The application of microanalytical techniques in isotope geochemistry: 1.~Single crystal argon-40/argon-39 dating of rhyolites in the Jemez volcanic field, New Mexico, with implications for evolution of the magma system. 2.~Towards development of a laser microprobe Fourier transform mass spectrometer for isotopic analysis of geologic samples.

摘要

Dating of single crystals from rhyolites in the Jemez Mountains volcanic field (JMVF) by the laser fusion {dollar}sp{lcub}40{rcub}{dollar}Ar/{dollar}sp{lcub}39{rcub}{dollar}Ar technique reveals phenocryst populations dominated by juvenile crystals, but often containing xenocrystic and altered crystals. Isochron plots of single crystal analyses allow identification of the eruptive age and trapped Ar in the sample. Explosive caldera forming events commenced in the JMVF at 1.78 Ma with eruption of the Sand Diego Canyon ignimbrites. Further caldera collapse events occurred with eruption of the lower Bandelier Tuff at 1.51 Ma (Toledo Caldera) and the upper Bandelier Tuff at 1.14 Ma (Valles Caldera). Postcollapse rhyolites of the Valles Caldera were erupted over an {dollar}sim{dollar}1 Ma interval from immediately following caldera formation until {dollar}sim{dollar}200 ka. Volcanism was periodic with eruptive activity at {dollar}sim{dollar}1.133 Ma, 975-915 ka, 800-787 ka, 557-521 ka, and {dollar}sim{dollar}300-170 ka. Most samples contain trapped atmospheric Ar, however several have apparent {dollar}sp{lcub}40{rcub}{dollar}Ar/{dollar}sp{lcub}36{rcub}{dollar}Ar ranging from 282 to 325. Approximately 30% of the postcollapse rhyolites yield {dollar}sp{lcub}40{rcub}{dollar}Ar/{dollar}sp{lcub}39{rcub}{dollar}Ar dates significantly older than previous K-Ar dates. This is most likely due to incomplete extraction of {dollar}sp{lcub}40{rcub}{dollar}Ar* from high-temperature alkali feldspars. Magmas erupted at 975-787 ka and 557-521 ka record differentiation sequences controlled by crystal-liquid fraction and minor assimilation, whereas those vented at 1.133 Ma and {dollar}sim{dollar}300-170 ka were distinct compositionally but show no differention. Nd isotopic compositions ({dollar}epsilonsb{lcub}rm Nd{rcub}{dollar} = {dollar}-{dollar}2.7 to {dollar}-{dollar}4.6) indicate that {dollar}sim{dollar}20-65% of these rhyolitic magmas was of mantle-derived origin. Sr isotopic values as low as 0.70464 and calculated magmatic {dollar}deltasp{lcub}18{rcub}{dollar} O of +6.6-7.0{dollar}perthous{dollar} suggest that granulitic lower crust of igneous origin was assimilated by basaltic magmas.; Work on the development of a Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer has established a performance baseline for the initial goal of in situ isotopic analysis. The application of the SWIFT excitation technique to eject more abundant ions should allow measurement of trace element isotope ratios with precision approaching that seen for abundant elements. (Abstract shortened with permission of author.)