Reconstructing the Chronology of Supernovae: Determining Major Variations in the History of the Cosmic-ray Flux Incident on the Earth's Surface by Measuring the Concentration of 22Ne in Halite

Details Reconstructing the Chronology of Supernovae: Determining Major Variations in the History of the Cosmic-ray Flux Incident on the Earth's Surface by Measuring the Concentration of 22Ne in Halite Reconstructing the Chronology of Supernovae: Determining Major Variations in the History of the Cosmic-ray Flux Incident on the Earth's Surface by Measuring the Concentration of 22Ne in Halite

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v53b2154n&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #V53B-2154

Mathematics

Logic

1140 Thermochronology, 1165 Sedimentary Geochronology, 3675 Sedimentary Petrology

Scientific paper

We plan to measure the inventory of cosmogenically produced 22Ne atoms preserved in the mineral lattice of halite in deposits of rock salt, and to use that inventory to measure variations in the cosmic-ray flux to enable us to reconstruct the history of supernovae. Bedded rock salt consists almost entirely of the mineral halite (NaCl). Any neon trapped in the halite crystals during precipitation is primarily 20Ne, with a 22Ne concentration of 9% or less. Any neon resulting from cosmic-ray interactions with 23Na is solely 22Ne; therefore, 22Ne atoms in excess of 9% of the total neon are cosmogenic in origin. Measurement of the 22Ne inventory in halite from deposits covering a range of geologic ages may enable us to document the systematic growth of 22Ne through geologic time and, thus, establish the cosmic-ray flux and a chronology of supernovae. The cosmic-ray flux is attenuated in direct proportion to the mass of material overlying a halite deposit. To adjust the 22Ne inventory to account for that attenuation, we must reconstruct the post-depositional history of accumulation and removal of superjacent sediment for each halite deposit we study. As an example of our procedure, we reconstruct here the shielding history of the Permian halite deposit, the Salado Formation, Delaware Basin, New Mexico. The stratigraphy of the Delaware Basin has been well documented via exploration and production wells drilled in search of oil and gas, exploration boreholes associated with potash mining, and comprehensive geologic site assessment of the DOE Waste Isolation Pilot Plant (WIPP). WIPP is a subsurface repository for the permanent disposal of transuranic wastes, located in southeastern New Mexico, 42 km east of Carlsbad and approximately 655 m beneath the surface in the Salado Fm. The Salado Fm is part of the Late Permian Ochoan Series, and consists of 1) a lower member, 2) the McNutt Potash Zone, and 3) an upper member. WIPP lies between marker bed (MB)139 and MB136 in the lower member of the Salado Fm. MB139 forms the floor of WIPP. At WIPP, the Rustler Fm and the Dewey Lake Red Beds, both Ochoan in age, directly overlie the Salado Fm. The Dewey Lake marks the end of Permian deposition. The Santa Rosa Fm of the Late Triassic (late Carnian) Chinle Group lies unconformably over the Ochoan series. The unconformity represents a depositional gap of at least 25 million years. Another major unconformity, representing at least 210 million years, separates the Santa Rosa Fm from the overlying Miocene-Pliocene Ogallala Fm. The Pleistocene Gatuña Fm conformably overlies the Ogallala, but in many places it unconformably overlies Ochoan Permian rocks. Strata from the Jurassic to Late Tertiary do not occur at WIPP. The stratigraphic section at WIPP is well documented, but we must account for the column of rock that was deposited over the site and subsequently removed by erosion since deposition of the Salado halite. From hydrocarbon maturation-data, Hills (1984) estimated the missing overburden as 1,200 m. From stratigraphic inferences, Powers and Holt (1995) estimated that 400 m of rock have been removed by erosion since deposition of the Salado Fm. Using thermal indicators such as vitrinite reflectance, fission-track analysis, U/Th-He, 40Ar/39Ar, and fluid-inclusion analysis, we plan to constrain the thermal history at the site. We will reconstruct the mass of material that accumulated above the Salado Formation, and the period of time during which that cover remained in place.

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