Rose City Research Articles

The metallic microstructures and thermal histories of severely reheated chondrites

The metallographic structures of eight severely reheated chondrites (Farmington, Ramsdorf, Orvinio, Wickenburg, Lubbock, Rose City, Arapahoe and Tadjera) have been studied by optical, scanning electron microscope and electron microprobe techniques. Unreheated chondrites and experimentally heat treated chondritic material have also been examined. The following metallographic characteristics can be used to estimate the post-shock residual temperature of reheated chondrites: melted appearance of metal-troilite, presence of martensite, P enrichment of the metal, and the averaging of central metal grain compositions. Metallographic characteristics used to estimate the cooling rates of the severely reheated chondrites are the Ni content of troilite, the Ni gradients in metal grain rims, and the presence of secondary kamacite and phosphides. Farmington, Ramsdorf, Orvinio, Lubbock, Rose City and several of the heat treatment specimens have substantial P in solution in the metal grains ( > 0.1 wt%). P enrichment is apparently caused by reduction of phosphates upon severe reheating and partial melting of metal-troilite areas in chondritic meteorites. The eight severely reheated chondrites studied showed evidence of reheating to temperatures ranging from ∼ 950°C to ∼ 1250°C. Ramsdorf has the highest reheating temperature (1200–1250°C) and the fastest cooling rate ∼ 100°C/day. Wickenberg has the lowest reheating temperature (950–1000°C) and the slowest cooling rate, ∼ 1°C/100yrs. Cooling rate estimates correspond to post-reheating burial depths of less than 1 to ∼ 1000m.

STRUCTURAL SIGNIFICANCE OF GRAVITY SURVEYS IN THE VIRGINIA CITY—MOUNT ROSE AREA, NEVADA AND CALIFORNIA

Gravity surveys were made in the Mount Rose and Virginia City quadrangles to investigate the concealed Cenozoic structure of the alluviated basins and to explore for significant gravity differences between basins and ranges. The gravity data have been computed as complete Bouguer anomalies, which incorporate corrections for all the accurately assessible factors affecting gravity values, including topographic irregularities but not isostatic compensation. Gravity variations in the basins are closely related to changes in thickness of Cenozoic sedimentary rocks and alluvium in the basins. The gravity data indicate that light Cenozoic sedimentary rocks in Washoe Valley are at least 1800 feet thick; that the sediments in the northern part of Steamboat Valley are only about 600 feet thick and thin gradually eastward but thin abruptly to the west near Steamboat Hills; that east of Reno, between Sparks and the Virginia Range, sediments are roughly 2800 feet thick and thin abruptly about half a mile west of the range; and that in the basin west of Reno sediments are locally 1800 feet or more thick. The variations in thickness reflect important structural deformation in the basins. The late Cenozoic structures that separate the basins from the mountains are a complex of contemporaneous faults and folds; folding is predominant in some areas and normal faulting in others. From the eastern boundary of the area surveyed, the gravity values decrease westward about 1 mgal per mile, a total of 30 mgals, to a minimum at the east side of the Carson Range of the Sierra Nevada and then increase toward the main Sierra block. There is little or no gravity expression of the Virginia Range or the Carson Range relative to sediment-free parts of the basins adjacent to the ranges. Gravity measurements of the Coast and Geodetic Survey combined with the present measurements demonstrate that the Bouguer anomaly from San Francisco eastward shows a general inverse relationship to elevation but is a minimum along the east side of the Carson Range rather than at the main summit of the Sierra. This gravity profile may be the result of a thickening of the earth9s crust eastward beneath the higher part of the continent, combined with a smaller effect from greater-than-average amounts of granitic rock in the crust of the Sierran region.