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Please use this identifier to cite or link to this item:
http://hdl.handle.net/10136/466
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| Title: | 40Ar/39Ar chronology of volcanic events in the AND-1B drillcore - implications for age models and glacial history of the Ross Embayment |
| Authors: | Ross, Jake I. |
| Keywords: | 40Ar/39Ar Geochronology,
ANDRILL
Cenozoic volcanism,
Ross Island
Antarctica |
| Issue Date: | 21-Aug-2009 |
| Abstract: | 40Ar/39Ar dating of a suite of volcanic clasts and tephra, collected
throughout the AND-1B drill core, significantly aids the development of an age-depth
model. High precision dates determined for a variety of volcanic materials
provide pinning points for constraining the chronostratigraphy of the drill core.
The volcanic materials we have dated include 1) felsic and basaltic tephra, 2)
interior of an ~3 m thick intermediate lava flow and 3) felsic and basaltic volcanic
clasts. Two felsic tephra, two basaltic tephra and the intermediate lava flow yield
precise and accurate depositional ages, whereas the volcanic clasts provide
maximum depositional ages. The ages for eight stratigraphic intervals are 1)
17.17-17.18 mbsf, basaltic clast (maximum depositional age 0.310±0.039 Ma, all
errors quoted at 2σ), 2) 52.80-52.82 mbsf, three basaltic clasts (maximum
depositional age 0.726±0.052 Ma), 3) 85.27-85.87 mbsf felsic tephra
(1.014±0.008 Ma), 4) ~112-145 mbsf sequence of basaltic tephra (1.633±0.057
to 1.683±0.055 Ma), 5) 480.97-481.96 mbsf pumice-rich mudstone (4.800±0.076
Ma), 6) 646.30-649.34 mbsf intermediate lava flow (6.48±0.13 Ma), 7) 822.78
mbsf kaersutite phenocrysts from volcanic clasts (maximum depositional age
8.53±0.53 Ma) and 8) ~1280 mbsf, three volcanic clasts (maximum depositional
age 13.57±0.13 Ma).
Incremental heating analyses of glass shards from the basaltic tephra
display a reproducible saddle-shaped spectrum with discordant apparent ages.
Two out of more than twenty-five basaltic glass analyses yield both a statistically
defined plateau and precise inverse isochron age. The remainder of the
analyses yield discordant apparent ages for all steps and statistically
unconstrained isochron ages. Recoil effects, excess 40Ar, and xenocrystic
contamination are possible explanations for the observed discordance.
Volcanic clasts provide the best means of constraining the age of the drill
core at depths >600 mbsf. Electron microprobe characterization of numerous
volcanic intervals below ~600 mbsf indicates high degrees of alteration in the
felsic and basaltic tephra, whereas the volcanic clasts show little to no alteration.
The paucity of fresh tephra at depths greater than ~600 mbsf requires age
constraints for this interval of the drill core to rely on dating populations of
volcanic clasts. Although volcanic clasts do not provide accurate time stratigraphic pinning points they do yield accurate maximum depositional ages
and are important for provenance studies. |
| URI: | http://hdl.handle.net/10136/466 |
| Appears in Collections: | Independent Studies
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Files in This Item:
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Description |
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| Ross, Jake I..pdf | | 7150Kb | Adobe PDF | View/Open |
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