Almandine-spessartine Garnet Research Articles

Variations in orientation relationships between rutile inclusions and garnet host relate to magmatic growth zoning

Rutile inclusions in almandine-spessartine garnet from a peraluminous pegmatoid from the Moldanubian zone (Bohemian Massif, AT) show distinct changes in aspect ratio, shape preferred orientations (SPO) and crystallographic orientation relationships (COR) along the transition between microstructurally different growth zones in the garnet core and rim. For identification of the COR characteristics we pool specific CORs based on their common axial relationship into three COR groups: Group 103R/111G, Group 001R/111G and Group 001R/100G. The rutile inclusions in the garnet core domains are elongated along the four Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document} directions and are dominated by COR Group 103R/111G. The garnet rim zone additionally contains rutile needles elongated along Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}100⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document}. Here, Group 001R/111G and 001R/100G are more abundant than in the garnet core. Needle-shaped rutile in the rim shows a systematic correlation between SPOs and CORs as needles elongated parallel to Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}111⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document} are dominated by Group 103R/111G and 001R/111G, whereas those needles elongated parallel to Grt⟨\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\langle$$\\end{document}100⟩\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\rangle$$\\end{document} exclusively pertain to CORs of 001R/100G. Furthermore, the frequency of each particular SPO in the garnet rim clearly depends on the local growth direction of the particular Grt{112} sector. Facet-specific variations in rutile SPO frequencies in different sectors and growth zones of garnet were observed even between equivalent directions, indicating that the microstructures and textures of rutile inclusions reflect varying parameters of garnet growth. The characteristic differences in COR groups of different garnet growth zones are referred to compositional changes in the bulk melt or compositional boundary layer, associated with magmatic fractional crystallisation.

Directed growth of a sector-zoned garnet in a pegmatoid from the Bohemian Massif, Austria

Almandine-spessartine garnet from the Moldanubian Zone in the Bohemian Massif (Austria) records microstructural and compositional features of crystal growth imprinted during the transition from pegmatoid magmatic to subsolidus state. Directed garnet growth results in asymmetric compositional, morphological and microstructural zoning. In addition, the garnet core shows sector zoning reflected by the predominance of phosphate inclusions in {110} sectors, and rutile inclusions in {112} sectors of garnet. These sector-specific abundances of inclusions are also expressed in the electron probe microanalytical data of the non-equivalent garnet sectors. The contribution of the submicron and nano-inclusions of phosphates and rutile is of crucial importance for thermobarometric methods based on P, Na and Ti in garnet. The sector-specific abundance of inclusions is referred to the surface configuration of individual garnet facets. Contrastingly, the microstructural growth banding observed in the {112} garnet sectors supposedly results from compositional fluctuations of a boundary layer at the garnet-melt interface. These fluctuations control the details of the inclusion microstructures, which differ even in crystallographically equivalent sectors.

(Fe,Mn)-(Ti,Sn)-(Nb,Ta) oxide assemblage in a little fractionated portion of a mixed (NYF + LCT) pegmatite from Piława Górna, the Sowie Mts. block, SW Poland

* �Correspondingauthor T extural relationships and compositional variations of (Fe,Mn)-(Ti,Sn)-(Nb,Ta) oxides and associated minerals from low- to moderately fractionated, mixed (NYF + LCT) anatectic pegmatites, which belong to the Julianna pegmatitic system, exposed at Pilawa Gorna, Lower Silesia, Poland, were studied to elucidate the geochemical evolution of the pegmatite-forming melt. The primary Nb-Ta oxides crystallized in the following sequence: (1) columbite-(Fe) evolving to columbite-(Mn), associated with Nb- and Ta-bearing ilmenite-pyrophanite + almandine-spessartine garnet + Ti-poor schorl, (2) columbite-(Fe), (3) tantalite-(Fe), (4) Ti- and Sn-bearing ixiolite ± ferrowodginite ± Nb- and Ta-bearing cassiterite ± columbite-(Fe), all crystallizing along with Ti-enriched schorl evolving to dravite. The most characteristic features of the mineral assemblages are the reversed Mn-Fe fractionation trends and increasing Ca concentrations at the final stage of crystallization. These compositional variations are explained by a possible pre-emplacement contamination of the pegmatite-forming melt and re-equilibration of biotite and plagioclase from the early-crystallized outer zones of the pegmatites with rapidly cooling pegmatitic melt. Local changes in melt composition were of minor importance. Late alterations, such as chloritization and muscovitization of biotite or albitization of Ca-rich plagioclase were significant only at the hydrothermal stage.

High-alumina titanite in mineral assemblages of the Berezitovy gold-base-metal deposit, Upper Amur Region

Grothite—a rare Al- and F-rich variety of titanite—was identified in two different gold-bearing mineral assemblages of the Berezitovy gold-base-metal deposit, Upper Amur Region, Russian Far East. Grothite is associated with quartz, orthoclase, chlorite, muscovite, tourmaline, almandine-spessartine garnet, ilmenite, pyrophanite, magnetite, fluorapatite, and sulfides. Grothite forms numerous scattered lamellar aggregates 20–100 μm in size with a relatively homogeneous structure. The lamellae grow in chlorite or between chlorite and orthoclase. According to microprobe analyses, variations in major elements of grothite are as follows (wt %): 30.56–34.07 SiO2, 7.91–12.71 Al2O3, 22.83–28.29 TiO2, 23.55–29.21 CaO, 0.52–4.25 FeO, and 2.19–6.16 F. It is suggested that grothite appeared in the gold-bearing mineral assemblages of the Berezitovy deposit due to the specifics of the primary composition of the host rocks and physicochemical conditions of the fluid regime at the final stage of deposit formation.

Petrology of the Coyaguayma ignimbrite, northern Puna of Argentina: Origin and evolution of a peraluminous high-SiO2 rhyolite magma

The Coyaguayma ignimbrite is a strongly peraluminous (SP), sillimanite, garnet-bearing silicic rhyolite which erupted in the northern Puna segment of the central Andean plateau in the Upper Miocene (~11Ma), a period that was characterized by the eruption of voluminous (100s to 1000s km3) dacitic ignimbrites of high-K calc-alkaline affinity. In this region, the SP magmatic rocks are both rare and small, but their importance is potentially much greater as rocks of this type are usually interpreted as products of crustal melting and therefore useful for addressing mantle addition vs crustal recycling in the central Andes.The phenocryst assemblage of the Coyaguayma ignimbrite comprises plagioclase (An26-18), quartz, Ba-rich and Ba-poor sanidine, minor Al-rich ferromagnesian minerals (AlT- and AlVI-rich biotite, almandine–spessartine garnet) and sillimanite, as well as accessory zircon and monazite. Textural relations suggest that the accessory and ferromagnesian phases crystallized before quartz and feldspars. Mineral equilibria suggest that crystallization of the rhyolite magma began at ~5kbar and 800°C, and continued almost isobarically to 720°C, causing the residual liquid to increase H2O contents from ~4–5% to ~7.5% before eruption.Most major features (e.g., high SiO2, A/CNK >1.3, low CaO, MgO, TiO2 and FeO) and trace element patterns (low Ba, Sr, Th, LREE and Eu/Eu*; high Rb, U, Y and Nb), along with the thermobarometric constraints on magmatic P, T and water contents are consistent with an origin by mica dehydration melting of metapelitic sources (e.g., typical biotite-muscovite gneisses from the outcropping S Puna basement). However, the relatively low initial 87Sr/86Sr (~0.7125), and high 143Nd/144Nd (~0.512200) ratios invalidate a pure crustal origin. Instead, we propose contamination of calc-alkaline dacitic magmas similar to typical Puna ignimbrites by metapelite at mid-crustal settings (≥18km depth). Geochemical modeling that satisfies the chemical and isotopic data suggests mixing of 70% dacite and ~30% of metapelite partial melts, followed by extensive (70%) fractionation of plagioclase, K-feldspar, quartz and biotite, with minor magnetite and apatite from the hybrid magma. Sillimanite in the Coyaguayma rhyolite is interpreted as a restite mineral or a product of incongruent melting of the metapelite, which was preserved intact in the hybrid melt due to local equilibrium.This petrogenetic model explains most characteristics of crystal-poor SP rhyolites from the Puna plateau (e.g., Tocomar, Ramadas rhyolites) and it may be more generally applicable to occurrences of SP magmas in Andean-type continental arcs dominated by calc-alkaline magmatism.

Origin and metamorphic evolution of Fe–Mn-rich garnetites (coticules) in the Desná Unit (Silesicum, NE Bohemian Massif)

The Fe–Mn-rich garnetites (coticules) associated with banded iron formations occur in the Neoproterozoic metamorphic complex of the Desná Unit. Their mineral assemblage consists of almandine-spessartine garnet, quartz, magnetite, epidote and sporadically biotite, chlorite, amphibole, plagioclase, calcite, ilmenite and apatite. The mineralogical, petrological and geochemical results demonstrate that the protolith of coticules and associated BIFs was formed during periods of submarine hydrothermal activity in the Late Proterozoic by differential precipitation of Fe and Mn from low-temperature solutions. The Fe–Mn-rich precipitates mixed to a varied degree with the detritic siliciclastic material. Subsequently the protolith underwent polyphase metamorphism reaching the amphibolite facies grade during the pre-Variscan (M1) and Variscan periods. The P–T conditions assumed for the peak Variscan metamorphism (M2) are T ∼540–580°C and P ∼600MPa, the M3 is characterized by an upper pressure limit 400MPa at a maximum temperature of 600°C and a metamorphic event M4 related to the hydration reactions which occurred during the late-stage of the Variscan orogenesis is characterized by T ∼150–300°C at P <200MPa.

The first find of grothite in gold ore deposits

1083 Grothite, representing an Aland Frich titanite variety that is described by the theoretical formula (Ca,REE)(Ti,Al,Fe)SiO4(O,OH,F) (1), is a relatively rare mineral, which was previously unknown in ores from noble metal deposits. Titanites with high concen� trations of Al (up to 14 wt %) and F (up to 2.5 wt %) are characteristic of eclogites, dolomites, and marbles of Austria (2, 3). Tinbearing minerals of the titanite- malayaite series (described by the theoretical formula CaSnO(SiO 4 )) are widespread in tin ore skarn deposits of the world, Russia included (4). Compositionally peculiar fluoraluminous titanite with respective Al2O3 and F contents ranging from 9.0 to 13.5 and from 3 to 5 wt %, which may be attributed to grothite, was estab� lished in metamorphosed carbonaceous-siliceous sediments of the cherty formation in the SikhoteAlin mountainous system (Primor'e region) (5). Grothite was first established in noblemetal objects during the mineralogical study of ores from the Berizitovoe gold-polymetallic deposit of the Verkhnii Amur region. The deposit is located in the northeast� ern flank of the Amur goldore province (6) in acti� vated structures of the southeastern North Asia Craton at its junction with surrounding structures of the Tuku� ringra-Dzhagda Terrane of the Mongol-Okhotsk orogenic belt (7). The ore body of this deposit is repre� sented by the nearmeridional zone of metasomatic rocks with sulfide mineralization localized in the mas� sif of Paleoproterozoic porphyric granodiorites. The zone consists of two steep funnelshaped bodies joined near the surface and pinching out downward (Fig. 1). In plan, the zone is characterized by a complex lentic� ular shape and dips at angles of 70°-75° in the south� western direction. It is 950 m long at the surface and from 10-15 to 110 m thick. Metasomatites of the deposit are represented by quartz-muscovite rocks with impregnated almand� ine-spessartine garnet and tourmaline. Accessory minerals with variable concentrations are orthoclase, chlorite, biotite, anorthite, zinc spinel (ferruginous gahnite), sphene, zircon, epidote, allanite (orthite), prehnite, fluorapatite, fluorite, and graphite. Polymetallic mineralization widespread in the metasomatic zone of the deposit occurs in the form of complex stringers and nests. The main ore minerals are sphalerite, galenite, pyrite, pyrrhotite, and magne� tite. Ores of the deposit are of the dominant zinc com� position with subordinate admixture of lead.

Long-term average mineral weathering rates from watershed geochemical mass balance methods: Using mineral modal abundances to solve more equations in more unknowns

The number of phases for which weathering rates can be determined by watershed geochemical mass balance is limited by the number of equations that can be constructed from elemental flux losses from the watershed and mineral stoichiometries. Mass balance studies of watershed weathering rates routinely use the flux losses of the six major cations SiO 2, Al, Na, K, Mg, and Ca. Analyses of these species in water are common, but following matrix algebraic methods limits the number of weathering rates that can be calculated to six. For the Brubaker Run watershed located in the northern Piedmont Physiographic Province of Pennsylvania (USA), long-term (10 3–10 6 year) watershed chemical flux losses have been determined using 10Be-derived total denudation rates and zirconium-normalized total chemical concentrations from bedrock and soils. Chemical flux losses calculated from solid-phase data have three advantages: They (1) Permit generation of a relatively large number of equations because both major and trace analyses are included; (2) eliminate the need for many years of regular (e.g., weekly) sampling and chemical analyses of stream water and atmospheric precipitation, and measurement of hydrologic parameters (i.e., precipitation, stream discharge, etc.); and (3) long-term weathering rate calculations need not address biomass. For Brubaker Run, eight minerals are involved in weathering; the five primary minerals are REE-rich epidote, ankerite, almandine–spessartine garnet, muscovite, chlorite, and the three secondary products are weathered muscovite, kaolinite, and gibbsite. The long-term average weathering rates of these minerals were calculated using the six major cations, and two trace elements selected from Rb, Sr, Ba, La, Pr, Nd, Sm, Gd, and Dy. Despite having the eight equations needed, geochemically reasonable weathering rates (e.g., positive primary mineral rates that reflect destruction) could not be achieved regardless of the two trace elements used in the mass balance calculations. For Brubaker Run, this is primarily attributable to the natural heterogeneity of the trace element concentrations within the host mineral grains, with trace element stoichiometries in some minerals varying by as much as an order of magnitude. Because the trace elements are hosted by a relatively small number of minerals, the computed weathering rates of other minerals become very sensitive to small variations in trace cation stoichiometry. REE-rich epidote, garnet, and ankerite within the Brubaker Run watershed together host nearly all of the Ca in the bedrock, and completely dissolve at or near the weathering front. Consequently, approximately all of the Ca in bedrock is lost from the regolith. In bedrock the mole-percentages of Ca hosted by REE-rich epidote, garnet, and ankerite are 49 mol%, 4 mol%, and 43 mol%, respectively, and are determined by the modal abundance of the mineral in the bedrock and its Ca stoichiometry. The weathering rates of REE-rich epidote, garnet, and ankerite can be determined by distributing to each mineral that fraction of the total watershed Ca flux loss for which it is responsible based on its mole-percent Ca in bedrock. By using a base cation that is completely lost from the regolith, and knowing the mole-percentage of that element in the mineral(s) undergoing weathering, additional equations may be added to the mass balance matrix. We term this technique the “flux distribution method.” The flux distribution method eliminates the need for additional equations established using trace elements. Based on the mineral weathering rates for the Brubaker Run watershed determined using the flux distribution method, the rates at which the weathering front penetrated the bedrock (the “saprolitization” rate) are 4.5 m Myr − 1 and 6.5 m Myr − 1 for chlorite and muscovite, respectively. These measured long-term average saprolitization rates compare very favorably with published theoretical values for the nearby northern Maryland Piedmont which range from 2.2 to 5.3 m Myr − 1 .

The generation and crystallization conditions of the Proterozoic Harney Peak Leucogranite, Black Hills, South Dakota, USA: Petrologic and geochemical constraints

The mineralogy, petrology and geochemistry of the Proterozoic Harney Peak Granite, Black Hills, South Dakota, were examined in view of experimentally determined phase equilibria applicable to granitic systems in order to place constraints on the progenesis of peraluminous leucogranites and commonly associated rare-element pegmatites. The granite was emplaced at 3–4 kbar as multiple sills and dikes into quartz-mica schists at the culmination of a regional high-temperature, low-pressure metamorphic event. Principally along the periphery of the main pluton and in satellite intrusions, the sills segregated into granite-pegmatite couplets. The major minerals include quartz, K-feldspar, sodic plagioclase and muscovite. Biotite-{Mg No. [Molar MgO/(MgO+FeO)]=0.32-0.38} is the predominant ferromagnesian mineral in the granite's core, whereas at the periphery of the main pluton and in the satellite intrusions tourmaline (Mg No.=0.18–0.48) is the dominant ferromagnesian phase. Almandine-spessartine garnet is also found in the outer intrusions. There is virtually a complete overlap in the wide concentration ranges of SiO2, CaO, MgO, FeO, Sr, Zr, W of the biotite- and tourmaline-bearing granite suites with no discernable differentiation trends on Harker diagrams, precluding the derivation of one suite from the other by differentiation following emplacement. This is consistent with the oxygen isotope compositions which are 11.5 ± 0.6‰ for the biotite granites and 13.2 ± 0.8‰ for the tourmaline granites, suggesting derivation from different sources. The concentrations of TiO2 and possibly Ba are higher and of MnO and B are lower in the biotite granites. The normative Orthoclase/Albite ratio is extremely variable ranging from 0.26 to 1.65 in the biotite granites to 0.01–1.75 in the tourmaline granites. Very few sample compositions fall near the high-pressure, watersaturated haplogranite minima-eutectic trend, indicating that the granites for the most part are not minimum melts generated under conditions with \(a_{{\text{H}}_{\text{2}} {\text{O}}}\)=1. Instead, most biotite granites are more potassic than the water-saturated minima and eutectics and in analogy with experimentally produced granitic melts, they are best explained by melting at ∼6 kbar, \(a_{{\text{H}}_{\text{2}} {\text{O}}}\)<1 and temperatures ∼800°C. Such high temperatures are also indicated by oxygen isotope equilibration among the constituent minerals (Nabelek et al. 1992). Several of the tourmaline granite samples contain virtually no K-feldspar and have oxygen isotope equilibration temperatures 716–775°C. Therefore, they must represent high-temperature accumulations of liquidus minerals crystallized under equilibrium conditions from melts more sodic than the water-saturated haplogranite minima or during fractionation of intruded melts into granite-pegmatite couplets accompanied by volatile-aided differentiation of the alkali elements. The indicated high temperatures, \(a_{{\text{H}}_{\text{2}} {\text{O}}}\)<1, the relatively high TiO2 and Ba concentrations and the relatively low \(\partial ^{18} {\text{O}}\) values of the biotite granites suggest that they were generated by high-extent, biotite-dehydration melting of an immature Archean metasedimentary source. The ascent of the hot melts may have triggered low-extent, muscovite-dehydration melting of schists higher in the crust producing the high-B, low-Ti melts comprising the periphery of the main pluton and the satellite intrusions. Alternatively, the different granite types may be the result of melting of a vertical section of the crust in response to the ascent of a thermal pulse, with the low-\(\partial ^{18} {\text{O}}\) biotite granites generated at a deeper, hotter region and the high-\(\partial ^{18} {\text{O}}\) tourmaline granites at a higher, cooler region of the crust. The low-Ti and high-B concentrations in the high-\(\partial ^{18} {\text{O}}\) melts resulted in the crystallization of tourmaline rather than biotite, which promoted the observed differentiation of the melts into the granitic and pegmatitic layers found along the periphery of the main pluton and the satellite intrusions.

Volcanic and intrusive history of the Pine Grove porphyry molybdenum system, southwestern Utah

The Pine Grove porphyry molybdenum deposit in the Wah Wah Mountains, southwestern Utah, occurs in the eroded vent of a lower Miocene ash flow (23-22 m.y. old). The vent, located in a tilted basin-and-range fault block, has been eroded about 1.7 km below the pre-eruption surface; consequently, the closest outcrops of related ash-flow tuff are 10 km away from the vent. Correlation of comagmatic ash-flow tuff and intrusive porphyry is made possible by the presence of accessory almandine-spessartine garnet of unique composition in both extrusive and intrusive units.Several lines of evidence suggest the presence of a parental magma chamber roughly 100 km 3 in size and compositionally zoned from rhyolite to dacite. Each eruptive episode began with a high silica content, rhyolitic magma containing quartz, sanidine, plagioclase, biotite, garnet, titanomagnetite, monazite, xenotime, ilmenorutile (?), and zircon. Presumably in response to continued rapid eruption and deeper drawdown in the parent chamber, some ash-flow eruptions also vented dacitic magma containing quartz, plagioclase, sanidine, biotite, hornblende, augite, titanomagnetite, ilmenite, sphene, apatite, and zircon.The dominant ore-related intrusion was emplaced immediately following concurrent intrusion of rhyolitic and dacitic magma into the vent. Multiple injections of trachyandesitic magma into the base of the chamber may have initiated convection or magma fracturing, surging of dacitic magma, and exsolution of volatiles that resulted, first, in ash-flow eruptions and, later, in episodes of mineralization. Therefore, an unusual combination of eruption mechanics and chamber volume and geometry allowed sampling of possibly the entire compositional range of the chamber during eruption but prohibited the complete loss of rhyolitic magma. Magmatic processes, such as late-stage convection, magma fracturing, and volatile fluxing may have been critical in the development of porphyry Mo mineralization.

Retrograde garnet zoning at low water pressure in metapelitic rocks from Kemi�, SW Finland

The compositional zoning of almandine-spessartine garnet has been studied in pelitic gneisses from the Kemio region, SW Finland. Garnets with inverse zoning (outward increase in Mn) abound in the low-grade musc+qz zone. Garnets with inverse zoning, normal zoning (outward decrease in Mn), and with a homogeneous composition occur in the higher-grade sill+ Kfs zone. The formation of garnet with inverse zoning is explained by continuous retrograde reactions in the model system K2O-Al2O3-FeO-MnO-MgO-SiO2-H2O at strongly reduced partial water pressures.

Fission track annealing characteristics of garnet

Fission track annealing characteristics of almandine-spessartine garnet have been investigated. The data indicate that the activation energy for thermal fading of tracks ranges from 2.6 to 6.0 eV with a mean of 4.1 eV. The annealing temperatures for this variety are lower than for the andradite-grossular variety.

The elastic properties of an almandine-spessartine garnet and elasticity in the garnet solid solution series

This paper reports the results of ultrasonic measurements of the second-order elastic constants of a single-crystal specimen of almandine-spessartine garnet as a function of pressure and temperature. The density of the sample, ρ0 = 4.2396 ± 0.0016 g/cm3, is consistent with the composition (Fe0.52, Mn0.46,Ca0.01) 3Al2Si3O12 as determined by microprobe analysis. The pertinent results, Ks = 1763.3 ± 2.3 kbar, (∂Ks/∂T)T = 4.59 ± 0.16, and (∂Ks/∂P)P = −0.172 ± 0.005 kbar/0C, where Ks is the adiabatic bulk modulus, are related systematically to other garnet results published by previous investigators. In addition to the foregoing conventional ultrasonic data we have obtained a measurement of the higher-order property (∂2Ks /∂T∂P) of 0.9±0.6×10−30C−1. The magnitude of this parameter suggests that it could significantly effect seismic velocity and density equations of state which are used commonly to model the earth's mantle. Assuming that the second-order elastic constants Cij of garnets are related linearly to the molar fraction of their constituent end-members, the present data have been combined with the work of previous investigators in order to derive the elastic properties throughout the garnet solid solution series. The relevant' results for Ks from the least squares analysis for the various end members are as follows (in kilobars); pyrope, 1658±13; almandine, 1801±7; spessartine, 1723±10; grossular, 1705±13. These data provide the basis for calculating the elastic wave velocities for any (Mg, Fe, Mn, Ca) 3Al2Si3O12 garnet.

Chemistry of the minerals of the granitic Murrumbidgee Batholith, Australian Capital Territory

Sixty-six mineral concentrates from the granitic Murrumbidgee Batholith, A.C.T., were analysed for major and trace elements to explore their chemical variation. Analyses of eighteen microcline samples show that Ba, Sr, Rb and Ga vary in a regular manner which can be correlated with the SiO 2 content of their host rock, but Pb and Y vary irregularly; all six trace elements vary independently of the major element composition of the microcline. Twenty-two analyses show a high correlation between the anorthite content of plagioclase and the CaO and SiO 2 contents of the host rocks. Ba and Sr are significantly correlated with the Ca content of the plagioclase, and therefore with the SiO 2 content of the host rocks, but Pb, Rb and Y behave irregularly. Ga content of the plagioclase decreases with increasing SiO 2 content of the host rock and shows a high correlation with Al content of the plagioclase. All of twenty-two biotites analysed are rich in Al and the Al content of both tetrahedral and octahedral sites appears to be related to host rock paragenesis and, therefore, to bulk rock composition. Fe 2+, Mn, Li, Nb, Zn, Ga, Rb and Pb contents of the biotites increase with increasing SiO 2 content of their host rocks, whilst Mg, V, Cr, Co and Na decrease; Cu, K and Ba show no correlation with host rock SiO 2 content. Analyses of two muscovites indicate preferential enrichment in Nb and Ga. Chemical similarities between hornblendes occurring in xenoliths and in enveloping tonalite at an isolated locality in the batholith suggest a xenocrystal origin for the hornblende in the tonalite. Almandine-spessartine garnet occurs in two leucogranite intrusions and is probably a primary mineral phase precipitated from Mn-rich magma. Regular patterns discerned in the partition of trace elements between coexisting mineral pairs suggest that a close approach to equilibrium has been attained in the intrusions, but that three different types of intrusions recognised in the batholith have equilibrated at different temperatures.