Statistics
Scientific paper
Dec 2006
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2006agufm.v23h..07m&link_type=abstract
American Geophysical Union, Fall Meeting 2006, abstract #V23H-07
Statistics
1025 Composition Of The Mantle, 1032 Mid-Oceanic Ridge Processes (3614, 8416), 1038 Mantle Processes (3621), 1040 Radiogenic Isotope Geochemistry, 1094 Instruments And Techniques
Scientific paper
Xenon isotopes provide unique insights into the sources of volatile material for planet Earth, the degassing of the mantle, and the chemical evolution of the mantle. Whether the Earth's mantle has solar or planetary heavy noble gases has remained a fundamental and outstanding question. Resolving this issue is crucial for planet accretion models and understanding how volatiles were incorporated into the solid Earth. Here we report the detection of solar, or possibly chondritic (Q), xenon in a gas-rich basalt glass. The sample was collected from the Hotu Matua seamount chain, located south of the Sojourn Ridge, during the 2001 Cook16MV expedition. Xenon was extracted by step crushing fresh basalt glass in vacuum, and xenon isotopes were measured using the Nu multicollector noble gas mass spectrometer at Harvard. Based on reproducibility of standards run over a period of 3 days, which were similar in size (3.5 x 10^{-14}cc of ^{130}Xe) to the sample, external precision for ^{124,126}Xe/^{130}Xe ratios are better than 2%, for ^{128}Xe/^{130}Xe is 7‰, and for ^{129}Xe/^{130}Xe and ^{136}Xe/^{130}Xe ratios are 4‰. These uncertainties are only marginally larger than predicted from counting statistics. A clear excess in ^{124,126,128}Xe was observed. The anomalies in non-radiogenic isotopes of xenon cannot result from instrumental mass fractionation or other experimental artifacts since excesses in ^{128}Xe are correlated with excesses in ^{129}Xe. In addition, the ^{129}Xe/^{130}Xe and ^{136}Xe/^{130}Xe ratios fall on the MORB line. Thus, we conclude that the anomalies in the non-radiogenic isotopes of xenon are a real feature of the mantle source of MORBs. Excesses in ^{124,126,128}Xe/^{130}Xe ratios plot on the air solar mixing line and indicate the presence of a solar xenon component in the MORB source. Since the non-radiogenic isotopic composition of solar and Q xenon are similar, a chondritic xenon component cannot be ruled out. Krypton isotopes can potentially distinguish between a solar and a Q component and additional high-precision Kr isotopic measurements are currently underway. The correlation between ^{128}Xe and ^{129}Xe excesses in our sample defines a steeper trajectory than the correlation seen in CO2 well gases, although given the small number of data points such a conclusion is tentative. Nevertheless, a steeper trend implies a significantly larger solar component than previously determined values of 20% (e.g., Holland and Ballentine, 2006). While we are in the process of making additional Xe measurements, a major implication of the current data is that subduction zones must form a significant barrier to the transport of atmospheric Xe into the mantle in order to preserve the solar signature. Alternatively, mantle domains that have been largely isolated from the convective mantle must exist. Leakage of xenon from such a reservoir would then provide the excess ^{129}Xe associated with the solar signature seen in the MORB source. [Ref:Holland and Ballentine, Nature 441, 186, 2006].
Langmuir Charles H.
Mukhopadhyay Subhadeep
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