Low 18O terranes tracking mesozoic climates in the South Pacific : a 25-year search in the BRF-5 Laboratory, Southern Section

Substantially negative delta 18O values of altered rocks are an unfailing guide to 18O-depleted meteoric water of low mean annual temperature, and therefore of cold climates at times of hydrothermal alteration. However, because water-rock interaction is often incomplete even where it has occurred, the lack of depleted values is poor evidence for lack of cold climate. A now established average surface rock composition of -6.6 per thousand delta 18O, with a lower limit of -16 per thousand, for an igneous and metamorphic terrane of some 6,000 km/sup 2/ in West Antarctic indicates Cretaceous meteoric water of less, or much less, than -20 per thousand. This new anomaly and similar anomalies in New Zealand date from before rifting along the Antarctic-Pacific Rise and are tracking terranes originating from the Mesozoic South polar archipelago. In this area subaerial, or subglacial, hydrothermal isotope exchange has been particularly effective, and/or meteoric waters were isotopically unusually depleted. The discovery, mapping and dating of further isotopically depleted zones in the geological record will improve constraints for Paleozoic and Mesozoic greenhouse climates. A brief account is given of practical and conceptual highlights in the running of the oxygen extraction line originally brought to New Zealand by Professor Bob Clayton. (author)