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International audienceIsotopic fingerprinting has long been used to trace magmatic processes and the components that contribute to magmas. Recent technological improvements have provided an opportunity to analyze isotopic compositions on the scale of individual crystals, and consequently to integrate isotopic and geochemical tracing with textural and petrographic observations. It has now become clear that mineral phases are commonly not in isotopic equilibrium with their host glass/groundmass. Isotopic ratios recorded from core to rim of a mineral grain reflect the progressive changes in the magma composition from which the mineral crystallized. The sense of these changes and the relationship between isotopic composition and petrographic features, such as dissolution surfaces, can be used to constrain magma evolution pathways involving open system processes such as magma mixing, contamination and recharge

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We 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 2 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, and sparse (c) GM-type grains (texturally similar to zircons from GM dome YA) and (d) 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 40 Ar/ 39 Ar (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 of 838.5 ± 8.8 ka (95% confidence), implying that the Bishop Tuff system was only active for ~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 3x concentration differences between sides and tips within the same growth zones.Contrasting trends in molar (Sc+Y+REE 3+ )/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 aTiO 2 . The bright-rim portions of grains are inferred to have crystallized from the same magma as generated the bright rims seen under cathodoluminescence or back-scattered electron imaging on quartz and feldspar, respectively. This less evolved, slightly hotter magma

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Taupo volcanic centre has been active for c. 300 000 years. Since the c. 26.5 ka caldera-forming Oruanui eruption 28 eruptions have occurred from Taupo, varying between 0.1 and >45 km 3 in minimum bulk volume, and with repose periods ranging from c. 20 to 6000 years. All magma erupted post-26.5 ka is compositionally and mineralogically distinct from pre-Oruanui and Oruanui eruptives, and is inferred to have formed at or after 26.5 ka. Four post-Oruanui magma types are identified on the basis of whole rock and mineral compositions: one dacitic, forming three eruptions between 20.5 ka and 17 ka, and three subtly distinct rhyolite compositions erupted in discrete periods from 11.8 to 9.95, 7.05 to 2.75 and 2.15 to 1.74 ka. Stepwise compositional variations between, and limited variation within, rhyolite groups suggest emplacement of three petrogenetically separate batches of magma within only 10 000 years. The 15-35 km 3 of magma erupted at 1.77 ka evidently appeared in <10 3 years; this short residence time may have contributed to the lack of zonation within this magma chamber. Taupo is unusual amongst large rhyolite volcanoes in terms of the high frequency of activity since 26.5 ka, rapid stepwise changes in rhyolite compositions, and insignificant differentiation within individual subgroups. These traits are attributed to the combined effects of the extensional arc setting, thermal energy from mafic magma, and the shallow slope of the plagioclase-saturated rhyolite liquidus.

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