Mathematics – Logic
Scientific paper
Sep 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995metic..30r.592w&link_type=abstract
Meteoritics, vol. 30, no. 5, page 592
Mathematics
Logic
4
Ages, Radiogenic, Angrite, Dating, Eucrite, Eucrites, Meteorites, Caldera, Chervony Kut, Differentiated, Ibitira, Juvinas, Primitive, Radionuclides, Extinct
Scientific paper
Caldera (CAL), a find from Chile, is one of only two known unbrecciated non-cumulate eucrites (the other being Ibitira). For this reason it was deemed of great importance to perform on this meteorite extensive radiochronological studies in an attempt to better constrain the time of solidification of basalts on the eucrite parent body (EPB) and, thus, the evolutionary time scale of planetesimals. Compared to other non-cumulate eucrites CAL has very coarse grain size. Unfortunately, there is extensive yellow-brown staining on almost all grain surfaces pervading throughout the whole meteorite. Because of the meteorite's unknown terrestrial residence time, and the possible terrestrial origin of this staining, it was clear that it had to be removed before reliable isotopic measurements on mineral separates could be attempted. This proved to be very difficult and up to 2 h of etching in 1.8N HCl was required to achieve satisfactory results. Mineral separates of plagioclase (Pl) and pyroxene (Px), though partially etched but mostly clean of the surface coating, together with an aliquot of an acid cleaned bulk sample and an unetched chip, were dissolved and analyzed for Sm-Nd isotopic systematics. There is a large spread in ^147Sm/^144Nd ratios between Pl and Px (0.0896 and 0.278, respectively). Excellent isochrons for both the ^147Sm-^143Nd and the ^146Sm-^142Nd systems were obtained. Note that even the data from the unetched bulk sample fall on the isochrons which indicates that only insignificant amounts of REE could have been introduced on earth. The ^147Sm-^143Nd system yields an age of 4.544+/-0.019 Ga with an initial ^143Nd/^144Nd = 0.506741=+/-24. The short lived ^146Sm-^142Nd system results in a ^146Sm/^144Sm ratio of 0.0075+/-0.0010 and an initial of epsilon (sub)142 = -3.1+/-0.5. All these results from Caldera are indistinguishable from data obtained previously in our laboratory on angrites [1,2] and the non-cumulate eucrites Juvinas [3] and Chervony Kut (CK) [4] and indicate that both Sm-Nd systems in these meteorites closed contemporaneously within the uncertainties afforded by the data (i.e. 20 to 30 My for ^147Sm-^143Nd and is greater than or equal to 15 My for ^146Sm-^142Nd). In contrast, the fine resolution of the ^53Mn-^53Cr system paints a different but, nonetheless, totally consistent picture. In spite of a wide range in Mn/Cr ratios from about 0 (Chromite) to about 7 (Px) the ^53Cr/^52Cr ratios in all samples measured (chromite, etched bulk, Px, and silicates) are the same. Although the typical errors are 10 to 12 ppm the range in the ^53Cr/^52Cr excesses is only 1.14 to 1.17 epsilon units (parts in 10^4 above the terrestrial ratio) with an average of 1.15 epsilon. Thus, the slope of the best fit line through these data points is 0 +/-1(2x10^-7). This means that ^53Mn was no longer extant when the Mn-Cr system closed in CAL. When compared to the ^53Mn/^55Mn ratio of 3.6x10^-6 found for CK [5] this indicates that formation of CAL occurred more than or equal to 15 My after that of CK. Similarly, if the angrite parent body [6] formed with the same initial 653Mn/^55Mn as the EPB then the angrites are more than or equal to 10 My older than CAL. Thus the true age of CAL has to be less than or equal to 4.548 Ga. This upper limit is totally consistent with the Sm-Nd results and probably very close to the true age since the lower limit on the obtained ^146Sm/^144Sm ratio will not allow an age much lower than 4.548 Ga. Note, however, that this "age" may well indicate the time of extensive re-crystallization[7], probably from a melt, when the Cr isotopes were totally equilibrated. References: [1] Lugmair G. W. and Marti K. (1977) EPSL, 35, 349. [2] Lugmair G. W. and Galer S. J. G. (1992) GCA, 56, 1673. [3] Lugmair G. W. et al. (1975) EPSL, 27, 79-84. [4] Wadhwa M. and Lugmair G. W. (1995) LPS XXVI, 1453-1454. [5] Lugmair G. W. et al. (1994) LPS XXV, 813-814. [6] Lugmair G. W. et al. (1992) LPS XXIII, 823-824. [7] Boctor N. Z., personal communication.
Lugmair Guenter W.
Wadhwa Meenakshi
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