Silica-rich Melts Research Articles

Genetic relationship between 1780 Ma dykes and coeval volcanics in the Lvliang area, North China

The 1780Ma dykes and coeval volcanics constitute a large igneous province in the North China Craton; however, whether these dykes and volcanic rocks of varying compositions originated from one or multiple sources remains controversial. The ca. 1780 Ma Lvliang dykes are characterized by widespread E-W-oriented dykes and minor N-W-oriented diabase dykes. The E-W-oriented dykes are bimodal and comprise two subgroups: one is dominated by acidic high-silica dykes (SiO2 >63wt%), while the other is dominated by mafic to intermediate relatively low-silica dykes (SiO2 <63wt%). The ca. 1780Ma volcanics (in the Xiaoliangling Group) in the Lvliang area are also bimodal and dominated by rhyolite to dacite volcanics and basalt to andesite volcanics with few clastic interlayers.SIMS U-Pb dating on zircons from one high-silica dyke and a rhyolite volcanic layer yields 207Pb/206Pb ages of 1783±7Ma and 1776±6Ma, respectively, and SIMS U-Pb dating on baddeleyites from a low-silica dyke yields a 207Pb/206Pb age of 1789±5Ma. They are all tholeiitic in composition (MgO: 0.3–6.1wt%; SiO2: 51–74wt%), enriched in light rare earth elements ((La/Yb)N = 7.9–20.2) and show negative anomalies in Nb, Ta, Sr and Eu (Eu/Eu*=0.3–0.9). They also have similar in-situ Hf isotopes of baddeleyite/zircon (εHf values of −14.5 to −1.8) and bulk rock εNd (−6.7 to −3.0) isotopes. Furthermore, immiscible textures are identified from the Xiaoliangling Group volcanics, which are characterized by some silica-rich melts globules in the Fe-rich magma.These spatiotemporal affinities and geochemical similarities, as well as the identified immiscible textures, reveal that the low-silicate dykes are conduits for the basalt-andesite with significant fractionation of clinopyroxene and plagioclase; while the high-silica dykes are conduits for the rhyolite-dacite with distinct immiscible segregation prior to eruption. In addition, there is a sharp change of εHf values at 1780–1730Ma, and their Hf TDM ages are ∼2750–1950Ma. As these rocks originated from the subcontinental lithospheric mantle, this may indicate that the subcontinental lithospheric mantle was metasomatized during this time period, possibly by a paleo-plume event.

High-Pressure Melting Experiments on Basalt-Peridotite Layered Source (KLB-1/N-MORB): Implications for Magma Genesis in Hawaii

In order to understand the melting processes that occur within recycled oceanic crust and mantle in a heterogeneous plume (e.g., that beneath the Hawaiian Islands), a series of high-pressure-high-temperature layered experiments were performed at 2.9 GPa, 5 GPa, and 8 GPa, from 1300°C to 1650°C, using a fertile peridotite KLB-1 and N-MORB. Our experiments at conditions below the dry peridotite solidus produced melt compositions that ranged from basaltic andesite to tholeiite. An Opx reaction band formed between eclogite and peridotite layers, likely via chemical reaction between a silica-rich eclogite-derived partial melt and olivine in the peridotite matrix. At temperatures at or above the dry peridotite solidus, substantial melting occurred in both basalt and peridotite layers, and fully molten basalt melt and melt pockets from the peridotite layer combined. In our layered experiments, major and minor element contents in reacted melts closely matched those of Hawaiian tholeiite and picrite, except for Fe. Partial melts of anhydrous run products had ~55 - 57 wt% SiO2 at low temperature (i.e., were andesitic) and had ~50 - 53 wt% SiO2 at high temperatures, slightly below the dry peridotite solidus (i.e., were tholeiitic, and similar to those that occur during the Hawaii shield-building stage). Based on the Fe- and LREE-enriched signature in Hawaiian tholeiites, we propose that recycled components in the Hawaiian plume are not modern N-MORB, but are Fe-rich tholeiite; a lithology that was common in the Archaean and early Proterozoic. We have demonstrated that the entire compositional spectrum of Hawaiian tholeiites (basalt to picrite) can be formed by basalt-peridotite reactive melting near the dry solidus of peridotite. Based on these results, we propose that the potential temperature of the sub-Hawaiian plume may be much lower than previously estimated.

Immiscible iron- and silica-rich melts and magnetite geochemistry at the El Laco volcano (northern Chile): Evidence for a magmatic origin for the magnetite deposits

The Pliocene El Laco volcano (northern Chile) hosts a large iron oxide deposit that occurs as stratiform bodies with underlying feeder zones (dykes) hosted by hydrothermally altered andesite lava flows. Phenocrysts in the andesite hosting the mineralization have abundant melt inclusions displaying strong evidence for melt immiscibility. Most of the pyroxene and the plagioclase phenocrysts contain abundant Fe-rich melt inclusions with low-Ti magnetite, pyroxene, apatite, and minor anhydrite. The glassy groundmass of these melt inclusions is chemically similar to sub-alkaline rhyolitic rocks (e.g., >70% SiO2; Si-Al-K-Na-rich). Large resorbed plagioclase phenocrysts also host another group of melt inclusions with a similar silica-rich groundmass but hosting evenly distributed immiscible and complex Fe-rich spheroidal globules that are enriched in Ti, P, Mg, Ca, and S. These immiscible globules and the Ti-poor magnetite daughter crystals in the melt inclusions in pyroxene phenocrysts are thought to be the parental iron oxide-melt liquid, after which their coalescence leads to the formation of magnetite melts, from which the magnetite that formed the El Laco iron oxide deposits crystallized and were emplaced.The mineralogy and chemistry of these melt inclusions tracks the chemical evolution of the Fe-Ox melts and supports a magmatic origin for the El Laco magnetite. This is mainly recognized by the continuum of compositions between volcanic, separate immiscible Fe-oxide melts, and magnetite ores. The trace element composition of the Fe-rich melt inclusions and of the magnetite in the host andesite and the ore bodies reveals a systematic depletion in most of the elements that are compatible in magnetite. The Ti content of the magnetite also records the separation and crystallization of the iron-rich melt. Typically, the magnetite in the orebodies and the melt inclusions is Ti-poor compared with titanomagnetite found in the andesite groundmass and that hosted by the hydrothermally-altered andesite; that is interpreted as being attributed to its crystallization under oxidizing and sulfur-rich conditions. The cause of this unusual chemical behaviour is probably due to the mechanism(s) governing the crystallization of the iron-rich melt, including the temperature, changes in chemistry prior to eruption, and an increase in the redox conditions of the silicate melt. Magma oxidation facilitates the prevalence of ferric iron and formation of Ti-poor magnetite, preventing the incorporation of Ti in the exsolving iron-rich melt. The likely reason for the formation of these large Ti-poor magnetite deposits is likely the contamination of the parental andesitic magma by oxidized crustal rocks and concomitant crystallization of magnetite under oxidizing conditions, something that had a significant impact on the elemental distribution.

Oxygen isotopes reveal crustal contamination and a large, still partially molten magma chamber in Chaîne des Puys (French Massif Central)

The two main magmatic properties associated with explosive eruptions are high viscosity of silica-rich magmas and/or high volatile contents. Magmatic processes responsible for the genesis of such magmas are differentiation through crystallization, and crustal contamination (or assimilation) as this process has the potential to enhance crystallization and add volatiles to the initial budget. In the Chaîne des Puy series (French Massif Central), silica- and H2O-rich magmas were only emitted during the most recent eruptions (ca. 6–15ka). Here, we use in situ measurements of oxygen isotopes in zircons from two of the main trachytic eruptions from the Chaîne des Puys to track the crustal contamination component in a sequence that was previously presented as an archetypal fractional crystallization series. Zircons from Sarcoui volcano and Puy de Dôme display homogeneous oxygen isotope compositions with δ18O=5.6±0.25‰ and 5.6±0.3‰, respectively, and have therefore crystallized from homogeneous melts with δ18Omelt=7.1±0.3‰. Compared to mantle derived melts resulting from pure fractional crystallization (δ18Odif.mant.=6.4±0.4‰), those δ18Omelt values are enriched in 18O and support a significant role of crustal contamination in the genesis of silica-rich melts in the Chaîne des Puys. Assimilation–fractional–crystallization models highlight that the degree of contamination was probably restricted to 5.5–9.5% with Rcrystallization/Rassimilation varying between 8 and 14. The very strong intra-site homogeneity of the isotopic data highlights that magmas were well homogenized before eruption, and consequently that crustal contamination was not the trigger of silica-rich eruptions in the Chaîne des Puys. The exceptionally strong inter-site homogeneity of the isotopic data brings to light that Sarcoui volcano and Puy de Dôme were fed by a single large magma chamber. Our results, together with recent thermo-kinetic models and an experimental simulation (Martel et al., 2013), support the existence of a large (~6–15km3), still partially molten mid-crustal reservoir (10–12km deep) that is filled with silica-rich magma.

Are Adakites Slab Melts or High-pressure Fractionated Mantle Melts?

Adakites are unusual felsic igneous rocks commonly associated with asthenospheric slab window opening or fast subduction of a young (<25 Ma) oceanic plate that may allow slab melting at shallow depths. Their genesis has been extensively debated, as they are also observed in other geodynamic settings where thermal models do not predict slab melting in the fore-arc region. Here, we present a new approach that provides new constraints on adakite petrogenesis in hot subduction zones (e.g. the Philippines) and above an asthenospheric window (e.g. Baja California, Mexico). We use amphibole compositions to estimate magma storage depths and the composition of the host melts to test the hypothesis that adakites are pristine slab melts. We find that adakites from the Philippines and Baja California fore-arcs formed in two distinct petrogenetic scenarios: in the Philippines, water-rich mantle melts stalled and crystallized within lower and upper crustal magma storage regions to produce silica-rich melts with an adakitic signature; in Baja California, slab melts that percolated through the mantle wedge mixed or mingled with water-rich mantle melts within a lower crustal (∼30 km depth) magma storage region before stalling in the upper arc crust (∼7–15 km depth). Alternatively, the Baja California adakites may represent mixing products between high-pressure differentiated mantle melts and mantle melts in a lower crustal magma reservoir, periodically refluxed by mantle melts. Thus, slab melting is not necessarily required to produce an adakitic geochemical fingerprint in hot subduction zones. The hot downgoing plate may cross the ‘adakitic window’ and melt in specific geodynamic settings such as the opening of a slab tear, as beneath Baja California.

Probing permeability and microstructure: Unravelling the role of a low-permeability dome on the explosivity of Merapi (Indonesia)

Low permeability dome rocks may contribute to conduit overpressure development in volcanic systems, indirectly abetting explosive activity. The permeability of dome-forming rocks is primarily controlled by the volume, type (vesicles and/or microcracks), and connectivity of the void space present. Here we investigate the permeability–porosity relationship of dome-forming rocks and pumice clasts from Merapi's 1888 to 2013 eruptions and assess their possible role in eruptive processes, with particular emphasis on the 2010 paroxysmal eruption. Rocks are divided into three simple field classifications common to all eruptions: Type 1 samples have low bulk density and are pumiceous in texture; Type 2 samples, ubiquitous to the 2010 eruption, are dark grey to black in hand sample and vary greatly in vesicularity; and Type 3 samples are weakly vesicular, light grey in hand sample, and are the only samples that contain cristobalite. Type 2 and Type 3 rocks are present in all eruptions and their permeability and porosity data define similar power law relationships, whereas data for Type 1 samples are clearly discontinuous from these trends. A compilation of permeability and porosity data for andesites and basaltic andesites with published values highlights two microstructural transitions that exert control on permeability, confirmed by modified Bayesian Information Criterion (BIC) analysis. Permeability is microcrack- and diktytaxitic-controlled at connected porosities, φc, <10.5vol.%; vesicle- and microcrack-controlled at 10.5<φc<31vol.%; and likely vesicle-controlled for φc>31vol.%. Type 3 basaltic andesites, the least permeable of the measured samples and therefore the most likely to have originated in the uppermost low-permeability dome, are identified as relicts of terminal domes (the last dome extruded prior to quiescence). Cristobalite commonly found in the voids of Type 3 blocks may not contribute significantly to the reduction of the permeability of these samples, mainly because it is associated with an extensive microporous, diktytaxitic texture. Indeed, the low permeability of these rocks is more likely associated with their lower fracture density. We propose that diktytaxitic textures may arise from late-stage gas filter pressing of a silica-rich melt phase, which leaves behind a microlite-supported groundmass and cristobalite in neighbouring vesicles. Due to the ubiquity of the Type 3 rocks in all Merapi eruptions, we do not invoke the emplacement of a low-permeability cap as having favoured a particularly high pressurization and subsequent high explosivity of the 2010 eruption. The debate as to the reasons for the highly explosive 2010 eruption rages on.

On progress and rate of the peritectic reaction Fo + SiO2 → En in natural andesitic arc magmas

Using a high resolution ion microprobe with SCAPS imaging, the peritectic reaction of forsterite+silica to enstatite was studied down to submicron level in a natural andesitic tephra from the Central Plateau of North Island, New Zealand. The fayalitic component of natural olivines is stable in high-silica melts, and therefore the reaction is in fact a two-step progress: 1. Dissolution of Mg-rich olivine, rate-limited by Fe–Mg interdiffusion at the crystal rim, results in enrichment of Fe in the crystal rim and of Mg in the c. 1μm wide melt boundary layer around the crystal. 2. Magnesian pyroxenes preferentially but not exclusively nucleate in the melt boundary layer and grow; as soon as these microlites touch the rim of the dissolving olivine, they shield the crystals from the silica-rich melt, thereby preventing further olivine dissolution. At this point, Fe–Mg interdiffusion begins to destroy the Fe-enrichment of the olivine rim. The reaction is completed when the dissolving olivine crystal is completely mantled by magnesian pyroxene microlites. Thick pyroxene mantles are likely the result of pyroxene overgrowth rather than due to peritectic transformation. The morphology of the olivine rim preserves information about the reaction history of the grain. Modeling of Fe–Mg interdiffusion in the olivine rim following its shielding from the melt by pyroxene overgrowth may yield the rates of olivine dissolution and the rates of pyroxene growth if temperature is known. For the tephra we have studied, microlite thermometry yields a temperature of 1137 (±41)°C, indicating an olivine dissolution rate of 3–6×10−11ms−1 and an initially volumetric pyroxene growth rate of 2.2–5.6×10−21m3s−1. This points to timescales between olivine crystal uptake into the SiO2-rich melt and explosive eruption at the surface of a few hours to at most a day.

The lunar Gruithuisen silicic extrusive domes: Topographic configuration, morphology, ages, and internal structure

The Gruithuisen domes, situated on the western portion of the Imbrium basin rim, form three tall mountains (NW, Gamma, Delta) totaling ∼780km3 in volume. The shapes of the domes are significantly different from that of mare-type domes elsewhere on the Moon. We use data from the Lunar Reconnaissance Orbiter (LRO) and Kaguya missions (LRO Lunar Orbiter Laser Altimeter, Lunar Reconnaissance Orbiter Camera, Diviner, and the Kaguya imager) to characterize the domes and assess models for their origin. The configuration of the domes (steep slopes, up to ∼18–20°) and their specific remote sensing characteristics (strong downturn in the UV, and results from the M3 and Diviner instruments) suggest that the domes formed by eruptions of highly viscous lava. The estimated surface volumes of the domes vary from ∼20km3 (NW dome) to ∼290km3 (Gamma dome) to ∼470km3 (Delta dome). The domes occur on the portion of the Imbrium basin rim that is overlain by ejecta from the post-Imbrium Iridum crater. In some areas, relatively high albedo smooth volcanic plains are seen within the Iridum ejecta near the Gruithuisen domes, and low albedo mare deposits surround and embay the domes and Iridum crater. Dating of different units and features by crater counts indicates that impact melts from the Iridum basin are ∼3.9Ga old, the domes Gamma and Delta are ∼3.8Ga, and the ages of the plains near the domes vary from ∼2.3 to ∼3.6Ga. A fresh impact crater exposes the internal structure of the Gamma dome. The most prominent features on the wall of the crater are rough, blocky layers that are typical of volcanic plains in the highlands and maria around the domes. The layers are interleaved with fine-grained materials of higher and lower albedo and the visible orientation of the layers changes over short (a few hundred meters) distances. These characteristics of the internal structure of the dome are consistent with eruptions of high viscosity lava (rough layers) that alternated with possible explosive activity (fine-grained materials). The spatial association of the Gruithuisen domes with the highland lava plains resembles the situation in which bimodal volcanism occur on Earth. The terrestrial association can be due to either fractional crystallization in basaltic magma reservoirs or remelting of high-silica crustal materials. In the first case, the evolved melts appear in later stages of volcanic activity and in the second case these melts are formed near the beginning of evolution of the magmatic systems. The age estimates of the Gruithuisen domes and the surrounding volcanic plains are more consistent with the crustal remelting scenario. However, remelting of primary anorthositic crust cannot readily produce the silica-rich melts and requires the presence of pre-existing granite-like materials. Formation of the domes by fractional crystallization avoids this difficulty but requires explanation of the older age of the domes relative to the volcanic plains in the surroundings. A third option is that the domes are unrelated genetically to the mare deposits.

The amount of recycled crust in sources of mantle-derived melts.

Plate tectonic processes introduce basaltic crust (as eclogite) into the peridotitic mantle. The proportions of these two sources in mantle melts are poorly understood. Silica-rich melts formed from eclogite react with peridotite, converting it to olivine-free pyroxenite. Partial melts of this hybrid pyroxenite are higher in nickel and silicon but poorer in manganese, calcium, and magnesium than melts of peridotite. Olivine phenocrysts' compositions record these differences and were used to quantify the contributions of pyroxenite-derived melts in mid-ocean ridge basalts (10 to 30%), ocean island and continental basalts (many >60%), and komatiites (20 to 30%). These results imply involvement of 2 to 20% (up to 28%) of recycled crust in mantle melting.

An Experimental Study of Trace Element Fluxes from Subducted Oceanic Crust

We have determined experimentally the hydrous phase relations and trace element partitioning behaviour of ocean floor basalt protoliths at pressures and temperatures (3 GPa, 750‐1000 � C) relevant to melting in subduction zones. To avoid potential complexities associated with trace element doping of starting materials we have used natural, pristine mid-ocean ridge basalt (MORB from Kolbeinsey Ridge) and altered oceanic crust (AOC from Deep Sea Drilling Project leg 46, � 20 � N Atlantic). Approximately 15 wt % water was added to starting materials to simulate fluid fluxing from dehydrating serpentinite underlying the oceanic crust. The vapour-saturated solidus is sensitive to basalt K2O content, decreasing from 825 6 25 � C in MORB (� 0� 04 wt % K2O) to � 750 � Ci n AOC (� 0� 25 wt % K2O). Textural evidence indicates that near-solidus fluids are sub-critical in nature. The residual solid assemblage in both MORB and AOC experiments is dominated by garnet and clinopyroxene, with accessory kyanite, epidote, Fe‐Ti oxide and rutile (plus quartz‐coesite, phengite and apatite below the solidus). Trace element analyses of quenched silica-rich melts show a strong temperature dependence of key trace elements. In contrast to the trace elementdoped starting materials of previous studies, we do not observe residual allanite. Instead, abundant residual epidote provides the host for thorium and light rare earth elements (LREE), preventing LREE from being released (RLREE 1500 and La/ SmPUM (where PUM indicates primitive upper mantle) � 1, most closely matching the geochemical signal of arc lavas worldwide, were generated from AOC at 800‐850 � C.

Partitioning of light lithophile elements during basalt eruptions on Earth and application to Martian shergottites

An enigmatic record of light lithophile element (LLE) zoning in pyroxenes in basaltic shergottite meteorites, whereby LLE concentrations decrease dramatically from the cores to the rims, has been interpreted as being due to partitioning of LLE into a hydrous vapor during magma ascent to the surface on Mars. These trends are used as evidence that Martian basaltic melts are water-rich (McSween et al., 2001). Lithium and boron are light lithophile elements (LLE) that partition into volcanic minerals and into vapor from silicate melts, making them potential tracers of degassing processes during magma ascent to the surface of Earth and of other planets. While LLE degassing behavior is relatively well understood for silica-rich melts, where water and LLE concentrations are relatively high, very little data exists for LLE abundance, heterogeneity and degassing in basaltic melts. The lack of data hampers interpretation of the trends in the shergottite meteorites. Through a geochemical study of LLE, volatile and trace elements in olivine-hosted melt inclusions from Kilauea Volcano, Hawaii, it can be demonstrated that lithium behaves similarly to the light to middle rare Earth elements during melting, magma mixing and fractionation. Considerable heterogeneity in lithium and boron is inherited from mantle-derived primary melts, which is dominant over the fractionation and degassing signal. Lithium and boron are only very weakly volatile in basaltic melt erupted from Kilauea Volcano, with vapor-melt partition coefficients <0.1. Degassing of LLE is further inhibited at high temperatures. Pyroxene and associated melt inclusion LLE concentrations from a range of volcanoes are used to quantify lithium pyroxene-melt partition coefficients, which correlate negatively with melt H2O content, ranging from 0.13 at low water contents to <0.08 at H2O contents >4 wt%. The observed terrestrial LLE partitioning behavior is extrapolated to Martian primitive melts through modeling. The zoning observed in the shergottite pyroxenes is only consistent with degassing of LLE from a Martian melt near its liquidus temperature if the vapor-melt partition coefficient was an order of magnitude larger than observed on Earth. The range in LLE and trace elements observed in shergottite pyroxenes are instead consistent with concurrent mixing and fractionation of heterogeneous melts from the mantle.

The effect of pressure and water concentration on the electrical conductivity of dacitic melts: Implication for magnetotelluric imaging in subduction areas

Silica-rich hydrous magmas are commonly stored in crustal reservoirs, but are also present at mantle depths in subduction contexts as a result of slab melting in the presence of considerable amounts of water and other volatile species. Magnetotelluric surveys frequently identify highly conductive zones at crustal or mantle depths possibly revealing the presence of such silica-rich melts and this can be used to trace the cycling of water in subduction zones and its relationship with arc-magmatism. The achievement of such a purpose is impeded by poor knowledge of the electrical conductivity of both dry and hydrous silica-rich melts at pressure. To fill this gap, we performed in situ electrical conductivity measurements on a dacitic melt using a 4-wire set up to 1300°C, 3.0GPa and H2O content up to 12wt.%. Melt conductivity is strongly correlated with its water content, and we reveal a complex effect of pressure being relatively small at low water contents and major at high water contents: with increasing water content, the activation volume ranges between 4 (dry) and 25cm3/mol (H2O=12wt.%) and the activation energy decreases from 96kJ (dry) to 62kJ (12wt.% H2O). By comparison with diffusivity data, sodium appears to be the main charge carrier, even at high (12wt.%) water content. A T–P–[H2O] model predicting the conductivity of dacitic melts shows that crustal and mantle wedge conductive bodies can be interpreted by the presence of silica-rich, hydrous, partially crystallized magma.

Experimental melting of phlogopite-bearing mantle at 1 GPa: Implications for potassic magmatism

We have experimentally investigated the fluid-absent melting of a phlogopite peridotite at 1.0 GPa (1000–1300 °C) to understand the source of K2O- and SiO2-rich magmas that occur in continental, post-collisional and island arc settings. Using a new extraction technique specially developed for hydrous conditions combined with iterative sandwich experiments, we have determined the composition of low- to high-degree melts (Φ=1.4 to 24.2 wt.%) of metasomatized lherzolite and harzburgite sources. Due to small amounts of adsorbed water in the starting material, amphibole crystallized at the lowest investigated temperatures. Amphibole breaks down at 1050–1075 °C, while phlogopite-breakdown occurs at 1150–1200 °C. This last temperature is higher than the previously determined in a mantle assemblage, due to the presence of stabilizing F and Ti. Phlogopite–lherzolite melts incongruently according to the continuous reaction: 0.49 phlogopite + 0.56 orthopyroxene + 0.47 clinopyroxene + 0.05 spinel = 0.58 olivine + 1.00 melt. In the phlogopite–harzburgite, the reaction is: 0.70 phlogopite + 1.24 orthopyroxene + 0.05 spinel = 0.99 olivine + 1.00 melt. The K2O content of water-undersaturated melts in equilibrium with residual phlogopite is buffered, depending on the source fertility: from ∼3.9 wt.% in lherzolite to ∼6.7 wt.% in harzburgite. Primary melts are silica-saturated and evolve from trachyte to basaltic andesite (63.5–52.1 wt.% SiO2) with increasing temperature. Calculations indicate that such silica-rich melts can readily be extracted from their mantle source, due to their low viscosity. Our results confirm that potassic, silica-rich magmas described worldwide in post-collisional settings are generated by melting of a metasomatized phlogopite-bearing mantle in the spinel stability field.

Melt–peridotite interaction in the shallow lithospheric mantle of the North China Craton: evidence from melt inclusions in the quartz-bearing orthopyroxene-rich websterite from Hannuoba

To better understand the origin, migration, and evolution of melts in the lithospheric mantle and their roles on the destruction of the North China Craton (NCC), we conducted a petrological and geochemical study on a quartz-bearing orthopyroxene-rich websterite xenolith from Hannuoba, the NCC, and its hosted melt and fluid inclusions. Both clinopyroxene and orthopyroxene in the xenolith contain lots of primary and secondary inclusions. High-temperature microthermometry of melt inclusions combined with Raman spectroscopy analyses of coexisting fluid inclusions shows that the entrapment temperature of the densest inclusions was ~1215°C and the pressure ~11.47 kbar, corresponding to a depth of ~38 km, i.e. within the stability of the spinel lherzolite. Intermediate pressure inclusions probably reflect progressive fluid entrapment over a range of depths during ascent, whereas the low-pressure inclusions (P < 2 kbar) may represent decrepitated primary inclusions. In situ laser-ablation ICP-MS analyses of major and trace elements on individual melt inclusions show that the compositions of these silicate melt inclusions in clinopyroxene and orthopyroxene are rich in SiO2, Al2O3, and alkalis but poor in TiO2 and strongly enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), with negative anomalies of high-field strength elements (HFSEs). These characteristics suggest that the silica-rich melts could be derived from the partial melting of subducted oceanic slab. Therefore, this kind of quartz-bearing orthopyroxene-rich websterite may be produced by interaction between the slab-derived melts with the mantle peridotite. This study provides direct evidence for the origin, migration, and evolution of melts in the lithospheric mantle, which may play an important role in the destruction of the NCC.

Prolonged magmatic activity on Mars inferred from the detection of felsic rocks

Felsic rocks have not been identified on Mars, a planet that lacks plate tectonics to drive the magmatic processes that lead to evolved silica-rich melts. Spectral observations by the Mars Reconnaissance Orbiter indicate that felsic lithologies occur at multiple localities on Mars and suggest prolonged magmatic activity on ancient Mars.

Crustal xenoliths from Tallante (Betic Cordillera, Spain): insights into the crust–mantle boundary

AbstractThe volcano of Tallante (Pliocene) in the Betic Cordillera (Spain) exhumed a heterogeneous xenolith association, including ultramafic mantle rocks and diverse crustal lithologies. The latter include metagabbroids and felsic rocks characterized by quartz-rich parageneses containing spinel ± garnet ± sillimanite ± feldspars. Pressure–temperature estimates for felsic xenoliths overlap (at 0.7–0.8 GPa) those recorded by the mantle-derived peridotite xenoliths. Therefore, we propose that an intimate association of interlayered crust and mantle lithologies characterizes the crust–mantle boundary in this area. This scenario conforms to evidence provided by the neighbouring massifs of Ronda and Beni Bousera (and by other peri-Mediterranean deep crust/mantle sections) where exhumation of fossil crust–mantle boundary reveals that this boundary is not sharp. The results are discussed on the basis of recent geophysical and petrological studies emphasizing that in non-cratonic regions the crust–mantle boundary is often characterized by a gradational nature showing inter-fingering of heterogeneous lithologies. Silica-rich melts formed within the crustal domains intruded the surrounding mantle and induced metasomatism. The resulting hybrid crust–mantle domains thus provide suitable sources for exotic magma types such as the Mediterranean lamproites.

Petrochemical features of Miocene volcanism around the Çubukludağ graben and Karaburun peninsula, western Turkey: Implications for crustal melting related silicic volcanism

Widespread Neogene volcanism, mainly intermediate and rarely mafic and felsic in composition, was controlled by the extensional tectonic regime in western Turkey. The Karaburun and Cumaovası volcanics are the cases for understanding the magma source(s) and petrological processes, producing the extension-related mafic and felsic volcanism. The Karaburun volcanics (KV) are mainly oriented north to south in the Karaburun peninsula and span a wide spectrum from basalt (20Ma) to rhyolite (16Ma), and younger trachyte and trachydacites (13Ma). The products of the subaerial silicic volcanism (the Cumaovası volcanics, CV; 17Ma) which are represented by cluster of rhyolite domes, related pyroclastics occur within the NE–SW trending Çubukludağ graben, and intermediate and mafic volcanic rocks are lack in this area. The lavas of the Cumaovası volcanics are high silica rhyolites and rare dacites which are calc alkaline, peralumious and enriched significantly in LILE. Extremely low Sr, Ba values, extremely Eu depletions and very low LaN/YbN ratios are typical for the rhyolites of CV, similar to the topaz rhyolites. The Karaburun volcanics, with the exception of the minor alkaline basaltic and trachytic lavas, are mainly calc alkaline and metaluminous intermediate lavas. 87Sr/86Sr ratios of the KV and dacitic samples of CV are close to each other and range from 0.708 to 0.709; while Sr isotopic ratios of the rhyolites are significantly high and variable (0.724–0.786). 143Nd/144Nd ratios of the CV and KV, except for the alkaline samples, are similar for both sequences vary from 0.51230 to 0.51242.Geological, geochemical, isotopic and radiochronologic data reveal that the KV and CV were formed in extensional tectonic setting, but evolved by different petrological processes in different magma chambers. During the Neogene, underplated mafic magma was injected into the crust and hybridized by mantle and crustal derived materials. Geochemical features and trace element modeling for the mafic members of the KV indicate that they were derived from enriched lithospheric mantle and modified by fractional crystallization from basalt to rhyolite (Helvacı et al., 2009). Unexpectedly, the felsic lavas from Cumaovası region have a unique chemical composition, and similar to the extension related rhyolites formed from small magma bodies. Our data reveal that extension related mafic inputs caused crustal anatectic melting and formed felsic melts that rapidly ascended into the upper crust. The Cumaovası felsic rocks were differentiated into the highly evolved silica-rich melts within the magma chambers trapped near the surface.

Compositional and kinetic controls on liquid immiscibility in ferrobasalt–rhyolite volcanic and plutonic series

We present major element compositions of basalts and their differentiation products for some major tholeiitic series. The dry, low-pressure liquid lines of descent are shown to approach or intersect the experimentally-defined compositional space of silicate liquid immiscibility. Ferrobasalt–rhyolite unmixing along tholeiitic trends in both volcanic and plutonic environments is supported by worldwide occurrence of immiscible globules in the mesostasis of erupted basalts, unmixed melt inclusions in cumulus phases of major layered intrusions such as Skaergaard and Sept Iles, and oxide-rich ferrogabbros closely associated with plagiogranites in the lower oceanic crust. Liquid immiscibility is promoted by low-pressure, anhydrous fractional crystallization that drives the low Al2O3, high FeO liquids into the two-liquid field. Kinetic controls can be important in the development of two-liquid separation. The undercooling that occurs at the slow cooling rates of plutonic environments promotes early development of liquid immiscibility at higher temperature. In contrast rapid cooling in erupted lavas leads to large undercoolings and liquid immiscibility develops at significantly lower temperatures. Unmixing leads to the development of a compositional gap characterized by the absence of intermediate compositions, a feature of many tholeiitic provinces. The compositions of experimental unmixed silica-rich melts coincide with those of natural rhyolites and plagiogranites with high FeOtot and low Al2O3, suggesting the potential role of large-scale separation of immiscible Si-rich liquid in the petrogenesis of late-stage residual melts. No trace of the paired ferrobasaltic melt is found in volcanic environments because of its uneruptable characteristics. Instead, Fe–Ti±P-rich gabbros are the cumulate products of immiscible Fe-rich melts in plutonic settings. The immiscibility process may be difficult to identify because both melts crystallize the same phases with the same compositions. The two liquids might form incompletely segregated emulsions so that both liquids continue to exchange as they crystallize and remain in equilibrium. Even if segregated, both melts evolve on the binodal surface and exsolve continuously with decreasing temperature. The two liquids do not differentiate independently and keep crystallizing the same phases with differentiation. Further evolution by fractional crystallization potentially drives the bulk liquid out of the two-liquid field so that very late-stage liquids could evolve into the single melt phase stability field.

Effect of the Na/K mixing on the structure and the rheology of tectosilicate silica-rich melts

Viscosity and structure of Na/K tectosilicate glasses containing 75 and 83mol% of SiO2 have been investigated. Viscosity increases non-linearly when K+ ions substitute Na+ ions in these melts. The viscosity variations depending on chemical composition cannot be reproduced using an ideal mixing model of the configurational entropy. Consequently, it appears that Na and K elements do not mix randomly in the studied aluminosilicate melts. Raman spectra of glasses show that, during the Na/K substitution, evolutions of both the ring distribution and the T-O-T mean angle, or force constant, occur. The proportion of small-membered (three, four) rings, compared to large-membered rings, is higher in K-rich glasses than in Na-rich glasses. Moreover, Raman spectra features suggest that two different TO2 environments exist and cohabit into the glass. They could represent two populations of six-membered rings differing by their force constant or their T-O-T angles. One of these environments evolves when K substitutes for Na, showing an increase of its mean T-O-T angle and force constant. The other environment remains unchanged. From the observations, we propose that Na/K mixed tectosilicate glasses contained two sub-networks: one composed of the Si, Al, O and K atoms, and another of the Si, Al, O and Na. We suggest that the different size of alkali elements combined to the charge balancing needs of Al3+ ions can be the source of the different clustering of alkali cations into different sub-networks. Furthermore, and as previously inferred by older studies, potassic tectosilicate glasses could present silica-like and leucite-rich regions, explaining notably the incongruent crystallization of the orthoclase liquid.

Decompression-induced Crystallization in Hydrated Silica-rich Melts: Empirical Models of Experimental Plagioclase Nucleation and Growth Kinetics

Isothermal and isobaric crystallization of plagioclase in a water-saturated synthetic rhyolitic melt is investigated through time series of decompression experiments. The experimental variables are the rate at which samples are initially decompressed (30, 150, and 1200 MPa/h) from 200 MPa and 875°C, final pressure (25 to 160 MPa), and holding time at final pressure (up to 17 days). Through textural measurements of the crystals, plagioclase crystallization kinetics has been characterized in terms of nucleation lag and rates of nucleation and growth. Plagioclase crystallization is markedly dependent on effective undercooling, Teff, and holding time at crystallization pressure. With Teff increasing from 55 to 110°C, (i) nucleation lag decreases from 1-2 days to ~15 min, (ii) maximum nucleation rates increase from ~10-3 to 10-2 mm-2.s-1, and (iii) maximum growth rates decrease from ~10-6 to 5x10-7 mm.s-1. The initial decompression rate (30, 150, and 1200 MPa/h) has no systematic control on crystallization at final pressure, except for the 1200-MPa/h series in which samples show nucleation difficulties. From the experimental data of Teff-constrained plagioclase number density, proportion, and morphology, we provide means to assess conditions of nucleation and growth of natural plagioclase microlites from rapidly-ascended rhyolitic melts, through the determination of plagioclase liquidus curve and Teff prevailing during crystallization.