Mineralogy and geochemistry of granitoids from Kinnaur region,Himachal Higher Himalaya,India: Implication on the nature of felsic magmatism in the collision tectonics

Felsic magmatism in the southern part of Himachal Higher Himalaya is constituted by Neoproterozoic granite gneiss (GGn), Early Palaeozoic granitoids (EPG) and Tertiary tourmaline-bearing leucogranite (TLg). Magnetic susceptibility values (<3 ×10^?3 SI), molar Al₂ O ₃/(CaO + Na₂ O + K ₂O) (≥1.1), mineral assemblage (bt–ms–pl–kf–qtz ± tur ± ap), and the presence of normative corundum relate these granitoids to peraluminous S-type, ilmenite series (reduced type) granites formed in a syncollisional tectonic setting. Plagioclase from GGn (An₁₀–An₃₁) and EPG (An₁₅–An₃₃) represents oligoclase to andesine and TLg (An₂–An₁₅) represents albite to oligoclase, whereas compositional ranges of K-feldspar are more-or-less similar (Or₈₈ to Or₉₅ in GGn, Or₈₆ to Or₉₇ in EPG and Or₈₇ to Or₉₄ in TLg). Biotites in GGn (Mg/Mg + Fe^t= 0.34–0.45), EPG (Mg/Mg + Fe^t= 0.27–0.47), and TLg (Mg/Mg + Fe^t= 0.25–0.30) are ferribiotites enriched in siderophyllite, which stabilised between FMQ and HM buffers and are characterised by dominant 3Fe\(\rightleftharpoons \)2Al, 3Mg\(\rightleftharpoons \)2Al substitutions typical of peraluminous (S-type), reducing felsic melts. Muscovite in GGn (Mg/Mg + Fe^t=0.58–0.66), EPG (Mg/Mg + Fe^t=0.31?0.59), and TLg (Mg/Mg + Fe^t=0.29–0.42) represent celadonite and paragonite solid solutions, and the tourmaline from EPG and TLg belongs to the schorl-elbaite series, which are characteristics of peraluminous, Li-poor, biotite-tourmaline granites. Geochemical features reveal that the GGn and EPG precursor melts were most likely derived from melting of biotite-rich metapelite and metagraywacke sources, whereas TLg melt appears to have formed from biotite-muscovite rich metapelite and metagraywacke sources. Major and trace elements modelling suggest that the GGn, EPG and TLg parental melts have experienced low degrees (～13, ～17 and ～13%, respectively) of kf–pl–bt fractionation, respectively, subsequent to partial melting. The GGn and EPG melts are the results of a pre-Himalayan, syn-collisional Pan-African felsic magmatic event, whereas the TLg is a magmatic product of Himalayan collision tectonics.     

Tourmaline in the Vetka porphyry copper-molybdenum deposit of the Chukchi Peninsula of Russia

Three generations of tourmaline have been identified in propylite in the Vetka porphyry copper-molybdenum deposit of the Chukchi Peninsula of Russia. Tourmaline-I is characterized by its Fe_tot/(Fe_tot + Mg) value, which ranges from 0.33 to 0.49. Tourmaline-II, which crystallizes at a lower temperature, overgrowing tourmaline-I or occurring as isolated crystals, is distinguished by a higher Fe_tot/(Fe_tot + Mg), which varies from 0.46 to 0.72. The Fe_tot/(Fe_tot + Mg) ratio in tourmaline-III, which overgrows tourmaline-II is lower (0.35–0.49), and is identical to that of the first tourmaline generation. This is probably caused by the beginning of sulfide deposition. Tourmalines in the deposit characterized by complex isomorphic substitutions can be attributed to the intermediate members of the dravite—“hydroxy-uvite”-“oxy-uvite” and schorl-“hydroxy-feruvite”-“oxy-feruvite” series. Tourmaline starts to crystallize at temperatures above 340°C. The fluid responsible for the tourmaline deposition was magmatic, with a significant admixture of meteoric water (δ¹⁸O_{H 2}O = −0.85 to −0.75‰). The high Fe³⁺/Fe_tot ratio (0.50) indicates high oxygen activity when the tourmaline precipitated. It has been established that the isomorphic substitution Fe_tot → Al is typomorphic of tourmalines from porphyry copper deposits worldwide.     

Stable isotope compositions of tourmalines from granites and related hydrothermal rocks of the Karagwe-Ankolean belt,northwest Tanzania

Tourmaline is a ubiquitous mineral in the Mid-Proterozoic, peraluminous, syn- to post-tectonic granites and aplites and the related hydrothermal rocks of the Karagwe-Ankolean belt in northwest Tanzania. Electron microprobe analysis indicates that tourmalines from all of the intrusive and hydrothermal lithologies: (1) belong to the schorl-dravite solid-solution series; and (2) plot within the field occupied by tourmaline from Li-poor granitoids on the Fe-Al-Mg classification diagram. Oxygen isotope compositions range from +12.2 to +11.6‰ (SMOW) for magmatic tourmalines and from +10.8 to +9.8‰ for those of hydrothermal origin. Hydrogen isotope compositions vary from −79 to − 65‰ (SMOW) for magmatic tourmalines and from −99 to −84‰ for hydrothermal tourmalines. Water contents measured by manometry are constant at 3.0–3.2 wt.%. Within the broad grouping there arc systematic variations in both chemical [particularly Fe_tot/(Fe_tot + Mg ratio)] and isotopic composition that relate to evolving magmatic and hydrothermal conditions. Igneous differentiation [increasing Fe_tot/(Fe_tot + Mg) in magmatic tourmaline] has produced trends with higher δ¹⁸O in quartz, lower δ¹⁸O in tourmaline, and larger Δ_QTZ.−TOUR.-values, that reflect a combination of a reduction of crystallization temperature and an increase of Fe_tot/ (Fe_tot + Mg) in the residual melt. Subsequent cooling and interaction of an exsolved, B-rich magmatic fluid with the pelitic country rocks, resulted in the deposition of hydrothermal tourmaline with increasing Fe_tot/(Fe_tot + Mg) ratios, and progressively lower δ¹⁸O and δD -values.     

Tourmaline from deposits of the Birgil’da-Tomino ore cluster, South Urals

Tourmalines from the Kalinovka porphyry copper deposit with epithermal bismuth-gold-basemetal mineralization and the Michurino gold-silver-base-metal prospect have been studied in the South Urals. Tourmaline from the Kalinovka deposit occurs as pockets and veinlets in quartz-sericite metasomatic rock and propylite. The early schorl-“oxy-schorl” [Fe_tot/(Fe_tot + Mg) = 0.66?0.81] enriched in Fe³⁺ is characterized by the homovalent isomorphic substitution of Fe³⁺ for Al typical of propylites at porphyry copper deposits. The overgrowing tourmalines of the second and third generations from propylite and quartz-sericite metasomatic rock are intermediate members of the dravite-magnesio-foitite solid solution series [Fe_tot/(Fe_tot + Mg) = 0.05?0.46] with homovalent substitution of Mg for Fe²⁺ and coupled substitution of ^X _? + ^YAl for ^XNa + ^YMg. These substitutions differ from the coupled substitution of ^YAl + ^WO^2? for ^YFe²⁺ + ^WOH^? in tourmaline from quartz-sericite rocks at porphyry copper deposits. At the Michurino prospect, the tourmaline hosted in the chlorite-pyrite-quartz veins and veinlets with Ag-Au-Cu-Pb-Zn mineralization is an intermediate member of the dravite-magnesio-foitite solid solution series [Fe_tot/(Fe_tot + Mg) = 0.20?0.31] with homovalent substitution of Mg for Fe²⁺ and coupled substitutions of ^X _? + ^YAl for ^XNa + ^YMg identical to that of late tourmaline at the Kalinovka deposit. Thus, tourmalines of the porphyry and epithermal stages are different in isomorphic substitutions, which allow us to consider tourmaline as an indicator of super- or juxtaposed mineralization.     

Solid solutions of (Mg,Fe<Superscript>2+</Superscript>)-cordierite: Synthesis,water content,and magnetic properties

The dependence of water concentration in synthetic (Mg, Fe²⁺)-cordierite on the composition of the solid solution was examined in experiments that lasted for 10 days at = 200–230 MPa, t = 600–700°C, and oxygen fugacity corresponding to the Fe-FeO buffer. Mass spectrometric data indicate that the dependence of water concentration in cordierite on its Fe mole fraction Fe²⁺/(Fe²⁺ + Mg) has maxima at compositions with F = 0.2–0.3. IR diffuse reflectance spectroscopic data and data on the structural setting of H₂O molecules in the structural channels of alkali-free (Mg, Fe²⁺)-cordierite indicate that the H-H vector of some H₂O molecules (H₂O-II) is perpendicular to [001] of the crystal. The dependence of the magnetic properties of synthetic (Mg, Fe²⁺)-cordierite was studied by static magnetization technique at 5–300 K in an external magnetic field up to 20 kOe in strength.     

Temporal, Spatial and Geochemical Discriminations of Granitoids in South Korea

Abstract: Five groups of the Phanerozoic granitoids in South Korea can be deduced from their temporal and spatial distributions: (1) Jurassic granitoids in the Gyeonggi massif, (2) Permo-Jurassic granitoids in the Ogcheon belt, (3) Permo-Jurassic granitoids in the Yeongnam massif, (4) Cretaceous granitoids in the Ogcheon belt, and (5) Cretaceous granitoids in the Gyeongsang basin. Though the granitoids of each group generally show calc-alkaline and orogenic natures, the petrological, geochemical and genetical features are different with each other. The Permo-Jurassic granitoids in the Ogcheon belt have lower contents of Al₂O₃, Fe₂O₃, CaO, P₂O₅, but higher of FeO, FeOT, MgO, K₂O than those in the Yeongnam massif. From higher feature of K₂O, Na₂O+K₂O and K₂O/Na₂O, the Ogcheon belt seems to have been located at closer continent side relative to the Yeongnam massif during Permo-Jurassic time. From lower values of Fe₂O₃/FeO and magnetic susceptibility the granitoids of the Ogcheon belt had been solidified under more reducing environment than those of the Yeongnam massif. The Cretaceous granitoids in the Ogcheon belt have lower contents of TiO₂, Fe₂O₃, FeO, FeOT, CaO and P₂O₅, but higher of MgO, K₂O, Na₂O+K₂O and K₂O/Na₂O than those in the Gyeongsang basin. This feature indicates that the Ogcheon belt would correspond to the continental environment of magma genesis during Cretaceous time. Higher values of Fe₂O₃/FeO and magnetic susceptibility in the Cretaceous granitoids in the Gyeongsang basin suggest that the granitoids had been solidified under highly oxidizing environment. From the particular chemical features of K₂O, Na₂O+K₂O and K₂O/Na₂O, the Permo-Jurassic granitoids in the Ogcheon belt, the Yeongnam massif as well as the Cretaceous ones in the Gyeongsang basin can be categorized to the continental margin type granite. The Jurassic granitoids in the Gyeonggi massif are possibly of collision type, and the Cretaceous granitoids in the Ogcheon belt of post–orogenic, intra–conti–nent type. The Jurassic granitoids in the Gyeonggi massif had been possibly generated by crustal melting during the collision of Gyeonggi massif to the northern Pyeongnam basin block. The Cretaceous granitoids in the Ogcheon belt had been emplaced at the hinterland of the continental margin during post-orogenic stage of the Honam Shear Zone. The Cretaceous granitoids in the Gyeongsang basin are often compared with Japanese Cretaceous?Paleogene granitoids in their geochemical and genetical natures. For the granitoid composition, the granitoids in the Gyeongsang basin are higher in Fe₂O₃, Fe₂O₃/FeO, Na₂O, K₂O, Na₂O+K₂O and K₂O/Na₂O, but lower in Al₂O₃, FeO, MnO, CaO and P₂O₅ than the Japanese granitoids. The contents of TiO₂, FeOT and MgO are similar in both granitoids. This geochemical contrast would imply that the Cretaceous granitoid magmas in the Gyeongsang basin had been originated at closer place to the continent side under more tensional field, and solidified under more oxidizing environment than the coeval Japanese granitoid magmas.     

An empirical model for the calculation of spinel-melt equilibria in mafic igneous systems at atmospheric pressure: 1. Chromian spinels

 In order to develop a model for simulating naturally occurring chromian spinel compositions, we have processed published experimental data on chromian spinel-melt equilibrium. Out of 259 co-existing spinel-melt experiments reported in the literature, we have selected 118 compositions on the basis of run time, melt composition and experimental technique. These data cover a range of temperatures 1150–1500° C, oxygen fugacities of −13<log f _O2< −0.7, and bulk compositions ranging from basalt and norite, to komatiite. Six major spinel components with Cr³⁺, Al³⁺, Ti⁴⁺, Mg²⁺, Fe³⁺ and Fe²⁺-bearing end-members were considered for the purpose of describing chromite saturation as a function of melt composition, temperature and oxygen fugacity at 1 atmosphere pressure (0.101 MPa). The empirically calibrated mineral-melt expression based on multiple linear regressions is: K ^Sp _i =A/T(K)+B log f _O2+C ln (Fe³⁺/Fe²⁺)_L+D ln R _L +E, where K ^Sp _i is an equilibrium constant and R _L is a melt structure-chemical parameter (MSCP). Twenty-eight forms of equilibrium constants were considered, including single distribution coefficients, exchange equilibrium constants, formation constants for AB₂O₄ components, as well as simple “spinel cation ratios”. For each form of the equilibrium constants, a set of 16 combinations of the MSCPs have been investigated. The MSCP is present in the form of composite ratios [e.g., Si/O, NBO/T,(Al+Si)/Si, or (Na+K)/Al] or as simple cation ratios (e.g., Mg/Fe²⁺). For the calculation of Fe³⁺ and Fe²⁺ species in silicate melts, we used existing equations, whereas the Fe³⁺/Fe²⁺ ratio of spinels was calculated from the spinel stoichiometry. The regression parameters that best repoduce the experimental data were for the following constants: (Fe³⁺/Fe²⁺) _Sp , (Mg/Fe²⁺) _Sp /(Mg/Fe²⁺) _L , (Cr/Al) _Sp / (Cr/Al) _L , K _FeCr2O4, and Ti _Sp /Ti _L . These expressions have been combined into a single program called SPINMELT, which calculates chromite crystallization temperature and composition at a given f _O2 with an average accuracy of ∼10° C and 1–2 mol%. An example of the use of SPINMELT is presented for a magma parental to the Bushveld Complex. Received: 30 May 1995/Accepted: 1 November 1995     

Early Paleozoic adakite in the Liuyuan area from the Beishan orogenic belt,NW Gansu Province: Petrogenesis and implication for tectonic setting

The Liuyuan area,which is located on the southern margin of the Beishan orogenic belt,develops abundant Early Paleozic granitoids.SHRIMP zircon U-Pb dating yielded a weighted mean 206Pb/238U age of 421±8 Ma for the Liuyuan granodiorite(Zhao Zehui et al.,2007),implying its Late Silurian intrusion.Geochemical compositions showed that the Liuyuan granodiorite is characterized by high SiO2(65.01%-67.31%),A12O3(17.17%-18.05%) and Na2O(Na2O/K2O=1.67-1.87) but low Mg# contents calculated as 100×Mg2+/(Mg2++∑Fe2+) from 28.77 to 31.15,as well as being enriched in Sr(472×10-6-517×10-6) but depleted in Yb(1.2×10-6-1.42×10-6) and Y(12.8×10-6-14×10-6).The REEs are characterized by right-inclined patterns with LREE enrichment,HREE depletion and slightly negative Eu anomalies(Eu/Eu*=0.91-0.97).Major and trace elements indicate that the granodiorite is an adakite.The Nb/Ta values of the granodiorite vary from 10.80 to 18.01 and Nb/U from 6.32 to 10.09,both lying between the values of the crust and the mantle.The rock has low εNd(t) values(-2.5--0.8) and high ISr(0.706321-0.706495).Geochemical and Sr-Nd isotopic compositions indicate that the Liuyuan granodiorite is possibly derived from partial melting of thickening lower crust,related to mantle underplating.The Yb-Ta and Y+Nb-Rb discriminant diagrams imply the Liuyuan granodiorite intruded in a local extensional tectonic setting during late collision.Combined with previous studies on geochronology,geochemistry and tectonic setting of granitoids,we interprete that the constraint of this adakite in the Liuyuan area indicates that the tectonic setting may have transformed from collision to extension during the Early Devonian.     

Behaviour of Fe<Subscript>4</Subscript>O<Subscript>5</Subscript>–Mg<Subscript>2</Subscript>Fe<Subscript>2</Subscript>O<Subscript>5</Subscript> solid solutions and their relation to coexisting Mg–Fe silicates and oxide phases

Experiments at high pressures and temperatures were carried out (1) to investigate the crystal-chemical behaviour of Fe₄O₅–Mg₂Fe₂O₅ solid solutions and (2) to explore the phase relations involving (Mg,Fe)₂Fe₂O₅ (denoted as O₅-phase) and Mg–Fe silicates. Multi-anvil experiments were performed at 11–20 GPa and 1100–1600 °C using different starting compositions including two that were Si-bearing. In Si-free experiments the O₅-phase coexists with Fe₂O₃, hp-(Mg,Fe)Fe₂O₄, (Mg,Fe)₃Fe₄O₉ or an unquenchable phase of different stoichiometry. Si-bearing experiments yielded phase assemblages consisting of the O₅-phase together with olivine, wadsleyite or ringwoodite, majoritic garnet or Fe³⁺-bearing phase B. However, (Mg,Fe)₂Fe₂O₅ does not incorporate Si. Electron microprobe analyses revealed that phase B incorporates significant amounts of Fe²⁺ and Fe³⁺ (at least ~?1.0 cations Fe per formula unit). Fe-L_2,3-edge energy-loss near-edge structure spectra confirm the presence of ferric iron [Fe³⁺/Fe_tot?=?~?0.41(4)] and indicate substitution according to the following charge-balanced exchange: [4]Si⁴⁺?+?[6]Mg²⁺?=?2Fe³⁺. The ability to accommodate Fe²⁺ and Fe³⁺ makes this potential “water-storing” mineral interesting since such substitutions should enlarge its stability field. The thermodynamic properties of Mg₂Fe₂O₅ have been refined, yielding H°_1bar,298?=???1981.5 kJ mol^??1. Solid solution is complete across the Fe₄O₅–Mg₂Fe₂O₅ binary. Molar volume decreases essentially linearly with increasing Mg content, consistent with ideal mixing behaviour. The partitioning of Mg and Fe²⁺ with silicates indicates that (Mg,Fe)₂Fe₂O₅ has a strong preference for Fe²⁺. Modelling of partitioning with olivine is consistent with the O₅-phase exhibiting ideal mixing behaviour. Mg–Fe²⁺ partitioning between (Mg,Fe)₂Fe₂O₅ and ringwoodite or wadsleyite is influenced by the presence of Fe³⁺ and OH incorporation in the silicate phases.     

Chemical mass transfer in magmatic processes IV. A revised and internally consistent thermodynamic model for the interpolation and extrapolation of liquid-solid equilibria in magmatic systems at elevated temperatures and pressures

A revised regular solution-type thermodynamic model for twelve-component silicate liquids in the system SiO₂-TiO₂-Al₂O₃-Fe₂O₃-Cr₂O₃-FeO-MgO-CaO-Na₂O-K₂O-P₂O₅-H₂O is calibrated. The model is referenced to previously published standard state thermodynamic properties and is derived from a set of internally consistent thermodynamic models for solid solutions of the igneous rock forming minerals, including: (Mg,Fe²⁺,Ca)-olivines, (Na,Mg,Fe²⁺,Ca)^M2 (Mg,Fe²⁺, Ti, Fe³⁺, Al)^M1 (Fe³⁺, Al,Si)₂ ^TETO₆-pyroxenes, (Na,Ca,K)-feldspars, (Mg,Fe²⁺) (Fe³⁺, Al, Cr)₂O₄-(Mg,Fe²⁺)₂ TiO₄ spinels and (Fe²⁺, Mg, Mn²⁺)TiO₃-Fe₂O₃ rhombohedral oxides. The calibration utilizes over 2,500 experimentally determined compositions of silicate liquids coexisting at known temperatures, pressures and oxygen fugacities with apatite ±feldspar ±leucite ±olivine ±pyroxene ±quartz ±rhombohedral oxides ±spinel ±whitlockite ±water. The model is applicable to natural magmatic compositions (both hydrous and anhydrous), ranging from potash ankaratrites to rhyolites, over the temperature (T) range 900°–1700°C and pressures (P) up to 4 GPa. The model is implemented as a software package (MELTS) which may be used to simulate igneous processes such as (1) equilibrium or fractional crystallization, (2) isothermal, isenthalpic or isochoric assimilation, and (3) degassing of volatiles. Phase equilibria are predicted using the MELTS package by specifying bulk composition of the system and either (1) T and P, (2) enthalpy (H) and P, (3) entropy (S) and P, or (4) T and volume (V). Phase relations in systems open to oxygen are determined by directly specifying the f _{o 2} or the T-P-f _{o 2} (or equivalently H-P-f _{o 2}, S-P-f _{o 2}, T-V-f _{o 2}) evolution path. Calculations are performed by constrained minimization of the appropriate thermodynamic potential. Compositions and proportions of solids and liquids in the equilibrium assemblage are computed.     

Crystal chemistry of a Mg-vesuvianite and implications of phase equilibria in the system CaO-MgO-Al2O3-SiO2-H2O-CO21

Abstract Chemical analysis (including H₂, F₂, FeO, Fe₂O₃) of a Mg-vesuvianite from Georgetown, Calif., USA, yields a formula, Ca_18.92Mg_1.88Fe³⁺_0.40Al_10.97Si_17.81- O_69.0.1(OH)_8.84F_0.14, in good agreement on a cation basis with the analysis reported by Pabst (1936). X-ray and electron diffraction reveal sharp reflections violating the space group P4/nnc as consistent with domains having space groups P4/n and P4nc. Refinement of the average crystal structure in space group P4/nnc is consistent with occupancy of the A site with Al, of the half-occupied B site by 0.8 Mg and 0.2 Fe, of the half-occupied C site by Ca, of the Ca (1,2,3) sites by Ca, and the OH and O(10) sites by OH and O. We infer an idealized formula for Mg-vesuvianite to be Ca₁₉Mg(MgAl₇)Al₄Si₁₈O₆₉(OH)₉, which is related to Fe^3+-vesuvianite by the substitutions Mg + OH = Fe³⁺+ O in the B and O(10) sites and Fe³⁺= Al in the AlFe site. Thermodynamic calculations using this formula for Mg-vesuvianite are consistent with the phase equilibria of Hochella, Liou, Keskinen & Kim (1982) but inconsistent with those of Olesch (1978). Further work is needed in determining the composition and entropy of synthetic vs natural vesuvianite before quantitative phase equilibria can be dependably generated. A qualitative analysis of reactions in the system CaO-MgO-Al₂O₃-SiO₂-H₂O-CO₂ shows that assemblages with Mg-vesuvianite are stable to high T in the absence of quartz and require water-rich conditions (XH₂O > 0.8). In the presence of wollastonite, Mg-vesuvianite requires very water-rich conditions (XH₂O > 0.97).     

Variation of infrared absorption spectra in the system Bi2Al4−x Fe x O9 (x = 0–4), structurally related to mullite

The crystal structure of Bi₂Al_4−x Fe _x O₉ compounds (x = 0–4) has striking similarities with the crystal structure of mullite. A complete substitution of Al by Fe³⁺ in both octahedral and tetrahedral sites is a particular structural feature. The infrared (IR) spectra of the Bi₂M₄O₉ compounds (M = Al, Fe³⁺) are characterised by three band groups with band maxima in the 900–800, 800–600 and 600–400 cm⁻¹ region. Based on the spectroscopic results obtained from mullite-type phases, the present study focuses on the composition-dependent analysis of the 900–800 cm⁻¹ band group, which is assigned to Al(Fe³⁺)–O stretching vibrations of the corner-sharing MO₄ tetrahedra. The Bi₂Al₄O₉ and Bi₂Fe₄O₉ endmembers display single bands with maxima centred at 922 and 812 cm⁻¹, respectively. Intermediate Bi₂Al_4−x Fe _x O₉ compounds exhibit a distinct splitting into three relatively sharp bands, which is interpreted in terms of ordering effects within the tetrahedral pairs. Thereby the high-energy component band of the band triplet relates to Al–O–Al conjunctions and the low-energy component band to Fe–O–Fe conjunctions. The intermediate band is assigned to stretching vibrations of Al–O–Fe or Fe–O–Al configurations of the corner-sharing tetrahedral pairs. Bands in the 800–600 cm⁻¹ range are assigned to low-energy stretching vibrations of the MO₄ tetrahedra and to M–O–M bending vibrations of the tetrahedral pairs. Absorptions in the 600–400 cm⁻¹ range are essentially determined by M–O stretching modes of the M cations in octahedral coordination.     

Redox equilibria and the structural role of iron in alumino-silicate melts

The relationship between the redox ratio Fe⁺²/(Fe⁺²+Fe⁺³) and the K₂O/(K₂O + Al₂O₃) ratio (K₂O^*) were experimentally investigated in silicate melts with 78 mol% SiO₂ in the system SiO₂-Al₂O₃-K₂O-FeO-Fe₂O₃, in air at 1,400° C. Quenched glass compositions were analyzed by electron microprobe and wet chemical microtitration techniques. Minimum values of the redox ratio were obtained at K₂O^*0.5. The redox ratio in peralkaline melts (K₂O^*>0.5) increases slightly with K₂O^* whereas this ratio increases dramatically in peraluminous melts (K₂O^*<0.5) as K₂O is replaced by Al₂O₃. These data indicate that all Fe⁺³ (and Al⁺³) occur as tetrahedral species charge balanced with K⁺ in peralkaline melts. In peraluminous melts, Fe⁺³ (and Al⁺³) probably occur as both tetrahedral species using Fe⁺² as a charge-balancing cation and as network-modifying cations associated with non-bridging oxygen.     

Trace element variations in olivine phenocrysts from Ugandan potassic rocks as clues to the chemical characteristics of parental magmas

Olivine phenocrysts in ugandite and leucite basanite from the western branch of the East African Rift have been analysed for up to 34 trace elements by Laser-ICP-MS with detection limits as low as 1 ppb. A combination of point analyses with varying ablation crater diameters and line scans allow the identification of subtle zonations from core to rim, as well as characterization of the chemical effects of contamination along cracks. Trace element concentrations are remarkably uniform between large and small phenocrysts; fractionated leucite basanites (Mg# 59) have higher D _Ca and D _Al, and less fractionated LREE/HREE than MgO-rich ugandites (Mg# 75–80). Minor zonation is seen in elements with cation charges from 5+ to 2+ (P, Ti, Zr, Cr, Al, Sc, V, Cu, Mn, Ni) and show correlation between Ti and Al, but not P. Early phenocryst cores with high Li or Ni, low Mn, or enrichments in many trace elements can be identified, whereas xenocrysts have exceptionally low Na, Cr, Ti, V and Co. Partition coefficients for Ni are 31–35, less than in lamproites, with which they demonstrate an approximately linear correlation with K₂O content, K₂O/Al₂O₃ and K₂O/Na₂O of the melt, but none with SiO₂ content or Mg#. D-values for Cr, Mn and Co overlap with those of basalts, whereas those for Sc (0.011–0.018), Zn (0.44–0.49) and Ga (0.006–0.007) are lower. D _V of various potassic rocks (0.015 in the Ugandan rocks) confirms the dependence on fO₂ calibrated by the Fe³⁺/(Fe³⁺+Fe²⁺) of spinels; the Ugandan potassic rocks crystallized at fO₂ = FMQ to FMQ + 1. The ugandite olivines have some trace element characteristics reminiscent of those in metasomatized Kaapvaal peridotites, but not ocean islands. Line scan analyses are contaminated in Al, Ca, Cu, Ga, Sr, Zr, Nb, La and Ce, elements that are also concentrated in microcracks between subgrains, indicating smearing out during polishing, and demonstrating that large spot analyses produce the best results.     

Coexisting hornblendes and biotites from Precambrian gneisses of the south coast of Western Australia

The compositions of coexisting hornblendes and biotites from amphibolite and granulite facies gneisses from the south coast of Western Australia were controlled by host rock composition, paragenesis, metamophic grade, pressure, and oxygen fugacity. The Mg/(Mg + Fe²⁺) and Mn/Fe²⁺ ratios in both minerals and possibly the Al^vi contents of the hornblendes are related to host rock compositions. Metamorphic grade appears to influence, perhaps only indirectly, the Ti, Mn, and Fe³⁺ contents of both minerals and possibly the hornblende Ca content. The higher Ti and lower Mn contents of the granulite facies hornblendes and biotites are attributed to their coexistence with pyroxenes, whereas their lower Fe³⁺/(Fe²⁺ + Fe³⁺) ratios are probably due to lower oxygen fugacity in the granulite facies environment. Grade-related colour variations in both minerals were controlled by their Ti/Fe²⁺ and Fe³⁺/(Fe²⁺ + Fe³⁺ ratios. The relatively low Al^vi contents of the hornblendes suggest low- to moderate-pressure metamorphism.Variations in element distribution coefficients are related to variations in mineral compositions rather than metamorphic grade. Thus K_{D(Al^iv ?Si)} is related to the Al^iv andedenite alkali contents of the hornblendes, K_{D(Fe²⁺ ?Mg)} to the distributions of Al^iv ?Si and Al^vi + Ti + Fe³⁺, K_D(Mn) to the Mn contents of both minerals, and K_D(Al^vi) to the Al^vi contents of the biotites.     

Petrography and major elements geochemistry of microgranular enclaves and neoproterozoic granitoids of south Khasi,Meghalaya: Evidence of magma mixing and alkali diffusion

Neoproterozoic (690±19 Ma) felsic magmatism in the south Khasi region of Precambrian northeast Indian shield, referred to as south Khasi granitoids (SKG), contains country-rock xenoliths and microgranular enclaves (ME). The mineral assemblages (pl-hbl-bt-kf-qtz-mag) of the ME and SKG are the same but differ in proportions and grain size. Modal composition of ME corresponds to quartz monzodiorite whereas SKG are quartz monzodiorite, quartz monzonite and monzogranite. The presence of acicular apatite, fine grains of mafic-felsic minerals, resorbed maficfelsic xenocrysts and ocellar quartz in ME strongly suggest magma-mixed and undercooled origin for ME. Molar Al₂O₃/CaO+Na₂O+K₂O (A/CNK) ratio of ME (0.68–0.94) and SKG (0.81–1.00) suggests their metaluminous (I-type) character. Linear to sub-linear variations of major elements (MgO, Fe₂O₃ ^t, P₂O₅, TiO₂, MnO and CaO against SiO₂) of ME and SKG and two-component mixing model constrain the origin of ME by mixing of mafic and felsic magmas in various proportions, which later mingled and undercooled as hybrid globules into cooler felsic (SKG) magma. However, rapid diffusion of mobile elements from felsic to mafic melt during mixing and mingling events has elevated the alkali contents of some ME.     

Complete chemical analyses of metamorphic hornblendes: implications for normalizations,calculated H2O activities,and thermobarometry

Twenty-two hornblendes separated from amphibolites and granulites of the Grenville Orogen of Ontario have been quantitatively analyzed for major and minor elements by electron microprobe, for FeO/Fe₂O₃ by wet chemistry, and for H₂O by manometric measurement as H₂. Hornblende formulae were calculated on the basis of 24O+OH+Cl+F. Most samples are magnesio-hornblendes, ferroan pargasitic hornblendes and ferroan hastingsitic hornblendes, with weight fractions of Fe³⁺/(Fe²⁺+Fe³⁺) ranging from 0.15 to 0.50. An oxy-amphibole component of 0–25 mol%, with an average value of 17 mol%, is obtained for these complete analyses. When compared with structural formulae determined solely from microprobe data, normalization based on 13=Si+Ti+Al+Fe+Mn+Mg cations provides the best approximation to hornblende formulae calculated from the complete analyses. Less satisfactory agreement is obtained from a normalization scheme based on 15=Si+Ti+Al+Fe+Mn+Mg+Ca, while worst agreement is obtained from normalization to 23 oxygens assuming all Fe is Fe²⁺. No normalization scheme based on microprobe data alone consistently replicates the measured FeO, Fe₂O₃, and H₂O; accurate determination of these values requires complete chemical analysies. Ionic solution models previously have been proposed to evaluate the activity of Ca₂Mg₅Si₈ O₂₂(OH)₂(a _Trem) in hornblende for use in equilibria that constrain the activity of H₂O (a _{H 2}O) in igneous and metamorphic rocks. Application of ionic models to typical hornblendes produces low a _Trem (usually<0.01), consequetly yielding extremely low a _{H 2}O. If an oxy-amphibole component is present, the calculated a _Trem and H₂O is further reduced. An oxy-amphibole component of 25% reduces the calculated H₂O activity and that of any hydroxyl-amphibole component by 50% below that calculated with simplified assumptions regarding X _OH in the hydroxyl site (i.e., X _OH=1, or X _OH=1–X _Cl–X _f). Thus, methods of amphibole normalizations appear to have a substantial effect on calculated amphibole and H₂O activites. Before quantitative hornblende thermobarometry can be calibrated and applied, the amounts of FeO, Fe₂O₃ and H₂O must be measured in order to fully characterize hornblende solid solutions.Contribution No. 478 from the Mineralogical Laboratory, University of Michigan     

High-<Emphasis Type="Italic">T</Emphasis> behaviour of gedrite: thermoelasticity,cation ordering and dehydrogenation

The thermoelastic behaviour of a natural gedrite having the crystal-chemical formula ^ANa_0.47 ^B(Na_0.03 Mg_1.05 Fe_0.86²⁺ Mn_0.02 Ca_0.04) ^C(Mg_3.44 Fe_0.36²⁺ Al_1.15 Ti_0.05⁴⁺) ^T(Si_6.31 Al_1.69)O₂₂ ^W(OH)₂ has been studied by single-crystal X-ray diffraction to 973 K (Stage 1). After data collection at 973 K, the crystal was heated to 1,173 K to induce dehydrogenation, which was registered by significant changes in unit-cell parameters, M1–O3 and M3–O3 bond lengths and refined site-scattering values of M1 and M4 sites. These changes and the crystal-chemical formula calculated from structure refinement show that all Fe²⁺ originally at M4 migrates into the ribbon of octahedrally coordinated sites, where most of it oxidises to Fe³⁺, and there is a corresponding exchange of Mg from the ribbon into M4. The resulting composition is that of an oxo-gedrite with an inferred crystal-chemical formula ^ANa_0.47 ^B(Na_0.03 Mg_1.93 Ca_0.04) ^C(Mg_2.56 Mn_0.02²⁺ Fe_0.10²⁺ Fe_1.22³⁺ Al_1.15 Ti_0.05⁴⁺) ^T(Si_6.31 Al_1.69) O₂₂ ^W[O_1.12²⁻ (OH)_0.88]. This marked redistribution of Mg and Fe is interpreted as being driven by rapid dehydrogenation at the H3A and H3B sites, such that all available Fe in the structure orders at M1 and M3 sites and is oxidised to Fe³⁺. Thermoelastic data are reported for gedrite and oxo-gedrite; the latter was measured during cooling from 1,173 to 298 K (Stage 2) and checked after further heating to 1,273 K (Stage 3). The thermoelastic properties of gedrite and oxo-gedrite are compared with each other and those of anthophyllite.     

P–T–X controls on phase stability and composition in LTMP metabasite rocks – a thermodynamic evaluation

The stability of pumpellyite + actinolite or riebeckite + epidote + hematite (with chlorite, albite, titanite, quartz and H₂O in excess) mineral assemblages in LTMP metabasite rocks is strongly dependent on bulk composition. By using a thermodynamic approach (THERMOCALC), the importance of CaO and Fe₂O₃ bulk contents on the stability of these phases is illustrated using P–T and P–X phase diagrams. This approach allowed P–T conditions of ～4.0 kbar and ～260 °C to be calculated for the growth of pumpellyite + actinolite or riebeckite + epidote + hematite assemblages in rocks containing variable bulk CaO and Fe₂O₃ contents. These rocks form part of an accretionary wedge that developed along the east Australian margin during the Carboniferous–Triassic New England Orogen. P–T and P–X diagrams show that sodic amphibole, epidote and hematite will grow at these conditions in Fe₂O₃‐saturated (6.16 wt%) metabasic rocks, whereas actinolite and pumpellyite will be stable in CaO‐rich (10.30 wt%) rocks. With intermediate Fe₂O₃ (～3.50 wt%) and CaO (～8.30 wt%) contents, sodic amphibole, actinolite and epidote can coexist at these P–T conditions. For Fe₂O₃‐saturated rocks, compositional isopleths for sodic amphibole (Al³⁺ and Fe³⁺ on the M2 site), epidote (Fe³⁺/Fe³⁺ + Al³⁺) and chlorite (Fe²⁺/Fe²⁺ + Mg) were calculated to evaluate the efficiency of these cation exchanges as thermobarometers in LTMP metabasic rocks. Based on these calculations, it is shown that Al³⁺ in sodic amphibole and epidote is an excellent barometer in chlorite, albite, hematite, quartz and titanite buffered assemblages. The effectiveness of these barometers decreases with the breakdown of albite. In higher‐P stability fields where albite is absent, Fe²⁺‐Mg ratios in chlorite may be dependent on pressure. The Fe³⁺/Al and Fe²⁺/Mg ratios in epidote and chlorite are reliable thermometers in actinolite, epidote, chlorite, albite, quartz, hematite and titanite buffered assemblages.     

Structural changes and valence states in the MgCr<Subscript>2</Subscript>O<Subscript>4</Subscript>–FeCr<Subscript>2</Subscript>O<Subscript>4</Subscript> solid solution series

The influence on the structure of Fe²⁺ Mg substitution was studied in synthetic single crystals belonging to the MgCr₂O₄–FeCr₂O₄ series produced by flux growth at 900–1200 °C in controlled atmosphere. Samples were analyzed by single-crystal X-ray diffraction, electron microprobe analyses, optical absorption-, infrared- and Mössbauer spectroscopy. The Mössbauer data show that iron occurs almost exclusively as ^IVFe²⁺. Only minor Fe³⁺ (<0.005 apfu) was observed in samples with very low total Fe. Optical absorption spectra show that chromium with few exceptions is present as a trivalent cation at the octahedral site. Additional absorption bands attributable to Cr²⁺ and Cr³⁺ at the tetrahedral site are evident in spectra of end-member magnesiochromite and solid-solution crystals with low ferrous contents. Structural parameters a₀, u and T–O increase with chromite content, while the M–O bond distance remains nearly constant, with an average value equal to 1.995(1) Å corresponding to the Cr³⁺ octahedral bond distance. The ideal trend between cell parameter, T–O bond length and Fe²⁺ content (apfu) is described by the following linear relations: a₀=8.3325(5) + 0.0443(8)Fe²⁺ (Å) and T–O=1.9645(6) + 0.033(1)Fe²⁺ (Å) Consequently, Fe²⁺ and Mg tetrahedral bond lengths are equal to 1.998(1) Å and 1.965(1) Å, respectively.