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Mount Somma–Vesuvius is a stratovolcano that represents a geological hazard to the population of the city of Naples and surrounding towns in southern Italy. Historically, volcanic eruptions at Mt. Somma–Vesuvius (SV) include high-magnitude Plinian eruptions, such as the infamous 79 CE eruption that occurred after 295 years of quiescence and killed thousands of people in Pompeii and surrounding towns and villages. The last eruption at SV was in 1944 and showed a Volcanic Explosivity Index (VEI) of 3 (0.01 km 3 of volcanic material erupted). Following the 1944 eruption, SV has been dormant for the past nearly 79 years, with only minor fumarolic and seismic activity. During its long history, centuries of dormancy at SV have ended with Plinian eruptions (VEI 6) that signal the beginning of a new cycle of eruptive activity. Thus, the current dormancy stage demands a need to better understand the mechanism involved in high-magnitude eruptions in order to better predict future eruption magnitude and style. Despite centuries of research on the SV volcanic system, many questions remain, including the evolution of magmatic volatiles from deep primitive magmas to shallower more evolved magmas. Developing a better understanding of the physical and chemical processes associated with volatile evolution at SV can provide insights into magma dynamics and the mechanisms that trigger highly explosive eruptions at SV. In this study, we present new data for the pre-eruptive volatile contents of magmas associated with four Plinian and two inter-Plinian eruptions at SV based on analyses of reheated melt inclusions (MIs) hosted in olivine. We correct the volatile contents of bubble-bearing MIs by taking into account the volatile contents of bubbles in the MIs. We recognize two groups of MIs: one group hosted in high-Fo olivine (Fo 85–90) and relatively rich in volatiles and the other group hosted in low-Fo olivine (Fo 70–69) and relatively depleted in volatiles. The correlation between volatile contents and compositions of host olivines suggests that magma fractionation took place under volatile-saturated conditions and that more differentiated magmas reside at shallower levels relative to less evolved/quasi-primitive magmas. Using the CO 2 contents of corrected MIs hosted in Fo 90 olivine from SV, we estimate that 347 to 686 t d -1 of magmatic CO 2 exsolved from SV magmas during the last 3 centuries (38–75 Mt in total) of volcanic activity. Although this study is limited to only few SV magmas, we suggest that further study applying similar methods could shed light on the apparent lack of correlation between the volatile contents of MIs and the style and age of eruptions. Further, such studies could provide additional constraints on the origin of CO 2 and the interaction between the carbonate platform and ascending magmas below SV. [ABSTRACT FROM AUTHOR]

Inclusions of basaltic melt trapped inside of olivine phenocrysts during igneous crystallization provide a rich, crystal-scale record of magmatic processes ranging from mantle melting to ascent, eruption, and quenching of magma during volcanic eruptions. Melt inclusions are particularly valuable for retaining information on volatiles such as H2O and CO2 that are normally lost by vesiculation and degassing as magma ascends and erupts. However, the record preserved in melt inclusions can be variably obscured by postentrapment processes, and thus melt inclusion research requires careful evaluation of the effects of such processes. Here we review processes by which melt inclusions are trapped and modified after trapping, describe new opportunities for studying the rates of magmatic and volcanic processes over a range of timescales using the kinetics of post-trapping processes, and describe recent developments in the use of volatile contents of melt inclusions to improve our understanding of how volcanoes work. Inclusions of silicate melt (magma) trapped inside of crystals formed by magma crystallization provide a rich, detailed record of what happens beneath volcanoes. These inclusions record information ranging from how magma forms deep inside Earth to its final hours as it ascends to the surface and erupts. The melt inclusion record, however, is complex and hazy because of many processes that modify the inclusions after they become trapped in crystals. Melt inclusions provide a primary archive of dissolved gases in magma, which are the key ingredients that make volcanoes erupt explosively. [ABSTRACT FROM AUTHOR]

The Tissint meteorite is a geochemically depleted, olivine-phyric shergottite. Olivine megacrysts contain 300-600 μm cores with uniform Mg# (~80 ± 1) followed by concentric zones of Fe-enrichment toward the rims. We applied a number of tests to distinguish the relationship of these megacrysts to the host rock. Major and trace element compositions of the Mg-rich core in olivine are in equilibrium with the bulk rock, within uncertainty, and rare earth element abundances of melt inclusions in Mg-rich olivines reported in the literature are similar to those of the bulk rock. Moreover, the P Kα intensity maps of two large olivine grains show no resorption between the uniform core and the rim. Taken together, these lines of evidence suggest the olivine megacrysts are phenocrysts. Among depleted olivine-phyric shergottites, Tissint is the first one that acts mostly as a closed system with olivine megacrysts being the phenocrysts. The texture and mineral chemistry of Tissint indicate a crystallization sequence of: olivine (Mg# 80 ± 1) → olivine (Mg# 76) + chromite → olivine (Mg# 74) + Ti-chromite → olivine (Mg# 74-63) + pyroxene (Mg# 76-65) + Cr-ulvöspinel → olivine (Mg# 63-35) + pyroxene (Mg# 65-60) + plagioclase, followed by late-stage ilmenite and phosphate. The crystallization of the Tissint meteorite likely occurred in two stages: uniform olivine cores likely crystallized under equilibrium conditions; and a fractional crystallization sequence that formed the rest of the rock. The two-stage crystallization without crystal settling is simulated using MELTS and the Tissint bulk composition, and can broadly reproduce the crystallization sequence and mineral chemistry measured in the Tissint samples. The transition between equilibrium and fractional crystallization is associated with a dramatic increase in cooling rate and might have been driven by an acceleration in the ascent rate or by encounter with a steep thermal gradient in the Martian crust. [ABSTRACT FROM AUTHOR]

Fluids exsolved from mafic melts are thought to be dominantly CO2-H2O ± S fluids. Curiously, although CO2 vapor occurs in bubbles of mafic melt inclusions (MI) at room temperature (T), the expected accompanying vapor and liquid H2O have not been found. We reheated olivine-hosted MI from Mt. Somma-Vesuvius, Italy, and quenched the MI to a bubble-bearing glassy state. Using Raman spectroscopy, we show that the volatiles exsolved after quenching include liquid H2O at room T and vapor H2O at 150 °C. We hypothesize that H2O initially present in the MI bubbles was lost to adjacent glass during local, sub-micrometer-scale devitrification prior to sample collection. During MI heating experiments, the H2O is redissolved into the vapor in the bubble, where it remains after quenching, at least on the relatively short time scales of our observations. These results indicate that (1) a significant amount of H2O may be stored in the vapor bubble of bubble-bearing MI and (2) the composition of magmatic fluids directly exsolving from mafic melts at Mt. Somma-Vesuvius may contain up to 29 wt% H2O. [ABSTRACT FROM AUTHOR]

Trace element concentrations in pyroxene, plagioclase, and olivine were measured in five diogenite breccias previously identified as containing distinct harzburgitic (ol+opx) and orthopyroxenitic (opx) lithologies (dimict). Three samples show two distinct populations of pyroxene trace element abundances, supporting their classification as dimict. These three meteorites show increases in Y, Yb, and HREE concentrations from harzburgitic to orthopyroxenitic pyroxenes, supporting the hypothesis that the lithologies are related through fractional crystallization whereby harzburgite olivine and pyroxene crystallized from the magma first followed by orthopyroxenite pyroxene. Depletions in LREE and Eu concentrations in the orthopyroxenitic lithology are most likely due to equilibration with LREE and Eu-rich phases, likely plagioclase, which is found primarily in that lithology. Two samples do not show evidence supporting a dimict classification. Large pyroxene trace element variation in one sample indicates that it is polymict, while uniform trace element distribution in the other suggests that it may be a monomict breccia. [ABSTRACT FROM AUTHOR]

- The absence of dunite (>90 vol% olivine) in the howardite, eucrite, and diogenite (HED) meteorite suite, when viewed with respect to spectroscopic and petrologic evidence for olivine on Vesta, is problematic. Herein, we present petrologic, geochemical, and isotopic evidence confirming that Miller Range (MIL) 03443, containing 91 vol% olivine, should be classified with the HED clan rather than with mesosiderites. Similarities in olivine and pyroxene FeO/MnO ratios, mineral compositions, and unusual mineral inclusions between MIL 03443 and the diogenites support their formation on a common parent body. This hypothesis is bolstered by oxygen isotopic and bulk geochemical data. Beyond evidence for its reclassification, we present observations and interpretations that MIL 03443 is probably a crustal cumulate rock like the diogenites, rather than a sample of the Vestan mantle. [ABSTRACT FROM AUTHOR]

• Primary and secondary sulfide melt inclusions occur in cumulate minerals. • Sulfide melt inclusions confirm sub-economic PGE tenor of magmatic sulfide mineralization. • Melt inclusions not impacted by post-solidus alteration/hydrothermal activity. • Low R factors likely limited the metal grades in mineralization. Magmatic Fe-Ni-Cu-Co sulfide melt inclusions are preserved within cumulate and intercumulate phases (apatite, plagioclase, olivine, ilmenite) in the mineralized Caribou Lake Gabbro, a marginal subalkaline-alkaline mafic-ultramafic rock assemblage that forms part of the multiphase Blatchford Lake intrusive suite, Northwest Territories, Canada. Three distinct styles of sulfide mineralization occur in the Caribou Lake Gabbro: (i) disseminated sulfides, (ii) semi-massive to massive sulfides, and (iii) trace sulfide "platinum-group element (PGE)-enriched" mineralization. Within all mineralization styles, the major mineral assemblage consists of pyrrhotite + chalcopyrite + pentlandite + magnetite + ilmenite. Trace metal phases identified in the disseminated and semi-massive to massive mineralization comprise sphalerite, galena, glaucodot-gersdorffite solid solution [(Co,Fe)AsS-NiAsS], molybdenite, tellurobismuthite (Bi 2 Te 3), altaite (PbTe), electrum, nickeline, melonite (NiTe 2), empressite (AgTe), and unidentified Pb-Se-Te, Bi-Te, and Pb-Te phases. No platinum-group minerals (PGM) were observed in any mineralization styles. Within coarse-grained olivine gabbro sulfide melt inclusions of primary origin occur in olivine and ilmenite, and secondary inclusions occur in plagioclase. Secondary sulfide melt inclusions also occur in cumulus apatite in pegmatitic gabbro. Sulfide melt inclusions consist predominantly of pyrrhotite, with lesser amounts and variable proportions of chalcopyrite, and cobaltian pentlandite (∼31–38 wt% Co), and magnetite. In a minority of inclusions, sphalerite, magnetite, and glaucodot-gersdorffite were also observed however, no PGM were observed in the sulfide melt inclusions. The metal tenors of sulfide melt inclusions and sulfide mineralization are comparable with respect to Cu concentration, with sulfide melt inclusions containing between 0.06 and 2.7 wt%, and mineralization containing between 0.54 and 2.9 wt% (recalculated to 100% sulfide). Cobalt concentrations are also similar in sulfide melt inclusions (950–4400 ppm) and mineralization (1350–3500 ppm). In contrast to Cu and Co, Ni is enriched in mineralization (0.94–2.1 wt%) compared to sulfide melt inclusions (0.13–0.59 wt%). The difference in Ni concentration suggests that the inclusions trapped a residual sulfide melt that had been compositionally modified by olivine crystallization and associated "trapped liquid shift effect" prior to inclusion entrapment. Notably, PGE concentrations in sulfide melt inclusions are very low (i.e., below limits of detection of LA-ICPMS; < ∼4 ppm), consistent with the observation of low PGE grades in the associated mineralization styles, and 1–2 orders of magnitude lower than in sulfide melt inclusions reported in economically mineralized layered mafic-ultramafic intrusions. Using Cu and Pd bulk rock compositions for various mineralization styles, together with estimates of starting magma composition (S-undepleted tholeiite, and local, fine-grained, fresh gabbro), R-factors were estimated numerically. S-undepleted tholeiite (from published data) is too Cu- and Pd-rich to be a suitable starting magma composition to source disseminated and semi-massive to massive sulfide, and would require an initial (earlier) stage of metal depletion. Using the local gabbro as a starting magma composition estimate, the metal tenors of disseminated, and semi-massive to massive sulfide mineralization are estimated to have resulted from an R-factor of <100. On the other hand, trace sulfide "PGE-enriched" mineralization could have formed from a S-undepleted tholeiite at an R-factor of at least 1000. It does not appear likely that all mineralization styles in the Caribou Lake Gabbro formed from the same starting silicate melt composition. Low metal tenors of sulfide mineralization within the Caribou Lake Gabbro are likely the result of a magmatic system with a low R-factor, supported by low Ni in olivine and modelling of Cu and Pd systematics. In this case, sulfide melt inclusions predict the sub-economic PGE tenor of the sulfide mineralization as being a primary feature rather than the result of post-solidus downgrading by later metamorphic or hydrothermal fluids. More broadly, sulfide melt inclusions may serve as a proxy for sulfide metal tenor at different stages in the history of a magmatic system, providing a potentially valuable tool for exploration where post-entrapment changes may have modified bulk metal tenors, or where sulfide mineralization has not yet been located. [ABSTRACT FROM AUTHOR]

Volatile-rich and typically oxidized carbonaceous chondrites, such as CV and CM chondrites, potentially respond to impacts differently than do other chondritic materials. Understanding impact melting of carbonaceous chondrites has been hampered by the dearth of recognized impact melt samples. In this study we identify five carbonaceous chondrite impact melt clasts in three host meteorites: a CV3 red chondrite, a CV3 oxA chondrite, and a regolithic howardite. The impact melt clasts in these meteorites respectively formed from CV3 red chondrite, CV3 oxA chondrite, and CM chondrite protoliths. We identified these impact melt clasts and interpreted their precursors based on their texture, mineral chemistry, silicate bulk elemental composition, and in the case of the CM chondrite impact melt clast, in situ measurement of oxygen three-isotope signatures in olivine. These impact melts typically contain euhedral–subhedral olivine microphenocrysts, sometimes with relict cores, in glassy groundmasses. Based on petrography and Raman spectroscopy, four of the impact melt clasts exhibit evidence for volatile loss: these melt clasts either contain vesicles or are depleted in H 2 O relative to their precursors. Volatile loss (i.e., H 2 O) may have reduced the redox state of the CM chondrite impact melt clast. The clasts that formed from the more oxidized precursors (CV3 oxA and CM chondrites) exhibit phase and bulk silicate elemental compositions consistent with higher intrinsic oxygen fugacities relative to the clast that formed from a more reduced precursor (CV3 red chondrite). The mineral chemistries and assemblages of the CV and CM chondrite impact melt clasts identified here provide a template for recognizing carbonaceous chondrite impact melts on the surfaces of asteroids. [ABSTRACT FROM AUTHOR]