HR: 14:55h
AN: V52D-06    [PDF]
TI: CO2 Well Gas, Noble Gases and the Origin of Volatiles in the Earth's Mantle
AU: * Ballentine, C J
EM: cballentine@man.ac.uk
AF: The University of Manchester, Dept. Earth Sciences, Manchester, M13 9PL United Kingdom
AU: Sherwood Lollar, B
EM: bsl@primus.ca
AF: University of Toronto, Dept. Geology, Canada, M5S 3B1 Canada
AU: Marty, B
EM: bmarty@crpg.cnrs-nancy.fr
AF: Centre de Recherches Petrographiques et Geochimiques (CRPG), 15 Rue Notre-Dame des Pauvres, Nancy, 54501 France
AU: Cassidy, M
EM: Jo1955mar@aol.com
AF: University of Houston, Dept. Geosciences, Houston, TX 77204 United States
AB: Neon isotopes provide a unique opportunity to distinguish between volatiles introduced into the mantle during the accretion of the Earth by equilibration between a magma ocean with a massive early atmosphere, or as a component trapped within accreting material. A massive early atmosphere would have a Solar Ne isotopic composition (20Ne/22Ne=13.8) while accreting material that has been irradiated by solar wind preserves a distinct composite mix, often called Ne-B (20Ne/22Ne=12.5). To date the highest reliable Ne isotopic values measured in mid ocean ridge basalt (MORB) samples reach a maximum at values of about 12.5. It is not possible to rule out using the MORB sample suite the possibility that ubiquitous air contamination (20Ne/22Ne=9.8) found in this sample type suppresses the maximum Ne value measured. We present noble gas data from 14 CO2 well gases collected from Harding County, New Mexico. With 3He/4He ranging from 0.76 to 3.7Ra (Ra= atmospheric ratio) and correlated 40Ar/36Ar ratios that vary from 4660 to 22600, these samples represent a mixture of crustal and mantle-derived noble gases with only a small groundwater-derived air component. The precision with which we can analyse these gases combined with a systematic and well-mixed crust/groundwater component allows us to resolve crustal, mantle and air components for He, Ne and Ar. The resolved elemental ratios of the mantle He/Ne and He/Ar components are similar to those resolved in volatile rich mid ocean ridge popping rocks. A near constant crust+air end-member component defines a Ne isotope mixing line that intersects with the MORB-air mixing line at 20Ne/22Ne=12.20 plus or minus 0.05. The small difference between the 20Ne/22Ne upper-limit resolved in the well gases compared to that observed in MORB can be accounted for by slightly different mantle He/Ne ratios. While this result provides the first unambiguous evidence for the upper mantle 20Ne/22Ne limit, sources other than Ne-B must also be considered. These include lowering the mantle Ne isotopic composition from Solar by the recycling of air-derived volatiles into the mantle and by enhanced nucleogenic production of 22Ne caused by the co-siting of F with U+Th in the mantle.
DE: 1025 Composition of the mantle
DE: 1040 Isotopic composition/chemistry
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting