INVESTIGATION OF THE EFFECTS OF DIAGENETIC ALTERATION ON THE CHEMISTRY OF APATITE PHENOCRYSTS IN VOLCANIC TUFF DEPOSITS OF PLEISTOCENE LAKE TECOPA, INYO COUNTY, CALIFORNIA

Primary bentonites are the result of devitrification of volcanic tuff deposits. They have long been recognized for their potential usefulness as chronostratigraphic markers because of their instantaneous deposition (in geologic time) and their large areal distributions. Correlations of bentonites on a regional scale have been limited because of difficulties in identifying specific beds in the field. Efforts to overcome these difficulties have led to techniques for finding chemical fingerprints in bentonites. One such technique is to use the REE chemistry of primary apatite phenocrysts. The underlying assumption made when using apatite chemistry is that the apatite phenocrysts are stable during alteration of the tuff. However, the effects of diagenetic alteration on apatite phenocrysts have not yet been thoroughly examined. The deposits of Pleistocene Lake Tecopa, located in southeastern Inyo County, California, contain interbedded mudstones and rhyolitic vitric tuffs. The tuffs in Lake Tecopa have been diagenetically altered so that recognized diagenetic facies of zeolite, potassium feldspar, fresh glass, and bentonite are each present in beds that have been physically correlated. Samples of each of the facies were gathered from two units to assess the effects of diagenetic alteration on apatite phenocrysts. The apatites were separated from the samples and single grains were dissolved and analyzed by ICP-MS. REE ratios were calculated and Tukey-Kramer Honestly Significant Difference tests were performed for each ratio to quantify the differences between samples. There are no statistical differences in REE chemistry among the Tecopa samples with the possible exceptions of two of the eleven samples. No observable changes in apatite REE chemistry through diagenetic facies were found. The apatites appear to have survived zeolitic, potassic, and bentonitic alteration without losing the REE chemical fingerprint inherited from the parent magma.