In search of the noble gas 3.52 Ga atmospheric signatures

Details In search of the noble gas 3.52 Ga atmospheric signatures In search of the noble gas 3.52 Ga atmospheric signatures

Dec 2008

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufm.v12b..07p&link_type=abstract

American Geophysical Union, Fall Meeting 2008, abstract #V12B-07

Physics

Nuclear Physics

1060 Planetary Geochemistry (5405, 5410, 5704, 5709, 6005, 6008), 1610 Atmosphere (0315, 0325)

Scientific paper

The isotopic signatures of noble gases in the Present-day mantle and in the atmosphere permit exceptional insight into the evolution of these reservoirs through time ([1]). However, related exchange models are under- constrained and would require direct measurements of the atmospheric composition long ago, e.g., in the Archaean. Drilling in the the 3.52 Ga chert-barite ([2]) of the Dresser formation(Pilbara Drilling Project) , North Pole, Pilbara craton (Western Australia), led to recovery of exceptionally fresh samples preserving primary fluid inclusions unaffected by surface weathering. The whole formation is considered to be an already established basin when hydrothermal processes started. The chemical composition of primary fluid inclusions trapped in hydrothermal quartz from vacuolar komatiitic basalt from 110 m depth were determined by synchrotron X-ray microfluorescence (ESRF, Grenoble,France). Data show that fluids are relatively homogenous, consisting of a Ba-rich fluid and a Fe (+Ba)-rich fluid of hydrothermal origin as concluded by Foriel et al.([3]). The isotopic compositions of xenon and argon trapped in these fluids were measured by mass spectrometry following vacuum crushing. The three argon isotopes show a homogeneous signature quite different from present-day Earth atmosphere but we cannot exclude the possibility that secondary nuclear reactions produced these anomalies. Despite this, the Xe isotopic trends indicate a less radiogenic signature than the Present-day atmosphere, and probably represent a remnant of the Archaean atmosphere. If this xenon composition is primitive then it implies that there is no cosmogenic production through time. However, argon ratios could be explained by cosmogenic production which implies significant surface exposure times. Cosmogenic production will produce correlated argon and xenon isotope signatures. Therefore it is necessary to differentiate primary from secondary composition. To investigate the effects of these nuclear reactions on Xe isotope production, barite from 30m shallower depth in the same core were analyzed. Variable excesses can be linked to spallogenic and cosmogenic reactions ([4] [5] [6]) which allow the primitive Xe isotopic signature to be isolated from subsequent secondary production. Models of the archaean atmospheric noble gas signature can thereby be compared with different theories on primitive atmospheric composition. [1] Staudacher T. Allègre C.J. (1982) EPSL 60, p 389-406 [2] Van Kranendonk MJ., Hickman A.H., Williams I.R. and Nijman W. (2001) Rec.-Geol. Surv. West. Aust. 2001/9, 134 [3] Foriel J., Philippot P., Rey P., Somogyi A., Banks D. and Ménez B. (2004) EPSL, 228, 451-463 [4]Srinivasan B. (1976) EPSL, 31, 129-141 [5]Charalambus S. (1971) Nuclear Physics, A166, 145 [6]Meshik A. P., Hohenberg C. M., Pravdivtseva O. V. and Kapusta Y. (2001) Phys. Rev., C 64, 035205-1 035205-6

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