Showing papers in "Contributions to Mineralogy and Petrology in 1978"

Experimental calibration of the partitioning of Fe and Mg between biotite and garnet

Abstract: The cation exchange reaction Fe3Al2Si3O12 +KMg3AlSi3O10(OH)2 = Mg3Al2Si3O12+KFe3-AlSi3 O10(OH)2 has been investigated by determining the partitioning of Fe and Mg between synthetic garnet, (Fe, Mg)3Al2Si3O12, and synthetic biotite, K(Fe, Mg)3AlSi3O10(OH)2. Experimental results at 2.07 kbar and 550 °–800 ° C are consistent with In [(Mg/Fe) garnet/(Mg/Fe) biotite] = -2109/T(°K) +0.782. The preferred estimates for Δ¯H and Δ¯S of the exchange reaction are 12,454 cal and 4.662 e.u., respectively. Mixtures of garnet and biotite in which the ratio garnet/biotite=49/1 were used in the cation exchange experiments. Consequently the composition of garnet-biotite pairs could approach equilibrium values in the experiments with minimal change in garnet composition (few tenths of a mole percent). Equilibrium was demonstrated at each temperature by reversal of the exchange reaction. Numerical analysis of the experimental data yields a geothermometer for rocks containing biotite and garnet that are close to binary Fe-Mg compounds.

Nature of alkalic volcanic rock series

Abstract: The alkalic rocks are here regarded as a category in a classification of igneous rock series (rock associations) and not as a class in petrographic systematics The alkalic series as a whole are characterized by higher Na2O+K2O content than the subalkalic series in the alkali vs SiO2 diagram At least three different trends (types) of differentiation appear to exist in large-scale alkalic volcanic associations One (here designated as the Kennedy trend) starts from weakly nepheline-normative basalt and shows increasing normative nepheline with advancing fractionation to reach a phonolitic composition Another (here called the Coombs trend) starts from hypersthene-normative basalt and shows increasing normative hypersthene and then normative quartz with advancing fractionation to reach a comenditic composition Besides these two trends, it seems that many alkalic associations exist which show a differentiation trend starting from nepheline-normative basaltic composition and leading to hypersthenenormative, and then to quartz-normative compositions (here designated as the straddle-B type) Alkalic rocks of these three trends are higher not only in Na2O+K2O but also in Rb, Ba, Sr and Zr than subalkalic rocks The alkalic basalts as a whole are characterized by higher contents of such elements and not by any degree of silica undersaturation It is widely believed that alkalic rocks are characterized by the presence of normative nepheline as well as by the absence of orthopyroxene and pigeonite Indeed such a relationship holds for the Kennedy trend, but it is not always valid for other types of alkalic associations Some alkalic rocks of the Coombs trend and straddle-B type have quartz (or other silica minerals) and orthopyroxene and pigeonite

Petrogenesis of Archaean ultrabasic and basic volcanics: Evidence from rare earth elements

Abstract: Rare earth element (REE) and major element data are presented on 44 Archaean samples which include spinifex textured ultramagnesian lavas (STPK) spinifex textured basalts (STB) and low MgO tholeiites. The samples come from the Yilgarn and Pilbara Blocks (W. Australia), Barberton (South Africa), Belingwe and Que Que (Rhodesia), Abitibi (Canada) and the 3.7 b.y. Isua Belt of Western Greenland. In addition REE data are given on three near primitive mid-ocean ridge basalts (MORB) and a glassy MORB-type basalt from Taiwan. We suggest that REE patterns, particularly the light REE and Eu, can be affected by metamorphism, but argue that the consistency of pattern from samples both within and between areas enables recognition of primary patterns. La/Sm ratios of 2.7 b.y. STPK are characterised by being lower than those of associated basalts. The 3.5 b.y. STPK Barberton material does not show this feature but instead displays significant heavy REE depletion. The separation of garnet from these liquids is suggested as a possible mechanism for the high CaO/Al2O3 ratios, (Al loss) and the heavy REE and Sc depletion. The REE data on Barberton material is equivocal on the derivation of the so-called basaltic komatiites from the peridotitic komatiites. However, REE analyses on STPK and high magnesian lavas from elsewhere suggests that crystal fractionation is not a viable mechanism to produce one from the other. We suggest instead, that varying amounts of partial melting of different sources is responsible for the spectrum of compositions. The STB appear to be an easily recognised rock type within the Archaean. They are characterised by quench (clinopyroxene) textures and a light REE enriched pattern. It is suggested that these are near primary melts and that their REE patterns mirror their mantle source. We propose a two stage model for the 2.7 b.y. mafic complexes, in which, prior to the generation of ultrabasic magmas, the source underwent a small amount of partial melting which resulted in the removal of a melt enriched in incompatible elements. The depletion process could be achieved either during mantle diapirism or by upward migration of interstitial melts into an Archaean low velocity zone. The spread of La/Sm ratios in STPK and STB is used as an argument that the Archaean mantle was chemically heterogeneous and that the degree of heterogeneity was similar to that observed in modern ocean volcanics. As a result, partial melting of the mantle under different P-T conditions produced a spectrum of magma types. The information presently available on Archaean mafic and silicic magmatism and the incompleteness of geochemical data on present day tectonic environments are two major obstacles in formulating Archaean tectonic models. In addition a comparison of present day and Archaean ultramafic and silicic rocks suggests that plate tectonic models as presently understood may not be suitable analogues for all Archaean tectonic environments.

Liquidus phase relations on the join diopside-forsterite-anorthite from 1 atm to 20 kbar: Their bearing on the generation and crystallization of basaltic magma

Abstract: In the system CaO-MgO-Al2O3-SiO2, the tetrahedron CaMgSi2O6(di)-Mg2SiO4(fo)-SiO2-CaAl2 SiO6(CaTs) forms a simplified basalt tetrahedron, and within this tetrahedron, the plane di-fo-CaAl2Si2O8(an) separates simplified tholeiitic from alkalic basalts. Liquidus phase relations on this join have been studied at 1 atm and at 7, 10, 15, and 20 kbar. The temperature maximum on the 1 atm isobaric quaternary univariant line along which forsterite, diopside, anorthite, and liquid are in equilibrium lies to the SiO2-rich side of the join di-fo-an. The isobaric quaternary invariant point at which forsterite, diopside, anorthite, spinel, and liquid are in equilibrium passes, with increasing pressure, from the silica-poor to the silica-rich side of the join di-fo-an, which causes the piercing points on this join to change from forsterite+diopside+anorthite+liquid and forsterite +spinel+anorthite+liquid below 5 kbar to forsterite +diopside+spinel+liquid and diopside +spinel+anorthite+liquid above 5 kbar. As pressure increases, the forsterite and anorthite fields contract and the diopside and corundum fields expand. The anorthite primary phase field disappears entirely from the join di-fo-an between 15 and 20 kbar. Below about 4 kbar, the join di-fo-an represents, in simplified form, a thermal divide between alkalic and tholeiitic basalts. From about 4 to at least 12 kbar, alkalic basalts can produce tholeiitic basalts by fractional crystallization, and at pressures above about 12 kbar, it is possible for alkalic basalt to be produced from oceanite by crystallization of both olivine and orthopyroxene. If alkalic basalts are primary melts from a lherzolite mantle, they must be produced at high pressures, probably greater than about 12 kbar.

Residual glasses and melt inclusions in basalts from DSDP Legs 45 and 46: Evidence for magma mixing

Abstract: Compositional relations among natural glasses in basalts recovered by Legs 45 and 46 (DSDP) provide powerful constraints on their differentiation histories. Residual glass compositions in the moderately evolved aphyric and abundantly phyric basalts within each site demonstrate that none of the units is mutually related to any other or to a common parent by simple fractional crystallization. At Site 396, where clinopyroxene phenocrysts are absent, progressively more evolved liquids (lower Mg/ (Mg+Fe) and higher TiO2) are characterized by lower calcium-aluminum ratios, which can only be generated by clinopyroxene fractionation. This paradox is amplified by some melt inclusions in olivine phenocrysts that have higher CaO/Al2O3 and lower TiO2 than any residual glasses. The occurrences of these distinctive compositions are correlated with the highly magnesian character of the host olivines (Fo90−89), and the melts are interpreted as trapped primitive liquids, parental to the more fractionated derivatives. Melt inclusions intermediate in composition between the residual glasses and the most primitive olivine melt inclusions are present in the cores of some plagioclase phenocrysts that have had a history of resorption. On the basis of a petrographic and microprobe analysis of the zoning relations in these phenocrysts, the inclusions are inferred to be melts entrapped at the time of extensive corrosion of the host crystals. Interpreted in conjunction with other mineral and geochemical data, the compositional trends in the glasses indicate that magma mixing has played a major role in the genesis of the Leg 45 and 46 basalts. The reality of mixing is demonstrated by extensive disequilibrium textures in the plagioclase phenocrysts and the presence in evolved lavas of refractory plagioclase and olivine phenocrysts bearing primitive melt inclusions. The chemical imprint of clinopyroxene fractionation despite the absence of clinopyroxene phenocrysts is believed to be accomplished by plating of gabbro on to the upper walls of the subvolcanic magma chamber as it evolves between mixing events. Repeated influxes of primitive magma batches will move the resultant hybrids alway from clinopyroxene saturation and generate olivine-plagioclase cotectic magmas. This model provides a physical buffering mechanism that accounts for the volumetric dominance of moderately evolved basalts among ocean floor tholeiites. Major and trace element models based on the combination of mixing and fractional crystallization also explain heretofore enigmatic geochemical characteristics of MORB.

Experimental determination of the isotopic fractionation between gaseous CO2 and carbon dissolved in tholeiitic magma

Abstract: Isotope fractionation of carbon between CO2 and carbon dissolved in a tholeiitic magma measured in the range 1120–1280 ° C, 7.0–8.4 Kb varies from 4.6 to 4‰ in favor of CO2. These results make possible to explain all deep seated 13C values from a restricted range of primary mantle 13C concentrations. They also suggest that carbon could be dissolved in basaltic magmas in a reduced form.

The role of fluids in the formation and subsequent development of early continental crust

Abstract: A petrogenetic model is developed to explain the evolution and geochemical character of granitic rocks in early Archean (pre 3.6 Gyr) continental crust taking into account the following important geological constraints, viz.: 1. High geothermal gradients (probably in excess of 90 ° C/km) and resulting widespread granulite facies metamorphism even at relatively shallow depths 2. The fractionation of certain major and trace elements under granulite facies conditions 3. The composition and geochemical behaviour of fluids which emanate from or pass through terrains undergoing granulite facies metamorphism viz. carbonic fluids containing significant amounts of SO2 and halogens.

Coalification and graphitization in high-pressure schists in New Caledonia

Abstract: The northern portion of the Tertiary high-pressure schist belt in New Caledonia contains, from west to east, a metamorphic progression from lawsonite-albite facies through glaucophanitic greenschists to eclogitic albite-epidote amphibolites. This belt is flanked to the west by Upper Cretaceous-Eocene metasediments, of prehnite-pumpellyite grade. Paraschists throughout this whole sequence contain abundant carbonaceous material which shows a progressive metamorphism from coal to graphite. Structural analysis of lithostatic load and oxygen isotope data have provided a PT profile for the carbon metamorphism. In the prehnite-pumpellyite metasediments, phytoclasts were progressively coalified to anthracite rank under PT conditions which extended up to 3 kb/255 ° C at the lawsonite isograd where graphite first appears. On the high grade side of the lawsonite isograd a transitional mixed zone of continued coalification and graphitization occurred within the PT range 3 kb/255 ° C to 5.5 kb/335 ° C which included the ferroglaucophane isograd. Immediately beyond this zone all phytoclasts were completely graphitized before the epidote isograd was reached at 6.3 kb/ 390 ° C. The prevailing metamorphic environment retarded coalification, but accelerated graphitization, under conditions of high pressure and a low temperature gradient (7 ° C/km) that had been generated within the sedimentary pile by rapid tectonic thickening and consequent deep burial.

Experimental determination of the spinel peridotite to garnet peridotite reaction in the system MgO-Al2O3-SiO2 in the range 900 °–1100 °C and Al2O3 isopleths of enstatite in the spinel field

Abstract: The univariant high-pressure reaction of aluminous enstatite and spinel to pyrope and forsterite in the MgO-Al2O3-SiO2 system has been determined in the temperature range 900 °–1100 °C by hydrothermal reversals in the piston-cylinder apparatus using the low-friction NaCl pressure medium. A mixture of synthetic minerals, including an enstatite with 6 wt% Al2O3, with product and reactant assemblages in nearly equal amounts, was the starting material. The equilibrium pressure of 19.3±0.3 kbar at 1000 ° C and average dP/dT slope of 8.0 bars/ ° C confirm the strong curvature of the equilibrium below 1200 ° C deduced by Obata (1976) from a theoretical study of experimental Al2O3 isopleths of enstatite in the garnet field. His prediction of an absolute minimum pressure near 18 kbar of the garnet peridotite assemblage in the ternary system is undoubtedly correct.

Melting of plagioclase in the system Ab − An − $${\text{H}}_{\text{2}} {\text{O}}$$ and Qz − Ab − An − $${\text{H}}_{\text{2}} {\text{O}}$$ at P H 2 O =5 kbars, an equilibrium problem

Abstract: Melting experiments were performed in the systems at P H 2 O =5 kbars. The compositions of the plagioclases coexisting with melt or with melt and quartz were determined. The data obtained in the plagioclase-water systems show that the reaction rate decreases exponentially with decreasing temperature. Plagioclase compositions do not change any further for run times ≧ 1 h at 1000° C and are considered to have reached equilibrium. At 900° C run times of approximately 200 h are necessary to approach equilibrium. At 800° C the reaction is too slow to attain constant compositions within reasonable times. The solidus of the system Qz-Ab-An-H2O was reversed using mixtures of 95% plagioclase and 5% quartz. This solidus gives the equilibrium compositions of plagioclase for any combination plagioclase-quartz-melt. A series of comparative experiments was also made with An20 and An40, mixed with quartz in the ratio plagioclase/quartz: 1/1, because theoretically the equilibrium solidus composition of plagioclase should be independent of the plagioclase/quartz ratio. However, at various run times and temperatures, the plagioclase compositions thus obtained approached the equilibrium solidus compositions only very slowly. It is extrapolated that at 730° C about 100,000 years are needed to reach equilibrium composition. The experimental data show that it is not possible to reach equilibrium in the investigated tonalite- and anorthosite-system around and below 800° C in run times available in the laboratory. It is also concluded that many investigations performed in the granite system Qz-Or-Ab-An-H2O did not reach equilibrium.

Age, origin, and thermal histories of some plutonic rocks from the Salinian block of California

Abstract: U-Pb isotopic measurements on zircons from some granitic rocks of the Salinian block indicate emplacement and crystallization of these rocks about 104 m.y. ago (mid-Cretaceous). The relatively radiogenic nature of initial Sr and common Pb in these rocks, and the presence of an inherited component of zircon in some of them strongly suggest the involvement of continental crust in the genesis of the magmas. Possibly the magmas were generated in a zone of melting that overlapped the boundary between the mantle and the continental crust. U-Pb measurements on sphene, apatite, and feldspar from the plutons, along with previously published K-Ar and fission-track ages shed light on the post-emplacement thermal histories of the plutons. Most of the samples from the northern part of the Salinian block (Bodega Head, Point Reyes) show relatively simple thermal histories. Evidently these plutons were emplaced at moderate levels in the crust, crystallized, and cooled to moderate temperatures over an interval of about 10–15 m.y. Plutons from the central Salinian block (Santa Lucia Range) show more complex and prolonged thermal histories. These plutons evidently were emplaced at greater depths in the crust than were the plutons from the northern Salinian block. They remained at elevated temperatures for ca. 25 m.y., then cooled fairly rapidly, probably as a result of rapid uplift and erosion. One sample from the northern Salinian block shows an even longer span of time between emplacement and cooling. The thermal evolution of the Salinian plutonic rocks predates major offset along the San Andreas fault zone and thus reflects the thermal evolution of the undisrupted source terrane of the Salinian block. Isotopic measurements of the type reported here might therefore prove valuable in correlations across the San Andreas fault zone. Moreover, detailed study of thermal evolution within the Salinian block could shed light on major offsets within the block.

The thermodynamic properties of reciprocal solid solutions

Abstract: A multisite solid solution of the type (A, B) (X, Y) has the four possible components AX, AY, BX, BY. Taking the standard state to be the pure phase at the pressure and temperature of interest, the mixing of these components is shown not to be ideal unless the condition: $$\Delta G^0 = (\mu _{AX}^0 + \mu _{BY}^0 - \mu _{AY}^0 - \mu _{BX}^0 = 0$$ applies. Even for the case in which mixing on each of the individual sublattices is ideal, ΔG 0 contributes terms of the following form to the activity coefficients of the constituent components: $$RT\ln \gamma _{AX} = - X_{B_1 } X_{Y_2 } \Delta G^0$$ (X Ji refers to the atomic fraction of J on sublattice i). The above equation, which assumes complete disorder on (A, B) sites and on (X, Y) sites is extended to the general n-component case. Methods of combining the “cross-site” or reciprocal terms with non-ideal terms for each of the individual sites are also described. The reciprocal terms appear to be significant in many geologically important solid solutions, and clinopyroxene, garnet and spinel solid solutions are all used as examples. Finally, it is shown that the assumption of complete disorder only applies under the condition: $$\Delta G^0 \ll zn_1 RT$$ where z is the number of nearest-neighbour (X, Y) sites around A and n 1 is the number of (A, B) sites in the formula unit. If ΔG 0 is relatively large, then substantial short range oder must occur and the activity coefficient is given by (ignoring individual site terms): $$\gamma _{AX} = \left( {\frac{{1 - X'_{Y_2 } }}{{1 - X_{Y_2 } }}} \right)^{zn_1 }$$ where X′Y2 is the equilibrium atomic fraction of Y atoms surrounding A atoms in the structure. The ordered model may be developed for multicomponent solutions and individual site interactions added, but numerical methods are needed to solve the simultaneous equations involved.

Temperature and pressure dependence of Fe-Mg partitioning between garnet and phengite, with particular reference to eclogites

Abstract: Experimental data combined with data from natural rocks have been used to calibrate a geothermometer based on the distribution of Fe2+ and Mg between coexisting garnets and phengites. The pressure effect on the K D -value appears to be considerable.

Plagioclase buoyancy in basaltic liquids as determined with a centrifuge furnace

Abstract: Plagioclase buoyancy experiments have been carried out in a high-temperature centrifuge furnace using seventeen basaltic liquids and plagioclase crystals of three compositions: An89, An76 and An55. The results show that the floating tendency of plagioclase in basaltic liquids is at least 0.03 g/cm3 greater than indicated by the calculations. If this correction factor is applied to calculations of plagioclase buoyancy in the Skaergaard Intrusion, it is found that the plagioclase crystals in the lower and middle zones were less dense than the coexisting liquids. Other phenomena relevant to crystal transport in basaltic liquids were observed in the centrifuge experiments. These included crystal flotation by rising bubbles, plagioclase sinking because of the formation of plagioclase-magnetite composite grains, graded bedding of olivine and magnetite, and more than 60% intercumulus basaltic liquid between settled olivine crystals.

Active ankerite cementation in the subsurface Eocene of southwest Texas

Abstract: In subsurface samples of Wilcox (Eocene) sandstones, calcite cements occur above 2315 m depths, whereas ankerites occur at depths from 2560 m (temperatures 125 ° C) to at least 4650 m (temperatures 210 ° C). Microprobe analyses indicate that some shallow ankerites have appreciable excess calcium, analogous to protodomites. Ankerites at depths greater than 3200 m have compositions of about CaMg0.5Fe0.5(CO3)2.

The tertiary dike swarms of the Kangerdlugssuaq area, East Greenland

Abstract: Detailed field studies supplemented by petrographic and petrochemical data of dikes in the Kangerdlugssuaq area lead to the recognition of six distinct generations, which may be correlated with the major structural and magmatic events in this part of the North Atlantic Province. In order of decreasing age these swarms are: 1. A broad coastparallel dike swarm of high Fe and Ti tholeiitic basalts, identified as feeders for the plateau basalts of East Greenland and intruded prior to coastal flexuring. 2. A radial tholeiitic dike swarm with MORB affinities, related both in chemistry and chronology to the major gabbroic plutons (including Skaergard) and penecontemporaneous with the coastparallel flexure of East Greenland. 3. A coastparallel alkaline dike swarm, most intense some 20 km inland, composed of picritic to hawaiitic rock types and intruded subsequent to flexuring. 4. Peralkaline rhyolites found in the immediate neighbourhood of the nordmarkitic syenites and clearly related to the major period of salic plutonism. 5. A radial strongly alkaline swarm, including several types of alkaline olivine basalts and trachybasalts differentiating to oversaturated trachytes. 6. Simultaneous with the last, a coastparallel dike swarm composed of transitional basalts differentiating to rhyolitic rock types.

The potential source of lead in the Permian Kupferschiefer bed of Europe and some selected Paleozoic mineral deposits in the Federal Republic of Germany

Abstract: New lead isotopic compositions have been measured for Paleozoic bedded and vein ore deposits of Europe by the high precision thermal emission (triple filament) technique. Eleven samples have been analyzed from the Upper Permian Kupferschiefer bed with representatives from Poland to England, three samples from the Middle Devonian Rammelsberg deposit and one from the Middle Devonian Meggen deposit, both of which are conformable ore lenses and are in the Federal Republic of Germany (FRG); and also two vein deposits from the FRG were analyzed, from Ramsbeck in Devonian host rocks and from Grund in Carboniferous host rocks. For Kupferschiefer bed samples from Germany, the mineralization is of variable lead isotopic composition and appears to have been derived about 250 m.y. ago from 1700 m.y. old sources, or detritus of this age, in Paleozoic sedimentary rocks. Samples from England, Holland, and Poland have different isotopic characteristics from the German samples, indicative of significantly different source material (perhaps older). The isotopic variability of the samples from the Kupferschiefer bed in Germany probably favors the lead containing waters coming from shoreward (where poor mixing is to be expected) rather than basinward (where better mixing is likely) directions. The data thus support the interpretation of the metal source already given by Wedepohl in 1964. Data on samples from Rammelsberg and Meggen tend to be slightly less radiogenic than for the Kupferschiefer, about the amount expected if the leads were all derived from the same source material but 100 to 150 m.y. apart in time. The vein galena from Ramsbeck is similar to that from Rammelsberg conformable ore lenses, both in rocks of Devonian age; vein galena from Grund in Upper Carboniferous country rocks is similar to some bedded Kupferschiefer mineralization in Permian rocks, as if the lead composition was formed at about the same time and from similar source material as the bedded deposits. Although heat has played a more significant role in the formation of some of these deposits (veins and Rammelsberg-Meggen) than in others (Kupferschiefer), there is no indication of radically different sources for the lead, all apparently coming from sedimentary source material containing Precambrian detritus. One feldspar lead sample from the Brocken-Oker Granite is not the same in isotopic composition as any of the ores analyzed.

Two billion years old relicts in the Hercynian belt of Western Europe

Abstract: Icart orthogneisses (Guernsey, Channel Islands) present, in spite of their petrographic homogeneity, Rb-Sr whole rock data strongly scattered between two reference isochrons at 2.55 and 1.91 b.y. We present a U-Pb zircon age of 2018±15m.y. (upper intersection on Concordia) which is probably close to the emplacement age of the whole massif. This age is so far the oldest obtained from a rock of igneous origin in the Hercynian belt of Western Europe.

40 Ar- 39 Ar ages of scandinavian impact structures: I Mien and Siljan

Abstract: 40Ar-39Ar ages have been obtained on samples of impact melt from the Lake Mien and Siljan hypervelocity impact structures in Sweden. Two samples of “rhyolite” from Lake Mien yield a plateau age of 118.7±2.3 m.y., which is consistent with earlier conventional K-Ar results. Two melt samples from a dikelet in the central granite of Siljan have an integrated age of 361.9±1.1 m.y.

Thermodynamic properties of almandine-grossular garnet solid solutions

Abstract: The mixing properties of Fe3Al2Si3O12-Ca3Al2Si3O12 garnet solid solutions have been studied in the temperature range 850–1100° C. The experimental method involves measuring the composition of garnet in equilibrium with an assemblage in which the activity of the Ca3Al2Si3O12 component is fixed. Experiments on the assemblage garnet solid solution, anorthite, Al2SiO5 polymorph and quartz at known pressure and temperature fix the activity of the Ca3Al2Si3O12 component through the equilibrium: 1 $$\begin{gathered} {\text{3CaAl}}_{\text{2}} {\text{Si}}_{\text{2}} {\text{O}}_{\text{8}} \rightleftarrows {\text{Ca}}_{\text{3}} {\text{Al}}_{\text{2}} {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} \hfill \\ {\text{Anorthite garnet}} \hfill \\ {\text{ + 2Al}}_{\text{2}} {\text{SiO}}_{\text{5}} {\text{ + SiO}}_{\text{2}} \hfill \\ {\text{ sillimanite/kyanite quartz}}{\text{.}} \hfill \\ \end{gathered}$$ This equilibrium, with either sillimanite or kyanite as the aluminosilicate mineral, was used to control $${\text{a}}_{{\text{Ca}}_{\text{3}} {\text{Al}}_{\text{2}} {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} }^{{\text{gt}}} $$ . The compositions of the garnet solutions produced were determined by measurement of their unit cell edges. At 1 bar Fe3Al2Si3O12-Ca3Al2Si3O12 garnets exhibit negative deviations from ideality at the Fe-rich end of the series and positive deviations at the calcium end. With increasing pressure the activity coefficients for the Ca3Al2Si3O12 component increase because the partial molar volume of this component is greater than the molar volume of pure grossular. Previous studies indicate that the activity coefficients for the Ca3Al2Si3O12 component also increase with increasing (Mg/Mg+Fe) ratio of the garnet. The region of negative deviation from ideality implies a tendency towards formation of a stable Fe-Ca garnet component. Evidence in support of this conclusion has been found in a natural Fe-rich garnet which was found to contain two different garnet phases of distinctly different compositions.

Fluid inclusions, deformation and recrystallization in granite tectonites

Abstract: An investigation has been made of the relationships between tectonic processes and fluid inclusions in quartz from variably deformed and syntectonically recrystallized granitic rocks from the Lachlan Fold Belt, eastern Australia. The quartz contains many fluid inclusions which decorate healed fractures introduced as a result of late-stage brittle deformation. The majority of small inclusions however, are associated with deformation band boundaries and deformation lamellae showing that they have been introduced during or subsequent to ductile deformation. Fluid inclusions disappear from the cores of sub-grains during recovery and before recrystallization, and new inclusions which form along sub-grain boundaries coalesce into stringers. Inclusions are eliminated from both sides of low angle boundaries showing that inclusions leak their contents either through the system of dislocations which accompanies grain interior slip, or by a dissolution-condensation process whereby inclusion contents move by lattice diffusion and condense on the boundaries.

Minor-element and Sr-isotope geochemistry of tertiary stocks, Colorado mineral belt

Abstract: Rocks of the northeast portion of the Colorado mineral belt form two petrographically, chemically and geographically distinct rock suites: (1) a silica oversaturated granodiorite suite; and (2) a silica saturated, high alkali monzonite suite. Rocks of the granodiorite suite generally have Sr contents less than 1000 ppm, subparallel REE patterns and initial 87Sr/ 86Sr ratios greater than 0.707. Rocks of the monzonite suite are restricted to the northeast part of the mineral belt, where few rocks of the granodiorite suite occur, and generally have Sr contents greater than 1000 ppm, highly variable REE patterns and 87Sr/86Sr initial ratios less than 0.706. Despite forming simple, smooth trends on major element variation diagrams, trace element data for rocks of the granodiorite suite indicate that they were not derived from a single magma. These rocks were derived from magmas having similar REE patterns, but variable Rb and Sr contents, and Rb/Sr ratios. The preferred explanation for these rocks is that they were derived by partial melting of a mixed source, which yielded pyroxene granulite or pyroxenite residues. The monzonite suite is chemically and petrographically more complex than the granodiorite suite. It is subdivided here into alkalic and mafic monzonites, and quartz syenites, based on the textural relations of their ferromagnesian phases and quartz. The geochemistry of these three rock types require derivation from separate and chemically distinct magma types. The preferred explanation for the alkalic monzonites is derivation from a heterogeneous mafic source, leaving a residue dominated by garnet and clinopyroxene. Early crystallization of sphene from these magmas was responsible for the severe depletion of the REE observed in the residual magmas. The lower Sr content and higher Rb/Sr ratios of the mafic monzonites requires a plagioclase-bearing source. The Sr-isotope systematics of the majority of these rocks are interpreted to be largely primary, and not the result of crustal contamination. The positive correlation of Rb/Sr and 87Sr/86Sr ratios for the least fractionated samples indicate that the sources from which parent magmas of both the granodiorite and monzonite suites were derived are Precambrian in age.

Uranium in Metamorphic Rocks

Abstract: The distribution of U has been studied in two metamorphic rock-series with a gradient of regional metamorphism. One series ranges from the lowest greenschist to amphibolite facies and the other one shows increasing metamorphic grade from amphibolite to granulite facies. Several medium and high pressure granulitic inclusions from alkali basalts were also analyzed. The abundances of U in the rocks do not appear to be affected by metamorphism below the granulite facies grade. Granulites are depleted in U in comparison with equivalent rocks of amphibolite facies grade. There are also differences in their U distribution, as the bulk of U in amphibolite facies rocks is located along the fractures and cleavage planes of ferro-magnesian minerals and in U-rich accessories, while in granulites, most of the U resides in accessory minerals. It seems that the depletion of U in granulites is due to a loss of U which is not located in accessory minerals or in the crystal structure of rock-forming minerals and may also be related to a migration of hydrous fluids, perhaps during dehydration.

Chemographic relationships in sapphirine-bearing rocks from Sonapahar, Assam, India

Abstract: Silica-deficient rocks in which sapphirine coexists with cordierite, gedrite, chondrodite, spinel, corundum and phlogopite in various parageneses, occur in association with cordierite-garnet-sillimanitebearing granulitic gneisses in a Precambrian metamorphic terrain displaying amphibolite-granulite transition facies. Brucite coexists with spinel and corundum and its unusual stability in this environment has been attributed to its significant fluorine content. Kornerupine occurs as armoured relics within cordierite in one sample of the sapphirine-spinel-cordierite-phlogopite-bearing rock. Sapphirines (X Mg 0.77 to 0.92) plot near the 7∶9∶3 composition.X Mg varies in the order: chondrodite and brucite>gedrite>cordierite >phlogopite>sapphirine>spinel.

Geochemistry and origin of massif-type anorthosites

Abstract: Samples of Proterozoic anorthosite complexes from the Adirondack Mountains of New York, Burwash Area of Ontario, and the Nain Complex of Labrador, ranging in composition from anorthosite to anorthositic gabbro, have been analyzed for major elements, Rb, Sr, Ba and nine rare-earth elements (REE), in order to set limits on the compositions and origins of their parent magmas. Similar rock types from the different areas have similar major and trace element compositions. The anorthosites have high Sr/Ba ratios, low REE abundances (Ce about 10, Yb about 0.5–1.5 times chondrites) and large positive Eu anomalies. The associated anorthositic gabbros have lower Sr/Ba ratios, REE abundances nearly an order of magnitude higher than the anorthosites, and small to negligible positive Eu anomalies.

The geochemical origin of sepiolite and kerolite at Amboseli, Kenya

Abstract: A massive white sepiolite deposit at Amboseli precipitated from magnesium and silica released during the ground water dolomitization of an earlier lacustrine sepiolite. Kerolite has since formed in proximity to the massive sepiolite as an alteration product of sepiolite and as a ground water precipitate when the pH is below 8. Authigenic sepiolite also occurs in the overlying younger sediments. Kerolite is likely to occur but has not yet been positively identified. Presently, ground waters within the Amboseli Basin are supersaturated with respect to sepiolite and kerolite. This supersaturation results from the weathering of alkaline olivine basalts on the edge of the basin. The precipitation of sepiolite and/or kerolite does not control ground water compositions in the basin. These reactions are slow compared to other aqueous-mineral reactions such as those maintaining carbonate mineral equilibria. Equilibrium between disordered-dolomite and calcite buffers the log a Mg2+/(a H+)2 as a function of log f CO 2 in ground waters in the proximity of the massive sepiolite. This reaction can help explain the presence of sepiolite associated with dolomites in other near-surface deposits besides Amboseli.

Rare earth evidence for the origin of the Nûk gneisses Buksefjorden region, southern West Greenland

Abstract: Rare earth element (REE) concentrations have been determined (by the INAA method) for the c. 2,800 m.y. old Nuk gneisses from the Buksefjorden region, southern West Greenland. Samples include dioritic to granodioritic gneisses and synplutonic mafic dykes; a Malene metagabbro and Qorqut granite were analysed for comparisons. The early Nuk gneisses, diorites and tonalites, have mildly fractionated REE patterns which are interpreted as resulting from partial melting of garnetbearing amphibolite or granulite. Early Nuk trondhjemitic gneisses possess downward convex patterns with prominent positive Eu anomalies; they may be related to the diorites and tonalites by the separation of hornblende in a residue of partial melting or fractional crystallization. Most of the later Nuk grey gneisses have extremely fractionated linear patterns which were derived from a source very rich in garnet, possibly eclogite. REE patterns measured in the late Nuk Ilivertalik granite complex are mildly fractionated but with a high overall abundance consistent with an origin by partial melting of mafic lower crustal material. Two sets of synplutonic mafic dykes have strongly fractionated patterns similar to those found in alkali basalts. The geochemical variations suggest that the igneous precursors of the Nuk gneisses were not cogenetic, but were derived from widely differing sources.

Microstructure and cleavage development in selected slates

Abstract: A detailed microstructural study of three slates by high voltage transmission electron microscopy is reported. The slates are mineralogically similar, come from minor fold cores and exhibit differing degrees of cleavage intensity. All three slates have domains of orientated phyllosilicates (cleavage lamellae) which contain only a low percentage of quartz and carbonate. Between these lamellae are lenticular domains which contain deformed phyllosilicates and which are enriched in secondary minerals. The initiation of cleavage lamellae can be clearly observed in electron micrographs from one of the slates studied. It occurs along zones of intense deformation, viz. along kinks and microfolds, which form from initial crenulations that are difficult to detect in a petrological microscope.

Isotopic ages and tectono-metamorphic history of the metamorphic basement of North-Eastern Sardinia

Abstract: New Rb/Sr and K/Ar data on minerals and whole-rocks from the metamorphic basement of Northeastern Sardinia are presented A formation of augen gneiss of volcano-sedimentary origin yields a Rb/Sr whole-rock age of 441±33 my; a Rb/Sr isochron age of 306±10 my is found for the minerals separated from one of these samples K/Ar measurements on micas also yield ages of 319–284 my A banded migmatite which originated through a process of metamorphic differentiation was analysed by the Rb/Sr method Six bands, treated as whole-rock samples, fit an isochron of 344±7 my Biotite and plagioclase from one of these bands yield an isochron age of about 300 my