Melt Pockets Research Articles

Evidence for the Disproportionation of Iron in a Eucrite Meteorite: Implications for Impact Processes on Vesta

AbstractPure metallic iron is a common component in extraterrestrial samples (e.g., ordinary chondrite, returned lunar samples), and is thought to be an indicator of extremely reducing conditions. However, tiny pure metallic iron (<1 mm) particles are rarely found in Howardite–Eucrite–Diogenite meteorites, and their formation mechanism on Vesta is poorly understood. Sub‐micron sized pure metallic iron particles were identified by transmission electron microscope studies of a melt pocket in the basaltic eucrite meteorite Northwest Africa 11592. Our results demonstrate that the pure metallic iron likely formed through a dissociation reaction of ferrous iron (i.e., 3Fe2+ = Fe0 + 2Fe3+) in pyroxene. Another disproportionation reaction product, ferric iron, was confirmed to be incorporated into Al‐rich clinopyroxene in the melt pocket by electron energy loss spectra analyses. The temperature conditions required for the formation of the melt pocket are estimated to be above 1,310°C, as indicated by the crystallization of nano‐sized hercynite within acicular plagioclase. Such high temperatures on Vesta could only have been reached if an impact occurred during peak thermal metamorphism, early in the asteroid's evolution. This formation mechanism of these pure metallic iron particles could play a role in redox environment of the Vesta.

Occurrence of tuite and ahrensite in Zagami and their significance for shock-histories recorded in martian meteorites

AbstractWe report on the discovery of two high-pressure minerals, tuite and ahrensite, located in two small shock-induced melt pockets (SIMP 1 and 2) in the Zagami martian meteorite, coexisting with granular and acicular stishovite and seifertite. Tuite identified in this study has two formation pathways: decomposition of apatite and transformation of merrillite under high-P-T conditions. Chlorine-bearing products, presumably derived from the decomposition of apatite, are concentrated along the grain boundaries of tuite grains. Nanocrystalline ahrensite in the pyroxene clast in SIMP 2 is likely to be a decomposition product of pigeonite under high-P-T conditions by a solid-state transformation mechanism. The pressure and temperature conditions estimated from the high-pressure minerals in the shock-induced melt pockets are ~12–22 GPa and ~1100–1500 °C, respectively, although previous estimates of peak shock pressure are higher. This discrepancy probably represents the shift of kinetic relative to thermodynamic phase boundaries, in particular the comparatively small region that we examine here, rather than a principal disagreement between the peak shock conditions.

Exploring the relationships between shear zones and granites: field and microstructural data for contrasting case studies of the Borborema Province (NE Brazil)

We discuss meso- and microstructural features of granites closely related to strike-slip shear zones in the Borborema Province, NE Brazil. The Riacho do Icó stock is an en-cornue intrusion aged at ca. 607 Ma. Magmatic fabric is recorded in the core of the granite, whilst increasing deformation is marked by the development of mylonitic fabrics towards the Afogados da Ingazeira shear zone, including magmatic foliation and lineation rotation. Early recrystallization of quartz and K-feldspar crystals is widespread as a fabric with well-developed granoblastic polygonal textures and lobate subgrain boundaries, heterogeneously deformed lenses and ameboid quartz ribbons, typical of igneous rocks submitted to deformation in deep crustal levels. On the other hand, the Espinho Branco-Santa Luzia leucogranitic belt is hosted along the Patos Lineament, aged between the ca. 575 – 565 Ma interval. These rocks show discordant relationships with the host migmatites and the main deformational fabric is characterized by a dominant magmatic foliation that is locally overprinted by structures that are typical of solid-state flow. Quartz melt pockets and interstitial quartz grains filling fractures in feldspar clasts are common. Such characteristics are compatible with granites that were injected in the continental crust along planar anisotropies (i.e., shear zones) formed during the late-stage partial melting events that originated the migmatites of the area. The case studies are proxies in the understanding of different episodes of magma emplacement along shear zones in this part of West Gondwana.

Exposure history, petrology, and shock‐induced sulfidization reaction of Alatage Mountain 001 strewn field samples

AbstractSeveral hundred meteorites with a total mass of over 100 kg were collected as the Alatage Mountain (AM) strewn field located in the Kumtag desert, Xinjiang Province, China. Twelve AM meteorites were studied in this work. Petrography, mineralogy, bulk chemistry, bulk oxygen isotopic compositions, and light noble gas concentrations and isotopic compositions were determined for all or for a selection of the meteorites. The studied meteorites are L‐chondrites that suffered a very strong impact; impact melt veins and melt pockets are widely distributed. More than 50% of the troilite exists in the form of blebs and veins in olivine and pyroxene. Some of these meteorites are impact melt recrystallized rocks (e.g., AM 037). The strong impact caused the decomposition of troilite, which led in AM 034 to the sulfidization reaction of olivine. The metal in most meteorites is almost completely altered, and the troilite has been significantly oxidized. Weathering resulted in the depletion of Mg, Fe, Co, and Ni, and the enrichment of Sr, Ba, Pb, and U in these meteorites. The cosmic ray exposure (CRE) ages measured for these specimens range between 6.2 ± 1.9 Ma and 9.0 ± 2.7 Ma, depending on the cosmogenic nuclide used. The average CRE age is 7.6 ± 1.3 Ma. Both 4He and 40Ar gas retention ages indicate that the strong impact which caused the shock effects occurred about 320 Ma ago.

The morphology, evolution and seismic visibility of partial melt at the core–mantle boundary: implications for ULVZs

SUMMARY Seismic observations indicate that the lowermost mantle above the core–mantle boundary (CMB) is strongly heterogeneous. Body waves reveal a variety of ultra-low velocity zones (ULVZs), which extend not more than 100 km above the CMB and have shear velocity reductions of up to 30 per cent. While the nature and origin of these ULVZs remain uncertain, some have suggested they are evidence of partial melting at the base of mantle plumes. Here we use coupled geodynamic/thermodynamic modelling to explore the hypothesis that present-day deep mantle melting creates ULVZs and introduces compositional heterogeneity in the mantle. Our models explore the generation and migration of melt in a deforming and compacting host rock at the base of a plume in the lowermost mantle. We test whether the balance of gravitational and viscous forces can generate partially molten zones that are consistent with the seismic observations. We find that for a wide range of plausible melt densities, permeabilities and viscosities, lower mantle melt is too dense to be stirred into convective flow and instead sinks down to form a completely molten layer, which is inconsistent with observations of ULVZs. Only if melt is less dense or at most ca. 1 per cent more dense than the solid, or if melt pockets are trapped within the solid, can melt remain suspended in the partial melt zone. In these cases, seismic velocities would be reduced in a cone at the base of the plume. Generally, we find partial melt alone does not explain the observed ULVZ morphologies and solid-state compositional variation is required to explain the anomalies. Our findings provide a framework for testing whether seismically observed ULVZ shapes are consistent with a partial melt origin, which is an important step towards constraining the nature of the heterogeneities in the lowermost mantle and their influence on the thermal, compositional and dynamic evolution of the Earth.

Conditions and Dynamics of Magma Storage in the Snæfellsnes Volcanic Zone, Western Iceland: Insights from the Búðahraun and Berserkjahraun Eruptions

AbstractEstablishing the conditions and dynamics of pre-eruptive magma storage and transfer within transient transcrustal storage networks is a major focus of quantitative volcanic petrology. In Iceland, the behaviour, conditions and timescales of magmatic processes within on-rift plumbing systems are increasingly well constrained. However, relatively little is known about magma storage and transfer in off-rift zones, despite off-rift volcanoes being able to generate hazardous explosive eruptions after centuries or millennia of dormancy (e.g. 2010 AD Eyjafjallajökull; 1362 AD Öræfajökull; 3.0 ka, 4.2 ka and 1104 AD Hekla). We present a combined geochemical and geothermobarometric study of magma storage and transfer recorded in the products of the postglacial Búðahraun (∼5.0–8.0 ka) and Berserkjahraun (∼4.0 ka) eruptions within the Snæfellsnes volcanic zone. The eruption products contain diverse and compositionally heterogeneous macrocryst cargoes recording complex petrogenetic histories of crystal evolution and inheritance from different parts of the sub-volcanic plumbing systems. Geothermobarometry indicates two compositionally and thermally heterogeneous magma storage regions located in the lower (20 ± 4 km) and upper-mid (11 ± 3 km) crust. Crystallization pressure and depth estimates coincide with comparable data from Vatnafell, a small sub-glacial table mountain (tuya) in the centre of the Snæfellsnes volcanic zone, indicating that the nature and conditions of magma storage have remained unchanged since the Upper Pleistocene. Trace element zoning of clinopyroxene macrocrysts indicates that mafic recharge into the upper-mid-crustal storage zone triggered the eruptions of Búðahraun and Berserkjahraun. Evidence for eruption-triggering mafic recharge and basaltic cannibalism involving the transfer and amalgamation of crystals with different evolutionary histories sets the Búðahraun and Berserkjahraun eruptions apart from other studied eruptions in Iceland. We propose that the compositional and textural diversity preserved within the crystal cargoes are a direct consequence of the reduced heat flow beneath the Snæfellsnes volcanic zone, which favours the formation of isolated melt pockets in which compositionally diverse macrocryst populations formed. Periodic flushes of primitive basaltic magma from depth promote widespread mixing with evolved melts, resulting in the assembly of crystals with diverse ancestries from different parts of the sub-volcanic systems. Insights gained from the diverse macrocryst cargoes of Búðahraun and Berserkjahraun and comparisons with recent off-rift volcanism in Iceland are essential for the development of future monitoring efforts and hazard evaluation. Although volcanism within the Snæfellsnes volcanic zone differs fundamentally from that in rift zones where eruptions are controlled by extensional spreading, magma ascent from depth still appears to follow pre-existing tectonic escape routes. This could result in extremely short advance warning times on the order of a few days.

Experimental investigation of the composition of incipient melts in upper mantle peridotites in the presence of CO2 and H2O

The compositions of mantle-derived magmas indicate a substantial variety in the abundances of volatiles in the upper mantle. CO2 and H2O depress the melting point of mantle peridotites considerably, delineating a pressure-temperature region of incipient melting where small degrees of melt exist over a large temperature range (~300 °C) before major melting begins. However, the chemical characterization of these melts in high-pressure experiments is challenging at low melt fractions (melt pockets may occupy volumes of only 10–50 μm3) because of analytical uncertainties related to the ubiquitous formation of metastable phases during quenching.This systematic partial melting study presents carefully determined compositions of incipient melts of a range of peridotites in the presence of CO2 + H2O mixtures at 2.5 to 7 GPa. Four different fertile and depleted peridotites were used: Hawaiian pyrolite, K2O-enriched pyrolite, MORB pyrolite and depleted lherzolite. To arrive at accurate melt compositions, we introduce the melt tomography method that integrates multiple area scans of melt pockets polished to several depths. Results confirm that incipient and low degree melts progress abruptly (within 25 °C) from carbonatitic towards melilititic-nephelinitic compositions at 2.5 GPa, whereas they progress gradually from carbonate-rich to carbonated silicate (aillikitic) compositions at 4–5 GPa. Melt compositions at near-solidus conditions are mainly controlled by the breakdown of carbonate, and hydrous phases such as pargasite and phlogopite, and become less siliceous and slightly more magnesian with increasing pressure at given melt fractions. Melts exhibit strong increases in SiO2 (2.75 to 44 wt%) with increasing temperature, whereas TiO2, Na2O and K2O decrease. The generally strongly potassic (K2O ≤ 6.63 wt%) and sodic (Na2O ≤ 3.06 wt%) character of the volatile-rich, incipient and low-degree melts indicate that these would act as reactive metasomatic agents that may transport large amounts of energy and induce chemical changes in large volumes of the upper mantle.

Shear wave splitting measurements in northeastern Uganda and southeastern Tanzania: corroborating evidence for sublithospheric mantle flow beneath East Africa

SUMMARY Several interpretations of shear wave splitting measurements in East Africa have been proposed, but a comprehensive understanding of seismic anisotropy is lacking vis-à-vis geodynamic processes affecting the African Plate, in part because of geographically limited data coverage. Here, we report new shear wave splitting observations using PKS, SKS and SKKS phases recorded on 17 temporary AfricaArray seismic stations in northeastern Uganda and southeastern Tanzania that help fill key gaps in data coverage in non-volcanic regions of East Africa. The stations in northeastern Uganda straddle the Aswa shear zone, which separates the Neoproterozoic Saharan Metacraton (to the east) and the Palaeoproterozoic Northern Uganda Terrane (to the west). The stations in southeastern Tanzania are located mostly in Karoo basins formed within the Neoproterozoic Mozambique Belt. Our results show NNE fast polarization directions in northeastern Uganda, with splitting times between 1.0 and 1.5 s, and NE directions in southeastern Tanzania, with splitting times between 0.6 and 1.3 s. The NNE fast polarization directions in Uganda cannot be explained with oriented melt pocket (OMP) anisotropy in the lithosphere, as the stations are far from any volcanic fields, or fossil anisotropy resulting from the Neoproterozoic East African orogeny. They are, however, parallel to the flow direction of the African superplume and corroborate previous measurements from northern Uganda that were used to argue that the overall northerly orientation of fast polarization directions across East Africa can be attributed to sublithospheric mantle flow associated with the African superplume. The NE fast polarization directions in Tanzania cannot be explained with SE-oriented fossil anisotropy expected from the Mesozoic rifting of Madagascar from Africa or OMP anisotropy in the lithosphere. However, they are consistent with the overall northerly orientation of fast polarization directions in East Africa and can be attributed to either fossil anisotropy in the Mozambique Belt lithosphere resulting from the East African Orogeny or sublithospheric mantle flow associated with the African superplume.

Using Volatile Element Concentration Profiles in Crystal‐Hosted Melt Embayments to Estimate Magma Decompression Rate: Assumptions and Inherited Errors

AbstractMagma decompression rate has profound impacts on volcanic eruption style as it determines the time available for most kinetic processes (e.g., volatile exsolution, crystal nucleation and growth) that influence the explosive‐effusive eruption transition. Thus, accurately quantifying decompression rate is a critical goal for understanding volcanic eruption dynamics. A recently developed technique uses crystal‐hosted pockets of melt that remain open to the host magma (melt embayments) to calculate an average decompression rate. Diffusion of volatile elements (e.g., H2O, CO2) out of the embayment during decompression creates “frozen” concentration gradients upon quenching that can be modeled to calculate the time needed to create the gradients. This geospeedometer is increasingly used, but inherent assumptions associated with the modeling and their impact on calculated decompression rates are poorly quantified. Therefore, we have conducted a numerical investigation to assess the impact of three common model simplifications pertaining to rhyolitic magmas: 1D diffusion models, equilibrium degassing, and isothermal decompression. We find that the greatest deviation between imposed and calculated decompression rates occur when 1D models are applied to “necked” embayments that have a constriction where the embayment joins with the far field melt. Simplifying to equilibrium degassing can also introduce modeling errors when disequilibrium conditions exist, though the prevalence of one or the other condition in nature is currently under debate. Assuming isothermal conditions introduces little error into modeled timescales. All of our modeling results are summarized into a list of best practices to minimize error in modeled timescales due to modeling assumptions.

Influence of element distribution on mechanical properties in the bonding zone of explosively welded steels

A melt pocket in the bonding zone of medium carbon steel bonded on low-carbon steel by explosive welding was investigated by laser assisted atom probe tomography. It was found that the structure is nanocrystalline and (sub-) grain boundaries are enriched with carbon. High hardness values in the melt pocket are attributed to a combination of (sub-) grain size and carbon distribution.

Jingsuiite, TiB2, a new mineral from the Cr-11 podiform chromitite orebody, Luobusa ophiolite, Tibet, China: Implications for recycling of boron

Abstract The new mineral jingsuiite (TiB2, IMA-2018-117b), together with osbornite-khamrabaevite solid solution (TiN-TiC), deltalumite, and a potential new mineral, hexagonal Ti10(Si,P,☐)7, constitute four inclusions up to 50 μm across in corundum recovered from the Cr-11 podiform chromitite orebody near Kangjinla, Luobusa ophiolite, Tibet, China. EELS, EDS, and 3D electron diffraction were applied to study the phases. In one inclusion, jingsuiite forms a rounded grain 40 μm across. Associated osbornite-khamrabaevite solid solution forms an irregular mass up to 10 μm across having the composition Ti(N0.5C0.5) and the Ti10(Si,P,☐)7 phase forms an incomplete overgrowth up to 20 μm thick around the grain of jingsuiite. In a second inclusion, jingsuiite, osbornite-khamrabaevite solid solution, Ti10(Si,P,☐)7 and deltalumite form a lamellar intergrowth 100 μm long composed of tablets of the four phases up to 50 μm long × 4 μm in thickness. Jingsuiite has a primitive hexagonal cell with a = 3.04(6), b = 3.04(6), c = 3.22(6) Å, α = 90°, β = 90°, γ = 120°, V = 25.8 (9) Å3, space group P6/mmm, Z = 1. Its structure was determined ab initio and dynamically refined on the basis of three-dimensional electron diffraction data; it is equivalent to that of synthetic TiB2. Results of EELS analyses of jingsuiite in foil no. 5357 (N = 20) gave B 61.87(1.22), C 1.53 (1.26), Ti 36.62 (1.45) at% from which an empirical formula of Ti1.10(B1.86C0.05)Σ1.91 was calculated on the basis of 3 atoms. The ideal formula is TiB2. Our preferred scenario is that corundum with entrapped Ti-Si-P-Fe intermetallic melts was precipitated from basaltic magmas during exhumation following deep subduction. Enrichment of B in the melt pockets is attributed to the highly reducing conditions that led to the segregation of siderophile elements into intermetallic melts and to the siderophile behavior of B, thereby concentrating it in the intermetallic melts in preference to silicate melt. Experimental work on the Ti-Fe-Si system indicates that minerals enclosed in corundum grains such as Ti, FeTiSi2, and TiSi2 could have crystallized from alloy melts at the lowest T accessible on the liquidus, i.e., <1300 °C. The presence of TiB2 in four inclusions in the Cr-11 orebody suggests incorporation of crustal sediments in the ophiolite followed by deep subduction to the Transition Zone where qingsongite (cubic BN) is inferred to have crystallized and subsequently exhumed to shallower levels where hexagonal BN and jingsuiite presumably crystallized.

Ti3+ in corundum traces crystal growth in a highly reduced magma

Aggregates of corundum crystals with skeletal to hopper morphology occur in pyroclastic rocks erupted from Cretaceous basaltic volcanoes on Mt Carmel, N. Israel. The rapid growth of the crystals trapped volumes of the parental Al2O3-supersaturated melt; phenocrysts of tistarite (Ti2O3) in the trapped melts indicate crystallization at oxygen fugacities 6–7 log units below the Iron-Wüstite buffer (fO2 = ΔIW − 6 to − 7), induced by fluxes of mantle-derived CH4-H2 fluids. Cathodoluminescence images reveal growth zoning within the individual crystals of the aggregates, related to the substitution of Ti3+ in the corundum structure. Ti contents are < 0.3 wt% initially, then increase first linearly, then exponentially, toward adjacent melt pockets to reach values > 2 wt%. Numerical modelling indicates that the first skeletal crystals grew in an open system, from a moving magma. The subsequent linear increase in Ti reflects growth in a partially closed system, with decreasing porosity; the exponential increase in Ti close to melt pockets reflects closed-system growth, leading to dramatic increases in incompatible-element concentrations in the residual melts. We suggest that the corundum aggregates grew in melt/fluid conduits; diffusion modelling implies timescales of days to years before crystallization was terminated by explosive eruption. These processes probably operate in explosive volcanic systems in several tectonic settings.

Controls on the scales of equilibrium during granulite facies metamorphism

AbstractPhase equilibrium modelling is commonly employed to constrain the pressure–temperature (P–T) evolution of granulite facies rocks, from which their geodynamic setting may be inferred. However, defining a suitable equilibrium volume in such rocks is non‐trivial due to heterogeneities in protolith composition and open system behaviour, including melt loss or gain and deformation. Consequently, equilibrium volumes and the mineral assemblages they contain may vary both temporally and spatially within a single rock. Additionally, despite the extreme temperatures they attained, granulites commonly contain microtextures indicating gradients in chemical potential after the metamorphic peak. This study assesses the processes that control compositional heterogeneity between equilibrium volumes in a suprasolidus granulite and the consequences for the development of a range of microstructures. An approach combining phase equilibrium forward modelling and chemical potential diagrams is used to simulate the suprasolidus evolution of two adjacent equilibrium volumes in an Mg‐rich metapelite. Assuming equilibrium within the two compositional domains, evolving granulite facies assemblages vary along a high T/P (125°C/kbar) clockwise P–T path representative of high T/P metamorphic terranes. Many retrograde microtextures in metapelitic rocks can be reproduced by considering chemical potential (µ) gradients in µMgO, µFeO and µCaO between phases, assuming (a) the presence of melt on grain boundaries, (b) that Al2O3 is perfectly immobile and (c) that K2O and Na2O are perfectly mobile. Additionally, documented microtextures in granulites imply local SiO2‐undersaturation, despite the presence of matrix quartz in the rock. Preserving such chemical potential gradients requires that solid phases are chemically and physically isolated from melt. Efficient prograde melt drainage may lead to a loss in melt interconnectivity at high temperature (&gt;900°C), meaning that equilibration of centimetre‐scale compositional domains is controlled by solid‐state diffusion. The presence of isolated pockets of viscous melt allows the formation of discontinuous intergranular reaction microtextures.

Zircon petrochronology in large igneous provinces reveals upper crustal contamination processes: new U\u2013Pb ages, Hf and O isotopes, and trace elements from the Central Atlantic magmatic province (CAMP)

Zircon occasionally crystallizes in evolved melt pockets in mafic large igneous province (LIP) magmas, and in these cases, it is used to provide high-precision age constraints on LIP events. The precision and accuracy of high-precision ages from LIPs are crucially important, because they may be implicated in mass extinctions. However, why zircon crystallizes in these magmas is not clearly understood, since their mafic compositions should limit zircon saturation. Here, we investigate the occurrence of zircon (and baddeleyite) in intrusive and extrusive mafic rocks from Central Atlantic Magmatic Province (CAMP) using petrography, trace-element analysis, Ti temperatures, Hf and oxygen isotopes, and high-precision U–Pb geochronology, along with petrological and thermal modeling. We provide new ages for CAMP sills that intruded into Paleozoic sediments in Brazil, indicating that the high and low Ti magmatism in this area occurred synchronously over 264 ± 57 ka. We show that upper crustal assimilation, especially of shales, during the emplacement of the CAMP likely led to zircon saturation. Assimilation of upper crustal sediments is also supported by high δ18O values and some rare negative εHf values in the zircon crystals. The only extrusive sample analyzed was the North Mountain basalt in Nova Scotia, Canada. This sample contains a large age variation in its zircon crystals (up to 4 Ma), and the older crystals have slightly more negative εHf values suggesting the presence of small (micron scale) xenocrystic cores associated with very late-stage sediment assimilation. However, the CAMP dataset as a whole suggests that the presence of xenocrystic cores is rare. Assuming no xenocrystic cores, and considering the zircon undersaturated nature of LIP mafic melts, the oldest zircon age clusters in a population should record the magma emplacement (or time when assimilation occurred), and the younger ages in a population are more likely to reflect Pb loss, especially given the high U concentrations of LIP zircon. Our identification of heterogeneous isotopic and elemental compositions in LIP zircon indicates that zircon in these magmas saturate in isolated minute melt pockets just before the system cools below its solidus.

Upper mantle seismic anisotropy beneath the Northern Transantarctic Mountains inferred from peridotite xenoliths near Mt. Melbourne, northern Victoria Land, Antarctica

Microstructural investigations of mantle xenoliths from the Mt. Melbourne area were undertaken to reveal the origin of S-wave splitting beneath northern Victoria Land, Antarctica. The six analyzed peridotites contain various deformation features. The rotated olivine maxima of [100] and [010] into horizontal and vertical orientations, respectively, are classified into five samples with a D-type crystallographic preferred orientation (CPO) and one sample as an A-type CPO. The D-type olivine fabric can be explained by multiple slip systems of {0kl}[100] at low-temperature and high-stress conditions; therefore, both compressional and extensional regimes during subduction and rifting, respectively, could be applied in this study. With an assumption that olivine a-axes are aligned along the direction of mantle flow to form maximum S-wave splitting, the observed delay time of 0.9–1.3 s beneath northern Victoria Land can be partially explained by the anisotropy in the mantle peridotites. The remaining seismic anisotropy can be explained by the presence of melt pockets trapped along tectonic faults that developed perpendicular to the fast S-wave splitting direction. This study therefore demonstrates that the NE–SW-trending S-wave splitting beneath northern Victoria Land, Antarctica, results from the existence of both mantle peridotites as well as melt pockets trapped along the tectonic faults.

Parageneses of TiB2 in corundum xenoliths from Mt. Carmel, Israel: Siderophile behavior of boron under reducing conditions

Abstract Titanium diboride (TiB2) is a minor but common phase in melt pockets trapped in the corundum aggregates that occur as xenoliths in Cretaceous basaltic volcanoes on Mt. Carmel, north Israel. These melt pockets show extensive textural evidence of immiscibility between metallic (Fe-Ti-C-Si) melts, Ca-Al-Mg-Si-O melts, and Ti-(oxy)nitride melts. The metallic melts commonly form spherules in the coexisting oxide glass. Most of the observed TiB2 crystallized from the Fe-Ti-C silicide melts and a smaller proportion from the oxide melts. The parageneses in the melt pockets of the xenoliths require fO2 ≤ ΔIW-6, probably generated through interaction between evolved silicate melts and mantle-derived CH4+H2 fluids near the crust-mantle boundary. Under these highly reducing conditions boron, like carbon and nitrogen, behaved mainly as a siderophile element during the separation of immiscible metallic and oxide melts. These parageneses have implications for the residence of boron in the peridotitic mantle and for the occurrence of TiB2 in other less well-constrained environments such as ophiolitic chromitites.

Donwilhelmsite, [CaAl4Si2O11], a new lunar high-pressure Ca-Al-silicate with relevance for subducted terrestrial sediments

AbstractWe report on the occurrence of a new high-pressure Ca-Al-silicate in localized shock melt pockets found in the feldspatic lunar meteorite Oued Awlitis 001 and discuss the implications of our discovery. The new mineral crystallized as tiny, micrometer-sized, acicular grains in shock melt pockets of roughly anorthitic bulk composition. Transmission electron microscopy based three-dimensional electron diffraction (3D ED) reveals that the CaAl4Si2O11 crystals are identical to the calcium aluminum silicate (CAS) phase first reported from static pressure experiments. The new mineral has a hexagonal structure, with a space group of P63/mmc and lattice parameters of a = 5.42(1) Å; c = 12.70(3) Å; V = 323(4) Å3; Z = 2. This is the first time 3D ED was applied to structure determination of an extraterrestrial mineral. The International Mineralogical Association (IMA) has approved this naturally formed CAS phase as the new mineral “donwilhelmsite” [CaAl4Si2O11], honoring the U.S. lunar geologist Don E. Wilhelms. On the Moon, donwilhelmsite can form from the primordial feldspathic crust during impact cratering events. In the feldspatic lunar meteorite Oued Awlitis 001, needles of donwilhelmsite crystallized in ~200 mm sized shock melt pockets of anorthositic-like chemical composition. These melt pockets quenched within milliseconds during declining shock pressures. Shock melt pockets in meteorites serve as natural crucibles mimicking the conditions expected in the Earth's mantle. Donwilhelmsite forms in the Earth's mantle during deep recycling of aluminous crustal materials, and is a key host for Al and Ca of subducted sediments in most of the transition zone and the uppermost lower mantle (460–700 km). Donwilhelmsite bridges the gap between kyanite and the Ca-component of clinopyroxene at low pressures and the Al-rich Ca-ferrite phase and Ca-perovskite at high-pressures. In ascending buoyant mantle plumes, at about 460 km depth, donwilhelmsite is expected to break down into minerals such as garnet, kyanite, and clinopyroxene. This process may trigger minor partial melting, releasing a range of incompatible minor and trace elements and contributing to the enriched mantle (EM1 and EM2) components associated with subducted sedimentary lithologies.

Tectonic constraints on a magmatic plumbing system: The Quetrupillán Volcanic Complex (39°30′ S, 71°43′ W), Southern Andes, Chile

The Quetrupillán Volcanic Complex is a composite system, active since the Pleistocene. We combine petrological and geochemical data from its erupted products with structural and geometrical constraints applied to the overall system and region. We conclude that a basaltic source melt is hindered on its ascent through the crust due to a compressional tectonic regime, influenced further by the structural control imposed by the Liquiñe-Ofqui Fault Zone. The stalled melt evolves by fractional crystallisation within the crust and undergoes a degree of crustal contamination, resulting in a network of trachytic melt pockets within a transcrustal magmatic system. Eruptions sourced from these pockets have generated numerous lava flows with trachytic compositions, which have occurred from the summit and flanks of Quetrupillán during the Holocene. Occasionally, some of the basaltic source melt has reached shallow levels within the plumbing system with minor interaction with the trachytic melt, resulting in the eruption of lavas with basalt, basaltic andesite and trachyandesite compositions. We propose a conceptual model for the magmatic system of Quetrupillán, in which the Liquiñe-Ofqui Fault Zone plays an important role in exerting a structural control on the crust on which Quetrupillán resides, influencing magma residence times and pathways to the surface.

Partial melting due to breakdown of phengite and amphibole in retrogressed eclogite of deep Precambrian crust: An example from the Algonquin terrane, western Grenville Province, Canada

Characterizing partial melting of eclogite in Precambrian orogens is critical to understanding the petrological processes that operate in the deep orogenic crust. Retrogressed eclogite from the Algonquin terrane in the western Grenville Province, Canada, was taken as an example to study such processes. The studied samples contained the assemblage of garnet, omphacite, amphibole, phengite, biotite, and rutile at peak pressure. Various exhumation- and melt-related textures formed show evidence for a complex post-peak pressure history. Rutile is only found in the core of garnet, whereas ilmenite-bearing assemblages predominate in the garnet rim and the matrix. Symplectite of diopside + plagioclase + amphibole replaced former omphacite. Symplectite of amphibole and plagioclase formed as a corona around garnet. Polymineralic inclusions of plagioclase + K-feldspar ± ilmenite ± biotite ± amphibole ± apatite ± sulfides in garnet crystallized from melt, which was captured during garnet growth and reacted with the host. Cusps of plagioclase into the boundaries of other phases (e.g. garnet and clinopyroxene) mimic the original melt-solid interface. Neoblasts of garnet with euhedral crystal faces were formed as a peritectic phase during eclogite melting. Leucocratic veins without external connection, present in the sampled mafic pod at outcrop scale, are interpreted as crystallized pockets of melt which derived internally. Pseudosection modeling of the studied samples along with compositional isopleths of chemically zoned garnet yielded an exhumation path from 1.7 ± 0.1 GPa, 680 ± 50 °C to 1.3 ± 0.1 GPa, 920 ± 50 °C. Along this path, the modal content of melt increased in response to the breakdown of phengite and amphibole along with involvement of omphacite and rutile. The melt crystallized during cooling and equilibrated at 0.6–0.8 GPa, 710–750 °C, estimated using empirical thermobarometry. This study shows that metabasite from Proterozoic collisional orogens can be partially melted on the exhumation and heating path through phengite- and amphibole-dehydration melting.

Crystal Chemistry of Barian Titanian Phlogopite from a Lamprophyre of the Gargano Promontory (Apulia, Southern Italy)

This study is focused on a barian titanian phlogopite found in an alkaline ultramafic dyke transecting Mesozoic limestones of the Gargano Promontory (Apulia, Italy). The rock containing the barian titanian phlogopite, an olivine-clinopyroxene-rich lamprophyre with nepheline and free of feldspars, has been classified as monchiquite. The present study combines chemical analyses, single crystal X-ray diffraction and Raman spectroscopy. Chemical variations suggest that the entry of Ba into the phlogopite structure can be explained by the exchange Ba + Al = K + Si. The crystal structure refinement indicates that the Ti uptake is consistent with the Ti–oxy exchange mechanism. The structural parameters associated with the oxy substitution mechanism are extremely enhanced and rarely reported in natural phlogopite: (a) displacement of M2 cation toward the O4 site (~0.7); (b) M2 octahedron bond-length distortion (~2.5); (c) very short c cell parameter (~10.14 Å). Raman analysis showed most prominent features in the 800–200 cm−1 region with the strongest peaks occurring at 773 and 735 cm−1. Only a weak, broad band was observed to occur in the OH-stretching region. As concerns the origin of the barian titanian phlogopite, the rock textural features clearly indicate that it crystallized from pockets of the interstitial melt. Here, Ba and Ti enrichment took place after major crystallization of olivine under fast-cooling conditions, close to the dyke margin.