THE DISTRIBUTION OF UNMELTED IMPACT EJECTA ASSOCIATED WITH THE UPPER EOCENE CLINOPYROXENE-BEARING (CPX) SPHERULE LAYER

At least two closely spaced (e.g., ~25 cm in core RC9-58) distal impact ejecta layers occur in upper Eocene marine sediments. The younger layer contains tektite fragments, microtektites, shocked grains of quartz, feldspar and zircon, and high-pressure polymorphs, e.g., coesite, stishovite and reidite. This impact ejecta occurs ~ 8 million square kilometers (~ 1.5 % of the Earth surface area) and appears to have been derived from the Chesapeake Bay impact structure. The slightly older layer contains cpx spherules and microtektites, and has an associated Ir anomaly. This layer has global distribution (Indian Ocean – central Pacific Ocean – Gulf of Mexico – Caribbean Sea – northern Atlantic Ocean – Southern Atlantic Ocean), and may be derived from the Popigai impact structure (Sr-Nd data; Whitehead et al., 2000). Recently, coesite and shocked quartz were discovered in the cpx spherule layer at ODP Site 709C in the Western Indian Ocean (Liu et al., 2002). We processed a 5.7-gram sample of the cpx spherule layer from DSDP Site 216 in the eastern Indian Ocean (~ 2800 km from ODP Site 709C) in order to search for unmelted impact ejecta. The sample was dried overnight in the oven, treated with 20% HCl, and then wet-sieved into different size fractions. The >125 micron and 63 – 125 micron size fractions were subjected to heavy liquid separation. Material from the > 2.0 gram/cc fraction was studied with binocular microscope. XRD (Gandolfi camera) of two white-opaque grains showed them to be coesite. We expect to identify additional unmelted impact ejecta in this sample with work in progress, and these data should help to define the nature of the target rock. Coesite has now been confirmed in the cpx spherule layer from two widely separated sites in the Indian Ocean.