Noble Gas Signature in Australian Cherts and Banded Iron Formations [2.5-3.5 Ga]: an Attempt to Detect Earth's Paleoatmosphere

D. L. Pinti Dept. of Earth and Space Science. Faculty of Science. Osaka University. 1-1 Machikaneyama. Toyonaka, Osaka 560, Japan

J. Matsuda Dept. of Earth and Space Science. Faculty of Science. Osaka University. 1-1 Machikaneyama. Toyonaka, Osaka 560, Japan

The noble gas signature of mantle and extraterrestrial samples has been used to constraint the early processes of accretion of the Earth and the evolution of his atmosphere (Azbel and Tolstikhin, 1993; Turner, 1989). Evidence of time evolution for paleoatmospheric noble gas isotopic signature (e.g., 40Ar/36Ar and 129Xe/130Xe) would impose unequivocable constraints on the Earth's atmosphere evolution. Several attempts have been done to find geological samples which could have incorporated atmosphere during their formation and retained it. Among them, cherts could have tightly trapped ancient air in their micro- and cryptocrystalline structure (Cadogan, 1977; Sano et al., 1994). However, early works showed that light noble gases have been totally or partially replaced by modern atmosphere and their isotopic composition is contaminated by radiogenic and nucleogenic contributions. Matsuda and Nagao (1986) showed that siliceous microfossils in deep-sea sediments were the main phase responsible for the enrichment of heavy noble gases. Further studies (Matsubara et al., 1988) reported that Xe was tightly trapped in amorphous silica and it was released at temperatures higher than 1100šC. These studies indicated the possibility that silica and cherts have still preserved the paleoatmospheric Xe.

Therefore, we planned to carry out precise measurements of noble gases in several cherts of different ages. Differently from previous works, which measured noble gases from samples of different geological environment and geographical setting, we decided to concentrate our effort on the cherts formation associated with the BIF (banded iron formations) of the Pilbara craton and Hamersley, Western Australia. These formations contain the oldest sedimentary record on the Earth's surface. Moreover, it is thought that BIF are formed by mixing of hydrothermal MOR fluids with oceanic water (Isley, 1995), and thus the analysis of their trapped components could give features about very old mantle composition. The samples are deep-sea cherts from Marble Bar Formation (3.5 Ga; Pilbara Craton), 3.5 Ga cherts associated with barite at North Pole Dome and containing the oldest fossils, and with 3.8 Ga and 2.5 Ga (Hamersley Group) Banded Iron Formations. The gas has been extracted by stepwise heating at 800šC, 1200šC and 1600šC. The noble gas measurements showed a multi-component release pattern, indicating at least two trapped components corresponding to the lowest (800šC) and higher (1600šC) temperature. The noble gas isotopic ratios show nucleogenic 21Ne* and radiogenic 40Ar* enrichment. The latter increase with the temperature, suggesting modern atmosphere contamination for the low-temperature trapped component. The Xe isotopic composition is the most interesting. In a 3.5 Ga deep sea chert from Marble Bar, the Xe isotopic pattern show, together with fissiogenic 131-136Xe, a slight enrichment in 129Xe. Repeated analyses have confirmed this enrichment, however the uncertainties are large. Therefore, crushing of larger amounts of this sample are in progress, to better establish the origin and entity of this xenon anomaly and its possible origin.


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