No abstract

U‐Pb ages of four zircons in thin‐sections of lunar breccia 73217 have been determined using the sensitive high‐resolution ion microprobe, SHRIMP. All crystals show remarkably little loss of radiogenic Pb and give U‐Pb ages that are within 10% of concordance at 4356−14+23 m.y. Two of the crystals show evidence of radiogenic initial Pb that evolved in a source with μ ∼2000. One zircon is an inclusion in an ilmenite of the clast assemblage, which shows that the zircons belong to that assemblage and strongly suggests that they date formation of the (igneous) rock from which the clasts were derived. The principal loss of radiogenic Pb from the zircons occurred significantly later than 3900 m.y. but probably earlier than 1000 m.y. Two of the zircons are optically and chemically zoned and only partly metamict. U, Th, Yb, Y, and P are relatively enriched in the metamict region of one.

The Ordovician Period, long considered a supergreenhouse state, saw one of the greatest radiations of life in Earth's history. Previous temperature estimates of up to approximately 70 degrees C have spawned controversial speculation that the oxygen isotopic composition of seawater must have evolved over geological time. We present a very different global climate record determined by ion microprobe oxygen isotope analyses of Early Ordovician-Silurian conodonts. This record shows a steady cooling trend through the Early Ordovician reaching modern equatorial temperatures that were sustained throughout the Middle and Late Ordovician. This favorable climate regime implies not only that the oxygen isotopic composition of Ordovician seawater was similar to that of today, but also that climate played an overarching role in promoting the unprecedented increases in biodiversity that characterized this period.