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| Titel |
Circumventing shallow air contamination in Mid Ocean Ridge Basalts |
| VerfasserIn |
Sujoy Mukhopadhyay, Rita Parai, Jonathan Tucker, Jennifer Middleton, Charles Langmuir |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250127706
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| Publikation (Nr.) |
 EGU/EGU2016-7610.pdf |
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| Zusammenfassung |
| Noble gases in mantle-derived basalts provide a rich portrait of mantle degassing and
surface-interior volatile exchange. However, the ubiquity of shallow-level air contamination
frequently obscures the mantle noble gas signal. In a majority of samples, shallow air
contamination dominates the noble gas budget. As a result, reconstructing the variability in
heavy noble gas mantle source compositions and inferring the history of deep recycling of
atmospheric noble gases is difficult. For example, in the gas-rich popping rock
2ΠD43, 129Xe/130Xe ratios reach 7.7±0.23 in individual step-crushes, but the bulk
composition of the sample is close to air (129Xe/130Xe of 6.7). Here, we present results
from experiments designed to elucidate the source of shallow air contamination in
MORBs.
Step-crushes were carried out to measure He, Ne, Ar and Xe isotopic compositions on
two aliquots of a depleted popping glass that was dredged from between the Kane and
Atlantis Fracture Zones of the Mid-Atlantic Ridge in May 2012. One aliquot was sealed in
ultrapure N2 after dredge retrieval, while the other aliquot was left exposed to air for 3.5
years. The bulk 20Ne/22Ne and 129Xe/130Xe ratios measured in the aliquot bottled in
ultrapure N2 are 12.3 and 7.6, respectively, and are nearly identical to the estimated mantle
source values. On the other hand, step crushes in the aliquot left exposed to air for several
years show Ne isotopic compositions that are shifted towards air, with a bulk 20Ne/22Ne of
11.5; the bulk 129Xe/130Xe, however, was close to 7.6. These results indicate that lighter
noble gases exchange more efficiently between the bubbles trapped in basalt glass and air,
suggesting a diffusive or kinetic mechanism for the incorporation of the shallow air
contamination. Importantly, in Ne-Ar or Ar-Xe space, step-crushes from the bottled
aliquot display a trend that can be easily fit with a simple two-component hyperbolic
mixing between mantle and atmosphere noble gases. Step-crushes in the aliquot
left exposed to air display significantly more scatter, which makes it difficult to fit
a two-component mixing hyperbola and obtain the mantle source value for this
aliquot.
In summary, our simple and inexpensive experiment demonstrates that at least in some
samples, significant air contamination is added after dredge retrieval from the ocean floor.
Bottling samples in ultrapure N2 upon dredge retrieval can largely eliminate this component
of shallow-level air contamination. As a result, the number of step crushes required to
characterize a sample decreases and estimating the mantle source compositions of the basalts
becomes significantly easier, which in turn leads to more refined estimates of mantle
degassing and regassing rates. |
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