Adiabatic Decompression Research Articles

Petrogenetic model of the Permian Tarim Large Igneous Province

Over the last two decades great strides have been made in characterizing the spatial distribution, time sequence, geochemical characteristics, mantle sources, and magma evolution processes for various igneous rocks in the Early Permian Tarim Large Igneous Province (TLIP). This work has laid a solid foundation for revealing the evolutionary processes and genetic models of large igneous provinces (LIPs). This study systematically demonstrates the two-stage melting model for the TLIP based on our previous research work and predecessor achievements, and highlights the two types of magmatic rocks within the TLIP. The two-stage melting model suggests that the formation of the TLIP is mantle plume related. The early hot mantle plume caused the low-degree partial melting of the lithosphere mantle, while in the later stage, the plume partially melted due to adiabatic uplift and decompression. Therefore, this model carries signatures of both the “Parana” and “Deccan” models in terms of mantle plume activity. During the early stage, the mantle plume provided the heat required for partial melting of sub-continental lithosphere mantle (SCLM), similar to the “Parana Model”, while later the plume acted as the main avenue for melting, as in the “Deccan Model”. Basalts that erupted in the first stage have higher 87Sr/86Sr, lower 143Nd/144Nd ratios, and are enriched in large ion lithophile elements and high field strength elements, indicating a possible origin from the enriched continental lithosphere mantle, similar to the Parana type geochemical features. The basic-ultrabasic intrusive rocks in the second stage exhibit lower 87Sr/86Sr, higher 143Nd/144Nd ratios relative to the basalts, consistent with the involvement of a more depleted asthenospheric material, such as a mantle plume, similar to the Deccan type geochemical features. The first stage basalts can be further subdivided into two categories, i.e., Group 1 and Group 2 basalts. Group 2 basalts have lower 87Sr/86Sr and higher 143Nd/144Nd ratios than Group 1 basalts, and lie between compositions of the Group 1 basalts and second stage magmatism. Group 2 basalts may be the intermediate component of the TLIP, and the whole TLIP is the result of plume and lithosphere interaction. Developing this petrogenetic model for the TLIP aids in comprehensively understanding its magmatism and deep geological and geodynamic processes. Furthermore, this work enriches the theories describing the origin of large igneous province and mantle plume activity.

Cold plutonism in the Arabian–Nubian Shield: evidence from the Abu Diab garnet-bearing leucogranite, central Eastern Desert, Egypt

The Neoproterozoic Abu Diab garnet-bearing leucogranite (SiO 2 73.7 – 76.8 wt%; total alkalis 8.1 – 9.3 wt%) was emplaced during the late to post Pan-African Orogeny. The pluton is almost homogeneous in composition, weakly peraluminous (molecular Al 2 O 3 /(CaO + Na 2 O + K 2 O) = 1.04 – 1.12) and approaches the minimum melt composition in the water-saturated Qz–Ab–Or system. The chondrite-normalized patterns are almost unfractionated, with weak tetrad effects and prominent negative Eu anomalies. Ba, Sr, P, Eu and Ti show strong negative anomalies in primitive-mantle normalized multi-element diagrams. Magmatic garnet is dominantly a spessartine (66 – 67 mol %)–almandine (27 – 29 mol %) solid solution. Our results indicate that the Abu Diab granite magma formed from low-degree eutectic melting of a felsic pelite source in the lower to middle continental crust. Zirconium saturation thermometry indicates magma generation temperatures of 732 ± 28°C (2σ), placing the Abu Diab leucogranite in the ‘cold granite’ group. This indicates that muscovite breakdown is the dominant reaction in the melt source. The high normative Or, the increasing Rb/Sr concomitant with decreasing Sr and Ba, and the sharp negative Eu anomalies are consistent with fluid-absent melting. Field relations and petrographic observations indicate the emplacement of Abu Diab granites after the closure of the Mozambique Ocean and collision of East and West Gondwana, most probably during fast adiabatic uplift and decompression of the thickened crust. Supplementary material: The locations of the peraluminous leucocratic granites in the Arabian-Nubian and other supplementary data for this paper are available at https://doi.org/10.6084/m9.figshare.c.3815293 .

Crustal recycling and juvenile addition during lithospheric wrenching: The Pontivy-Rostrenen magmatic complex, Armorican Massif (France), Variscan belt

The South Armorican Shear Zone (SASZ), in the French Armorican Variscan belt, is a lithospheric wrench fault that acted during the Late Carboniferous as a transition zone between two distinct domains: a thickened domain to the south affected by extension and crustal magmatism, and a weakly thickened domain to the north subjected to dextral wrenching and crust- and mantle-derived magmatism. The Pontivy-Rostrenen complex is a composite intrusion emplaced along the SASZ. To the south, the complex is made of leucogranites whereas, to the north, monzogranites outcrop together with small intrusions of quartz monzodiorite. U-Pb dating of magmatic zircon by LA-ICP-MS reveal that most magmatic rocks were emplaced at ca. 315Ma (between 316.7±2.5Ma and 310.3±4.7Ma), excepted a late leucogranitic intrusion that was emplaced at 304.7±2.7Ma. The leucogranites (−4.8<εNd (T)<2.1; presence of Archean to Paleozoic inherited zircon) are strongly peraluminous (A/CNK>1.1) and formed by partial melting of metasediments and peraluminous orthogneisses. The monzogranite (−4.0<εNd (T)<−3.2; scarce Paleozoic inherited zircon) is moderately peraluminous (1<A/CNK<1.3) and formed by partial melting of an orthogneiss with a probable metaluminous composition whereas the quartz monzodiorite (−3.2<εNd (T)<−2.2; no inherited zircon) is metaluminous (0.7<A/CNK<1.1) and formed by partial melting of a metasomatized lithospheric mantle. The evolution of the magmas was controlled by fractional crystallization, magma mixing and/or peritectic mineral entrainment. At the scale of the Armorican Variscan belt, crustal partial melting, to the south of the SASZ, was triggered by lithospheric thinning and adiabatic decompression during extension. Conversely, to the north, asthenosphere upwelling during strike-slip deformation and subsequent slab tearing, as suggested by tomographic data, induced the melting of both the crust and the mantle fertilized during previous subduction events. This process is likely not exclusive to the Armorican Massif and may be applied to other regions in the Variscan belt, such as Iberia.

Majorite-olivine–high-Ca pyroxene assemblage in the shock-melt veins of Pervomaisky L6 chondrite

High-pressure minerals—majorite-pyrope garnet and jadeite—were found in the Pervomaisky L6 ordinary chondrite. Majorite-pyrope (79 mol% majorite) was observed within the fine-grained silicate matrix of a shock-melt vein (SMV), coexisting with olivine and high-Ca pyroxene. This is the first report of a garnet–olivine–high-Ca pyroxene assemblage that crystallized from the melt in the SMV matrix of meteorite. P-T conditions of the formation of the SMV matrix with olivine fragments are 13.5–15.0 GPa and 1750–2150 °C, the lowest parameters among all known majorite-bearing (H,L)-chondrites. The estimated conditions include the olivine/(olivine + ringwoodite) phase boundary and there is a possibility that observed olivine is the result of wadsleyite/ringwoodite back-transformation during a cooling and decompression stage. In the framework of this hypothesis, we discuss the problem of survival of the high-pressure phases at the post-shock stage in the meteorites and propose two possible P-T paths: (1) the high-pressure mineral is transformed to a low-pressure one during adiabatic decompression above the critical temperature of direct transformation; and (2) quenching below the critical temperature of direct transformation within the stability field of the high-pressure phase and further decompression. The aggregates with plagioclase composition (Ab 81.1 An 14.9 Or 4.1 ) occur in host-rock fragments near (or inside) of the SMV, and have a radial, concentric “spherulite-like” microstructure previously described in the Novosibirsk meteorite, and that is very similar to the texture of tissintite in the Tissint martian meteorite. It is likely that jadeite is related to crystallization of the SMV and could have formed from albitic feldspar (plagioclase) melt at 13.5–15.0 GPa and ~2000 °C.

The relationship between glaciation and volcanism: Numerical simulation and the Holocene volcanism in Kamchatka

This study is concerned with numerical simulation of the strain due to glaciation and glacial melting, when a magma zone (a layer containing inclusions of magma and magma cumulates) is present at the crust–mantle boundary. According to analytical solutions of this problem that involves viscous relaxation of an uncompensated depression at the place of the molten glacier, the depth to the zone of increased shear stresses beneath the depression is proportional to its width, while the relaxation duration is proportional to viscosity of the lithosphere and is a few thousand years. These fundamental estimates are corroborated by our numerical simulation. According to it, the magma zone at the Moho boundary shields the zone of increased shear stresses, limiting it from below. The maximum values (12–25 MPa) with glacial thickness 500–1000 m are reached at the top of this layer of low viscosity. The directions of maximum compression (s1) as calculated for the time after the melting indicate that the magma that rises along dikes is displaced from the center of the magma lens toward its periphery. It is found that glacial unloading makes the dipping faults in the crust above the low-viscosity layer attractors for the rising magma. Glacial unloading accelerates, by factors of a few times, the magma generation in the mantle that occurs following the mechanism of adiabatic decompression, as well as facilitating the accumulation of mantle fluids in the zone of increased shear stresses at the boundary of the low viscosity layer. The magma traverses this deep fluid collector and increases the intensity and explosivity of eruptions at the beginning of an interglacial period. Our numerical simulation results are in general agreement with published data on Early Holocene volcanic eruptions that occurred after the second phase of the Late Pleistocene glaciation in Kamchatka.

Primary magmas and mantle temperatures through time

AbstractChemical composition of mafic magmas is a critical indicator of physicochemical conditions, such as pressure, temperature, and fluid availability, accompanying melt production in the mantle and its evolution in the continental or oceanic lithosphere. Recovering this information has fundamental implications in constraining the thermal state of the mantle and the physics of mantle convection throughout the Earth's history. Here a statistical approach is applied to a geochemical database of about 22,000 samples from the mafic magma record. Potential temperatures (Tps) of the mantle derived from this database, assuming melting by adiabatic decompression and a Ti‐dependent (Fe2O3/TiO2 = 0.5) or constant redox condition (Fe2+/∑Fe = 0.9 or 0.8) in the magmatic source, are thought to be representative of different thermal “horizons” (or thermal heterogeneities) in the ambient mantle, ranging in depth from a shallow sublithospheric mantle (Tp minima) to a lower thermal boundary layer (Tp maxima). The difference of temperature (ΔTp) observed between Tp maxima and minima did not change significantly with time (∼170°C). Conversely, a progressive but limited cooling of ∼150°C is proposed since ∼2.5 Gyr for the Earth's ambient mantle, which falls in the lower limit proposed by Herzberg et al. [2010] (∼150–250°C hotter than today). Cooling of the ambient mantle after 2.5 Ga is preceded by a high‐temperature plateau evolution and a transition from dominant plumes to a plate tectonics geodynamic regime, suggesting that subductions stabilized temperatures in the Archaean mantle that was in warming mode at that time.

Boninite-like intraplate magmas from Manihiki Plateau require ultra-depleted and enriched source components

The Ontong Java and Manihiki oceanic plateaus are believed to have formed through high-degree melting of a mantle plume head. Boninite-like, low-Ti basement rocks at Manihiki, however, imply a more complex magma genesis compared with Ontong Java basement lavas that can be generated by ∼30% melting of a primitive mantle source. Here we show that the trace element and isotope compositions of low-Ti Manihiki rocks can best be explained by re-melting of an ultra-depleted source (possibly a common mantle component in the Ontong Java and Manihiki plume sources) re-enriched by ≤1% of an ocean-island-basalt-like melt component. Unlike boninites formed via hydrous flux melting of refractory mantle at subduction zones, these boninite-like intraplate rocks formed through adiabatic decompression melting of refractory plume material that has been metasomatized by ocean-island-basalt-like melts. Our results suggest that caution is required before assuming all Archaean boninites were formed in association with subduction processes.

Comb layering monitors decompressing and fractionating hydrous mafic magmas in subvolcanic plumbing systems (Fisher Lake, Sierra Nevada, USA)

AbstractUnderstanding the dynamics of volcanic eruptions requires constraints on the processes of melt extraction in shallow magma reservoirs. This study focuses on the formation of intraplutonic igneous layering known as comb layers and orbicules within the Fisher Lake Pluton, Sierra Nevada. Comb layers are mineralogically and texturally diverse, ranging from plagioclase‐amphibole comb layers to plagioclase‐pyroxene and olivine comb layers. The majority of individual layers are related to the initiation of plagioclase nucleation and growth on preexisting host rocks and remobilized xenoliths. We use simple phase diagrams to illustrate the formation of comb layers through crystal fractionation along conduit walls and xenoliths, with plagioclase as the liquidus phase. Near‐adiabatic ascent of water‐undersaturated melts through the crust will lead to superheating and crystal resorption, which upon saturation of volatiles at shallow depth can induce heterogeneous nucleation of plagioclase. The rarity of orbicules and comb layers worldwide suggests that adiabatic decompression of crystal‐poor, moderately hydrous melts through the crust is rare. Most melts in intermediate/silicic volcanoes crystallize in multiple reservoirs within the crust prior to emplacement at shallow depth as crystal‐bearing melts. Comb layers and orbicules preserve evidence of multiple melt injections where a sequence of texturally and mineralogically similar comb layers and orbicules reflect the timescale of an individual melt pulse. Short timescales on the order of months to years for the growth of these sequences are estimated using experimental plagioclase growth rates and are interpreted as representing the lifetime of a melt injection within a long‐lived, subvolcanic melt extraction zone.

(Garnet)-spinel peridotite xenoliths from Mega (Ethiopia): Evidence for rejuvenation and dynamic thinning of the lithosphere beneath the southern Main Ethiopian Rift

Petrological and geothermobarometric data from (garnet)-spinel peridotite and pyroxenite xenoliths in Quaternary alkali-basalts at Mega (Sidamo region, Ethiopia) provide evidence for refertilization and thinning of the lower subcontinental lithospheric mantle (SCLM) during Tertiary rifting events in the southern Main Ethiopian Rift (MER). Samples of lherzolites, harzburgites, and olivine websterites contain spinel-pyroxene symplectites that were formed by garnet breakdown reactions. P-T equilibration conditions, in part calculated using the reconstructed garnet composition (pyrope), indicate the evolution of peridotites from the P-T conditions from the garnet stability field (2.9–2.2GPa; 945–1025°C), through garnet-spinel transition (< 2.1–1.7GPa at 906–1017°C), to the final equilibration in the spinel stability field (1.3GPa at 862–1056°C), corresponding to a nearly adiabatic decompression path of about 50km. Peridotite chemical compositions plot in an off-craton trend, with variable fertility degrees, only in part due to partial melting processes. Rocks show variable CaO contents (0.96–4.12wt%), CaO/Al2O3 ratios (1.42–4.46), and clinopyroxene contents (6–31vol.%), reaching compositions more enriched than pyrolite. Clinopyroxene shows evident LREE, U, Th, and Sr enrichments, particularly in harzburgites. Geochemical data suggest heterogeneous refertilization of a refractory lower SCLM, induced by infiltration and migration through porous flow of silicate-carbonate melts. Metasomatic processes drove melt compositions towards low-viscosity highly-mobile C-O-H-rich melts, inducing cryptic and hydrous modal metasomatism in refractory peridotites. Heterogeneous refertilization of the SCLM beneath the southern MER seems to have been induced by upwelling of asthenospheric melts and/or by reactivation of ancient lithospheric metasomes. Rejuvenation processes could have rheologically weakened the base of SCLM through gravitational instabilities, and increased the extensional stresses, causing a destabilizing effect at the onset of the lithospheric thinning and Tertiary rifting.

The role of decompression and micro-jetting in shock wave synthesis experiments

In a set of shock experiments under comparable porosities at pressures of about 35 GPa the melting behavior of porous copper was investigated. All experiments were performed with the impedance corrected sample recovery system and different degrees of decompression were used. It was possible to reduce the degree of molten metal in parts of the sample after sample recovery down to zero. The avoiding of melting was possible only by avoiding larger degrees of adiabatic decompression. This behavior implies a complete dependence of the melting on the release path for porous copper under the given conditions. The zones, where the melting processes are avoided, include also areas with intense micro jetting. Because also in these zones melting does not occur, it is possible that the melting curve of copper along the Hugoniot is not yet solved. The experiments have verified, that it is possible to use equation-of-state calculations for the solid state only, concerning the pressure area of currently commercial interest for the production of nitrides and diamond with copper-powder as pressure medium. Furthermore in this work the role of different parts of the sample recovery capsule is described to improve the comparability of shock wave synthesis experiments. On the other hand, the experimentally results given in this work show significant differences to data, obtained by a number of simulations.

REEBOXPRO: A forward model simulating melting of thermally and lithologically variable upwelling mantle

AbstractThe compositions and volumes of basalts erupted in divergent margin environments provide a record of the thermal, chemical, and dynamical state of their mantle source regions. To relate basalt compositions and volumes to the underlying thermochemical and dynamical state of their mantle source regions, we have developed REEBOX PRO, a compiled stand‐alone application that simulates adiabatic decompression melting of passively or actively upwelling mantle containing up to five distinct lithologies. The model calculates melt compositions using thermodynamic and experimental constraints on the melting behaviors and mineral‐melt partitioning behavior of homogeneous and lithologically heterogeneous mantle sources containing anhydrous peridotite, hydrous peridotite, harzburgite, and/or silica‐saturated/‐undersaturated pyroxenite. Key model outputs include the mean composition and crustal thickness for the bulk basaltic crust, calculated for passive and active upwelling scenarios. Here, we present the mathematical formulations underlying the model and benchmark it against existing hydrous melting models and models for mid‐ocean ridge basalt formation. We show that the hydrous and anhydrous peridotite melting models incorporated in REEBOX PRO capture the essential differences in basalt composition and volume demonstrated by previous models, and constrain the ambient mantle beneath the global spreading ridge system to be between 1319 and 1366°C, depending on the relative fertility and/or water content of the mid‐ocean ridge mantle source. We also show how model outputs may be manipulated outside of the modeling program to calculate nontraditional melt mixing scenarios. These examples highlight the flexibility of REEBOX PRO for simulating melt generation within a range of geodynamical contexts.

Silica-enriched mantle sources of subalkaline picrite-boninite-andesite island arc magmas

Primary arc melts may form through fluxed or adiabatic decompression melting in the mantle wedge, or via a combination of both processes. Major limitations to our understanding of the formation of primary arc melts stem from the fact that most arc lavas are aggregated blends of individual magma batches, further modified by differentiation processes in the sub-arc mantle lithosphere and overlying crust. Primary melt generation is thus masked by these types of second-stage processes. Magma-hosted peridotites sampled as xenoliths in subduction zone magmas are possible remnants of sub-arc mantle and magma generation processes, but are rarely sampled in active arcs. Published studies have emphasised the predominantly harzburgitic lithologies with particularly high modal orthopyroxene in these xenoliths; the former characteristic reflects the refractory nature of these materials consequent to extensive melt depletion of a lherzolitic protolith whereas the latter feature requires additional explanation.Here we present major and minor element data for pristine, mantle-derived, lava-hosted spinel-bearing harzburgite and dunite xenoliths and associated primitive melts from the active Kamchatka and Bismarck arcs. We show that these peridotite suites, and other mantle xenoliths sampled in circum-Pacific arcs, are a distinctive peridotite type not found in other tectonic settings, and are melting residues from hydrous melting of silica-enriched mantle sources. We explore the ability of experimental studies allied with mantle melting parameterisations (pMELTS, Petrolog3) to reproduce the compositions of these arc peridotites, and present a protolith (‘hybrid mantle wedge’) composition that satisfies the available constraints. The composition of peridotite xenoliths recovered from erupted arc magmas plausibly requires their formation initially via interaction of slab-derived components with refractory mantle prior to or during the formation of primary arc melts. The liquid compositions extracted from these hybrid sources are higher in normative quartz and hypersthene (i.e., they have a more silica-saturated character) in comparison with basalts derived from prior melt-depleted asthenospheric mantle beneath ridges. These primary arc melts range from silica-rich picrite to boninite and high-Mg basaltic andesite along a residual spinel harzburgite cotectic. Silica enrichment in the mantle sources of arc-related, subalkaline picrite-boninite-andesite suites coupled with the amount of water and depth of melting, are important for the formation of medium-Fe (‘calc-alkaline’) andesite-dacite-rhyolite suites, key lithologies forming the continental crust.

Water undersaturated mantle plume volcanism on present‐day Mars

AbstractBased on meteorite evidence, the present‐day Martian mantle has a combined abundance of up to a few hundred ppm of H2O, Cl, and F, which lowers the solidus and enhances the magma production rate. Adiabatic decompression melting in upwelling mantle plumes is the best explanation for young (last 200 Myr) volcanism on Mars. We explore water undersaturated mantle plume volcanism using a finite element mantle convection model coupled to a model of hydrous peridotite melting. Relative to a dry mantle, the reduction in solidus temperature due to water increases the magma production rate by a factor of 1.3–1.7 at 50 ppm water and by a factor of 1.9–3.2 at 200 ppm water. Mantle water also decreases the viscosity and increases the vigor of convection, which indirectly increases the magma production rate by thinning the thermal boundary layer and increasing the flow velocity. At conditions relevant to Mars, these indirect effects can cause an order of magnitude increase in the magma production rate. Using geologic and geophysical observations of the Late Amazonian magma production rate and geochemical observations of melt fractions in shergottite meteorites, present‐day Mars is constrained to have a core–mantle boundary temperature of ~1750 to 1800 °C and a volume‐averaged thermal Rayleigh number of 2 × 106 to 107, indicating that moderately vigorous mantle convection has persisted to the present day. Melting occurs at depths of 2.5–6 GPa and is controlled by the Rayleigh number at the low pressure end and by the mantle water concentration at high pressure.

Origin and geodynamic setting of Late Cenozoic granitoids in Sulawesi, Indonesia

Late Cenozoic granitoids are widespread in a 1600km long belt forming the Western and Northern Sulawesi tectono-magmatic provinces. They can be divided into three rock series: shoshonitic (HK), high-K felsic calc-alkaline (CAK), and normal calc-alkaline to tholeiitic (CA-TH). Representative samples collected from eleven plutons, which were subjected to petrography, major element, trace element, Sr, Nd, Pb isotope and whole-rock δ18O analyses, are all I-type and metaluminous to weakly peraluminous. The occurrence of the two K-rich series is restricted to Western Sulawesi, where they formed in an extensional, post-subduction tectonic setting with astenospheric upwelling providing thermal perturbation and adiabatic decompression. Two parental magma sources are proposed: enriched mantle or lower crustal equivalent for HK magmas, and Triassic igneous rocks in a Gondwana-derived fragment thrust beneath the cental and northern parts of Western Sulawesi for CAK magmas. The latter interpretation is based on striking similarities in radiogenic isotope and trace element signatures. CA-TH granitoids are found mostly in Northern Sulawesi. Partial melting of lower-middle crust amphibolites in an active subduction environment is the proposed origin of these rocks. Fractional crystallization and crustal contamination have played a significant role in magma petrogenesis, particularly in the case of the HK and CAK series. Contamination by organic carbon-bearing sedimentary rocks of the HK and CAK granitoids in the central part of Western Sulawesi is suggested by their ilmenite-series (reduced) character. The CAK granitoids further to the north and CA-TH granitoids in Northern Sulawesi are typical magnetite-series (oxidized). This may explain differences in mineralization styles in the two regions.

Interpretación geológica y geofísica de la Elevación de Río Grande, margen brasileño suroriental: tectónica extensional y rifting de las cortezas continental y oceánica

This paper discusses the geological and geophysical interpretation of rift structures in the region extending from the Rio Grande Rise, in the Southeastern Brazilian margin, towards the Cabo Frio High, which separates the Campos and Santos basins. We have analysed potential field data (gravity and magnetic) from the Argentine to the Brazilian oceanic basins and extending over the Pelotas, Santos and Campos basins. The Rio Grande Rise shows a relatively negative Bouguer anomaly in an area that corresponds to a major positive bathymetric feature between the Argentine and Brazil basins. North–south propagators related to the early spreading centres of the Atlantic Ocean are observed from Argentina towards the southern Santos Basin, which is characterized by an elevated basement topography relative to the Pelotas Basin. The region adjacent to the Florianopolis Fracture Zone between the Santos and Pelotas basins is also characterized by an elevated basement region aligned in an east–west direction, and locally it is marked by rift structures aligned along a NW–SE direction, forming a lineament or shear zone (Cruzeiro do Sul lineament) that extends from the Cabo Frio High towards the Rio Grande Rise, thus involving both continental and oceanic crusts. The Rio Grande Rise is associated with the east–west-trending fracture zones, which are characterized by several aligned magnetic anomalies in the southern Santos Basin. The Rio Grande Fracture Zone continues landward as the Sao Paulo Ridge, and extends towards the platform as the Florianopolis High. Oceanic propagators are identified from Argentina towards the Pelotas and Santos basins, and locally we observe rupturing of the salt layer by igneous intrusions or possibly by mantle exhumation. The Florianopolis (or Rio Grande) Fracture Zone is marked by an abrupt topographic offset separating the Pelotas Basin from the southern Santos Basin, and the associated volcanic belts limit the southernmost occurrence of the late Aptian evaporate sequence. The evaporite sequence in this segment of the continental margin shows remarkable layering of halite, anhydrite and carnalite. Conjugate to the Rio Grande Rise, the Walvis Ridge, offshore Namibia, is similarly a topographic high, but rift structures as observed in the Brazilian side are apparently unique in the South Atlantic. Alternative interpretations for the origin of the Rio Grande Rise include: a volcanic edifice or plateau rooted in the mantle; an intraplate shear zone affecting both continental and oceanic crust; an oceanic area of igneous over-productivity caused by a hotspot or a thermal anomaly in the mantle; a palaeo-spreading centre in the Cretaceous Atlantic Ocean; an area of excessive volcanic activity resulting from mantle differentiation due to adiabatic decompression; or perhaps an isolated remnant of continental crust left outboard of the Brazilian continental margin during the drifting process.

Links between arc volcanoes and porphyry-epithermal ore deposits

Porphyry and epithermal ore deposits, which are the products of magmatic hydrothermal fluids, are intimately associated with volcanoes in continental and island arcs above subduction zones, but the exact nature of this relationship has remained enigmatic. Although metal deposition is usually thought to occur during the waning stages of volcanism, numerous ore deposits have been demonstrated to be synvolcanic. Here we show how the formation of these deposits is tied to volcanic cycles. We relate the chemical variations in vapors from Merapi volcano, Indonesia, to different stages of its eruptive cycle. The chemical compositions of volcanic vapors from subduction zone volcanoes are then compared globally to those of fluid inclusions from porphyry-epithermal deposits. These data show that adiabatic decompression is the principal control on mineralization. The data also suggest that volcanic and subvolcanic magmatic-hydrothermal systems are under lithostatic pressure during quiescence but decompress rapidly during injections of mafic magma and explosive eruptions. During quiescence, the magma evolves through fractional crystallization and devolatilization, gradually becoming oxidized and enriched in gold and other incompatible metals. Upon the injection of sulfur-rich mafic magmas, subvolcanic intrusions brecciate the overlying rocks, the systems are depressurized, the volcanoes erupt explosively, supercritical fluids unmix into vapor and brine, and base metal sulfides precipitate.

Halide based shock-wave treatment of fluid-rich natural phases

For the synthesis of high pressure phases from natural minerals and the shock wave treatment of fluid bearing phases a halide based method was developed. The experiments were performed in the pressure range between 25 and 162 GPa with a success rate for the new method of 100% for the new method. Based on the Impedance Corrected Sample Recovery Capsule under avoiding the adiabatic decompression a direct comparison between different loading paths and sample holder geometries is possible. The recovered samples show neither indications of melting in the case of kaolinite and very limited degassing in the case of carbonates. The recovery of amorphous water bearing Al-Si-phases with Aluminum in four-, five- and six-fold coordination was possible. The samples were analyzed with scanning electron microscopy, x-ray diffraction, nuclear-magnetic-resonance- and infra-red-spectroscopy and the results were directly compared.

40Ar/39Ar ages and petrogenesis of the West Iberian Margin onshore magmatism at the Jurassic–Cretaceous transition: Geodynamic implications and assessment of open-system processes involving saline materials

The West Iberian Margin (WIM) preserves onshore testimonies of three Mesozoic magmatic cycles. In this paper we present and discuss 40Ar/39Ar ages and geochemical data for the second cycle, which occurred at least from 148Ma to 140Ma, during the late stages of an important extensional event associated with the Iberia–Newfoundland rifting. The related lithospheric stretching induced magma genesis by adiabatic decompression. Primitive rocks are mildly alkaline but evolved to SiO2-saturated and oversaturated rocks at “high” pressure. Magmas sampled a source of fairly homogenous composition characterized by Sr and Nd isotopic compositions (εNdi from +1.6 to +4.2), more enriched than the typical N-MORB source. Magmas were generated at the top of the garnet zone. Considering the thickness of the lithosphere and the geochemical constraints, an origin by melting of a metasomatized domain of the lithosphere is favored. The composition of these onshore magmas is somewhat distinct from the quasi coeval magmas emplaced offshore, which is interpreted as a result of the less important onshore lithospheric stretching, leading to lower degrees of partial melting. This favored the contribution of lithospheric metasomatized domains to onshore magmas. Rocks intruded two sectors of the Lusitanian Basin separated by the Nazaré Fault and characterized by distinct subsidence rates during the Jurassic. The fact that the rocks to the north of the Nazaré Fault are significantly more evolved indicates the more important development of magma chambers in the north, suggesting distinct thermal profiles for those two sectors. Such magma chambers enabled the “high-pressure” fractionation necessary to drive magma compositions from Ne-normative to SiO2- saturated and -over-saturated. The rocks cropping out south of the Nazaré fault are clearly less evolved, and its variability is mostly due to different partial melting events. Some rocks present evidence of post-magmatic processes involving neighboring Jurassic evaporite materials, leading to an increase in the Na2O content and 87Sr/86Sr ratio.

Sub-continental lithospheric mantle structure beneath the Adamawa plateau inferred from the petrology of ultramafic xenoliths from Ngaoundéré (Adamawa plateau, Cameroon, Central Africa)

Ultramafic xenoliths (lherzolite, harzburgite and olivine websterite) have been discovered in basanites close to Ngaoundéré in Adamawa plateau. Xenoliths exhibit protogranular texture (lherzolite and olivine websterite) or porphyroclastic texture (harzburgite). They are composed of olivine Fo89-90, orthopyroxene, clinopyroxene and spinel. According to geothermometers, lherzolites have been equilibrated at 880–1060 °C; equilibrium temperatures of harzburgite are rather higher (880–1160 °C), while those of olivine websterite are bracketed between 820 and 1010 °C. The corresponding pressures are 1.8–1.9 GPa, 0.8–1.0 GPa and 1.9–2.5 GPa, respectively, which suggests that xenoliths have been sampled respectively at depths of 59–63 km, 26–33 km and 63–83 km. Texture and chemical compositional variations of xenoliths with temperature, pressure and depth on regional scale may be ascribed to the complex history undergone by the sub-continental mantle beneath the Adamawa plateau during its evolution. This may involve a limited asthenosphere uprise, concomitantly with plastic deformation and partial melting due to adiabatic decompression processes. Chemical compositional heterogeneities are also proposed in the sub-continental lithospheric mantle under the Adamawa plateau, as previously suggested for the whole Cameroon Volcanic Line.

Eocene accretion at Kamchatka and a pulse of mantle plume magmatism

The paper presents newly obtained chemical analyses of metamorphosed mafic volcanic rocks from the upper reaches of the Krutogorova River, southwestern Kamchatka. According to their geochemistry, these mafic rocks and the orebearing mafic rocks of th e cortlandite-gabbro-norite association at the Kuval� orog occurrence of Ni mineralization in Kamchatka were produced by melting undepleted mantle at its adi� abatic decompression (without involvement of subductionrelated fluids and melts). The significant crystal� lization depths of the small cortlandite-gabbro-norite intrusions are confirmed by our pressure evaluations ( P = 8 kbar) by a newly developed amphibole barometer; the crystallization depths of the small subvolcanic intrusions was much shallower. The physical mechanism of derivation of the primitive partial mantle melts is discussed using simple numerical simulations of processes related to the onset of subduction and oceanic slab breakoff. Geochemical data and results of our numeri cal simulations testify that the picrobasaltic magmas were generated in ascending flows induced in the upper mantle by complicated geodynamic processes during the accretion of the Achaivaam-Valaginskii island arc to the Eurasian continent in the Eocene.