Biotite Compositions Research Articles

Mineral chemistry (biotite, muscovite, garnet, and plagioclase) in the Kathmandu and Gosainkund regions, central Nepal Himalaya

The greenschist- to granulite-facies rocks in the Kathmandu and Gosainkund regions are divided into three tectonic units on the basis of structure, lithology, and metamorphism. The Gosainkund Crystalline Nappe (GCN) corresponds to the southward extension of the Higher Himalayan Crystallines (HHC), which thrusts over the Kathmandu Crystalline Nappe (KCN) along the Main Central Thrust (MCT). The GCN and KCN thrust over the Lesser Himalaya (LH) along the MCT and the Mahabharat Thrust (MT), respectively.
 Systematic traverses with the microprobe of four minerals (i.e. biotite, muscovite, garnet, and plagioclase) from all the three units were carried out to study their chemical variations. Most of the biotite compositions from all units belong to the annite compositional field. There is an inverse relationship between the Ti and the Mg number (Mg2+/ (Mg2++Fe2+) and the latter increases from the top to bottom section of the KCN. The LH biotites resemble the bottom composition of the KCN whereas the GCN biotite compositions are quite scattered. However, the Ti-Mg number trend does not seem to be related directly to the grade of metamorphism. The muscovites from the KCN are rich in FeO while the muscovites from other two units are rich in Al2O3. In the KCN, the almandine and pyrope contents in garnet decrease from bottom to top section, while the grossular and spessartine contents in garnet increase. This trend is consistent with the prograde metamorphic evolution observed in the field. The composition of garnets from the bottom to the top section of the GCN does not show any systematic variation, but in the upper section, where sillimanite appears, the almandine con tent decreases and the spessartine content slightly increases. This variation in composition suggests a polyphase metamorphic evolution. The albite content of plagioclase decreases from the lower to upper section in the KCN while there is not any systematic variation in the GCN.
 The P-T conditions record the good preservation of inverse metamorphism in the LH below the MCT. The comparison of P-T results between the KCN and the GCN suggests that exhumation of the KCN was followed by the exhumation of the GCN.

Fluid Evolution in a Subvolcanic Granodiorite Pluton Related to Fe andPb-Zn Mineralization, Banska Stiavnica Ore District, Slovakia

The central zone of the Miocene Stiavnica stratovolcano (Western Carpathians) hosts an extensive subvolcanic intrusive complex dominated by a granodiorite pluton (14.5–15.5 Ma). The granodiorite intrusion has a bell-jar form and was emplaced mainly into basement rocks, although its central part extends to the base of a subvolcanic and volcanic andesitic complex, 1.5 to 3 km below the paleosurface. In the more uplifted western part of the complex, the margin of the granodiorite includes numerous apophyses with associated magnetite skarns, whereas in the central, apical part it contains an irregular network of fractures hosting base metal stockwork mineralization that extends to a depth of 250 m. The granodiorite intrusion, especially in the marginal facies, is altered. High-temperature subsolidus deuteric (K-feldspar, secondary biotite, and amphibole) and endoskarn (pyroxene) alteration, intimately related to magnetite skarns, resulted in depletion in Fetotal and increase in CaO and MgO in the intrusion. Low-temperature, hydrothermal alteration (epidote, chlorite, K-feldspar, sericite), closely related to the stockwork mineralization, is characterized by enrichment in K2O, Pb, Zn, Cu, and MnO, and depletion in Na2O. The overlying andesites are affected by acid leaching. Fluid inclusions and stable isotopes (O,H) were studied in samples of granodiorite related to the skarn and to the stockwork, from the top of the pluton down to 1,230 m in depth. Magmatic quartz contains hypersaline fluid inclusions (up to 71 wt % NaCl equiv), vapor-rich saline and low-salinity inclusions, as well as moderate- to low-salinity liquid-rich aqueous inclusions with homogenization temperatures (Th) ranging from 161° to> 600°C. Brine inclusions from magmatic quartz in marginal phases of the granodiorite show signs of NaCl saturation during entrapment, with the majority probably having been captured under vapor unsaturated conditions. Most vapor-rich inclusions represent early stages of fluid immiscibility, whereas the low-salinity liquids are products of dilution by meteoric fluid and/or late-stage portions of magmatic fluids. Inclusion-bearing minerals in the skarn assemblage contain moderate- to low-salinity fluid inclusions (Th = 215°–371°C) with evidence of late-stage boiling at low hydrostatic pressure. Hydrothermal minerals from base metal stockwork and massive silica-rich rock contains only low-salinity inclusions (up to 5 wt % NaCl equiv, Th = 191°–367°C). The isotopic composition of magmatic biotite and hornblende from granodiorite indicates open magmatic degassing (deuterium depletion) during granodiorite crystallization, influenced by reequilibration to variable degrees. Fluids in equilibrium with skarn and stockwork-related minerals show a clear progressive mixing trend with isotopically relatively heavy, possibly δ 18O-shifted, meteoric water. Magnetite skarns and zones of high-temperature subsolidus deuteric alteration are related to ponding of magmatic brines in the roof of the intrusion, resulting from fluid immiscibility at relatively low pressures. The associated escaping vapor was responsible for acid leaching in andesites above the apical part of the pluton. Stockwork mineralization and associated low-temperature potassic alteration are probably related to the large convective hydrothermal system and upflow of late-stage, deep, exsolved supercritical fluids, using the increased permeability (fracturing) of the central apical part of the intrusion.

Petrographic, geochemical and isotopic constraints on magma dynamics and mixing in the Miocene Monte Capanne monzogranite (Elba Island, Italy)

The Monte Capanne pluton (Tuscan Magmatic Province, Elba Island, Italy) displays a great variety of magmatic products. Mafic microgranular enclaves (MME) are widespread and display variable size, texture and composition. Petrographic and mineralogical features highlight the importance of magma mixing in their genesis. In particular, patchy-zoned phenocrysts and corroded An-rich plagioclase are common in the MME, which, together with their abundance in the monzogranitic host, suggest that mixing between mantle and crustal-derived magmas occurred relatively early in the crystallisation history of the pluton. A gabbroic enclave is characterised by the abundance of An-rich plagioclase and amphibole, the latter possibly replacing former clinopyroxene. Differences in biotite composition between MME reflect the heterogeneous nature of the mafic magmas prior to the enclave formation. The importance of magma mixing is also evident in K-feldspar megacrysts, which commonly exhibit resorption surfaces followed rimward by a high-Ba zone. Hybrid Orano dykes, which are the latest magmas injected into the Monte Capanne pluton, display specific disequilibrium textures (i.e. sieve-textured plagioclase phenocrysts) and mineral content (i.e. abundance of amphibole) compared to the MME. The genesis of most MME, which display a wide spectrum of major and trace element compositions compared to the host monzogranite, involves a complex interplay between mixing and crystallisation following the input of mantle-derived magma into a silicic magma chamber. Possible loss of the liquid phase from the evolving basic magma may have produced MME distinctively depleted in LREE and enriched in MgO. Fingerprinting the chemical and isotopic signature of the basic magma involved in the genesis of the MME is further complicated by subsequent (but limited) exchange between MME and host. The basic magma is inferred to be similar to coeval high-K calc-alkaline volcanic rocks occurring on Capraia Island. Isotopically, the monzogranite and its MME display distinctive trends pointing towards a low- ɛ Nd end-member, represented by a garnet mica schist xenolith. We propose that these trends, together with the abundance of variably digested metasedimentary xenoliths dispersed throughout the pluton, reflect assimilation of metapelitic crust early in the evolution of the pluton during storage of the magma within the Tuscan basement, i.e. prior to the final emplacement of the pluton at upper crustal levels. The hybrid Orano dykes, which were injected at the end of the Monte Capanne crystallisation history, display distinctive trace element compositions, similar to REE-Sr-Ba-rich andesites on Capraia. The Orano dykes originated from a deeper level of magma storage and interacted with a crystal-rich enclave-bearing monzogranitic mixture. The proposed scenario involving periodic recharge by mantle-derived magmas and crustal assimilation is also in agreement with isotopic variations observed within a zoned K-feldspar megacryst, which exhibits a core with high 87Sr/ 86Sr and a rimward decrease of 87Sr/ 86Sr, arguing for a high 87Sr/ 86Sr silicic system replenished by lower 87Sr/ 86Sr mantle-derived magmas.

The low-pressure partial-melting behaviour of natural boron-bearing metapelites from the Mt. Stafford area, central Australia

This study has examined the ~300 MPa partial melting behaviour of four metapelites collected from the highest grade rocks occurring below the anatectic zone of the Mt. Stafford area, Arunta Inlier, central Australia. In this area, metasediments are interpreted to have undergone partial melting within the andalusite stability field; possibly as a result of a lowering of the metapelite solidus by the presence of boron in the rocks. Two of the samples were two mica metapelites (MTS70 and MTS71). These both contained significant quantities of tourmaline and were thus boron enriched. The other two samples are biotite metapelites. One of these rocks contains only a trace of tourmaline (MTS8) and the other is tourmaline free (MTS7). Despite expectations that muscovite in the two mica samples would break down via a subsolidus reaction, muscovite was stable to above 750°C due to the incorporation of Ti, phengitic and possibly F components into its structure. Between 750 and 800°C, muscovite melted out completely via a coupled muscovite + biotite fluid-absent incongruent reaction. Tourmaline was partially consumed in this reaction, with the elbaitic component being preferentially consumed. In the most mica-rich sample this reaction produced ~60% melt at 800°C. In the biotite metapelites, biotite melting began at a temperature below 800°C and was accompanied by very modest melt production at this low temperature. In contrast to the two mica metapelites, the main pulse of melt production in these samples occurred at a temperature between 850 and 950°C. In both these samples biotite + melt coexisted over a temperature range in excess of 150°C, and in MTS8, biotite was still in the run products at 950°C. The very refractory nature of these evolved biotite compositions is most likely a consequence of both the presence of a Ti buffering phase in the assemblage (ilmenite) and the essentially plagioclase-free nature of the starting compositions. Under the fluid-absent conditions of this study, tourmaline is clearly a reactant in the partial melting process, but does not appear to shift the fluid-absent incongruent melting reactions markedly. In the tourmaline-rich two mica metapelites, tourmaline only disappears from the run products at a temperature above 850°C, where it coexisted with a substantial melt proportion. This appears to coincide with the point of maximum boron concentration in the melts.

Rhythmic Layering in the Apatite (‐Magnetite) Deposit in the Fanshan Complex, North China

Abstract. The Fanshan complex consists of three (First‐ to Third‐) Phases of intrusion. The First‐Phase Intrusive is composed of rhythmic layers. The Fanshan apatite (‐magnetite) deposit is situated near the top of no. 6 rhythmic Unit of the First‐Phase Intrusive. Two sketch maps were drawn through the deposit along no. 25 cross‐cut on 425 mL and no. 1 crosscut on 491 mL in the Fanshan mine. Through this mapping, a small‐scaled rhythmic layering (called sub‐rhythmic layering) was newly found. The sub‐rhythmic layering consists of five typical rocks: biotite clinopyroxenite, biotite rock, biotite‐apatite rock, alternation biotite‐apatite rock and apatite rock. The thickness of layers varies from four millimeters to several meters. Six units of sub‐rhythmic layering were distinguished. Each unit consists of biotite clinopyroxenite (or biotite rock and biotite‐apatite rock) layer at the bottom and apatite rock layer at the top.Major element compositions of biotite and clinopyroxene were analyzed. The trends of Mg/(Mg+Fe*+Mn) atomic ratio (Fe*: total iron) of biotite and clinopyroxene indicate that the magma evolved markedly from relatively magnesian bottom layer to less magnesian top layer in each sub‐rhythmic unit. A general trend, however, through the whole sub‐rhythmic layering sequence shows that the both minerals become relatively magnesian upwards. The hypotheses of liquid immiscibili‐ty, gravitational settling and magma mixing are against with the results obtained in the present study. This study suggests that the apatite (‐magnetite) deposit is formed through fractional crystallization process.

Discovery and Genetic Mechanism of Basic Granulite in the Altay Orogenic Belt, Xinjiang, NW China

Abstract Altay granulite (AG), which represents the product of high‐grade metamorphism in the lower crust, was newly found in the Wuqiagou area, Fuyun County in the Altay orogenic belt, Northwest China. It is composed mainly of hypersthene, augite, basic plagioclase, amphibole and brown biotite. Its mineral compositions of amphibole and biotite are rich in Mg/(Mg+Fe2+) and Ti. Geochemically, the AG is enriched in Mg/(Mg+Fe2+) and Al2O3, and poor in CaO, with depletion of U, Th, K and Rb contents. Furthermore, geochemical data reflect that the protolith of the AG is igneous‐genetic calc‐alkaline basalt formed under an island arc environment. The AG has ΣREE of 92.38–96.58 ppm and enriched LREE model with weak positive Eu anomaly of 1.09–1.15. In the MORB normalized spider diagram, the AG shows tri‐doming pattern with a strong negative Nb anomaly and medium negative P and Ti anomalies, reflecting that the AG has tectonic relation with subduction or subduction‐related materials. The P‐T conditions of peak metamorphism of the AG are 750–780°C and >0.6–0.7 GPa. Retrograde metamorphism implies that the protolith of the Altay granulite might undergo a metamorphic process along a clockwise P‐T trajectory. Therefore, the formation and evolution of the AG may have a genetic association with continental collision/orogeny and the AG was taken into the Late Paleozoic meta‐strata by way of tectonic emplacement.

Aluminous granulites from the Pulur complex, NE Turkey: a case of partial melting, efficient melt extraction and crystallisation

In the Pulur complex, NE Turkey, a heterogeneous rock sequence ranging from quartz-rich mesocratic gneisses to silica- and alkali-deficient, Fe-, Mg- and Al-rich melanocratic rocks is characterized by granulite-facies assemblages involving garnet, cordierite, sillimanite, ilmenite, ±spinel, ±plagioclase, ±quartz, ±biotite, ±corundum, rutile and monazite. Textural evidence for partial melting in the aluminous granulites, particularly leucosomes, is largely absent or strongly obliterated by a late-stage hydrothermal overprint. However, inclusion relations, high peak P– T conditions, the refractory modes, bulk and biotite compositions of the melanocratic rocks strongly support a model of partial melting. The melt was almost completely removed from the melanocratic rocks and crystallised within the adjacent mesocratic gneisses which are silica-rich, bear evidence of former feldspar and show a large range in major element concentrations as well as a negative correlation of most elements with SiO 2. Peak conditions are estimated to be ≥800 °C and 0.7–0.8 GPa. Subsequent near-isothermal decompression to ∼0.4–0.5 GPa at 800–730 °C is suggested by the formation of cordierite coronas and cordierite–spinel symplectites around garnet and in the matrix. Sm–Nd, Rb–Sr and 40Ar/ 39Ar isotope data indicate peak conditions at ∼330 Ma and cooling below 300 °C at ∼310 Ma.

An Elle-based 2-D model for cation exchange reaction between garnet and biotite

We have developed a model for diffusion zoning development in garnet employing a numerical modeling system, Elle [Jessell et al. 2001]. The Elle system simulates textural development at thin section scale with an assumption that long term textural changes can be achieved by adding incremental processes. Many numerical models have been developed for the diffusion zoning pattern of garnet, but these models considered only lattice diffusion without allowing grain boundary diffusion. However, our model simulates the diffusion zoning in garnets by considering not only lattice diffusion but also grain boundary/lattice reaction and grain boundary diffusion. Sub-processes for our garnet-biotite system include (1) lattice diffusion, (2) grain-boundary diffusion, and (3) grain boundary/lattice reaction. The finite difference explicit method was used for the numerical calculation of lattice and boundary diffusion. For diffusion calculation, chemical concentrations of individual nodal points are used to calculate the amount of transported cations with diffusion coefficients determined from experiments. For lattice/boundary reaction, the lowest energy compositions of garnet and biotite were calculated using the distribution coefficient. Experimental runs were performed to test the consistency and reproducibility of the results. The results demonstrate the consistency and reproducibility, but show somewhat different zoning from that of Spear (1991), due to reduced dimension (3D in nature vs 2D in our model) and the arbitrarily chosen reaction constant. In spite of these limitations, we feel that the model presented here is at least a good starting point for diffusive mass transfer in a material with grain boundary structure. Journal of the Virtual Explorer, 2004 Volume 15 Paper 6 http://virtualexplorer.com.au/ An Elle-based 2-D model for cation exchange reaction between garnet and biotite Page 2 Journal of the Virtual Explorer, 2004 Volume 15 Paper 6 http://virtualexplorer.com.au/ An Elle-based 2-D model for cation exchange reaction between garnet and biotite Page 3

COMPOSITION OF BIOTITE FROM GRANITIC ROCKS OF THE CANADIAN APPALACHIAN OROGEN: A POTENTIAL TECTONOMAGMATIC INDICATOR?

Biotite, the dominant ferromagnesian mineral in Paleozoic granitic rocks of the Canadian Appalachians, has been analyzed with an electron microprobe (wavelength dispersion) for major elements and by 57Fe Mossbauer spectroscopy. We sampled a wide variety of rock types, ranging from gabbro, diorite, syenite to granite, but by far mostly granitic ( sensu lato ). The most pronounced variations are in total Al contents and Fe/(Fe + Mg) values. In the biotite quadrilateral (annite –siderophyllite –phlogopite – eastonite), biotite from A-type granites of the Humber and Avalon zones in Gaspe (Quebec) and New Brunswick is characterized by low mean Al contents, ~1.15 atoms per formula unit ( apfu ), and variable Fe/(Fe + Mg) values in the range 0.4 to 0.9. In the granites of the Notre Dame arc of the Dunnage zone in Newfoundland, biotite has moderate mean values of Al (~1.40 apfu ) and Fe/(Fe + Mg) (~0.58). In granites of the Gander zone of New Brunswick and Newfoundland, biotite has a mean Fe/(Fe + Mg) value of 0.6 and shows a pronounced trend of increasing total Al (1.05 to 1.75 apfu ), confirming significant contributions of aluminous supracrustal material to the magmas, either by assimilation or anatexis. Finally, in granites of the Meguma zone, derived entirely from metasedimentary material, biotite exhibits a remarkable increase in total Al (1.30 to 2.00 apfu ) and considerable iron-enrichment [Fe/(Fe + Mg) in the range 0.4 to 1], with compositions nearing the siderophyllite end-member. The biotite from most zones plots on or above the NNO buffer, indicating moderately oxidizing conditions, whereas that from the Meguma zone plots mainly between the QFM and NNO buffers, implying fairly reducing conditions during crystallization. Assuming a reasonable range of crystallization temperatures of 750 to 900°C, oxygen fugacities ranged from 10−10 to 10−16.9 bars during crystallization. The composition of biotite reflects primarily the nature of the host magmas. It cannot readily be used for tectonomagmatic characterization of these rocks without the aid of other types of data.

Evaluating micas in petrologic and metallogenic aspect: I–definitions and structure of the computer program MICA +

Micas are significant ferromagnesian minerals in felsic to mafic igneous, metamorphic, and hydrothermal rocks. Because of their considerable potential to reveal the physicochemical conditions of magmas in terms of petrologic and metallogenic aspects, mica chemistry is used extensively in the earth sciences. For example, the composition of phlogopite and biotite can be used to evaluate the intensive thermodynamic parameters of temperature ( T, °C), oxygen fugacity ( fO 2), and water fugacity ( fH 2O) of magmatic rocks. The halogen contents of micas permit the estimation of the fluorine and chlorine fugacities that may be used in understanding the metal transportation and deposition processes in hydrothermal ore deposits. The Mica + computer program has been written to edit and store electron-microprobe or wet-chemical mica analyses. This software calculates structural formulae and shares out the calculated anions into the I, M, T, and A sites. Mica + classifies micas in terms of composition and octahedral site-occupancy. It also calculates the intensive parameters such as fO 2, T, and fH 2O from the composition of biotite in equilibrium with K-feldspar and magnetite. Using the calculated F–OH and Cl–OH exchange systematics and various log ratios ( fH 2O/ fHF, fH 2O/ fHCl, fHCl/ fHF, X Cl/ X OH, X F/ X OH, X F/ X Cl) of mica analyses. Mica + gives valuable determinations about the characteristics of hydrothermal fluids associated with alteration and mineralization processes. The program output is generally in the form of screen outputs. However, by using the “Grf” files that come up with this program they can be visualized under the Grapher software both as binary and ternary diagrams. Mica analyses subjected to the Mica + program were calculated on the basis of 22+ z positive charges taking into account the procedure by the Commission on New Mineral Names Mica Subcommittee of 1998.

Geochemistry of biotite in metabentonites as an age discriminant, indicator of regional magma sources and potential correlating tool

AbstractThe chemical composition of unaltered biotites extracted from 18 Lower Palaeozoic metabentonites from Sweden, Norway and Scotland was determined. Discrimination based on FeO, MgO and TiO2proved able to separate the metabentonites on the basis of their stratigraphical age. A triangular discrimination diagram using MgO, FeO and Al2O3classified the biotites as calc-alkaline. Independent support for the discriminant power of FeO, MgO and TiO2in biotite comes from Sr isotope ratios and Sr concentration data from cognate apatite crystals. Within this dataset Caradocian and Llandoverian metabentonites have unique biotite compositions, while Wenlockian samples can be characterized to a lesser degree. Biotite composition also supported the correlation of a pair of metabentonites from Norway known to form part of the same horizon. Biotite in metabentonites preserves the chemical signature of the host magmas in which it crystallized and can be used to discriminate between metabentonites of different ages and provenance.

Stability of phengite and biotite in eclogites and characteristics of biotite- or orthopyroxene-bearing eclogites

Stability of phengite and biotite in eclogite is discussed using petrological data of natural eclogites, and the observational data are examined by thermodynamic calculations. Generally, phengite is a major K phase in natural eclogite and is stable in wide range of bulk composition. However, in eclogites from several localities of the Caledonides, biotite occurs as a stable eclogite-facies mineral, and is often associated with orthopyroxene. Bulk compositions of such biotite- or orthopyroxene-bearing eclogites are compared with those of eclogites from the Dabie–Sulu region, China, where phengite is a major K phase in eclogite. The biotite- or orthopyroxene-bearing eclogites from the Western Gneiss Region of the Caledonides are rich in MgO (10–15 wt%) and relatively poor in CaO (7–8 wt%) and Al2O3 (12–16 wt%). The CaO/MgO ratios of the biotite- or orthopyroxene-bearing eclogites are clearly lower than those of eclogites from the Dabie–Sulu region, indicating that MgO-rich and CaO-poor environments should be important for stabilizing of biotite and orthopyroxene in eclogite. Biotite-bearing eclogite from the North-East Greenland Eclogite Province is rich in MgO (≈16 wt%) and CaO (≈15.5 wt%) and extremely poor in Al2O3 (≈8 wt%). To stabilize biotite in eclogite, Al2O3-poor environments are also important. Bulk compositions of these biotite- or orthopyroxene-bearing eclogites are similar to picrite basaltic compositions. To examine these observational data, thermodynamic calculations were carried out in a seven-component system KH2O1.5–Na2O–CaO–FeO–MgO–Al2O3–SiO2, which includes garnet, kyanite, phengite, biotite, quartz, omphacite, orthopyroxene and olivine in conjunction with mass-balance calculations. Firstly, calculations were performed on the average bulk composition of eclogites from the Dabie–Sulu region to lherzolite (KLB-1). The calculation results confirmed that phengite should be stable in eclogite with 'ordinary' basaltic composition, whereas biotite and orthopyroxene should be stable in picrite basaltic compositions (e.g. MgO >11.0 wt%, CaO <9.8 wt%, Al2O3 <15.2 wt% at 700 °C, 2.5 GPa). Further calculations in basaltic system confirmed that increase of MgO content and decrease of CaO and Al2O3 contents were important to stabilize biotite and orthopyroxene in eclogite. Thus, mineral assemblage in picrite basalt system should be completely different from that in normal basaltic system.

Biotite composition as a tool for the identification of Quaternary tephra beds

Stratigraphically important Quaternary rhyolitic tephra deposits that erupted from the Okataina and Taupo volcanic centers in New Zealand can be geochemically identified using the FeO and MgO contents of their biotite phenocrysts. The FeO/MgO ratio in biotite does not correlate with FeO/MgO in the coexisting glass phase so that tephra beds with similar glass compositions can be discriminated by their different biotite compositions. Some individual tephra deposits display sequential changes in biotite composition that allow separate phases of the eruption to be identified, greatly increasing the potential precision for correlation. In addition, devitrified lavas that are unsuitable for glass analysis can be correlated to coeval tephra deposits by their biotite compositions. Biotite is common in high-K 2O (>4 wt%) tephra beds and is widely dispersed in ash plumes because of its platy form, thus making it important in correlation studies.

Biotite dissolution processes and mechanisms in the laboratory and in nature: Early stage weathering environment and vermiculitization

Biotite dissolution in the laboratory and in nature was examined and compared to elucidate certain aspects of the weathering processes. Batch dissolution experiments of fresh biotite [(K 0.91 Na 0.01 )(Mg 0.40 Fe 2.07 Mn 0.05 Al 0.14 Ti 0.19 )(Si 2.82 Al 1.18 )O 10 (OH) 2 ] in granite were carried out at 150 ℃ for 1 to 56 days. Examination by scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectrometer (EDS) revealed that, in the early stage, dissolution proceeds from the edges of crystals inward and secondary minerals such as Fe oxide are precipitated mostly at the edges, with only a few secondary minerals found on the basal surfaces. A dissolution experiment using a mixture of biotite and muscovite, done at 150 ℃ for 7 days, indicated that hematite crystals formed mostly at the edges of biotite but not on muscovite. This observation suggests that released Fe is precipitated before it diffuses into the bulk solution. Because a dissolution rate at the edge is larger by two orders of magnitude than at the basal surface (Turpault and Trotignon 1994), precipitation before diffusion of dissolved elements to bulk solution well explains the preferential secondary mineralization at the edges in the laboratory experiments. SEM-EDS of fresh to slightly weathered biotites revealed that early stage weathering proceeds in the same way as in the laboratory with the edges being preferentially weathered and secondary minerals being precipitated mostly at the edges. Because of the similarity between the occurrence of secondary minerals in the laboratory experiments and in nature, the laboratory results elucidate the early stage weathering conditions, namely that (1) supersaturation with respect to secondary minerals in a solution occurs around biotite, i.e., the solution is poorly connected to a main flow pathway of water, and (2) once supersaturation is achieved, secondary minerals are precipitated mainly at the edge before some released elements diffuse into the solution. The immobility of water immediately adjacent to primary minerals partly explains the large difference in dissolution rate between the laboratory and natural samples. A phlogopite dissolution experiment was done at 150 ℃ for 5.5 days to examine the effect of biotite composition on vermiculitization. High-resolution transmission electron microscopy revealed that vermiculite layers are formed by layer-by-layer transformation within phlogopite after 5.5 day dissolution but not within biotite after 56 day dissolution or within slightly weathered biotite. A comparison of our data with results of other studies indicates that the formation of vermiculite layers between biotite layers occurs even in the early stage when biotite is not Fe-rich [more than 0.8 Mg per O 10 (OH) 2 ]. When biotite is Fe-rich [less than 0.4 Mg per O 10 (OH) 2 ]-like the present one- released Fe is precipitated as, for example, Fe hydroxide, and vermiculite is rarely formed because of limited availability of Mg. Thus, a higher Mg content in biotite facilitates the formation of vermiculite, at least in the early stage. Because vermiculite dissolves at a much slower rate than biotite, Mg-rich biotite dissolves at a slower rate than Fe-rich biotite. In the late stage, biotite dissolution still continues from the edge and within biotite, which results in a fine comb-like texture for weathered biotite; vermiculite occurs as a domain within chemically and structurally altered biotite as well, as at the edge even during weathering of Mg-poor biotite.

Mineralogy and Chemistry of Biotites from Eastern Pontide Granitoid Rocks, NE-Turkey: Some Petrological Implications for Granitoid Magmas

Abstract The Eocene Zigana Granitoid in the northern zone and the Middle-Upper Eocene Dolek and Saricicek Plutons in the southern zone of the Eastern Pontides (NE-Turkey) consist of different rock series ranging in composition from intermediate to felsic compositions containing from <1% to 9% of biotite. The biotites are Mg-rich ( X Fe = Fe 2+/Fe 2+ + Mg) = 0.28 – 0.49) in the southern zone plutons and Mg- to Fe-rich ( X Fe = Fe 2+/Fe 2+ + Mg) = 0.33 – 0.78) in the northern zone pluton. They coexist with K-feldspar, plagioclase, quartz, amphibole, pyroxene, low titanium magnetite, minor ilmenite ± titanite. Biotites have been buffered by the Ni-NiO. The plutons equilibrated under low oxygen fugacities (Log fO 2) ranging from –16.01 to –20.23 bars for Dolek and Saricicek, and –16.70 to –17.90 bars for Zigana. EPMA indicate that the composition of biotite is related to the mineral assemblages of Dolek, Saricicek and Zigana biotites as a consequence of chemistry of the magma and the physical conditions during crystallization. The biotite compositions were used to discriminate the tectonic setting of the plutons. The intrusions appear to be related to calc-alkaline to high-K calc-alkaline orogenic suites. Furthermore, Mg [6]-Fe tot substitution has played a significant role during the crystallization of the biotites.

Precision in Estimating Mineral Proportions from Chemical Analyses

The precision of estimating rock mineralogy by CIPW norm calculation and the least-squares analysis method were examined in this study. Five hundred granitoid rocks having varying compositions of quartz, K-feldspar, plagioclase, hornblende, and biotite were randomly generated by a computer program. Random errors of up to 5% were added to both rock and mineral compositions to determine the applicability of these two methods. Results indicate that the least-squares analysis method gave better-estimated rock mineralogy than did the CIPW norm calculations. Precision of estimating mineral proportions are 6.5% for quartz, 7.5% for K-feldspar, and 8.1% for plagioclase in the least-squares analysis, but for the CIPW norm calculation they are 14.0% for quartz, 13.1% for K-feldspar, and 17.4% for plagioclase. Correlation coefficients of true mineral mass proportion versus estimated mineral mass proportion range from 0.995 to 0.997 in the least-squares analysis, whereas they range from 0.896 to 0.936 in the norm calculation. These results show that the least-squares analysis method consistently yields better estimates of mineral mass proportions from chemical analyses.

The production of ectomycorrhizal mycelium in forests: Relation between forest nutrient status and local mineral sources

Due to acid rain and nitrogen deposition, there is growing concern that other mineral nutrients, primarily potassium and phosphorus, might limit forest production in boreal forests. Ectomycorrhizal (EcM) fungi are important for the acquisition of potassium and phosphorus by trees. In a field investigation, the effects of poor potassium and phosphorus status of forest trees on the production of EcM mycelium were examined. The production of EcM mycelium was estimated in mesh bags containing sand, which were buried in the soil of forests of different potassium and phosphorus status. Mesh bags with 2% biotite or 1% apatite in sand were also buried to estimate the effect of local sources of nutrients on the production of EcM mycelium. No clear relation could be found between the production of EcM mycelium and nutrient status of the trees. Apatite stimulated the mycelial production, while biotite had no significant effect. EcM root production at the mesh bag surfaces was stimulated by apatite amendment in a forest with poor phosphorus status. The contribution of EcM fungi to apatite weathering was estimated by using rare earth elements ( REE) as marker elements. The concentration of REE was 10 times higher in EcM roots, which had grown in contact with the outer surface of apatite-amended mesh bags than in EcM roots grown in contact with the biotite amended or sand-filled mesh bags. In a laboratory study, it was confirmed that REE accumulated in the roots with very low amounts (<1 %) translocated to the shoots. The short-term effect of EcM mycelium on the elemental composition of biotite and apatite was investigated and compared with biotite- and apatite-amended mesh bags buried in trenched soil plots, which were free from EcM fungi. The mesh bags subjected to EcM fungi showed no difference in chemical composition after 17 months in the field. This study suggests that trees respond to phosphorus limitation by increased exploitation of phosphorus-containing minerals by ectomycorrhiza. However, the potential to ameliorate potassium limitation in a similar way appears to be low. (Less)

Architecture of a 1.38–1.34 Ga granite–rhyolite complex as revealed by geochronology and isotopic and elemental geochemistry of subsurface samples from west Texas, USA

During the evolution of Laurentia, a Mesoproterozoic felsic igneous belt extended from Fennoscandia through Canada to the southwestern United States. This belt, referred to as the granite–rhyolite province in North America, forms much of the west Texas and eastern New Mexico basement. We present data from 41 closely spaced wells in west Texas that penetrated several hundred meters into the southern granite–rhyolite province and provide the first opportunity to develop a three-dimensional view of the basement from subsurface samples. The felsic rock types include hornblende-bearing quartz monzonite, ignimbritic rhyolite, and comagmatic granite with eutectic textures. These rocks plot in the high-K to ultra-high-K field. Quartz syenite, which plots exclusively in the ultra-high-K field, is the final felsic phase of magmatism. All of the felsic magmas were variably oxidizing based on biotite compositions. UPb chronology supports the intrusive relations established by petrography: the quartz monzonite is oldest (∼1380 Ma) and is followed by the granite–rhyolite sequence (∼1360 Ma) and the quartz syenite (∼1340 Ma). Nd model ages range from 1520 to 1740 Ma, indicating involvement of older Proterozoic crust. In many wells, thick mafic sills intrude the granite–rhyolite sequence. Nd model ages for the mafic rocks range from 1560 to 1440 Ma, implying that they are Mesoproterozoic in age. The mafic rocks include an alkaline, OIB-like suite, which is not supportive of a subduction origin. UPb ages are also reported for the Mescalero Well #1 in eastern New Mexico. In this well, metasedimentary and metavolcanic rocks of the Debaca sequence unconformably overlie a quartz syenite similar to the quartz syenite from the west Texas wells. Detrital zircons from the basal meta-arkose of the sequence are ∼1690 and ∼1320 Ma, indicating a maximum age of ∼1300 Ma for this sequence.

An electron microprobe study of P645/T390: Evidence for an Early Helladic III Lerna–Aegina connection

AbstractMiddle Helladic Aeginetan Ware is widely distributed throughout the Aegean. It is characterized by a “gold mica or volcanic ash” fabric and is abundant at Lerna on the Argive plain, Greece, in the Middle Helladic and Late Helladic I periods. A group of Early Helladic III ceramics at Lerna includes P645/T390, which also has a volcanic fabric. We have sampled all the major rock types of the separate volcanic episodes on Aegina, Methana, and Poros and analyzed the constituent minerals with the electron microprobe. These analyses provide a reference standard against which the mineralogy of P645/T390 and other potential Aeginetan wares can be compared. The compositions of amphibole, clinopyroxene, plagioclase, and biotite in P645/T390 are identical to the same minerals in dacites on Aegina, suggesting an origin on that island. Compositional mismatches with the minerals of Methana and Poros indicated the ceramic was not manufactured at these locations. This narrow‐necked jar represents the earliest occurrence of Aeginetan Ware on mainland Greece that has been confirmed by a quantitative method. © 2002 Wiley Periodicals, Inc.

Chemical variations in hydrothermal minerals of the Los Humeros geothermal system, Mexico

The Los Humeros geothermal system is composed of more than 2200 m of Quaternary altered volcanic rocks and an underlying Cretaceous sedimentary sequence. The low salinity of the fluids discharged at present (Na + and Cl − concentrations <500 ppm), and the excess steam, indicate that the reservoir contains a mixture of steam and dilute groundwater. Water-rock equilibrium is not attained. Hydrothermal minerals are present in veinlets, vugs, and replacing primary minerals. Three mineral zones are recognized: 1) a shallow argillic zone (<400 m depth), 2) a propylitic zone (ranging between 500 and 1800 m) and 3) a skarn zone (>1800 m). Petrographic examination of cuttings from five wells and temperature data indicate at least two stages of hydrothermal activity. Temperature is the main factor that affects the chemical composition of chlorite, epidote and biotite. Fe 2+ and Al IV increase in chlorite with temperature [from 1.4 formula position unit (fpu) to 2.8, and from 0.7 to 2.4 fpu, respectively]. The pistacite content of epidote varies from 18 to 33 mol% in high-temperature regions (>270 °C) and from 13 to 26 mol% in low-temperature regions (<250 °C). Biotite displays a slight increase in Al IV contents (1.55–2.8) and octahedral occupancy (5.93–6.0 fpu) with temperature. Whole rock composition and variations in oxygen fugacity conditions are factors that also affect the concentrations of Fe, Al and Mg in the octahedral sites of chlorite, epidote, biotite and amphiboles. Chemical variations observed in alteration minerals at different depths in the Colapso Central-Xalapazco region could be used as indicators of relict physico-chemical conditions in the reservoir, before the present economic exploitation.