High-temperature Metasomatism Research Articles

The evolution of refertilized lithospheric mantle beneath the northeastern Siberian craton: Links between mantle metasomatism, thermal state and diamond potential

Although the diamond potential of cratons is linked mainly to thick and depleted Archean lithospheric keels, there are examples of craton-edge locations and circum-cratonic Proterozoic terranes underlain by diamondiferous mantle. Here, we use the results of comprehensive major and trace-element studies of detrital garnets from diamond-rich Late Triassic (Carnian) sedimentary rocks in the northeastern Siberia to constrain the thermal and chemical state of the pre-Triassic mantle and its ability to sustain the diamond storage. The studied detrital mantle-derived garnets are dominated by low- to medium-Cr lherzolitic (∼45%) and low-Cr megacrystic (∼39%) chemistries, with a significant proportion of eclogitic garnets (∼11%), and only subordinate contribution from harzburgitic garnets (∼5%) with variable Cr2O3 contents (1.2–8.4 wt.%). Low-Cr megacrysts display uniform, “normal” rare-earth element (REE) patterns with no Eu/Eu* anomalies, systematic Zr and Ti enrichment (mainly within 2.5–5), which are evidence of their crystallization from deep metasomatic melts. Lherzolitic (G9) garnets exhibit normal or humped to MREE-depleted sinusoidal REE patterns and elevated Nd/Y (up to 0.33–0.41) and Zr/Y ratios (up to 7.62). Rare low- to high-Cr harzburgitic (G10) garnets have primarily “depleted”, sinusoidal REE-patterns, low Ti, Y and HREE, but vary significantly in Zr-Hf, Ti and MREE-HREE contents, Nd/Y (within 0.1–2.4) and Zr/Y (1.53–19.9) ratios. The observed trends of chemical enrichment from the most depleted, harzburgitic garnets towards lherzolitic (including high-Ti high-Cr G11-type) garnets and megacrysts result from either voluminous high-temperature metasomatism by plume-derived silicate melts or recurrent mobilization of less voluminous kimberlitic or related carbonated mantle melts, rather than the initially primitive, fertile nature of the Proterozoic SCLM. Calculated Ni-in-garnet temperatures (primarily within ∼1150–1250 °C) indicate their derivation from at least ∼220 km thick Cr-undersaturated lithosphere at the relevant Devonian to Triassic thermal flow of ∼45 mW/m2 or cooler. We suggest the existence of rare harzburgitic domains in the primarily lherzolitic diamond-facies SCLM beneath the northeastern Siberian craton at least by Triassic, whereas the abundance of eclogitic garnets, predominance of E-type inclusions in placer diamonds and specific morphologies argue for diamondiferous eclogites occurring within a ∼50–65 kbar diamond window of the Olenek province by the same time.

ПЕТРОЛОГИЯ КЫЗЫР-БУРЛЮКСКОГО МАФИТ-УЛЬТРАМАФИТОВОГО МАССИВА (СЕВЕРО-ВОСТОК ЗАПАДНОГО САЯНА)

Актуальность исследования. В Западном Саяне установлены многочисленные интрузивные мафит-ультрамафитовые тела. Их формационная принадлежность и металлогеническая специализация до настоящего времени остаются до конца не выясненными и постоянно привлекают внимание как производственных, так и научных организаций, в связи с их металлогенической специализацией на хром, никель, медь, благородные металлы. Для выяснения их формационной принадлежности нами получены оригинальные результаты петрологического исследования мафит-ультрамафитового Кызыр-Бурлюкского массива, которые позволяют выявить условия его формирования, последующего преобразования в процессе длительной эволюции на уровнях верхней мантии–земная кора. Объекты: мафит-ультрамафитовый Кызыр-Бурлюкский массив. Он расположен в северо-восточной части Западного Саяна и, совместно с Калнинским и Эргакским массивами, является составной частью потенциально хромитоносных массивов Амыльского рудного района. Методы. Петрографическое изучение ультрамафитов проводилось на поляризационном микроскопе AxioScope-40 Carl Zeiss. Диагностика химического состава силикатных и рудных минералов выполнена методом рентгеноспектрального микроанализа с применением электронного сканирующего микроскопа Tescan Vega II LMU, оборудованного энергодисперсионным Oxford INCA Energy 350 и волнодисперсионным Oxford INCA Wave 700 спектрометрами. Результаты. Согласно проведенному петрологическому исследованию Кызыр-Бурлюкский массив представлен главным образом предельно деплетированными дунитами, сформировавшимися в верхней мантии, на уровнях которой они претерпели интенсивные высокотемпературные пластические деформации внутрикристаллическим трансляционным скольжением при подчиненной роли синтектонической рекристаллизации. В коровых условиях, под воздействием более поздних габброидных интрузивов, дуниты подвергались высокотемпературному метасоматозому с образованием пород верлитклинопироксенитовой ассоциации.

Experimental Study of Amphibole Interaction with H2O–HCl Fluid at 650–750°C, 5–7 Kbar: Implications for High-Temperature Metasomatism of Metabasites

Interaction between amphibole and H2O–HCl fluid was experimentally studied in an internally heated pressure vessel (IHPV) at 650–800°C and 5–7 kbar. The stability of the anorthite–amphibole association was approved to be constrained at T = 650–800°C, (P = 5–7 kbar) within a range of HCl fugacity (fHCl) 50 100 µm) corundum and quartz crystals. Based on experimental data, it is demonstrated that interaction between metabasite with acidified fluid resulted in Ca, Fe, and no so much Mg removal from the metabasite (as inferred for rocks cropping out on Kii Island in the White Sea). The HCl fugacity in the metasomatizing fluid was approximately 50–200 bar at T = 650 – 800°C, P = 5–7 kbar. Rock depletion in bases led to that the residual matrix enriched in Al2O3 and SiO2, and this was favorable for the origin of corundum, on the one hand, and highly silicic rocks, like quartzite, on the other.

Multi-stage rodingitization of ophiolitic bodies from Northern Apennines (Italy): Constraints from petrography, geochemistry and thermodynamic modelling

The investigated mantle bodies from the External Ligurians (Groppo di Gorro and Mt. Rocchetta) show evidences of a complex evolution determined by an early high temperature metasomatism, due to percolating melts of asthenospheric origin, and a later metasomatism at relatively high temperature by hydrothermal fluids, with formation of rodingites. At Groppo di Gorro, the serpentinization and chloritization processes obliterated totally the pyroxenite protolith, whereas at Mt. Rocchetta relics of peridotite and pyroxenite protoliths were preserved from serpentinization. The rodingite parageneses consist of diopside ​+ ​vesuvianite ​+ ​garnet ​+ ​calcite ​+ ​chlorite at Groppo di Gorro and garnet ​+ ​diopside ​+ ​serpentine ​± ​vesuvianite ​± ​prehnite ​± ​chlorite ​± ​pumpellyite at Mt. Rocchetta. Fluid inclusion measurements show that rodingitization occurred at relatively high temperatures (264–334 ​°C at 500 ​bar and 300–380 ​°C at 1 ​kbar). Garnet, the first phase of rodingite to form, consists of abundant hydrogarnet component at Groppo di Gorro, whereas it is mainly composed of grossular and andradite at Mt. Rocchetta. The last stage of rodingitization is characterized by the vesuvianite formation. Hydrogarnet nucleation requires high Ca and low silica fluids, whereas the formation of vesuvianite does not need CO2-poor fluids. The formation of calcite at Groppo di Gorro points to mildly oxidizing conditions compatible with hydrothermal fluids; the presence of andradite associated with serpentine and magnetite at Mt. Rocchetta suggests Fe3+-bearing fluids with fO2 slightly higher than iron-magnetite buffer. We propose that the formation of the studied rodingite could be related to different pulses of hydrothermal fluids mainly occurring in an ocean-continent transitional setting and, locally, in an accretionary prism associated with intra-oceanic subduction.

Granitization and High-Temperature Metasomatism in Mafic Rocks: Comparison between Experimental Data and Natural Observations

The paper reports newly obtained data that append older results of experimental modeling of granitization processes. The experiments were aimed at modeling high-temperature metasomatism of mafic rocks, a process that involves the transfer of major components at 750°C and 500 MPa at a pressure gradient. The source of the transported Si, Ca, and Mg in the experiments was garnet. The solution was pure H2O and 25 wt % NaCl aqueous solution. In the experiments, garnet was decomposed into pyroxenes, amphiboles, plagioclase, and minor amounts of melt, ilmenite, and iron oxides. The associated partial dissolution led to the transfer and redeposition of the dissolved components on the surface of a gabbroanorthosite underlay and to the development of mineral rims, which were analogous to those produced at garnet decomposition. The compositions of the newly formed minerals in the rims were identical to those produced at metamorphism of gabbroanorthosite at Т ≥ 750°C, P > 700 MPa. When the mineral rim was formed, some elements are removed, and this process was controlled by the composition of the fluid phase. The pure H2O fluid removed Fe, Ca, and Mg. The aqueous fluid containing NaCl (XNaCl ≈ 0.1) did not extract Ca from minerals. This indicates that no high NaCl concentrations are typical of fluid in processes that form basificates at granitization. The experiments have shown that H2O and H2O–NaCl fluids remove more Fe that other elements. Preferable Fe extraction from naturally occurring associations is evident from the elevated Fe mole fractions of the mafic minerals and from the fact that the basificates typically contain magnetite and hematite.

Granitization and high-temperature metasomatism in mafic rocks: comparison of experimental and natural data

The paper reports newly obtained data that append older results of experimental modeling of granitization processes. The experiments were aimed at modeling high-temperature metasomatism of mafic rocks, a process that involves the transfer of major components at 750°C and 500 MPa at a pressure gradient. The source of the transported Si, Ca, and Mg in the experiments was garnet. The solution was pure H2O and 25 wt % NaCl aqueous solution. In the experiments, garnet was decomposed into pyroxenes, amphiboles, plagioclase, and minor amounts of melt, ilmenite, and iron oxides. The associated partial dissolution led to the transfer and redeposition of the dissolved components on the surface of a gabbroanorthosite underlay and to the development of mineral rims, which were analogous to those produced at garnet decomposition. The compositions of the newly formed minerals in the rims were identical to those produced at metamorphism of gabbroanorthosite at Т ≥ 750°C, P > 700 MPa. When the mineral rim was formed, some elements are removed, and this process was controlled by the composition of the fluid phase. The pure H2O fluid removed Fe, Ca, and Mg. The aqueous fluid containing NaCl (XNaCl ≈ 0.1) did not extract Ca from minerals. This indicates that no high NaCl concentrations are typical of fluid in processes that form basificates at granitization. The experiments have shown that H2O and H2O-NaCl fluids remove more Fe that other elements. Preferable Fe extraction from naturally occurring associations is evident from the elevated Fe mole fractions of the mafic minerals and from the fact that the basificates typically contain magnetite and hematite.

High-Temperature Metasomatism of the Layered Mafic–Ultramafic Massif in Kiy Island, Belomorian Mobile Belt

The paper presents results of a detailed petrologic study of metasomatites and their host metagabbroids in the northwestern part of Kiy Island, Onega Bay, White Sea. The first evidence is acquired that coronitization and amphibolization of the host rocks took place at the peak of Svecofennian metamorphism at Т = 700–640°C, Р = 9–10 kbar, and $${a_{{H_2}O}}$$ = 0.2–0.3. Accompanying metasomatism has formed a number of long (up to several meters long) melanocratic hornblendite and garnet–amphibole veins 0.3–2 m thick. In this area, metasomatites of another type make up single relatively thin amphibole–zoisite lenses that sometimes host ruby-like corundum. The fluid phase that induced metasomatism was poor in salts (Na,K)Cl, and hence, the rocks do not contain sodic plagioclase, and their amphibole is tschermakite but not pargasite. The compositions of the metasomatites of the two types are proved to be complementary, and this indicates that they were most likely produced by high-temperature metasomatism but not via the removal of components by fluid from migmatization zones.

Fluid-induced high-temperature metasomatism at Rundvågshetta in the Lützow-Holm Complex, East Antarctica: Implications for the role of brine during granulite formation

We report new petrological, phase equilibria modeling, and fluid inclusion data for pelitic and mafic granulites from Rundvågshetta in the highest-grade region of the Neoproterozoic Lützow-Holm Complex (LHC), East Antarctica, and provide unequivocal evidence for fluid-rock interaction and high-temperature metasomatism in the presence of brine fluid. The studied locality is composed dominantly of well-foliated pelitic granulite (K-feldspar + quartz + sillimanite + garnet + ilmenite) with foliation-parallel bands and/or layers of mafic granulite (plagioclase + orthopyroxene + garnet + ilmenite + quartz + biotite). The boundary between the two lithologies is defined by thin (about 1–20 cm in thick) garnet-rich layers with a common mineral assemblage of garnet + plagioclase + quartz + ilmenite + biotite ± orthopyroxene. Systematic increase of grossular and decrease of pyrope contents in garnet as well as decreasing Mg/(Fe + Mg) ratio of biotite from the pelitic granulite to garnet-rich rock and mafic granulite suggest that the garnet-rich layer was formed by metasomatic interaction between the two granulite lithologies. Phase equilibria modeling in the system NCKFMASHTO demonstrates that the metasomatism took place at 850–860 °C, which is slightly lower than the peak metamorphism of this region, and the modal abundance of garnet is the highest along the metapelite–metabasite boundary (up to 40%), which is consistent with the field and thin section observations. The occurrence of brine (7.0–10.9 wt.% NaCleq for ice melting or 25.1–25.5 wt.% NaCleq for hydrohalite melting) fluid inclusions as a primary phase trapped within plagioclase in the garnet-rich layer and the occurrence of Cl-rich biotite (Cl = 0.22–0.60 wt.%) in the metasomatic rock compared to that in pelitic (0.15–0.24 wt.%) and mafic (0.06–0.13 wt.%) granulites suggest infiltration of brine fluid could have given rise to the high-temperature metasomatism. The fluid might have been derived from external sources possibly related to the formation of major suture zones formed during the Gondwana amalgamation.

Silica-undersaturated reaction zones at a crust–mantle interface in the Highland Complex, Sri Lanka: Mass transfer and melt infiltration during high-temperature metasomatism

Sri Lanka is a crucial Gondwana fragment mostly composed of granulitic rocks in the Highland Complex surrounded by rocks with granulite to amphibolite grade in the Vijayan and Wanni Complex that were structurally juxtaposed during Pan-African orogeny. Fluids associated with granulite-facies metamorphism are thought to have controlled various lower crustal processes such as dehydration/hydration reactions, partial melting, and high-temperature metasomatism. Chemical disequilibrium in the hybrid contact zone between a near peak post-tectonic ultramafic enclave and siliceous granulitic gneiss at Rupaha within the Highland Complex produced metasomatic reaction zones under the presence of melt. Different reaction zones observed in the contact zone show the mineral assemblages phlogopite+spinel+sapphirine (zone A), spinel+sapphirine+corundum (zone B), corundum (~30%)+biotite+plagioclase zone (zone C) and plagioclase+biotite+corundum (~5%) zone (zone D). Chemical potential diagrams and mass balance reveal that the addition of Mg from ultramafic rocks and removal of Si from siliceous granulitic gneiss gave rise to residual enrichment of Al in the metasomatized mineral assemblages. We propose that contact metasomatism between the two units, promoted by melt influx, caused steady state diffusional transport across the profile. Corundum growth was promoted by the strong residual Al enrichment and Si depletion in reaction zone whereas sapphirine may have been formed under high Mg activity near the ultramafic rocks. Modelling also indicated that metasomatic alteration occurred at ca. 850°C at 9kbar, which is consistent with post-peak metamorphic conditions reached during the initial stage of exhumation in the lower crust and with temperature calculations based on conventional geothermometry.

Alteration Facies Linkages Among Iron Oxide Copper-Gold, Iron Oxide-Apatite, and Affiliated Deposits in the Great Bear Magmatic Zone, Northwest Territories, Canada

High-temperature metasomatism driven by ascent of voluminous, saline fluid columns in the upper crust plays a major role in the genesis of iron oxide-alkali alteration ore systems but fundamental questions remain on genetic linkages among iron oxide copper-gold (IOCG), iron oxide-apatite (IOA), albitite-hosted uranium, and skarn deposits that they produce. Excellent surface exposures of such systems in the Great Bear magmatic zone of northernwestern Canada record the depth to paleosurface, prograde evolution of iron oxide-alkali alteration facies, and mineralization. Across the belt, albitite corridors that are tens of kilometers in length record the earliest reactions between highly saline fluids and host rocks along fault zones and subvolcanic intrusions. Pervasive albitization partitioned metals from the host rocks into the ascending fluid column, leaving behind structurally weakened corridors of porous albitite. These corridors were cut, replaced, and overprinted by amphibole- and magnetite-bearing, calcic-iron alteration assemblages. In extreme cases, the discharge of calcium, iron, and specialized metals formed iron oxide-apatite deposits (±vanadium ± rare earth elements) while recharging the outgoing fluids in sodium, potassium, and base and precious metals. As temperatures declined and fluid chemistry evolved through fluid-rock reactions, the formation of potassic-iron alteration assemblages, breccias, and sulfides resulted in magnetite- and hematite-group IOCG mineralization. Within carbonate units, skarns formed prior to, are replaced by, and evolved to calcic-iron alteration facies. Skarns can locally host base metal mineralization. Tectonically uplifted albitite breccias replaced by potassic-iron alteration assemblages became a preferential host for uranium mineralization. The results of this study also illustrate that permutations and cyclical build-up of alteration products can arise from a combination of faulting, differential uplift, and renewed magmatism. Framed within an alteration-facies deposit model, alteration zones and mineral occurrences play a pivotal role in predicting the mineral potential of iron oxide and alkali-altered systems at district to deposit scales.

Generations of Melt Extraction, Melt–Rock Interaction and High-Temperature Metasomatism Preserved in Peridotites of the ∼497 Ma Leka Ophiolite Complex, Norway

Ophiolites allow spatial and temporal assessment of the causes and length-scales of upper mantle compositional heterogeneity because they permit field-based observations to be coupled with geochemical investigations of upper mantle lithologies. The � 497 Ma Leka Ophiolite Complex (Norway) comprises a section of early Palaeozoic (Iapetus) oceanic lithosphere with well-exposed mantle and lower crustal sections and generally low degrees of serpentinization. The Leka upper mantle section is heterogeneous at the centimetre to metre scale, manifested by abundant dunite lenses and sheets in harzburgitic host-rock, especially within � 500 m of the palaeo Moho. Abundant chromitite (� 60 vol. % Cr-spinel) and pyroxenite lenses and layers also occur in the uppermost 200‐300 m of the mantle section. These diverse mantle lithologies probably developed in a suprasubduction-zone (SSZ) setting, as a result of fluid-assisted melt extraction, offering an opportunity to interrogate the nature of chemical heterogeneities developed in such rocks. At � 497 Ma, the Os isotopic compositions of Leka harzburgites averaged � 2% more radiogenic than the projected average for abyssal peridotites at that time, yet they exhibit nearly chondritic relative abundances of the highly siderophile elements (HSE). Several of the harzburgites are characterized by low initial 187 Os/ 188 Os (<0� 121), reflecting Proterozoic melt depletion. Preservation of Os isotopic compositions consistent with ancient (<0� 5 to 2 Ga) melt depletion episodes is a common characteristic of melt-depleted oceanic peridotites. There is no clear evidence that SSZ melt extraction had a discernible impact on the bulk Os isotopic composition of the Iapetus oceanic mantle, as represented by the Leka harzburgites. By contrast, non-harzburgitic lithologies are generally characterized by more radiogenic initial 187 Os/ 188 Os and more variable HSE abundances. The dunites, chromitites and pyroxenites of the LOC can be separated into two groups on the basis of their trace element geochemistry and the Re-Os isotope errorchrons that they define, yielding ages of 485 6 32 Ma and 589 6 15 Ma, respectively. The former age corresponds, within error, to the accepted age of the ophiolite (497 6 2 Ma). The meaning of the latter age is uncertain, but possibly corresponds to the early stages of Iapetus opening. The Leka ophiolite reveals the importance of oceanic lithosphere formation processes for mantle heterogeneity at metre to kilometre scales, but also emphasizes the robustness of Os isotopes in recording older melt-depletion events.

Experimental study of the interaction between amphibole and H2O-NaCl fluid at 900°C and 500 MPa: Application to high-temperature metasomatism of metabasic rocks

Experimental study of the interaction between amphibole and H2O-NaCl fluid at 900°C and 500 MPa: Application to high-temperature metasomatism of metabasic rocks

High- to ultrahigh-temperature metasomatism related to brine infiltration in the Neoarchean Limpopo Complex: Petrology and phase equilibrium modeling

Fluids associated with granulite-facies metamorphism are considered to have controlled various lower crustal processes such as dehydration/hydration reactions, partial melting, material circulation, and high-temperature metasomatism. Here, we report new petrological data obtained from enderbite and its metasomatized equivalents from the Southern Marginal Zone (SMZ) of the Neoarchean Limpopo Complex, South Africa, and evaluate the alteration process caused by infiltration of high-salinity aqueous fluids during granulite-facies metamorphism. At the study area on Farm Commissiedraai, enderbite (plagioclase+quartz+K-feldspar+orthopyroxene) is partly metasomatized with the formation of continuous alteration zones comprising plagioclase+quartz+K-feldspar+orthopyroxene+garnet, plagioclase+quartz+K-feldspar+garnet, and finally mesoperthite/antiperthite+quartz+garnet+sillimanite. Bulk K2O increases, and CaO, MgO, FeO, and TiO2 decreases continuously from enderbite to the most intensely metasomatized rock. The occurrence of aqueous fluid inclusions with final melting temperatures of −29.2°C to −21.6°C trapped as a primary phase within quartz in the highly altered sample indicates that a high-salinity brine (probably NaCl rich) with additional components such as MgCl2 and CaCl2 controlled metasomatism. We employed a phase equilibrium modeling technique to evaluate the metasomatic process in the system NCKFMASH using THERMOCALC 3.3, and discovered that addition of K to and removal of Ca and Mg from the enderbite gave rise to the metasomatized mineral assemblages. Modeling also indicated that metasomatic alteration occurred at ca. 900°C, which is consistent with post-peak metamorphic conditions reached during the initial stage of exhumation in the SMZ, and with temperature calculations based on ternary-feldspar geothermometry of antiperthite in the highly metasomatized sample. The brine fluid that accompanied metasomatism infiltrated along the high-temperature Petronella Shear Zone, and was probably derived from devolatilization of low-grade greenstone materials of the Kaapvaal Craton as the result of thrusting hot granulite over cool granite-greenstone belts. Our results therefore suggest that metasomatism at this locality occurred closely after the peak metamorphism having been reached at ∼1000°C in the root zone of the Limpopo orogeny at ∼2.72Ga.

High-temperature metasomatism related to infiltration of brine in the Limpopo Complex, South Africa

High-temperature metasomatism related to infiltration of brine in the Limpopo Complex, South Africa

Calcic amphiboles in peridotite xenoliths from Avacha volcano, Kamchatka, and their implications for metasomatic conditions in the mantle wedge

Abstract Highly metasomatized parts of peridotite xenoliths from Avacha volcano, Kamchatka, Russia, characteristically contain calcic amphiboles, especially tremolites. They are rich in metasomatic pyroxenes with high Mg-number (= Mg/(Mg+Fe) atomic ratio), up to 0.94–0.98, and contain Cr-poor aluminous spinels. They have the spinel lherzolite mineral assemblage and equilibrium temperatures of 900–1000 °C or higher, beyond the stability field of tremolite. The tremolite was therefore retrogressively formed after the peak of high-temperature metasomatism. The high-Mg-number, low-alkali environment facilitates the formation of tremolite instead of Al-rich calcic amphiboles. A sulphur-bearing silicic melt derived from a slab is a probable agent involved in the metasomatism. High f O 2 recorded in the highly metasomatized peridotites is consistent with this process. This type of metasomatism can produce high-Mg-number peridotites and pyroxenites with low-Cr-number spinel within the mantle wedge where the Mg-number of silicates is positively correlated with the Cr-number of spinel in ambient peridotites.

Fluids in early stage hydrothermal alteration of high-sulfidation epithermal systems: A view from the Vulcano active hydrothermal system (Aeolian Island, Italy)

High-sulfidation (HS) epithermal systems have elements in common with passively degassing volcanoes associated with high T, acid fumarole fields or acid crater lakes. They are considered to form in two stages, the first of which involves advanced argillic alteration resulting from intense, strongly acidic fluid–rock interaction. The La Fossa hydrothermal system (Vulcano Island) represents a classic example of such an active HS system and can be considered as a modern analogue of this early stage of alteration, resulting in a core of intense silicic (90–95% pure SiO 2) alteration surrounded by alunitic alteration zones. This paper focuses on a geochemical and stable isotope study of the surficial alteration facies of Vulcano – particularly the horizon characterized by strong silicic alteration – and on deep seated xenoliths ejected during the last eruption of La Fossa volcano (1888–90) that can be considered as representative of fragments of the deep conduit system of La Fossa volcano. Using directly measured temperatures at the sites of sampling, we have calculated fluid composition in isotopic equilibrium with the alteration products. The large range of measured silica δ 18O (12.3 to 29‰) reflects the wide range of formation temperatures (80–240 °C). The fluid compositions calculated for intense silicic alteration vary from − 0.9 to + 6.5‰. These are significantly heavier than local meteoric water (− 6‰), and are consistent with derivation from the condensation of high-temperature fumarolic gases, dominated by magmatic fluids and rich in acid gases (SO 2, H 2S, HCl, HF), into shallow groundwaters of meteoric origin, with dynamically variable ratios of fumarolic steam/meteoric water. The calculated δ 18O and δD of water in equilibrium with alunite also suggest the mixing of magmatic and meteoric waters for the fluids involved in the genesis of advanced argillic alteration facies. The calculated δ 18O of water in equilibrium with hedenbergitic clinopyroxene, found in a veinlet in a metasomatized xenolith is + 8.9‰. This value cannot reasonably result from water–rock interaction between the host volcanic rocks and surface water. Instead, it most likely represents a fluid (brine) exsolved from magma, which was responsible for high temperature metasomatism in the deep conduit system.

Metasomatism and ore formation at contacts of dolerite with saliferous rocks in the sedimentary cover of the southern Siberian platform

The data on the mineral composition and crystallization conditions of magnesian skarn and magnetite ore at contacts of dolerite with rock salt and dolomite in ore-bearing volcanic—tectonic structures of the Angara—Ilim type have been integrated and systematized. Optical microscopy, scanning and transmission electron microscopy, electron microprobe analysis, electron paramagnetic resonance, Raman and IR spectroscopy, and methods of mineralogical thermometry were used for studying minerals and inclusions contained therein. The most diverse products of metasomatic reactions are found in the vicinity of a shallow-seated magma chamber that was formed in Lower Cambrian carbonate and saliferous rocks under a screen of terrigenous sequences. Conformable lodes of spinel-forsterite skarn and calciphyre impregnated with magnesian magnetite replaced dolomite near the central magma conduit and apical portions of igneous bodies. At the postmagmatic stage, the following mineral assemblages were formed at contacts of dolerite with dolomite: (1) spinel + fassaite + forsterite + magnetite (T = 820−740°C), (2) phlogopite + titanite + pargasite + magnetite (T = 600–500°C), And (3) clinochlore + serpentine + pyrrhotite (T = 450°C and lower). Rock salt is transformed at the contact into halitite as an analogue of calciphyre. The specific features of sedimentary, contact-metasomatic, and hydrothermal generations of halite have been established. The primary sedimentary halite contains solid inclusions of sylvite, carnallite, anhydrite, polyhalite, quartz, astrakhanite, and antarcticite; nitrogen, methane, and complex hydrocarbons have been detected in gas inclusions; and the liquid inclusions are largely aqueous, with local hydrocarbon films. The contact-metasomatic halite is distinguished by a fine-grained structure and the occurrence of anhydrous salt phases (CaCl2 · KCl, CaCl2, nMgCl2 · mCaCl2) and high-density gases (CO2, H2S, N2, CH4, etc.) as inclusions. The low-temperature hydrothermal halite, which occurs in skarnified and unaltered silicate rocks and in ore, is characterized by a low salinity of aqueous inclusions and the absence of solid inclusions. The composition and aggregative state of inclusions in halite and forsterite indicate that salt melt-solution as a product of melting and dissolution of salt was the main agent of high-temperature metasomatism. Its total salinity was not lower than 60%. The composition and microstructure of magnetite systematically change in different mineral assemblages. Magnetite is formed as a result of extraction of iron together with silicon and phosphorus from dolerite. The first generation of magnetite is represented by mixed crystals, products of exsolution in the Fe-Mg-Al-Ti-Mn-O system. The Ti content is higher at the contact of dolerite with rock salt, whereas, at the contact with dolomite, magnetite is enriched in Mg. The second generation of magnetite does not contain structural admixtures. The distribution of boron minerals and complex crystal hydrates shows that connate water of sedimentary rocks could have participated in hydrothermal metasomatic processes.

Contact metamorphism and selective metasomatism of the layered Bellerophon Formation in the eastern Monzoni contact aureole, northern Italy

The Monzoni intrusive complex in the central Western Dolomites comprises a series of small monzogabbro, monzodiorite and pyroxenite intrusions, which were emplaced in previously unmetamorphosed Permo-Triassic sediments during the mid-Triassic. Intrusion depth was less than five kilometres, and solidus temperatures were 900–1000 °C. An about one kilometre wide contact aureole developed in the country rocks for which peak metamorphic temperatures of about 800 and 500 °C are derived for the immediate intrusive contact and for a distance of 380 m from the contact, respectively. The Upper Permian Bellerophon Formation is an alternating sequence of dolomite-poor and dolomite-rich limestone layers. These two lithologies behaved differently during contact metamorphism. Whereas the layers derived from dolomite-poor precursors behaved as closed systems, the layers derived from dolomite-rich lithologies were much more susceptible to the infiltration of fluids or melts. The supply of substantial amounts of silica, alumina and ferric iron from the nearby intrusions lead to high temperature metasomatism of these lithologies. Substantial chemical alteration is discernible up to 65 m away from the intrusive contact and produced clinopyroxene rich skarns, which preferentially replaced the layers derived from dolomite-rich protoliths. The presence of clintonite and melilite in the skarn assemblages indicates formation temperatures in excess of 750 °C. In contrast, the high-grade equivalents of the originally dolomite-poor lithologies largely preserved their original chemical and stable isotope compositions and do not show any evidence of pervasive fluid or melt infiltration at high-grade conditions. The syn-metamorphic permeability structure of the Bellerophon Formation appears to have been largely controlled by the lithological layering and was heterogeneous on a decimetre scale. Together with a complex folding of the country rocks this makes large scale syn-metamorphic thermally driven fluid circulation unlikely in the Bellerophon Formation.

The effect of metasomatism on the Cr-PGE mineralization in the Finero Complex, Ivrea Zone, Southern Alps

The magmatic metasomatism that was responsible for producing chromitite–dunite bodies in the unusual phlogopite peridotite of the Finero Complex in Permian to Triassic times also influenced the Cr-platinum group elements (PGE) mineralization. At least the end stages of this metasomatism are recorded in compositional zoning of chromite grains in the podiform chromitite. Metasomatic melt, with or without vapor, reacted with chromite to produce core-to-rim Cr enrichment of extant chromite grains and was concurrent with pyroxene crystallization. Under conditions of lower melt/rock ratio, metasomatism resulted in core-to-rim Al enrichment in chromite and crystallization of amphibole between chromite and clinopyroxene. This early, high-temperature metasomatism is unrelated to the later and pervasive K-metasomatism that crystallized phlogopite and was associated with the intrusion of clinopyroxenite dikes that cut the peridotite. Much later, serpentinization of olivine locally depleted chromite in Al and enriched it in Fe and formed minor amounts of magnetite. The PGE, which are present mainly as laurite inclusions in chromite, were remobilized by the early metasomatism. This resulted in substantial variation in the PGE contents of chromitites and imposed a characteristic PGE pattern in which chondrite-normalized Os, Ir, Ru and Rh contents are high but Pt and Pd contents are low. The slopes of PGE chondrite-normalized concentration patterns are systematically related to absolute PGE abundance and to rock mode. Chromitites with low modal orthopyroxene, clinopyroxene, and amphibole exhibit negative PGE slopes and contain relatively high PGE concentrations, whereas chromitites rich in these silicate minerals have positive slopes and low PGE contents.

Evidence of a contact metamorphic aureole with high-temperature metasomatism in the deepest part of the active geothermal field of Larderello, Italy

The deep part (1.5–4.5 km) of the Larderello geothermal field (Tuscany, Italy) consists of rocks that were metamorphosed by the Hercynian and Alpine orogenies, and which were thermally metamorphosed in the same place during the emplacement of granite intrusions of 3.8–1.0 Ma age. These rocks are potential deep-seated reservoirs and could be the target of future exploitation. The petrographic, geochemical and fluid inclusion data indicate that thermally metamorphosed phyllite, micaschist, gneiss, amphibolite and carbonates underwent a recrystallisation at temperatures of 425–670 °C, under a lithostatic pressure regime of 95-130 MPa. Li–Na-rich fluids of magmatic origin, and aqueous-carbonic fluids with varying proportions of H 2O and CO 2 that formed during the contact metamorphism, were present during this stage. The fluids present during the contact metamorphic event were responsible for a widespread B-metasomatism and local F-metasomatism. In some cases, high-temperature metamorphism of graphitic schists can control the composition of the aqueous-carbonic fluids. A late-stage, lower temperature hydrothermal activity was responsible for both the propylitic and sericitic alterations and for the replacement of the high-temperature mineral assemblage. Stable isotope ( δ 18O and δD) data on the thermally metamorphosed rocks and granites indicate that these rocks underwent depletion in the heavy isotopes. Magma degassing and dehydration metamorphic reactions can explain the isotopic values of these rocks. Late-hydrothermal fluid of meteoric origin may also have contributed to the depletion of the heavy isotopes from the rocks. Under contact metamorphism conditions the rocks were plastic and impermeable. A transition from plastic to brittle conditions occurred as a consequence of the cooling of the system at depths of <4 km. The brittle-plastic transition at Larderello now occurs at a depth of 4.5–5 km, where present-day temperatures are in the range 400–450 °C. Deep-seated fluids probably occur in this zone, as suggested by the geophysical data.