Experimental examination of two-pyroxene graphical thermometers using natural pyroxenes with application to metaigneous pyroxenes from the Adirondack Mountains,New York

Clinopyroxene with exsolved orthopyroxene and coexisting orthopyroxene with exsolved clinopyroxene (inverted pigeonite) in metaigneous rocks from the Adirondacks, New York, were experimentally homogenized at temperatures near those inferred for their original crystallization. The purposes were several: (1) to test the graphical two-pyroxene geothermometer of Lindsley (1983); (2) to test the hypothesis of Bohlen and Essene (1978) that these were originally igneous pyroxenes; and (3) to test whether modal recombination of complexly exsolved pyroxenes yields realistic compositions. Experiments on Fe-rich compositions at 930° and 870° C (1 GPa) are compatible with the graphical thermometer of Lindsley (1983); however, this graphical thermometer yields apparent temperatures approximately 50° C too high for experiments at 1050° C and 1100° C (0 MPa). This suggests that at intermediate Mg/Fe the augite isotherms for these temperatures lie at lower wollastonite compositions than shown by Lindsley. The results are, however, in good agreement with isotherms derived from the solution model of Davidson (1985). When these isotherms are applied to a variety of terrestrial and lunar igneous rocks and the metaigneous rocks from the Adirondacks, temperatures given by augite and pigeonite compositions from coexisting pairs are similar. Comparison of the experimentally homogenized compositions with modally recombined compositions of Bohlen and Essene (1978) show that discrepancies between augite and pigeonite temperatures may nevertheless arise if pyroxene grains formed by granular exsolution are not correctly reintegrated.     

The use of coexisting calcite-ankerite solid solutions as a geothermometer

Limited solid solution of Mg and Fe²⁺ occurs in calcite coexisting with dolomiteankerite. This substitution is strongly temperature-dependent. Experimentally determined calcite compositions co-existing with a dolomite phase are available in the binary system CaCO₃-MgCO₃ between 500° C and 900° C (Harker and Tuttle, 1955). This information is extrapolated to lower temperatures and is combined with three synthetic calcite-ankerite pairs determined at 400° C, 450° C and 500° C (Rosenberg, 1967).The compositions of six naturally occurring calcites coexisting with ankerites from Sofala, N.S.W. are not accurately known, but X-ray determined compositional limits assuming firstly Mg substitution then Fe substitution yield maximum and minimum values for both possibilities. These limits are plotted on the ternary CaCO₃-MgCO₃-FeCO₃ together with the experimentally derived solvus isotherms. Assuming equilibration at constant temperature, actual compositions of these natural calcites plot along the 415° C isotherm.     

Mineralogy and petrology of silicate inclusions in iron meteorites

Silicate inclusions in 17 iron meteorites have been analyzed by the electron microprobe and classified, according to their phase assemblages, compositions, and textures, into three major types: Odessa, Copiapo, and Weekeroo Station, and three miscellaneous types: Enon, Kendall County, and Netschaëvo. Phase compositions in both Odessa- and Copiapo-type inclusions are very similar, but the two types are different in texture and constituent phases. Weekeroo Station-type inclusions are very different in every respect from other inclusions.For Odessa- and Copiapo-type inclusions, the distribution coefficients of Fe²⁺ and Mg in coexisting orthopyroxene and clinopyroxene indicate equilibration temperatures of 1,000° C, and the Ca/(Ca+Mg) ratios indicate temperatures of 900° C to 1,000° C. Equilibration temperatures determined for chromite-olivine pairs have a higher range of 1,154° C to 1,335° C. Minor element distributions among coexisting ferromagnesian silicates in these inclusions follow consistent patterns and are constant for any given sample, suggesting equilibrium assemblages. Major and minor element distributions for Weekeroo Station inclusions are anomalous, indicating nonequilibrium.Compositional data, the fragmentary shapes of many inclusions, the highly differentiated characteristic of two types of inclusions, the apparent disequilibrium between kamacite in inclusions and kamacite of the iron host, and the relict chondrules found in Netschaëvo suggest that many of the inclusions did not form cogenetically with the iron host, but represent pre-existing stony material that was taken up by an iron melt, probably not in the core of the parent body (or bodies).     

Oxygen isotope geothermometry and stability of lawsonite and pumpellyite in the Shuksan Suite,North Cascades,Washington

Metamorphic temperatures of 330°–400° C are inferred for rocks from the Shuksan blueschist terrane in the North Cascades, Washington. The temperatures are calculated from ¹⁸O fractionations between coexisting quartz and magnetite using the equations of Bottinga and Javoy (1973). Pressures of approximately 7 kilobars are indicated by the Jadeite content of clinopyroxene coexisting with quartz+albite. Published experimental and theoretical studies of the stability of lawsonite and pumpellyite are consistent with the oxygen isotope temperatures and occurrence of these minerals in the Shuksan Suite.     

Igneous pyroxenes from metamorphosed anorthosite massifs

Anorthosites, mangerites and charnockites from metamorphosed anorthosite massifs (the Adirondacks and elsewhere) commonly contain coarsely exsolved pyroxenes with substantial amounts of exsolved orthopyroxene (in clinopyroxene) and clinopyroxene (in orthopyroxene). Electron microprobe reintegration of such pyroxenes yields compositions which indicate that pigeonite and subcalcic augite coexisted before metamorphic reequilibration. Equilibration temperatures of 1100 °±100 °C for anorthosite and 1000 °C±100 °C for mangerites and charnockites are inferred from the solvus of Ross and Huebner (1975). These temperatures constrain minimum magmatic thermal conditions and suggest that the magmas were relatively dry. Exsolution lamellae of coarse pyroxenes and small equant coexisting pyroxenes (with little or no exsolution) yield temperatures of 750 °C, consistent with equilibration during granulite facies metamorphism. Relict igneous textures and compositions persisted through the metamorphic event due to dry P(H₂O) P(solid) metamorphic conditions. The reintegrated pyroxene compositions provide a window through the metamorphism and yield constraints on the pre-metamorphic igneous events.Contribution No. 340 from the Mineralogical Laboratory, Department of Geology and Mineralogy, The University of Michigan, Ann Arbor, Michigan, 48109, U.S.A.     

Shonkin Sag laccolith,Montana

A variety of rock types is developed in the Shonkin Sag laccolith, with extreme compositions represented by the porphyritic pseudoleucite-bearing chilled margin, shonkinitic in composition, and by the final differentiate, chemically a nepheline syenite. During differentiation the pyroxenes changed in composition from Ca-rich varieties through aegirine-augite to acmite; there is no evidence of an immiscibility gap between Ca-rich and Na-rich pyroxenes. Olivine compositions changed from approximately Fa₂₀ to Fa₄₀, but in marked contrast the coexisting biotites exhibit a more extensive compositional range, from annite₂₄ to annite₁₀₀; the crystal margins of annites in the most evolved rocks are manganophyllite-rich. Titanomagnetites are TiO₂-poor varieties. Arfvedsonite and melanite occur in the most evolved syenites.From mineralogical and thermodynamic data initial and final temperatures of crystallization of the various rock types have been calculated. At an estimated total vapor pressure of 310 bars, the temperature of intrusion was 985° C. Final crystallization of the laccolith took place below 700° C, and crystallization intervals for most rock types are of the order of 170° C. Over the total magmatic temperature range the activity of silica in the melt decreased from 0.13 to 0.09. Oxygen fugacity falls with temperature approximately parallel to the synthetic fayalite-magnetite-quartz oxygen buffer until olivine disappears. The crystallization of Na-rich pyroxenes does not demand an increase in the fugacity of oxygen, but rather requires that the oxygen fugacity fall less rapidly with temperature than would be the case if olivine and magnetite were present.     

Melting of plagioclase-quartz assemblages at 2 kbar water pressure

The melting of plagioclase and quartz has been investigated at P _H2O =2kb. A single crystal of plagioclase was surrounded by quartz powder and water. A reaction rim consisting of glass and of An-rich plagioclase developed around unchanged starting plagioclase. Microprobe determinations of melt and coexisting plagioclase compositions reveal a strong fractionation of plagioclase components between melt and new plagioclase. For example at 850° C the approximate X _An of melt is 0.3 and that of plagioclase is 0.8. The temperature interval between beginning of melting and complete melting of cotectic compositions is 100° C or more for quartz-plagioclase mixtures with plagioclases between An 40 and An 75. In comparison to the system Ab-An-H₂O the plagioclase melting loop is somewhat wider in the investigated system Qz-Ab-An-H₂O but the temperature interval is much smaller (100° C) than in the system Ab-An-H₂O (200° C). The solidus data indicated by the new plagioclase compositions are practically identical with those observed in beginning of melting experiments. The results show that fractionation of plagioclase components between partial melts and restite plagioclase can be more pronounced in multi-component rocks than in the pure plagioclase system. This finding is important for the development of albite rich rocks from more basic compositions.     

Kaledonische Intrusivgesteine des Stavanger-Gebietes

An empirical approach has been taken to develop a geothermometer based on plagioclase-magmatic liquid equilibrium. Compositions of coexisting plagioclase and liquid (glass) obtained by electron microprobe analysis of quenched samples from equilibrium melting experiments of natural granitic rocks at water pressures of 0.5 and 1.0 kilobars have been used along with data from the equilibrium experiments of Bowen (1913, 1915), Prince (1943) and Yoder et al. (1957) to calibrate this geothermometer. Applications of this geothermometer to natural occurrences demonstrate that it can provide useful information on temperature of equilibration of coexisting plagioclase and liquid in rocks ranging in composition from basalt to rhyolite. The plagioclase geothermometer is in good general agreement with other geothermometers wherever these are applicable. Where temperatures are known from other sources it can be used to predict the equilibrium compositions of plagioclase in magmas as well as to provide a rough estimate of water pressure.     

Voluminous low temperature rhyolitic magmas in New Zealand

The glassy acid pumices, lavas and ignimbrites of the Pleistocene-Recent Central volcanic region of New Zealand contain iron-titanium oxide microphenocrysts, whose composition has been determined. Thirteen coexisting titanomagnetite-ilmenite pairs (all onephase and homogeneous) give two groups of equilibration temperatures: 735–780° C for amphibole-bearing rhyolites, and 860–890° C for very young non-amphibole-bearing pumices. O¹⁸/O¹⁶ analyses of coexisting phenocrysts from five amphibole-bearing rhyolites give estimated temperatures in the range 695–860° C, with an average of approximately 780° C.Using Zen's (1971) thermodynamic data on anthophyllite, the fugacity of water has been calculated for two rhyolites with cummingtonite-orthopyroxene-quartz phenocrysts; at 735° C and 745° C, fH₂O is 1100 and 1300 bars respectively. These values are consistent with that derived for an analysed phenocryst assemblage of biotite-sanidine-magnetite. As all the investigated rhyolites contain phenocrysts of orthopyroxene and magnetite, it is suggested that the small increase in of the low-temperature amphibole assemblages in comparison to the amphibole-free assemblages is caused by higher silica activity, as quartz phenocrysts are absent in the high-temperature amphibole-free rhyolites.The existence of large-scale rhyolitic magmas, with phenocryst contents ranging from 0–40 %, at temperatures in the lower part of the magmatic range, is interpreted to be consistent with previously suggested models of upper crustal anatexis in New Zealand, in which the breakdown of micas contributed the water necessary for partial melting.Details of the occurrence and chemistry of the cummingtonite phenocrysts are given in the appendix.     

The thermal history of equilibrated ordinary chondrites and the relationship between textural maturity and temperature

The compositions and textures of phases in eleven equilibrated ordinary chondrites from the H, L, and LL groups spanning petrographic types 4-6 were studied and used to constrain the thermal histories of their parent bodies. Based on Fe-Mg exchange between olivine and spinel, average equilibration temperatures for type 4-6 chondrites encompass a small range, 586-777 °C, relative to what is commonly assumed for peak temperatures (600-950 °C). The maximum temperatures recorded by individual chondrites, which are minima relative to peak metamorphic temperatures, increase subtly but systematically with metamorphic type and are tightly clustered for H4-6 (733-754 °C) and LL4-6 (670-777 °C). For the Ls, Ausson (L5) records a higher maximum olivine-spinel temperature (761 °C) than does the L4 chondrite Saratov (673 °C) or the L6 chondrite Glatton (712 °C). Our data combined with olivine-spinel equilibration temperatures calculated for other equilibrated ordinary chondrites using mineral compositions from the literature demonstrate that, in general, type 4 chondrites within each chemical group record temperatures lower than or equal to those of types 5-6 chondrites.For H chondrites, the olivine-spinel closure temperature is a function of spinel grain size, such that larger grains, abundant in types 5-6 chondrites, record temperatures of ∼740 °C or more while smaller grains, rare in types 5-6 but abundant in type 4 chondrites, record lower temperatures. Olivine-spinel temperatures in the type 6 chondrites Guareña and Glatton are consistent with rapid (50-100 °C/Myr) cooling from high temperatures in the ordinary chondrite parent bodies. With one exception (∼500 °C/Myr), olivine-spinel data for St.-Séverin (LL6) are consistent with similar cooling rates. Cooling rates of order 100 °C/Myr at ∼750 °C for type 6 chondrites are considerably higher than previously determined cooling rates for lower temperatures (?550 °C) based on metallography, fission tracks, and geochronology. For H chondrites, current thermal models of an “onion shell” parent body are inconsistent with a small range of peak temperatures based on olivine-spinel and two pyroxene thermometry combined with a wide dispersion of cooling rates at low temperatures. Equilibrated chondrites may have sampled regions near a major transition in physical properties such as near the base of a regolith pile.     

Phengite geobarometry based on the limiting assemblage with K-feldspar,phlogopite, and quartz

Following and extending the early work of Velde (1965) the pressure-temperature dependence of the compositions of potassic white micas coexisting with K-feldspar, quartz, and phlogopite in the model system K₂O-MgO-Al₂O₃-SiO₂-H₂O was investigated up to fluid pressures of 24 kbar by synthesis experiments. There is a strong, almost linear increase of the Si content per formula unit (p.f.u.) of phengite, ideally KAl_2–xMg_x[Al_1–xSi_3+xO₁₀] (OH)₂ with pressure, as well as a moderate decrease of Si (or x) with temperature. The most siliceous phengite with Si near 3.8 p.f.u. becomes stable near 20 kbar depending on temperature. However, contrary to Velde's assumption, these phengites coexisting with the limiting assemblage are invariably not of an ideal dioctahedral composition (as given by the above formula) but have total octahedral occupancies as high as about 2.1 p.f.u.The stability field of the critical assemblage phengite — K-feldspar — phlogopite — quartz ranges, in the presence of excess H₂O, from at least 350° C to about 700° C but has an upper pressure limit in the range 16–22 kbar, when K-feldspar and phlogopite react to form phengite and a K, Mg-rich siliceous fluid.For the purpose of using these phase relationships as a new geobarometer for natural rocks, the influence of other components in the phengite (F, Fe, Na) is evaluated on the basis of literature data. Water activities below unity shift the Si isopleths of phengite towards higher pressures and lower temperatures, but the effects are relatively small. Tests of the new geobarometer with published analytical and PT data on natural phengite-bearing rocks are handicapped by the paucity of reliable values, but also by the obvious lack of equilibration of phengite compositions in many rocks that show zonation of their phengites or even more than one generation of potassic white micas with different compositions. From natural phengites that do not coexist with the limiting assemblage studied here but still with a Mg, Fe-silicate, at least minimum pressures can be derived with the use of the data presented.     

Compositional zonation in garnets in peridotite xenoliths

Garnets in 42 peridotite xenoliths, most from southern Africa, have been analyzed by electron probe to seek correlations between compositional zonation and rock history. Xenoliths have been placed into the following 6 groups, based primarily upon zonation in garnet: I (12 rocks)-zonation dominated by enrichment of Ti and other incompatible elements in garnet rims; II (10 rocks)-garnet nearly homogeneous; III (8 rocks)-rims depleted in Cr, with little or no related zonation of Ti; IV (3 rocks)-slight Ti zonation sympathetic to that of Cr; V (3 rocks)-garnet rims depleted or enriched in Cr, and chromite included in garnet; VI (6 rocks)-garnets with other characteristics. Element partitioning between olivine, pyroxene, and garnet rims generally is consistent with the assumption of equilibrium before eruption. Although one analyzed rock contains olivine and pyroxene that may have non-equilibrated oxygen isotopes, no corresponding departures from chemical equilibrium were noted. Causes of zoning include melt infiltration and changes in temperature and pressure. Zonation was caused or heavily influenced by melt infiltration in garnets of Group I. In Groups III, IV, and V, most compositional gradients in garnets are attributed to changes in temperature, pressure, or both, and gradients of Cr are characteristic. There are no simple relationships among wt% Cr₂O₃ in garnet, calculated temperature, and the presence of compositional gradients. Rather, garnets nearly homogeneous in Cr are present in rocks with calculated equilibration temperatures that span the range 800–1500 °C. Although the most prominent Cr gradients are found in relatively Cr-rich garnets of rocks for which calculated temperatures are below 1050 °C, gradients are well-defined in a Group IV rock with T1300 °C. The variety of Cr gradients in garnets erupted from a range of temperatures indicates that the zonations record diverse histories. Petrologic histories have been investigated by simulated cooling of model rock compositions in the system CaO–MgO–Al₂O₃–SiO₂–Cr₂O₃. Proportions and compositions of pyroxene and garnet were calculated as functions of P and T. The most common pattern of zonation in Groups III and IV, a decrease of less than 1 wt% Cr₂O₃ core-to-rim, can be simulated by cooling of less than 200 °C or pressure decreases of less than 1 GPa. The preservation of growth zonation in garnets with calculated temperatures near 1300 °C implies that these garnets grew within a geologically short time before eruption, probably in response to fast cooling after crystallization of a small intrusion nearby. Progress in interpreting garnet zonations in part will depend upon determinations of diffusion rates for Cr. Zonation formed by diffusion within garnet cannot always be distinguished from that formed by growth, but Ca–Cr correlations unlike those typical of peridotite suite garnets may document diffusion.     

Sulfide petrology of basal chilled margins in layered sills of the Archean Deer Lake Complex,Minnesota

Sulfide minerals in amounts up to 3 vol% are found in basal, chilled marginal zones of two layered peridotite-pyroxenite-gabbro sills in the Early Precambrian Deer Lake Complex, northcentral Minnesota. The sulfides occur interstitially to silicate minerals, and consist of pyrrhotite with minor exsolved cobaltian pentlandite, chalcopyrite, gersdorffite, and marcasite±pyrite as an alteration product of pyrrhotite.The basal chilled units (3–6 m) of the sills are divisable into three zones based primarily on textures. The lowermost zone is an equigranular basalt, whereas the overlying zone is characterized by skeletal, spinifex-like actinolite after clinopyroxene. The upper zone of the basal margins contains elongate and swallow tail plagioclase, and is barren of sulfide minerals.Electron microprobe analyses of sulfide minerals and modal data suggest that sulfide bulk compositions at 1,100–1,000 ° C represent a pyrrhotite solid solution and a coexisting Cu-rich sulfide liquid. Cooling of the Cu-rich liquid and low temperature transformations are thought to have produced chalcopyrite or chalcopyrite plus pyrrhotite. The sulfide minerals have reequlibrated to temperatures near 300 ° C or less.Analyses of sulfur content and ³⁴S values suggest that assimilation of sulfur from adjacent country rocks was the principal mechanism responsible for anomalous concentrations of sulfides in the basal chilled margins. The distribution of sulfides in the peridotite-pyroxenite-gabbro portions of the sills, and calculations of settling rate preclude an origin involving gravitational settling of immiscible droplets through the magma body.     

Application of arsenopyrite geothermometry and sphalerite geobarometry to the Taebaek Pb-Zn(-Ag) deposit at Yeonhwa I mine,Republic of Korea

The Taebaek Pb-Zn(-Ag) deposit of the Yeonhwa I mine, Republic of Korea, occurs in a broadly folded and reverse-faulted terrain of Paleozoic sedimentary rocks: the Taebaeksan basin. The orebodies consist of several thin tabular orebodies of hydrothermal replacement type where they are hosted by carbonate rocks. The Pb-Zn(-Ag) mineralization can be divided into four distinct stages based upon the mode of occurrence of ore minerals, ore textural relationships and their composition. Based on temperatures inferred from arsenopyrite compositions by means of electron microprobe and fluid inclusions, the estimated temperatures for the stages I, II, III and IV reach 330 to 350 °C, 270 to 340 °C, 230 to 250 °C, and <220 °C, respectively. The sulphur activity (atm) of ore formation at the Taebaek deposit was estimated for each stage as 10^–11 to 10^–11.5, 10^–9.5 to 10^–13, 10^–13.5 to 10^–15, and <10^–15, respectively. Even though application of sphalerite geobarometry is problematic because of the absence of good mineral assemblages, sphalerite coexisting with pyrite but not with pyrrhotite was used to estimate the minimum mineralization pressure (about 1 kbar).     

A regular solution model for met-aluminous silicate liquids: Applications to geothermometry,immiscibility, and the source regions of basic magmas

Adopting a set of multioxide components and using published compositional data on olivineand plagioclase-liquid equilibria we have developed a 17 component regular solution model for met-aluminous silicate liquids. The partial molar excess free energies predicted from this model can be used together with phenocryst compositions as an effective geothermometer, with an approximate error of 20 °C (30 °C for olivine, 12 °C for plagioclase). The regular solution formulation is also successful in predicting liquid immiscibility at (1) high mole fractions of silica commonly observed in phase diagrams, and at (2) lower temperatures in lunar basalts and intermediate lavas. The model yields activities of silica which are consistent with those obtained from solid-liquid silica buffers in rocks which contain olivine and enstatite or quartz. From predicted activities of KAlSi₃O₈ in liquids coexisting with plagioclase a value is obtained for the limiting Henry's law activity coefficient of KAlSi₃O₈ in the solid. This coefficient agrees well with that inferred from plagioclase-sanidine equilibrium phenocryst assemblages in rhyolites. The activities of silica obtained from this model are used to place constraints on the pressure-temperature regions where various types of basic magmas are generated. In conjunction with plagioclase geothermometry an application is given where the pressure, temperature, and water content of an olivine andesite is predicted from the activity of silica.     

Spinel-olivine geothermometry in peridotites from ultramafic complexes

An empirical calibration of the spinel-olivine geothermometer (Evans and Frost, 1975) is attempted and applied to parageneses of lherzolitic rocks. In ultramafic complexes, most of the derived temperature estimates range between 700 ° and 850 °C, and appear generally lower than those given by other geothermometers, based upon Ca or Al contents of coexisting pyroxenes. A comparison of the different thermometric data in two well studied occurrences, the Ronda ultramafic complex and the San Carlos xenolith field, suggests that differential rates of diffusion and recrystalization may account for the large range of temperatures (between 700 ° and 1200 °C) determined by various mineral geothermometers in peridotites. The equilibrium exchange of Mg and Fe between spinel and olivine would be the fastest one and would continue effectively during the cooling down to relatively low temperatures, while other exchange reactions used as geothermometers in peridotites, like partitioning of Ca and Al in coexisting pyroxenes, are blocked at higher temperatures.     

Feldspar geothermometry of the Hell Canyon Pluton,Boulder Batholith,Montana

The bulk compositions of the groundmass alkali feldspar from the Hell Canyon Pluton is 0.146mole% albite. The composition of the outermost zone of the oscillatory zoned plagioclase is 0.686 mole% albite, whereas the most calcic cores have a composition of 0.43 mole% albite. The structural state of the alkali feldspar is near orthoclase. Both composition of coexisting feldspars and structural state of the alkali feldspar are nearly constant throughout the pluton.Exsolved albite in the alkali feldspar have a composition of 0.965 mole% albite and the orthoclase host has a composition of 0.032 mole%. Singe crystal X-ray studies indicate that the albite intergrowths are coherent with the host.Equilibrium temperatures derived from the coexisting feldspar average 554 ° C; about 150 ° C, too low for the minimum solidus temperatures for reasonable emplacement pressures (2 kb). If this minimum solidus temperature is assumed, then the alkali feldspar has lost about 0.15 mole% albite. This loss was most likely caused by hydrothermal solutions associated with the crystallizing magma and equilibrated at about 550 ° C. However, based on the coherent albite intergrowths and the orthoclase structure state it can be inferred that the system was relatively free of volatiles below 500 ° C. Final equilibirium between orthoclase host and albite intergrowths occurred at about 311 ° C.     

The volcanoes and caldera of Talasea,New Britain: Mineralogy

The Talasea Peninsula is composed of a chain of Quaternary volcanoes whose lavas range from basalt to rhyolite. The peninsula is situated in an orogenic environment and the lavas, while essentially calc-alkaline, show some differences from other orogenic suites on the Pacific rim. The most distinctive feature of the Talasea series is absolute iron enrichment in some lavas. Mineralogically, the andesites are characterized by phenocrysts of plagioclase, orthopyroxene, clinopyroxene and titanomagnetite, while the basalts lack titanomagnetite phenocrysts but contain olivine. The acid rocks have a mineralogy similar to that of the andesites, but also contain quartz, amphibole, biotite and ilmenite. The compositions of coexisting titanomagnetite and ilmenite in the acid lavas indicate equilibration temperatures in the range 920° to 860° C and oxygen fugacities ( ) above those of the fayalitemagnetite-quartz buffer assemblage. The mineralogical evidence supports the hypothesis of a crystal fractionation origin for this series and there is a possibility that the was more or less constant during the early stages of its evolution. The iron enriched lavas may be an offshoot from the main line of descent, resulting from near-surface fractionation, with the dominance of plagioclase in the crystal residuum producing an iron-rich liquid.     

Garnet peridotite from Colorado Plateau ultramafic diatremes: Hydrates,carbonates, and comparative geothermometry

Crystal fragments of pyrope from diatremes of ultramafic microbreccia in the Navajo Province of the Colorado Plateau contain inclusions of olivine, pyroxene, spinel, chlorite, amphibole, chlorapatite, and dolomite. The included suite supports earlier hypotheses that hydrous phases and carbonates were primary parts of some garnet peridotite assemblages in the Plateau lithosphere. Garnets with spinel and orthopyroxene inclusions likely all were sampled at pressures less than 36 kb and perhaps as low as 15–20 kb; no evidence was found for inclusions from greater depths. Temperature estimates are 800°–900° C for garnet-clinopyroxene equilibration, but only 500°–700° C for garnetolivine equilibration. Inherent differences between geothermometry methods account for only part of the discrepancy. Pronounced Fe-Mg zoning in garnet at olivine contacts and the lack of such zoning at clinopyroxene contacts are evidence that the difference in part relates to relative reequilibration rates with cooling. The garnet-olivine temperature estimates may be the best approximations to mantle temperatures before eruption. Our data are compatible both with the hypothesis that the garnet peridotite was emplaced in the mantle by large-scale, horizontal transport in the lithosphere and with the hypothesis that rocks were sampled from Precambrian lithosphere cooled to temperatures like those along a low heat flow geotherm. Discordances between the geothermometers here and in other lherzolite localities may be keys to evaluating tectonic histories of lherzolite masses.     

Compatible P-T conditions for eclogites and surrounding gneisses in the Kristiansund area,western Norway

Analyses of coexisting garnets, clinopyroxenes and plagioclases from eclogites and high pressure granulitic gneisses in the Kristiansund area within the west Norwegian basal gneiss region are used to establish the P-T conditions for the metamorphic peak for these rocks. Based on the distribution of Fe and Mg between coexisting garnet and clinopyroxene in both eclogite and granulites, equilibrium temperatures are estimated to 750 °±50 ° C. Pressures are derived from the absence of orthopyroxenes in the granulites, and from the assemblage clinopyroxene +plagioclase+quartz present in the gneisses. Equilibrium pressures are estimated to 18.5±3.0 kilobars, and these equilibrium conditions are thus compatible with equilibrium conditions derived for both orthopyroxene-free and most orthopyroxene-bearing country-rock eclogites from adjacent areas.