In situ Sr isotope measurement of small glass samples using multiple-Faraday collector inductively coupled plasma mass spectrometry with 10^12Ω resistor high gain Faraday amplifiers

An analytical protocol was developed for correcting Kr baseline-induced bias and Rb isobaric overlap factors to analyse Sr isotope ratios for small glass samples using excimer laser ablation (LA) with an Aridus II desolvating nebuliser dual-intake system and multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS). The combined use of a low-oxide interface setup, along with high-gain Faraday amplifiers with a 1012 Ω resistor, enabled precise determination of Sr isotope ratios from 50-100 μm diameter craters at 10 Hz laser repetition rate. Residual analytical biases of 84Sr/86Sr and 87Sr/86Sr isotope ratios, obtained from Kr baseline suppressions (Kimura et al., 2013, Journal of Analytical Atomic Spectrometry, 28, 945-957), were found to be nonlinear, but the correction method was applicable to 50-200 μm/10 Hz craters. We also found that the 85Rb/87Rb overlap correction factor changed with time with a change in the surface condition of the sampler-skimmer cones. The correction factor of 85Rb/87Rb was thus determined at least once per five unknown measurements using the Aridus solution intake line. We determined 87Sr/86Sr isotope ratios from MkAn anorthite (Sr = 305 ppm, Rb = 0.07 ppm), BHVO-2G, KL2-G, ML3B-G (Sr = 312-396 ppm, Rb = 5.8-9.2 ppm), and BCR-2G (Sr = 337 ppm, Rb = 48.5 ppm) basalt glasses using a 50-100 μm/10 Hz crater. The results agree well with their reference values, determined by thermal ionisation mass spectrometry, even with the high Rb/Sr ratio (0.14) in the BCR-2G glass. The internal/intermediate precisions were ±0.0002 (two-standard deviation: 2SD) for 100 μm craters and ±0.0005 for 50 μm craters. The new instrument settings and analytical protocol improved the precision by a factor of two compared to the previous report using LA-(sector field)-ICP-MS and enables the analysis of sample volumes that are ten times smaller than those used in previous LA-MC-ICP-MS analyses with equal precision

Determination of relative Faraday cup efficiency factor using exponential law mass fractionation model for multiple collector thermal ionization mass spectrometry

A new method for determining the relative Faraday cup efficiency (RFCE) for nine Faraday cups has been developed using 86Sr, 87Sr, and 88Sr isotope signals, and the exponential mass fractionation law. Ten different Faraday cup configurations, combined with a nonlinear solver calculation module in an Excel spreadsheet, allowed for accurate RFCE determination. This method serves as a useful diagnostic tool for characterizing the functionality of Faraday cups

Determination of relative Faraday cup efficiency factor using exponential law mass fractionation model for multiple collector thermal ionization mass spectrometry

A new method for determining the relative Faraday cup efficiency (RFCE) for nine Faraday cups has been developed using 86Sr, 87Sr, and 88Sr isotope signals, and the exponential mass fractionation law. Ten different Faraday cup configurations, combined with a nonlinear solver calculation module in an Excel spreadsheet, allowed for accurate RFCE determination. This method serves as a useful diagnostic tool for characterizing the functionality of Faraday cups

Geochemical variations in primitive lavas from the Kibblewhite volcano, Kermadec arc

Poster session abstract GU2018-1639, European Geosciences Union (EGU) General Assembly 2018 (8-13 April, 2018, Vienna, Austria

Voluminous magma formation for the 30-ka Aira caldera-forming eruption in SW Japan: contributions of crust-derived felsic and mafic magmas

Understanding the origin, assembly, and evolution of voluminous magma that erupts in catastrophic caldera-forming eruptions (CCFEs) is a community imperative. A CCFE of the Aira caldera at 30 ka discharged over 350 km3 of magma, which can be grouped into petrographically and geochemically distinct types: voluminous rhyolite, small amounts of rhyodacite, and andesite magmas. To further understand the magma plumbing system of the Aira CCFE, we examined the geochemical characteristics of whole rock and plagioclase from its eruptive deposits. The trace element and 87Sr/86Sr signatures recorded in the plagioclase phenocrysts of these magmas indicate that the three magmas were originally produced by partially melting an identical source rock, which was estimated to be a mafic amphibolite with an 87Sr/86Sr signature of ∼0.7055 that comprised the lower crust. Melting of mafic amphibolite produced both felsic and mafic magmas by low and high degrees of partial melting, respectively. The mafic magma assimilated uppermost crustal materials and crystallized to produce an andesite magma type. The andesitic magma consists of phenocrysts (∼39 vol%) and melt with a dacitic (∼70 wt% SiO2) composition. The felsic magma mixed with ∼10% of the andesite magma and crystallized, forming the rhyolite magma. The mixing between the andesite and rhyolite magmas before the Aira CCFE produced the rhyodacite magma. The 30-ka Aira CCFE magmas were generated only by melting two kinds of crustal materials with different geochemical characteristics and had geochemical variations due to different conditions of partial melting and mixing between various crustal melts. The lack of definitive evidence of the mantle component mixing with the Aira CCFE magmas suggests that the mantle-derived magmas worked only as a heat source for crustal melting

Missing western half of the Pacific Plate: Geochemical nature of the Izanagi-Pacific Ridge interaction with a stationary boundary between the Indian and Pacific mantles

The source mantle of the basaltic ocean crust on the western half of the Pacific Plate was examined using Pb–Nd–Hf isotopes. The results showed that the subducted Izanagi–Pacific Ridge (IPR) formed from both Pacific (180–∼80 Ma) and Indian (∼80–70 Ma) mantles. The western Pacific Plate becomes younger westward and is thought to have formed from the IPR. The ridge was subducted along the Kurile–Japan–Nankai–Ryukyu (KJNR) Trench at 60–55 Ma and leading edge of the Pacific Plate is currently stagnated in the mantle transition zone. Conversely, the entire eastern half of the Pacific Plate, formed from isotopically distinct Pacific mantle along the East Pacific Rise and the Juan de Fuca Ridge, largely remains on the seafloor. The subducted IPR is inaccessible; therefore, questions regarding which mantle might be responsible for the formation of the western half of the Pacific Plate remain controversial. Knowing the source of the IPR basalts provides insight into the Indian–Pacific mantle boundary before the Cenozoic. Isotopic compositions of the basalts from borehole cores (165–130 Ma) in the western Pacific show that the surface oceanic crust is of Pacific mantle origin. However, the accreted ocean floor basalts (∼80–70 Ma) in the accretionary prism along the KJNR Trench have Indian mantle signatures. This indicates the younger western Pacific Plate of IPR origin formed partly from Indian mantle and that the Indian–Pacific mantle boundary has been stationary in the western Pacific at least since the Cretaceous