Partial Melting Of Peridotite Research Articles

Eocene mafic alkaline volcanism in north Central Iran; Signatures of development and subsequent melting of a pyroxenite mantle source

Volcanic rocks from the Miami-Davarzan area are basalts to trachyandesites interlayered with fossiliferous beds and bear significant implications on the Eocene geodynamic evolution of northern central Iran. The rocks constitute two distinct groups, a low-Nb and a high-Nb suite. The least evolved low-Nb suite includes clinopyroxene-olivine phyric samples and is characterized by higher CaO and lower Al2O3 as compared to peridotite partial melts. The high-Nb suite includes more evolved plagioclase-clinopyroxene phyric samples. Fractional crystallization modeling using least-squares mass balance calculations for major elements suggests that the high-Nb suite was formed from a low-Nb parental melt by differentiation of a clinopyroxene-dominated mineral assemblage. The Miami-Davarzan volcanic rocks are characterized by significant large ion lithophile elements (LILE) and light rare earth elements (LREE) enrichment and high field strength elements (HFSE) depletion, which are typical signatures of subduction-related settings. Immobile, highly to moderately incompatible elements such as NbTa, LaCe, PrNd are higher in the Miami-Davarzan volcanic rocks as compared to the typical magmatic arcs, which is an indication of slab melt involvement and metasomatism of the mantle source. The Sr-Nd-Pb isotopic data also confirm a common mantle source involving the addition of subducted sediments into a depleted mantle source for both the low-Nb and high-Nb suites. The geochemical characteristics such as high CaO, low Al2O3, and the isotopic data can be best explained by derivation of the low-Nb suite parental melt from a pyroxenite mantle source. The pyroxenite mantle source was probably developed as a result of interaction between shallow depleted mantle and slab plus sediment-derived melts. It is suggested that the extensional phase that dominated the northern central Iran during the Eocene prompted partial melting of a pyroxenite mantle source.

Extremely enriched lithospheric mantle–derived magmas in the central Lhasa Terrane, southern Tibet

ABSTRACT Plate tectonics plays a crucial role in the evolution of Earth’s continents and oceans. However, the impact of oceanic subduction and continental collision on the continental lithospheric mantle within collisional orogenic belts is still a matter of debate. To address this issue, we performed a thorough analysis of mafic rocks in the Nixiong area within the central Lhasa Terrane, southern Tibet. The mafic rocks are composed of diabases and basalts formed during syn-collisional period (~50 Ma). The diabases have low SiO2 and moderate MgO contents, and are enriched in large ion lithophile elements and depleted in high field strength elements. They show high 87Sr/86Sri (0.711451–0.712084) and low εNd(t) (−7.9 to −6.44), as well as enriched Hf-Pb isotope compositions (εHf(t) = -7.58 to −6.59; 206Pb/204Pbi = 18.59–18.71; 207Pb/204Pbi = 15.71–15.72; 208Pb/204Pbi = 39.33–39.53). Geochemical modelling suggests that the diabases were formed through the partial melting of peridotite at high temperatures and shallow depths, with significant addition of subduction-related components to the primary melts. The diabases probably originated from a metasomatized lithospheric mantle due to subduction of Neo-Tethyan slab. The basalts display similar SiO2 but lower MgO contents compare with the diabases. They are also characterized by extremely high 87Sr/86Sri (0.713332–0.713448), low εNd(t) (−10.32 to −9.96), and significantly enriched Hf-Pb isotope compositions (εHf(t) = -12.30 to −12.05; 206Pb/204Pbi = 18.81–18.85; 207Pb/204Pbi = 15.74–15.75; 208Pb/204Pbi = 39.81–39.88), corresponding to an extremely enriched lithospheric mantle. Integrated with the previous studies, the Nixiong mafic rocks exhibit the most enriched isotopic compositions among the arc- or syn-collision-related mafic rocks in southern Tibet. These extremely isotopically enriched components were most likely derived from the ancient continental lithospheric mantle beneath the central Lhasa Terrane. Our results provided unambiguous evidence that the pre-existing ancient continental lithospheric mantle could persist locally during the prolonged oceanic subduction and subsequent continental collision.

The estimated water content of the Abaga basalts (Inner Mongolia, China) and compositions of the recycled materials in the source: Implication for magma genesis

The Abaga volcanic rocks are intraplate basalts that formed in the late Cenozoic period and are located in eastern central Inner Mongolia. Previous studies have discussed the genesis of the studied basalts. However, limited information about subducted recycled materials (the water content, sediments, and melts of the subducted plate), which control the generation of basalts, in the studied basalts has been reported. In this study, the major element abundances and crystallization temperatures and pressures of clinopyroxenes have been reported to estimate the water content of the studied magma, which has been applied to the Wulanhada volcanic field. The calculated result displays that the water content in the Abaga melts are between 0.5% and 1.7% (standard error estimate: 0.45%; average value: 1.3%). These estimated values are in range of the Aershan–Chaihe basalts (0.19–2.7%), while it is higher than those of the mid-ocean ridge basalts (MORBs: 0.16–0.47%). The high-water content of the studied basalts may have a close relationship with the mantle transition zone, which has also been reported to contain high concentrations of H2O. Subduction related sediments and slab may contribute to high-water content of magma. In addition, the clinopyroxene-melt thermobarometer shows that clinopyroxenes generally crystallized at 1113–1246 °C at the crustal level (3.8–12.4 kbar), while olivines formed at 1243–1392 °C (assuming pressure is 13 kbar, H2O = 1.3%). We also studied whole-rock major and trace elements and Sr–Nd–Hf–O isotope compositions of the Abaga basalts. High (Ta/U)N and (Nb/Th)N values that are higher than 1, NdHf isotopic values that plot between sediments and oceanic crust, and low δ18O values (4.51–5.59‰) of the alkali basalts illustrate that a subducted oceanic slab may have been added to the studied mantle source. In addition, we decipher that sediments may also exist in the source with Ba/Th, Nb/La, Ce/Pb and Ba/La ratios and NdHf isotopic values locating between sediments and altered oceanic crust. We use sediments, depleted MORB mantle (DMM), and recycled oceanic crust to simulate mixing features among these endmembers, which demonstrates that 80% of depleted MORB mantle, 15% of altered oceanic crust, and 5% of global subducting sediment (GLOSS), with 4–5% partial melting of peridotite or pyroxenite is a proper result with which to interpret the genesis of the studied alkali basalts.

Quaternary post-collisional high Nb-like basalts from Bijar-Qorveh, NW Iran: A metasomatized lithospheric mantle source

The Quaternary Bijar-Qorveh mafic volcanic rocks (BQMVR) occur in the Sanandaj-Sirjan Zone (SSZ) of Iran, a segment of the Alpine-Himalayan Orogeny with a Gondwana-affinity basement. Phases 1 and 2 of BQMVR are silica-undersaturated (basanite, tephrite, and phonotephrite) and form a highly alkaline or HA suite. The last phase, phase 3, of BQMVR is silica-saturated (alkali olivine basalt and hawaiite) and is called the mildly alkaline or MA suite. BQMVR share well-known subduction-related negative Nb Ta anomalies, and their Nb abundances are high enough to be classified as ‘high-Nb like basalts’. However, they do not show geochemical characteristics of typical high-Nb basalts. Trace element abundances and patterns of the MA suite mimic garnet-bearing spinel peridotite partial melts, whereas the HA suite is too rich in incompatible trace elements to match normal mantle partial melts. Enriched trace element patterns of the HA suite are akin to hornblendite experimental partial melts, and are likely derived from a garnet-amphibole peridotite source. Sr-Nd-Pb isotopic characteristics of the BQMVR indicate a subcontinental lithospheric mantle affinity. The HA suite tends toward the EM1 array, whereas the MA suite follows the EM2 array. The highly radiogenic Sr Nd isotopes of the MA suite indicate significant crustal contamination. Lithospheric delamination beneath the SSZ that followed Arabia-Eurasia collision, triggered garnet-amphibole peridotite partial melting that interacted with 10–30% of the asthenospheric component to have formed the HA suite. Mantle melting at shallower lithosphere depths followed and produced the MA suite. MELTS modeling shows the BQMVR underwent fractional crystallization of clinopyroxene ± olivine. Whole-rock thermobarometric calculations suggest derivation of the primary melts at a depth of ~97 km, however the Cpx-thermobarometer indicates that such melts equilibrated at lower crust depths at a depth of 45 km close to the Moho. The present Quaternary volcanostratigraphic study provides substantial clues for major post-collisional alkaline high-Nb mafic volcanism bearing signatures of two distinct magmatic stages; an early phase (phases 1 and 2) rooted in a deeper garnet amphibole peridotite followed by phase 3 rooted in a garnet-poor spinel peridotite. • Quaternary alkaline mafic volcanism in western Iran is post collisional. • Highly alkaline (HA) suite is derived from juvenile amphibole rich mantle. • Mildly alkaline (MA) suite is derived from shallower, ancient lithospheric mantle. • EM1 and EM2 affinities of the HA and MA suites respectively comply with the SCLM. • MELTS and isotopes revealed FC and AFC roles for HA and MA suites respectively.

Carbon recycling efficiency in subduction zones constrained by the effects of H2O-CO2 fluids on partial melt compositions in the mantle wedge

The extent of CO2 transfer from subducting lithologies to the overlying mantle wedge in general and to the arc magma source regions in particular remains debated. The limit of CO2 transfer to the sub-arc mantle could be estimated if the effects of CO2 on the primary hydrous melt compositions of mantle wedge can be assessed in relation to the observed compositions of primitive arc magmas. Here we present new piston cylinder and multi-anvil experiments using Au75Pd25 and Au capsules on four depleted peridotite + H2O ± CO2 starting compositions with 3.5 wt.% H2O and XCO2 [=molar CO2/ (CO2+H2O)] of 0–0.17. Experiments were performed at 2–4 GPa and 1200°C to constrain how the presence of variable CO2 in slab-derived aqueous fluids affects the composition of peridotite partial melts. All experiments consisted of low degree melts (<10 wt.%) in equilibrium with olivine + orthopyroxene ± clinopyroxene. Melts at 2–4 GPa are basaltic for XCO2 of 0–0.10 and become SiO2-poor and CaO-rich at XCO2 > 0.10. Comparison between our experimental partial melt compositions with a global dataset of the most primitive arc magmas suggests that the upper limit of XCO2 in fluids inducing melting in mantle wedges is ∼0.10 at 2–4 GPa. We apply these new constrains to an H2O and CO2 mass balance model for subduction zones and estimate that at least 34–86% of CO2 entering subduction zones bypasses the sub-arc melt generation zone and is subducted to the convecting mantle, either carried by the slab or by the down-dragged limb of the mantle wedge directly above the slab.

Global trends in novel stable isotopes in basalts: Theory and observations

The geochemistry of global mantle melts suggests that both mid-ocean ridge basalts (MORB) and ocean island basalts (OIB) sample lithological and temperature heterogeneities originating in both the upper and lower mantle. Recently, non-traditional stable isotopes have been suggested as a new tool to complement existing tracers of mantle heterogeneity (e.g., major and trace elements, radiogenic isotopes), because mineral- and redox-specific equilibrium stable isotope fractionation effects can link the stable isotope ratios of melts to their source mineralogy and melting degree. Here, we investigate five stable isotope systems (Mg-Ca-Fe-V-Cr) that have shown promise in models or natural samples as tracers of mantle temperature and/or lithological heterogeneity. We use a quantitative model, combining thermodynamically self-consistent mantle melting and equilibrium isotope fractionation models, to explore the behaviour of the isotope ratios of these elements during melting of three mantle lithologies (peridotite, and silica-excess and silica-deficient pyroxenites), responding to changes in mantle mineralogy, oxygen fugacity, temperature and pressure. We find that, given current analytical precision, the stable isotope systems examined here are not predicted to be sensitive to mantle potential temperature variations through equilibrium isotope fractionation processes. By contrast, source lithological heterogeneity is predicted to be detectable in some cases in the stable isotope ratios of erupted basalts, although generally only at proportions of >10% MORB-like pyroxenite in the mantle source, given current analytical precision. Magnesium and Ca stable isotopes show most sensitivity to a garnet-bearing source lithology, and Fe and Cr stable isotopes are potentially sensitive to the presence of MORB-like pyroxenite in the mantle source, although the behaviour of Cr isotopes is comparatively under-constrained and requires further work to be applied with confidence to mantle melts. When comparing the magnitude and direction of predicted equilibrium isotopic fractionation of peridotite and pyroxenite melts to natural MORB and OIB data, we find that aspects of the natural data (including the mean Mg-Ca-Fe-V isotopic composition of MORB, the range of Mg-Ca isotopic compositions seen in MORB data, the mean Mg-Ca-Cr isotopic composition of OIB, and the range of Mg-V-Cr isotopic compositions in OIB data) can be matched by equilibrium isotope fractionation during partial melting of peridotite and pyroxenite sources – with pyroxenite required even for some MORB data. However, even when considering analytical uncertainty on natural sample measurements, the range in stable isotope compositions seen across the global MORB and OIB datasets suggests that kinetic isotope fractionation, or processes modifying the isotopic composition of recycled crustal material such that it is distinct from MORB, may be required to explain all the natural data. We conclude that the five stable isotope systems considered here have potential to be powerful complementary tracers to other geochemical tracers of the source lithology of erupted basalts. However, continued improvements in analytical precision in conjunction with experimental and theoretical predictions of isotopic fractionation between mantle minerals and melts are required before these novel stable isotopes can be unambiguously used to understand source heterogeneity in erupted basalts.

Partitioning of Fe2O3 in peridotite partial melting experiments over a range of oxygen fugacities elucidates ferric iron systematics in mid-ocean ridge basalts and ferric iron content of the upper mantle

Basalts and peridotites from mid-ocean ridges record fO2 near the quartz-fayalite-magnetite buffer (QFM), but peridotite partial melting experiments have mostly been performed in graphite capsules (~ QFM-3), precluding evaluation of ferric iron’s behavior during basalt generation. We performed experiments at 1.5 GPa, 1350–1400 °C, and fO2 from about QFM-3 to QFM+3 to investigate the anhydrous partitioning behavior of Fe2O3 between silicate melts and coexisting peridotite mineral phases. We find spinel/melt partitioning of Fe2O3 ({D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{spl}/mathrm{melt}}) increases as spinel Fe2O3 concentrations increase, independent of increases in fO2, and decreases with temperature, which is consistent with new and previous experiments at 0.1 MPa. We find {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} = 0.63 ± 0.10 and {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{cpx}/mathrm{melt}} = 0.78 ± 0.30. MORB Fe2O3 and Na2O concentrations are consistent with a modeled MORB source with Fe2O3 = 0.48 ± 0.03 wt% (Fe3+/ΣFe = 0.053 ± 0.003) at potential temperatures (TP) from 1320 to 1440 °C. The temperature-dependence of the {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{spl}/mathrm{melt}} function alone allows ~ 40% of the variation in MORB compositions. If we allow {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} and {D}_{mathrm{Fe}2mathrm{O}3}^{mathrm{opx}/mathrm{melt}} to also vary with temperature by tying them to spinel Fe2O3 through intermineral partitioning, then all the MORB data are within error of the model. Our model Fe2O3 concentration for the MORB source would require that the convecting mantle be more oxidized at a given depth than recorded by continental mantle xenoliths. Our result is supported by thermodynamic models of mantle with Fe3+/ΣFe = 0.03 that predict fO2 of ~ QFM-1 near the garnet-spinel transition, which is inconsistent with fO2 of MORB. Our results support previous suggestions that redox melting may occur between 200 and 250 km depth.

Partial melt generation and evolution of magma reservoir conditions at the Paka volcanic complex in Kenya: Constraints from geochemistry, petrology and geophysics

In this study, we determine magma generation and evolution of magma storage conditions beneath Paka volcano located in northern segment of the Kenya Rift. The surface geology of the area is characterised by five main sequences including pre-Paka formations and volcanic sequence 1–4, all built up by basalt, intermediate and trachyte rocks spanning 582–8 ka. Numerical trace element modelling suggests that the primary magma in Paka resulted principally from 5 to 10% garnet–peridotite partial melting and possibly 0.1–1.5% partial melting of spinel-lherzolite. The parental magma was probably held at the Moho (~30 km) and evolved by ~76% gabbro fractional crystallization to generate a liquid of similar chemistry as that of erupted Paka basalts. We propose that under moderately oxidized conditions (∆FMQ +1) the primary magma underwent a further ~70% fractional crystallization and ~ 20% assimilation of crustal syenitic material to generate Paka trachyte lavas. Basaltic and intermediate magmas were held at polybaric storage pressure conditions ranging between 1.4 and 7 kbar (5–25 km), at temperatures between 1086 and 1204 °C with relatively low water content of <1.8 wt%. The trachyte magma resided at low pressures, i.e., 0.3–2.5 kbar (1–9 km) at 901–980 °C with water contents of 3.5–5.1 wt%. Based on our data eruption source depths remained largely the same for a period spanning from 582 to 11 ka. Geophysical data show correlation with magma depths beneath Paka volcano.

Zircon U–Pb geochronology, Hf isotope composition, and petrochemical characteristics of Paleocene granitoids in the western Gangdese Belt, Tibet

Abstract —The research team studied the petrology, whole-rock geochemistry, zircon U–Pb age, and stable isotopic characteristics of the Rongguo Longba and Garongcuo granites of the Nuocang area to understand better the impact of Neo-Tethys ocean subduction and In-dia–Eurasia continental collision on Paleocene tectonomagmatic processes along the southern margin of the Gangdese Belt. The Rongguo Longba granite and Garongcuo granite porphyry formed at 61.86 and 62.17 Ma, respectively. The Nuocang granitoids are characterized by (1) high SiO2, NaO2, and Al2O3 contents and low FeOtot, MgO, and TiO2 contents; (2) LREE and LILE enrichment and HREE and HFSE (Nb, P, and Ti) depletion; and (3) obvious negative Eu anomalies. These features indicate that the Nuocang granites are of the high-K calc-alkaline and peraluminous granite types. Furthermore, their zircon Hf isotope characteristics suggest that the magma source region has an ancient crystalline basement. The basaltic andesitic crystal tuff is the product of garnet–peridotite partial melting and crust contamination from rising magma emplacement.

Pyroxene does not always preserve its source hydrogen concentration: Clues from some peridotite xenoliths

Water is key to many geodynamical processes in the Earth's upper mantle, yet its preservation in mantle minerals is still debated. To throw some light on this problem, we here carried out an integrated study of whole-rock and mineral chemistry, and hydrogen concentrations in olivine, orthopyroxene, and clinopyroxene within 18 spinel lherzolite samples from three localities (Lianshan, Panshishan, and Tashan) in the Nanjing area, eastern China. Whole-rock and mineral compositions suggest that the studied peridotite samples interacted with melt at different melt/rock ratios following various degrees of partial melting (up to 11%). Fourier transform infrared (FTIR) measurements show that olivine is almost dry (<1 wt ppm H2O) while the cores of orthopyroxene and clinopyroxene contain 14–151 wt ppm H2O and 41–218 wt ppm H2O, respectively. Profile analyses of >70 orthopyroxene grains, which are homogeneous in major-element compositions, covering all the studied samples show hydrogen-depleted rims, indicative of hydrogen diffusional loss. This hydrogen zonation is probably caused by hydrogen chemical diffusion controlled by the mobility of trivalent cations (most likely Al3+) in response to magma degassing or partial melting of peridotite during ascent, or interactions of peridotite with melt, or a combination of these processes. By contrast, no hydrogen zonation is observed in clinopyroxene. Based upon the comparison of chemical compositions (especially Fe and AlIV contents) of clinopyroxene within our samples with those in diffusion experiments, it is inferred that the hydrogen diffusivity in clinopyroxene should be larger than that in orthopyroxene from our samples. This inference points to that clinopyroxene within the studied samples must have experienced diffusional loss of hydrogen as well, suggesting that water concentrations in the lithospheric mantle beneath the study area are probably underestimated. Furthermore, it also implies that orthopyroxene instead of clinopyroxene most likely preserves the in-situ water concentrations at depth, at least at its core. The absence of hydrogen zonation in clinopyroxene can be attributed to its fine-grained nature and fast hydrogen diffusivity. Our FTIR data also show that Lianshan and Tashan samples have water concentration ratio between clinopyroxene and orthopyroxene (RCpx/Opx) of ∼2, similar to mantle xenoliths from eastern China and other localities worldwide, yet Panshishan samples have higher RCpx/Opx values (2.3–5.9). Since hydrogen loss is suggested for both pyroxenes, RCpx/Opx of ∼2 thus cannot be taken as a reliable indicator of preservation of original water concentration of mantle source and equilibrium partitioning of hydrogen between pyroxene, as opposed to previous suggestions.

SSZ Semail Ophiolite vs MORB Masirah Ophiolite: A Perspective from Podiform Chromitites

AbstractOne of the major topics of debate in ophiolite geology is the original tectonic setting of ophiolites. New studies show that most ophiolites are formed more frequently in a supra‐subduction zone (SSZ) environment and that only a very small number of ophiolites have formed in an oceanic range (MOR). The Masirah ophiolite is one of the few oceanic ridge ophiolites that have been preserved, and the evidence that was formed in a subduction environment is missing (Moseley and Abbotts 1979, Dilek and Furnes, 2011; Rollinson, 2017). Masirah Island, the Batain and Ras Madrah areas of eastern Oman are almost entirely composed of a well‐developed ophiolite, known as the Masirah ophiolite (Fig. 1), which is, however, completely unrelated to the nearby Semail Ophiolite in the northern Oman Mountains (Fig. 2). The Masirah ophiolite is Jurassic in age and represents oceanic lithosphere derived from the Indian Ocean, but is about 15–20 Myr later than emplacement of mid‐Cretaceous Semail ophiolite in northern Oman. The presence of basaltic to rhyolitic lavas of calc‐alkaline affinity and boninites in the lava sequence of the Semail ophiolite led several researchers to propose a back‐arc basin model for this ophiolite (e.g. Tamura and Arai, 2006; Godard et al., 2008; Rollinson and Adetunji, 2015).The Masirah Ophiolite shows close affinities with MORB peridotites in general. Most of the olivine from the Masirah harzburgites show Fo contents that are similar to those of olivine from MORB. Both pyroxenes in these harzburgites have similar Mg# values, Al2O3 and Cr2O3 contents to those of pyroxenes from MORB peridotites. The observed primitive mantle‐normalized REE patterns showing enrichment in LREEs indicate that the Masirah peridotites have been modified by fluids or melts enriched in LREEs in a MORB environment.Podiform chromitites housed in ophiolites today interpreted as magmatic deposits formed during the reaction of molten rock in environments spike in the middle of the ocean (MOR) or suprasubduccion zone (SSZ) (Arai and Matsukage, 1998; Rollinson and Adetunji, 2015). The Masirah chromitites has a mineral chemistry similar to the mineral chemistry of chromite crystallized from MOR magmas. The Cr# values of chromite in the Masirah chromatite are similar to those of MOR peridotites. These findings suggest that the ultramafic and mafic cumulate rock assemblages overlying the upper mantle peridotites in the Masirah ophiolite represent the products of magma evolution in a MOR initiation stage within the proto Indian Ocean.Coexisting high‐ and low‐Cr# associations of chromitite and dunite have been found in the Semail ophiolite, which illustrates the common situation of ophiolites having both SSZ and MOR geochemical signatures. Cr# varies from 40–60 for shallow chromite bodies, and over the range 70–80 for the deep locations. This diversity of chromitite types suggests two stages of magmatic activity were responsible for the chromitite genesis, in response to a switch of tectonic setting. The first is residual from lower degree, partial melting of peridotite, which produced low‐Cr# chromitites at the Moho transition zone, possibly in a mid‐ocean‐ridge setting. The second chromitite‐forming event involves higher degree partial melting, which produced high‐Cr# discordant chromitite in the upper mantle, possibly in a supra‐subduction zone setting.Assemblages of mono‐ and poly‐phase silicate inclusions (including olivine, orthopyroxene, clinopyroxene, amphibole, phlogopite, serpentine, native Fe, FeO, alloy, sulfide, calcite, laurite, celestine and halite) within chromite have been observed in the low Cr# podiform chromitites from the Semail and Masirah ophiolites. The existence of hydrous silicate inclusions in the chromite calls for a role of hydration during chromite genesis. High‐T bright green hornblende–edenite included in the chromites is evidence of the introduction of water in the magma at the end of the chromite crystallization. Such paragenesis points to the presence of hydrous fluids during the activity of the shear bands.

Fractionation of Ruthenium from Other Highly Siderophile Elements (HSEs) in Forearc Mantle

AbstractThe geochemical characteristics and behaviors of highly siderophile elements (HSEs) in forearc peridotites remain poorly constrained due to the scarcity of data. Here, we report HSE abundances of mantle peridotites from the New Caledonia ophiolites, a classical ophiolite generated in a forearc setting. Those peridotites show non‐chondritic, strongly fractionated HSE patterns and can be classified into two distinct types (namely Group I and Group II). Group I peridotites have higher HSE contents than Group II peridotites, which might be because intergranular sulfides were completely removed but sulfide inclusions were retained during partial melting of peridotites in a forearc environment, and meanwhile the distribution of sulfide inclusions are not uniform in mantle. Moreover, Group I peridotites display flat patterns from Os to Pt but strongly depleted in Pd, which resemble those patterns of some mantle wedge xenoliths. The Pt–Pd decoupling can be attributed to high degrees of partial melting. However, Group II peridotites are characterized by strongly positive Ru anomaly with highly super‐chondritic Ru/Os and Ru/Ir ratios. Such characteristics are the first reported cases for forearc peridotites. The fractionation of Ru from other HSEs might reflect the stability of refractory Ru‐rich phases in mantle wedge peridotites during different processes, e.g., partial melting and melt/fluid‐rock reactions.

High-temperature inter-mineral Cr isotope fractionation: A comparison of ionic model predictions and experimental investigations of mantle xenoliths from the North China Craton

Recent works have proposed that chromium (Cr) isotopes could be fractionated during peridotite partial melting and basaltic magma crystallization. However, until now, inter-mineral Cr isotope fractionation behavior for major mantle minerals has been poorly constrained. To investigate the mechanism and magnitude of equilibrium inter-mineral Cr isotope fractionation and to explore its implications for planetary mantle and crust lithology evolutions, we performed a systematic study of equilibrium Cr isotope fractionation between major mantle minerals by coupling theoretical ionic modeling with laboratory isotope analyses of natural samples and Cr valence determinations by X-ray absorption near edge spectroscopy (XANES). The ionic model considers both charges and coordination environments of Cr species in mantle minerals, which have proven to be critical factors affecting inter-mineral isotope fractionation. The ionic modeling results predict a general order of spinel (Spl) > pyroxene (Py) ≥ olivine (Ol) in 53Cr/52Cr. Systematic Cr isotope analyses of coexisting mantle minerals of seventeen peridotite xenoliths from Beiyan in the North China craton were performed. Chromium isotope results for different mineral pairs of lherzolites (e.g., Δ53CrSpl–Ol of 0.11‰ to 0.16‰, Δ53CrSpl–Py of 0.04‰ to 0.11‰ and Δ53CrPy–Ol of 0.05‰ to 0.10‰ at 870 °C to 970 °C), document measurable and systematic inter-mineral Cr isotope fractionation, in excellent agreement with the modeling results predicted with XANES-determined Cr2+/ΣCr values of the mineral separates. This fractionation order could essentially account for Cr isotope behaviors during partial melting and magmatic crystallization observed in terrestrial peridotites and lunar basalts.In contrast, we found that metasomatisms could influence Cr isotope compositions of minerals from Beiyan metasomatized clinopyroxene- (Cpx-) rich lherzolites and wehrlites by means of mineral–melt interaction and/or kinetic diffusion, leading to disequilibrium inter-mineral Cr isotope fractionation.

The behaviour of ferric iron during partial melting of peridotite

In order to better understand the behaviour of Fe3+ during partial melting of a mid-ocean-ridge-basalt (MORB) mantle source, we performed partial melting experiments of a spinel peridotite at 1.5 GPa, between 1320 and 1450 °C, and over a range of oxygen fugacities (fO2) varying from FMQ-3.5 to FMQ+6 (log-units relative to FMQ = Fayalite-Magnetite-Quartz oxygen buffer). Changes in fO2 were achieved using different capsule materials and by varying the oxidation state of the starting material. Fe3+ contents of spinels and glasses were examined using Fe K-edge X-ray Absorption Near Edge Structure (XANES) spectroscopy in full-field mode. Our results show that this method allows Fe3+/ƩFe measurements to be made of spinels in our experimental charges but is not suitable for quantifying Fe3+/ƩFe of glasses, particularly at relatively low fO2 where melt Fe2O3 contents are small. We therefore employed several methods (Fe3+/ƩFe ratios of spinels measured by XANES and calculated by stoichiometry, and Fe-alloy sliding redox sensors) to assess the fO2 of the samples. These were then used to calculate the Fe3+/ƩFe ratios of glasses using published models, which were further constrained by Mössbauer spectroscopy measurements on an additional set of partial melting experiments.For a particular initial redox state, the Fe3+/ΣFe ratio in melts remains approximately constant with partial melting degree. This is explained by a slight decrease of the bulk partition coefficient of Fe2O3 (DFe2O3peridotite-melt) as the degree of partial melting increases. The Fe3+/ΣFe ratios of resulting melts are thus restricted to a relatively narrow range over large degrees of melting, as observed for global MORB glass analyses. This observation is also confirmed by pMELTS calculations employing different bulk Fe3+/ΣFe ratios. We conclude that the Fe2O3 content in MORB can be accounted for through partial melting of a mantle source containing 0.1–0.5 wt% Fe2O3 and a bulk DFe2O3peridotite-melt ranging from 0.3 to 0.1.

Differential partial melting process for temporal variations of Shandong basalts revealed by melt inclusions and their host olivines

The link between the geochemical variability of lavas and their source lithologies is very important in understanding the nature of mantle sources and melting processes. The Cenozoic basalts of Shandong province are ideal targets for addressing this issue. Previous studies have revealed two different types of basalts in Shandong province. The first type includes older, slightly alkalic basalts (SAB) with EMI oceanic island basalt (OIB)-like trace element and isotope compositions, whereas the second type includes younger, highly alkalic basalts (HAB) with HIMU OIB-like trace element compositions, but depleted isotope compositions. However, the source lithologies and temporal variations of these basalts are still debated. In this study, we present data from olivine-hosted melt inclusions and whole rock samples for the Cenozoic Yujiagou basalts of Shandong province in the North China Craton. The Yujiagou basalts are alkalic basalts with trace element compositions more like those of EMI OIB, suggesting that the Yujiagou basalts are SAB. In contrast, the olivine-hosted melt inclusions have more variable compositions, straddling the compositional field between the SAB and the HAB. In particular, the melt inclusions exhibit variable Pb isotope compositions ranging from the HAB to the SAB, arguing for the coexistence of HAB and SAB melts in the Yujiagou basalts. Compared with olivines crystallized from partial melts of peridotite, olivines from the Yujiagou basalts show lower Ca concentrations, but higher Ni abundances and Fe/Mn ratios. Combined with the low CaO contents of the bulk rocks and melt inclusions, this evidence suggests that the Yujiagou basalts represent the products of partial melting of pyroxenite. The mantle potential temperature and crystallizing temperatures of primary magmas calculated based on melt inclusions within high Fo olivines (Fo=~90) are >100°C higher than those of the ambient mantle, suggesting a probable thermal anomaly in the mantle. Combined with experimental results, we deduce that the temporal change in Cenozoic basalt compositions in Shandong province is due to differential partial melting processes that could be attributable to a thermal anomaly.

Mantle metasomatism and magma formation in continental lithosphere: Data on xenoliths in alkali basalts from the Makhtesh Ramon, Negev desert, Israel

Mantle xenoliths (lherzolites, clinopyroxene dunites, wehrlites, and clinopyroxenites) in the Early Cretaceous volcanic rocks of Makhtesh Ramon (alkali olivine basalts, basanites, and nephelinites) represent metasomatized mantle, which served as a source of basaltic melts. The xenoliths bear signs of partial melting and previous metasomatic transformations. The latter include the replacement of orthopyroxene by clinopyroxene in the lherzolites and, respectively, the wide development of wehrlites and olivine clinopyoroxenites. Metasomatic alteration of the peridotites is accompanied by a sharp decrease in Mg, Cr, and Ni, and increase of Ti, Al, Ca contents and 3+Fe/2+Fe ratio, as well as the growth of trace V, Sc, Zr, Nb, and Y contents. The compositional features of the rocks such as the growth of 3+Fe/2+Fe and the wide development of Ti-magnetite in combination with the complete absence of sulfides indicate the high oxygen fugacity during metasomatism and the low sulfur concentration, which is a distinctive signature of fluid mode during formation of the Makhtesh Ramon alkali basaltic magma. Partial melting of peridotites and clinopyroxenites is accompanied by the formation of basanite or alkali basaltic melt. Clino- and orthopyroxenes are subjected to melting. The crystallization products of melt preserved in the mantle rock are localized in the interstices and consist mainly of fine-grained clinopyroxene, which together with Ti-magnetite, ilmenite, amphibole, rhenite, feldspar, and nepheline, is cemented by glass corresponding to quartz–orthopyroxene, olivine–orthopyroxene, quartz–feldspar, or nepheline–feldspar mixtures of the corresponding normative minerals. The mineral assemblages of xenoliths correspond to high temperatures. The high-Al and high-Ti clinopyroxene, calcium olivine, feldspar, and feldspathoids, amphibole, Ti-magnetite, and ilmenite are formed at 900–1000°. The study of melt and fluid inclusions in minerals from xenoliths indicate liquidus temperatures of 1200–1250°C, solidus temperatures of 1000–1100°C, and pressure of 5.9–9.5 kbar. Based on the amphibole–plagioclase barometer, amphibole and coexisting plagioclase were crystallized in clinopyroxenites at 6.5–7.0 kbar.

Europium and strontium anomalies in the MORB source mantle

Lower crustal recycling depletes the continental crust of Eu and Sr and returns Eu and Sr enriched materials into the mantle (e.g., Tang et al., 2015, Geology). To test the hypothesis that the MORB source mantle balances the Eu and Sr deficits in the continental crust, we carried out high precision Eu/Eu∗ and Sr/Sr∗ measurement for 72 MORB glasses with MgO >8.5% from the Pacific, Indian, and Atlantic mid-ocean ridges. MORB glasses with MgO⩾9wt.% have a mean Eu/Eu∗ of 1.025±0.025 (2 σm, n=46) and Sr/Sr∗ of 1.242±0.093 (2 σm, n=41) and these ratios are positively correlated. These samples show both positive and negative Eu and Sr anomalies, with no correlations between Eu/Eu∗ vs. MgO or Sr/Sr∗ vs. MgO, suggesting that the anomalies are not produced by plagioclase fractionation at MgO >9wt.% and, thus, other processes must be responsible for generating the anomalies. We term these MORB samples primitive MORBs, as they record the melt Eu/Eu∗ and Sr/Sr∗ before plagioclase fractionation. Consequently, the mean oceanic crust, including cumulates, has a bulk Eu/Eu∗ of ∼1 and 20% Sr excess.Considering that divalent Sr and Eu(II) diffuse faster than trivalent Pr, Nd, Sm, and Gd, we evaluated this kinetic effect on Sm–Eu–Gd and Pr–Sr–Nd fractionations during spinel peridotite partial melting in the MORB source mantle. Our modeling shows that the correlated Eu and Sr anomalies seen in primitive MORBs may result from disequilibrium mantle melting. Melt fractions produced during early- and late-stage melting may carry positive and negative Eu and Sr anomalies, respectively, that overlap with the ranges documented in primitive MORBs. Because the net effect of disequilibrium melting is to produce partial melts with bulk positive Eu and Sr anomalies, the MORB source mantle must have Eu/Eu∗<1.025±0.025 (2 σm) and Sr/Sr∗<1.242±0.093 (2 σm). Although we cannot rule out the possibility that recycled lower continental crustal materials, which have positive Eu and Sr anomalies, are partially mixed into the upper mantle (i.e., MORB source region), a significant amount of this crustal component must have been sequestered into the deep mantle, as supported by the negative 206Pb/204Pb–Eu/Eu∗ and 206Pb/204Pb–Sr/Sr∗ correlations in ocean island basalts.

Key new pieces of the HIMU puzzle from olivines and diamond inclusions.

Mantle melting, which leads to the formation of oceanic and continental crust, together with crust recycling through plate tectonics, are the primary processes that drive the chemical differentiation of the silicate Earth. The present-day mantle, as sampled by oceanic basalts, shows large chemical and isotopic variability bounded by a few end-member compositions. Among these, the HIMU end-member (having a high U/Pb ratio, μ) has been generally considered to represent subducted/recycled basaltic oceanic crust. However, this concept has been challenged by recent studies of the mantle source of HIMU magmas. For example, analyses of olivine phenocrysts in HIMU lavas indicate derivation from the partial melting of peridotite, rather than from the pyroxenitic remnants of recycled oceanic basalt. Here we report data that elucidate the source of these lavas: high-precision trace-element analyses of olivine phenocrysts point to peridotite that has been metasomatized by carbonatite fluids. Moreover, similarities in the trace-element patterns of carbonatitic melt inclusions in diamonds and HIMU lavas indicate that the metasomatism occurred in the subcontinental lithospheric mantle, fused to the base of the continental crust and isolated from mantle convection. Taking into account evidence from sulfur isotope data for Archean to early Proterozoic surface material in the deep HIMU mantle source, a multi-stage evolution is revealed for the HIMU end-member, spanning more than half of Earth's history. Before entrainment in the convecting mantle, storage in a boundary layer, upwelling as a mantle plume and partial melting to become ocean island basalt, the HIMU source formed as Archean-early Proterozoic subduction-related carbonatite-metasomatized subcontinental lithospheric mantle.

Petrogenesis of high-CaO lavas from Mauna Kea, Hawaii: Constraints from trace element abundances

The role of a mafic component in the petrogenesis of Oceanic Island Basalts (OIBs) is highly debated. As the best studied OIB, Hawaiian lavas provide critical insights into OIB genesis. At a given MgO content, the CaO content in the melt has been used to distinguish between partial melts of peridotite and garnet pyroxenite/eclogite. However, calculations using the BATCH program show that CaO contents in volatile-free melts saturated with all four phases, garnet, clinopyroxene, orthopyroxene and olivine, are controlled by both degrees of partial melting and source compositions, and low melt CaO content is not diagnostic of partial melts from garnet pyroxenite/eclogite. This is an important consideration in understanding the origin of high-CaO lavas recovered from the Hawaii Scientific Drilling Project (HSDP). Detailed geochemical and isotopic studies have been focused on the HSDP high- and low-SiO2 group lavas, and high-CaO lavas were not well studied because they were not included in the original reference suite samples. Here, we report trace element abundances obtained on a suite of high-CaO glasses and compared the trace element abundances of high-CaO lavas to those in high- and low-SiO2 lavas. When normalized to the average composition of low-SiO2 lavas, high-CaO lavas form a U-shaped trace element pattern, enriched in both the most incompatible (Nb, Th) and the least incompatible (Sc, V) elements. This compositional distinction is best explained if high-CaO parental magma represents a mixture of a low degree partial melt of the low-SiO2 mantle source with a high degree (>80%) partial melt derived from a mafic cumulate component. This mafic cumulate must be clinopyroxene-rich, and it could be delaminated mafic cumulate formed under arcs during continent formation, lower continental crust, recycled lower oceanic crust, or high pressure cumulates from a magma chamber.

Petrogenesis of Late Cenozoic basalts from North Hainan Island: Constraints from melt inclusions and their host olivines

Melt inclusions and their host olivines in basaltic lavas provide important information about the nature of their mantle source. We present the first analyzed chemical data of olivine-hosted melt inclusions in Cenozoic basalts from the North Hainan Island and report the discovery of both tholeiitic and alkalic melt inclusions in a single rock sample. Cenozoic basalts from the Hainan Island are predominantly tholeiites with only small amounts of alkali basalts. There is a much broader compositional variation in melt inclusions than whole rocks. Compared to partial melts of mantle peridotite, the Hainan basalts have lower CaO, Na2O/TiO2, CaO/Al2O3 and Co/Fe, and higher TiO2, FeO∗, Fe/Mn, Zn/Fe and Zn/Mn. The olivine phenocrysts from the Hainan basalts contain lower Ca and Mn, and higher Ni and Fe/Mn than those of olivines crystallized from partial melts of peridotite. Projections from or towards olivine into the plane CS-MS-A for melt inclusions and whole rocks with MgO >7.5wt% imply that the residual minerals in the source of the tholeiites are mainly clinopyroxene and garnet, possibly with some orthopyroxene, while in the source of the alkali basalts they are dominated by clinopyroxene and garnet. This indicates that a pyroxenite component could serve as the source lithology of the Hainan basalts. The OIB-like trace element compositions, with Ba, Sr, Nb and Ta positive anomalies, and Th and U negative anomalies, of the Hainan basalts suggest that a recycled oceanic crust component was involved in the source of the Hainan basalts. Based on a CMAS projection of primary magma compositions of the whole rocks and melt inclusions, we infer that a stage-2 silica-deficient pyroxenite derived from melt–peridotite reaction or mechanical mixing between recycled oceanic crust and peridotite can serve as the source lithology. Partial melts derived from such a source can match the overall compositions of the Hainan basalts better than those of MORB-eclogite and fertile peridotite. The compositional range from tholeiitic to alkalic basalts is ascribed to decreasing degrees of melting of a similar silica-deficient pyroxenite source, which is consistent with the lower incompatible trace element and REE abundances in the tholeiites compared to the alkali basalts. Both subalkaline and alkaline melt inclusions are present in a single sample, which indicates that magma mixing had occurred in the deep magma chamber prior to eruption below the Hainan Island.