No abstract

A fundamental but unquantified assumption in U-Th/He dating of apatite is that grains do not incorporate extraneous helium by solution or other processes, but large age dispersion seen in some samples suggests that this assumption might be violated. Our laboratory experiments show that helium solubility in apatite is quite low and unlikely to lead to age dispersion in most samples. However, in some samples highly variable and sometimes large helium uptake suggests that apatite grains can trap helium in microvoids that could be derived from fluid inclusions or other microstructures, a conclusion supported by crushing and step-heating experiments. The presence of such microvoids raises the possibility that closure and age systematics could be complicated either by trapping of internally generated radiogenic helium and/or alteration of helium diffusion kinetics by impeding diffusion.

Standing over 2 km above the surrounding topography and flanked by orogen-scale strike-slip faults, the Hangay Dome in central Mongolia is characterized by long wavelength high topography, basaltic eruptions spanning 30 million years, and an abundance of flat-topped summit plateaus. However, despite decades of research, the origin and timing of the intraplate

Epeirogenic histories of highland areas have confounded earth scientists for decades, as there are few sedimentary records of paleoelevation in eroding highlands. For example, mechanisms that have led to the high elevations of the Hangay Mountains in central Mongolia are not clear, nor is it well understood how the epeirogenic history of central Mongolia is connected to that of a broader region of high elevation that extends hundreds of kilometers to the north, east, and west. However, preserved basaltic lava flows record paleoelevation in the size distributions of vesicles at the tops and bottoms of flow units. As an initial step toward better understanding the tectonics of this part of Asia, we collected and analyzed samples from several basaltic lava flows from throughout the Hangay Mountains to use as a paleoaltimeter on the basis of lava flow vesicularity. Samples were dated and scanned with x-ray tomography to provide quantitative information regarding their internal vesicle size distributions. This yielded the pressure difference between the top and bottom of each flow for paleoelevation calculation. Results suggest that the Hangay Mountains experienced uplift of more than 1 km sometime during the past 9 m.yr. The magnitude of uplift of the Hangay, in addition to the composition of its lavas, the geomorphology of the region, its drainage pattern history, and other proxies, bears on possible mechanisms for uplift of this part of central Asia.