasse N. (1989) GCA, 53, 197-214.
Bansal Bhavtosh
Nyquist Larry E.
Shih Chia-You
Wiesmann Henry
No associations
LandOfFree
Isotopic Studies of Angrite LEW 86010 and the Early History of Its Parent Body does not yet have a rating. At this time, there are no reviews or comments for this scientific paper.
If you have personal experience with Isotopic Studies of Angrite LEW 86010 and the Early History of Its Parent Body, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Isotopic Studies of Angrite LEW 86010 and the Early History of Its Parent Body will most certainly appreciate the feedback.
Profile ID: LFWR-SCP-O-1073082