Stefan Nicolescu

Late Miocene exhumation and uplift of the Washington Cascade Range

The Washington Cascade Range is a complex, polygenetic mountain range that dominates the topographic, climatic, and cultural configurations of Washington State. Although it has been the locus of ongoing arc magmatism since the Eocene, most of the range is distinct from the southern part of the arc in Oregon and California in that bedrock uplift has produced high surface elevations and topographic relief, rather than volcanic burial or edifice construction. (U-Th)/He and fission-track ages of bedrock samples on the east flank of the range record relatively rapid cooling in the early Tertiary, but slow exhumation rates (∼0.2 km/m.y.) through most of the Oligocene. Samples on the west flank suggest rapid cooling in the late Miocene (8–12 Ma), and age variations in vertical transects are consistent with a pulse of rapid exhumation (0.5–1.0 km/m.y.) at that time. Apatite He ages as young as 1–5 Ma in several areas suggest that high cooling and possibly exhumation rates persist locally. Accelerated exhumation rates ca. 10 Ma are also observed in the Coast Mountains of British Columbia and southeast Alaska, ∼1500 km to the north, suggesting a large-scale mechanism for the exhumation pulse at that time.

Age and temperature of shock metamorphism of Martian meteorite Los Angeles from (U-Th)/He thermochronometry

Mineralogic features attributed to impact-induced shock metamorphism are commonly observed in meteorites and terrestrial impact craters. Partly because the duration of shock metamorphism is very short, constraining the timing and temperature of shock events has been problematic. We measured (U-Th)/He ages of single grains of merrillite and chlorapatite from the Martian meteorite Los Angeles (LA). Merrillite and chlorapatite ages cluster at 3.28 6 0.15 Ma (2s) and 2.18 6 0.19 (2s) Ma, respectively. The mean age of the merrillites, which are larger than chlorapatites, is indistinguishable from cosmic-ray exposure ages (3.1 6 0.2 Ma), suggesting that impact-induced shock metamorphism was coeval with ejection of the LA precursor from Mars. To constrain the initial temperature of shock metamorphism in the LA precursor body, we modeled diffusive loss of He from merrillite as a function of diffusion domain size, LA precursor body size, and ablation depth. From these calculations, we suggest that the metamorphic temperature of the shock event was higher than 450 8C. These results support the idea that shock pressures of the Martian meteorite Shergotty were higher than 45 GPa, as inferred from the presence of post-stishovite SiO2 polymorphs. Single-grain (U-Th)/He dating of phosphates may provide unique constraints on the timing and pressure-temperature dynamics of shock metamorphism in a wide variety of extraterrestrial materials.

Samarskite Rediscovered at the Spinelli Prospect, Glastonbury, Connecticut

ABSTRACT The Spinelli prospect, located in Glastonbury, Hartford County, Connecticut, USA, was developed at the beginning of the 20th century on a small pegmatite outcrop, most likely for small-scale feldspar and mica mining. Its significance among many other Connecticut pegmatites was augmented by the pivotal role samarskite from the prospect played in the development of U-Pb dating and by the use of other minerals from the same location in a variety of other radioisotopic dating measurements. Old documents, some never published, reveal that in the early days of radioisotopic dating it was Wilbur Foye of Wesleyan University who made the Spinelli prospect samarskite available to scientists at Harvard University, and from there the US Geological Survey and National Research Councils Committee on the Measurement of Geologic Time. Very little has been published about the detailed mineralogy of the host pegmatite and the chemistry of Spinelli samarskite, and almost nothing about the prospects ownership history. The present paper fills these gaps and includes historical aspects, an updated map, a radiological survey of samarskite distribution in the outcrop, the paragenesis of the Spinelli prospect pegmatite and the geochemistry of its most famous mineral.