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The advent of double-Wien filter-selection-aperture and hexapole-collision-cell technologies coupled to laser ablation multicollector inductively coupled plasma mass spectrometry (LA-MC-ICP-MS/MS) enables in situ analysis of 87Sr variations produced by the decay of radioactive 87Rb (t1/2 = 49.61 ± 0.16 Ga). We present methodologies to acquire in situ87Rb–87Sr data using a Thermo Scientific™ Neoma™ MC-ICP-MS coupled to a laser ablation system. The ions first pass through a mass filter that blocks ions outside of the Rb–Sr mass region. The ions then travel through a hexapole collision cell filled with SF6, inducing the fluorination of Sr+ to form SrF+. Strontium isotopes are measured as SrF+ free of interferences, while rubidium isotopes are measured as Rb+. Formulas are presented to calculate the error ellipses of 87Rb/86Sr and 87Sr/86Sr ratios corrected for instrumental fractionation by standard bracketing. While LA-MC-ICP-MS/MS is not as precise as Thermal Ionization Mass Spectrometry (TIMS), it is less destructive and sample throughput is higher. It is therefore particularly well suited to analyze small and precious samples, or to examine population characteristics. We have analyzed several terrestrial and extraterrestrial materials to showcase the unique capabilities of LA-MC-ICP-MS/MS in Sr isotopic analyses: (1) an orthoclase megacryst and other minerals from the 397 Ma Shap granite, (2) feldspar grains from the 26.5 ka Oruanui supereruption in New Zealand, (3) Durango apatite, (4) highly refractory hibonite inclusions from the Murchison meteorite, and (5) the martian meteorite NWA 7034 also known as Black Beauty. Black Beauty is a polymict breccia that contains zircons as old as 4.4 Ga but whose 40Ar/39Ar age was partially reset at 1.4 Ga. All K-feldspar grains analyzed in Black Beauty give an 87Rb–87Sr age of 2.189 ± 0.059 Ga. Most likely, the 2.2 Ga 87Rb–87Sr age represents the age of lithification. This study demonstrates the great potential of in situ87Rb–87Sr dating for analyzing samples returned by planetary exploration missions, such as those currently collected by the Perseverance rover on Mars, or those that will be returned from Phobos by the MMX mission. [ABSTRACT FROM AUTHOR]

There is lively debate on whether Asian plate material was involved in southward flow of mid-lower crust in a ductile channel beneath southern Tibet. One argument against such involvement is the apparent absence of material derived from Asian lithosphere within the High Himalayan Series (Indian plate) that could represent the putative channel. A north-south--trending mid-Miocene dike swarm that intrudes the Tethyan sedimentary cover of the Sakya gneiss dome (Indian plate) yields new Sr-Nd isotopic data ([sup.87]Sr/[sup.86]Sr = 0.7071-0.7079;[[epsilon].sub.Nd]-4 to-6) indicating that these melts share the same source as Miocene dacitic dikes from north of the Indus-Tsangpo suture. Moreover, dikes on both sides of this suture represent crustal melts derived largely from mid-lower crust of the Asian plate, exposed today as the Nyainqentanglha gneisses that underlie the Gangdese batholith. We infer that melting of the Asian lithosphere extended south of the surface trace of the suture, requiring southward propagation of anatectic Asian middle crustal material during the Miocene. The emplacement ages of the southern dike swarm (12-9 Ma) thus delimit the timing of active southward ductile flow of Asian material. Keywords: Himalaya, isotope geochemistry, tectonics, channel flow.

The determination of accurate and precise isochron ages for igneous rocks requires that the initial isotope ratios of the analyzed minerals are identical at the time of eruption or emplacement. Studies of young volcanic rocks at the mineral scale have shown this assumption to be invalid in many instances. Variations in initial isotope ratios can result in erroneous or imprecise ages. Nevertheless, it is possible for initial isotope ratio variation to be obscured in a statistically acceptable isochron. Independent age determinations and critical appraisal of petrography are needed to evaluate isotope data. If initial isotope ratio variability can be demonstrated, however, it can be used to constrain petrogenetic pathways. Keywords: geochronology, igneous rocks, isotope geochemistry, isochrons.

Taupō volcano (New Zealand) is distinguished as the source of Earth's youngest supereruption (∼25.5 ka), with Lake Taupō occupying the resulting caldera. Taupō has also produced eruptions of a wide variety of sizes, styles and associated landscape responses over a ∼350 kyr period. Early Taupō (>54 ka) is poorly demarcated, merging with Maroa to the north, and is represented by widely scattered, geochemically distinct, effusive domes and explosive eruption products from vents all around the modern lake. Taupō had two independent magmatic systems from 54–25.5 ka, one that led to the Oruanui event focussed beneath the area of the modern lake and a second, northeast of the lake that has remained active to the present. Following the Oruanui supereruption, the rebuilt modern hyperactive Taupō magmatic system is primarily focussed beneath the lake and has generated 25 rhyolitic eruptions since ∼12 ka. The young rhyolite magmas come from an evolving silicic magma reservoir, but vary widely in their eruptive sizes and destructive potential. In the modern era Taupō experiences unrest every decade or so, but uncertainties remain over the nature of the magma reservoir and the processes that drive unrest or eruptive activity that require new geophysical data and interpretations. [ABSTRACT FROM AUTHOR]

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 [is greater than] 45 [km.sup.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.sup.3] of magma erupted at 1.77 ka evidently appeared in [is less than] [10.sup.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. Keywords: Taupo volcanic centre, explosive volcanism, rhyolite, magmatism.

Isotope ratios of elements such as Sr, Nd, Pb and Hf can be used as tracers of magmatic sources and processes. Analytical capabilities have evolved so that isotope ratios can now be analysed in situ, and isotopic tracers can therefore be used within single minerals to track the changing magmatic environment in which a given mineral grew. This contribution shows that Sr isotope ratios in feldspars that make up plutonic rocks will typically preserve initial isotopic variations, provided precise and accurate age corrections can be applied. Variations in initial isotope ratio can give a core-to-rim record of magmatic evolution and can be used to diagnose open system events such as contamination and magma recharge and mixing.New single grain Sr isotope data are presented from the Dais Intrusion, Antarctica, which reflect an open system origin for the crystals. The crystal cargo appears to be aggregated and assembled during transport and emplacement. This model, as opposed to a magma body crystallising post emplacement, may be more applicable to plutonic rocks in general, and is testable using the in situ isotopic determination methods described here. [ABSTRACT FROM PUBLISHER]

The timescales over which moderate to large bodies of silicic magma are generated and stored are addressed here by studies of two geographically adjacent, successive eruption deposits in the Taupo Volcanic Zone, New Zealand. The earlier, caldera-forming Rotoiti eruption (>100 km3 magma) at Okataina volcano was followed, within months at most, by the Earthquake Flat eruption (∼10 km3 magma) from nearby Kapenga volcano; both generated non-welded ignimbrite and coeval widespread fall deposits. The Rotoiti and Earthquake Flat deposits are both crystal-rich high-silica rhyolites, with sparse glass-bearing granitoid fragments also occurring in Rotoiti lag breccias generated during caldera collapse. Here we report 238U–230Th disequilibrium data on whole rocks and mineral separates from representative Rotoiti and Earthquake Flat pumices and the co-eruptive Rotoiti granitoid fragments using TIMS and in situ zircon analyses by SIMS. Multiple-grain zircon-controlled crystallisation ages measured by TIMS from the Rotoiti pumice range from 69±3 ka (<63 μm size fraction) to 76±6 (125–250 μm fraction), with a weighted mean of all size fractions of 71±2 ka. SIMS model ages from single zircons in pumice range from 50±24 ka to >350 ka, with a pronounced peak at 70–90 ka. The weighted mean of isochrons is 83±14 ka, in accord with the TIMS data. One glass-bearing Rotoiti granitoid clast yielded an age of 57±8 ka by TIMS (controlled by Th-rich phases that, however, are not apparently present in the juvenile pumices). Another glass-bearing Rotoiti granitoid yielded SIMS zircon model ages peaking at 60–90 ka, having a similar age distribution to the pumice. Age data from pumices are consistent with a published 64±4 ka eruptive age (now modified to 62±2 ka), but chemical and/or mineralogical data imply that the granitoid lithics are not largely crystalline Rotoiti rhyolite, but instead represent contemporaneous partly molten intrusions reflecting different sources in their chemistries and mineralogies. Similarly, although the Earthquake Flat eruption immediately followed (and probably was triggered by) the Rotoiti event, age data from juvenile material are significantly different. A multiple-grain zircon-controlled crystallisation age measured by TIMS from a representative pumice is 173±5 ka, while SIMS model ages range from 70−26+34 ka to >350 ka, with a peak at 105 ka. These age data coupled with previously published geochemical and isotopic data show that the Rotoiti and Earthquake Flat deposits were erupted from independent, unconnected magma bodies. [Copyright &y& Elsevier]