Geology topics

J. Gleason

Publications and source records attributed to J. Gleason.

4 recordsLinked to original sources

USGS2017

Isotopic evidence of Holocene climatic change in the San Juan Mountains, Colorado

The ??D of cellulose from 14C-dated wood, collected in the San Juan Mountains of southwestern Colorado, decreased by about 45??? from 9600 to 3100 yr B.P. and an additional 25??? to the present. The wood samples are from trees that grew above present-day tree line and reflect a time of warmer average summer temperatures. These changes in ??D are interpreted to indicate a major change during the Holocene in the sources of moisture, in the seasonality of precipitation, or in both. ?? 1988.

Quaternary Research↗

USGS1992

Modeling of ancient climate from deuterium content of water in volcanic glass

The explosive nature of the eruptions that produced rhyolitic tephras resulted in the ash being distributed over large areas. This ash, within a few thousand years after deposition, incorporated relatively large amounts of environmental water (up to 3.5%) into the glass structure. This hydration water is shown to retain its original deuterium concentration through time, and because the deuterium content of precipitation has been used for climate characterization, the hydration water, which is related to ancient precipitation, can be used to investigate ancient climates. Based on the analysis of ash samples dated at 13,700, 11,200 and 8500 BP, the climate in the states of Washington and Montana may have been about 3-6??C cooler at the end of the Pleistocene or Early Holocene than the present. We observe no change in the deuterium concentration of surface waters, and hence climate, in that region post-8500 BP. ?? 1992.

Quaternary International↗

USGS1990

Natural occurrence of silicon carbide in a diamondiferous kimberlite from Fuxian

Considerable debate surrounds the existence of silicon carbide in nature, mostly owing to the problem of possible contamination by man-made SiC. Recently, Gurney 1 reviewed reports of rare SiC inclusions in diamonds, and noted that SiC can only be regarded as a probable rather than proven cogenetic mineral. Here we report our observation of clusters of SiC coexisting with diamond in a kimberlite from Fuxian, China. Macrocrysts of α-SiC are overgrown epitaxially by β-SiC, and both polymorphs are structurally well ordered. We have also measured the carbon isotope compositions of SiC and diamonds from Fuxian. We find that SiC is more enriched in 12 C than diamond by 20‰, relative to the PDB standard. Isotope fractionation might have occurred through an isotope exchange reaction in a common carbon reservoir. Silicon carbide may thus ultimately provide information on carbon cycling in the Earth's mantle.

Nature↗

USGS1985

Petrogenesis of the magmatic complex at Mount Ascutney, Vermont, USA - I. Assimilation of crust by mafic magmas based on Sr and O isotopic and major element relationships

The Ascutney Mountain igneous complex in eastern Vermont, USA, is composed of three principal units with compositions ranging from gabbro to granite. Sr and O isotopic and major element relationships for mafic rocks, granites, and nearby gneissic and schistose country rock have been investigated in order to describe the petrogenesis of the mafic suite which ranges from gabbro to diorite. The entire complex appears to have been formed within a short interval 122.2??1.2 m.y. ago. The granites with ??18O near +7.8??? had an initial 87Sr/86Sr of 0.70395(??6) which is indistinguishable from the initial ratio of the most primitive gabbro. Initial 87Sr/86Sr ratios and ??18O values for the mafic rocks range from 0.7039 to 0.7057 and +6.1 to +8.6???, respectively. The isotopic ratios are highly correlated with major element trends and reflect considerable crustal contamination of a mantle-derived basaltic parent magma. The likely contaminant was Precambrian gneiss similar to exposed bedrock into which the basic rocks were emplaced. A new approach to modelling of assimilation during the formation of a cogenetic igneous rock suite is illustrated. Chemical and isotopic modelling indicate that the mafic rocks were produced by simultaneous assimilation and fractional crystallization. The relative amounts of fractionation and assimilation varied considerably. The mafic suite was not produced by a single batch of magma undergoing progressive contamination; rather, the various rocks probably were derived from separate batches of magma each of which followed a separate course of evolution. The late stage granite was apparently derived from basaltic magma by fractionation with little or no crustal assimilation. The early intrusive phases are much more highly contaminated than the final one. The observed relationships have important implications for the formation of comagmatic complexes and for isotopic modelling of crustal contamination. ?? 1985 Springer-Verlag.

Contributions to Mineralogy and Petrology↗