Evaluation of the sealed-tube low-temperature combustion method for the C-13/C-12 and H-2/H-1 ratio determinations of cellulose nitrate

Traditionally-suggested combustion time of 1 h at 550degreesC with the sealed-tube combustion method for determining the C-13/C-12 ratio of cellulose nitrate or other nitrogen-containing components could produce large negative deviation up to 1%.. Three types of cellulose are used to ascertain possible causes. The presence of nitrous oxide (N2O) formed during combustion is most likely responsible for this deviation. Prolongation of the combustion time (at least 5 h at 550degreesC) and intimate contact between copper oxide and organic matter can greatly improve the analysis precision and effectively reduce this deviation to an acceptable level. Regardless of scattered carbon isotope data, hydrogen isotope data are all reproducible within 2% when this method is coupled with the high temperature uranium reduction method. Thus, care should be taken for determining carbon and nitrogen isotope compositions of nitrogen-containing substances using the low temperature sealed-tube combustion method

Rapid emergence of life shown by discovery of 3,700-million-year-old microbial structures

Biological activity is a major factor in Earth\u27s chemical cycles, including facilitating CO2 sequestration and providing climate feedbacks. Thus a key question in Earth\u27s evolution is when did life arise and impact hydrosphere-atmosphere-lithosphere chemical cycles? Until now, evidence for the oldest life on Earth focused on debated stable isotopic signatures of 3,800-3,700 million year (Myr)-old metamorphosed sedimentary rocks and minerals1,2 from the Isua supracrustal belt (ISB), southwest Greenland3. Here we report evidence for ancient life from a newly exposed outcrop of 3,700-Myr-old metacarbonate rocks in the ISB that contain 1-4-cm-high stromatolites-macroscopically layered structures produced by microbial communities. The ISB stromatolites grew in a shallow marine environment, as indicated by seawater-like rare-earth element plus yttrium trace element signatures of the metacarbonates, and by interlayered detrital sedimentary rocks with cross-lamination and storm-wave generated breccias. The ISB stromatolites predate by 220 Myr the previous most convincing and generally accepted multidisciplinary evidence for oldest life remains in the 3,480-Myr-old Dresser Formation of the Pilbara Craton, Australia4,5. The presence of the ISB stromatolites demonstrates the establishment of shallow marine carbonate production with biotic CO2 sequestration by 3,700 million years ago (Ma), near the start of Earth\u27s sedimentary record. A sophistication of life by 3,700 Ma is in accord with genetic molecular clock studies placing life\u27s origin in the Hadean eon (\u3e4,000 Ma)6

Hydrogen isotope ratios in lunar rocks indicate delivery of cometary water to the Moon

Water plays a critical role in the evolution of planetary bodies, and determination of the amount and sources of lunar water has profound implications for our understanding of the history of the Earth-Moon system. During the Apollo programme, the lunar samples were found to be devoid of indigenous water. The severe depletion of lunar volatiles, including water, has long been seen as strong support for the giant-impact origin of the Moon. Recent studies have found water in lunar volcanic glasses and in lunar apatite, but the sources of lunar water have not been determined. Here we report ion microprobe measurements of water and hydrogen isotopes in the hydrous mineral apatite, found in crystalline lunar mare basalts and highlands rocks collected during the Apollo missions. We find significant water in apatite from both mare and highlands rocks, indicating a role for water during all phases of the Moon's magmatic history. Variations of hydrogen isotope ratios in apatite suggest the lunar mantle, solar wind protons, and comets as possible sources for water in lunar rocks and imply a significant delivery of cometary water to the Earth-Moon system shortly after the Moon-forming impact

Geology and vein tin mineralization in the Dadoushan deposit, Gejiu district, SW China

Vein-type tin mineralization in the Dadoushan deposit, Laochang ore field, Gejiu district, SW China, is predominantly hosted in Triassic carbonate rocks (Gejiu Formation) over cupolas of the unexposed Laochang equigranular granite intrusion. The most common vein mineral is tourmaline, accompanied by skarn minerals (garnet, diopside, epidote, phlogopite) and beryl. The main ore mineral is cassiterite, accompanied by minor chalcopyrite, pyrrhotite, and pyrite, as well as scheelite. The tin ore grade varies with depth, with the highest grades (similar to 1.2 % Sn) prevalent in the lower part of the vein zone. Muscovite ⁴⁰Ar-³⁹Ar dating yielded a plateau age of 82.7 ± 0.7 Ma which defines the age of the vein-type mineralization. Measured sulfur isotope compositions (δ³⁴S = -4.1 to 3.9 ‰) of the sulfides (arsenopyrite, chalcopyrite, pyrite, and pyrrhotite) indicate that the sulfur in veins is mainly derived from a magmatic source. The sulfur isotope values of the ores are consistent with those from the underlying granite (Laochang equigranular granite, -3.7 to 0.1 ‰) but are different from the carbonate wall rocks of the Gejiu Formation (7.1 to 11.1 ‰). The calculated and measured oxygen and hydrogen isotope compositions of the ore-forming fluids (δ¹⁸O(H₂O) = -2.4 to 5.5 ‰, δD = -86 to -77 ‰) suggest an initially magmatic fluid which gradually evolved towards meteoric water during tin mineralization