ABSTRACT
Dating of single crystals from rhyolites in the Jemez Mountains
volcanic field (JMVF) by the laser fusion 40Ar/39Ar 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 San Diego
Canyon ignimbrites. Xenocrystic material in these units was
apparently responsible for the anomalously old K-Ar ages
(2.84-3.64 Ma) previously obtained. 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). These eruptions record the chemical evolution of
a large, open system, upper crustal, silicic magma chamber.
Postcollapse rhyolites of the Valles Caldera were erupted over an
~1 Ma interval from immediately following caldera formation until
~200 ka. Volcanism was periodic with eruptive activity at ~1.133
Ma, 973-915 ka, 800-787 ka, 557-521 ka, and ~300-170 ka. Most
samples contain trapped atmospheric Ar, however several have
apparent 40Ar/36Ar ranging from 282 to 325. Approximately 30% of
the postcollapse rhyolites yield 40Ar/39Ar dates significantly
older than previous K-Ar dates. This is most likely due to
incomplete extraction of 40Ar* from high-temperature alkali
feldspars. Variations in petrographic, geochemical, and isotopic
characteristics indicate that the discrete intervals of volcanic
activity are related to the emplacement of shallow upper crustal
magma chambers. Magmas erupted at 973-787 ka and 557-521 ka record
differentiation sequences controlled by crystal-liquid fraction
and minor assimilation, whereas those vented at 1.133 Ma and
~300-170 ka were distinct compositionally but show no
differention. Nd isotopic compositions (eNd = -2.7 to -4.6)
indicate that ~20-65% of these rhyolitic magmas was of
mantle-derived origin. Sr isotopic values as low as 0.70464 and
calculated magmatic d18O of +6.6-7.0 °/oo 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
levels of precision for isotope ratio measurements of Kr gas using
electron beam ionization provide a measure of the capabilities of
FT-ICR under ideal conditions. Ratios of major isotopes are
measured to better than 0.1% whereas those involving minor
isotopes are reproducible to ±0.4%. Laser ionization (LI)
experiments yield significantly lower levels of precision due to
variations in ion number from shot to shot, mass fractionation at
the sample surface, and a larger spread in ion kinetic energy. LI
experiments involving isotope ratios of abundant elements
(metallic Ti) give precisions on the order of 1-4%, whereas those
involving trace elements (Pb in zircon or monazite) are measured
at 9-12%. 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.
Spell, T.L., 1991. The application of microanalytical techniques
in isotope geochemistry: 1. Single crystal 40Ar/39Ar 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.
Unpublished PhD dissertation, State University of New York at
Albany. 264pp., +xi
University at Albany Science Library call number: SCIENCE
MIC Film QE 40 Z899 1991 S64
Copies of this PhD dissertation can be ordered
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