The Los Angeles martian diabase: Phosphate U-Th-Pb geochronology and mantle source constraints

1Christopher R.M.McFarlane,1,2John G.Spray
Geochimica et Cosmochimica Acta (in Press) Link to Article [https://doi.org/10.1016/j.gca.2022.04.006]
1Department of Earth Sciences, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada
2Planetary and Space Science Centre, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada
Copyright Elsevier

In situ laser ablation inductively coupled mass spectrometry (LA ICP-MS) is used to determine the U-Th-Pb age of the phosphates ferromerrillite and apatite in the Los Angeles shergottitic meteorite. The initial 207Pb/206Pb was refined by analyzing K-rich diaplectic glass. LA ICP-MS mapping was used to document zones of elevated U and Th content and to establish textural controls on isotope ages. By critically assessing dispersion in the U-Th-Pb dataset due to Pb-diffusion in phosphates during high-temperature shock metamorphism, and as a result of subsequent terrestrial contamination, we obtain a best-estimate U-Pb age of 169 ± 5 Ma anchored at an initial 207Pb/206Pb of 0.98390 ± 0.00018. This is statistically indistinguishable from a joint-isochron age of 179 ± 6 with initial 208Pb/206Pb of 2.5151 ± 0.0028. These results complement previously determined Rb-Sr and Sm-Nd isotope ages and provide independent evidence for LA having crystallized as a medium-grained basic rock from a thick lava flow or high-level intrusion in the late Amazonian at ∼170 Ma. In the context of martian mantle evolution, the initial common-Pb values suggest that Los Angeles originated from a source (µ2 ∼3.2) that is similar to enriched members of the shergottite meteorite clan. The U-Th-Pb systematics of both ferromerrillite and apatite were locally affected by diffusive Pb-loss in thin U-enriched marginal domains and more profoundly in shock-induced melt pockets where temperatures briefly exceeded 2000°C. The results reveal: (1) how precise U-Pb ages can be attained from phosphates; (2) the importance of microtextural contextualization of isotope data; (3) that the timescales of cooling from shock conditions were sufficient to promote local diffusive re-equilibration of Pb over 10s of microns; and (4) that LA ICP-MS mapping can be used to locate domains with the highest U/Pb and Th/Pb, which increases precision on lower intercept ages and isochron regression lines.

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