Anorthite Content Research Articles

P-T-t evolution of Neoarchaean to Paleoproterozoic pelitic granulites from the Jidong terrane, eastern North China Craton

Four metamorphic mineral assemblages, formed at different metamorphic stages, are identified in pelitic granulites exposed in the Jidong terrane, east Hebei Province, North China Craton (NCC). Prograde mineral assemblages (M1) are represented by biotite, plagioclase, quartz, and ilmenite, preserved as mineral inclusions within garnet porphyroblasts. Peak mineral assemblages (M2), which are represented by the garnet cores, rutile, pseudomorphs of sillimanite after kyanite, and some matrix minerals, formed at P–T conditions of ∼12–13kbar/820–850°C. Post-peak mineral assemblages (M3), which consist of the inner rims of garnets and some matrix minerals, formed at P–T conditions of ∼11kbar/850°C. The final retrograde mineral assemblages (M4) are mainly represented by matrix biotite without rutile exsolution, and minor matrix plagioclase with relatively high anorthite content. High-resolution secondary ion mass spectrometry (SIMS) U−Pb dating of metamorphic zircons demonstrates that the pelitic granulites possibly reached the peak metamorphic stage at ∼2.5Ga. Younger metamorphic ages (2.41–2.45Ga) possibly represent the cooling ages of the granulites. Accordingly, the pelitic granulites exposed in the Jidong terrane record clockwise P–T–t paths with sequential isothermal decompression (ITD) and isobaric cooling (IBC) segments, likely caused by a subduction–collision event during the late Neoarchean to early Palaeoproterozoic in the eastern NCC.

Plagioclase twins in a basalt: an electron backscatter diffraction study

Twins in plagioclase, which are abundant in rocks, have important implications for the growth history and subsequent transformation. There are many twin laws in plagioclase and some of them are difficult to identify. This paper presents an electron backscatter diffraction (EBSD) analysis combined with electron microprobe analysis of plagioclase twinning in the Niutoushan alkali basalt. Theoretical pole figures of 12 different twin laws for plagioclase were generated. By comparing the pole figures obtained from EBSD with the theoretical ones, 11 twin laws have been identified in the alkali basalt, and their frequency has also been determined: Albite (28%), Carlsbad (25%), Albite–Carlsbad (34%), Pericline (3%), Ala (2%), Manebach (1%), Albite–Ala (1%), Prism {110} (2%), Prism {1\bar 10} (1%), Prism {130} (1%) and Prism {1\bar 30} (1%). All the plagioclase twins in the alkali basalt are growth twins. The anorthite content of the plagioclase has a negative correlation with the frequency of the Pericline and Albite twin laws but a positive correlation with the frequency of all the other twin laws, which is consistent with previous results. The theoretical pole figures of twin laws for plagioclase introduced in this paper can be applied to investigating plagioclase twin laws in all plagioclase-bearing rocks, and the relationship between twin pattern and crystallization conditions of the rocks can be established.

Volatile dilution during magma injections and implications for volcano explosivity

Abstract Magma reservoirs underneath volcanoes grow through episodic emplacement of magma batches. These pulsed magma injections can substantially alter the physical state of the resident magma by changing its temperature, pressure, composition, and volatile content. Here we examine plagioclase phenocrysts in pumice from the 2014 Plinian eruption of Kelud (Indonesia) that record the progressive capture of small melt inclusions within concentric growth zones during crystallization inside a magma reservoir. High-spatial-resolution Raman spectroscopic measurements reveal the concentration of dissolved H2O within the melt inclusions, and provide insights into melt-volatile behavior at the single crystal scale. H2O contents within melt inclusions range from ∼0.45 to 2.27 wt% and do not correlate with melt inclusion size or distance from the crystal rim, suggesting that minimal H2O was lost via diffusion. Instead, inclusion H2O contents vary systematically with anorthite content of the host plagioclase (R2 = 0.51), whereby high anorthite content zones are associated with low H2O contents and vice versa. This relationship suggests that injections of hot and H2O-poor magma can increase the reservoir temperature, leading to the dilution of melt H2O contents. In addition to recording hot and H2O-poor conditions after these injections, plagioclase crystals also record relatively cold and H2O-rich conditions such as prior to the explosive 2014 eruption. In this case, the elevated H2O content and increased viscosity may have contributed to the high explosivity of the eruption. The point at which an eruption occurs within such repeating hot and cool cycles may therefore have important implications for explaining alternating eruptive styles.

Grain size reduction due to fracturing and subsequent grain-size-sensitive creep in a lower crustal shear zone in the presence of a CO2-bearing fluid

To understand rheological weakening in the lower continental crust, we studied mylonites in the Paleoproterozoic Eidsfjord anorthosite, northern Norway. The zones of anorthositic mylonites range from a few millimeters to several meters thick, and include ultramylonites and protomylonites. They contain syn-kinematic metamorphic minerals, including Cl-bearing amphibole and scapolite. Thermodynamic analysis reveals that syn-deformational hydration reactions occurred at ∼600 °C and ∼700 MPa under CO2-bearing conditions. The protomylonites contain many fragmented plagioclase porphyroclasts. The fractures in porphyroclasts are filled with fine-grained plagioclase, suggesting that fracturing is a common mechanism of grain size reduction. The anorthite contents of fine-grained polygonal matrix plagioclase are different from those of porphyroclastic plagioclase, suggesting that the matrix grains nucleated and grew during syn-kinematic metamorphism. Plagioclase aggregates in the matrices of mylonites do not exhibit a distinct crystallographic preferred orientation, which implies that the dominant deformation mechanism was grain-size-sensitive creep. Consequently, in the lower crustal anorthositic mylonites, grain size reduction occurred via fracturing, rather than through dynamic recrystallization, leading to grain-size-sensitive creep. The syn-kinematic recrystallization of minor phases at plagioclase grain boundaries may suppress the growth of plagioclase and contribute to the development of grain-size-sensitive creep.

Melting carbonated epidote eclogites: carbonatites from subducting slabs

Current knowledge on the solidus temperature for carbonated eclogites suggests that carbonatitic liquids should not form from a subducted oceanic lithosphere at sub-arc depth. However, the oceanic crust includes a range of gabbroic rocks, altered on rifts and transforms, with large amounts of anorthite-rich plagioclase forming epidote on metamorphism. Epidote disappearance with pressure depends on the normative anorthite content of the bulk composition; we therefore expect that altered gabbros might display a much wider pressure range where epidote persists, potentially affecting the solidus relationships. A set of experimental data up to 4.6 GPa, and 1000 °C, including new syntheses on mafic eclogites with 36.8 % normative anorthite, is discussed to unravel the effect of variable bulk and volatile compositions in model eclogites, enriched in the normative anorthite component (An 37 and An 45). Experiments are performed in piston cylinder and multianvil machines. Garnet, clinopyroxene, and coesite form in all syntheses. Lawsonite was found to persist at 3.7 GPa, 750 °C, with both dolomite and magnesite; at 3.8 GPa, 775–800 °C, fluid-saturated conditions, epidote coexists with kyanite, dolomite, and magnesite. The anhydrous assemblage garnet, omphacite, aragonite, and kyanite is found at 4.2 GPa, 850 °C. At 900 °C, a silicate glass of granitoid composition, a carbonatitic precipitate, and Na-carbonate are observed. Precipitates are interpreted as evidence of hydrous carbonatitic liquids at run conditions; these liquids produced are richer in Ca compared to experimental carbonatites from anhydrous experiments, consistently with the dramatic role of H2O in depressing the solidus temperature for CaCO3. The fluid-absent melting of the assemblage epidote + dolomite, enlarged in its pressure stability for An-rich gabbros, is expected to promote the generation of carbonatitic liquids. The subsolidus breakdown of epidote in the presence of carbonates at depths exceeding 120 km provides a major source of C-O-H volatiles at sub-arc depth. In warm subduction zones, the possibility of extracting carbonatitic liquids from a variety of gabbroic rocks and epidosites offers new scenarios on the metasomatic processes in the lithospheric wedge of subduction zones and a new mechanism for recycling carbon.

Quantitative analysis of mass transfer during polymetamorphism in pelites of the Transangarian Yenisei Ridge

The study provides geological, structural, mineralogical, petrological, and geochronological evidence for polymetamorphic evolution of gneisses from the Garevka complex of the Yenisei Ridge. The results of the study provide significant insight into the geochemical behavior of major and trace elements in zoned garnet crystals and mineral inclusions formed during prograde and retrograde metamorphism of pelitic rocks. It was shown that the concentrations of Y and HREE in garnet decrease with increasing P and T and increase with decreasing pressure and temperature. The combined study of multicomponent chemical zoning patterns of coexisting minerals and metamorphic mineral reactions in metapelites was conducted. The results show that the main reason for a drastic increase in CaO content in garnets during collisional metamorphism is a mass exchange between garnet and plagioclase. The deviation from this trend, as indicated by the concurrent increase inthe grossular content of garnet and anorthite content of plagioclase, arises from the breakdown of epidote. The calculated metamorphic reactions, mass balance analysis, and changes in mineral chemistry during metamorphism reinforce the evidence for the isochemical character of processes with respect to most components of the system. The minimum volume of the system in which chemical exchange between reacting phases is balanced for all major and trace elements did not exceed ~1 mm3. The total HREE balance requires a greater reaction volume (up to ~8 mm ) involved in the redistribution of these elements, which provide evidence for their relatively higher mobility during metamorphism relative to other rare earth elements. The specific distribution and quite substantial mass transport of HREE are controlled by heterovalent isomorphic substitution between these elements and CaO in garnet.

Formation of the lunar highlands Mg-suite as told by spinel

Two competing hypotheses suggest lunar Mg-suite parental melts formed: (1) by shallow-level partial melting of a hybridized source region (containing ultramafic cumulates, plagioclase-bearing rocks, and KREEP), producing a plagioclase-saturated, MgO-rich melt, or (2) when plagioclase-undersaturated, MgO-rich melts were brought to plagioclase saturation during magma-wallrock interactions within the anorthositic crust. To further constrain the existing models, phase equilibria experiments have been performed on a range of Mg-suite parental melt compositions to investigate which composition can best reproduce two distinct spinel populations found within the Mg-suite troctolites—chromite-bearing (FeCr2O4) troctolites and the more rare pink spinel (MgAl2O4 or Mg-spinel) troctolites (PST). Phase equilibria experiments at 1 atm pressure were conducted under reducing conditions ![Formula][1] and magmatic temperatures (1225–1400 °C) to explore the spinel compositions produced from melts predicted by the models above. Additionally, the experimental data are used to calculate a Sp-Ol, Fe-Mg equilibrium exchange coe to cient to correct natural spinel for sub-solidus re-equilibration with olivine in planetary samples: Sp-Ol ![Formula][2] (R2 = 0.956). Melts from each model (≥50% normative anorthite) produce olivine, plagioclase, and Mg-spinel compositionally consistent with PST samples. However, chromite was not produced in any of the experiments testing current Mg-suite parental melt compositions. The lack of chromite in the experiments indicates that current estimates of Mg-suite parental melts can produce Mg-spinel bearing PST, but not chromite-bearing troctolites and dunites. Instead, model calculations using the MAGPOX equilibrium crystallization program predict chromite production from plagioclase-undersaturated melts (<20% normative anorthite). If so, experimental and model results suggest chromite in Mg-suite crystallized from plagioclase-undersaturated parental melts, whereas Mg-spinel in the PST is an indicator of magma-wallrock interactions within the lunar crust (a mechanism that increases the normative anorthite contents of initially plagioclase-undersaturated Mg-suite parental melts, eventually producing Mg-spinel). The constraints for magmatic chromite crystallization suggest Mg-suite parental melts were initially plagioclase-undersaturated. In turn, a plagioclase-undersaturated Mg-suite parent is consistent with mantle overturn models that predict Mg-suite parent magmas resulted from decompression melting of early ultramafic cumulates produced during the differentiation of a global lunar magma ocean. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif

Crystal residence times from trace element zoning in plagioclase reveal changes in magma transfer dynamics at Mt. Etna during the last 400 years

Trace element zoning in plagioclase of selected alkaline lavas from the historic (1607–1892AD) and recent (1983–2013AD) activity of Mt. Etna volcano has been used to explore the possible role that volcano-tectonics exert on magma transfer dynamics. The observed textural characteristics of crystals include near-equilibrium textures (i.e., oscillatory zoning) and textures with variable extent of disequilibrium (patchy zoning, coarse sieve textures and dissolved cores). Historic crystals exhibit lower K concentrations at lower anorthite contents, a feature in agreement with the general more potassic character of the recent lavas if compared to the historic products. Historic plagioclases have statistically higher Ba and lower Sr concentrations than the recent crystals, which result in different Sr/Ba ratios for the two suites of plagioclase. Variations in the anorthite content along core-to-rim profiles obtained on crystals with different types of textures for both the historic and recent eruptive periods were evaluated particularly versus Sr/Ba. At comparable average An contents, crystals characterized by oscillatory zoning, which are representative of near-equilibrium crystallization from the magma, display distinct Sr/Ba ratios. We suggest that these features are primarily related to recharge of a new, geochemically-distinct magma into the storage and transport system of the volcano. In addition to distinct trace element and textural characteristics of plagioclase, Sr diffusion modeling for plagioclase suggests that residence times are generally shorter for crystals found in recently erupted lavas (25–77 years, average 43years) compared to those of the historic products (43–163 years, average 99years). Shorter residences times correlate with gradual increases in eruption volume and eruption frequency rates through time. We attribute these features to an increasing influence, since the 17th century, of extensional tectonic structures within the upper 10km of the Etnean crust, which have promoted shorter residence times and higher eruption frequency.

Ca/Al of plagioclase-hosted melt inclusions as an indicator for post-entrapment processes at mid-ocean ridges?

The composition of melt inclusions in basalts erupted at mid-ocean ridges may be modified by post-entrapment processes, so the present composition of melt inclusions may not represent their original composition at the time of entrapment. By combining the melt inclusion composition in samples from the South Mid-Atlantic Ridge at 19oS analyzed in this study, and from the Petrological Database, we found that post-entrapment crystallization processes resulted in higher Ca/Al, Mg#[100°—atomic Mg 2+ /(Mg 2+ +Fe 2+ )], MgO and FeO contents, and lower CaO and Al 2 O 3 contents of plagioclase-hosted melt inclusions relative to those hosted in olivine. In addition, melt inclusions hosted in plagioclase with anorthite content larger than 80mol.% had been modified more readily than others. By discussing the relationships between Ca/Al and fractional crystallization, post-entrapment crystallization, and the original melt composition, we propose that Ca/Al can be regarded as an indicator of the effect of post-entrapment processes on melt inclusion composition. Specifically, i) when Ca/Al 1.0, the compositions of melt inclusions do not reflect the original melt composition nor preserve information about the mantle source. According to these criteria, plagioclase-hosted melt inclusions with Ca/Al>1.0 in basalts from the South Mid-Atlantic Ridge at 19oS cannot represent the composition of the melt at the moment of their entrapment.

Back‐arc magma processes in the Okinawa Trough: new insights from textural and compositional variations of plagioclase in basalts

AbstractThe evolution of magma during ascent through the crust in the region of the Okinawa Trough is recorded by its mineralogy. In this study, we utilized the characteristics of plagioclase minerals in basaltic magma to deduce magma generation processes, by examining (via back‐scattered electron images) the compositional features of plagioclase phenocrysts in basalts dredged from the middle and southern portions of the Okinawa Trough, which provided a detailed account of the evolution and crystallization of the magma. The crystallization and evolutionary histories of magmas from the middle Okinawa Trough are distinct from those in the southern Okinawa Trough. In the middle Okinawa Trough, we identified seven textural varieties of plagioclase: homogeneous‐core, fine oscillatory‐zoned, core‐sieved, mantle‐sieved, patchy‐core, glomerocrystic and microlitic. Anorthite (An) contents in the cores of plagioclase crystals are 80–87%, while concentrations of Fe, Mg and Sr vary more strongly, especially in patchy‐core plagioclase. The homogeneous‐core, fine oscillatory‐zoned and glomerocrystic plagioclase crystallized in deep‐seated chambers. In contrast, the core‐sieved, mantle‐sieved and patchy‐core textures developed when magmas ascended slowly and were injected into shallow chambers, during which plagioclase was resorbed and dissolved due to decompression. In the southern Okinawa Trough, homogeneous‐core plagioclase crystallized when magma ascended rapidly through deep conduits and was injected into a shallow chamber; under these conditions, only homogeneous‐core and core‐sieved plagioclase phenocrysts crystallized in the shallow chamber. The core of homogeneous‐core plagioclase (An86–An93) was partially resorbed and contaminated by crustal material. Degassing and undercooling promoted the rapid crystallization of microlitic plagioclase, especially in the outermost rims, during and after the extraction of magma from shallow chambers, in both the middle and southern Okinawa Trough. Copyright © 2016 John Wiley &amp; Sons, Ltd.

Uncommon behavior of plagioclase and the ancient lunar crust

AbstractCalcic plagioclase, the dominant mineral of the anorthositic lunar crust, fails to show the Na enrichment during cooling that is typical of magmatic plagioclase. We show that this enigmatic behavior may arise during fractionation of highly calcic plagioclase at depths greater than ~70 km in the lunar magma ocean because of the development of a negative azeotropic configuration at high anorthite contents that impedes and may even reverse the standard plagioclase albite enrichment with dropping temperature. This result supports a high‐pressure origin of this plagioclase consistent with the lunar magma ocean model. It also provides a new mechanism for forming lunar lithologies with sodic plagioclase from a highly Na‐depleted Moon through gravitational settling of spinel and refines the compositional characteristics of the late stage residual liquids of the lunar magma ocean.

Discovery of Neoarchean suprasubduction zone ophiolite suite from Yishui Complex in the North China Craton

The Archean tectonic realm of the North China Craton (NCC) is considered in recent models as a collage of several microblocks which were amalgamated along zones of ocean closure during late Neoarchean. Here we report the finding of a dismembered ophiolite suite from the southern margin of the Jiaoliao microblock in the interior of the unified Eastern Block of the NCC. The suite is composed of lherzolite, pyroxenite, noritic and hornblende gabbro, and hornblendite intruded by veins and sheets of leuco granite. Together with transposed layers and bands of metavolcanics and amphibolites, banded iron formation (BIF), and diabase dykes in the adjacent locations, the Yishui complex corresponds well with a dismembered suprasubduction zone ophiolite suite. Clinopyroxene in the pyroxenite and gabbroic rocks is Mg rich and range in composition from augite to diopside. Among orthopyroxenes, those in lherzolite show the highest XMg of 0.84–0.85. Plagioclase in hornblende gabbro shows high anorthite content (An50–64). Calcic amphiboles in the gabbroic rocks range in composition from ferropargasite to ferro-edenite, edenite and pargasite. Spinel inclusions in lherzolite are Cr-rich magnesiospinel. Geochemically, the mafic rocks from Yishui complex show subalkaline basaltic source, whereas the granitoids show volcanic arc affinity. The hornblende gabbro and gabbro, lherzolite and hornblendite show compositional similarity to E-MORB and N-MORB respectively. The lherzolite and hornblendite possess arc-related ultramafic cumulate nature, with overall features straddling the fields of IAT and IAT-MORB. The geochemical features are consistent with evolution in a suprasubduction regime with no significant crustal contamination. The majority of zircon grains in the Yishui suite exhibit magmatic texture and high Th/U ratios. Zircon grains from hornblendite define 207Pb/206Pb upper intercept age of 2538±30Ma. Zircons from the granite show ages of 2538±16Ma and 2503±21Ma, and those from the gabbros yield ages of 2503±16Ma and 2495±10Ma. The well defined major age peak at 2500Ma is broadly coeval with Neoarchean ages reported from the microblocks in the North China Craton. The zircon Lu–Hf data from the Yishui suite display εHf(t) values between −2.5 and 5.0, with corresponding model ages suggesting magma derivation from Neoarchean juvenile sources together with limited reworked Paleo-Mesoarchean crustal components.Our study is the first report of Neoarchean suprasubduction-type ophiolites from a locality far from the margins of the major crustal blocks and suture zones in the NCC and strengthens the concept that the craton is a mosaic of several microblocks with intervening oceans that closed along multiple subduction zones. We envisage that the amalgamation between the Xuhuai and the Jiaoliao microblocks resulted in the accretion of the Yishui suprasubduction zone ophiolitic assemblages onto the southern margin of the Jiaoliao microblock. The Neoarchean microblock amalgamation in the North China Craton provides new insights into continental growth in the early Earth and confirms that modern style plate tectonics might have been initiated early in the history of our planet.

Metamorphic P–T paths and Zircon U–Pb age data for the Paleoproterozoic metabasic dykes of high-pressure granulite facies from Eastern Hebei, North China Craton

Metamorphic P–T paths and age of metabasic dykes from the Eastern Hebei are very significant in revealing the tectonic framework and evolution of the North China Craton. These dykes crosscut the foliation of late Archean tonalitic–trondhjemitic–granodioritic (TTG) gneisses and supracrustal rocks and commonly develop a typical high-pressure granulite texture with an assemblage of garnet, clinopyroxene, plagioclase, amphibole, quartz and ilmenite. Garnet is weakly zoned where pyrope and grossular contents decrease from core to rim. Plagioclase occurs in two types, coarse-grained exhibiting composite zoning and fine-grained exhibiting simple zoning with core-rim increasing in anorthite content. Phase modeling using THERMOCALC, together with petrographic observations, suggest that these metabasic dykes may have experienced a clockwise P–T path with peak P/T conditions of 11–13kbar/790–810°C. The dykes show weakly right-declined REE patterns and depleted in Nb, Ta and Ti, similar to the metabasic dykes in the Trans-North China Orogen (TNCO). LA-ICP-MS analyses of zircon grains for two samples yield a metamorphic age of ∼1.82Ga, which corresponds to the ∼1.85Ga from granulite in the TNCO and was interpreted to represent the uplifting and cooling age of granulite terranes following a crustal thickening event. Thus, the Paleoproterozoic tectonic event involving activity of metabasic dykes and metamorphism occurred in the TNCO may have also extended to the Eastern Hebei region.

Amphibole reaction rims as a record of pre-eruptive magmatic heating: An experimental approach

Magmatic minerals record the pre-eruptive timescales of magma ascent and mixing in crustal reservoirs and conduits. Investigations of the mineral records of magmatic processes are fundamental to our understanding of what controls eruption style, as ascent rates and magma mixing processes are well known to control and/or trigger potentially hazardous explosive eruptions. Thus, amphibole reaction rims are often used to infer pre-eruptive magma dynamics, and in particular to estimate magma ascent rates. However, while several experimental studies have investigated amphibole destabilization during decompression, only two investigated thermal destabilization relevant to magma mixing processes. This study examines amphibole decomposition experimentally through isobaric heating of magnesio-hornblende phenocrysts within a natural high-silica andesite glass. The experiments first equilibrated for 24 h at 870 °C and 140 MPa at H2O-saturated conditions and ƒO2 ∼ Re–ReO prior to rapid heating to 880, 900, or 920 °C and hold times of 3–48 h. At 920 °C, rim thicknesses increased from 17 μm after 3 h, to 55 μm after 12 h, and became pseudomorphs after longer durations. At 900 °C, rim thicknesses increased from 7 μm after 3 h, to 80 μm after 24 h, to pseudomorphs after longer durations. At 880 °C, rim thicknesses increased from 7 μm after 3 h, to 18 μm after 36 h, to pseudomorphs after 48 h. Reaction rim microlites vary from 5–16 μm in size, with no systematic relationship between crystal size and the duration or magnitude of heating. Time-averaged rim microlite growth rates decrease steadily with increasing experimental duration (from 3.97×10−7 mms−1 to 3.1 to 3.5×10−8 mms−1). Time-averaged microlite nucleation rates also decrease with increasing experimental duration (from 1.2×103 mm−3s−1 to 5.3 mm−3 s−1). There is no systematic relationship between time-averaged growth or nucleation rates and the magnitude of the heating step. Ortho- and clinopyroxene together constitute 57–90 modal % mineralogy in each reaction rim. At constant temperature, clinopyroxene abundances decrease with increasing experimental duration, from 72 modal % (3 h at 900 °C) to 0% (48 h at 880 °C, and 36 h at 900 and 920 °C). Fe–Ti oxides increase from 6–12 modal % (after 3–6 h) to 26–34 modal % (after 36–48 h). Plagioclase occurs in relatively minor amounts (<1–11 modal %), with anorthite contents that increase from An56 to An88 from 3 to 36 h of heating. Distal glass compositions (>500 μm from reacted amphibole) are consistent with inter-microlite rim glasses (71.3–77.7 wt.% SiO2) within a given experiment and there is a weakly positive correlation between increasing run duration and inter-microlite melt SiO2 (68.9–78.5 wt.%). Our results indicate that experimental heating-induced amphibole reaction rims have thicknesses, textures, and mineralogies consistent with many of the natural reaction rims seen at arc-andesite volcanoes. They are also texturally consistent with experimental decompression reaction rims. On this basis it may be challenging to distinguish between decompression and heating mechanisms in nature.

Experimental investigation of the effect of Ca, Fe and Ti on cotectic compositions of the rhyolitic system

The effect of the normative anorthite content on the position of cotectic curves in the Qz–Ab–Or–An system has been investigated at 200 MPa and a water activity of 0.5. To simulate compositions as close as possible to those of natural high-silica rhyolites, all investigated compositions also contained ~ 1 wt% FeO and 0.2 wt% TiO 2. The position of the cotectic curves was deduced from crystallization experiments carried out between 790 and 850°C and using fourteen starting glass compositions containing ~ 3 wt% H 2 O. The liquidus phase of the different starting materials was used to constrain the primary fields of quartz, plagioclase and sanidine. The compositions of residual melts coexisting with solid phases were used to define the position of cotectic curves. Compared to the haplogranite system, the eutectic point is shifted away from the Ab apex, and its composition is estimated to be Qz 42 Ab 21 Or 37 when projected onto the haplogranite system. The implications for the estimation of the depth of magma storage conditions are discussed on the basis of an example from the Snake River Plain high-silica rhyolites.

Anatexis at the roof of an oceanic magma chamber at IODP Site 1256 (equatorial Pacific): an experimental study

Replenished axial melt lenses at fast-spreading mid-oceanic ridges may move upward and intrude into the overlying hydrothermally altered sheeted dikes, resulting in high-grade contact metamorphism with the potential to trigger anatexis in the roof rocks. Assumed products of this process are anatectic melts of felsic composition and granoblastic, two-pyroxene hornfels, representing the residue after partial melting. Integrated Ocean Drilling Program Expeditions 309, 312, and 335 at Site 1256 (eastern equatorial Pacific) sampled such a fossilized oceanic magma chamber. In this study, we simulated magma chamber roof rock anatectic processes by performing partial melting experiments using six different protoliths from the Site 1256 sheeted dike complex, spanning a lithological range from poorly to strongly altered basalts to partially or fully recrystallized granoblastic hornfels. Results show that extensively altered starting material lacking primary magmatic minerals cannot reproduce the chemistry of natural felsic rocks recovered in ridge environments, especially elements sensitive to hydrothermal alteration (e.g., K, Cl). Natural geochemical trends are reproduced through partial melting of moderately altered basalts from the lower sheeted dikes. Two-pyroxene hornfels, the assumed residue, were reproduced only at low melting degrees (<20 vol%). The overall amphibole absence in the experiments confirms the natural observation that amphibole is not produced during peak metamorphism. Comparing experimental products with the natural equivalents reveals that water activity (aH2O) was significantly reduced during anatectic processes, mainly based on lower melt aluminum oxide and lower plagioclase anorthite content at lower aH2O. High silica melt at the expected temperature (1000–1050 °C; peak thermal overprint of two-pyroxene hornfels) could only be reproduced in the experimental series performed at aH2O = 0.1.

Augite-anorthite glass-ceramics from residues of basalt quarry and ceramic wastes

Dark brown glasses were prepared from residues of basalt quarries and wastes of ceramic factories. Addition of CaF2, Cr2O3 and their mixture CaF2-Cr2O3 were used as nucleation catalysts. Generally, structures with augite and anorthite as major phases and small amount of magnetite and olivine phases were developed through the crystallization process. In the samples heat treated at 900?C the dominant phase is augite, whereas the content of anorthite usually overcomes the augite at higher temperature (1100?C). Fine to medium homogenous microstructures were detected in the prepared glass-ceramic samples. The coefficient of thermal expansion and microhardnessmeasurements of the glass-ceramic samples were from 6.16?10-6 to 8.96?10-6?C-1 (in the 20-500?C) and 5.58 to 7.16GP, respectively.

Ab initio calculations of elastic constants of plagioclase feldspars

Plagioclase feldspars comprise a large portion of the Earth’s crust and are very anisotropic, making accurate knowledge of their elastic properties important for understanding the crust’s anisotropic seismic signature. However, except for albite, existing elastic constants for plagioclase feldspars are derived from measurements that cannot resolve the triclinic symmetry. We calculate elastic constants for plagioclase end-members albite NaAlSi3O8 and anorthite CaAl2Si2O8 and intermediate andesine/labradorite NaCaAl3Si5O16 using density functional theory to compare with and improve existing elastic constants and to study trends in elasticity with changing composition. We obtain elastic constants similar to measured elastic constants and find that anisotropy decreases with anorthite content.

Microstructural and Metamorphic Constraints on the Thermal Evolution of the Southern Region of the Lewisian Gneiss Complex, NW Scotland

Felsic metagranitoids form a major part of the crust, but the metamorphic story they record is difficult to decipher because of a lack of index minerals. The microstructures and metamorphic assemblages of felsic gneisses and metadolerite dykes from the Lewisian Gneiss complex, NW Scotland, have been examined to estimate the pressure^temperature^time (P^T^t) history of the region. Characteristic geometries and compositions of zoned epidote and plagioclase from the gneisses and amphibole from the dykes provide key information. Bulk-rock compositions are modelled to constrain the likely metamorphic conditions experienced by the rocks. P^T^t paths are refined using a novel model for fractionation during grain-recycling of the plagioclase. In the gneisses, plagioclase grains have relatively albitic cores (An10^12) grading to more anorthitic rims (An20^30). The equant grain shapes of the plagioclase and asymmetry of the zoning across grain boundaries are consistent with the zoning having formed during coarsening, or grain-recycling, following deformation.The increase in anorthite content is due to the breakdown of epidote to release Ca and Al. Sharp boundaries between Fe-poor cores and Fe-rich rims in epidote result from the resorption of epidote whilst the plagioclase is growing followed by later regrowth.The possible P^Tconditions for the end of deformation and start of grain-recycling are restricted to those that occur along the plagioclase isopleth with the same value as the core compositions. These starting conditions are explored along with P^T^t path orientations over a range of values. The results are compared with the observed compositions and grain sizes to determine the best-fit P^T^t path. Most of the best-fit paths are dominated by decompression rather than heating (both of which result in epidote breakdown). Starting conditions are probably between 9 and 10·2 kbar at around 5808C along a quasi-linear path ending at around 7·5^8 kbar at 600^6208C.The timescale of decompression is poorly constrained owing to the uncertainty in the grain-recycling parameters. Rates of exhumation are between 0·14 and 2 mm a, which are reasonable within the range of present-day processes. Scourie dyke assemblages and mineral zoning broadly corroborate this P^T^t path. The path is similar to those recorded in Phanerozoic orogenic cycles but the significance of this work lies in our new methods for elucidating the metamorphic histories of metagranitoids.

Terminal particle from Stardust track 130: Probable Al-rich chondrule fragment from comet Wild 2

A 4×6μm terminal particle from Stardust track 130, named Bidi, is composed of a refractory assemblage of Fo97 olivine, Al- and Ti-bearing clinopyroxene and anorthite feldspar (An97). Mineralogically, Bidi resembles a number of components found in primitive chondritic meteorites including Al-rich chondrules, plagioclase-bearing type I ferromagnesian chondrules and amoeboid olivine aggregates (AOAs). Measured widths of augite/pigeonite lamellae in the clinopyroxene indicate fast cooling rates suggesting that Bidi is more likely to be a chondrule fragment than an AOA. Bulk element concentrations, including an Al2O3 content of 10.2wt%, further suggests that Bidi is more akin to Al-rich rather than ferromagnesian chondrules. This is supported by high anorthite content of the plagioclase feldspar, overall bulk composition and petrogenetic analysis using a cosmochemical Al2O3–Ca2SiO4–Mg2SiO4 phase diagram. Measured minor element abundances of individual minerals in Bidi generally support an Al-rich chondrule origin but are not definitive between any of the object types. Oxygen isotope ratios obtained from olivine (+minor high-Ca pyroxene)fall between the TF and CCAM lines and overlap similar minerals from chondrules in primitive chondrites but are generally distinct from pristine AOA minerals. Oxygen isotope ratios are similar to some minerals from both Al-rich and type I ferromagnesian chondrules in unequilibrated carbonaceous, enstatite and ordinary chondrites. Although no single piece of evidence uniquely identifies Bidi as a particular object type, the preponderance of data, including mineral assemblage, bulk composition, mineral chemistry, inferred cooling rates and oxygen isotope ratios, suggest that Bidi is more closely matched to Al-rich chondrules than AOAs or plagioclase-bearing type I ferromagnesian chondrules and likely originated in a chondrule-forming region in the inner solar system.