Roberto Clocchiatti

Cogenetic silica-rich and carbonate-rich melts trapped in mantle minerals in Kerguelen ultramafic xenoliths: Implications for metasomatism in the oceanic upper mantle

In an attempt to characterize metasomatic agents for the oceanic upper mantle, we have undertaken a study of melt and fluid inclusions trapped in metasomatized peridotite nodules (anhydrous spinel lherzolites and harzburgites) from the Kerguelen Islands (southern Indian Ocean). These xenoliths contain three types of genetically related inclusions hosted by olivine, clinopyroxene and orthopyroxene. These are silicate melt inclusions, carbonate-rich inclusions and CO2 fluid inclusions. These inclusions are secondary in nature and form trails along fracture planes in the sheared peridotites. Heating experiments conducted on silicate melt inclusions give an estimation of the entrapment temperatures ( ≈ 1250°C) and indicate that there is no genetic relationship between the inclusions and their host minerals. The chemical composition of the silicate melt inclusions is characterized by normative quartz and feldspar components, with SiO2 ≈ 60wt%, Al2O3 ≈ 20wt%, Na2O and K2O each ≈ 4–5wt%, FeO and MgO 1000 ppm, H2O ⩾ 1.2%, and oversaturation of the melt with CO2. The trace element signature is characterized by LREE enrichment, negative HFSE (Ti and Zr) anomalies and a TiZr value of 17. The trapped melt has crystallized the following minerals: K-rich amphibole (kaersutite), diopside, rutile, ilmenite and carbonate (magnesite). Carbonate-rich inclusions, interpreted as trapped carbonate melt, have crystallized calcite. The carbonate-rich inclusions are often physically connected with the silicate melt inclusions, indicating the former existence of a homogeneous melt which later unmixed into two separate melts. Cogenetic relationships between CO2 inclusions and both carbonate melt inclusions and silicate melt inclusions yield a minimum trapping pressure for all types of inclusions of 12.5 kbar at 1250°C, corresponding to upper mantle depths. Based on their daughter mineral types, their chemical composition and high volatile element contents, the silicate-carbonate melt inclusions trapped in the ultrabasic xenoliths of the Kerguelen Islands are interpreted as small amounts of a metasomatic melt phase. These melt inclusions cannot result from melting of the anhydrous peridotite assemblages in which they have been trapped. They must represent an exotic, migrating metasomatic melt phase in the oceanic lithosphere below the Kerguelen Islands.

The compatibility of rhenium and osmium in natural olivine and their behaviour during mantle melting and basalt genesis

Rhenium and osmium (Re–Os) elemental abundances have been obtained for magmatic olivine from a range of host basalt compositions, for mantle olivine and coexisting phases (silicate and sulphide) from a spinel–peridotite, and olivine and Fe–Ni metal from Pallasite meteorites. These data indicate that Re and Os concentrations in olivine are low in both mantle and magmatic environments, and both elements preferentially partition into silicate melt, sulphide or Fe–Ni metal, relative to olivine. For magmatic olivine the partition coefficients for Re and Os correlate with the MgO content of the olivine (like Fe, Mn and Ni), which suggests that the observed partitioning reflects substitution onto crystallographic sites, rather than defects or the presence of included phases. These data indicate that Os is extremely incompatible (that is, excluded from the silicate structure) in magmatic olivine, which suggests that olivine crystallisation alone cannot be responsible for the low Os contents of some oceanic basalts. Rather, olivine crystallisation is itself responsible for sulphide precipitation (in which Os is highly compatible), by producing sulphur saturation of the melt, and it is the coupled crystallisation of these phases that effects the Os–Mg–Ni co-variations observed in oceanic basalts. Rhenium is also incompatible in magmatic olivine but the data suggest that for Fe-rich olivine compositions Re may become compatible, which may explain, at least in part, the compatible behaviour of this element during basalt petrogenesis on other planetary bodies, such as Mars and the Moon. Preliminary data for mantle olivine, not demonstrably contaminated by included phases, suggest that the high Os concentrations (relative to magmatic olivine) relate to partitioning with a sulphide, rather than silicate melt.

Melt and fluid inclusions in basalts and xenoliths from Tahaa Island, Society archipelago : evidence for a metasomatized upper mantle

Abstract Peridotites and basalts from Tahaa (Society Archipelago, French Polynesia), have unusual REE contents. The peridotites exhibit LREE enrichments, whereas some of the basalts have HREE enriched patterns. The study of melt and fluid inclusions trapped in peridotitic and basaltic minerals indicate two different types of trapped melts. Type I melt inclusions contain daughter minerals (Fe Ti oxide, salite and plagioclase An 48–65 ) and occur in olivine and pyroxene phenocrysts from basalts. After homogenization (T = 1200°C), glass composition is similar to the average whole-rock composition of the host basalt (SiO 2 6.5 wt%,TiO 2 > 2.5 wt%,K 2 O/Na 2 O= 0.6,S≈ 1200 ppm andCl 2 > 7.3 wt%), pargasite, rutile, ilmenite, salite, carbonate, apatite and plagioclase (An 88 ). After homogenization (T = 1220°C), the silicate melt has the following composition:SiO 2 > 60 wt%,TiO 2 2 O> 6 wt%,K 2 O/Na 2 O> 1 and high contents of H 2 O, CO 2 and Cl. Co-genetic relationships between CO 2 fluid inclusions and type II melt inclusions yield a minimum trapping pressure of 7 kbar at 1220°C, corresponding to entrapment at oceanic upper mantle depths. This silicate melt is interpreted as being a trapped metasomatic melt resulting from a partial melting event of the peridotitic assemblage induces by deep seated fluids (H 2 O + CO 2 + Cl).

Earlier alkaline and transitional magmatic pulsation of Mt Etna volcano

Abstract The observation of glass inclusions with alkalic and transitional compositions in single olivine crystals from Aci Castello hyaloclastites indicates that small volumes of alkalic melts preceded tholeiite eruptions during the onset of Mt Etna volcanism. The gradual shift in composition from alkalic to transitional melts is explained by simple partial melting of a compositionally uniform source, with the degree of melting increasing by a factor of 3–4 during this early melting process. Such variation controls both the chemical and mineralogical signatures of emitted lavas. The source shows trace-element and isotopic signatures which may be representative of a HIMU-type plume for Etnean lavas.

The composition of melt inclusions in minerals at the garnet–spinel transition zone

High-pressure melts are preserved as inclusions in automorphous spinel crystals from reaction coronas surrounding pyrope garnets in lherzolites from Lashaine volcano, northern Tanzania. The inclusions are primary; i.e. they contain liquids that have formed in thermodynamic equilibrium with their host spinels and the trapping pressures of the melts correspond to those of the host spinel growths. Calibrations of the spinel^garnet phase equilibrium transition suggest that the spinel containing the melt inclusions formed at pressures around 2.5 GPa, and mass balance calculations indicate that the inclusions contain melts formed by low-degree of melting (FW0.04) during garnet breakdown. Despite the possibility that full equilibration was not attained with all the mineral phases found in the nodules, the melt inclusions, after homogenization by high-T experiments, display compositions remarkably consistent with experimental melts obtained at pressures around 2.5 GPa on natural and model peridotite systems. They are characterized by lower SiO2 (47.5 wt% on average) and higher MgO and FeO contents, at a given Na2O+K2O content, than low-degree melt inclusions formed around 1.0 GPa and preserved in spinel-peridotite minerals. This provides direct evidence for the hypothesis that for low-degree melts, the decrease in SiO2 with pressure is superimposed on a decrease of the effect of alkali oxides on the silica activity coefficient, thereby generating melts with a composition more akin to erupted alkali basalt than at shallow depths. fl 2000 Elsevier Science B.V. All rights reserved.

Study of hydrogen in melt inclusions trapped in quartz with a nuclear microprobe

Abstract Elastic recoil spectrometry induced by a 3 MeV 4He microbeam has been used to determine the hydrogen distribution within melt inclusions trapped in quartz. These minerals were selected from different geological environments: Guadeloupe (West Indies), Pantelleria Island (South Sicily, Italy) and San Pietro (South Sardinia, Italy). Bulk hydrogen contents are calculated (H assumed to be in H2O form). The knowledge of hydrogen distribution assists both in a better understanding and in the establishment of volcanic dynamism hypotheses. Finally, hydrogen-rich fluid inclusions are evidenced, the H concentration profile was obtained from simulation of the data and reported for the first time in a glass inclusion.

Silicic glasses in hydrous and anhydrous mantle xenoliths from Western Victoria, Australia: at least two different sources

Abstract Glasses in a hydrous wehrlite and in anhydrous lherzolites from Western Victoria, Australia, are present as interstitial glasses and secondary glass inclusions. Interconnections between each other, generally observed as thin necks, are still preserved. These petrographic characteristics are suitable for establishing a space–time relationship. Glasses in hydrous and anhydrous xenoliths show continuous chemical trends apparently governed by different processes. Glass patches in the hydrous wehrlite are interpreted as the product of decompressional breakdown of hydrous phases like amphibole and phlogopite. However, abundances of some elements suggest mixing and the involvement of an additional source. After precipitation of secondary phases (e.g., olivine, clinopyroxene and spinel), the brown microlite-free melt migrated and reacted with primary clinopyroxene and in rare cases was trapped as glass inclusions. The observed chemical trend can be explained by crystallisation of secondary phases of the amphibole breakdown melt with addition of an alkali-volatile-rich phase. In the anhydrous lherzolites, petrographic and chemical evidences suggest the existence of two glasses: a silica-rich glass (glass A) and a very silica-rich glass (glass B). The silica-rich glass A (SiO 2 : 60–65 wt.%) is interpreted as an initial silicic melt, possibly generated at mantle depths, with a continuous chemical trend toward low-silica glasses (SiO 2 : 52 wt.%). This evolution is possibly governed by increasing melt fractions and dissolution of original apatite. Glass inclusions formed by this melt are rich in CO 2 and characterised by a feldspar–diopside–olivine normative composition. Furthermore, in the proximity to orthopyroxene and, due to a later event possibly related to the ascent of the xenolith, the silica-rich glass acquired a very silicic composition (glass B) by reaction with orthopyroxene and crystallisation of microlites [Zinngrebe, E., Foley, S.F., 1995. Metasomatism in mantle xenoliths from Gees, West Eifel, Germany: evidences for the genesis of calc-alkaline glasses and metasomatic Ca-enrichment. Contrib. Mineral. Petrol., 122, 76–96]. The rare glass inclusions formed by this melt are CO 2 -free and have a quartz-feldspar normative composition.

Fluid inclusions in upper mantle xenoliths from Northern Patagonia, Argentina: Evidence for an upper mantle diapir

SummaryThree generations of fluid inclusions can be recognized in upper mantle xenoliths from alkali basalts of the Somoncura Massif, Northern Patagonia, Argentina. The first (“early”, “primary”) one consists of dense CO2 inclusions which were trapped in the mantle-crust boundary zone (22–36 km minimum trapping depth). Their co-genetic relationship with silicate melt inclusions enables us to constrain their minimum trapping temperature at 1200°C, indicating a high temperature event in a cooler environment. The “late” (“pseudosecondary” and “secondary”) generations of fluid inclusions were classified in accordance with their homogenization temperature to liquid CO2 (L1) and vapor CO2 (L2) phase. The minimum trapping depth for the first of the late inclusions (L1) is about 16 km. In spite of the uncertainties related to this value, L1 inclusions indicate that the upper mantle rocks, of which samples were delivered by the basalts, had some residence time in the middle crust where they experienced a metasomatic event. The fact that this event did not destroy the earlier inclusions, places severe constraints on its duration. The second late inclusions (L2) are low-pressure CO2 inclusions with a minimum trapping depth of only 2 km, presumably a shallow magma chamber of the host basalts. The succession of fluid inclusions strongly points toward a fairly fast uprising upper mantle underneath Northern Patagonia. The petrology and mineral chemistry of the peridotitic xenoliths support this view. Extensive partial melting and loss of these melts is indicated by the preponderance of harzburgites in the upper mantle underneath Northern Patagonia, a fairly unusual feature for a continental upper mantle. That depletion event as well as several metasomatic events — including those which left traces of fluid inclusions — are possibly related to a high-speed diapiric uprise of the upper mantle in this area. The path can be traced from the garnet peridotite stability field into the middle crust, a journey which must have been unusually fast. Differences in rock, mineral, and fluid inclusion properties between geographic locations suggest a diffuse and differential type of diapirism. Future studies will hopefully help to map the full extent and the highs and lows of this diapir and elucidate questions related to its origin and future.ZusammenfassungErdmantel - Xenolithe in Alkali-Basalten des Somoncure Massivs, Nord-Patagonien, Argentinien, führen drei Generationen von Fluid-Einschlüssen. Die erste (“frühe”, “primäre”) Generation besteht aus dichten CO2-Einschlüssen, welche offenbar in der Mantel-Kruste Grenzzone (22–36 km Minimum-Tiefe) eingeschlossen wurden. CO2-Einschlüsse sind kogenetisch mit Silikat-Schmelzeinschlüssen. Dies erlaubt die Abschätzung der Einschließ-Temperatur mit minimal 1200°C, was auf ein Hochtemperatur-Ereignis in einer deutlich kühleren Umgebung hinweist. Die “späten” (“pseudosekundäre” und „sekundäre”) CO2- Fluid-Einschlüsse bilden zwei Generationen von denen die eine in die flüssige (L1), die andere in die Dampfphase (L2) homogenisieren. Die minimale Einschließ-Tiefe für die L1 Generation ist etwa 16 km. Dies bedeutet - auch bei Berücksichtigung der mit diesem Wert verbundenen Ungenauigkeit - daß diese Erdmantel-Gesteine einige Zeit in der mittleren Erdkruste verbrachten und ein metasomatisches Ereignis erlebten, bevor sie von den Basalten zur Erdoberfläche gebracht wurden. Die Tatsache, daß dieses Ereignis die frühen Einschlüsse nicht zerstörte, kann nur bedeuten, daß es von kurzer Dauer war. Die L2-Generation besteht aus Niedrigdruck CO2-Einschlüssen mit einer Minimum-Einschließtiefe von nur 2 km. Dies könnte in einer seichten Magmakammer des Wirt Basaltes geschehen sein.Die Abfolge von Fluid-Einschlüssen deutet auf einen relativ schnell aufsteigenden oberen Erdmantel unterhalb von Patagonien hin. Die Petrologie und Mineralchemie der peridotitischen Xenolithe unterstützen das. Die Vorherrschaft von Harzburgiten im Erdmantel unterhalb von Nord-Patagonien deutet auf umfangreiche Bildung partieller Schmelzen und deren Abfuhr hin — eine für einen kontinentalen Mantel ungewöhnliche Situation. Sowohl die Verarmungsereignisse, als auch die metasomatischen Veränderungen (einschließlich jene, welche Spuren in Form von Fluid Einschlüssen hinterließen) machen das Vorhandensein eines schnell aufsteigenden Daipirs im oberen Erdmantel dieser Gegend wahrscheinlich. Der Aufstieg kann vom Stabilitätsbereich der Granat-Peridotite bis in die mittlere Kruste verfolgt werden und muß daher relativ schnell erfolgt sein. Unterschiede in Gesteins-, Mineral und Fluid-Eigenschaften zwischen verschiedenen Lokalitäten legen einen diffusen und differenziellen Diapirismus nahe. Zukünftige Studien sollten es ermöglichen, das Gesamtausmaß und die unterschiedlichen Aufstiegshöhen des Diapirs zu kartieren und Hinweise auf seine Entstehung und zukünftige Entwicklung zu erhalten.

Micro PIXE and micro SXRF: comparison of the two methods and application to glass inclusions from Vulcano (Eolian Islands — Italy)

PIXE with a 2.5 MeV proton beam of 10 × 10 μm2 beam size and Synchrotron Radiation Induced X-ray Emission (SXRF) with photons of 11.8 keV energy and a focal spot of 5 × 2.5 μm2 with especially high intensity of 2 × 109 photons/s have been performed on synthetic and natural volcanic glasses. A comparison of the possibilities and performances of these microanalytical methods is made. MDLs are given from the acquisitions obtained on synthetic glasses. Micro PIXE and micro SXRF were jointly applied to the study of trace elements in glass inclusions from the active volcanological system of Vulcano (Eolian Islands — Italy).

In situ study of magmatic processes: a new experimental approach

We present an internally heated autoclave, modified in order to permit in situ studies at pressure up to 0.5 GPa and temperature up to 1000 °C. It is equipped with transparent sapphire windows, allowing the observation of the whole experiment along the horizontal axis. In the experimental cell, the sample is held between two thick transparent plates of sapphire or diamond, placed in the furnace cylinder. The experimental volume is about 0.01 cm3. Video records are made during the whole experiment. This tool is developed mainly to study the magmatic processes, as the working pressures and temperatures are appropriate for subvolcanic magma reservoirs. However, other applications are possible, such as the study of subsolidus phase equilibria as we have used well-known phase transitions, such as the system of AgI, to calibrate the apparatus with respect to pressure and temperature. The principle of the apparatus is detailed. Applications are presented, such as studies of melt inclusions at pressure and temperature and an in situ study of magma degassing through the investigation of nucleation and growth processes of gas bubbles in a silicate melt during decompression.