Low-degree partial melting trends recorded in upper mantle minerals

The study of glass inclusions inside mantle minerals provides direct information about the chemistry of naturally occurring mantle-derived melts and the fine-scale complexity of the melting process responsible for their genesis. Minerals in a spinel lherzolite nodule from Grande Comore island contain glass inclusions which, after homogenization by heating, exhibit a continuous suite of chemical compositions clearly distinct from that of the host basanitic lava. The compositions range from silicic, with nepheline–olivine normative, 64 wt% SiO₂ and 11 wt% alkali oxides, to almost basaltic, with quartz normative, 50 wt% SiO₂ and 1–2 wt% alkali oxides. Within a single mineral phase, olivine, the inferred primary melt composition varies from 54 to 64 wt% SiO₂ for MgO content ranging from 8 to 0.8 wt%. An experimental study of the glass and fluid inclusions indicates that trapped melts represent liquids that are in equilibrium with their host phases at moderate temperature and pressure (T≈1230°C and P≈1.0 Gpa for melts trapped in olivine). Quantitative modelling of the compositional trends defined in the suite shows that all of the glasses are part of a cogenetic set of melts formed by fractional melting of spinel lherzolite, with F varying between 0.2 and 5%. The initial highly silicic, alkali-rich melts preserved in Mg-rich olivine become richer in FeO, MgO, CaO and Cr₂O₃ and poorer in SiO₂, K₂O, Na₂O, Al₂O₃ and Cl with increasing melt fractions, evolving toward the basaltic melts found in clinopyroxene. These results confirm the connection between glass inclusions inside mantle minerals and partial mantle melts, and indicate that primary melts with SiO₂ >60 wt%, alkali oxides >11%, FeO <1 wt% and MgO <1 wt% are generated during incipient melting of spinel peridotite. The composition of the primary melts is inferred to be dependent on pressure, and to reflect both the speciation of dissolved CO₂ and the effect of alkali oxides on the silica activity coefficient in the melt. At pressures around 1 GPa, low-degree melts are characterized by alkali and silica-rich compositions, with a limited effect of dissolved CO₂ and a decreased silica activity coefficient caused by the presence of alkali oxides, whereas at higher pressures alkali oxides form complexes with carbonates and, consequently, alkali-rich silica-poor melts will be generated.     

Rare earth elements in garnets and clinopyroxenes from garnet lherzolite nodules in kimberlites

Six pairs of coexisting garnets and clinopyroxenes were separated from the sheared and granular garnet lherzolite nodules in kimberlites and analyzed for rare earth elements (REE). The sheared and granular nodules can be distinguished in terms of REE pattern of both clinopyroxene and garnet. However, there are no significant differences in REE partitioning between clinopyroxene and garnet, indicating that the partitioning may be insensitive toP, T and composition. REE partition coefficients between garnet and liquid were estimated by using clinopyroxene-liquid partition coefficients found in the literature and clinopyroxene-garnet partitioning reported here. The estimated values agree with those reported by Philpotts et al. (1972). The estimated whole-rock REE pattern for the sheared nodules is similar to a chondritic pattern suggesting that the sheared nodules appear to be close to the primary mantle material. The REE data suggest that the granular nodules were originally garnet-free assemblages equilibrated with kimberlitic or nepheline-melilite basalt-like liquid, and later recrystallized as a garnet lherzolite assemblage.     

Prediction of siderophile element metal-silicate partition coefficients to 20 GPa and 2800°C: the effects of pressure, temperature, oxygen fugacity, and silicate and metallic melt compositions

We report new metal-silicate partition coefficients for Ni, Co and P at 7.0 GPa (1650–1750°C), and Ni, Co, Mo, W and P at 0.8, 1.0 and 1.5 GPa (1300–1400°C). Guided by thermodynamics, all available metal-silicate partition coefficients, D(i), where i is Ni, Co, P, Mo and W, are regressed against 1/T, P/T, lnf(O₂), ln(1 − X_s) (X_S is mole fraction of S in metallic liquid) and nbo/t (non-bridging oxygen/tetrahedral cation ratio, a silicate melt compositional-structural parameter) to derive equations of the following form: ln D(i) = aln f(O₂) + (b/T) + (cP/T) + d(nbo/t) + eln(1 − X_S) + f. Expressions for solid metal-liquid silicate and liquid metal-liquid silicate partition coefficients are derived for S-free and S-bearing systems.

We investigate whether Earth's upper-mantle siderophile element abundances can be reconciled with simple metal-silicate equilibrium. Sulfur-free metallic compositions do not allow a good fit. However, Ni, Co, Mo, W and P abundances in the upper mantle are consistent with simple metal-silicate equilibrium at mantle pressures and temperatures (27 GPa, 2200 K, ΔIW(iron-wüstite) = −0.15, nbo/t = 2.7; X_S = 0.15). Although these conditions are near the anhydrous peridotite solidus, they are well above the hydrous solidus and probably closer to the liquidus. A hydrous magma ocean and early mantle are consistent with predicted planetary accretion models. These results suggest that siderophile element abundances in Earth's upper mantle were established by liquid metal-liquid silicate equilibrium near the upper-mantle-lower-mantle boundary.     

Emplacement conditions of komatiite magmas from the 3.49 Ga Komati Formation, Barberton Greenstone Belt, South Africa

This paper provides new constraints on the crystallization conditions of the 3.49 Ga Barberton komatiites. The compositional evidence from igneous pyroxene in the olivine spinifex komatiite units indicates that the magma contained significant quantities of dissolved H₂O. Estimates are made from comparisons of the compositions of pyroxene preserved in Barberton komatiites with pyroxene produced in laboratory experiments at 0.1 MPa (1 bar) under anhydrous conditions and at 100 and 200 MPa (1 and 2 kbar) under H₂O-saturated conditions on an analog Barberton composition. Pyroxene thermobarometry on high-Ca clinopyroxene compositions from ten samples requires a range of minimum magmatic water contents of 6 wt.% or greater at the time of pyroxene crystallization and minimum emplacement pressures of 190 MPa (6 km depth). Since high-Ca pyroxene appears after 30% crystallization of olivine and spinel, the liquidus H₂O contents could be 4 to 6 wt.% H₂O. The liquidus temperature of the Barberton komatiite composition studied is between 1370 and 1400°C at 200 MPa under H₂O-saturated conditions. When compared to the temperature-depth regime of modern melt generation environments, the komatiite mantle source temperatures are 200°C higher than the hydrous mantle melting temperatures inferred in modern subduction zone environments and 100°C higher than mean mantle melting temperatures estimated at mid-ocean ridges. When compared to previous estimates of komatiite liquidus temperatures, melting under hydrous conditions occurs at temperatures that are 250°C lower than previous estimates for anhydrous komatiite. Mantle melting by near-fractional, adiabatic decompression takes place in a melting column that spans 38 km depth range under hydrous conditions. This depth interval for melting is only slightly greater than that observed in modern mid-ocean ridge environments. In contrast, anhydrous fractional melting models of komatiite occur over a larger depth range ( 130 km) and place the base of the melting column into the transition zone.     

Melt–wall rock interaction in the mantle shown by silicate melt inclusions in peridotite xenoliths from the central Pannonian Basin (western Hungary)

In this paper we present a detailed textural and geochemical study of two equigranular textured amphibole-bearing spinel lherzolite xenoliths from Szigliget, Bakony–Balaton Highland Volcanic Field (BBHVF, western Hungary) containing abundant primary silicate melt inclusions (SMIs) in clinopyroxene rims and secondary SMIs in orthopyroxene (and rarely spinel) along healed fractures. The SMIs are dominantly composed of silicate glass and CO₂-rich bubbles. Clinopyroxene and orthopyroxene are zoned in both studied xenoliths, especially with respect to Fe, Mg, Na, and Al contents. Cores of clinopyroxenes in both xenoliths show trace element distribution close to primitive mantle. Rims of clinopyroxenes are enriched in Th, U, light rare earth elements (LREEs) and medium REEs (MREEs). Amphiboles in the Szg08 xenolith exhibit elevated Rb, Ba, Nb, Ta, LREE, and MREE contents. The composition of silicate glass in the SMIs covers a wide range from the basaltic trachyandesite and andesite to phonolitic compositions. The glasses are particularly rich in P₂O₅. Both primary and secondary SMIs are strongly enriched in incompatible trace elements (mostly U, Th, La, Zr) and display a slight negative Hf anomaly. The development of zoned pyroxenes, as well as the entrapment of primary SMIs in the clinopyroxene rims, happened after partial melting and subsequent crystallization of clinopyroxenes, most probably due to an interaction between hot volatile-bearing evolved melt and mantle wall-rocks. This silicate melt filled microfractures in orthopyroxenes (and rarely spinels) resulting in secondary SMIs.     

Rare earth abundances in hornblende and clinopyroxene of a hornblende lherzolite xenolith: Implications for upper mantle fractionation processes

Hornblende and clinopyroxene of one of a suite of hornblende lherzolite xenoliths probably derived from the upper mantle are rich in light rare earths compared with clinopyroxenes from spinel lherzolite xenoliths. Partial melting of this hornblende lherzolite assemblage could yield a liquid with the rare earths, uranium and potassium abundances of a nephelinite.     

Melting relations of a magnesian abyssal tholeiite and the origin of MORBs

Melting relations of a glassy magnesian olivine tholeiite from the FAMOUS area have been studied within the pressure range 1 atm to 15 kbar. From 1 atm to 10 kbar, olivine is the liquidus phase, followed by plagioclase and Ca-rich clinopyroxene. Above 10 kbar, Ca-rich clinopyroxene appears on the liquidus, followed by orthopyroxene and spinel. Near 10 kbar, olivine, orthopyroxene, clinopyroxene, spinel and plagioclase crystallize within 10°C of the liquidus. This indicates that a liquid of this magnesian olivine tholeiite composition could coexist with mantle peridotite at about 10 kbar. This result is in agreement with the geochemistry of Ni; the Ni concentration of the studied sample corresponds to the theoretical concentration in a primary magma [14,15].These data suggest that at least some magnesian mid-oceanic ridge basalts (MORBs) could be primary melts segregated from the mantle at depths near the transition zone between plagioclase lherzolite and spinel lherzolite (about 10 kbar). Based on this model, the residual mantle after extraction of MORBs should be lherzolite, not harzburgite.High-pressure (7–10 kbar) fractionation models involving olivine, plagioclase and clinopyroxene, which have been proposed by several workers (e.g. [36]) to explain the varieties of MORBs, were re-emphasized based on this melting study. The rare occurrence of clinopyroxene as a phenocryst phase in MORBs is explained by precipitation in a magma chamber at high pressure, or by dissolution of clinopyroxene formed earlier at high pressure.     

Uranium and thorium diffusion in diopside

This paper presents new experimental data on the tracer diffusion rates of U and Th in diopside at 1 atm and 1150–1300°C. Diffusion couples were prepared by depositing a thin layer of U–Th oxide onto the polished surface of a natural diopside single crystal, and diffusion profiles were measured by ion microprobe depth profiling. For diffusion parallel to [001] the following Arrhenius relations were obtained: log₁₀D_U=(−5.75±0.98)−(418±28 kJ/mol)/2.303RT log₁₀D_Th=(−7.77±0.92)−(356±26 kJ/mol)/2.303RT. The diffusion data are used to assess the extent to which equilibrium is obtained during near fractional melting of a high-Ca pyroxene bearing mantle peridotite. We find that the diffusion rates for both elements are slow and that disequilibrium between solid and melt will occur under certain melting conditions. For near-fractional adiabatic decompression melting at ascent rates >3 cm/yr, high-Ca pyroxene will exhibit disequilibrium effects. High-Ca pyroxene will become zoned in U and Th and the melts extracted will be depleted in these incompatible elements relative to melts produced by equilibrium fractional melting. U and Th diffusivities in high-Ca pyroxene are similar, and diffusive fractionation of these elements will be limited. Numerical solutions to a dynamic melting model with diffusion-controlled chemical equilibration indicate that the activity ratio [²³⁰Th/²³⁸U] in a partial melt of spinel peridotite will be slightly less than 1 for a broad range of melting parameters. This result reinforces the already widely accepted conclusion that melting of spinel peridotite cannot account for ²³⁰Th excesses in mid-ocean ridge and ocean island basalts, and that garnet must therefore be present over part of the melting column.     

Source depletion and extent of melting in the Tongan sub-arc mantle

The fluid immobile High Field Strength Elements (HFSE) Nb and Ta can be used to distinguish between the effects of variable extents of melting and prior source depletion of the Tongan sub-arc mantle. Melting of spinel lherzolite beneath the Lau Basin back-arc spreading centres has the ability to fractionate Nb from Ta due to the greater compatibility of the latter in clinopyroxene. The identified spatial variation in plate velocities and separation of melt extraction zones, combined with extremely depleted lavas make Tonga an ideal setting in which to test models for arc melt generation and the role of back-arc magmatism.We present new data acquired by laser ablation-ICPMS of fused sample glasses produced without the use of a melt fluxing agent. The results show an arc trend towards strongly sub-chondritic Nb/Ta (< 17) with values as low as 7.2. Melting models show that large degree melts of depleted MORB mantle fail to reproduce the observed Nb/Ta. Alternatively, incorporation of residual back-arc mantle that has undergone less than 1% melting into the sub-arc melting regime reproduces arc values. However, the extent of partial melting required to produce the composition of the Lau Basin back-arc basalts averages 7%. This apparent discrepancy can be explained if only the lowermost 4 km of the residua from the mantle melt column beneath the back-arc is added to the source of arc magmas. We have identified that the degree of arc/back-arc coupling displayed in the rock record provides an index of the depth of hydrous melting beneath the arc. In this case, this would imply a depth of ~ 75 km for generation of arc magmas, indicating that hydrous melting in the mantle wedge is triggered by the breakdown of hydrous phases in the subducting slab.     

Origin of high-magnesian andesites in the Setouchi volcanic belt,southwest Japan,II. Melting phase relations at high pressures

Melting phase relations of an augite-olivine high-magnesian andesite and an augite-olivine basalt from the Miocene Setouchi volcanic belt in southwest Japan have been studied under water-saturated, water-undersaturated and under anhydrous conditions. Both the andesite and the basalt are characterized by low FeO*/MgO ratios (0.86 and 0.76 in weight, respectively) and qualify as primary magmas derived from the upper mantle.The andesite melt coexists with olivine, orthopyroxene and clinopyroxene at 15 kbar and 1030°C under water-saturated conditions, and at 10 kbar and 1070°C under water-undersaturated conditions (7 wt.% H₂O in the melt). The basalt-melt also coexists with the above three phases at 11 kbar and 1305°C under anhydrous conditions, and at 15 kbar and 1205°C in the presence of 4 wt.% water.Present studies indicate that high-magnesian andesite magmas may be produced even under water-undersaturated conditions by partial melting of mantle peridotite. It is suggested that two types of high-magnesian andesites in the Setouchi volcanic belt (augite-olivine and bronzite-olivine andesites) were produced by different degrees of partial melting; augite-olivine andesite magmas, whose mantle residual is lherzolite, were formed by lower degrees of partial melting than bronzite-olivine andesite magmas, which coexist with harzburgite. The basalt magmas, which were often extruded in close proximity to the high-magnesian andesite magmas, are not partial melting products of a mantle peridotite which had previously melted to yield high-magnesian andesite magmas.     

Petrological feature of spinel lherzolite xenolith from Oki-Dogo Island: An implication for variety of the upper mantle peridotite beneath southwestern Japan

Abstract   Spinel lherzolite is a minor component of the deep-seated xenolith suite in the Oki-Dogo alkaline basalts, whereas other types of ultramafic (e.g. pyroxenite and dunite) and mafic (e.g. granulite and gabbro) xenoliths are abundant. All spinel lherzolite xenoliths have spinel with a low Cr number (Cr#; < 0.26). They are anhydrous and are free of modal metasomatism. Their mineral assemblages and microtextures, combined with the high NiO content in olivine, suggest that they are of residual origin. But the Mg numbers of silicate minerals are lower (e.g. down to Fo₈₆) in some spinel lherzolites than in typical upper mantle residual peridotites. The clinopyroxene in the spinel lherzolite shows U-shaped chondrite-normalized rare-earth element (REE) patterns. The abundance of Fe-rich ultramafic and mafic cumulate xenoliths in Oki-Dogo alkali basalts suggests that the later formation of those Fe-rich cumulates from alkaline magma was the cause of Fe- and light REE (LREE)-enrichment in residual peridotite. The similar REE patterns are observed in spinel peridotite xenoliths from Kurose and also in those from the South-west Japan arc, which are non-metasomatized in terms of major-element chemistry (e.g. Fo > 89), and are rarely associated with Fe-rich cumulus mafic and ultramafic xenoliths. This indicates that the LREE-enrichment in mantle rocks has been more prominent and prevalent than Fe and other major-element enrichment during the metasomatism.     

Element partitioning between metallic liquid, silicate liquid, and lower-mantle minerals: implications for core formation of the Earth

We determined the partition coefficients of 19 elements between metallic liquid and silicate liquid at 20 GPa and 2500°C, and between metallic liquid and silicate perovskite at 27 GPa and 2200°C. Remarkable differences were observed in the partitioning behaviors of Si, P, W, Re, and Pb among the silicate liquid, perovskite, and magnesiowüstite coexisting with metallic liquid, reflecting incompatibility of the elements in the silicate or oxide phase. We could not observe any significant difference in the partitioning behaviors of V, Cr, Mn, Co, Ni, and Cu among the phases coexisting with metallic liquid.

Comparison of the present partitioning data with those obtained previously at lower pressure and temperature suggests that the exchange partition coefficients, K_met/sil, of Co, Ni, Mo, and W decrease, whereas those of V, Cr, and Mn increase and tend to approach unity with increasing pressure and temperature. We also made preliminary experiments to clarify the effect of sulfur on the partitioning behaviors. Sulfur lowers the exchange partition coefficients, K_met/sil, of Mo and W between metallic liquid and silicate liquid significantly at 20 GPa and 2300°C.

The mantle abundances of Co, Ni, Cu, Mo, and W calculated for the metal-silicate equilibrium model are lower than those of the real mantle, whereas P, K, and Mn are overabundant in the calculated mantle. The discrepancies in the abundances of Co and Ni could be explained by the chemical equilibrium at higher pressure and temperature. Large discrepancies in Mo and W between the calculated and real mantles could be accounted for by the effect of sulfur combined with the effects of pressure and temperature on the chemical equilibrium. The mantle abundances of P, K, and Cu could be accounted for by volatile loss in the nebula, perhaps before accretion of the Earth, combined with the chemical equilibrium at higher pressure and temperature. Thus the observed mantle abundances of P, K, Co, Ni, Cu, Mo, and W may be consistent with a model of sulfur-bearing metal-silicate equilibrium in lower-mantle conditions.     

Podiform chromitites in the lherzolite-dominant mantle section of the Isabela ophiolite, the Philippines

Abstract The Isabela ophiolite, the Philippines, is characterized by a lherzolite‐dominant mantle section, which was probably formed beneath a slow‐spreading mid‐ocean ridge. Several podiform chromitites occur in the mantle section and grade into harzburgite to lherzolite. The chromitites show massive, nodular, layered and disseminated textures. Clinopyroxene (±orthopyroxene/amphibole) inclusions within chromian spinel (chromite hereafter) are commonly found in the massive‐type chromitites. Large chromitites are found in relatively depleted harzburgite hosts having high‐Cr? (Cr/(Cr + Al) atomic ratio = ～0.5) chromite. Light rare earth element (LREE) contents of clinopyroxenes in harzburgites near the chromitites are higher than those in lherzolite with low‐Cr? chromite, whereas heavy REE (HREE) contents of clinopyroxenes are lower in harzburgite than in lherzolite. The harzburgite near the chromitites is not a residual peridotite after simple melt extraction from lherzolite but is formed by open‐system melting (partial melting associated with influx of primitive basaltic melt of deeper origin). Clinopyroxene inclusions within chromite in chromitites exhibit convex‐shaped REE patterns with low HREE and high LREE (+Sr) abundances compared to the host peridotites. The chromitites were formed from a hybridized melt enriched with Cr, Si and incompatible elements (Na, LREE, Sr and H₂O). The melt was produced by mixing of secondary melts after melt–rock interaction and the primitive basaltic melts in large melt conduits, probably coupled with a zone‐refining effect. The Cr? of chromites in the chromitites ranges from 0.65 to 0.75 and is similar to those of arc‐related magmas. The upper mantle section of the Isabela ophiolite was initially formed beneath a slow‐spreading mid‐ocean ridge, later introduced by arc‐related magmatisms in response to a switch in tectonic setting during its obduction at a convergent margin.     

Electrical conductivity of a spinel lherzolite and a garnet peridotite to 1550°C: relevance to the effects of partial melting

Electrical conductivity σ of two ultramafic rocks (a spinel lherzolite and a garnet peridotite) has been investigated to melting temperature at 1 bar under known oxygen fugacity environment. The electrical conductivity of the two rocks is found to increase with degree of partial melting and an ～ 15% melt fraction is necessary for the electrical conductivity to increase by ～ 1 order of magnitude. For a given melt fraction electrical conductivity of a spinel lherzolite is lower than that of a garnet peridotite and may be attributed to the differences in the composition of the melts formed.     

REE diffusion in calcite

Chemical diffusion of four rare-earth elements (La, Nd, Dy and Yb) has been measured in natural calcite under anhydrous conditions, using rare-earth carbonate powders as the source of diffusants. Experiments were run in sealed silica capsules along with finely ground calcite to ensure stability of the single-crystal samples during diffusion anneals. Rutherford backscattering spectroscopy (RBS) was used to measure diffusion profiles. The following Arrhenius relations were obtained over the temperature range 600–850°C: D_La =2.6×10⁻¹⁴ exp(−147±14 kJ mol⁻¹/RT) m² s⁻¹, D_Nd =2.4×10⁻¹⁴ exp(−150±13 kJ mol⁻¹/RT) m² s⁻¹, D_Dy =2.9×10⁻¹⁴ exp(−145±25 kJ mol⁻¹/RT) m² s⁻¹, D_Yb =3.9×10⁻¹² exp(−186±23 kJ mol⁻¹/RT) m² s⁻¹. In contrast to previous findings for refractory silicates (e.g. zircon), differences in transport rates among the REE are not pronounced over the range of temperature conditions investigated in this study. Diffusion of the REE is significantly slower than diffusion of the divalent cations Sr and Pb and slower than transport of Ca and C at temperatures above 650°C. Fine-scale zoning and isotopic and REE chemical signatures may be retained in calcites under many conditions if diffusion is the dominant process affecting alteration.     

Ultramafic inclusions from San Carlos,Arizona: Petrologic and geochemical data bearing on their petrogenesis

Ultramafic inclusions from San Carlos, Arizona, are classified into two groups. Group I inclusions are dominated by magnesian (Mg/Mg + ΣFe= 0.86 – 0.91), olivine-rich peridotites containing Cr-rich clinopyroxene and spinel. The less abundant Group I pyroxenites (containing Mg- and Cr-rich pyroxenes) occur as discrete inclusions and as portions of composite inclusions where they have a sharp, planar interface with lherzolite. Group II inclusions are dominated by clinopyroxene-rich peridotites containing Al- and Ti-rich augite and commonly abundant, Al-rich spinel. Compared to Group I inclusions, they are more Fe-rich (Mg/Mg + ΣFe= 0.62 – 0.78) and more hetereogeneous in composition and modal proportions. Similar groups occur at many ultramafic inclusion localities.Our petrographic and geochemical results lead to the following conclusions. Olivine-rich Group I inclusions are not genetically related to the host basanite, and they are formed from two components. Component A is a partial melting residue; it comprises the major portion of these inclusions and determines the modal mineralogy and major and compatible trace element composition. Component B results from a small degree (<5%) of garnet peridotite melting (probably, within the low-velocity zone). This highly LIL-element-enriched melt has migrated upwards into the overlying component A where it crystallized primarily as clinopyroxene and amphibole, and thus, introduced LIL elements into the residual component A. Subsequent cooling and subsolidus recrystallization have removed textural evidence of this mixing. This model has also been proposed for olivine-rich Group I inclusions from Victoria, Australia. At Victoria and San Carlos some relatively clinopyroxene-rich Group I lherzolites are not contaminated by component B, and they represent the best estimates of upper mantle composition prior to melting. Group I orthopyroxenites may be fragments of tectonic layers formed in lherzolite, but they could also be early cumulates (now metamorphosed) from the melt in equilibrium with component A. Group I clinopyroxenites have geochemical features of clinopyroxene in equilibrium with a magma. Thus, they could also represent early cumulates (now metamorphosed) from a magma unrelated to the host basanite. Alternatively, their geochemical characteristics could result from more complex models such as residues from partial remelting of pyroxenite dikes and veins or intradike segregation processes such as filter pressing. All Group II inclusions studied appear to be cumulates derived from a SiO₂-undersaturated magma, possibly an early magma in the same volcanic episode which culminated with eruption of the host basanite. The poikilitic texture of amphibole-rich (kaersutite) inclusions is consistent with a cumulate origin. The bulk compositions of Group II inclusions are not equivalent to typical basaltic compositions.     

Origins of the mafic and ultramafic rocks in the Ronda peridotite

The Ronda peridotite massif in southern Spain originated from the upper mantle, evidently as a rapidly rising diapir. Major and trace element abundance trends of the peridotites reflect their origin as residues from partial melting of garnet lherzolite. About 5% of the massif consists of mafic rocks, mainly pyroxenites and gabbros. They occur as concordant layers amidst the peridotites, and these layers do not cross-cut each other. However, major and trace element data show that the mafic layers do not have the geochemical characteristics of primary melts. We conclude that crystal/liquid fractionation occurred at high pressures ( > 19 kbar) as melts migrated through magma conduits towards the cooler exterior portion of the diapir. This process generated a sequence of “cumulates” (mainly clinopyroxene + orthopyroxene + spinel and clinopyroxene + garnet) along the walls of the conduits which are now represented by the mafic layers.     

Water and partial melting of Earth’s mantle

Water plays a crucial role in the melting of Earth’s mantle. Mantle magmatisms mostly occur at plate boundaries (including subduction zones and mid-ocean ridges) and in some intraplate regions with thermal anomaly. At oceanic subduction zones, water released by the subducted slab may induce melting of the overlying mantle wedge or even the slab itself, giving rise to arc magmatism, or may evolve into a supercritical fluid. The physicochemical conditions for the formation of slab melt and supercritical fluid are still under debate. At mid-ocean ridges and intraplate hot zones, water and CO₂ cause melting of the upwelling mantle to occur at greater depths and in greater extents. Low degree melting of the mantle may occur at boundaries between Earth’s internal spheres, including the lithosphere-asthenosphere boundary (LAB), the upper mantletransition zone boundary, and the transition zone-lower mantle boundary, usually attributed to contrasting water storage capacity across the boundary. The origin for the stimulating effect of water on melting lies in that water as an incompatible component has a strong tendency to be enriched in the melt (i.e., with a mineral-melt partition coefficient much smaller than unity), thereby lowering the Gibbs free energy of the melt. The partitioning of water between melt and mantle minerals such as olivine, pyroxenes and garnet has been investigated extensively, but the effects of hydration on the density and transport properties of silicate melts require further assessments by experimental and computational approaches.     

Garnet clinopyroxenite xenolith from Dish Hill,California

A single garnet clinopyroxenite xenolith found at the Dish Hill basanite cone near Ludlow, California, has well developed unmixing and reaction textures like those found in garnet pyroxenite inclusions in Hawaiian, African and Australian basalts and like those of pyroxenites in some European alpine peridotites. Reconstructed pyroxene compositions suggest that before unmixing the rock consisted of clinopyroxene and about 10% garnet plus spinel, but all of the garnet may have been dissolved in clinopyroxene. Most or all of the garnet formed by exsolution from clinopyroxene and by reaction between clinopyroxene and spinel in an open system. Following exsolution, the rock was deformed and partly recrystallized in the solid state. Similarity of compositions of exsolved and recrystallized minerals suggests recrystallization at P-T conditions similar to those of exsolution.The rock is not the chemical equivalent of the host basanite and cannot represent magma of basanitic composition crystallized in the mantle. Its history of deformation and recrystallization, like that of accompanying spinel lherzolite inclusions, supports the idea that the garnet clinopyroxenite is an accidental inclusion derived from the upper mantle.     

How chalcophile is rhenium? An experimental study of the solubility of Re in sulphide mattes

Although the Re/Os isotopic system has proved to be a valuable aid in understanding the evolution of the Earth's mantle, interpretation of the data is currently hindered by limited knowledge of the high-temperature geochemical behaviour of Re. In particular, the extent to which Re in the mantle is hosted by sulphide or silicate phases is poorly known. We report the results of an experimental study of the solubility of Re in sulphide melts (called here “mattes”) coexisting with a Re-rich Re–Fe alloy in the system Fe–Re–S–O over a range of fO₂, fS₂ and temperatures, which allow extrapolation to conditions pertinent to the Earth's mantle. The solubility of Re in mattes increases with increasing fS₂, with Re dissolving as Re⁴⁺ at high fS₂ and Re⁰ at low fS₂. The effect of fO₂ is negligible except at high fO₂ where O in the matte becomes important. At constant fS₂, an increase in temperature leads to an increase in the solubility of Re⁰ in the matte, but a decrease in the solubility of Re⁴⁺. These results, coupled with data for the Re solubility in silicate melts taken from the literature, allow the calculation of Re matte/silicate–melt partition coefficients (D_Re^matte/sil) for a range of conditions. The calculated D_Re^matte/sil show a large dependence on fO₂, and a lesser dependency on fS₂, with Re behaving as a chalcophile element for relatively reduced MORB-type mantle and as a lithophile element for the oxidised sources of island-arc basalts. The sensitivity of Re to fS₂ and fO₂ reconciles the apparent discrepancies between previous estimates of this parameter, which can vary by more than five orders of magnitude within the range of fS₂ and fO₂ covered by terrestrial basaltic magmas.