Dating Desert Meteorites: The Los Angeles Experience

The basaltic shergottite Los Angeles (LA) consists of two stones that are slightly to negligibly weathered. LA is petrologically similar to Shergotty and Zagami, both observed falls. We dated both stones by the Rb-Sr and Sm-Nd techniques. Our sample of LA#1 had visible patches of caliche on its surface. These were scraped away, and the sample coarsely crushed. A bulk sample was leached for 10 minutes in warm, 2N HCl, and the leachate and residue analysed. A second bulk sample was analysed without leaching. A third, more finely ground bulk sample also was analysed following 2N HCl leaching. Mineral separates of plagioclase and pyroxene were prepared by magnetic separation and handpicking, leached, and analysed. The Rb-Sr data showed the effects of terrestrial contamination, with data for the leachates forming a mixing line leading towards the Rb-Sr isotopic composition of plagioclase. The caliche was analysed and found to contain ~5260 ppm of Sr, but only ~34 ppm of Nd. A second set of mineral separates were leached more aggressively. The resultant data define a Rb-Sr isochron age of 165+/-11 Ma, in excellent agreement with previously determined Rb-Sr ages for Shergotty and Zagami. With one exception, Sm-Nd data for leachates of LA#1, combined with data for bulk samples and a plagioclase separate, give a Sm-Nd age of 172+/-8 Ma, in good agreement with the Rb-Sr age. A second plagioclase separate contained an isotopically distinct Nd contaminant, more radiogenic than caliche Nd. The sample of LA#2 was precleaned in 4N HCl and 4N HNO₃ prior to crushing. About 30 mg of the sample dissolved during this procedure. Its Sr and Nd concentrations were ~170 and ~70 ppm, resp. The Rb-Sr data showed the presence of caliche, but the Sm-Nd isotopic data are colinear with other data on a Sm-Nd isochron diagram. This step extracted a large fraction of the REE, apparently from sample phosphates. Pyroxene mineral separates were further treated with (2N HCl + 5% HF), and the residues analysed. All the Sm-Nd data, including that for plagioclase (unleached), the preclean leachate, a leach/residue pair for a bulk sample, an unleached bulk sample, and the pyroxene residues, fall along the same isochron, defining a Sm-Nd age of 174+/-12 Ma. Chondrite normalized (Nd/Sm)_CN = 1.7 for caliche, and ~0.45, ~0.85, and ~1.3 for LA pyroxene, bulk rock and leachates (phosphates), and plagioclase, respectively. These values for pyroxene and bulk rock are the same as for corresponding samples of Zagami; Zagami plagioclase separates were relatively impure, precluding comparison. Contributions from caliche to the Sr inventories of the LA samples varied from 0 to ~80%, as calculated from their measured ⁸⁷Sr/⁸⁶Sr ratios and the ⁸⁷Sr/⁸⁶Sr ratios of caliche and plagioclase. The (Nd/Sm)_CN values for the samples do not vary with Sr contamination within this range, further suggesting that the Sm-Nd systematics of LA are virtually unaffected by the presence of caliche. The Sr isotopic data suggest that weathering of meteoritic plagioclase played a role in development of the caliche. A higher Nd/Sr ratio in caliche than in plagioclase may reflect some contribution from meteoritic phosphates as well.