Daniele L. Pinti

Nitrogen and argon signatures in 3.8 to 2.8 Ga metasediments: Clues on the chemical state of the Archean ocean and the deep biosphere

Abstract N and Ar elemental and isotopic analyses were conducted on Archean metasediments of Isukasia, West Greenland and Pilbara Craton, Western Australia, in order to investigate the N isotopic evolution during the first half of Earth’s history. The selected samples are deep-sea sediments and hydrothermal deposits having ages from 3.8 to 2.8 Ga and affected by different degrees of metamorphism. The release patterns of N and Ar obtained by high-resolution stepped combustion show the occurrence of at least two trapped components. The first is released at 600°C and it is likely contained in fluid inclusions. N is released together with primordial 36Ar and shows a δ15N value of −1.3 ± 1.0‰, close to that of modern atmospheric N2 (δ15N = 0‰). This component is well preserved in hydrothermal-vent silica deposits of North Pole, Pilbara Craton, and nitrogen may represent ammonium salt dissolved in deep-sea hydrothermal fluids. The second N component, released at temperatures higher than 1000°C, is accompanied by radiogenic 40Ar∗, and shows a δ15N value of −7.4 ± 1.0‰ in a kerogen-rich chert from North Pole, Pilbara Craton. This N is likely biogenic and negative 15N values may reflect a metabolic isotopic fractionation induced by chemosynthetic bacteria using inorganic NH4+ contained in hydrothermal fluids. This 15N-depleted biogenic component may occur in Isukasia Banded Iron Formation (δ15N ∼ −1.7‰), but further data are needed to confirm such a hypothesis. In all other samples, metamorphic-induced Rayleigh distillation has altered the pristine N isotopic signature.

Atmosphere-derived noble gas evidence for the preservation of ancient waters in sedimentary basins

Hydrological modeling of the Middle Jurassic carbonate aquifer of the Paris Basin indicated fluid residence time from 1 k.y. to 100 k.y., whereas the chemistry of waters suggested longer residence time. Here, we report independent water age estimates on the basis of atmosphere-derived noble-gas recharge paleotemperatures. The calculated paleotemperatures range from 15 to 26°C and are related to the stable isotope composition and the salinity of the waters, and the distance from the recharge. The spatial distribution of the paleotemperatures suggests (1) occurrence of old saline waters in the center of the basin which infiltrated during the warmer climate interval in the Tertiary (Eocene) and (2) dilution by recent fresh waters (early Pleistocene) flowing from the recharge. These results strongly suggest that deep aquifers in sedimentary basins can store waters for very long periods of time, in contrast to the conclusions reached from hydrological modeling.

Anomalous xenon in Archean cherts from Pilbara Craton, Western Australia

Abstract We report the discovery of anomalies in the xenon isotopic composition of Archean cherts belonging to the metasedimentary sequences of Pilbara Craton, Western Australia. The cherts show contemporary excesses of 129 Xe and 131–136 Xe, compared to atmospheric Xe. The 129 Xe excesses are comparable to those observed in mantle-derived samples, such as MORBs, xenoliths, diamonds and CO 2 -rich well gases and correlate with excesses of 238 U fissiogenic-produced 136 Xe, close to the mantle ratio. However, isotope 131 Xe is in excess of 238 U fissiogenic-Xe production. The 129 Xe/ 131 Xe ratio ranges from 0.7 to 3.2 and it is consistent with production of these two isotopes by neutron-capture reactions with tellurium. These anomalies are analogous to those measured in metamorphic rocks (gneisses and amphibolites) in the KTB borehole in Germany and probably were produced by reactions with Te.

Neon excess in pumice : volcanological implications

Abstract We measured the noble gas abundance and Ne and Ar isotopic compositions in pumice samples from Japan and the Eolian Islands, Italy. The goal was to give clues on the mechanism of air incorporation into pumice and its relationship with eruption processes. The samples contain large excesses of atmospheric Ne over Ar, with 20 Ne / 36 Ar ratios up to 600 times the atmospheric ratio. This excess is produced by the preferential diffusion of Ne over Ar in pumice vesicles during volcanic eruptions. The 20 Ne / 36 Ar ratios measured in pumice reflect the initial quenching temperature, the cooling rate of the eruption column and the bubbles wall thickness. The final Ne amount is likely to be proportional to the volume of preserved vesicles in pumice. Calculations indicate that the fraction of isolated vesicles are less than 10% of the total, suggesting that a few amount of gas is preserved in pumice after deposition.

PALEOTEMP: a Mathematica program for evaluating paleotemperatures from the concentration of atmosphere-derived noble gases in ground water

Abstract During the recharge of an aquifer, atmosphere-derived noble gases are dissolved in ground water. The dissolved amount of gas depends on the relative noble gas solubilities, which in turn are inversely proportional to the ambient temperature. Therefore, paleotemperatures prevailing at the time of the aquifer recharge can be calculated from precise measurements of the amount of noble gases in ground water samples. However, the initial noble gas concentrations can be modified by several physical factors, such as the NaCl content of water, the elevation of the recharge area, the radiogenic in situ production of noble gas isotopes, air contamination and degassing, all of which make paleotemperatures difficult to estimate. The amount of air added or gas lost are unknown parameters which must be evaluated through iterative methods. Here, we present a program written with Mathematica ®, which calculates paleotemperatures using atmosphere-derived noble gas corrected from the above mentioned factors. The program involves root finding methods and minimization of functions which has been specifically developed for this application.