Melt Inclusions In Olivine Research Articles

Subduction cycling of volatiles and trace elements through the Central American volcanic arc: evidence from melt inclusions

Compositions of melt inclusions in olivine (Fo90-64) from 11 localities in Guatemala, Nicaragua and Cost Rica along the Central American Volcanic Arc are used to constrain combined systematics of major and trace elements and volatile components (H2O, S, Cl, F) in parental melts and to estimate volcanic fluxes of volatile elements. The melt inclusions cover the entire range of compositions reported for whole rocks from Central America. They point to large heterogeneity of magma sources on local and regional scales, related to variable contributions of diverse crustal (from the subducting and overriding plates) and mantle (from the wedge and incoming plate) components involved in magma genesis. Water in parental melts correlates inversely with Ti, Y and Na and positively with Ba/La and B/La (with the exception of Irazu Volcano), which indicates mantle melting fluxed by Ba-, B- and H2O-rich, possibly, serpentinite-derived fluid beneath most parts of the arc. Different components with melt-like characteristics (high LREE, La/Nb and probably also Cl, S and F and low Ba/La) control the geochemical peculiarities of Guatemalan and Costa Rican magmas. The composition of parental magmas together with published data on volcanic volumes and total SO2 flux from satellite measurements are used to constrain fluxes of volatile components and to estimate total magmatic flux in Central America. We found that volcanic flux accounts for only 13% of total magmatic and volatile fluxes. The remaining 87% of magmas remained in the lithosphere to form cumulates (∼39%) and intrusives (∼48%). The intrusive fraction of magmatic flux may be significantly larger beneath Nicaragua compared to Costa Rica. Interestingly, total fluxes of magmas and volatiles in Central America are quite similar to the global average estimates.

Melt inclusions in olivine phenocrysts in unaltered kimberlites from the Udachnaya-East pipe, Yakutia: Some aspects of kimberlite magma evolution during late crystallization stages

The results of a complex study of melt inclusions in olivine phenocrysts contained in unaltered kimberlites from the Udachnaya-East pipe indicate that the inclusions were captured late during the magmatic stage, perhaps, under a pressure of <1 kbar and a temperature of ≤800°C. The inclusions consist of fine crystalline aggregates (carbonates + sulfates + chlorides) + gas ± crystalline phases. Minerals identified among the transparent daughter phases of the inclusions are silicates (tetraferriphlogopite, olivine, humite or clinohumite, diopside, and monticellite), carbonates (calcite, dolomite, siderite, northupite, and Na-Ca carbonates), Na and K chlorides, and alkali sulfates. The ore phases are magnetite, djerfisherite, and monosulfide solid solution. The inclusions are derivatives of the kimberlite melt. The complex silicate-carbonate-salt composition of the secondary melt inclusions in olivine from the kimberlite suggests that the composition of the kimberlite melt near the surface differed from that of the initial melt composition in having higher contents of CaO, FeO, alkalis, and volatiles (CO2, H2O, F, Cl, and S) at lower concentrations of SiO2, MgO, Al2O3, Cr2O3, and TiO2. Hence, when crystallizing, the kimberlite melt evolved toward carbonatite compositions. The last derivatives of the kimberlite melt had an alkaline carbonatite composition.

Evolution of Mafic Alkaline Melts Crystallized in the Uppermost Lithospheric Mantle: a Melt Inclusion Study of Olivine-Clinopyroxenite Xenoliths, Northern Hungary

Olivine-clinopyroxenite xenoliths exhumed in alkali basalts (sensu lato) in the Nograd^Go« mo« r Volcanic Field (NGVF), northern Hungary, contain abundant silicate melt inclusions. Geothermobarometric calculations indicate that these xenoliths crystallized as cumulates in the upper mantle near the Moho. These cumulate xenoliths are considered to represent a period of Moho underplating by mafic alkaline magmas prior to the onset of Late Tertiary alkaline volcanism in the Carpathian^Pannonian region. The major and trace element compositions of silicate melt inclusions in olivine display an evolutionary trend characterized by a strong decrease in CaO/Al2O3. The parental melt of the cumulates was a basanite formed by low-degree (� 2%) partial melting of a garnet peridotite source. The compositional trend of the silicate melt inclu- sions, textural features, and modelling with pMELTS show that the parental melt evolved by major clinopyroxene and minor olivine crys- tallization followed by the appearance of amphibole simultaneously with significant resorption of the earlier clinopyroxene and olivine. The resulting residual melt was highly enriched in Al2O3, alkalis and most incompatible trace elements. This type of melt is likely to infiltrate and react with surrounding mantle peridotite as a metaso- matic agent. It might also form high-pressure pegmatite-like bodies in the mantle that might be the source of the amphibole and sanidine megacrysts also found in the alkali basalts of the NGVF. Preferential remelting of the later-formed (i.e. lower temperature) mineral assemblage (amphibole, sanidine, residual glass) might have signif- icantly contaminated the host alkaline mafic lavas, increasing their Al2O3 and total alkali contents and, therefore, reducing their MgO, FeO and CaO content.

Constraints on mantle melting and composition and nature of slab components in volcanic arcs from volatiles (H 2O, S, Cl, F) and trace elements in melt inclusions from the Kamchatka Arc

New and published data on the composition of melt inclusions in olivine (Fo 73–91) from volcanoes of the Kamchatka and northern Kurile Arc are used 1) to evaluate the combined systematics of volatiles (H 2O, S, Cl, F) and incompatible trace elements in their parental magmas and mantle sources, 2) to constrain thermal conditions of mantle melting, and 3) to estimate the composition of slab-derived components. We demonstrate that typical Kamchatkan arc-type magmas originate through 5–14% melting of sources similar or slightly more depleted in HFSE (with up to ∼ 1 wt.% previous melt extraction) compared to MORB-source mantle, but strongly enriched in H 2O, B, Be, Li, Cl, F, LILE, LREE, Th and U. Mean H 2O in parental melts (1.8–2.6 wt.%) decreases with increasing depth to the subducting slab and correlates negatively with both ‘fluid-immobile’ (e.g. Ti, Na, LREE) and most ‘fluid-mobile’ (e.g. LILE, S, Cl, F) incompatible elements, implying that solubility in hydrous fluids or amount of water does not directly control the abundance of ‘fluid-mobile’ incompatible elements. Strong correlation is observed between H 2O/Ce and B/Zr (or B/LREE) ratios. Both, calculated H 2O in mantle sources (0.1–0.4%) and degrees of melting (5–14%) decrease with increasing depth to the slab indicating that the ultimate source of water in the sub-arc mantle is the subducting oceanic plate and that water flux (together with mantle temperature) governs the extent of mantle melting beneath Kamchatka. A parameterized hydrous melting model [Katz et al. 2003, G 3, 4(9), 1073] is utilized to estimate that mantle melting beneath Kamchatka occurs at or below the dry peridotite solidus (1245–1330 °C at 1.5–2.0 GPa). Relatively high mantle temperatures (yet lower than beneath back-arc basins and ocean ridges) suggest substantial corner flow driven mantle upwelling beneath Kamchatka in agreement with numerical models implying non-isoviscous mantle wedge rheology. Data from Kamchatka, Mexico and Central America indicate that < 5% melting would take place beneath continental arcs without water flux from the subducting slab. A broad negative correlation appears to exist between crustal thickness and the temperature of magma generation beneath volcanic arcs with larger amounts of decompression melting occurring beneath thinner arc crust (lithosphere). In agreement with the high mantle temperatures, we observe a systematic change in the composition of slab components with increasing slab depth from solute-poor hydrous fluid beneath the volcanic front to solute-rich hydrous melt or supercritical liquid at deeper depths beneath the rear arc. The solute-rich slab component dominates the budget of LILE, LREE, Th and U in the magmas and originates through wet-melting of subducted sediments and/or altered oceanic crust at ≥ 120 km depth. Melting of the upper parts of subducting plates under water flux from deeper lithosphere (e.g. serpentinites), combined with high temperatures in the mantle wedge, may be a more common process beneath volcanic arcs than has been previously recognized.

Coexisting High- and Low-Calcium Melts Identified by Mineral and Melt Inclusion Studies of a Subduction-Influenced Syn-collisional Magma from South Sulawesi, Indonesia

Mineral and melt inclusions in olivines from the most Mg-rich magma from the southern West Sulawesi Volcanic Province indicate that two distinct melts contributed to its petrogenesis. The contribution that dominates the whole-rock composition comes from a liquid with high CaO (up to 16 wt %) and low Al2O3 contents (CaO/Al2O3 up to 1), in equilibrium with spinel, olivine (Fo85–91; CaO 0·35–0·5 wt %; NiO 0·2–0·30 wt %) and clinopyroxene. The other component is richer in SiO2 (>50 wt %) and Al2O3 (19–21 wt %), but contains significantly less CaO (<4 wt %); it is in equilibrium with Cr-rich spinel with a low TiO2 content, olivine with low CaO and high NiO content (Fo90–94; CaO 0·05–0·20 wt %; NiO 0·35–0·5 wt %), and orthopyroxene. Both the high- and low-CaO melts are potassium-rich (>3 wt % K2O). The high-CaO melt has a normalized trace element pattern that is typical for subduction-related volcanic rocks, with negative Ta–Nb and Ti anomalies, positive K, Pb and Sr anomalies, and a relatively flat heavy rare earth element (HREE) pattern. The low-CaO melt shows Y and HREE depletion (Gdn/Ybn ≤41), but its trace element pattern resembles that of the whole-rock and high-CaO melt in other respects, suggesting only small distinctions in source areas between the two components. We propose that the depth of melting and the dominance of H2O- or CO2-bearing fluids were the main controls on generating these contrasting magmas in a syn-collisional environment. The composition of the low-CaO magma does not have any obvious rock equivalent, and it is possible that this type of magma does not easily reach the Earth's surface without the assistance of a water-poor carrier magma.

Melt evolution during the crystallization of basanites of the Tergesh pipe, northern Minusinsk Depression

In this paper we describe the mineralogy and geochemistry of basanites and melt inclusions in minerals from the Tergesh pipe, northern Minusinsk Depression. The rocks are composed of olivine and clinopyroxene phenocrysts and a groundmass of olivine, clinopyroxene, titanomagnetite, plagioclase, apatite, ilmenite, and glass. Melt inclusions were found only in the olivine and clinopyroxene phenocrysts. Primary melt inclusions in olivine contain glass, rhonite, clinopyroxene, a sulfide globule, and low-density fluid. The phase composition of melt inclusions in clinopyroxene is glass + low-density fluid ± xenogenous magnetite. According to thermometric investigations, the olivine phenocrysts began crystallizing at T = 1280–1320°C and P > 3.5 kbar, whereas groundmass minerals were formed under near-surface conditions at T ≤ 1200°C. The oxygen fugacity gradually changed during basanite crystallization from oxidizing (NNO) to more reducing conditions (QFM). The investigation of glass compositions (heated and unheated inclusions in phenocrysts and groundmass) showed that the evolution of a basanite melt during its crystallization included mainly an increase in SiO2, Al2O3, and alkalis, while a decrease in femic components, and the melt composition moved gradually toward tephriphonolite and trachyandesite. Geochemical evidence suggests that the primary basanite melt was derived from a mantle source affected by differentiation.

FIB, TEM and LA–ICPMS investigations on melt inclusions in Martian meteorites—Analytical capabilities and geochemical insights

In order to obtain full information coverage on melt inclusions in Martian meteorites (subgroup nakhlites) complementary micro-analytical techniques were used, i.e. focused ion beam, transmission electron microscopy and laser ablation. Using focused ion beam several lamellae for transmission electron microscopy were prepared and secondary electron images of cross-sections could be acquired. Laser ablation–inductively coupled plasma mass spectrometry analyses were performed on selected inclusions to obtain mass-oriented bulk composition of inclusions at depth. The differences in composition between melt inclusions in olivine and augite crystals would suggest a xenocrystic origin for olivine. Furthermore, electron diffraction patterns clearly indicated that the SiO 2-rich phase in inclusions from augite in meteorites from Northwest Africa site is re-crystallized, whereas it is still vitreous in the inclusions from Nakhla sampling site. Therefore, different post-entrapment evolutions were active for the two nakhlite meteorite sets, the Nakhla and the NWA817 set. Melt inclusions in Nakhla olivine presented alteration veins, which were presumably produced before their landing on Earth. If this is the case, this would indicate a alteration stage already on Mars with all the consequence in terms of climate history. Melt inclusions in Nakhla augite resulted unaffected by any alteration or modification following the entrapment, and therefore represent the best candidate to indicate the pristine magma composition.

Vigorous Exsolution of Volatiles in the Magma Chamber beneath a Hydrothermal System on the Modern Sea Floor of the Eastern Manus Back-Arc Basin, Western Pacific: Evidence from Melt Inclusions

Image analyses and ion probe analyses were carried out on melt inclusions in olivine, pyroxene, and plagioclase phenocrysts of vesicular volcanic rocks that host the actively forming massive sulfides at the PACMANUS hydrothermal field in the eastern Manus back-arc basin, western Pacific. The melt inclusions, ranging in size from 150 μm, contain one or more bubbles. Most of them have bubble/melt inclusion volume ratios (Vb/Vm) of >5.5 vol percent, higher than those for the melt inclusions with their bubbles generated from shrinkage (Vb/Vm, 3.6 vol %) or postentrapment exsolution of volatiles (Vb/Vm, 3.7 vol %). This, together with the presence of vapor bubbles with little melt (Vb/Vm, 80 to >95 vol %) in pyroxene and olivine phenocrysts, suggests that a vapor phase was formed from vigorous exsolution of volatiles or the boiling of a magma in the crystallizing magma chamber. The Vb/Vm ratios show distinct distribution patterns in the phenocryst minerals, indicating that there was more than one vesiculation event in the magma chamber. Melt inclusions with no visible bubbles have the highest contents of H2O (2.1–2.5 wt %). The H2O contents of the glass in the melt inclusions are inversely related to the size of the coeval bubbles. The significant variation of H2O contents (1.0–2.5%) in the melt inclusions suggests that the H2O was degassed from the crystallizing magma. The positive correlations of S, Cl, and possibly F with H2O indicate that all these volatiles were exsolved from the melt into the vapor phase. The negative correlation of H2O with alkali contents in the glass of melt inclusions suggests that the magma was crystallizing in the presence of a fluid phase. It is estimated that the magma might have lost up to 65 percent of its volatiles while crystallizing. Significant amounts of magmatic fluid can be degassed from a magma during its crystallization in a shallow magma chamber. An individual vesiculation event may last at least 5 to 48 years, as estimated from the growth of the phenocrysts. This estimate is a minimum because not all the phenocrysts crystallize at the same time. The estimated life of a black smoker chimney is 2 to 10 years, which is within the duration of an individual vesiculation event. This implies that the magmatic fluid could be a major source of ore metals during the growth of individual sulfide chimneys. Through multiple vesiculation events in a crystallizing magma chamber, the magma could provide large quantities of ore metals and volatiles to a hydrothermal system over a long period of time to form a large massive sulfide deposit on the sea floor.

Immiscible iron- and silica-rich melts in basalt petrogenesis documented in the Skaergaard intrusion

Silicate liquid immiscibility in basalt petrogenesis is a contentious issue. Immiscible iron- and silica-rich liquids were reported in melt inclusions of lunar basalt and in groundmass glasses of terrestrial volcanics. In fully crystallized plutonic rocks, however, silicate liquid immiscibility has yet to be proven. Here we report the first finding of natural, immiscible iron- and silica-rich melts in a plutonic environment documented in the Skaergaard intrusion, East Greenland. Primary melt inclusions (now finely crystallized) in apatite are either dark or light colored. The predominant dark colored type contains 30.9 ± 4.2 wt% FeO t and 40.7 ± 3.6 wt% SiO 2 , whereas the light colored type contains 8.6 ± 5.9 wt% FeO t and 65.6 ± 7.3 wt% SiO 2 . Similar light colored melt inclusions in olivine and fine-grained dark and light colored interstitial pockets also give evidence of crystallization from emulsion of silica and iron-rich liquids. On the outcrop scale, silica-rich (melanogranophyre) pods and layers in iron-rich ferrodiorite of the Upper Zone of the Skaergaard intrusion witness segregation of the two liquids. These findings demand that silicate immiscibility is considered in basalt petrogenesis. Some granitic rocks may represent unmixed silica-rich melt, whereas the dense, iron-rich melt is likely to sink in the crust and could mix with hot mantle-derived magma to form unusual rocks, like ferropicrites, otherwise interpreted as products of heterogeneous mantle sources.

Sayh al Uhaymir 094: A new martian meteorite from the Oman desert

Abstract— Sayhal Uhaymir (SaU) 094 is a 223.3 g, partially crusted, strongly to very strongly shocked melanocratic olivine‐porphyric rock of the shergottite group showing a microgabbroic texture. The rock consists of pyroxene (52.0–58.2 vol%)—dominantly prismatic pigeonite (En60–68Fs20–27Wo7–9) associated with minor augite (En46–49Fs15–16Wo28–31)—brown (shock‐oxidized) olivine (Fo65–69; 22.1–31%), completely isotropic interstitial plagioclase glass (maskelynite; An50–64Or0.3‐0.9; 8.6–13.0%), chromite and titanian magnesian chromite (0.9‐1.0%), traces of ilmenite (Ilm80–86), pyrrhotite (Fe92–100; 0.1‐0.2%), merrillite (&lt;&lt;0.1%), and pockets (4.8‐6.7%) consisting of green basaltic to basaltic andesitic shock glass that is partially devitrified into a brown to black product along boundaries with the primary minerals. The average maximum dimensions of minerals are: olivine (1.5 mm), pyroxene (0.3 mm) and maskelynite (0.3 mm). Primary melt inclusions in olivine and chromite are common and account for 0.1‐0.6% of the rock. X‐ray tomography revealed that the specimen contains ˜0.4 vol% of shock‐melt associated vesicles, up to 3 mm in size, which show a preferred orientation. Fluidization of the maskelynite, melting and recrystallization of pyroxene, olivine and pyrrhotite indicate shock stage S6. Minor terrestrial weathering resulted in calcite‐veining and minor oxidation of sulfides. The meteorite is interpreted as paired with SaU 005/008/051. The modal composition is similar to Dar al Gani 476/489/670/735/876, with the exception that neither mesostasis nor titanomagnetite nor apatite are present and that all phases show little zonation. The restricted mineral composition, predominance of chromite among the oxides, and abundance of olivine indicate affinities to the lherzolitic shergottites.

High-temperature experiments on silicate melt inclusions in olivine at 1 atm: inference on temperatures of homogenization and H 2O concentrations

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 2O of melt inclusions in olivine phenocrysts (Fo 82–89). The samples were collected at different volcanic sites (Piton de la Fournaise [PdF], Réunion 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 2O 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 2O. 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 2O decreases with time. The H 2O-rich melt inclusions may have lost from 20% to 80% H 2O 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.

Reduced magmatic fluids in basalt from the island of Disko, central West Greenland

Fluid and melt inclusions in minerals of two native iron-bearing dyke rocks from Disko, Greenland, were investigated using heating and cooling microscopic techniques. The melt inclusions in olivine are homogenized at temperatures of 1170–1200 °C. The melts are characterized by high SiO 2, K 2O and TiO 2 and low MgO contents. The co-existence of combined melt–fluid (gas) inclusions in olivine implies that the olivine phenocrysts crystallized from a heterogeneous magma at low pressure. Gas bubbles were investigated in the glass of the rock. Water and four crystalline phases were observed at room and low temperatures on the walls of the bubbles: serpentine, naphthalene, clathrate, and an unknown phase. The complex composition of trapped volatiles is reflected in considerable ranges of clathrate melting and solidus temperatures (20 to 7.5 °C and −82.6 to −73 °C, respectively). Melt equilibrium with metallic iron at T=1450 K suggests that log f O 2 =−13.95. The calculated composition of C–O–H fluid in equilibrium with Fe, Fe 3C and FeS is characterized by high CH 4 and H 2 contents. During cooling at the postmagmatic stage, polymerization proceeds, and heavy hydrocarbons are formed inside vesicles. The presence of organic compounds in inclusions trapped at a magmatic stage is a natural consequence of unusual reducing condition during crystallization.

The role of apatite fractionation and REE distribution in alkaline rocks from Mt. Etna, Sicily

Melt inclusions in olivine and apatite, and REE distribution of apatite were studied in one of the least differentiated members of the oldest alkaline succession of Mt. Etna. Apatite occurs both as microphenocrysts and as inclusions in olivine crystals, even in the most Mg-rich ones (Fo82). In addition phenocrysts and groundmass are composed of plagioclase, clinopyroxene, olivine and magnetite.

Petrogenesis of the crust‐mantle transition zone and the origin of lower crustal wehrlite in the Oman ophiolite

We studied trace element geochemistry and petrology of the crust‐mantle transition zone (MTZ) in the Samail massif of the Oman ophiolite to constrain the location where different primitive magmas mix beneath an oceanic spreading ridge. The MTZ is the deepest location where crystallization took place and thus is an ideal place to determine the compositional diversity of melts leaving the mantle, with various sources and degrees of depletion. We have reached three main conclusions: (1) More than 90% of our samples record equilibration with compositionally indistinguishable parental melts, similar to mid‐ocean ridge basalts (MORB) and the melts that formed the crust in Oman. This suggests that mixing of diverse, polybaric partial melts of mantle peridotite occurred at or below the depth of the MTZ. The presence of distinct heterogeneity in less than 10% of our samples is similar to the nature and frequency of heterogeneity observed in melt inclusions in olivines from MORB. (2) Among the samples recording trace element equilibrium with MORB‐like liquids are wehrlitic rocks, previously suggested to be cumulates from an unusual parental melt on the basis of petrological observation. (3) Systematics of Eu distribution among plagioclase and clinopyroxene in “impregnated peridotites” demonstrate that these minerals did not crystallize from “trapped melt.” As a consequence, it is not possible to use the modal proportion or texture of plagioclase + clinopyroxene impregnations to estimate the instantaneous melt porosity or the shape of melt pores at any time during the formation of these rocks.

Cr-spinel, an excellent micro-container for retaining primitive melts – implications for a hydrous plume origin for komatiites

Ultramafic melt inclusions were discovered in Cr-spinels of 2.7 Ga Al-undepleted komatiites from the Belingwe Greenstone Belt, Zimbabwe. The inclusions consist of glass and sub-micrometer-size quench crystals of olivine and clinopyroxene. Homogenized melt inclusions are highly magnesian, ranging from 12.5 to 19.5 wt% in MgO content, and are also close to host komatiitic magma in other compositions. This fact indicates entrapment of melt into host spinel during the early stages of crystallization. The water content of two melt inclusions was determined using an ion probe; the high magnesian melt inclusion, 17.5 wt% in MgO, contains 1.1 wt% H2O, whereas the moderately magnesian melt inclusion, 11.8 wt% in MgO, has 1.7 wt% H2O. This evidence suggests that the primary komatiite melt contained 0.8–0.9 wt% H2O and 23.4–25.0 wt% MgO. The water content is about five times greater than previous estimates from melt inclusions in olivine [McDonough and Danyushevsky, EOS Trans. AGU 76 (1995) S266]. In addition, even the high H2O content preserved in melt inclusions within Cr-spinel may represent the minimum estimates of the parental composition, because part of the water should be dehydrated from parental magma during crystallization in the magma chamber. If the komatiite melt was formed by a high degree of partial melting of a peridotite, the source mantle should contain considerable amounts of water (∼0.5 wt%). However, recent melting experiments of hydrous peridotite indicate that the addition of 0.5 wt% H2O to mantle peridotite would not significantly decrease komatiite liquidus temperature [Asahara et al., Geophys. Res. Lett. 25 (1998) 2201–2204]. Petrological study of melt inclusions together with experimental data suggest that Belingwe komatiites were formed from a hydrous plume at high temperatures.

Melt Generation and Movement beneath Theistareykir, NE Iceland

A detailed study of the volume and composition of all the lavas from the Theistareykir segment of the Northern Volcanic Zone of Iceland was designed to study basaltic melt generation and movement beneath a spreading ridge. The trace element compositions of the lavas are variable, and those of melt inclusions in olivine, clinopyroxene and plagioclase phenocrysts even more so. We show that this variability can be produced by mixing instantaneous melts produced by isentropic decompression of mantle whose initial potential temperature is 1480°C, and that the calculated volume and composition of the average melt is consistent with geophysical and petrological observations. Pressure and temperature estimates suggest that the phenocrysts form in the upper mantle, at depths of 30–40 km, and trap melts formed at greater depths. Some mixing of the instantaneous melts occurs before the melt is trapped, and more mixing occurs before the lavas are erupted. A similar model can account for the composition of melt inclusions from the FAMOUS area of the Mid-Atlantic Ridge, and from the Gorda and Juan de Fuca Ridges.

Early evolution of Martian volatiles: Nitrogen and noble gas components in ALH84001 and Chassigny

Studies on SNC meteorites have permitted the characterization of modern Martian atmospheric components as well as indigenous Martian nitrogen and solar‐type xenon. New isotopic and elemental abundances of noble gases and nitrogen in ALH84001 and Chassigny provide important constraints on the early evolution of the planet. A primitive solar Xe component (Chass‐S) and an evolved Xe component (Chass‐E), augmented with fission Xe are identified in Chassigny. Both components represent interior reservoirs of Mars and are characterized by low 129Xe/132Xe (&lt;1.07) and by distinct elemental ratios 36Ar/132Xe &lt;5 and &gt;130, respectively. Light nitrogen (δ15N = −30‰) is associated with the Chass‐S component and is enriched in melt inclusions in olivine. An ancient (presumably incorporated ∼4 Gyr ago) evolved Martian atmospheric component is identified in ALH84001 and has the following signatures: 129Xe/132Xe = 2.16, 36Ar/38Ar ≥ 5.0, 36Ar/132Xe = ∼50, 84Kr/132Xe = ∼6, and δ15N = 7‰. The trapped Xe component in ALH84001 is not isotopically fractionated. We observe major shifts in nitrogen signatures due to cosmogenic N component in both Chassigny and ALH84001. A heavy nitrogen component of comparable magnitude (δ15N&gt;150‰) has previously been interpreted as (heavy) Martian atmospheric N. In situ produced fission Xe components, due to 244Pu in ALH84001 and due to 238U in Chassigny, are identified. The ALH84001 data strongly constrain exchanges of Martian atmospheric and interior reservoirs. Mars retained abundant fission Xe components, and this may account for the low observed fission Xe component in the modern Martian atmosphere. Chronometric information regarding the evolution of the early Martian atmosphere can be secured from the relative abundances of radiogenic and fission Xe, as ∼80% of the Martian 129Xer is observed in the atmospheric 129Xe/132Xe ratio ∼4 Gyr ago.

Melt inclusions in olivine and pyroxene phenocrysts from lamprophyres of Chhaktalao Area, Madhya Pradesh, India

Melt inclusions in olivine and pyroxene phenocrysts in kersantite and camptonite at Chhaktalao in Madhya Pradesh, India are mainly of the evolved type forming daughter minerals of olivine, pyroxene, plagioclase, spinel, mica, titanomagnetite and sulphides. Heating studies exhibit a temperature range from 1215° to 1245°C for the melt inclusions in olivine in camptonite and 1220–1245°C for olivine in kersantite. The temperature for melt inclusions in pyroxene ranged from 1000° to 1150°C in camptonite and 850–1100°C for pyroxene in kersantite. The bubble inside these melt inclusions is mainly CO 2. The T h°C of CO 2 into liquid phase occurred between 26° and 31°C in olivine and 25–30°C in pyroxene from kersantite and camptonite. The maximum density estimated is 0.72 g/cm 3 and the minimum is 0.45 g/cm 3. The depth of entrapment of the melt inclusion is estimated between 10–15 km. The pressure of entrapment of melt inclusion in olvine is 4.6 kbar where as that in pyroxene is 3.7 kbar. The lamprophyres in the Chhaktalao area are considered to be derived from low depth and low pressure region, possibly within spinel lherzolite zone.

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 (Fo 80–91) are present as neoblasts or in finely recrystallized patches. The most magnesian olivines (Fo 89.7–91, CaO < 0.1 wt%) contain primary silicic melt inclusions (SiO 2 = 63.5–60 wt%) with high sulfur concentrations (1440–2540 ppm S), associated with MSS (Monosulfide Solide Solution) globules. The occurrence of both S-rich silicic melts saturated with respect to sulfur, at fO 2 close to Ni–NiO buffer, and multiphase fluid inclusions composed of an S–H 2O-rich vapor phase, a sulfide globule and casual anhydrite strongly suggests high fS 2. The melt and the composite inclusions are interpreted as resulting from the immiscibility between hydrous S-rich silicic melt, sulfide melt, and H 2O–S rich vapor. The S-rich fluid inclusions also indicate the possible existence of an exsolved fluid phase containing sulfur at depth. The moderately silicic melt inclusions (SiO 2 = 62–56.6 wt%) in the Fe-rich olivine (Fo 88.6–80) are depleted with respect to sulfur (S = 430 to 790 ppm), the concentration of which is controlled by the vapor phase. Because the silicic melt inclusions have been previously interpreted as produced by variable extent of melting of the subducted oceanic crust, sulfur in the silicic melts, in the sulfide globules and in the H 2O-rich phase, should therefore be inherited and recycled from the subducted slab through S–H 2O-silicate rich fluids or supercritical fluids (or melts). In the calc-alkaline basalt sample, the olivine phenocrysts (Fo > 85, CaO = 0.25 wt%) and their melt inclusions (CaO/Al 2O 3 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 (1 σ=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, H 2O-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 fO 2 and fS 2.

Magma–conduit interaction at Piton de la Fournaise volcano (Réunion Island): a melt and fluid inclusion study

Major-element, Cl, S, F analyses have been performed on a wide selection of melt inclusions trapped in olivine (Fo 81–87) from scoria and crystal-rich lapilli samples of Piton de la Fournaise volcano. As a whole, they display a transitional basaltic composition. The melt inclusions (8–9 wt.% MgO, 0.62–0.73 wt.% K 2O) are in equilibrium with olivines (Fo 81–85) in the samples from the Central Feeding Zone and the South-East Feeding Zone and show a slight alkaline affinity. The melt inclusions in olivines (Fo 85–87) from the North-West Rift (NWR) contain 9.3–9.7 wt.% MgO and 0.54–0.58 wt.% K 2O, with a more tholeiitic tendency. In oceanitic lavas and crystal-rich lapilli, the olivine xenocrysts are recognisable by the presence of one or more secondary shear plane fracture(s) filled up with CO 2 and alkali-rich basaltic melt inclusions. In dunite nodules, olivines present also contain several secondary shear plane fracture(s) filled up with CO 2 and high-SiO 2 melt inclusions. Secondary CO 2-rich fluid inclusions in olivine (Fo 85–87) from the NWR samples indicate PCO 2 up to 500 MPa whereas, PCO 2 ranges from 95 MPa to few tenths of bars in the other samples. Both the primary melt inclusions and the secondary fluid inclusions strongly suggest that the olivine crystallises and accumulates over a wide depth range (15 km). It is envisioned that cumulative pockets with low residual porosity are repeatedly percolated with a CO 2-rich fluid phase, possibly associated with basaltic to SiO 2-rich melts, and are finally disrupted and entrained to the surface when vigorous magma transfer occurs. The SiO 2-rich residual melts in early-formed dunitic or gabbroic bodies may have acted as contaminant agents for the more alkali character of magmas vented through the central feeding system, where a well-developed cumulative system is thought to exist. Finally, the existence of secondary fluid and melt inclusions in olivines implies that the dunitic bodies are weakened on the micrometric scale.