Parental Silicate Melt Research Articles

The Calatrava paradox to decipher the origin of carbonatites: A petrological insight on Finca La Nava, Calatrava Province (central Spain)

The paper presents a detailed petrographic and geochemical study of the pyroclastites outcropping at La Nava maar (Calatrava Volcanic Province-CVP). Here, extrusive carbonatites mix with melilite nephelinite lapilli and bombs, crustal and mantle debris. Nephelinite and carbonatite are not characterised by a HFSE distribution typical of immiscible pairs. ΣREE and La/Lu ratios reflect primitive mantle carbonates and kimberlites. Igneous carbonates have δ13C and δ18O isotopic ratios in the range of extrusive carbonatites and differ from local sedimentary carbonates. Carbonatites have 87Sr/86Sr and 144Nd/143Nd isotopic ratios similar to Calatrava leucitites but different to melilite nephelinite. The difference indicates that melilite nephelinites and carbonatites are co-eruptive but not comagmatic. The mingled magmatic convoy, underwent decarbonation processes, releasing Ca and Mg that reacted with parental silicate melt. Consequently, the nephelinitic melt evolved towards a melilititic composition producing Ca-rich overgrowths on mafic phenocrysts and reacted to produce high Mg#, low Cr + Ni forsterite. Mass balance calculations indicats that the nephelinite magma assimilated up to 30% of the carbonatite magma. Mixing modelling based on the Sr and Nd isotopic composition of mantle xenoliths and carbonatites indicate that La Nava carbonatite may represent the mantle metasomatic agent.

The Nature and Composition of the J-M Reef, Stillwater Complex, Montana, USA

Abstract In this contribution, we analyze 30 years of mine development data and quantitatively identify the processes that control the grade and tenor of the mineralized rock. An assay database of more than 60,000 samples was used to examine variations in ore grade and tenor of the sulfide mineralization in the J-M reef horizon of the Stillwater Complex along the strike and down the dip of the deposit in the area of the Stillwater mine. We compare these results with data from the East Boulder mine and whole-rock lithogeochemistry of samples collected along the entire strike length of the complex. We find significant variation in the composition of the reef sulfides in different spatial domains of the Stillwater mine area and between the Stillwater and East Boulder mines. Most of the variation in the grade and tenor can be explained by a variation in the mass of silicate magma with which the sulfide liquid equilibrated (i.e., R factor); however, geochemical and textural evidence suggests that parts of the reef may have experienced significant S loss following initial sulfide melt segregation. Some variability in the reef tenor and grade can be attributed to variable amounts of sulfur loss due to low-temperature hydrothermal fluids and the overestimation or underestimation of metal concentrations in reef assays due to the nugget effect. Furthermore, we address the Pd/Pt ratio of the reef samples and suggest that the lower solubility of Pt in the parental silicate melt may have caused the crystallization and removal of Pt alloys at some point before the melt reached sulfide saturation and Pt could partition into the sulfide liquid. This disparity between the prior evolution of Pt and Pd in the silicate melt resulted in the observed Pd/Pt ratio of ~3.65 across all areas of the reef—a value significantly larger than anticipated for primitive mantle-derived magmas.

Composition, crystallization conditions and genesis of sulfide-saturated parental melts of olivine-phyric rocks from Kamchatsky Mys (Kamchatka, Russia)

Sulfide liquids that immiscibly separate from silicate melts in different magmatic processes accumulate chalcophile metals and may represent important sources of the metals in Earth's crust for the formation of ore deposits. Sulfide phases commonly found in some primitive mid-ocean ridge basalts (MORB) may support the occurrence of sulfide immiscibility in the crust without requiring magma contamination and/or extensive fractionation. However, the records of incipient sulfide melts in equilibrium with primitive high-Mg olivine and Cr-spinel are scarce. Sulfide globules in olivine phenocrysts in picritic rocks of MORB-affinity at Kamchatsky Mys (Eastern Kamchatka, Russia) represent a well-documented example of natural immiscibility in primitive oceanic magmas. Our study examines the conditions of silicate-sulfide immiscibility in these magmas by reporting high precision data on the compositions of Cr-spinel and silicate melt inclusions, hosted in Mg-rich olivine (86.9–90 mol% Fo), which also contain globules of magmatic sulfide melt. Major and trace element contents of reconstructed parental silicate melts, redox conditions (ΔQFM = +0.1 ± 0.16 (1σ) log. units) and crystallization temperature (1200–1285 °C), as well as mantle potential temperatures (~1350 °C), correspond to typical MORB values. We show that nearly 50% of sulfur could be captured in daughter sulfide globules even in reheated melt inclusions, which could lead to a significant underestimation of sulfur content in reconstructed silicate melts. The saturation of these melts in sulfur appears to be unrelated to the effects of melt crystallization and crustal assimilation, so we discuss the reasons for the S variations in reconstructed melts and the influence of pressure and other parameters on the SCSS (Sulfur Content at Sulfide Saturation).

New Magmatic Oxybarometer Using Trace Elements in Zircon

Abstract We derive a novel method for determining the oxidation state of a magma as zircon crystallized, with a standard error of ±0·6 log unit ƒO2, using ratios of Ce, U, and Ti in zircon, without explicit determination of the ionic charge of any of them, and without independent determination of crystallization temperature or pressure or parental melt composition. It yields results in good agreement with oxybarometry on Fe–Ti oxide phenocrysts and hornblende phenocrysts quenched in eruptive I- and A-type dacites and rhyolites, but our zircon oxybarometer is also applicable to slowly cooled plutonic rocks and applicable to detrital and xenocrystic zircons. Zircon/melt partition coefficients of Ce and U vary oppositely with ƒO2 variation in the silicate melt. The Ce/U ratio in zircon also varies with the Ce/U element ratio in the silicate melt. During mafic-to-felsic magmatic differentiation, Ce and U are incorporated mainly in calcium-dominated lattice sites of clinopyroxene, hornblende, apatite, and occasionally titanite and/or allanite, all of which have a similar degree of preference for Ce over U. We employ the U/Ti ratio in zircon and in silicate melt as a magmatic differentiation index. Convergent- and divergent-plate-margin differentiation series consistently follow the relation log (Ce/U) ≈ –0·5 log (U/Ti) + C' in silicate melts of basaltic to rhyolitic composition. That correlation permits thermodynamic derivation of the oxybarometry relation among those elements in zircon: log fO2(sample)−log fO2(FMQ)≈42n+1log[Ce/(Ui×Ti)z]+C, wherein Ui denotes age-corrected initial U content, FMQ represents the reference buffer fayalite + magnetite + quartz, superscript z denotes zircon, and n varies with the average valence of uranium in the zircon’s parental silicate melt. We empirically calibrate this relation, using 1042 analysed zircons in 85 natural populations having independently constrained log ƒO2 in the range FMQ – 4·9 to FMQ + 2·9, to obtain the equation log fO2(sample)−log fO2(FMQ)=3·998(±0·124) log[Ce/(Ui×Ti)z]+2·284(±0·101) with a correlation coefficient R = 0·963 and standard error of 0·6 log unit ƒO2 in calc-alkalic, tholeiitic, adakitic, and shoshonitic, metaluminous to mildly peraluminous and mildly peralkaline melts in the composition range from kimberlite to rhyolite. Thermodynamic assessment and empirical tests indicate that our formulation is insensitive to varying crystallization temperature and pressure at lithospheric conditions. We present a revised equation for Ti-in-zircon thermometry that accounts appropriately for pressure as well as reduced activity of TiO2 and SiO2 in rutile- and quartz-undersaturated melts. It can be used to retrieve absolute values of ƒO2 from values of ΔFMQ obtained from a zircon analysis.

The petrology of kimberlites from South Australia: Linking olivine macrocrystic and micaceous kimberlites

Kimberlites of Jurassic age occur in various parts of South Australia. Thirty-nine of these kimberlites, which are mostly new discoveries, were studied to characterize their structural setting, their petrography, and the composition of their constituent minerals. Although some of the kimberlites in South Australia occur on the Archean to Paleoproterozoic Gawler Block, most are part of a northwest-trending, semi-continuous kimberlite dike swarm located in the Adelaide Fold Belt. The kimberlites typically occur as dikes or sills, but diatremes are also present. In the Adelaide Fold Belt, diatremes are restricted to the hinge zones of regional-scale folds within thick sedimentary sequences of the Adelaidean Supergroup. Despite widespread and severe alteration, coherent and pyroclastic kimberlites can be readily distinguished. U-Pb and Sr/Nd isotopic compositions of groundmass perovskite indicate that all kimberlites belong to the same age group (177–197 Ma) and formed in a near-primitive mantle environment (87Sr/86Sr: 0.7038–0.7052, εNd: −0.07 to +2.97). However, the kimberlites in South Australia are compositionally diverse, and range from olivine-dominated varieties (macrocrystic kimberlites) to olivine-poor, phlogopite-dominated varieties (micaceous kimberlites). Macrocrystic kimberlites contain magnesium-rich groundmass phlogopite and spinel, and they are typically olivine macrocryst-rich. Micaceous kimberlites, in contrast, contain more iron- and titanium-rich groundmass phlogopite and less magnesian spinel, and olivine macrocrysts are rare or absent. Correlations between phlogopite and spinel compositions with modal abundances of olivine, indicate that the contrast between macrocrystic and micaceous kimberlites is primarily linked to the amount of mantle components that were incorporated into a compositionally uniform parental mafic silicate melt. We propose that assimilation of xenocrystic magnesite and incorporation of xenocrystic olivine from dunitic source rocks were the key processes that modified the parental silicate melt and created the unique hybrid (carbonate-silicate) character of kimberlites. Based on the composition of xenoliths and xenocrysts, the lithospheric mantle sampled by the South Australian kimberlites is relatively uniform, and extends to depths of 160–170 km, which is slightly below the diamond stability field. Only beneath the Eurelia area does the lithosphere appear thicker (>175 km), which is consistent with the presence of diamonds in some of the Eurelia kimberlites.

Noble metals potential of sulfide-saturated melts from the subcontinental lithosphere

The origin of vast accumulations of nickel and platinum in some continental magmatic rocks is still enigmatic, but ultimately linked to silicate-sulfide liquid immiscibility. The exact composition of pristine sulfide melts has proved extremely difficult to document and understand, largely because of the ephemeral, reactive qualities and small quantities of such melts. Here we report the discovery of Fe-Ni sulfide melt globules highly enriched in noble metals (Pt, Pd, Au; 120 ppm total platinum group elements [PGE]) within an unusual high-Mg andesitic glass (8.2 wt% MgO, 57.3 wt% SiO2) dredged from the southern Mid-Atlantic Ridge, near the Bouvet triple junction. The composition of this glass indicates derivation of its parental silicate melt from a garnet pyroxenite mantle source with pronounced “continental” isotopic (Pb, Sr, Nd, Hf, Os, O) signatures. We infer that the chemical properties of this high-temperature (1250 °C) melt, notably high SiO2 and Ni (310 ppm) contents, promoted sulfide saturation at low pressures in a purely oceanic setting, and propose that this unique example, with its likely origin in the continental lithospheric mantle, may be a useful analogue for incipient Ni-PGE-sulfide melt generation and magmatic PGE enrichment.

Liquid immiscibility between silicate, carbonate and sulfide melts in melt inclusions hosted in co-precipitated minerals from Kerimasi volcano (Tanzania): evolution of carbonated nephelinitic magma

The evolution of a carbonated nephelinitic magma can be followed by the study of a statistically significant number of melt inclusions, entrapped in co-precipitated perovskite, nepheline and magnetite in a clinopyroxene- and nepheline-rich rock (afrikandite) from Kerimasi volcano (Tanzania). Temperatures are estimated to be 1,100°C for the early stage of the melt evolution of the magma, which formed the rock. During evolution, the magma became enriched in CaO, depleted in SiO2 and Al2O3, resulting in immiscibility at ~1,050°C and crustal pressures (0.5–1 GPa) with the formation of three fluid-saturated melts: an alkali- and MgO-bearing, CaO- and FeO-rich silicate melt; an alkali- and F-bearing, CaO- and P2O5-rich carbonate melt; and a Cu–Fe sulfide melt. The sulfide and the carbonate melt could be physically separated from their silicate parent and form a Cu–Fe–S ore and a carbonatite rock. The separated carbonate melt could initially crystallize calciocarbonatite and ultimately become alkali rich in composition and similar to natrocarbonatite, demonstrating an evolution from nephelinite to natrocarbonatite through Ca-rich carbonatite magma. The distribution of major elements between perovskite-hosted coexisting immiscible silicate and carbonate melts shows strong partitioning of Ca, P and F relative to FeT, Si, Al, Mn, Ti and Mg in the carbonate melt, suggesting that immiscibility occurred at crustal pressures and plays a significant role in explaining the dominance of calciocarbonatites (sovites) relative to dolomitic or sideritic carbonatites. Our data suggest that Cu–Fe–S compositions are characteristic of immiscible sulfide melts originating from the parental silicate melts of alkaline silicate–carbonatite complexes.

Geochemistry of sulfides in Hawaiian garnet pyroxenite xenoliths: Implications for highly siderophile elements in the oceanic mantle

Two dominant petrographic types of sulfides occur in Hawaiian garnet pyroxenite xenoliths — Type I occurs as globular, poikilitic inclusions only in clinopyroxenes, and Type II occurs interstitially between silicate phases. Chemically, the two types are essentially identical and their compositions are consistent with Fe–Ni monosulfide solid solution (Fe = 55–57%, Ni = 3–10%, Cu = 0.5–2.0%, S = 35–37% and Zn = 0.01–0.5%). Both types of pyroxenitic sulfides have a factor of 10 to 1000 lower PGE contents than sulfides found in peridotites elsewhere (e.g. Os ∼ 1 to < 0.01 ppm), show fractionated PGE patterns (Pd ( n )/Ir ( n ) ∼ 1–35; n = chondrite-normalized) and very high Re ( n )/Os ( n ) ratios (∼ 10–400). The Ni and the total PGE (Os + Ir + Pt + Pd) contents of the sulfides correlate positively with the Mg# of clinopyroxene in the host rock, which points to a genetic link between the sulfides and silicates and a high-temperature mantle origin. The Hawaiian sulfides formed from an immiscible sulfide melt that separated from a silicate melt, which formed the host garnet pyroxenite. Experimental constraints place this sulfide–silicate melt immiscibility at 1530 ± 100 °C and 3.1 ± 0.6 GPa, i.e. near the base or slightly below the Pacific lithosphere. Melt/mineral partition calculations suggest that the parental silicate melts (prior to immiscibility) were similar to the Honolulu Volcanics (HV) alkali lavas that host the xenoliths, and that limited (0.1–0.3%) sulfide fractionation from a parental HV-type melt can account for the observed HSE variability in the sulfides. The relatively low Pt/Re ratios (< 10) suggest that, upon “aging”, such sulfides cannot generate the coupled 186Os– 187Os isotope enrichments observed in Hawaiian lavas and some komatiites. Therefore, recycling of mantle sulfides of pyroxenitic parentage is unlikely to explain the enriched Pt–Re–Os isotope systematics of plume-derived lavas.

Genesis of the Khaluta alkaline-basic Ba-Sr carbonatite complex (West Transbaikala, Russia)

The Khaluta carbonatite complex comprizes fenites, alkaline syenites and shonkinites, and calcite and dolomite carbonatites. Textural and compositional criteria, melt inclusions, geochemical and isotopic data, and comparisons with relevant experimental systems show that the complex formed by liquid immiscibility of a carbonate-saturated parental silicate melt. Mineral and stable isotope geothermometers and melt inclusion measurements for the silicate rocks and carbonatite all give temperatures of crystallization of 915–1,000°C and 890–470°C, respectively. Melt inclusions containing sulphate minerals, and sulphate-rich minerals, most notably apatite and monazite, occur in all of the lithologies in the Khaluta complex. All lithologies, from fenites through shonkinites and syenites to calcite and dolomite carbonatites, and to hydrothermal mineralisation are further characterized by high Ba and Sr activity, as well as that of SO3 with formation of the sulphate minerals baryte, celestine and baryte-celestine. Thus, the characteristic features of the Khaluta parental melt were elevated concentrations of SO3, Ba and Sr. In addition to the presence of SO3, calculated fO2 for magnetites indicate a high oxygen fugacity and that Fe+3>Fe+2 in the Khaluta parental melt. Our findings suggest that the mantle source for Khaluta carbonatite and associated rocks, as well as for other carbonatites of the West Transbaikalia carbonatite province, were SO3-rich and characterized by high oxygen fugacity.

HYDROUS SULFIDE MELTING: EXPERIMENTAL EVIDENCE FOR THE SOLUBILITY OF H2OIN SULFIDE MELTS

The effect of H 2 O on sulfide melting temperatures has been investigated in the FeS-PbS-ZnS system at 1.5 GPa, revealing that the addition of H 2 O results in a 35°C drop in melting temperature from 900° to 865°C. In addition to the melting point depression, the solubility of H 2 O is confirmed by the presence of vesicles in the quenched melt. No oxide phases were present in any of the run products, ruling out oxidation as a cause of the melting point depression. Confirmation of the solubility of H 2 O in sulfide melts is consistent with the recent suggestion by Mungall and Brenan (2003) of a magmatic origin for halogen-rich alteration associated with magmatic sulfide ore deposits, as the hydrous component of the alteration may similarly originate in the fractionating sulfide melt. Anatectic sulfide melts could be expected to contain more H 2 O than magmatic sulfide melts, owing to the lack of a parental silicate melt that buffers the H 2 O content of magmatic sulfide melts. Fluids expelled from the cooling anatectic melts, such as that present during granulite facies metamorphism of sulfide deposits at Broken Hill, Australia, may have been responsible for associated retrograde hydrothermal alteration.

Trace element geochemistry of Amba Dongar carbonatite complex, India: Evidence for fractional crystallization and silicate-carbonate melt immiscibility

Carbonatites are believed to have crystallized either from mantle-derived primary carbonate magmas or from secondary melts derived from carbonated silicate magmas through liquid immiscibility or from residual melts of fractional crystallization of silicate magmas. Although the observed coexistence of carbonatites and alkaline silicate rocks in most complexes, their coeval emplacement in many, and overlapping initial87Sr/86Sr and143Nd/144Nd ratios are supportive of their cogenesis; there have been few efforts to devise a quantitative method to identify the magmatic processes. In the present study we have made an attempt to accomplish this by modeling the trace element contents of carbonatites and coeval alkaline silicate rocks of Amba Dongar complex, India. Trace element data suggest that the carbonatites and alkaline silicate rocks of this complex are products of fractional crystallization of two separate parental melts. Using the available silicate melt-carbonate melt partition coefficients for various trace elements, and the observed data from carbonatites, we have tried to simulate trace element distribution pattern for the parental silicate melt. The results of the modeling not only support the hypothesis of silicate-carbonate melt immiscibility for the evolution of Amba Dongar but also establish a procedure to test the above hypothesis in such complexes.

Sulphide Segregation in Ferropicrites from the Pechenga Complex, Kola Peninsula, Russia

parental ferropicritic magma. In differentiated flows and intrusions The Palaeoproterozoic Ni–Cu sulphide deposits of the Pechenga the sulphide liquids segregated and accumulated at the base of these Complex, Kola Peninsula, occur in the lower parts of ferropicritic bodies, but because of a low silicate–sulphide mass ratio the sulphide intrusions emplaced into the phyllitic and tuffaceous sedimentary liquids had a low PGE tenor and Pt/Ir and Cu/Ir ratios similar unit of the Pilgujarvi Zone. The intrusive rocks are comagmatic to the parental silicate melts. During cooling the sulphide liquid with extrusive ferropicrites of the overlying volcanic formation. crystallized 40–50% of monosulphide solid solution (mss) and the Massive lavas and chilled margins from layered flows and intrusions residual sulphide liquid became enriched in Cu, Pt and Pd and contain 100). Most of the massive and breccia ores represent during crystallization of the parental magma. Compared with the mixtures of mss and residual Cu-rich sulphide liquid whereas PGE-depleted central zones of differentiated flows (spinifex and chalcopyrite-rich veins formed when the Cu-rich sulphide liquids clinopyroxene cumulate zones) the olivine cumulate zones at the were squeezed out into the surrounding sediments. base contain elevated PGE abundances up to 10 ng/g Pd and Pt. A similar pattern is displayed in intrusive bodies, such as the Kammikivi sill and the Pilgujarvi intrusion. The olivine cumulates at the base of these bodies contain massive and disseminated Ni–Cu

The nature of multicomponent aluminosilicate melts

The parental silicate melts of the most abundant igneous rocks in the earth's crust may be regarded as multicomponent solutions of as many as 15 chemically discrete, electrostatically neutral complexes or species. These species arecorrelated with thermodynamic components and, by adoption of a “quasi-crystalline” model, are identified, both in terms of chemical composition (stoichiometry) and their basic structural components, with the crystalline phases that form on the liquidus under equilibrium conditions. Among the approximately 10 end-member components (species) in any one of these igneous-rock melts, the three aluminosilicate (feldspar-like) components, NaAlSi 3O 8 ( ab), CaAl 2Si 2O 8 ( an), and KAlSi 3O 8 ( or), together comprise more than 50 mole % of the solution, hence they may be regarded as a multicomponent solvent in which the several other non-aluminosilicate species are dissolved. The three components of this multicomponent solvent, moreover, are found from the solution behavior of H 2O to mix essentially ideally with each other. Similarly, the non-aluminosilicate component, Si 4O 8 ( qz), is found to mix ideally with the aluminosilicates of the solvent. The thermodynamic behavior of the end-member components as a function of liquidus temperature, pressure, and composition (including H 2O content), interpreted in accordance with the quasi-crystalline model, lead to the recognition of two basic types of homogeneous melt reactions that result in formation of other species from the end-member components. These end-member components—especially the feldspar-like aluminosilicates ( a)—may undergo partial dissociation, or the aluminosilicates may interact with the non-aluminosilicate components. In either type, the reaction may involve no change in the coordination number of the Al atoms ( IVAlO 4 5− in the feldspar-like species), or it may involve a change to higher coordination numbers ( VAlO 5 7− and VIAlO 6 9−) for at least part of the Al atoms. This tendency toward variable coordination of Al is regarded as the principal factor in determining the nature and extent of speciation reactions in aluminosilicate-rich melts, especially at high pressures where higher coordination numbers are favored. The basis for these interpretations is the difference between the mole fraction of end-member component i ( X i am), as obtained from published experimental liquidus data, and the activity of i ( a i am), calculated from internally consistent sets of Gibbs free energies and volumes of melting for crystalline material of pure i composition. These differences for the end-member aluminosilicate components, X a am − a a am, are either zero (no speciation reaction involving a) or greater than zero (speciation reaction consumes a) in solutions containing such chemically and structurally dissimilar species as Si 4O 8 ( qz) and Mg 4Si 2O 8 ( fo), a forsterite-like species. The thermodynamic consequences of attributing the differences to speciation reactions are to return the aluminosilicate components to Raoultian behavior in melts where aluminosilicates crystallize on the liquidus. By the same procedures, Si 4O 8 and Mg 4Si 2O 8 also become Raoultian in behavior when mixed with the aluminosilicates, but more experimental data are needed to evaluate the mixing properties of other geochemically important melt components. The relationships between X i m and a i am, when expressed in simple empirical equations of the form X i am − a i am = X i am ƒ(P, X j am, X w m), contain the information required to calculate liquidus equilibrium relations in hydrous ( X w m) and anhydrous ( X i,j,… am) melts of geochemical interest over a wide range of conditions. When interpreted in terms of the quasi-crystalline model, they also provide new insights into the nature of multicomponent aluminosilicate melts and into numerous magmatic phenomena.