LASER ABLATION INDUCTIVELY COUPLED PLASMA MASS SPECTROMETRY OF THE PLEISTOCENE WILSON CREEK ASH BEDS, INYO COUNTY, CALIFORNIA

Volcanic ashes expelled from the same volcanic vent or field are intra-volcanic ashes that often yield glass shards with a similar major-element geochemical composition that are generally indistinguishable using standard microprobe analytical techniques. The Wilson Creek Formation near Mono Craters in eastern California contains 19 volcanic ash layers that represent a family of intra-volcanic eruptions indistinguishable by major-element composition and ranging in age from 32 to 13 ka. Differentiating these ash layers, which are found from eastern California to Utah, is an important tephrochronologic problem, as they offer great potential as widespread marker beds. Recent research has shown that the Wilson Creek ash layers are distinguishable by INAA-measured minor and trace-element composition. Ash #15, important for its correspondence with the Mono Lake paleomagnetic excursion, has a particularly distinctive composition. Earlier INAA analyses showed that Ash 15 is closest in composition to Ash 1, but is separable from Ash 1 by Ba, La, Ce, Yb, and U concentrations. In this study, we analyzed 12-20 glass shards each from 17 of 19 Wilson Creek ash beds by laser ablation, inductively-coupled-plasma mass spectrometry (LA-ICP-MS). Our preliminary interpretation is that the LA-ICP-MS results are comparable to the INAA data. Significantly, Ash 15 is distinguishable from the remaining Wilson Creek ash beds by LA-ICP-MS. The results of this study highlight the use of LA-ICP-MS as a fast, cost effective means of determining trace and minor elemental composition of chemically homogenous tephra.