Abstract
Pristine, rhyolitic melt inclusions occur in chemically resistant
phenocrysts in Paleozoic bentonites that are preserved in the
Lower Devonian Helderberg Group limestones of New York State and
the Central Appalachian Basin, and in the Esopus Formation in New
York State. These melt inclusions have been chemically analyzed in
order to (1) obtain constraints on their magmatic sources and (2)
to chemically "fingerprint" the host bentonites so that they can
be used as time horizons for stratigraphic correlation.
High-precision electron microprobe and instrumental neutron
activation techniques have been developed in order to obtain
major-, minor-, and trace-element abundances. Using these
techniques, the abundances of these elements have been obtained
with adequate precision not only to distinguish one bentonite from
another, but to confidently distinguish differences between the
composition of different melt inclusions from a single bentonite.
Pre-entrapment or post-entrapment processes may alter the
chemistry of melt inclusions so that they are not reliable samples
of the original melt. Boundary layer enrichment of
slowly-diffusing incompatible elements in the liquid during
crystal growth has been modeled numerically in order to assess the
effect of this process on melt inclusion chemistry. The morphology
of host-quartz crystals and the textures of melt inclusions within
the host phenocrysts have been described in an effort to constrain
inclusion entrapment processes. Post-entrapment crystallization
has been identified in some bentonites, and an experimental
technique has been developed to rehomogenize crystallized melt
inclusions so that they can be analyzed by electron microprobe.
Four groups of Lower Devonian bentonites have been distinguished
based on the similarity in the minor- and trace-element content of
the melt inclusions. The bentonites within these groups apparently
share a common source that may be similar to more recent
continental ash flow tuffs such as the Bishop or Bandelier Tuffs.
Geochemical fingerprinting of inclusions has been successful in
establishing important stratigraphic correlations between
bentonites. The boundary between different bentonite groups can be
recognized across wide geographic regions and has been used as a
basin-wide correlative feature. These correlations are compared to
the stratigraphic framework previously established by
stratigraphers.
Hanson, B., 1995. A geochemical study of rhyolitic melt
inclusions in igneous phenocrysts from Lower Devonian bentonites.
Unpublished PhD dissertation, State University of New York at
Albany. 483pp., +xviii; 2 folded plates (stratigraphic charts)
University at Albany Science Library call number: SCIENCE
MIC Film QE 40 Z899 1995 H35
Copies of this PhD dissertation can be ordered
from Proquest UMI
Front matter (title,
table of contents, abstract, acknowledgements) - 0.6MB pdf
file
Photo pages in dissertation
(colour
and greyscale photos with captions): - 23.7MB pdf file
Plate 1 - Bentonite
Correlation Chart for the Lower Devonian rocks from the
northeastern United States
(lithostratigraphic columnar correlation chart
with location map) - pdf file 1.3MB
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