Coupled trace element and Sr-Nd-(Pb) isotopes in olivine-hosted melt inclusions from the Mariana arc

The Mariana arc forms part of the 2500 km Izu–Bonin–Mariana arc system caused by westward subduction of the Pacific Plate beneath the Philippine Sea Plate over the last ~45 Myr. The magmatism produced in this comparatively simple arc setting records a moderate flux of fluids and sediments from the downgoing plate, however, the low MgO (<6 wt.%) of the lavas imply that magma mixing, crystal fractionation and crustal assimilation mask the primitive melt compositions. Olivine-hosted melt inclusions (MIs), in contrast, provide access to melt trapped deep in the magmatic plumbing system allowing more precise determination of the nature and quantity of recycled components. Here we analyse coupled trace element and Sr-Nd(-Pb) isotope compositions of olivine-hosted MIs in five samples from four islands within the Central Island province: Agrigan (AGR2, AGR6), Pagan (PAG3), Guguan (GUG6) and Sarigan (SAG1). Bulk rock MgO contents range from 4 to 5.7 wt.% [1]. We specifically target melt inclusions in olivine phenocrysts with the highest fortsterite content (Fo = Mg/(Mg+Fe)*100): AGR: 80-86; PAG: 76-81; GUG: 76-88; SAG: 84-88 mol%. Trace element contents and ratios of the selected MIs record marked differences between islands and show larger variability compared to published bulk rock and MI compositions [2]. Ba/La and Th/Nb or La/Sm ratios – indicators of slab fluids versus sediment melts, respectively – confirm that Guguan inclusions are dominated by a fluid component whereas Agrigan and Sarigan reflect a larger sediment contribution. Pagan inclusions show intermediate compositions and a restricted range indicating the influence of both fluids and sediments. Sr-Nd-(Pb) isotope compositions of individual and pooled melt inclusions will be determined by wet chemistry and TIMS techniques using 10^13 Ω amplifier technology [3] to further identify and quantify the recycled components. [1] Elliott et al. (1997). Journal of Geophysical Research, 102: 14991-15019. [2] Kent & Elliott (2002). Chemical Geology, 183: 263-286. [3] Koornneef et al. (2019). Nature Communications 10, 323

Subduction-related mantle heterogeneity recorded by Sr-Nd-Pb isotopes in melt inclusions from central Italy

Subduction of Earth’s surface material at convergent plate boundaries exerts fundamental control on global element fluxes that shape long-term planet evolution. However, the extent and mechanisms of subduction recycling remain ambiguous, especially in continental subduction zones where sediment input is highest. To resolve this issue, we examine the complex subduction setting of peninsular Italy, whose diverse post-collisional magmatic products reflect substantial temporal and spatial variations in subducted material in the mantle source. Primary melt systematics are potentially masked in bulk lavas by magma mixing and assimilation in the crust. Hence analyses are conducted on olivine-hosted melt inclusions (MIs), which more fully record the geochemical heterogeneity of the mantle source. Recent advances in TIMS technology, i.e. the use of 10^13 Ω amplifiers [1], now allow isotope analysis of MIs with exceedingly low abundances of Sr (2 ng), Nd (30 pg) and Pb (100 pg). Coupled Sr-Nd-Pb isotope, major and trace element data are presented on ~20 homogenized, high-potassium (HKS) to melilitite melt inclusions hosted by primitive (Fo92–90) olivines from three key Quaternary volcanic centers (Vulsini, Sabatini and Alban Hills) in the Roman Magmatic Province, central Italy. Systematic covariations are recorded in the MIs between proxies for sediment metasomatism such as K2O, U/Th, U/Nb, Cs/Rb, Be and 87Sr/86Sr. Furthermore, the MIs exhibit striking unradiogenic Pb isotope compositions, which have not been reported for volcanic rocks from this area before. These primitive melt compositions indicate the involvement of isotopically distinct and trace element–enriched mantle domains below central Italy. We infer that the covariations reflect melt extraction from a small-scale heterogeneous mantle source that was modified by sediment melts derived from the subducted Adriatic slab. As such, these findings will help to better constrain mantle modification and recycling fluxes in subduction zones

Optimised techniques to determine coupled trace element and Sr-Nd-Pb isotope ratios in olivine-hosted melt inclusions

Melt inclusions (MIs) in deeply formed magmatic minerals are typically characterised by larger variability in major and trace element- and isotope compositions compared to bulk lavas. The larger geochemical variability reflects that MIs represent partial melts that have escaped post-entrapment melt mixing that homogenises bulk lava compositions. Thus melt inclusions more realistically record the compositional heterogeneity of a mantle source and can be used to infer the presence of enriched or depleted source components in various tectonic settings. Recent development of 10^13 Ω resistors in the feedback loop of Faraday cup amplifiers used in thermal ionisation mass spectrometry (TIMS) [1] now allows determining combined Sr-Nd-Pb isotopes in individual melt inclusions (<300 µm). Data on MIs from peninsular Italy confirm the strength of these techniques to identify source components previously unresolved by bulk lava geochemistry [2, 3]. We further optimised the combined wet chemistry and TIMS analytical techniques applied to individual olivine- hosted melt inclusions to determine coupled 1) trace element ratios by ICPMS; 2) Sr-Nd-Pb concentrations by isotope dilution; and 3) isotopic compositions by TIMS. For Pb isotope analyses we use an optimised 207Pb–204Pb double spike technique [4]. Total procedural blanks (<20 pg Sr; <1 pg Nd; <10 pg Pb) can be corrected for using the analysed isotope compositions, but currently are the main limiting factor for application to tectonic settings that generate more depleted magma compositions. The improved procedures along with data on reference materials and melt inclusions from the Roman Magmatic Province in Italy are presented to evaluate the accuracy and reproducibility as a function of the amount of material analysed

Mantle sources of recent Anatolian intraplate magmatism: A regional plume or local tectonic origin?

We present an extensive study of rehomogenized olivine-hosted melt inclusions, olivine phenocrysts, and chromian spinel inclusions to explore the link between geodynamic conditions and the origin and composition of Pliocene–Quaternary intraplate magmatism in Anatolia at Kula, Ceyhan-Osmaniye, and Karacadağ. Exceptional compositional variability of these products reveals early and incomplete mixing of distinct parental melts in each volcanic center, reflecting asthenospheric and lithospheric mantle sources. The studied primitive magmas consist of (1) two variably enriched ocean island basalt (OIB)-type melts in Kula; (2) both OIB-type and plume mid-ocean ridge basalt (P-MORB)-like melts beneath Toprakkale and Üçtepeler (Ceyhan-Osmaniye); and (3) two variably enriched OIB-type melts beneath Karacadağ. Estimated conditions of primary melt generation are 23–9 kbar, 75–30 km, and 1415–1215 °C for Kula; 28–19 kbar, 90–65 km, and 1430–1350 °C for Toprakkale; 23–18 kbar, 75–60 km, and 1400–1355 °C for Üçtepeler; and 35–27 kbar, 115–90 km, and 1530–1455 °C for Karacadağ, the deepest levels of which correspond to the depth of the lithosphere-asthenosphere boundary in all regions. Although magma ascent was likely facilitated by local deformation structures, recent Anatolian intraplate magmatism seems to be triggered by large-scale mantle flow that also affects the wider Arabian and North African regions. We infer that these volcanics form part of a much wider Arabian-North African intraplate volcanic province, which was able to invade the Anatolian upper plate through slab gaps

Silicate melt inclusions in the new millennium: A review of recommended practices for preparation, analysis, and data presentation

International audienceMineral-hosted melt inclusions have become an important source of information on magmatic processes. As the number of melt inclusion studies increases, so does the need to establish recommended practice guidelines for collecting and reporting melt inclusion data. These guidelines are intended to ensure certain quality criteria are met and to achieve consistency among published melt inclusion data in order to maximize their utility in the future. Indeed, with the improvement of analytical techniques, new processes affecting melt inclusions are identified. It is thus critical to be able to reprocess any previously published data, such that reporting the raw data is one of the first “recommended practices” for authors and a publication-criteria that reviewers should be sensitive to. Our guidelines start with melt inclusion selection, which is a critical first step, and then continue on to melt inclusion preparation and analysis, covering the entire field of methods applicable to melt inclusions

Silicate melt inclusions in the new millennium:a review of recommended practices for preparation, analysis, and data presentation

Mineral-hosted melt inclusions have become an important source of information on magmatic processes. As the number of melt inclusion studies increases, so does the need to establish recommended practice guidelines for collecting and reporting melt inclusion data. These guidelines are intended to ensure certain quality criteria are met and to achieve consistency among published melt inclusion data in order to maximize their utility in the future. Indeed, with the improvement of analytical techniques, new processes affecting melt inclusions are identified. It is thus critical to be able to reprocess any previously published data, such that reporting the raw data is one of the first “recommended practices” for authors and a publication-criteria that reviewers should be sensitive to. Our guidelines start with melt inclusion selection, which is a critical first step, and then continue on to melt inclusion preparation and analysis, covering the entire field of methods applicable to melt inclusions