Ba- and Ti-rich oxymica from nephelinites in the Middle Atlas Volcanic Province,northern Morocco

Mineralogy and Petrology - A Ba- and Ti-rich mica (up to 14.0 wt% BaO and 13.1 wt% TiO2) occurs in nephelinites from the Middle Atlas Volcanic Province, Morocco. The rocks show a...     

Phase relations of iron–silicon alloys at high pressure and high temperature

The phase relations of Fe-6.4 wt% Si and Fe-9.9 wt% Si have been investigated up to 130 GPa and 2,600 K based on in situ synchrotron X-ray diffraction measurements in a laser-heated diamond-anvil cell along with chemical analysis of the quenched samples using a field-emission electron probe microanalyzer. We found that the maximum solubility of silicon in solid hcp-iron increases with increasing pressure. Linear extrapolation of the phase boundary between hcp + B2 and hcp phases for Fe-9.9 wt% Si suggests that the solid hcp-iron can include more than 9.9 wt% Si at the Earth’s inner-core conditions. If silicon is a major light element in the outer core, a substantial amount of silicon may be incorporated into the inner core during inner-core solidification.     

Hydrous peridotites with Ti-rich chromian spinel as a low-temperature forearc mantle facies: evidence from the Happo-O’ne metaperidotites (Japan)

The Happo-O’ne peridotite complex is situated in the northeastern part of the Hida Marginal Tectonic Zone, central Japan, characterized by the high-P/T Renge metamorphism, and is considered as a serpentinite mélange of Paleozoic age. Peridotitic rocks, being massive or foliated, have been subjected to hydration and metamorphism. Their protoliths are mostly lherzolites to harzburgites with subordinate dunites. We found a characteristic mineral assemblage, olivine + orthopyroxene + tremolite + chlorite + chromian spinel, being stable at low-T, from 650 to 750°C, and high-P, from 16 to 20 kbar, tremolite–chlorite peridotites of the tremolite zone. Olivines are Fo₈₈–Fo_91, and orthopyroxenes (Mg# = 0.91) show low and homogenous distributions of Al₂O₃ (up to 0.25 wt%), Cr₂O₃ (up to 0.25 wt%), CaO (up to 0.36 wt%) and TiO₂ (up to 0.06 wt%) due to the low equilibration temperature. Chromian spinels, which are euhedral and enclosed mainly in the orthopyroxenes, have high TiO_2, 3.1 wt% (up to 5.7 wt%) on average, and high Cr# [=Cr/(Cr + Al) atomic ratio], 0.95 on average but low Fe³⁺ [=Fe³⁺/(Cr + Al + Fe³⁺) atomic ratio, <0.3]. The bulk-rock chemistry shows that the Happo-O’ne metaperidotites with this peculiar spinel are low in TiO₂ (0.01–0.02 wt%), indicating no addition of TiO₂ from the outside source during the metamorphism; the high TiO₂ of the peculiar spinel has been accomplished by Ti release from Ti-bearing high-T pyroxenes during the formation of low-T, low-Ti silicates (<0.1 wt% TiO₂) during cooling. Some dunites are intact from hydration: their olivine is Fo₉₂ and spinel shows high Cr#, 0.72. The Happo-O’ne metaperidotites (tremolite–chlorite peridotites), being in the corner of the mantle wedge, are representative of a hydrous low-T, high-P mantle peridotite facies transitional from a higher T anhydrous peridotite facies (spinel peridotites) formed by in situ retrograde metamorphism influenced by fluids from the subducting slab. They have suffered from low-T (<600°C) retrogressive metamorphism to form antigorite and diopside during exhumation of the Renge metamorphic belt.     

The intermediate step in fractionation trends of mildly alkaline volcanic suites: An experimental insight from the Pavin trachyandesite (Massif Central,France)

We examined magma storage conditions and eruptive dynamics for the trachyandesite (~58 wt% SiO₂, 9–10 wt% alkalis) of the Pavin monogenetic volcano, a maar-like explosive crater belonging to a small group of youngest volcanoes in the Massif Central. By confronting the natural samples to experimental products, we constrained pre-eruptive conditions around 950–975 °C, 150–200 MPa (~5.5–7.0 km in depth), NNO+1.5, and 4.5–5.5 wt% melt H₂O. There is petrological evidence of magma crystallization in the conduit up to shallow levels (~50 MPa; 2 km in depth) before fragmentation into pumice clasts in the last kilometre of ascent. The experiments highlight the role of biotite and of crystallization pressure in defining separate compositional trends of residual liquids, i.e. alkaline (trachytes) versus sub-alkaline (dacite-rhyolite).     

Melting of Fe–Ni–Si and Fe–Ni–S alloys at megabar pressures: implications for the core–mantle boundary temperature

High pressure melting behavior of three Fe-alloys containing 5 wt% Ni and (1) 10 wt% Si, (2) 15 wt% Si or (3) 12 wt% S was investigated up to megabar pressures by in situ X-ray diffraction and laser-heated diamond anvil cell techniques. We observe a decrease in melting temperature with increasing Si content over the entire investigated pressure range. This trend is used to discuss the melting curve of pure Fe. Moreover, our measurements of eutectic melting in the Fe–Fe₃S system show a change in slope around 50 GPa concomitant with the fcc–hcp phase transition in pure solid iron. Extrapolations of our melting curve up to the core–mantle boundary pressure yield values of 3,600–3,750 K for the freezing temperature of plausible outer core compositions.     

Nickel enrichment in mantle olivine beneath a volcanic front

We found abnormally Ni-rich olivine (Fo = 93) with up to 5.3 wt% of NiO, ten times higher than the ordinary mantle value (0.4 wt%), in a highly metasomatized mantle peridotite xenolith from Avacha volcano, the Kamchatka arc, Russia. The Ni enrichment displays outward diffuse circular domains (<1 mm across) in fine-grained (mostly <100 μm) olivine-rich parts. Associated metasomatic orthopyroxene also shows high NiO (<1.1 wt%). Such high Ni concentrations in olivine cannot be attained in ordinary residual or cumulus peridotites, but are achievable via diffusion from Ni-rich sulfide melt. Sulfur-bearing silicic melt, the main metasomatic agent for the Avacha peridotites, separated sulfide melt, which was fractionated to be Ni-rich at relatively low temperatures. This is a new way of mobility and redistribution of Ni in the mantle, which is active in the mantle wedge, especially beneath a volcanic front.     

Melting of iron–silicon alloy up to the core–mantle boundary pressure: implications to the thermal structure of the Earth’s core

The melting temperature of Fe–18 wt% Si alloy was determined up to 119 GPa based on a change of laser heating efficiency and the texture of the recovered samples in the laser-heated diamond anvil cell experiments. We have also investigated the subsolidus phase relations of Fe–18 wt% Si alloy by the in-situ X-ray diffraction method and confirmed that the bcc phase is stable at least up to 57 GPa and high temperature. The melting curve of the alloy was fitted by the Simon’s equation, P(GPa)/a = (T _m(K)/T ₀) ^c , with parameters, T ₀ = 1,473 K, a = 3.5 ± 1.1 GPa, and c = 4.5 ± 0.4. The melting temperature of bcc Fe–18 wt% Si alloy is comparable with that of pure iron in the pressure range of this work. The melting temperature of Fe–18 wt% Si alloy is estimated to be 3,300–3,500 K at 135 GPa, and 4,000–4,200 K at around 330 GPa, which may provide the lower bound of the temperatures at the core–mantle boundary and the inner core–outer core boundary if the light element in the core is silicon.     

Bulk chemical compositions of Al-rich objects from Rumuruti (R) chondrites: Implications for their origin

R chondrites are a distinct group of chondritic meteorites with unique mineralogical and chemical compositions. They contain various types of Al-rich objects [Ca,Al-rich inclusions (CAIs), Al-rich chondrules and fragments], whose mineralogical compositions and classifications were previously determined by Rout and Bischoff (2008). Here, we report on the bulk compositions of 126 such Al-rich objects determined by broad-beam electron probe microanalysis.Most of the CAIs, except a few, are significantly altered by complex nebular and/or parent body processes and the determination of their pristine composition is difficult. We found that the simple concentric spinel-rich inclusions have high Al₂O₃ (21–72 wt%) correlated with their high modal spinel. The subgroup of simple concentric spinel-rich CAIs have a high Al₂O₃ (21–57 wt%) and also sometimes high FeO (up to 36 wt%), due to a high hercynitic component in the spinel. One simple concentric spinel- and hibonite-rich CAI H030/L, identified as a HAL-type CAI by isotopic studies reported elsewhere, has a highly refractory composition (Al₂O₃～72 wt%). Most of the simple concentric spinel- and fassaite-rich CAIs have consistently high CaO (～2.5–17 wt%) compared to other simple concentric spinel-rich inclusions group, where only some inclusions have high CaO (up to 15 wt%). Simple concentric spinel- and Na,Al-alteration product-rich CAIs are heavily altered and have high Na₂O (up to ～12.5 wt%).The three analyzed fassaite-rich spherules have high CaO and Al₂O₃, and complex spinel- and fassaite-rich CAIs have high CaO (up to 23 wt%) and SiO₂ (up to 41 wt%). Most of the complex spinel- and plagioclase-rich CAIs are altered and contain high amounts of secondary oligoclase. However, a few are less affected by secondary alteration and these are characterized by relatively high CaO (up to 24 wt%) and Al₂O₃ (18–33 wt%); complex spinel and Na,Al-alteration product-rich CAIs are similar to the concentric spinel- and Na,Al-alteration product-rich CAIs. Due to Fe- and alkali-metasomatism, the vast majority of the inclusions in this subgroup were heavily altered, either in a nebular or parent body environment. As a result of this alteration, they contain high FeO and Na₂O+K₂O+Cl.Almost all inclusions have a Ca/Al-ratio below the solar ratio. This suggests that significant Ca/Al fractionation occurred during the formation of most CAIs, most probably due to disequilibrium condensation of spinel prior to melilite. However, a distillation process cannot be ruled out for some CAIs in producing the spinel enrichment. Some porous and fine grained CAIs may have been produced by agglomeration of refractory dust rich in spinel and fassaite. The HAL-type CAI, H030/L, most likely formed by distillation, similar to most of the HAL-type inclusions. Phase equilibrium analysis, in the CMAS system, shows that the fassaite-bearing spherules most likely formed by metastable crystallization and disequilibrium processes. Al-rich chondrules are characterized by >10 wt% Al₂O₃, and most of these also have high FeO and Na₂O. Considering their bulk compositions, their precursors seem to have been a mixture of a ferromagnesian chondrule component rich in olivine and an anorthite–spinel–pyroxene–nepheline-rich CAI component. The mineral assemblages of some of the less altered Al-rich chondrules conform to those predicted by phase equilibrium studies.     

Phase relations and formation of sodium-rich majoritic garnet in the system Mg3Al2Si3O12–Na2MgSi5O12 at 7.0 and 8.5 GPa

The pseudo-binary system Mg₃Al₂Si₃O₁₂–Na₂MgSi₅O₁₂ modelling the sodium-bearing garnet solid solutions has been studied at 7 and 8.5 GPa and 1,500–1,950°C. The Na-bearing garnet is a liquidus phase of the system up to 60 mol% Na₂MgSi₅O₁₂ (NaGrt). At higher content of NaGrt in the system, enstatite (up to ∼80 mol%) and then coesite are observed as liquidus phases. Our experiments provided evidence for a stable sodium incorporation in garnet (0.3–0.6 wt% Na₂O) and its control by temperature and pressure. The highest sodium contents were obtained in experiments at P = 8.5 GPa. Near the liquidus (T = 1,840°C), the equilibrium concentration of Na₂O in garnet is 0.7–0.8 wt% (∼6 mol% Na₂MgSi₅O₁₂). With the temperature decrease, Na concentration in Grt increases, and the maximal Na₂MgSi₅O₁₂ content of ∼12 mol% (1.52 wt% Na₂O) is gained at the solidus of the system (T = 1,760°С). The data obtained show that most of natural diamonds, with inclusions of Na-bearing garnets usually containing <0.4 wt% Na₂O, could be formed from sodium-rich melts at pressures lower than 7 GPa. Majoritic garnets with higher sodium concentrations (>1 wt% Na₂O) may crystallize at a pressure range of 7.0–8.5 GPa. However the upper pressure limit for the formation of naturally occurring Na-bearing garnets is restricted by the eclogite/garnetite bulk composition.     

Mineralogical and geochemical investigations of pyrite-rich mine waste from a kyanite mine in Central Virginia with comments on recycling

A pyrite-rich waste stream is one of three types generated from a kyanite mine in central Virginia near the town of Dillwyn, Buckingham County. Currently, ore consists of approximately 3% pyrite waste and an estimated 382,000 tons of this waste stream has been generated over the past 60 years. The mineralogy of the waste stream consists of variable amounts of pyrite (70–>99%), talc (1–20%), quartz (1–10%), kyanite (0.5–5%) with minor or trace amounts of magnetite, hematite, galena, anorthite and rare earth phosphate. Energy dispersive spectroscopy analysis indicates that talc has minor amounts of Al up to 1.57 wt% and Fe up to 4.29 wt% and pyrite grains have no impurities above detection limit of approximately 0.1 wt%. Bulk chemical analysis of selected elements using inductively coupled plasma-mass spectrometry analysis indicate that Zn (28.82–367.71 ppm), As (8.94–18.26 ppm), Se (44.62–64.50 ppm), Cd (0.19–1.03 ppm), Hg (0.87–35.91 ppm), and Pb (65.10–189.66 ppm) occur at levels of some environmental concern. Au and Ag concentrations are negligible. Currently the waste stream is well managed and sold, but for a low price. Talc is of sufficient quality to be of interest for recycling but the estimated 540 tons generated per year is not a suitable quantity to be economically viable. Currently, the waste stream is not viable for recycling for higher monetary value; however, the characteristics of the pyrite may enable such recycling in the future for solar energy technologies. This and other associated waste streams show long term promise for integrated recycling and may play important economic roles in an economically disadvantaged region.     

An assessment of Portland cement,cement kiln dust and Class C fly ash for the immobilization of Zn in contaminated soils

Zn-contaminated soils obtained from a steel company in the Republic of Korea were stabilized using Portland cement (PC), cement kiln dust (CKD) and Class C fly ash (FA). The effectiveness of the treatment was evaluated by the United States Environmental Protection Agency toxicity characteristic leaching procedure (TCLP) and the Universal Treatment Standard (UTS) of 4.3 mg/L. X-ray powder diffraction (XRPD) analyses were performed to investigate the crystalline phases associated with Zn immobilization. Scanning electron microscopy (SEM)–energy dispersive X-ray (EDX) analyses were utilized to support the XRPD results. The treatment results showed that the TCLP-Zn concentrations obtained from the 10 wt% PC and 15 wt% CKD treated samples were less than the UTS, after 7 days curing. However, the FA treatment (up to 30 wt%) was not effective in meeting the UTS even after 28 days curing. All PC–CKD treatment combinations were effective in reducing the TCLP-Zn concentrations below the UTS criteria. Moreover, a 20 wt% dose of a PC-FA treatment combination (75/25 PC-FA) was successful in reducing the TCLP-Zn concentrations below 4.3 mg/L after 1 day. The XRPD results showed that ettringite and Zn₆Al₂(OH)₁₆CO₃·4H₂O were the possible phases associated with Zn immobilization upon PC and CKD treatment. The SEM–EDX results confirmed the presence of ettringite, while Zn₆Al₂(OH)₁₆CO₃·4H₂O was not identified.     

Mineralogical and geochemical investigations of silicate-rich mine waste from a kyanite mine in central Virginia: implications for mine waste recycling

A kyanite mine in central Virginia produces a silicate-rich waste stream which accumulates at a rate of 450,000–600,000 tons per year. An estimated 27 million tons of this waste stream has accumulated over the past 60 years. Grain size distribution varies between 1.000 and 0.053 mm, and is commonly bimodal with modes typically being 0.425 and 0.250 mm and uniformity coefficients vary from 2.000 to 2.333. Hydraulic conductivity values vary from 0.017 to 0.047 cm/s. Mineralogy of the waste stream consists of quartz, muscovite, kyanite and hematite. Muscovite grains have distinct chemical compositions with significant Na₂O content (1.12–2.66 wt%), TiO₂ content (0.63–1.68 wt% TiO₂) and Fe content, expressed as Fe₂O₃ (up to 1.37 wt%). Major element compositions of samples were dominated by SiO₂ (87.894–90.997 wt%), Al₂O₃ (6.759–7.741 wt%), Fe₂O₃ (1.136–1.283 wt%), and K₂O (0.369–0.606 wt%) with other components being <1.000 wt%. Elements of environmental concern (V, Cr, Ni, Cu, Zn, As, Ag, Sn, Sb, Ba, Hg, Tl, and Pb) were detected; however, the concentrations of all elements except Ni were below that of the kyanite quartzites in the region from which the waste is derived. Both major and trace element compositions indicate minimal variation in composition. The waste stream has potential for recycling. Muscovite is suitable for recycling as a paint pigment or other industrial applications. Muscovite and hematite are commonly intergrown and are interpreted to be material where much of the elements of environmental concern are concentrated. Reprocessing of the waste stream to separate muscovite from other components may enable the waste stream to be used as constructed wetland media for Virginia and nearby states. Recycling of this mine waste may have a positive impact on the local economy of Buckingham County and aid in mitigation of wetland loss.     

Dissolution-precipitation reactions controlling fast formation of dolomite under hydrothermal conditions

In laboratory experiments, the precipitation of dolomite at ambient temperature is virtually impossible due to strong solvation shells of magnesium ions in aqueous media and probably also due to the existence of a more intrinsic crystallization barrier that prevents the formation of long-range ordered crystallographic structures at ambient surface conditions. Conversely, dolomite can easily form at high temperature (>100 °C), but its precipitation and growth requires several days or weeks depending on experimental conditions. In the present study, experiments were performed to assess how a single heat-ageing step promotes the formation of dolomite under high-carbonate alkaline conditions via dissolution-precipitation reactions. This reaction pathway is relevant for the so-called hydrothermal dolomite frequently observed in carbonate platforms, but still ill-defined and understood. Our precipitation route is summarized by two main sequential reactions: (1) precipitation of Mg-calcite at low temperature (∼20 °C) by aqueous carbonation of synthetic portlandite (Ca(OH)₂) in a highly alkaline medium (1 M of NaOH and 1 M of MgCl₂), leading to precipitation of oriented nanoparticles of low- and high-Mg calcite (∼79 wt%) coexisting with aragonite (∼18 wt%) and brucite (∼3 wt%) after 24 h; (2) fast dolomitization process starting from 1 h of reaction by a single heat-ageing step from ∼20 to 200, 250 and 300 °C. Here, the Mg-calcite acts as a precursor that lowers the overall kinetics barrier for dolomite formation. Moreover, it is an important component in some bio-minerals (e.g. corals and seashells). Quantitative Rietveld refinements of XRD patterns, FESEM observations and FTIR measurements on the sequentially collected samples suggest fast dolomite precipitation coupled with dissolution of transient mineral phases such as low-Mg calcite (Mg < 4 mol%), high-Mg calcite (Mg > 4 mol%), proto-dolomite (or disordered dolomite; Mg > 40 mol%) and Ca-magnesite. In this case, the dolomite formation rate and the time-dependent mineral composition strongly depend on reaction temperature. For example, high-purity dolomitic material (87 wt% of dolomite mixed with 13 wt% of magnesite) was obtained at 300 °C after 48 h of reaction. Conversely, a lower proportion of dolomite (37 wt%), mixed with proto-dolomite (43 wt%), Ca-magnesite (16 wt%) and high-Mg calcite (4 wt%), was obtained at 200 °C after 72 h. The present experiments provide an additional mechanism for the massive dolomite formation in sedimentary environments (ex. deep sea organic-rich carbonate-sediments) if such sediments are subjected to significant temperature variations, for example by hot fluid circulations related to volcanic activity. In such systems, organic degradation increases the carbonate alkalinity (HCO₃⁻) necessary to induce the dolomitization process at low and high temperature.     

Mineralogy and geochemistry of laterites from the Morro dos Seis Lagos Nb (Ti,REE) deposit (Amazonas,Brazil)

The Morro dos Seis Lagos niobium deposit (2897.9 Mt at 2.81 wt% Nb₂O₅) is associated with laterites formed by the weathering of siderite carbonatite. This iron-rich lateritic profile (>100 m in thickness) is divided into six textural and compositional types, which from the top to the base of the sequence is: (1) pisolitic laterite, (2) fragmented laterite, (3) mottled laterite, (4) purple laterite, (5) manganiferous laterite, and (6) brown laterite. All the laterites are composed mainly of goethite (predominant in the lower and upper varieties) and hematite (predominant in the intermediate types, formed from goethite dehydroxylation). The upper laterites were reworked, resulting in goethite formation. In the manganiferous laterite (10 m thick), the manganese oxides (mainly hollandite, with associated cerianite) occur as veins or irregular masses, formed in a late event during the development of the lateritic profile, precipitated from a solution with higher oxidation potential than that for Fe oxides, closer to the water table. Siderite is the source for the Mn. The main Nb ore mineral is Nb-rich rutile (with 11.26–22.23 wt% Nb₂O₅), which occurs in all of the laterites and formed at expense of a former secondary pyrochlore, together with Ce-pyrochlore (last pyrochore before final breakdown), Nb-rich goethite and minor cerianite. The paragenesis results of lateritization have been extremely intense. Minor Nb-rich brookite formed from Nb-rich rutile occurs as broken spherules with an “oolitic” (or Liesegang ring structure). Nb-rich rutile and Nb-rich brookite incorporate Nb following the [Fe³⁺ + (Nb, Ta) for 2Ti] substitution and both contain up to 2 wt% WO₃. The laterites have an average Nb₂O₅ content of 2.91 wt% and average TiO₂ 5.00 wt% in the upper parts of the sequence. Average CeO₂ concentration increases with increasing depth, from 0.12 wt% in the pisolitic type to 3.50 wt% in the brown laterite. HREE concentration is very low.     

Stability and crystal chemistry of iron-bearing dense hydrous magnesium silicates

Dense hydrous magnesium silicates (DHMS) are supposed to be key phases in planetary water cycles because of their ability to carry water to deep mantle regions in subduction slab environments. In order to understand water cycles in iron-enriched planetary systems such as Mars knowledge of the water content and stability of iron-bearing DHMS is required. Iron-bearing DHMS were synthesized based on two starting compositions, MgFeSiO₄ + 9.5 wt% H₂O system and a simple hydrous Martian mantle composition containing Fe, Mg, Al and Si + 12.35 wt% H₂O (hydrous FMAS system). Compared to literature data on phase D, iron-bearing phase D shows analogous variations in water contents as Mg-phase D but appears to be stable at higher temperatures than Mg-phase D for both starting compositions used in this study. Iron-bearing superhydrous phase B contains up to 7 wt% H₂O and shows an extended thermal stability in the hydrous FMAS system. The high-temperature stability of iron-bearing DHMS with a Mars-like bulk composition indicates that these hydrous phases could host significant amounts of water at core-mantle boundary conditions (1500 °C and 23 GPa) in a hydrous Martian mantle.     

Revisiting the compositions and volatile contents of olivine-hosted melt inclusions from the Mount Shasta region: implications for the formation of high-Mg andesites

Primitive chemical characteristics of high-Mg andesites (HMA) suggest equilibration with mantle wedge peridotite, and they may form through either shallow, wet partial melting of the mantle or re-equilibration of slab melts migrating through the wedge. We have re-examined a well-studied example of HMA from near Mt. Shasta, CA, because petrographic evidence for magma mixing has stimulated a recent debate over whether HMA magmas have a mantle origin. We examined naturally quenched, glassy, olivine-hosted (Fo_87–94) melt inclusions from this locality and analyzed the samples by FTIR, LA-ICPMS, and electron probe. Compositions (uncorrected for post-entrapment modification) are highly variable and can be divided into high-CaO (>10 wt%) melts only found in Fo > 91 olivines and low-CaO (<10 wt%) melts in Fo 87–94 olivine hosts. There is evidence for extensive post-entrapment modification in many inclusions. High-CaO inclusions experienced 1.4–3.5 wt% FeO^T loss through diffusive re-equilibration with the host olivine and 13–28 wt% post-entrapment olivine crystallization. Low-CaO inclusions experienced 1–16 wt% olivine crystallization with <2 wt% FeO^T loss experienced by inclusions in Fo > 90 olivines. Restored low-CaO melt inclusions are HMAs (57–61 wt% SiO₂; 4.9–10.9 wt% MgO), whereas high-CaO inclusions are primitive basaltic andesites (PBA) (51–56 wt% SiO₂; 9.8–15.1 wt% MgO). HMA and PBA inclusions have distinct trace element characteristics. Importantly, both types of inclusions are volatile-rich, with maximum values in HMA and PBA melt inclusions of 3.5 and 5.6 wt% H₂O, 830 and 2,900 ppm S, 1,590 and 2,580 ppm Cl, and 500 and 820 ppm CO₂, respectively. PBA melts are comparable to experimental hydrous melts in equilibrium with harzburgite. Two-component mixing between PBA and dacitic magma (59:41) is able to produce a primitive HMA composition, but the predicted mixture shows some small but significant major and trace element discrepancies from published whole-rock analyses from the Shasta locality. An alternative model that involves incorporation of xenocrysts (high-Mg olivine from PBA and pyroxenes from dacite) into a primary (mantle-derived) HMA magma can explain the phenocryst and melt inclusion compositions but is difficult to evaluate quantitatively because of the complex crystal populations. Our results suggest that a spectrum of mantle-derived melts, including both PBA and HMA, may be produced beneath the Shasta region. Compositional similarities between Shasta parental melts and boninites imply similar magma generation processes related to the presence of refractory harzburgite in the shallow mantle.     

Ultra-depleted peridotite xenoliths in the Northern Taihang Mountains: Implications for the nature of the lithospheric mantle beneath the North China Craton

We report the finding of peridotite xenoliths in the Early Cretaceous Longmengou olivine-bearing diabase (138 Ma) in the Northern Taihang Mountains in the central North China Craton. Based on the modal proportions of olivine, clinopyroxene, amphibole and anorthite, these peridotite xenoliths can be divided into three zones: clinopyroxene-bearing olivine zone (COZ), olivine-clinopyroxene zone (OCZ), and amphibole-bearing anorthite-clinopyroxene zone (AACZ). The core of olivine grains in clinopyroxene-bearing olivine zone have higher Mg^# (> 95), SiO₂ (41.80–42.53 wt%) and lower CaO (< 0.07 wt%), FeO (3.91–4.54 wt%) than the rim (Mg^# = 92.5–93.4, SiO₂ = 41.27–41.98 wt%, CaO = 0.20–0.34 wt%, and FeO = 7.02–8.87 wt%), suggesting that rim is reaction product. The core of olivine grains with higher Mg^# (> 95) and lower NiO content (< 0.04 wt%) in the clinopyroxene-bearing olivine zone was derived from ultra-depleted mantle subsequently altered by high Mg^# melts/magma with low Ni. Two generations of olivine grains occur in the OCZ where the first generation shows exsolution of ilmenite and magnetite rods containing up to 0.35 wt% TiO₂, and was likely derived from garnet peridotite hydrated by water. The second generation shows high Mg^# (96.2–97.1) and cataclastic texture, and was possibly formed by decomposition of the COZ. The occurrence of aluminous spinel suggests the role of melts with extremely high Al and Mg. Clinopyroxene in the AACZ shows systematic core-rim compositional variation with CaO and SiO₂ contents increasing towards the rim, and MgO and Fe₂O₃ concentrations decreasing from the core to the rim, indicating that the amphibole-bearing anorthite-clinopyroxene zone is a product of the reaction between mantle xenoliths and mafic magma. Plagioclase with high An value (92.0–99.95, average 97.79) indicates that the metasomatic melts have high Ca/Na and Al/Si ratios, possibly produced by the partial melting of ultra-depleted mantle under “wet” conditions. Combined with the data on other mantle xenoliths discovered in the NCC, our results suggest that the Mesozoic lithospheric mantle beneath the North Taihang Mountains within the central NCC is composed of ultra-depleted Archean and Paleoproterozoic peridotites and dunites modified by complex melts. We also propose that the destruction of eastern part of the NCC mainly occurred during Early Cretaceous, and that the boundary of the lithospheric destruction coincides with the Taihang Mountains.     

S-rich apatite-hosted glass inclusions in xenoliths from La Palma: constraints on the volatile partitioning in evolved alkaline magmas

The composition of S-rich apatite, of volatile-rich glass inclusions in apatite, and of interstitial glasses in alkaline xenoliths from the 1949 basanite eruption in La Palma has been investigated to constrain the partitioning of volatiles between apatite and alkali-rich melts. The xenoliths are interpreted as cumulates from alkaline La Palma magmas. Apatite contains up to 0.89 wt% SO₃ (3560 ppm S), 0.31 wt% Cl, and 0.66 wt% Ce₂O₃. Sulfur is incorporated in apatite via several independent exchange reactions involving (P⁵⁺, Ca²⁺) vs. (S⁶⁺, Si⁴⁺, Na⁺, and Ce³⁺). The concentration of halogens in phonolitic to trachytic glasses ranges from 0.15 to 0.44 wt% for Cl and from <0.07 to 0.65 wt% for F. The sulfur concentration in the glasses ranges from 0.06 to 0.23 wt% SO₃ (sulfate-saturated systems). The chlorine partition coefficients (D_Cl^{apatite/glass}) range from 0.4 to 1.3 (average D_Cl^{apatite/glass} = 0.8), in good agreement with the results of experimental data in mafic and rhyolitic system with low Cl concentrations. With increasing F in glass inclusions D_F^{apatite/glass} decreases from 35 to 3. However, most of our data display a high partition coefficient (~30) close to D_F^{apatite/glass} determined experimentally in felsic rock. D_S^{apatite/glass} decreases from 9.1 to 2.9 with increasing SO₃ in glass inclusions. The combination of natural and experimental data reveals that the S partition coefficient tends toward a value of 2 for high S content in the glass (>0.2 wt% SO₃). D_S^{apatite/glass} is only slightly dependent on the melt composition and can be expressed as: SO_{3 apatite} (wt%) = 0.157 * ln SO_{3 glass} (wt%) + 0.9834. The phonolitic compositions of glass inclusions in amphibole and haüyne are very similar to evolved melts erupted on La Palma. The lower sulfur content and the higher Cl content in the phonolitic melt compared to basaltic magmas erupted in La Palma suggest that during magma evolution the crystallization of haüyne and pyrrhotite probably buffered the sulfur content of the melt, whereas the evolution of Cl concentration reflects an incompatible behavior. Trachytic compositions similar to those of the (water-rich) glass inclusions analyzed in apatite and clinopyroxene are not found as erupted products. These compositions are interpreted to be formed by the reaction between water-rich phonolitic melt and peridotite wall-rock.     

Garnet-bearing tonalitic porphyry from East Kunlun,Northeast Tibetan Plateau: implications for adakite and magmas from the MASH Zone

A garnet-bearing tonalitic porphyry from the Achiq Kol area, northeast Tibetan Plateau has been dated by SHRIMP U-Pb zircon techniques and gives a Late Triassic age of 213 ± 3 Ma. The porphyry contains phenocrysts of Ca-rich, Mn-poor garnet (CaO > 5 wt%; MnO < 3 wt%), Al-rich hornblende (Al₂O₃ ~ 15.9 wt%), plagioclase and quartz, and pressure estimates for hornblende enclosing the garnet phenocrysts yield values of 8–10 kbar, indicating a minimum pressure for the garnet. The rock has SiO₂ of 60–63 wt%, low MgO (<2.0 wt%), K₂O (<1.3 wt%), but high Al₂O₃ (>17 wt%) contents, and is metaluminous to slightly peraluminous (ACNK = 0.89–1.05). The rock samples are enriched in LILE and LREE but depleted in Nb and Ti, showing typical features of subduction-related magmas. The relatively high Sr/Y (~38) ratios and low HREE (Yb < 0.8 ppm) contents suggest that garnet is a residual phase, while suppressed crystallization of plagioclase and lack of negative Eu anomalies indicate a high water fugacity in the magma. Nd–Sr isotope compositions of the rock (εNd_T = −1.38 to −2.33; ⁸⁷Sr/⁸⁶Sr_i = 0.7065–0.7067) suggest that both mantle- and crust-derived materials were involved in the petrogenesis, which is consistent with the reverse compositional zoning of plagioclase, interpreted to indicate magma mixing. Both garnet phenocrysts and their ilmenite inclusions contain low MgO contents which, in combination with the oxygen isotope composition of garnet separates (+6.23‰), suggests that these minerals formed in a lower crust-derived felsic melt probably in the MASH zone. Although the rock samples are similar to adakitic rocks in many aspects, their moderate Sr contents (<260 ppm) and La/Yb ratios (mostly 16–21) are significantly lower than those of adakitic rocks. Because of high partition coefficients for Sr and LREE, fractionation of apatite at an early stage in the evolution of the magma may have effectively decreased both Sr and LREE in the residual melt. It is suggested that extensive crystallization of apatite as an early phase may prevent some arc magmas from evolving into adakitic rocks even under high water fugacity.     

Mn-oxides and sequestration of heavy metals in a suburban catchment basin of the Chesapeake Bay watershed

The Chesapeake Bay is greatly impacted by numerous pollutants including heavy metals and understanding the controls on the distribution of heavy metals in the watershed is critical to mitigation and remediation efforts in controlling this type of pollution. Clasts from a stormwater catchment basin draining a subdivision near George Mason University, Fairfax VA (38°50.090°N 78°19.204°W) were investigated using X-ray diffraction (XRD), Scanning electron microcopy (SEM) and energy dispersive spectroscopy (EDS) to determine the nature of Mn-oxide coatings and relationship to bound heavy metals. Mn-oxides are poorly crystalline and occur as subhedral to anhedral platy particles and more rarely as euhedral plates. Micronodules are a commonly observed texture. Chemical compositions of coatings are variable with average major constituent concentrations being Mn (33.38 wt%), Fe (11.88 wt%), Si (7.33 wt%), Al (5.03 wt%), and Ba (0.90 wt%). Heavy metals are found in the coatings with Zn being most prevalent, occurring in approximately 58% of analyses with an average concentration of (0.66 wt%). Minor amounts of Co, Ni, Pb, and Cl are observed. Heavy metals and Cl are interpreted as being derived from road pollution. Mn-oxides can serve as a sequestration mechanism for pollution but may also release heavy metals. Field and laboratory observations indicate Mn-oxides occurring on the surface of the clasts can be mechanically mobilized. This is a mechanism for transporting heavy metals into the Chesapeake Bay watershed. Deicing agents may serve as a mechanism to release heavy metals through cation exchange and increased ionic strength. This is the first detailed mineralogical investigation of Mn-oxides and the roles they may play in pollution in the Chesapeake Bay.