Melt Pockets Research Articles

Raman spectroscopy of high-pressure phases in shocked L6 chondrite NWA 5011

In the paper we present results of studies of thick shock melt veins in NWA 5011 L6 chondrite. The veins contain a wide variety of high-pressure phases that correspond to contrast values of pressure-temperature parameters on equilibrium phase diagrams. Olivine was transformed to ringwoodite and wadsleyte, orthopyroxene to majorite, akimotoite, and bridgmanite glass, maskelenite is converted to jadeite (+SiO2) and lingunite, apatite to tuite, and chromite to the phase with the calcium ferrite (mCF-FeCr2O4) structure. ) The peak PT shock parameters for NWA 5011 seem highest among the ones for other shocked chondrites according to wide occurrence of lingunite and bridgmanite glass and are considerable higher than 25 GPa and 2500 K. Akimotoite crystals in a quenched matrix of shock melt veins were found for the first time. Probably, they initially crystallized as bridgmanite, since akimotoite is not a liquidus phase in related systems. Plagioclase-chromite aggregates have been established, which characterize the late stages of the shock process and are formed during successive crystallization from isolated pockets of the impact melt.

Liquid Immiscibility in Regions of Localized Shock-Induced Melting in the Elga Meteorite

Abstract—The regions of localized shock melting (melt pockets) in one of silicate inclusions in IIE Elga iron meteorite were investigated with EMPA, SEM, TEM and Raman spectroscopy. It has been established that the mechanism of formation of melt pockets in Elga is of a mixed nature, being associated not only with melting in situ of the silicate matrix, but also with the intrusion of portions of the melted schreibersite–oxide rim into the silicate inclusion. Melt pockets have an emulsion texture, which is a sign of phase separation by liquid immiscibility in high-temperature shock melts. The emulsion texture formed by droplet-shaped exsolutions of siderite in the schreibersite matrix of one of the melt pockets has all the features of phase separation by liquid immiscibility at superliquidus temperatures. This convincingly indicates the extraterrestrial origin of siderite in the Elga meteorite.

Enigmatic super-reduced phases in corundum from natural rocks: Possible contamination from artificial abrasive materials or metallurgical slags

Recently a number of reports claimed enigmatic appearance of high-pressure and super-reduced minerals in ophiolitic chromitite and peridotite. Diamond, moissanite, various metal alloys, and native metals, carbides and nitrides were found in mineral separates from bulk rock probes of chromitite and peridotite from Tibet, Polar Ural and other localities. Similar findings of super-reduced phases were reported for pyroclastic rocks and alluvial deposits of Mt. Carmel in Israel. We performed the study of microinclusions in corundum grains from abrasive materials produced industrially in an electric arc furnace and found that they are very similar to microinclusions in corundum grains from natural samples. The key similar phases are Ti3+4Al2(Zr,Ti4+)4O11 carmeltazite, Ti2O3 tistarite, TiN1-x, TiC1-x, FeSi and Fe-Si-Ti alloys, hibonite, grossite, anorthite, and residual feldspatic glass. Although some differences between abrasive corundum and corundum from Tibet and Mt. Carmel are obvious, the morphology of melt pockets and amount of mineralogical similarities is more than critical to suggest that they can be of the same origin. The additional possible source of corundum grains with super-reduced inclusions is various Al-bearing slags after aluminothermic reactions during steelmaking. Moreover, taking into account the spectacular similarity of diamonds from ophiolite with synthetic diamonds, we claim for a thorough reconsideration of ultrahigh-pressure and super-reduced phases in natural rocks and argue that we need to find criteria for discrimination between natural and artificial samples. One of the strongest criteria would be textural and structural features that clearly demonstrate the indigenous character of the host minerals.

The potential of phosphorus in clinopyroxene as a geospeedometer: Examples from mantle xenoliths

We investigate the potential to use concentrations and zoning patterns of phosphorus (P) in clinopyroxene as indicators of the rates of igneous and metasomatic processes, comparable to recent applications of P in olivine but applicable to more evolved rocks and lower temperatures of crystallization. Few high-P pyroxenes have been previously reported, and none have been analyzed in detail for the mechanism of P enrichment or the implications for mineral growth kinetics. Here, we report the discovery and characteristics of exotic phosphorus-rich secondary clinopyroxene in glassy pockets and veins in composite mantle xenoliths from the Cima Volcanic Field (California, USA) and the Middle Atlas Mountains (Morocco, West Africa). These glass-bearing xenoliths preserve evidence of melt infiltration events and the contrasting behavior of P in their pyroxene crystals constrains the different rates of reaction and extents of equilibration that characterized infiltration in each setting. We report optical petrography and chemical analysis of glasses and minerals for major elements by electron microprobe microanalyzer and trace elements by laser-ablation Inductively Coupled Plasma Mass Spectrometry. The Cima Volcanic Field specimen shows one end-member behavior, with unzoned P-rich clinopyroxene in a melt pocket. We attribute this occurrence to a slow crystallization process that occurred after the melt temperature reached near-equilibrium with the host rock and during which the P concentration in the melt was buffered by apatite saturation. In the Morocco xenolith, by contrast, clinopyroxene exhibits zonation with P increasing all the way to the rim, in contact with the glass. We ascribe this feature to a rapid growth process in which excess P was incorporated into the growing clinopyroxene from a diffusive boundary layer. We demonstrate quantitative agreement between the enrichment of P and other trace elements and their expected diffusion and partitioning behavior during rapid growth. We suggest that P has not been widely reported in clinopyroxene in large part because it has rarely been looked for and that its analysis offers considerable promise as a kinetic indicator both in xenoliths and volcanic rocks.

In Situ Chalcophile and Siderophile Element Behavior in Sulfides from Moroccan Middle Atlas Spinel Peridotite Xenoliths during Metasomatism and Weathering

In situ chalcophile and siderophile major and trace elements were analyzed in sulfides from eight Moroccan Middle Atlas lherzolite xenoliths using electron microprobe and laser ablation inductively coupled plasma mass spectrometry. The sulfides occur enclosed in primary silicates, interstitial in the peridotite matrix, and associated with glass-bearing melt pockets. Monosulfide solid solutions are enriched in these xenoliths relative to pentlandite and intermediate solid solutions. Regardless of the textural occurrence, sulfide platinum-group element (PGE) patterns are distinguished into residual ([Pd/Ir]N < 1 and [Pt/Pd]N > 1 or [Pt/Pd]N < 1), melt-like ([Pd/Ir]N > 1), and unfractionated patterns. The coexistence of both residual and melt-like PGE signatures on a cm scale in a single sample implies that sulfides may record initial depletion and subsequent re-enrichment more effectively than constituent silicates do. Chalcophile and siderophile trace elements other than the PGEs are fractionated between the precipitated sulfide phases, but do not vary systematically with the PGE signatures, suggesting that the PGEs are comparatively sensitive to melting and depletion. In addition, Fe-rich hydroxides generated by sulfide breakdown due to atmospheric weathering display PGE systematics almost identical to their precursor sulfides, implying that they may be reliable tracers of mantle processes even after extensive weathering.

Petrogenesis of plagiogranites from the Troodos Ophiolite Complex, Cyprus

Small volumes of felsic melt, commonly known as oceanic plagiogranites, appear as melt pockets or dikes within the gabbroic section and the sheeted dikes root zone of the oceanic crust. Plagiogranites from the Troodos Ophiolite Complex on Cyprus are among the best exposures of felsic rocks that are embedded in a complete section of obducted oceanic lithosphere. Nevertheless, their exact petrogenesis is still a matter of debate, largely due to limited high-quality trace element and radiogenic isotope data. Previously proposed models for Troodos plagiogranites have included both low-pressure dehydration melting and fractional crystallisation at deeper levels of the oceanic crust. To evaluate both models, oceanic plagiogranites from the Troodos Ophiolite Complex were analysed in this study for their major and trace elements, and for the first time also for Hf–Nd–Sr isotope compositions. The trace element measurements also include for the first time high-precision measurements of high-field-strength element (HFSE) abundances that now permit to better unravel the petrogenesis of the Troodos plagiogranites and their possible mantle sources. In general, the Troodos plagiogranites exhibit a narrow range of Nb/Ta and Zr/Hf that overlap the compositions of mid-ocean ridge basalts (MORB). In line with earlier studies, three compositional groups can be identified: two groups formed by either fractional crystallisation or combined fractional crystallisation and wall rock assimilation, and one group derived from partial melting of slightly altered oceanic crust. The majority of the Troodos plagiogranites (Main Group) are the product of extensive fractional crystallisation of ambient arc-tholeiitic mafic melts. A second group of plagiogranites (Spilia Group) is generated by fractional crystallisation of boninitic precursor melts and the assimilation of arc-tholeiitic crustal material. Variable HFSE concentrations and diagnostic Hf–Nd isotope signatures that are unique to both suites allow discriminating between the two parental melts and fractionation processes. A small group of plagiogranites (Zoopigi Group) is interpreted to derive from partial melting in the conductive layer of active magma chamber lenses (AML). Elevated Nb/Ta, Zr/Hf, and light rare-earth element (LREE) enrichments in these rocks are in support of this model. Collectively, our data suggest that low-pressure fractional crystallisation (also in combination with assimilation of wall rocks) might be the predominant process controlling the formation of felsic rocks on Cyprus, whereas dehydration melting appears to be less important. If compared to Archean tonalitic–trondhjemitic–granodioritic suites (TTGs), compositions of plagiogranites from Cyprus mirror shallow-level processes in thin oceanic crust, which is illustrated by their narrow, MORB-like range of HFSE ratios and their distinct enrichment in heavy rare-earth elements (HREE) that distinguishes them from the Archean TTGs.

Petrogenesis and In Situ U‐Pb Geochronology of a Strongly Shocked L‐Melt Rock Northwest Africa 11042

AbstractNorthwest Africa (NWA) 11042, originally classified as a primitive achondrite with no chondritic relicts, is rather a unique L‐melt rock. It is a severely shocked, igneous‐textured ultramafic rock composed of euhedral to subhedral olivine (Fa25.1 ± 0.5) and pyroxenes (low‐Ca pyroxene Fs20.7 ± 0.8Wo4.2 ± 1.0 and Ca‐rich pyroxene Fs11.5 ± 0.5Wo37.6 ± 1.2) with interstitial albitic plagioclase (Ab80.7 ± 1.7Or5.0 ± 0.7) that has been completely converted to maskelynite. Mineral compositions are similar to those of equilibrated L‐chondrites. Melt pockets are scattered throughout the sample, containing high‐pressure minerals including ringwoodite, wadsleyite, jadeite, and lingunite. Merrillite and apatite in NWA 11042 contain significantly higher REE abundances than those of ordinary chondrites, indicative of igneous fractional crystallization. In situ U‐Pb dating of apatite in NWA 11042 reveals an upper intercept age of 4,479 ± 43 Myr and a lower intercept age of 465 ± 47 Myr on the normal U‐Pb concordia diagram. The upper intercept age recorded the time when NWA 11042 initially crystallized. This age is much younger than when the decay of short‐lived nuclides (e.g., 26Al) would act as a major heat source, suggesting that melting and crystallization of NWA 11042 could be otherwise triggered by an impact event. The lower intercept age represents a reset age due to a later impact event, which is in coincidence with the disruption event of L‐chondrite parent body at ~470 Myr. NWA 11042 is an excellent example to link igneous‐textured meteorites with a chondritic parent body through shock‐induced melting.

Melt segregation and the architecture of magmatic reservoirs: insights from the Muroto sill (Japan)

The separation of melt from crystals is a fundamental process driving the chemical differentiation of magmas and can lead to the formation of pockets of potentially eruptible magmas in highly crystallised magma reservoirs. While geochemical and geophysical evidence exists for the presence of such isolated pockets of eruptible melt, the processes that control their volume and spatial arrangement remain unclear. The Muroto sill in Japan provides an excellent opportunity to study these processes as it is perfectly exposed and shows clear evidence for melt segregation. We collected geochemical and structural data across the sill and performed thermal modelling to quantify extraction timescales and to constrain the range of crystallinity at which melt extraction occurred. Our data and calculations show that the middle–lower portion of the sill experienced melt extraction at crystal fractions between 0.65 and 0.8 over 100–150 years, until magma was too crystalline for further segregation to occur. We propose a new approach that can be used to invert measured geochemical profiles and identify the range of crystallinity at which melt extraction takes place. With this approach, the results we obtain for the Muroto sill can be generalised to magma reservoirs of different sizes and chemistries. Our calculations, and the comparison with natural magmatic systems, show that the volume of melt-rich pockets in a magma reservoir is proportional to the reservoir volume, while their spatial arrangement depends on the physiochemical properties of magmas. The results of this study increase our understanding of the factors controlling the distribution and volume of pockets of eruptible magmas in large magma reservoirs. Our calculations show that eruptible magma in dacitic and rhyolitic magma reservoirs, which are responsible for some of the largest eruptions on Earth, tend to be distributed in lenses of small volume within highly crystallised magma. Such architecture diminishes our capacity of identifying eruptible magma in large magma reservoirs such as Yellowstone using geophysical methods, and jeopardises our capacity of assessing the potential of a reservoir to feed a large eruption.

Monazite as a monitor for melt‐rock interaction during cooling and exhumation

AbstractGranulite facies cordierite–garnet–biotite gneisses from the southeastern Reynolds Range, central Australia, contain both orthopyroxene‐bearing and orthopyroxene‐free quartzofeldspathic leucosomes. Mineral reaction microstructures at the interface of gneiss and leucosome observed in outcrop and petrographically, reflect melt‐rock interaction during crystallization. Accessory monazite, susceptible to fluid alteration, dissolution and recrystallization at high temperature, is tested for its applicability to constrain the chemical and P–T–time evolution of melt‐rock reactions during crystallization upon cooling. Bulk rock geochemistry and phase equilibria modelling constrain peak pressure and temperature conditions to 6.5–7.5 kbar and ~850°C, and U–Pb geochronology constrains the timing of monazite crystallization to 1.55 Ga, coeval with the Chewings Orogeny. Modelling predicts the presence of up to 15 vol.% melt at peak metamorphic conditions. Upon cooling below 800°C, melt extraction and in situ crystallization of melt decrease the melt volume to less than 7%, at which time it becomes entrapped and melt pockets induce replacement reactions in the adjacent host rock. Replacement reactions of garnet, orthopyroxene and K‐feldspar liberate Y, REE, Eu and U in addition to Mg, Fe, Al, Si and K.We demonstrate that distinguishing between monazite varieties solely on the basis of U–Pb ages cannot solve the chronological order of events in this study, nor does it tie monazite to the evolution of melt or stability of rock‐forming minerals. Rather, we argue that analyses of various internal monazite textures, their composition and overprinting relations allow us to identify the chronology of events following the metamorphic peak. We infer that retrograde reactions involving garnet, orthopyroxene and K‐feldspar can be attributed to melt‐rock interaction subsequent to partial melting, which is reflected in the development of compositionally distinct monazite textural domains. Internal monazite textures and their composition are consistent with dissolution and precipitation reactions induced by a high‐T melt. Monazite rims enriched in Y, HREE, Eu and U indicate an increased availability of these elements, consistent with the breakdown of orthopyroxene, garnet and K‐feldspar observed petrographically. Our study indicates that compositional and textural analysis of monazite in relation to major rock‐forming minerals can be used to infer the post‐peak chemical evolution of partial melts during high‐ to ultrahigh‐temperature metamorphism.

Petrology and oxygen isotopic composition of large igneous inclusions in ordinary chondrites: Early solar system igneous processes and oxygen reservoirs

Large (>3.5 mm and up to 4 cm across) igneous inclusions poor in metal and sulfide are a minor but not uncommon component in ordinary chondrites, and have implications for the nature of physiochemical and melting processes in the early solar system. We obtained petrographic-chemical data for forty-two large igneous inclusions in ordinary chondrites of various groups (H, L, LL) and petrographic types (3–6) and oxygen isotope data for a subset of twelve of these inclusions and their host chondrites. Different inclusions formed both before and after the thermal metamorphism experienced by their host chondrites. The bulk chemical compositions of the inclusions vary broadly around whole-rock chondrite composition, comprise four main chemical types and some other variants, and show little evidence of having formed as igneous differentiates. Oxygen isotope compositions overlap ordinary chondrite compositions and are related to inclusion chemical type. Most prevalent in type 3 and 4 chondrites are inclusions, often droplets, of the vapor-fractionated (Vfr) chemical type, either enriched in refractory lithophile elements, or depleted in volatile lithophile elements, or both. These inclusions have low Δ17O (∼0.1–0.6‰) and high δ18O (∼4–8‰) values and formed in reservoirs with Δ17O lower than their hosts, primarily as evaporative melts and mixtures that probably experienced kinetic isotopic fractionation. Another chemical type (Unfr + K) has unfractionated abundances of lithophile elements except for being strongly enriched in K, a signature also found in some impact melts from melt rocks and melt breccias. These inclusions formed by impact melting of chondritic material and accompanying K enrichment. Inclusions with unfractionated (Unfr) lithophile element abundances are present in type 3–6 chondrites and are prevalent in type 5 and 6. Some are spatially associated with coarse metal-sulfide nodules in the chondrites and likely formed by in situ impact melting. Others were melted prior to thermal metamorphism and were chemically but not isotopically homogenized during metamorphism; they are xenoliths that formed in oxygen reservoirs different than the hosts in which they were metamorphosed. The latter inclusions provide evidence for nebular or collisional mixing of primitive materials prior to thermal metamorphism of asteroid bodies, including transport of H-like source materials to the L body, LL-like source materials to the L body, and low-Δ17O materials to the LL body. Feldspar-rich (FldR) inclusions have compositions similar to melt pockets and could have formed by disequilibrium melting and concentration of feldspar during an impact event to form large droplets or large masses. Overall, the results of this study point to important and varied roles for both “planetary” impact melting and “nebular” evaporative melting processes to form different large igneous inclusions in ordinary chondrites. Chondrules may have formed by processes similar to those inferred for large inclusions, but there are important differences in the populations of these objects.

The lateral growth and coalesence of magma systems.

Thermal and mechanical models of magma reservoir growth need to be reconciled with deformation patterns and structural relationships observed at active magma systems. Geophysical observations provide a series of short time-scale snap-shots (100–102 years) of the long-term growth of magmatic bodies (103–106 years). In this paper, we first review evidence for the growth of magmatic systems along structural features and the associated deformation patterns. We then define three distinct growth stages, (1) aligned melt pockets, (2) coalesced reservoirs, (3) highly evolved systems, which can be distinguished using short-term surface observations. We use two-dimensional thermal models to provide first-order constraints on the time scales and conditions associated with coalescence of individual magma bodies into large-scale reservoirs. We find that closely spaced intrusions (less than 1 km apart) can develop combined viscoelastic shells over time scales of 10s kyr and form laterally extensive mush systems over time scales of 10–100 kyr. The highest temperatures and melt fractions occur during a period of thermal relaxation after melt injection has ceased, suggesting that caldera-forming eruptions may preferentially occur long after the main intrusive activity. The coalescence of eruptible melt-rich chambers only occurs for the highest melt supply rates and deepest systems. Thus, these models indicate that, in most cases, conductive heat transfer alone is not sufficient for a full coalescence of magma chambers and that other processes involving mechanical ruptures and mush mobilization are necessary; individual melt lenses can remain isolated for long periods within growing mush systems, and will only mix during eruption or other catastrophic events. The long-term history of the magmatic system is therefore critical in determining rheological structure and hence short-term behaviour. This framework for the development of magmatic systems in the continental crust provides a mechanical basis for the interpretation of unrest at the world's largest volcanoes.This article is part of the Theo Murphy meeting issue ‘Magma reservoir architecture and dynamics'.

Mineralized biosignatures in ALH-77005 Shergottite - Clues to Martian Life?

Abstract The ALH-77005 Martian meteorite was found in Allan Hills on Antarctica during the Japanese National Institute of Polar Research (1977-1978) mission. One thin section sample was studied by optical microscopy for microtexture and by FTIR-ATR microscopy for interpretation of biogenic minerals and embedded organic materials. The geochemical data (biogenic elements, δ13C) of ALH-77005 meteorite from literature implementing recent results were compared to terrestrial geological samples. The ALH-77005 has poikilitic textures with coarse pyroxenes and brown olivines, and with recrystallized melt pocket. The coarse-grained minerals do not contain any alteration along the grain boundaries. Melt pocket and vicinity of opaque minerals contain biogenic signatures as filamentous, coccoidal forms of iron-oxidizing bacteria. The biosignatures were determined by 1) coccoidal, filamentous forms, 2) presence of embedded organic material, 3) presence of biogenic minerals, like ferrihydrite, goethite, and hematite. The other signatures for biogenicity of this meteorite are strong negative δ13C, enrichment of Fe, Mn, P, Zn in shock melt support scenario. This study proposes presence of microbial mediation on Mars.

Petrography, mineral chemistry and shock metamorphism of the Mangui meteorite

The Mangui meteorite, also known as the Xishuangbanna meteorite, fell in Menghai, Xishuangbanna Dai Nationality Autonomous Prefecture, Yunnan Province, China at ~9: 45 pm on June 1, 2018. More than 500 fragments of Mangui were collected a total mass of ~50 kg. The mass of the largest fragment is ~1228 g. Mangui has a black fusion crust, a light gray exposure surface, and contains many black shock melt veins and pockets. In this work, the classification of Mingui and its shock history are investigated by studying its petrology, mineral chemistry, and the nature of its shock assemblages. Mangui is mainly composed of olivine, pyroxene, feldspar, Fe-Ni metal, troilite, chromite, merrillite and apatite, showing typical petrologic features of an ordinary chondrite. Relict chondrules are observed in hand specimens but their boundaries are less clear under FE-SEM. Electron Probe Micro Analysis (EPMA) shows that the chemical compositions of olivine (Fa25.1±0.3 mol%( n =71)), low-Ca pyroxene (Fs21.1±0.3Wo1.5±0.2 mol% ( n =58)) and plagioclase Ab84.1±0.7Or7.3±0.4 mol%( n =49)) in Mangui are relatively homogeneous. The petrographic type of Mangui is classified as type 6 by the coarse grains of secondary plagioclase, the homogeneous compositions of olivine and pyroxene, the recrystallization of matrix and blurry boundaries of relict chondrules. The chemical group of Mangui is L based on the average Fa content of olivine (Fe/(Fe+Mg)×100, atomic percentage, 25.1±0.3, n =71) and the average Fs content of pyroxene (Fe/ (Fe+Mg+Ca)×100, atomic percentage, 21.1±0.3, n =58). Therefore, the Mangui meteorite is classified as an L6 ordinary chondrite. The Mangui parent body experienced strong shock metamorphism with the widespread distribution of shock melt veins with widths up to 600 μm. Shock induced melt pockets were also observed. Micron-sized olivine and pyroxene in the center of the shock melt veins display chemical zonation, suggesting that they recrystallized from the shock induced melt. Plagioclase in the shock melt veins has transformed into maskelynite. Two high pressure polymorphs, jadeite and majorite, occur in the shock melt veins and melt pockets as measured by Raman analysis. Jadeite usually displays granular textures in these assemblages as euhedral to sub-euhedral crystals. Majorite was only found in one jadeite assemblage, occurring as inclusions in outer rim of an assemblage. The co-occurrence of majorite and jadeite indicate shock pressures from 17–20 GPa at temperatures ranging from 1900–2000°C. However, ringwoodite, whitlockite and other high pressure polymorphs have not been identified in the shock melt veins pockets. The occurrences of maskelynite, jadeite and majorite, and widespread distribution of shock melt veins and pockets suggest the shock stage of Mangui as S5. The absence of ringwoodite and tuite probably attribute to slow post-shock cooling rates, suggesting that the Mangui parent body experienced strong shock metamorphism with a subsequent slow post-shock cooling history. TEM and XRD work will be performed to better constrain the formation mechanisms of jadeite and majorite and their P - T - t histories. Furthermore, we aim on modelling the impact conditions on the Mangui parent body such as impact velocity, and parent body-impactor sizes by further study of the high-pressure minerals in shock-melt veins of the Mangui meteorite.

Evidence for magma–evaporite interactions during the emplacement of the Central Atlantic Magmatic Province (CAMP) in Brazil

Mafic sills of the Central Atlantic Magmatic Province (CAMP) in Brazil are temporally linked to the end-Triassic extinction (ETE), one of the largest mass extinctions of the Phanerozoic. The sills were emplaced into the volatile-rich sedimentary rocks of the Amazonas and Solimões basins in northern Brazil. Here we present a new geochemical study of 26 dolerite samples from 6 deep boreholes in the Brazilian basins, including whole-rock major and trace elements, whole-rock Sr–Nd isotopes and detailed biotite mineral chemistry. The bulk of the dolerites are characterized by phenocrysts of clinopyroxene and plagioclase in subophitic to intergranular textures, Fe–Ti oxides, and rare olivine and orthopyroxene. A different mineralogical assemblage (microphenocrysts of alkali-feldspar, quartz, biotite and apatite) is found in small independent domains, localized within the framework of coarser plagioclase and clinopyroxene laths. These fine-grained evolved domains crystallized in late-stage, evolved melt pockets in the interstitial spaces between earlier crystallized coarser grained crystals. The majority of the studied dolerites are generally evolved tholeiitic basalts and basaltic andesites with low TiO2 concentrations (<2.0 wt.%). Four samples have high TiO2 concentrations (>2.0 wt.%), and are found in the eastern part of the Amazonas Basin. Whole-rock major and trace element and Sr–Nd isotope geochemistry of both low- and high-Ti sills is similar to that of previously published CAMP rocks from the two magma types. Low-Ti sills show enriched isotopic signatures (143Nd/144Nd201Ma 0.51215 to 0.51244; 87Sr/86Sr201Ma 0.70568 to 0.70756), coupled with crustal-like characteristics in the incompatible element patters (e.g. depletion in Nb and Ta). Unaltered high-Ti samples show more depleted isotopic signatures (143Nd/144Nd201Ma 0.51260 to 0.51262; 87Sr/86Sr201Ma 0.70363 to 0.70398). Low-Ti dolerites from both the Amazonas and Solimões basins contain biotite with extremely high Cl concentrations (up to 4.7 wt.%). We show that there is a strong correlation between host-rock lithology and Cl concentrations in biotite from the dolerites, and interpret this to reflect large-scale crustal contamination of the low-Ti magmas by halite-rich evaporites. Our findings support the hypothesis that sill-evaporite interactions increased volatile release during the emplacement of CAMP. This strengthens the case for active involvement of this LIP in the end-Triassic crisis, and suggests that the sub-volcanic part of a LIP can be of major importance in driving climate change and mass extinctions.

The effect of dynamic recrystallisation on the rheology and microstructures of partially molten rocks

The present study is based on a series of two-dimensional simple shear numerical simulations of two-phase non-linear viscous materials used to investigate the mechanical behaviour of two-phase aggregates representing partially molten rocks. These simulations couple viscoplastic deformation with dynamic recrystallisation (DRX). The aim of these simulations is to investigate the competition between deformation and recrystallisation, and how they affect the mechanical behaviour and resulting microstructures of the deforming material. We systematically vary the melt to solid rock ratio, the dihedral angle of melt and the ratio of DRX vs. deformation. The results show that the amount of DRX and the dihedral angle have a first-order impact on the bulk rheology and the melt distribution in the aggregate. The numerical results allow defining two regimes, depending on the relative contribution of deformation and DRX: (1) a deformation-dominated regime at high strain rates (i.e., with a low ratio of recrystallisation vs. viscoplastic deformation) and (2) a recrystallisation-dominated regime at low strain rates (i.e., with a high ratio of recrystallisation vs. viscoplastic deformation). The first case results in systems bearing large connected melt pockets whose viscous flow controls the deformation of the aggregate, while disconnected smaller melt pockets develop in models where dynamic recrystallisation dominates. The results of this study allow us to better understand the development of connected melt pockets, which may focus melt flow. The distribution of the melt phase plays a key role in the formation of larger-scale melt-enriched shear bands, which in turn has a direct influence on large-scale convective mantle flow.

Partial melting due to breakdown of an epidote‐group mineral during exhumation of ultrahigh‐pressure eclogite: An example from the North‐East Greenland Caledonides

AbstractIn the North‐East Greenland Caledonides, P–T conditions and textures are consistent with partial melting of ultrahigh‐pressure (UHP) eclogite during exhumation. The eclogite contains a peak assemblage of garnet, omphacite, kyanite, coesite, rutile, and clinozoisite; in addition, phengite is inferred to have been present at peak conditions. An isochemical phase equilibrium diagram, along with garnet isopleths, constrains peak P–T conditions to be subsolidus at 3.4 GPa and 940°C. Zr‐in‐rutile thermometry on inclusions in garnet yields values of ~820°C at 3.4 GPa. In the eclogite, plagioclase may exhibit cuspate textures against surrounding omphacite and has low dihedral angles in plagioclase–clinopyroxene–garnet aggregates, features that are consistent with former melt–solid–solid boundaries and crystallized melt pockets. Graphic intergrowths of plagioclase and amphibole are present in the matrix. Small euhedral neoblasts of garnet against plagioclase are interpreted as formed from a peritectic reaction during partial melting. Polymineralic inclusions of albite+K‐feldspar and clinopyroxene+quartz±kyanite±plagioclase in large anhedral garnet display plagioclase cusps pointing into the host, which are interpreted as crystallized melt pockets. These textures, along with the mineral composition, suggest partial melting of the eclogite by reactions involving phengite and, to a large extent, an epidote‐group mineral. Calculated and experimentally determined phase relations from the literature reveal that partial melting occurred on the exhumation path, at pressures below the coesite to quartz transition. A calculated P–T phase diagram for a former melt‐bearing domain shows that the formation of the peritectic garnet rim occurred at 1.4 GPa and 900°C, with an assemblage of clinopyroxene, amphibole, and plagioclase equilibrated at 1.3 GPa and 720°C. Isochemical phase equilibrium modelling of a symplectite of clinopyroxene, plagioclase, and amphibole after omphacite, combined with the mineral composition, yields a P–T range at 1.0–1. 6 GPa, 680–1,000°C. The assemblage of amphibole and plagioclase is estimated to reach equilibrium at 717–732°C, calculated by amphibole–plagioclase thermometry for the former melt‐bearing domain and symplectite respectively. The results of this study demonstrate that partial melt formed in the UHP eclogite through breakdown of an epidote‐group mineral with minor involvement of phengite during exhumation from peak pressure; melt was subsequently crystallized on the cooling path.

The timing of high-temperature conditions and ductile shearing in the footwall of the Naxos extensional fault system, Aegean Sea, Greece

We present eight Rb-Sr multi-mineral isochron ages showing that high-temperature metamorphic conditions and partial melting during top-to-the-NNE extensional shearing in the footwall of the Naxos extensional fault system (i.e. Naxos metamorphic core complex) lasted until about 14–12 Ma. One migmatite sample yielded an age of 14.34 ± 0.2 Ma (2σ uncertainty) for crystallization of migmatization-related melt pockets. Four pegmatite samples, which are in part associated with partial melting of their host rocks, provided overlapping ages ranging from 13.81 to 12.23 Ma (age range includes 2σ uncertainty). Additional three samples of amphibolite-facies schist supplied Rb-Sr ages of around 14 Ma. Samples showing fluid- and/or deformation-assisted white mica and biotite reworking gave Rb-Sr mineral apparent ages of 11.1 ± 2.7, 10.16 ± 0.24, 9.7 ± 0.7 and 9.6 ± 0.15 Ma. These ages are interpreted to be associated with late stages of extensional shearing under greenschist-facies metamorphic conditions. Together with published U-Pb zircon ages of migmatite, and S- and I-type granite crystallization, the data indicate that the presence of melt in the footwall of the Naxos extensional fault system lasted for at least 7 Ma (from ~18 to ~11 Ma). This demonstrates that high temperatures and crustal melting resulting from and aiding extensional deformation was a long-lived and not a transient event. We conclude that melt-assisted deformation facilitated large-scale displacement on the Naxos extensional fault system by drastically weakening the extending crust for long periods of time.

Yakutites from the Popigai meteorite crater

For the first time, 60 large diamond aggregates were found inside the Popigai meteorite crater during washing of alluvial deposits along the Dogoi river crossing the crater. These aggregates are similar in appearance to yakutites from the placers of Northern Yakutia (YPY), and we regard them as yakutites from the Popigai crater (YPC). The structure and optical properties of Popigai impact diamonds from the impact melt rocks (tagamites) in the crater (PIDT) and yakutites YPC/YPY were compared in detail. In all these cases, a polycrystalline structure consisting of nanoscale grains of cubic and twinned cubic diamond (lonsdaleite) was found. This is the result of a solid-phase graphite-diamond transition due to an impact event 35 million years ago. The diamond aggregates show the following features: a red shift of the short-wave edge of the transmission, broadening of the diamond Raman peaks, signals from other diamond polytypes and numerous inclusions of other minerals in the Raman spectra, and a dominant broadband photoluminescence (PL). PL in the N3 system associated with N3V centers in PIDT diamonds indicates a high-temperature annealing of these aggregates with resulting aggregation of impurities during the prolonged cooling of large impact melt pockets and pools. It is assumed that some of the impact diamonds were ejected from the crater during the impact event and experienced rapid cooling. Some of these diamonds fell back into the crater (YPC yakutites), others have been deposited outside the crater and displaced during erosion (YPY yakutites). Difference in size and shape between the PIDTs and yakutites YPC/YPY is due to the difference in size of original graphite flakes or aggregates and/or due to the fundamentally different technologies of diamond extraction.

Geochemistry of Rock-Forming Minerals in Mantle Xenoliths from Basalts of Sverre Volcano, Spitsbergen Archipelago

The paper reports the results of SIMS and SEM-EDS study of rock-forming minerals from melt pockets in the central part of a spinel peridotite xenolith taken from Quaternary alkaline basalts of Sverre Volcano in the northwestern part of West Spitsbergen Island. Olivine and clinopyroxene are analyzed to trace changes related to the metasomatic interaction between spinel lherzolite and a carbonate melt with formation of corresponding secondary minerals and silicate glass. It is established that the metasomatic interaction of the carbonate melt with minerals of host spinel lherzolite is accompanied by partial recrystallization of olivine and clinopyroxene, or crystallization of the second generation of these minerals. Percolating carbonate melt caused significant changes in the major, trace, and rare-earth element composition of the considered minerals, thus placing constraints on the use of the composition of these minerals for calculation of P–T parameters, estimating equilibrium, and modeling petrological processes in mantle.

Deep earthquakes in subducting slabs hosted in highly anisotropic rock fabric

Analysis of deep subduction-zone earthquakes, those at depths greater than 60 km, reveals the physical and chemical properties of a descending oceanic lithosphere at mantle depths. Over the past five decades, it has been observed that a large fraction of deep earthquakes has non-double-couple (non-DC) seismic radiation patterns. In contrast, shallow earthquakes tend to have DC radiation patterns due to mechanisms of shear faulting. These observations have been used to argue that deep earthquakes rupture differently from shallow earthquakes. Here we show that the observed global distribution of non-DC deep earthquakes could be caused by shear faulting mechanisms, but in a highly anisotropic laminated rock fabric that surrounds the deep earthquakes within subducted slabs. For intermediate-depth earthquakes (~60–300 km), we found a large shear-wave anisotropy of ~25%, possibly caused by laminated fabric or aligned melt pockets oriented parallel to the slab interface, which provides new supporting evidence for the metamorphic dehydration reactions in slabs. However, at deep-focus depths (>300 km), the putative metastable phase-change mechanism alone cannot explain the seismic anisotropy. Instead, our results and those from recent experiments suggest materials such as magnesite, or perhaps carbonatite melt, may play a role in generating deep-focus earthquakes. Radiation patterns for deep earthquakes could be a result of shear faulting mechanisms—similar to those for shallow earthquakes—but in highly anisotropic rock fabric, suggest seismic analyses.