B. Carter Hearn

40Ar/39Ar ages from the rhyolite of Alder Creek, California: Age of the Cobb Mountain Normal-Polarity Subchron revisited

New 40 Ar/ 39 Ar age determinations on sanidine from the rhyolite of Alder Creek, California, indicate a 1.186 ±0.006 Ma age for the Cobb Mountain Normal-Polarity Subchron. The new age is statistically older (α = 0.05) than the previously reported K-Ar age (1.12 ±0.02 Ma) and agrees with the age suggested by the astronomical polarity time scale. Incomplete extraction of radiogenic 40 Ar ( 40 Ar*) from the sanidine is the most likely reason for the disparity between the 40 Ar/ 39 Ar and K-Ar ages. Because the Cobb Mountain subchron is a worldwide, short-duration event, and because no widely used interlaboratory 40 Ar/ 39 Ar standard younger than 27 Ma exists, we propose that sanidine from the rhyolite of Alder Creek be considered for use as a new Quaternary 40 Ar/ 39 Ar mineral standard.

Garnet peridotite xenoliths in a Montana, U.S.A., kimberlite

Abstract Within a swarm of late middle Eocene subsilicic-alkalic diatremes, one diatreme 270 by 370 m and an associated dike contain common xenoliths of granulite and rare xenoliths of spinel peridotite and garnet peridotite. Six garnet lherzolite xenoliths have been found and these show a range of textures. Four are granular, and two are intensely sheared. Phlogopite is absent from the intensely sheared xenoliths and is thought to be primary in part in the granular xenoliths. Estimated temperatures and depths of equilibration of xenolith pyroxenes range from 920°C, 106 km (32 kbar) to 1315°C, 148 km (47 kbar). The xenoliths show increasing amounts of deformation with greater inferred depths of origin. The temperature-depth points suggest a segment of an Eocene geotherm for Montana which is similar in slope to the steep portion of the pyroxene-determined Lesotho geotherm (B oyd and N ixon , this volume) and is considerably steeper than typical calculated shield and continental geotherms at present. The steep trend could be a result of plate-tectonic shearing and magma ascension within an Eocene low-velocity zone. Preservation of intensely sheared textures requires rapid transport of material from about 150 km depth during active deformation of relatively dry rock. The occurrence of monticellite peridotite in this kimberlite diatreme suggests that magmas which crystallized to monticellite peridotite at relatively shallow depth could be one of the primitive types of kimberlite magma.

Origin and Significance of Ilmenite Megacrysts and Macrocrysts from Kimberlite

Ilmenite populations (megacrysts and macrocrysts) from 26 kimberlites in North America have been characterized by electron microprobe analysis to assist in the understanding of the origin and significance of ilmenite in kimberlites worldwide. Most belong to the Cr-poor megacryst suite. Geochemical trends in Cr-poor-suite ilmenites are consistent with a mantle fractional crystallization origin, with ilmenite forming only a minor proportion of the crystallizing assemblage. Coprecipitating magnesite is inferred to be an important host for Mg, with its crystallization causing Mg depletion in coexisting ilmenite. Decrepitation of magnesite megacrysts during kimberlite ascent may have enriched kimberlite hosts in Mg, contributing to the Mg increase characteristic of ilmenite rims. Ilmenite rims commonly have lower hematite contents than do cores, suggesting that the oxidation state of the kimberlite, and thus its potential for diamond resorption, can be overestimated if core compositions alone are considered. N...

Interpretation of an Aeromagnetic Strip across the Northwestern United States

This report discusses the results of aeromagnetic survey bounded by latitudes 45°30′ N. and 47°00′ N. and extending from the Rocky Mountains, to approximately 120 mi offshore in the Pacific Ocean. East of the Rocky Mountains, a larger area has been surveyed in the Great Plains bounded approximately by latitudes 44°50′ N. and 48°10′ N. and by longitudes 104°W. and 110°W. Throughout the area of the survey, the magnetic map is marked by conspicuous northeast and northwest anomaly trends, lineaments, and breaks in the anomaly pattern. Their regional distribution, over-all magnetic character, and geologic evidence suggest that they are major structural features in the basement rocks. The close correspondence of structural and geologic features in younger rocks with these basement magnetic and structural trends suggest that basement trends controlled or at least greatly influenced intrusion, deposition, and structural history of younger rocks. In some cases, evidence suggests that basement structures have been reactivated during later tectonic activity. Perhaps even more striking than the northeast- and northwest-trending features are large east-west magnetic discontinuities which, in some cases, extend completely across the strip to the edge of the shelf, and which, in some cases, can be correlated with large-scale discontinuities dating back to the Precambrian.

Geothermal Prospecting in the Geysers-Clear Lake Area, Northern California

The geochemistry of thermal waters combined with mapping and geophysical studies have been used to define boundaries of the geothermal steam field in The Geysers-Clear Lake area, California. Chemical geothermometry and drill hole data imply that a 200/sup 0/C hot-water geothermal system underlies much of the 2.0 to 0.01-m.y.-old Clear Lake Volcanics. Chloride concentrations and electrical resistivity define the concealed extent of Great Valley sedimentary rocks underlying the young volcanic cover. It is suggested that the 80 or more vents in the volcanic field act as permeable conduits for water recharge and thus prevent creation of a widespread vapor-dominated system northeast of the Collayomi fault zone.