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dc.contributor.authorChamberlain, Katy J.en
dc.contributor.authorWilson, Colin J. N.en
dc.contributor.authorWooden, Joseph L.en
dc.contributor.authorCharlier, Bruce L. A.en
dc.contributor.authorIreland, Trevor R.en
dc.date.accessioned2018-03-13T12:08:23Z
dc.date.available2018-03-13T12:08:23Z
dc.date.issued2013-12-19
dc.identifier.citationChamberlain, K. J. et al (2014) 'New Perspectives on the Bishop Tuff from Zircon Textures, Ages and Trace Elements', Journal of Petrology, 55 (2):395.en
dc.identifier.issn14602415
dc.identifier.doi10.1093/petrology/egt072
dc.identifier.urihttp://hdl.handle.net/10545/622292
dc.description.abstractWe present zircon textural, trace element and U–Pb age data obtained by secondary ion mass spectrometry (SIMS) (SHRIMP-RG: sensitive high-resolution ion microprobe, reverse geometry) from 15 stratigraphically controlled Bishop Tuff samples and two Glass Mountain (GM) lava samples (domes OD and YA). Bishop zircon textures divide into four suites: (a) dominant sector-zoned grains, with (b) subordinate grains showing bright rims [lower U, Th, rare earth elements (REE)] in CL imaging, (c) sparse GM-type grains (texturally similar to zircons from GM dome YA) and (d) sparse Mesozoic xenocrysts from Sierran granitoid country rocks. All Bishop zircons from suites (a)–(c) combined have a weighted mean age of 777·9 ± 2·2 ka (95% confidence) and a tail back to ∼845 ka. Our eruption age estimate using the weighted mean of 166 rim ages of 766·6 ± 3·1 ka (95% confidence) is identical within uncertainty to published estimates from isotope-dilution thermal ionization mass spectrometry (ID-TIMS) (767·1 ± 0·9 ka, 2σ) and 40Ar/39Ar (767·4 ± 2·2 ka, 2σ) techniques, the latter using the 28·172 Ma age for the Fish Canyon sanidine standard. We estimate also an eruption age for GM dome YA of 862 ± 23 ka (95% confidence), significantly older than the currently accepted 790 ± 20 ka K–Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean age of 838·5 ± 8·8 ka (95% confidence), implying that the Bishop Tuff system was active for only ∼80 kyr, and had effectively no temporal overlap with the GM system. Trace element variations in Bishop zircons are influenced strongly for many elements by sector zoning, producing up to 3× concentration differences between sides and tips within the same growth zone. Contrasting trends in molar (Sc + Y + REE3+)/P ratios between sides and tips indicate contrasting mechanisms of substitution in different sectors of the same crystal. Concentrations of Ti in tips are double those in the sides of crystals, hindering applicability of the Ti-in-zircon thermometer, in addition to variations inherent to the 0·15–0·67 range in values proposed for aTiO2. The bright-rim portions of grains are inferred to have crystallized from the same magma as that which generated the bright rims seen under cathodoluminescence or back-scattered electron imaging on quartz and feldspar, respectively. This less evolved, slightly hotter magma invaded the deeper parts of the chamber represented in the late-erupted northern units possibly up to ∼10 kyr prior to eruption, but invaded shallower levels only very shortly before eruption, as shown by our textural information and previously proposed from the sharp delineation of quartz bright rims. By obtaining a large number of analyses from zircon separates that systematically cover the entire Bishop Tuff eruption sequence we can produce an eruption age estimate using SIMS to the same precision and accuracy as ID-TIMS and 40Ar/39Ar techniques.
dc.description.sponsorshipK.J.C. was supported by an NZ International Doctoral Research Scholarship administered by Education New Zealand, and acknowledges with thanks a Jack Kleinman Award administered by the US Geological Survey and two Victoria University of Wellington Faculty Strategic Research Grants. C.J.N.W. acknowledges support from the Royal Society of New Zealand through Marsden Fund grant VUW0813 and award of a James Cook Fellowship.en
dc.language.isoenen
dc.publisherOxford Academicen
dc.relation.urlhttps://academic.oup.com/petrology/article-lookup/doi/10.1093/petrology/egt072en
dc.rightsArchived with thanks to Journal of Petrologyen
dc.subjectGeologyen
dc.titleNew perspectives on the Bishop Tuff from zircon textures, ages and trace elements.en
dc.typeArticleen
dc.identifier.eissn00223530
dc.contributor.departmentVictoria University of Wellingtonen
dc.contributor.departmentStanford Universityen
dc.contributor.departmentThe Open Universityen
dc.contributor.departmentThe Australian National Universityen
dc.identifier.journalJournal of Petrologyen
refterms.dateFOA2018-06-13T00:00:00Z
html.description.abstractWe present zircon textural, trace element and U–Pb age data obtained by secondary ion mass spectrometry (SIMS) (SHRIMP-RG: sensitive high-resolution ion microprobe, reverse geometry) from 15 stratigraphically controlled Bishop Tuff samples and two Glass Mountain (GM) lava samples (domes OD and YA). Bishop zircon textures divide into four suites: (a) dominant sector-zoned grains, with (b) subordinate grains showing bright rims [lower U, Th, rare earth elements (REE)] in CL imaging, (c) sparse GM-type grains (texturally similar to zircons from GM dome YA) and (d) sparse Mesozoic xenocrysts from Sierran granitoid country rocks. All Bishop zircons from suites (a)–(c) combined have a weighted mean age of 777·9 ± 2·2 ka (95% confidence) and a tail back to ∼845 ka. Our eruption age estimate using the weighted mean of 166 rim ages of 766·6 ± 3·1 ka (95% confidence) is identical within uncertainty to published estimates from isotope-dilution thermal ionization mass spectrometry (ID-TIMS) (767·1 ± 0·9 ka, 2σ) and 40Ar/39Ar (767·4 ± 2·2 ka, 2σ) techniques, the latter using the 28·172 Ma age for the Fish Canyon sanidine standard. We estimate also an eruption age for GM dome YA of 862 ± 23 ka (95% confidence), significantly older than the currently accepted 790 ± 20 ka K–Ar age. The oldest zircon cores from late-erupted Bishop material (including those with GM-type textures) have a weighted mean age of 838·5 ± 8·8 ka (95% confidence), implying that the Bishop Tuff system was active for only ∼80 kyr, and had effectively no temporal overlap with the GM system. Trace element variations in Bishop zircons are influenced strongly for many elements by sector zoning, producing up to 3× concentration differences between sides and tips within the same growth zone. Contrasting trends in molar (Sc + Y + REE3+)/P ratios between sides and tips indicate contrasting mechanisms of substitution in different sectors of the same crystal. Concentrations of Ti in tips are double those in the sides of crystals, hindering applicability of the Ti-in-zircon thermometer, in addition to variations inherent to the 0·15–0·67 range in values proposed for aTiO2. The bright-rim portions of grains are inferred to have crystallized from the same magma as that which generated the bright rims seen under cathodoluminescence or back-scattered electron imaging on quartz and feldspar, respectively. This less evolved, slightly hotter magma invaded the deeper parts of the chamber represented in the late-erupted northern units possibly up to ∼10 kyr prior to eruption, but invaded shallower levels only very shortly before eruption, as shown by our textural information and previously proposed from the sharp delineation of quartz bright rims. By obtaining a large number of analyses from zircon separates that systematically cover the entire Bishop Tuff eruption sequence we can produce an eruption age estimate using SIMS to the same precision and accuracy as ID-TIMS and 40Ar/39Ar techniques.


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