Robert Clocchiatti

Sulfur abundance and its speciation in oxidized alkaline melts

Abstract The sulfur concentrations and the relative proportions of S2− and S6+ were measured by electron microprobe in a series of melt inclusions trapped in phenocrysts from different subduction-related and within-plate volcanoes. The melt inclusions correspond to potassic and shoshonitic primary melts to tholeiitic and hawaiitic primitive melts. In the tholeiitic and the transitional basaltic melt inclusions, sulfur is mainly present as S2− ( S 6+ S total = 0.03 to 0.08 ), and varies from 0.13 to 0.18 wt%. The occurrence of immiscible sulfides attests to their saturation. In shoshonitic and potassic primary melts, sulfur (S = 0.12 to 0.32 wt%) is dissolved as both S2− and S6+ ( S 6+ S total = 0.3 to 0.7 ). Their oxygen fugacity, estimated from the S 6+ S total ratios, ranges from NiNiO to NiNiO + 1 log unit. Hawaiitic melts may also dissolve up to 0.3 wt% sulfur possibly because of their oxidation state close to NiNiO, as illustrated by samples from Mt. Etna, Italy. Variations of sulfur, at constant S 6+ S total ratio and temperature in both the potassic and shoshonitic primary melts, indicate that these melts are undersaturated, with respect to a S-rich condensed phase, in agreement with their relatively high oxidation state. It strongly suggests that sulfur behavior in relatively oxidized primary alkaline melts is controlled by the mantle source melting conditions.

High-temperature experiments on silicate melt inclusions in olivine at 1 atm: inference on temperatures of homogenization and H2O concentrations

Abstract A series of heating stage experiments, at ambient atmosphere and high temperature, were conducted in order to detail the change in the homogenization temperature and the behavior of H 2 O of melt inclusions in olivine phenocrysts (Fo 82–89 ). The samples were collected at different volcanic sites (Piton de la Fournaise [PdF], Reunion Island; Stromboli, Aeolian Islands; FAMOUS zone) expected to have a range of magmatic water contents. The melt inclusions vary in composition from basaltic to shoshonitic, with H 2 O content from 0.14 to 2.9 wt.%. Temperatures of homogenization ( T h ) of melt inclusions systematically increase with time during heating experiments, regardless of their major element composition and their H 2 O. It is proposed that T h changes with time in response to the deformation of the host crystal and change in the volume of the cavity. The FTIR spectra successively acquired on the same inclusion repeatedly heated at constant temperature clearly demonstrated that the relative absorbance (Abs. n /Abs. 0 ) measured at 3535 cm −1 corresponding to hydroxyl group and molecular H 2 O decreases with time. The H 2 O-rich melt inclusions may have lost from 20% to 80% H 2 O after the first homogenization and almost total dehydration of melt inclusions may occur within few hours or less, at 1 atm. Water loss driven out off olivine-hosted melt inclusions possibly exists to some extent in natural large dunitic bodies or in mantle xenoliths.

Transfer of sulfur in subduction settings: an example from Batan Island (Luzon volcanic arc, Philippines)

Sulfur abundances have been determined in silicic and basaltic melt inclusions in olivines from harzburgitic xenoliths and a basaltic lava sample, all from Batan Island. In mantle xenoliths, olivines (Fo80–91) are present as neoblasts or in finely recrystallized patches. The most magnesian olivines (Fo89.7–91, CaO 85, CaO = 0.25 wt%) and their melt inclusions (CaO/Al2O3 from 0.8 to 1.15) have recorded early stages of crystallization. The sulfur concentrations for these calc-alkaline basaltic melts are estimated between 1720 and 3200 ppm, with a mean value at 2550 ppm (1σ=390) and S/Cl ratio at nearly 1. This is in agreement with the idea that arc basaltic melts may contain high concentrations of sulfur (S > 2000 ppm), at 1200°C. However, the heterogeneous distribution of S and its partitioning between silicate melts, H2O-rich vapor and S-bearing solid phases as illustrated by the Batan mantle xenoliths would result in highly variable sulfur concentrations in island arc basaltic magmas, mostly controlled by fO2 and fS2.

Assessment of a shallow magmatic system: the 1888–90 eruption, Vulcano Island, Italy

The magmatic system feeding the last eruption of the volcano La Fossa, Vulcano Island, Italy was studied. The petrogenetic mechanisms controlling the differentiation of erupted rocks were investigated through petrography, mineral chemistry, major, trace and rare earth element and Sr, Nd and Pb isotopic geochemistry. In addition, melt inclusion and fluid inclusion data were collected on both juvenile material and xenolithic partially melted metamorphic clasts to quantify the P-T conditions of the magma chamber feeding the eruption. A regular and continuous chemical zoning has been highlighted: rhyolites are the first erupted products, followed by trachytes and latites, whereas rhyolitic compositions were also found in the upper part of the sequence. The chemical and isotopic composition of the rhyolites indicates that they originated by fractional crystallization from latitic magmas plus the assimilation of crustal material; the trachytes represent hybrid magmas resulting from the mixing of latites and rhyolites, contaminated in the shallow magmatic system. The erupted products, primarily compositionally zoned from latites to rhyolites, are heterogeneous due to syn-eruptive mingling. The occurrence of magmacrust interaction processes, evidenced by isotopic variations (87Sr/86Sr=0.70474±3 to 0.70511±3; 143Nd/144Nd=0.512550±6 to 0.512614±8; 206Pb/204Pb=19.318–19.489; 207Pb/204Pb=15.642–15.782; 208Pb/204Pb=39.175–39.613), is confirmed by the presence of partially melted metamorphic xenoliths, with 87Sr/86Sr=0.71633±6 to 0.72505±2 and 143Nd/144Nd=0.51229±7, in rhyolites and trachytes. AFC calculations indicate a few percentage contribution of crustal material to the differentiating magmas. Thermometric measurements on melt inclusions indicate that the crystallization temperatures of the latites and trachytes were in the range of 1050–1100° C, whereas the temperature of the rhyolites appears to have been around 1000°C at the time of the eruption. Compositional data on melt inclusions reveal that the magmas involved in the eruption contained about 1–1.5 wt.% dissolved H2O in pre-eruptive conditions. Secondary fluid inclusions found in metamorphic xenoliths give low equilibration pressure data (30–60 MPa), giving the location of the higher portions of the chamber at around 1500–2000 m of depth.

The 1989–1990 activity of Etna magma mingling and ascent of H2OClSrich basaltic magma. Evidence from melt inclusions

Abstract The melt inclusions in olivine phenocrysts from basaltic tephra erupted by Etna volcano during the 1989–1990 eruptive period were investigated for major elements, Cl and S by electron microprobe, fluorine by PIGME (Particule Induced Gamma-ray Emission) and H 2 O by SIMS. The results indicate the ascent of basaltic magmas, with different K 2 O concentrations possibly related to different feeding-dykes. “LK” basaltic melt inclusions, rich in Cl, S and H 2 O, are trapped in olivine (Fo 84 ) from the 1990 tephra and represent the early crystallization stage. Their composition in major elements is quite similar to those of most of the historic and even pre-etnean alkali-basalts, but significantly lower in K 2 O compared with the 1989–1990 lavas. Only scarce “HK” basaltic melt inclusions were observed in the outer rim (Fo 79.6 ) of reversely-zoned olivine phenocrysts and correspond to late crystallization-stage, at low pressure. High sulfur concentration ( S ≈ 0.3 wt.%) is a general characteristic of the etnean basaltic magmas, with H 2 O/S ratio as high as 7. The volatile abundances of the Etna primitive basalts were computed at 1.45 wt.% for H 2 O, 0.17 wt.% for Cl and 0.047 wt.% for F, with a high Cl/F ratio close to 3.6. The 1989 lava samples with Cl at 660 ± 90 ppm and F at 770 ± 30 ppm have suffered a significant Cl loss. The possible behaviour of the volatile phase as CO 2 , H 2 O even Cl is discussed, with H 2 O and chlorine late degassing and an active role of H 2 O during the lava fountaining activity.

Primitive magmatism of Mt. Etna: insights from mineralogy and melt inclusions

Data on the mineralogy and petrology of primitive ancient and prehistoric basaltic magmas of Mt Etna (Sicily) are scarce. A systematic study of the mineralogy and magmatic inclusions in olivine phenocrysts from the most Mg-rich tholeiitic (Aci Castello, Aci Trezza and Adrano), transitional (Paterno) and alkaline (Mt. Maletto, Mt. Spagnolo and Timpa di Acireale) basalts has been undertaken, using the electron microprobe and heating/freezing stages.The maximum Fo content in olivine increases from 86 mol.% in tholeiites, to 88 mol.% in transitional basalt and up to 89-90.5 mol.% in alkaline basalts. The relationship between Fo of the most primitive olivine from alkaline samples and the Cr# of coexisting spinel indicates that their parental melts were near primary. Clinopyroxene Mg# values (maximum and range) correspond closely to those of olivine, suggesting their early co-crystallization. Orthopyroxene (Mg# 86 mol.%) has been found as inclusions in the most magnesian olivine in tholeiitic lavas only.Crystallization temperatures determined from melt inclusion studies revealed no contrast between samples of different affinity, and range from 1240 degrees to 1100 oC. The pressure of crystallization is believed to be higher than 2 kbar (up to 6 kbar), as indicated by the density of primary CO2 inclusions in olivine, and indirectly, by the early clinopyroxene crystallization.Melt inclusions in olivine from tholeiitic lavas form a continuous trend from Q-normative, low potassium and low phosphorus (0.2 wt%) to Si-undersaturated, Ne-normative compositions, enriched in K2O and P2O5 (similar to 1.6 wt%). The range in melt inclusion compositions cannot be accounted for by crystal fractionation and was probably generated during partial melting.The melts reconstructed from melt inclusions in olivine phenocrysts from a Mt. Maletto alkaline lava define a compositional trend consistent with early clinopyroxene + olivine fractionation. The melt equilibrated with the most primitive olivine (Fo(90.5)), clinopyroxene (Mg# 92) and Cr-spinel (Cr# 80) is considered to be a near-primary melt (Mg# 71-74 mol.%; CaO/Al2O3 ~ 1.3), formed by the melting of a clinopyroxene-rich source. The likelihood of a single magma parental to both the tholeiitic and alkaline suites is confidently ruled out.Comparison of characteristic features of the magmatism of Mt. Etna and adjacent areas (Mt. Iblei and the Aeolian are) testifies to progressive depletion of a mantle source by continuous magma extraction, and its heterogeneous chemical and modal modification.

Primitive basaltic melts included in podiform chromites from the Oman Ophiolite

Abstract In an attempt to characterize the composition of the parental melts of ophiolitic chromitites and their tectonic setting, we have undertaken a study of polymineralic solid inclusions trapped in chromites from the Oman Ophiolite (Sumail nappe). High-temperature experiments performed on inclusions show that they result from post-entrapment crystallization of homogeneous basaltic melts with primitive compositions (Mg# = 63.5–66.8). The primary nature of the inclusions, demonstrated by their distribution outlining the crystallographic zones of mineral growth, indicates that the trapped melts represent small amounts of the parental liquids of the host chromites. Homogenised melt inclusions show depleted trace element patterns, with significant Nb depletions, characteristic of a subduction-related origin. These observations indicate that chromite deposits from Oman harzburgitic ophiolites may have formed in a geodynamic setting akin to present-day back-arc basins.

The relationship between potassic, calc-alkaline and Na-alkaline magmatism in South Italy volcanoes: A melt inclusion approach ☆

The present-day tectonic setting of the Tyrrhenian Sea is dominated by the eastward migration of the Tyrrhenian^ Appenines subduction system and the existence of a contemporaneous and parallel extensional^compressional regime. This complex setting leads to the occurrence of a wide spectrum of magma-types in the South Italy volcanoes. Here, major and trace-element data for primitive melt inclusions preserved in olivine phenocrysts have been obtained in order to add constraints on the origin of the calc-alkaline magmas from the Aeolian arc (Stromboli and Vulcano islands), the potassic magmas from the Campania Province (Vesuvius and Phlegraean Fields) and the Na-alkaline magmas from Ustica Island. The approach used to determine the possible mantle sources of the trapped melts for each population of melt inclusions is based on the determination of the trace-element incompatibility sequence taken as the relative order of increasing bulk partition coefficients, which depends on the mineralogy of the source and gives direct information about minerals residual at the time of melting. Compositional similarities between the melt inclusions and their host lavas suggest that shallow-level magma contamination did not contribute significantly to the geochemical characteristics of the magma-types encountered in the region. Results of the trace-element modelling indicate that the melt inclusions from the Aeolian Islands and Campania Province volcanoes originate from mantle sources strongly affected by subduction-related metasomatic processes. Trace-element relationships of melt inclusions from Vulcano and Stromboli reflect melting of peridotitic sources that have been enriched by a slab-derived, aqueous fluid formed during dehydratation of K-free phases at shallow to intermediate depths. The negative high-field strength elements (HFSE) anomalies of these inclusions were generated in the absence of any residual phase in which HFSE might be compatible. In addition, their major element characteristics require the involvement of a clinopyroxene-rich source component in their genesis. With regard to the Vesuvius and Phlegraean Fields melt inclusions, their calculated incompatibility sequences point to a common phlogopite-bearing mantle source likely to be the result of interaction and hybridisation reactions with K2O and H2O-rich phases released from the slab at larger depths. Finally, trace

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.

Magmatology of Mt. Pelée (Martinique, F.W.I.). II: petrology of gabbroic and dioritic cumulates

Abstract The study of 24 plutonic xenoliths collected from Mt. Pelee pyroclastic deposits shows that they can be subdivided into: anorthite-pargasite-salite-bearing gabbroic adcumulates (type I: olivine-bearing; type II: magnetite-bearing); and bytownite-hornblende-hypersthene-augite-bearing dioritic orthocumulates. Additionally, olivine, diopside, pargasite and anorthite from a basaltic megacryst-rich ash layer have been considered. Mineralogical data show continuous and progressive variations in the compositions of the solid phases with increase of Fe/(Fe+Mg) ratios during the petrographic evolution of the investigated cumulate xenoliths. Thermobarometric data on minerals or mineral/liquid pairs indicate a decrease of crystallization temperatures from about 1090°C in gabbroic assemblages to about 850°C in dioritic assemblages. The gabbroic assemblage is believed to form in a water-bearing environment from a derivative calc-alkaline liquid at sufficient depth to stabilize Al-rich pargasite (i.e.: 8–9 kbar). We suggest that the materials investigated may represent, respectively, the bottom (megacrysts), the walls (type-I and -II gabbroic adcumulates), and the roof (dioritic orthocumulates) of stratified magma chamber(s) underlying Mt. Pelee volcano.