MINOR AND TRACE ELEMENT GLASS COMPOSITION OF LATEST PLEISTOCENE TEPHRA LAYERS FROM THE WILSON CREEK FORMATION, MONO LAKE, CALIFORNIA, USING INSTRUMENTAL NEUTRON ACTIVATION

At the type locality along Wilson Creek, northwest of Mono Lake, California, the Pleistocene Wilson Creek Formation is composed of mudstones interbedded with 19 tephra layers ranging in age from 32,000 to 13,000 years old (uncalibrated ¹⁴C ages). The eruptive source of the rhyolitic Wilson Creek tephra layers are the nearby Mono Craters, south of Mono Lake. One thick, basaltic ash bed near the top of the section was erupted from Black Point, at the north shore of the lake. The rhyolitic tephra layers are found as far east as Utah and are potentially valuable marker beds for the Pleistocene of the western United States. Ash bed #15 is also associated with the Mono Lake paleomagnetic excursion. However, the distribution of individual Mono Craters tephra layers is difficult to assess because the major-element composition of the glass shards are very similar; however, a trend in Ca and Fe allows differentiation of groups of tephra layers.. In this study, we calculated similarity coefficients (SC) for the major-element concentrations measured by electron microprobe and minor and trace-element concentrations measured by instrumental neutron activation analysis (INAA) for each tephra layer. The SC for elements (Si, Al, Fe, Ca, Ti), SC ranged from 0.99 to 0.82). In several cases, an SC of 1.0-0.93 is generally the range for replicate analyses. The SCs for elements (Sc, Mn, Rb, Cs, La, Ce, Sm, Tb, Yb, Lu, Hf, Th; including Fe [measured by microprobe]) ranged from 0.98-0.81. For ash bed #15, the major element data show that #15 shards have SC of >0.93 when compared to seven other Wilson Creek tephra layers. Using the INAA-measured minor and trace element concentrations, #15 has a SC <0.93 when compared to all the other Wilson Creek tephra layers. In addition, the light rare-earth fractionation of #15, shown by plots of Sm_N vs. La_N/Sm_N is unique. Using the minor and trace element data, we correlate the Bridgeport Creek (SC=0.94) and Rush Creek (SC=0.94) ash layers with Wilson Creek #19 (SC=0.94) and the Carson Sink (SC=0.96) and Upper Wilson Creek ash E (SC=0.95) layers with Wilson Creek #15.