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

Petrology of Long-Lived Mantle Plume Magmatism: Hawaii, Pacific, and Reunion Island, Indian Ocean

Mineralogy, geochemistry, and magmatic inclusions in minerals were investigated from represen­ tative samples of major volcanic series, and magmatic processes were numerically simulated to determine the compositions, formation conditions, and evolution of the primary melts of intraplate magmatism in the Hawai­ ian Islands and Reunion Island. It is demonstrated that the primary melts of the Hawaiian tholeiitic series and the Hawaiian and Reunion transitional series had an ultramafic composition (18.5- 21 wt% MgO- tholeiitic series and 16 - 20 wt % MgO- transitional series) and were generated by the melting of mantle peridotite at 30- 40 kbar and 1650- 1500°C. The primary melts of the Hawaiian basanite and nephelinite series approx­ { imate the composition of alkalic picrites (15.5 and 13.2 wt % MgO, respectively) and originated as a result ' of the partial melting of mantle peridotite at 30- 40 kbar and 1595 - 1485°C and 1575- 1465°C, respectively. The early crystallization of primary magma occurred in shallow, intermediate reservoirs. The depth at which the primary magma separated from peridotite was controlled by an oceanic lithosphere thickness of 100 ± 20 km. The successive temperature decrease of the Hawaiian primary magma formation in the sequence of the tholeii­ tic-transitional-basanite-nephelinitic series corresponds, considering the approximately similar depth of their formation (100- 130 km), to the model of lateral zoning of the Hawaiian mantle plume. It is demonstrated that the P-T conditions of intraplate magma formation are considerably higher than the values of tholeiites from oceanic rifts. This indicates that the mantle is, on a global scale, vertically heterogeneous and that intraplate magmas are generated in deeper mantle reservoirs as compared to MORB magmas.The pressure and temperature values obtained in this study for the primary magma generation suggest that the Hawaiian and Reunion mantle plumes originated in the lower mantle at a depth below 900 km. This is direct and independent evidence in sup­ port of the hotspot concept

Mineralogical evidence of the magma mixing at Hualalai volcano, Hawaii

A detailed study of olivine phenocrysts and the primary magmatic inclusions in them has revealed that the alkali olivine basalt of the alkali series of Hualalai Volcano, Hawaii contains two compositionally different olivine-spinel assemblages that originated as a result of crystallization of compositionally different magmas. It is concluded that the parent magmas of the Hualalai alkali series mixed not long before the eruption with low-Ca, high-Ti magma that had crystallized at a depth level of the crust-mantle interface

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

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