Global hydrogen reservoirs in basement and basins

BACKGROUND

Hydrogen is known to occur in the groundwaters of some ancient cratons. Where associated gases have been dated, their age extends up to a billion years, and the hydrogen is assumed also to be very old. These observations are interpreted to represent the radiolysis of water and hydration reactions and migration of hydrogen into fracture systems. A hitherto untested implication is that the overwhelming bulk of the ancient low-permeability basement, which is not adjacent to cross-cutting fractures, constitutes a reservoir for hydrogen.

RESULTS

New data obtained from cold crushing to liberate volatiles from fluid inclusions confirm that granites and gneiss of Archean and Palaeoproterozoic (>1600 Ma) age typically contain an order of magnitude greater hydrogen in their entrained fluid than very young (<200 Ma) granites. Sedimentary rocks containing clasts of old basement also include a greater proportion of hydrogen than the young granites.

CONCLUSIONS

The data support the case for a global reservoir of hydrogen in both the ancient basement and in the extensive derived sediments. These reservoirs are susceptible to the release of hydrogen through a variety of mechanisms, including deformation, attrition to reduce grain size and diagenetic alteration, thereby contributing to the hydrogen required by chemolithoautotrophs in the deep biosphere.

A Binary Source Model for Extension-Related Magmatism in the Great Basin, Western North America

Models for extension-related magmatism based on decompression melting of asthenospheric mantle poorly simulate fluxes and bulk compositions of magmas produced during early stages of continental extension. For the Great Basin of western North America, it is proposed that magmatism proceeded in two stages, the first involving melting of lithospheric mantle sources between 40 and approximately 5 million years ago (Ma), followed (since approximately 5 Ma) by melting of upwelling asthenospheric mantle in areas where extension has exceeded about 100 percent. This transition in magma sources is diachronous, depending on initial variations in lithosphere thickness and on rates of lithospheric thinning.