Phenocryst Compositions Research Articles

Recycling of crustal Os in nascent subduction zones revealed by ophiolitic boninites

Olivine-rich lavas in the Margi area, Cyprus, are part of the boninitic Upper Pillow Lava series of the Troodos ophiolite. Re-Os isotope compositions of both whole rock and olivine phenocrysts of these lavas have been measured to determine the initial 187Os/188Os isotope compositions of the primary boninitic magma. The Margi lavas have high and variable whole rock Re-Os isotope compositions (187Re/188Os: 1.060–8.613; 187Os/188Os: 0.1307–0.1599), with (87Sr/88Sr)90Ma (0.705936 ± 0.001021), εNd(90 Ma) (5.6 ± 2.5), and δ18O (8.59 ± 0.38 ‰). In contrast, olivine phenocrysts, which host spinel and melt inclusions, have low and constant 187Os/188Os from 0.1301 to 0.1336 and low 187Re/188Os from 0.1350 to 1.557. Initial 187Os/188Os of these olivine phenocrysts (0.1300 ± 0.0041) is lower than that of whole rock (0.1373 ± 0.0135). These facts indicate that the olivine phenocrysts faithfully record initial 187Os/188Os of the primary boninitic magmas, which are comparable to the 187Os/188Os of primitive mantle, whereas whole rock Re-Os isotopes have been affected by secondary processes after olivine crystallization. Further geochemical modeling indicates that, although Os can be mobilized during slab dehydration, the contribution of crustal Os is insufficient to significantly raise the 187Os/188Os of the mantle wedge in a nascent subduction zone. Overall, our findings provide new insights about Os recycling in the early stages of subduction.

Mineral Chemistry of Olivine, Oxy-Spinel, and Clinopyroxene in Lavas and Xenoliths from the Canary, Azores, and Cape Verde Islands (Macaronesia, North Atlantic Ocean): New Data and Comparisons with the Literature

An electron microprobe study was carried out on olivine, clinopyroxene, and oxy-spinel occurring in basalts and dunite xenoliths from the archipelagos of the Azores, the Canary Islands, and Cape Verde. By comparing our results with previously published data from the volcanic islands of Macaronesia, we confirmed the validity of the compositions of olivine, clinopyroxene, and oxy-spinel as geochemical tracers. The origin of olivine, i.e., crystallized in the lithospheric mantle or in volcanic rocks, was successfully discriminated. Olivine from Lanzarote dunite xenoliths, which represent fragments of the mantle transported to the surface by host magmas, exhibited higher Fo% values (Fo91.02 to Fo91.94) and a different distribution of minor elements Ca, Ni, and Mn (CaO up to 0.42 wt%, NiO 0.07–0.41 wt%, MnO 0.06–0.3 wt%) when compared with olivine occurring as phenocrysts in basaltic lavas from the Macaronesian islands. The highly variable forsterite contents (Fo75.1 to Fo94.4) in olivine from gabbro and peridotite xenoliths found across the islands of Macaronesia were attributed to fractional crystallization that started in a deep magma reservoir, suggesting that these xenoliths represent cumulate rocks and not mantle fragments. Alternatively, these xenoliths may have been affected by the interaction with metasomatic fluids. The composition of clinopyroxene phenocrysts was used to decipher formation conditions under extensional tectonics. Their composition suggests that the host lavas have an alkaline to calc-alkaline signature. Furthermore, clinopyroxene euhedral shapes and compositions suggest an origin by fractional crystallization in a closed magmatic system. The composition alone of oxy-spinel from Macaronesian basalts and xenoliths was not sufficient to draw conclusions about the geodynamic environment where they were formed. Nevertheless, the relationship between oxy-spinel and olivine crystallized in equilibrium was successfully used as oxybarometers and geothermometers. The oxy-spinel–olivine pairs show evidence that the basaltic lavas were crystallized from melts with higher oxygen fugacity and different cooling histories than those of the mantle xenoliths, as the latter crystallized and re-equilibrated much slower than the basalts.

Translithospheric magmatic plumbing system of a late Early Cretaceous intraplate volcano in NE China: Insights from geochemistry and phenocryst composition

The magmatic plumbing systems of intraplate volcanoes are key to understanding the structure of intraplate continental crust. A late Early Cretaceous volcano in NE China provides an opportunity to reconstruct the magmatic plumbing system of an intraplate paleo-volcano. ArAr and zircon SIMS UPb dating results indicate that olivine basalts formed at 102.81 ± 0.17 Ma and hornblende andesitic rocks formed at 101.1 ± 1.1 Ma–102.84 ± 1.2 Ma. Compared with coeval olivine basalts, the hornblende andesitic rocks have lower rare earth element (REE) abundance and parallel REE patterns, indicating that the basalts and andesitic rocks have independent origins. Olivine phenocrysts only occur in the basalts and are often zoned, with cores of Fo78–86 and rims of Fo62–68, the crystalline temperatures of olivine cores are 1204–1234 °C. The plagioclases from the basalt groundmass have An = 62–63. There are two types of plagioclase phenocryst in the andesitic rocks. One has An = 62–50, the other has An = 27–34.The former is normally zoned crystals (An = 62–50 from the core to rim) with high-An rims (60–63), similar in composition to plagioclase in the groundmass of the coeval basalt. They formed at temperatures of 1140–1162 °C and pressures of 4.7–7.0 kbar, implying that they were crystallized from basaltic melt with H2O contents of 0.60–0.82 wt%. The latter is low-An phenocrysts (27–34) with high-An (∼44) rims, suggesting that they were crystallized from felsic melt and that the injection of mafic magma had happened. The orthopyroxene phenocrysts only occur in andesitic rocks and have Mg# of 80–82, and are estimated to have crystallized at 6.7–8.5 kbar, and 1206–1222 °C, implying that they were crystallized from basaltic melt. The hornblende phenocrysts in andesitic rocks are pargasite, tschermakite, magnesio-hornblende, and magnesio-hastingsite and formed at 2.4–3.5 kbar and 883–957 °C. The estimated pressure and temperature of orthopyroxene and plagioclase (An>55) phenocrysts indicate that mafic magma chambers was located at depths of over 31.0 km, whereas those of hornblende phenocrysts show that felsic magma chamber was located at 8.9–12.8 km and has H2O content of 5.8–7.3 wt%. Using these data, we construct a new model of translithospheric magmatic plumbing system for intraplate volcanoes. Mantle-derived mafic magma chamber was emplaced at the crust–mantle boundary and causes partial melting of the lower crust, which in turn produces felsic magma that would be stored in a magma chamber in the upper crust. The mafic magma chamber at the crust-mantle boundary does not crystallize in-situ, as mafic magma could occasionally rise and be repeatedly injected and mix into the felsic magma chamber in the upper crust.

Water contents and hydrogen isotope compositions of amphibole in aillikites from the Tarim large igneous province, NW China: Insight into Earth’s deep water cycle

Water is known to play a crucial role in the generation of many large igneous provinces (LIP) worldwide, but the amount and origin of the water in their sources is still under debate. To address this question, this paper presents in situ major-, trace-element, and Sr isotope data coupled with bulk-mineral O-H-He isotope analyses of amphibole in a suite of aillikites from the Tarim LIP (NW China). The cores of zoned macrocrysts and microcrysts display partially overlapping compositions ranging between edenite and pargasite (75−83 versus 75−80 Mg#), which suggest a common origin from an evolving magma. The rims (Mg# = 75−80) of both macrocrysts and microcrysts are very similar to their cores for many elements, except for higher Sr and Ba contents. All the amphibole zones show similar primitive mantle−normalized trace element patterns, suggesting that they crystallized at different stages during magmatic evolution. This interpretation is confirmed by the homogenous Sr isotope compositions (87Sr/86Sr(i) = 0.70298−0.70394) of these amphiboles, which overlap with those of magmatic apatites and perovskites in these aillikites. The hydrogen isotope compositions (δD = −120‰ to −140‰) of the amphiboles are significantly lower than average mantle values. Given the low water contents (<0.66 wt%) of these minerals, the low H isotope signatures of the amphiboles might be caused by variable H2O loss during magma ascent. However, open-system Rayleigh fractionation modeling suggests that the hydrogen isotope compositions of these amphibole phenocrysts cannot be fully reproduced by crystallization following magmatic degassing. These low δD values require incorporation of recycled altered oceanic crust containing hydrous components in the plume source of these aillikites, which is consistent with the previously published moderately radiogenic He isotope ratios of olivine separates and bulk-rock Os and Pb isotope data for these same samples. We conclude that water in these magmas was derived from a plume source containing recycled water-bearing oceanic crust.

Composition of Phenocrysts in Lamproites of Gaussberg Volcano, East Antarctica

—This paper presents numerous new data on the geochemical composition of olivine, clinopyroxene, and leucite phenocrysts, as well as spinel inclusions in olivine and quench glass from lamproites of Gaussberg volcano (East Antarctica). Most of the olivine phenocrysts in the Gaussberg lamproites are high Mg varieties (Fo89–91) with elevated Ni contents (up to 4900 ppm) and high Ni/Co ratios. According to data of about 320 clinopyroxene analyses, two groups of diopsidic phenocrysts have been established. Group I consists mainly of high-Mg varieties (Mg#>80), while group II clinopyroxenes are less magnesian (Mg# 52–80). The main difference between the clinopyroxenes of the two groups is the elevated contents of Al2O3, FeO and reduced TiO2, Cr2O3, and NiO in the compositions of group II compared to group I, as well as different contents of trace elements, which may reflect their crystallization from different types of primary melts. According to the study of ~550 grains of leucite phenocrysts in the Gaussberg lamproites, it was shown that they correspond to the ideal stoichiometry of leucite K[AlSi2O6] and are enriched in Na2O (0.05–0.35 wt %), but depleted in K2O (19.9–20.9 wt %) compared to leucites from lamproites of other provinces. The BaO content reaches 0.3 wt %, SrO –0.04 wt %. The iron content in most leucite phenocrysts varies within 0.7–1.2 wt % Fe2O3, but some grains have the low Fe2O3 contents (<0.5 wt %). In leucite microlites of the groundmass and rims of phenocrysts, the Fe2O3 content can reach 2.4 wt %, which may indicate more oxidized conditions at lava eruption. Based on the study of natural samples, existing experimental data and numerical models, the order and conditions of crystallization of the Gaussberg lamproites were obtained. Crystallization proceeded in the following order: chromian spinel → chromian spinel + olivine → olivine + leucite (± chromian spinel) → olivine + leucite + clinopyroxene (± chromian spinel). The near-liquidus assemblage represented by high-Mg olivine phenocrysts with inclusions of Cr-spinel was formed in the temperature range from 1180 to 1250°C. Further crystallization of the melt with the formation of an association of olivine+leucite+clinopyroxene phenocrysts could occur at pressures below 2 GPa and temperatures of 1070–1180°C, corresponding to the presence of water in the magmatic system. Estimates of the redox conditions of crystallization of lamproites obtained using different oxybarometers vary in a wide range from QFM-0.5 to QFM+2.3. The elevated Ni contents in liquidus olivines of Gaussberg indicate the high nickel contents in the source. It is shown that the formation of ultra-alkaline magmas in the Gaussberg volcano area is likely related to melting of the continental lithosphere, which was heterogeneous and included both the peridotite mantle and hydrous pyroxenite fragments.

COMPOSITION OF PHENOCRYSTS OF LAMPROITE LAVA, GAUSSBERG VOLCANO, EAST ANTARCTICA

This paper presents numerous new data on the geochemical composition of olivine, clinopyroxene, and leucite phenocrysts, as well as spinel inclusions in olivine and quench glass from lamproites of the Gaussberg Volcano (East Antarctica). Most of the olivine phenocrysts in the Gaussberg lamproites are high Mg varieties (Fo 89–91) with elevated Ni contents (up to 4900 ppm) and high Ni/Co ratios. According to the data of about 320 analyzes of clinopyroxenes, two groups of phenocrysts belonging to the diopside group have been established. Group I consists mainly of high-Mg varieties (Mg#80), while group II clinopyroxenes are less magnesian (Mg# 52–80). The main difference between the clinopyroxenes of the two groups is manifested in the increased contents of Al2O3, FeO and reduced TiO2, Cr2O3, and NiO in the compositions of group II compared to group I, as well as different contents of trace elements, which may reflect their crystallization from different types of primary melts. According to the study of about 550 grains of leucite phenocrysts in Gaussberg lamproites, it was shown that they correspond to the ideal stoichiometry of leucite K[AlSi2O6] and are enriched in Na2O (0.05–0.35 wt %), but depleted in K2O (19.9–20.9 wt %) compared to with leucites from lamproites of other provinces. The BaO content reaches 0.3 wt %. %, SrO –0.04 wt. %. The iron content in most leucite phenocrysts varies within 0.7–1.2 wt % Fe2O3, with individual grains with low Fe2O3 contents (0.5 wt %). In microlites of leucite in the groundmass and rims of phenocrysts, the iron content can reach 2.4 wt % Fe2O3, which may indicate more oxidized conditions at the time of lava eruption.Based on the study of natural samples, existing experimental data and computational models, the order and conditions of crystallization of Gaussberg lamproites were restored. Crystallization proceeded in the following order: chrome spinel - chrome spinel + olivine - olivine + leucite (± chrome spinel) - olivine + leucite + clinopyroxene (± chrome spinel). The near-liquidus association, represented by high-Mg olivine phenocrysts with inclusions of Cr-spinel, was formed in the temperature range from 1180 to 1250°C. Further crystallization of the melt with the formation of an association of olivine+leucite+clinopyroxene phenocrysts could occur at pressures below 2 GPa and temperatures of 1070–1180°C, corresponding to the presence of water in the magmatic system. Estimates of the redox conditions of crystallization of lamproites, obtained using different oxybarometers, vary in a wide range from QFM-0.5 to QFM+2.3.Elevated Ni contents in liquidus olivines of Gaussberg indicate high nickel contents in the source. It is shown that the possible formation of ultraalkaline magmas in the region of Gaussberg Volcano occurred during the melting of the continental lithosphere, which was heterogeneous and included both the peridotite mantle and hydrous pyroxenite fragments.

Magma process of the ore-forming gabbrodiorite porphyry of the Meishan iron deposit, eastern China: Insights from the phenocrysts chemistry

Mesozoic iron deposits in the Ningwu volcanic basin in eastern China are genetically linked to the gabbrodiorite porphyries, which have long been regarded as the subvolcanic intrusive of the andesitic volcanics. Compared with abundant research conducted on the mineralization process, the magmatic process of the ore-forming intrusion has been poorly understood. This study performed petrographic and in situ analyses of representative phenocrysts from the ore-forming gabbrodiorite porphyry of the Meishan iron deposit to gain new insights into the magmatic process. The phenocrysts are composed of clinopyroxene and plagioclase, which commonly exhibit disequilibrium features, such as reverse zoning and sieved textures, suggesting that the gabbrodiorite porphyry evolved in an open-system andesitic-magma reservoir into which mafic magma was repeatedly recharged. The zoned clinopyroxene phenocrysts show consistent trace-element distribution patterns and a narrow range of 87Sr/86Sr of 0.7037–0.7049, which suggest that the recharged mafic magma was the primitive basaltic magma and crystal fractionation dominated the evolution from basaltic magma to andesitic magma. Pressure estimations based on clinopyroxene phenocryst compositions indicate that the magma reservoirs were emplaced at the uppermost mantle. Comparison of clinopyroxene phenocryst compositions between the Meishan gabbrodiorite porphyry and published andesitic volcanics shows distinct rare earth element distribution patterns, which don't support a volcanic–subvolcanic relationship between the gabbrodiorite porphyry and andesitic volcanics. Estimated pressures indicate that the primitive magma of andesitic volcanics evolved in lower crust reservoir, which was much shallower than the reservoirs of the ore-forming gabbrodiorite porphyry. Therefore, the Meishan gabbrodiorite porphyry was not the subvolcanic intrusive of the andesitic volcanics and probably represented an independent magmatic pulse. This understanding suggests that the exploration target of iron deposits in the Ningwu basin is not necessarily limited to the contact zone between gabbrodiorite porphyry and andesitic volcanics.

Conditions and Magmas Sources of Summit and Flank Eruptions of the Klyuchevskoy Volcano in 2020–2021: Isotope (Sr-Nd-Pb-O)-Geochemical Data

In 2021 a unique event occurred on Klyuchevskoy volcano (Kamchatka). After more than 30 years dominance of terminal eruptions a flank vent was formed. It was named after corresponding member of the Academy of Sciences USSR G.S. Gorshkov. The eruption began immediately after the end of the summit crater activation in 2020–2021 at an altitude of 2850 m in the northwestern part of the volcano, where no manifestations of flank volcanism were observed earlier. This paper presents geochemical and isotopic Sr-Nd-Pb-O data for lavas of the summit and flank eruptions of the Klyuchevskoy volcano in 2020–2021. A comparative petrographic analysis was carried out and the chemical composition of the Ol, Cpx, Pl phenocrysts of these lavas was analyzed. The lavas of both eruptions are alumina andesitic basalts of normal alkalinity. Variations of basic oxides in the lavas of the summit eruption and in the lavas of the G.S. Gorshkov vent are SiO2 53.1–53.2 wt. % and 51.6–53.2 wt. %, MgO 5.6 wt. % and 5.5–6.0 wt. %; respectively. Estimates of temperatures and pressures of plagioclase crystallization showed that for lavas of the summit eruption they were 1210–1118°С and 12.3–3.6 kbar, for lavas of the flank vent 1203–1119°С and 9.0–3.3 kbar. The content of the main elements, the proximity of the conditions of plagioclase generations, and the features of variations in the chemical composition of Ol, Cpx, and Pl phenocrysts in the lavas of both eruptions indicate the genetic relationship of the magmas that fed the summit and flank eruptions. The lavas of the summit eruptions of 2016, 2020–2021, as well as the lavas of the previous summit eruptions of the Klyuchevsky volcano, are characterized by highly sustained Sr-Nd-Pb isotopic characteristics: 87Sr/86Sr = 0.703625–0.703626, 143Nd/144Nd = 0.513085–0.513102, 206Pb/204Pb = 18.3148–18.3179). Isotopic ratios 207Pb/204Pb (15.5022–15.5107) and 208Pb/204Pb (37.9597–38.0143) are significantly higher for the lavas of the last summit and flank eruptions than for all Klyuchevskoy’s lavas of the past, which indicates more complex processes of magma evolution at crustal levels. The values of δ18O = 6.49–7.39 rel. units (SMOW) in the lavas of the considered eruptions are consistent with previously published data on the Klyuchevsky volcano. The lavas of the Gorshkov vent are enriched with Ba, Zr, Sr and other incompatible elements at constant values of MgO in comparison with the lavas of the last summit eruptions, which indicates differences in the ways of evolution of their magmas. Sharply increased values of the 87Sr/86Sr ratio (0.703673–0.703743) in the lavas of the G.S. Gorshkov vent, not previously observed in the lavas of the Klyuchevsky volcano, testifies to intensive assimilation of crustal matter by the initial melts in the northwestern part of the volcano.

Major and Trace Element Compositions of Clinopyroxene Phenocrysts in Altered Basaltic Rocks from Yüksekova Complex within Bitlis Suture Zone (Elazığ, Eastern Turkey): Implications for the Tholeiitic to Calc-Alkaline Magmatism

This paper presents major and trace element compositions of the clinopyroxene phenocrysts from variably altered basaltic rocks from the Yüksekova Complex from E Anatolia (Elazığ) to reveal the geochemical affinity of the basaltic magmatism and the intensive parameters during magmatic crystallization. The Yüksekova Complex crops out over extensive areas to the north of the Bitlis suture. The investigated samples come from an area of ca. 2500 km2 in the Elazığ province. Petrographically, the basaltic rocks are represented by basalt, porphyric basalt and dolerite. The rocks are characterized by plagioclase, clinopyroxene, magnetite and ilmenite and are devoid of any hornblende and biotite, whereby clinopyroxene and plagioclase form phenocrysts. Generally, the groundmass is variably altered, whereas clinophenocrysts are comparatively fresh. The clinopyroxene is represented by the composition Wo37–48En43–53Fs4–17, with Mg/(Mg + Fe2+) ratios of 0.93–0.73. Compositionally, the clinopyroxenes resemble those from island arc tholeiitic, calc-alkaline and anorogenic tholeiitic rocks. Clinopyroxene-melt geothermobarometry yielded temperatures of 1146 and 1193 °C and pressures of 1.4 to 7.0 kbar, suggesting that the clinopyroxene phenocrysts started to grow at variable crustal depths. There is no apparent difference of magma temperatures between different magma affinities. The Fe–Mg partition coefficient between clinopyroxene and melt (KD Fe−MgCpx-Melt = 0.27) suggests that the parental magmas in equilibrium with clinopyroxene were mostly primitive, with Mg# of 65–79. The REE patterns of parental melts in equilibrium with clinopyroxene phenocrysts resemble those of low-K tholeiitic to middle- to high-K calc-alkaline rocks from the magmatic arcs. The results of this study are mostly consistent with the inferences from the whole-rock geochemistry, and indicate that the major and trace element geochemistry can be effectively used to infer the geochemical affinities of the highly altered basaltic rocks.

Olivines and Their Melt Inclusions in Potassic Volcanic Rocks Record Mantle Heterogeneity beneath the Southern Tibet

Abstract Constraints on mantle heterogeneity are important for understanding geochemical differentiation and recycling in Earth’s interior. Alkali basalts are of particular interest in this context due to the clear signatures of recycled components that they yield. Previous studies have focused more on oceanic basalts than on continental mafic rocks because the chemical and isotopic characteristics of the latter are often modified by crustal contamination; however, high-forsterite olivine crystals and olivine-hosted melt inclusions may record the compositions of primitive melts before extensive crustal contamination, providing valuable insights into mantle heterogeneity. Here we report the results of high-precision major-, minor-, and trace-element and Pb isotope analyses of olivine phenocrysts and melt inclusions from the Sailipu potassic alkali basalts from southern Tibet, which constrain the mantle source of continental potassic alkali basalts. The minor- and trace-element compositions of the olivine phenocrysts (e.g. Ni and Ca contents and 100 × Mn/Fe, 10 000 × Zn/Fe, and Ni/Co ratios) suggest that the primary magmas were derived from a phlogopite-bearing pyroxenite source in which peridotite residues were also present. The low- and high-Li contents of the olivine phenocrysts suggest the recycling of continental crustal material in their mantle source. Comparing the major-element compositions (e.g. SiO2, TiO2, Al2O3, MnO, CaO, and Na2O contents), FCMS and FCKANTMS parameters, and pseudo-ternary projections of the primary Sailipu magmas with experimental data suggests that the primary magmas were partial melts of olivine-poor or carbonated olivine-poor lithologies with a small amount of peridotite residue. The negative Nb and Ta and positive Pb anomalies seen in primitive mantle-normalized trace-element patterns and the extremely low Nb/U ratios of the melt inclusions suggest that recycling of continental crust was the main process involved in enriching the mantle source of the Sailipu magmas. The heterogeneous Pb isotopic compositions of the melt inclusions (208Pb/206Pb = 2.089 – 2.158; 207Pb/206Pb = 0.851 – 0.858) are consistent with the presence of peridotite residue in this source. The subduction of Indian continental crust introduced enriched material (including carbonatites) into the mantle.

Partitioning Behaviors of Cobalt and Manganese along Diverse Melting Paths of Peridotitic and MORB-Like Pyroxenitic Mantle

Abstract The Co, Mn, Fe, and Ni contents of olivine phenocrysts and host basalts are sensitive to source mantle lithology, which suggests they may be used to constrain the processes of mantle melting and identify basalts formed from non-peridotitic (i.e. pyroxenitic) mantle sources. Here, we use a new comprehensive, forward model involving multiple parameters to simulate partitioning of Co and Mn during partial melting of the mantle in different tectonic settings: (1) polybaric continuous melting of peridotite mantle in mid-ocean ridges can generate melts that show decreasing Co and Mn with increasing degrees of melting so that the mid-ocean ridge basalts (MORBs) contain ~39–84 μg/g Co and ~900–1600 μg/g Mn; (2) flux-melting of the mantle wedge in subduction zones tends to produce a melt that has Co increasing from ~24 to 55 μg/g and Mn from ~500 to 1110 μg/g with increasing temperature; (3) melts produced by isobaric melting of the subcontinental lithospheric mantle are also sensitive to increasing temperature and have ~35–160 μg/g Co and ~800–2600 μg/g Mn; (4) decompression melting of peridotite related to the mantle plume generates melts containing ~45–140 μg/g Co and ~1000–2000 μg/g Mn, and the abundances of these metals decrease with increasing degrees of melting; and (5) partitioning behaviors of Co, Mn, and Ni during decompression melting of MORB-like pyroxenite contrast with those during decompression melting of peridotite due to the different mineralogy and compositions in mantle lithologies, and the MORB-like pyroxenite-derived melt is metal-poor with ~25–60 μg/g Co, ~290–1600 μg/g Mn, and ~160–340 μg/g Ni. Although high-Ni, low-Mn forsteritic olivine phenocrysts and high melt Fe/Mn ratio have been proposed as diagnostic indicators of pyroxenitic components in the mantle, our models show that these features can be also generated by melting of peridotite at greater depth (i.e. a high pressure and temperature). To quantify the effect of high-pressure melting of peridotite on these diagnostic indicators, we modeled the correlations of melt Fe/Mn and olivine Co, Mn, and Ni contents with melting depth along the decompression melting path of a thermal plume. When Fe/Mn ratios of basalts and/or compositions of olivine phenocrysts deviate significantly from our modeled correlation lines, high-pressure melting of peridotite cannot explain these data, and the existence of pyroxenitic component in the mantle source is likely required. The pyroxenite-derived melt is modeled to be Ni-poor, but mixing with a peridotite-derived melt can strongly increase the partition coefficient of Ni between olivine and mixed melt, resulting in the generation of high-Ni olivine phenocrysts in plume-associated magmatic suites.

Binary mixing of lithospheric mantle and asthenosphere beneath Tengchong volcano, SE Tibet: evidence from noble gas isotopic signatures

ABSTRACT The Miocene–Quaternary Tengchong volcanic field in SE Tibet was generated after the main stage of continental collision between India and Eurasia. Consensus on the origin of Tengchong volcanism has not yet been achieved. In this study, we analysed the He-Ar isotopic compositions of olivine and pyroxene phenocrysts, whole-rock major and trace elements and Sr-Nd-Pb isotopic compositions. The 3He/4He ratios range from 4.1 to 8.2 Ra (Ra = 1.4 × 10−6), overlapping the values of mid-ocean ridge basalt (7.0–9.0 Ra) and subcontinental lithospheric mantle (SCLM, 5.2–7.0 Ra). The low 3He/4He (< 7.0 Ra) basalts have high 87Sr/86Sr (average of 0.707928) and La/Yb (average of 22.0) ratios and low Nb/La (average of 0.36) ratios, whereas the high 3He/4He (> 7.0 Ra) basalts exhibit relatively low 87Sr/86Sr (average of 0.706708) and La/Yb (average of 17.0) ratios and relatively high Nb/La (average of 0.52) ratios. These observations indicate that the primitive magmas originated from a mixture of metasomatized SCLM and enriched asthenospheric mantle. The SCLM was likely metasomatized by the subducted Neo-Tethyan oceanic plate, while the asthenospheric mantle was enriched by the subducting Indian oceanic plate. The increasing trend of 3He/4He ratios and decreasing trend of 87Sr/86Sr ratios over time suggest that the contribution of the metasomatized SCLM decreased after the late Pleistocene relative to that of the enriched asthenosphere, reflecting progressive lithospheric extension and thinning. Our results reveal that magmatic He isotopes can be used to constrain deep dynamic processes.

Platinum-group elements in Late Quaternary high-Mg basalts of eastern Kamchatka: Evidence for minor cryptic sulfide fractionation in primitive arc magmas

The geochemical variations of magmas across and along supra-subduction zones (SSZ) have been commonly attributed to profound changes in the phase and chemical compositions of the mantle source and subduction-derived melt and fluid fluxes, as well as the physical parameters (e.g. depth, temperature, oxygen fugacity etc) of slab dehydration, mineral breakdown and melting. Here we test the variability of the Late Quaternary primitive magmas in the southern and northern parts of the meridionally oriented Eastern Volcanic Belt (EVB) of Kamchatka, with a slab depth varying from 60 to 160 km. Eight high-Mg (Mg# > 60 mol%) basalts were characterized for major, trace and platinum-group element (PGE) abundances, as well as the compositions of olivine phenocrysts and olivine-hosted spinel inclusions. The basalts in our study are geochemically typical of SSZ magmas and contain similar liquidus assemblages of forsteritic olivine (Mg# 78–92 mol%), low-Ti Cr-spinel and clinopyroxene. Although the absolute abundances of major and trace elements, and their ratios, in the basalts fluctuate to some extent, the observed variability cannot be correlated with any of considered parameters in the geometry of the Kamchatka SSZ and conditions of melting. This unexpected result led to the evaluation of the platinum-group element (PGE) systematics against the lithophile and chalcophile trace element geochemistry and the compositions of phenocrysts. Total whole-rock PGE content varies from 2.3 to 11.7 ppb, whereas the normalized PGE concentration patterns are typical for supra-subduction zones magmas and broadly similar in all studied samples. They are enriched in Rh, Pd and Pt relative to mid-ocean ridge basalts (MORB) and have nearly identical concentrations of Ir-group PGE. The only parameter that correlates well with PGE contents is the average Mg# of olivine phenocrysts from 84 to 90.3 mol%. This is interpreted to result from minor cryptic fractionation of sulfide melt, together with primitive olivine, in low-to-mid crustal conditions. Negative Ru anomalies on chondrite-normalized diagrams correspond to the Fe2+/Fe3+ ratios in spinel (a proxy for magma redox conditions), which reflects a replacement of monosulfide solid solution by laurite in the mantle wedge during oxidation.

An eclogitic component in the Pitcairn mantle plume: Evidence from olivine compositions and Fe isotopes of basalts

Subducted oceanic crust can transform into eclogite in the upper mantle, and generate chemical heterogeneity of mantle plumes, as recorded by elemental and radiogenic isotopic variations in oceanic island basalts (OIBs). The secondary pyroxenite produced by the reaction between eclogite-derived melt and peridotite is increasingly regarded as a major source component of OIBs, as well as peridotite. However, it remains unclear whether eclogite can be a direct source component of OIBs. To test this possibility, we present high-precision whole-rock Fe isotopes and the chemical compositions of olivine phenocrysts from well-characterized EM1-type basalts from Pitcairn Island. The Pitcairn basalts are characterized by moderate 87Sr/86Sr, low 143Nd/144Nd and 206Pb/204Pb isotopic ratios, and lowest δ26Mg values among OIBs, suggesting a contribution from recycled ancient crustal components (oceanic crust plus sediment). For comparison, we also report the Fe isotope compositions of FOZO-type basalts (with low 87Sr/86Sr, moderately high 143Nd/144Nd and moderate 206Pb/204Pb ratios) from the Louisville hot spot track, which were suggested to be a typical peridotite-derived OIB. The Louisville basalts have MORB-like δ57Fe values (0.06‰–0.15‰), whereas the Pitcairn lavas have substantially heavier Fe isotopic compositions (δ57Fe = 0.17‰–0.31‰) than MORBs. Quantitative evaluations suggest that magmatic differentiation, partial melting, and elevated oxygen fugacity in the source are insufficient to generate the heavy Fe isotopic compositions of the Pitcairn basalts.A good correlation between δ57Fe and εNd(i) (or δ26Mg) values in the Pitcairn basalts indicates binary mixing, and the melts derived from the EM1 endmember have unusually heavy Fe and light Mg isotopic compositions. In order to explain the origin of the Fe and Mg isotopic compositions, we calculated the Fe, Mg, and Nd isotopic compositions of partial melts of eclogite, secondary pyroxenite and peridotite, respectively. The results indicate that eclogite is the only suitable candidate to generate melts with both heavy Fe and light Mg isotopes. This inference is strengthened by the major and minor elemental compositions of olivine phenocrysts from the Pitcairn basalts, which show low Fo (73.2–82.5) and Ni contents (620–1949 ppm), and low Mn/Fe (0.011–0.015) and Ni/(Mg/Fe) (524–1041) ratios. These characteristics are markedly different from those of olivines from MORBs and the Koolau lavas from Hawaii, which represent phenocrysts that equilibrated with peridotite-derived and secondary pyroxenite-derived melts, respectively. We therefore argue that eclogite is the source lithology of the EM1 endmember of the Pitcairn basalts. Binary mixing between our modelled eclogite- and peridotite-derived melts produced magmas with relatively low Mg# value, Mg/Fe ratios and moderate Ni contents characteristics, which are preserved in the low-Fo Pitcairn olivines. Our results highlight that, in addition to peridotite and secondary pyroxenite, eclogite may survive in mantle plumes at shallow depths and make a substantial contribution to the source of OIBs.

Decoding multiple zoning patterns in clinopyroxene phenocrysts at Vulcano Island: A record of dynamic crystallization through interconnected reservoirs

Here we document how the different growth features and intracrystalline distributions of both major and trace cations in clinopyroxene phenocrysts are important recorders of the intricate magma dynamics at Vulcano Island (Aeolian Arc, Italy). The compositions of clinopyroxene phenocrysts from products erupted over the last ~54 ka cluster at different degrees of evolution, paralleling the polybaric-polythermal differentiation of mantle-derived mafic magmas into more evolved silicic melts. The hotter lower crust is the most favorable location for the storage of mafic magmas and the early crystallization of diopsidic (Mg#91) clinopyroxene (Pmax ≈ 750 MPa and Tmax ≈ 1220 °C). Diopsidic phenocrysts are depleted in both rare earth elements (REE) and high field strength elements (HFSE) but are enriched in transition elements (TE). The transfer and accumulation of primitive magmas in the colder upper crustal regions lead to the formation of an interconnected series of more differentiated magmatic reservoirs (P ≈ 100–450 MPa and T ≈ 1100–1180 °C) hosting discrete populations of clinopyroxene (Mg#84–85) with a broad spectrum of zonations and dissolution features. Recharge bands in clinopyroxene are markers of multiple inputs of primitive REE-HFSE-poor, TE-rich magmas from depth. Augitic phenocrysts (Mg#82) with strong negative Eu anomaly and REE + HFSE enrichments crystallizes from highly differentiated trachytic and rhyolitic melts stored at very shallow crustal conditions (P ≤ 50 MPa and T ≤ 1100 °C). These silicic reservoirs represent residual melts trapped-extracted from crystal-dominated mush regions in the uppermost part of the plumbing system. The residence time of clinopyroxene increases from ∼0.1 to ∼44 years from basalt to rhyolite, together with an increasing number of recharge bands. The mineral assemblage in more silicic and viscous mush melts is sufficiently resilient to record numerous mafic injections and high degrees of magma mixing, hybridization, and crystallization before eruption. Overall, the compositional zoning pattern of clinopyroxene presents a picture of plumbing system that extends through the crust and is characterized by distributions of melts and crystals which are progressively more evolved and heterogeneous in both space and time.

The Origin of Late Cenozoic Magmatism in the South China Sea and Southeast Asia

AbstractBasaltic lavas sample recycled crustal materials from their mantle source. Constraining the location and residence time of these recycled materials in the mantle is critical to understand global mantle dynamics. In this study, we present new whole‐rock major and trace element abundances, Sr‐Nd‐Mo‐Os isotopes, water contents and He isotopes of volcanic glasses, U‐Pb ages of zircons, and compositions of melt inclusions, spinels and olivines from the South China Sea (SCS) seamounts lavas. These new data are compared with literature data from intraplate volcanism of similar age from Southeast (SE) Asia. The isotope data of late Cenozoic lavas from the SCS seamounts and SE Asia can be explained by mixing between enriched mantle 2 (EM2) and depleted mid‐ocean ridge basalt mantle components. Our data are consistent with the EM2 signature of late Cenozoic lavas derived from recycled young oceanic crust and sediments. The compositions of olivine phenocrysts indicate an olivine‐dominated (peridotitic) mantle source. There is currently no evidence for a high‐3He/4He mantle plume component beneath the SCS. Our results combined with geophysical data and plate reconstructions suggest that the late Cenozoic magmatism is related to the upwelling of instabilities from the mantle transition zone (MTZ) triggered by a stagnant slab. The SCS seamount lavas sample an enriched MTZ containing young recycled materials, consistent with regional past subduction. Our study provides additional evidence that storage and recycling of crustal materials in or near the MTZ is an important mechanism to develop global mantle heterogeneities sampled by intraplate volcanoes.

Enrichment Nature of Ultrapotassic Rocks in Southern Tibet Inherited from their Mantle Source

Abstract Post-collisional ultrapotassic rocks (UPRs) in the Tibetan Plateau exhibit extreme enrichment in incompatible elements and radiogenic isotopes. Such enrichment is considered to be either inherited from a mantle source or developed during crustal evolution. In this study, to solve this debate we combined mineral textures and in situ geochemical composition of clinopyroxene phenocrysts in UPRs from southern Tibet to reveal their crustal evolution, enrichment cause and constrain metasomatism in their mantle source. Results show that the UPRs experienced an array of crustal processes, i.e., fractional crystallization, mixing, and assimilation. Fractional crystallization is indicated by decreases in Mg# and Ni and enrichment in incompatible elements (e.g. rare earth element (REE), Sr, Zr) toward the rims of normally zoned clinopyroxene phenocrysts (type-I). Magma mixing is evidenced by the presence of some clinopyroxene phenocrysts (type-II, -III) showing disequilibrium textures (e.g. reversed and overgrowth zoning), but in situ Sr isotope and trace element analysis of those disequilibrium zones indicate that late-stage recharged mafic magmas are depleted (87Sr/86Sr: 0.70659–0.71977) compared with the primitive ultrapotassic magmas (87Sr/86Sr: 0.70929–0.72553). Assimilation is revealed by the common presence of crustal xenoliths in southern Tibetan UPRs. Considering the much lower 87Sr/86Sr values (0.707759–0.709718) and incompatible element contents of these crustal xenoliths relative to their host UPRs, assimilation should have resulted in geochemical depletion of southern Tibetan UPRs rather than enrichment. The diluting impact of both assimilation and mixing is also supported by the modeling results based on the EC-E′RAχFC model combining the growth history of clinopyroxene. Trace elements ratios in clinopyroxenes also imply that the mantle source of southern Tibetan UPRs suffered an enriched and carbonatite-dominated metasomatism. Thus, we conclude that enrichment of southern Tibetan UPRs was inherited from the mantle source.

Clarifying source assemblages and metasomatic agents for basaltic rocks in eastern Australia using olivine phenocryst compositions

Many Cenozoic basaltic rocks in Eastern Australia exhibit an age-progressive trend from north to south, leading to the suggestion that one or more mantle plumes passed beneath the Australian plate. Trace element patterns indicate that the source regions have been metasomatised by infiltrating melts, but the source rock assemblages have never been closely identified. Here, trace element analyses of olivine and whole rock geochemistry for several occurrences in New South Wales (Dubbo, Ebor, Bingara/Inverell, and Barrington) are combined with samples from Buckland (Queensland) to characterize the mineralogy of the source and identify the nature of the melts that caused the metasomatic enrichment. According to Ni/Mg against Mn/Fe and Zn/Fe ratios in olivines, Zn/Fe and FC3MS (FeOT/CaO-3*MgO/SiO2) parameters in whole rocks, tholeiite, alkali basalt, and basanite rich in olivine xenocrysts from Dubbo were derived from pyroxenite-dominated mixed source, mixed pyroxenite + peridotite source, and peridotite-dominated source, respectively. Similarly, basalts from Ebor and Bingara/Inverell are suggested to originate from a mixed pyroxenite + peridotite source based on their high FC3MS values. In contrast, the source of basanite and picrobasalt from Barrington was peridotite-dominated with less pyroxenite. High Li and Zn in olivines, high P2O5/TiO2 and Zr/Hf at low Ti/Eu in whole rocks illustrate that the pyroxenite sources of basanites from Bingara/Inverell, Barrington, Dubbo and Buckland resulted from variable degrees of carbonatitic metasomatism. Partial melting of peridotite metasomatised by carbonatite melts at around the spinel-garnet peridotite transition depth produced basalts and basanites from Dubbo, Ebor, Bingara/Inverell, Barrington and Buckland. Carbonatitic metasomatism is widespread in the eastern Australian mantle lithosphere, concentrated on the ledge of the thick lithosphere beneath the Australian continent that stretches from Queensland, through New South Wales to Victoria.

The Volsci Volcanic Field (central Italy): eruptive history, magma system and implications on continental subduction processes

Here, we report on the Quaternary Volsci Volcanic Field (VVF, central Italy). In light of new 40Ar/39Ar geochronological data and compositional characterization of juvenile eruptive products, we refine the history of VVF activity, and outline the implications on the pre-eruptive magma system and the continental subduction processes involved. Different from the nearby volcanic districts of the Roman and Campanian Provinces, the VVF was characterized by small-volume (0.01–0.1 km3) eruptions from a network of monogenetic centers (mostly tuff rings and scoria cones, with subordinate lava occurrences), clustered along high-angle faults of lithospheric depth. Leucite-bearing, high-K (HKS) magmas (for which we report for the first time the phlogopite phenocryst compositions) mostly fed the early phase of activity (∼761–539 ka), then primitive, plagioclase-bearing (KS) magmas appeared during the climactic phase (∼424–349 ka), partially overlapping with HKS ones, and then prevailed during the late phase of activity (∼300–231 ka). The fast ascent of primitive magma batches is typical of a tectonically controlled volcanic field, where the very low magma flux is a passive byproduct of regional tectonic strain. We suggest that the dominant compressive stress field acting at depth was accompanied by an extensional regime in the upper crust, associated with the gravity spreading of the Apennine chain, allowing the fast ascent of magma from the mantle source with limited stationing in shallow reservoirs.

A tale of five enclaves: Mineral perspectives on origins of mafic enclaves in the Tuolumne Intrusive Complex

AbstractThe widespread occurrence of mafic magmatic enclaves (mme) in arc volcanic rocks attests to hybridization of mafic-intermediate magmas with felsic ones. Typically, mme and their hosts differ in mineral assemblage and the compositions of phenocrysts and matrix glass. In contrast, in many arc plutons, the mineral assemblages in mme are the same as in their host granitic rocks, and major-element mineral compositions are similar or identical. These similarities lead to difficulties in identifying mixing end members except through the use of bulk-rock compositions, which themselves may reflect various degrees of hybridization and potentially melt loss. This work describes the variety of enclave types and occurrences in the equigranular Half Dome unit (eHD) of the Tuolumne Intrusive Complex and then focuses on textural and mineral composition data on five porphyritic mme from the eHD. Specifically, major- and trace-element compositions and zoning patterns of plagioclase and hornblende were measured in the mme and their adjacent host granitic rocks. In each case, the majority of plagioclase phenocrysts in the mme (i.e., large crystals) were derived from a rhyolitic end member. The trace-element compositions and zoning patterns in these plagioclase phenocrysts indicate that each mme formed by hybridization with a distinct rhyolitic magma. In some cases, hybridization involved a single mixing event, whereas in others, evidence for at least two mixing events is preserved. In contrast, some hornblende phenocrysts grew from the enclave magma, and others were derived from the rhyolitic end member. Moreover, the composition of hornblende in the immediately adjacent host rock is distinct from hornblende typically observed in the eHD. Although primary basaltic magmas are thought to be parental to the mme, little or no evidence of such parents is preserved in the enclaves. Instead, the data indicate that hybridization of already hybrid andesitic enclave magmas with rhyolitic magmas in the eHD involved multiple andesitic and rhyolitic end members, which in turn is consistent with the eHD representing an amalgamation of numerous, compositionally distinct magma reservoirs. This conclusion applies to enclaves sampled &amp;lt;30 m from one another. Moreover, during amalgamation of various rhyolitic reservoirs, some mme were evidently disrupted from a surrounding mush and thus carried remnants of that mush as their immediately adjacent host. We suggest that detailed study of mineral compositions and zoning in plutonic mme provides a means to identify magmatic processes that cannot be deciphered from bulk-rock analysis.