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Showing papers in "Contributions to Mineralogy and Petrology in 1977"


Journal ArticleDOI
TL;DR: In this article, a semi-empirical equation of state for the diopside enstatite miscibility gap has been proposed for ortho-and clinopyroxene solid solutions.
Abstract: Simple mixing models have been applied to ortho- and clinopyroxene solid solutions and a semi-empirical equation of state extracted from the available experimental data for the diopside-enstatite miscibility gap. This equation successfully reproduces the miscibility gap over a temperature range of 800 °C to 1700 °C and is apparently also applicable to aluminous pyroxenes in the system CaSiO3-MgSiO3-Al2O3. The effect of iron solubility in the pyroxenes has been calibrated empirically using most of the available experimental data for multicomponent pyroxenes. This semi-empirical model reproduces most of the experimental data within 70 °C. Temperatures calculated for naturally equilibrated Mg-rich two-pyroxene assemblages deviate markedly from those estimated using the thermometer of Wood and Banno (1973). These discrepancies can be attributed to large inaccuracies in the thermometer of Wood and Banno (1973) for Mg-rich compositions.

1,467 citations


Journal ArticleDOI
TL;DR: In this paper, a P-X diagram for the Fe-Mg analogue of this reaction at 675 ° C is consistent with a naturally occuring cordierite-biotite K====== D ≥ 0.53 if Al content of biotite and cordierites water of hydration are taken into account.
Abstract: Stability relations of Fe-Mg cordierite with K feldspar have been determined for conditions of muscovite-quartz instability, applicable to highgrade metamorphism of pelitic rocks. Fe cordierite, K feldspar, and water break down to Fe biotite, sillimanite, and quartz at pressures above a line through 640 ° C, 2kbar and 710 ° C, 2.7 kbar. A P-X diagram for the Fe-Mg analogue of this reaction at 675 ° C is consistent with a naturally occuring cordierite-biotite K D value of 0.53 if Al content of biotite and cordierite water of hydration are taken into account. At higher temperatures Fe cordierite breaks down alone to almandine, sillimanite, quartz and water at pressures above a line through 650 ° C, 3.41 kbar and 760 ° C, 2.9 kbar. For the Fe-Mg reaction, P-X data up to 4 kbar may be extrapolated with use of natural K D values increasing toward one with increasing temperatures. Lines of constant cordierite composition for the two reactions intersect in an Fe-Mg univariant reaction of sillimanite-biotite-quartz to cordieritealmandine-K feldspar-water which is metastable relative to melt at $$P_{{\text{H}}_{\text{2}} {\text{O}}} $$ = P tot Reduced water pressure and impurities in the garnet and K feldspar greatly reduce the temperature of this reaction so that it becomes a reasonable reaction for upper amphibolite and granulite facies conditions. The results demonstrate that (1) cordierite may be used as a geobarometer if temperature and approximate $$X_{{\text{H}}_{\text{2}} {\text{O}}} $$ can be estimated, (2) almandine low in Mn and Ca does not participate in cordierite reactions where muscovite is present, and (3) the reaction which forms cordierite, almandine, and K feldspar is a possible melt-forming reaction which, under reduced $$P_{{\text{H}}_{\text{2}} {\text{O}}} $$ , occurs about 50 ° C above the muscovite melting reaction.

568 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used micro-probe analysis of relict grains of metamorphosed and weathered basalts to identify the magma type of the host lava, showing that any attempt to classify an unknown clinopyroxene as either an ocean-floor basalt or a volcanic arc basalt, within plate tholeiite or a within plate alkali basalt was most likely to be successful.
Abstract: Many metamorphosed and weathered basalts contain fresh clinopyroxene crystals set in an altered groundmass. Microprobe analysis of these relict grains can be used to identify the magma type of the host lava. Statistical discrimination of clinopyroxenes from known magma types provides a test of the effectiveness of this method, showing that any attempt to classify an unknown clinopyroxene as either an ocean-floor basalt, a volcanic arc basalt, a within plate tholeiite or a within plate alkali basalt magma type should have a 70% chance of success. Identification of within plate alkali basalts is most likely to be successful because their pyroxenes characteristically have high Na and Ti and low Si contents. Within plate tholeiites can usually be distinguished from volcanic arc basalts because their pyroxenes contain more Ti, Fe and Mn. However, neither of these last two magma types can be easily distinguished from ocean floor basalts on the basis of pyroxene analyses. Diagrams of pyroxene composition based on discriminant functions and on Na2O vs MnO vs TiO2, SiO2 vs TiO2 and SiO2 vs Al2O3 provide the basis for visual discrimination. The discrimination achieved is mainly due to differences in the bulk chemistry of the host magmas and in the partitioning of cations into the pyroxene lattice; differences in temperature and crystallization histroy of the magmas are of lesser, but nevertheless finite, importance. Application of this technique to pyroxenes in metabasalts from Othris, Greece gave results consistent with, but more ambiguous than, results obtained from immobile trace element studies.

363 citations


Journal ArticleDOI
TL;DR: In this paper, the composition of spinel Iherzolites has been studied and it is suggested that the compositions of spinels represent both undepleted and depleted compositions, and a representative composition for the primitive mantle is proposed on this basis.
Abstract: The compilation of analyses of continental and oceanic spinel Iherzolites show that these two types of Iherzolites have very similar compositions. Their composition range differ from that of African garnet Iherzolites, and the data suggest that the mantle beneath Africa has an anomalous composition. If the composition of the upper mantle may be estimated from that of Iherzolites, the compositions of spinel Iherzolite should form the basis for this estimate. It is suggested that the compositions of spinel Iherzolite represent both undepleted and depleted compositions, and a representative composition for the primitive mantle is proposed on this basis.

312 citations


Journal ArticleDOI
TL;DR: In this paper, an extended application of the Rayleigh distillation law to trace element behavior in a fractional crystallization sequence is described, using a trace element with a very low bulk partition coefficient as a reference (as suggested by Anderson and Greenland, 1969, and extended by Treuil and Varet, 1973).
Abstract: This paper describes an extended application of the Rayleigh distillation law to trace element behavior in a fractional crystallization sequence. Using a trace element with a very low bulk partition coefficient as a reference (as suggested by Anderson and Greenland, 1969, and extended by Treuil and Varet, 1973), we can derive bulk partition coefficients for other elements and, in turn, the mineralogical composition of the cumulates. Trace elements with large D, such as Ni and Cr, further constrain the system, and we can deduce the initial composition of the magma. An example of this technique is shown for Terceira Island in the Azores.

256 citations


Journal ArticleDOI
TL;DR: In this paper, seven high-purity cumulate clinopyroxenes from 2.7 b.y.m. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and 87Sr/86Sr ratio.
Abstract: Seven high-purity cumulate clinopyroxenes from 2.7 b.y. maficultramafic rock associations from the Abitibi belt, Superior Province, Canada, have been analyzed for major elements and K, Rb, Cs, Ba, Sr and 87Sr/86Sr ratio. Attempts to reconstruct the trace element patterns of the original parent magmas were partially successful; Sr contents (140 ppm), K/Rb (470) and K/Ba (16) ratios are similar to those of modern low-K island arc tholeiites. K/Cs ratios (2700) are significantly lower than island arc tholeiites (17,000) or oceanic island and oceanic ridge basalts (> 30,000); the presentday mantle seems to be more depleted in Cs than in Archean times. Initial Sr isotope ratios of the 7 Archean clinopyroxenes average 0.70114±13(2σ) with relatively little variation; this value is in good agreement with initial ratios published for felsic and mafic rocks of the same age, though the latter show much larger variations and uncertainties. The pyroxene Sr isotope data, in conjunction with data for rocks of other ages, defines the following simple model for mantle evolution:

193 citations


Journal ArticleDOI
TL;DR: In this paper, a consistent arrangement of solidus curves for reactions involving white mica, alkali feldspar, Al2SiO5, quartz and H2O is presented.
Abstract: A consistent arrangement of solidus curves for reactions involving white mica, alkali feldspar, Al2SiO5, quartz and H2O, as expressed by the components KAlO2-NaAlO2-Al2O3-SiO2-H2O, provides a generally accurate and useful model for the origin of migmatites and peraluminous granites through partial melting of pelitic rocks The complexities of the univariant (Na-K) reactions may be easily seen on the projection from SiO2 and H2O onto the Ab-Or-Al2O3 plane Alternative topologies for the appearance of muscovite, paragonite and Al2SiO5 on the quartz and H2O-saturated liquidus may be eliminated by a consideration of possible compositions of liquid and solid phases and Schreinemakers' analysis around (Na-K) invariant points Paragonite is not likely to be an important phase on the liquidus for most melting paths Melting paths for natural assemblages may be easily constructed appropriate to P-T-aH2O paths in the model system Such paths are consistent with observed textures in natural assemblages Although general models can be developed for decreased aH2O, it is difficult to separate such effects from those involving participation of a calcic component without careful examination of natural assemblages and experimental calibration of appropriate reactions

189 citations


Journal ArticleDOI
TL;DR: In this article, the problem of estimating the various parameters of a model so as to provide a best fit to observed trace element paths is treated as an inverse problem, where the trace element path (TEP) is defined as a representation of the solution to the direct problem.
Abstract: Trace element concentration data can be used in a systematic way for the study of igneous processes by means of constructing models of such processes which satisfactorily account for the observations We propose to treat the problem as an inverse problem The concept of trace element paths (TEP) is introduced as a representation of the solution to the direct problem The inverse problem consists of estimating, by a resolution of the equations, the various parameters of a model so as to provide a best fit to observed TEP A detailed account of the theory is given in the case of equilibrium fractional crystallization The estimated parameters are then those figuring in the Rayleigh distillation law, namely, 1) the initial concentrations of trace elements in the parental magma, 2) the bulk partition coefficients of the elements, and 3) the degree of crystallization corresponding to each sample of the magmatic suite analyzed

178 citations


Journal ArticleDOI
TL;DR: High-alumina basalts from seven High-Andean stratovolcanoes (37°30′S to 41°S) have major and trace element (including rare earth elements, REE) that are consistent with derivation by partial melting (typically 10−15%) of garnet-free peridotite followed by fractional crystallization of olivine and pyroxene as discussed by the authors.
Abstract: High-alumina basalts from seven High-Andean stratovolcanoes (37 °30′S to 41 °S) have major and trace element (including rare earth elements, REE) that are consistent with derivation by partial melting (typically 10–15%) of garnet-free peridotite followed by fractional crystallization of olivine and pyroxene. High-alumina basalts from two stratovolconoes require significantly lower degrees of melting (<5%) or melting of an incompatible, element-enriched source. However, a poorly understood feature of all of these basalts-and calc-alkaline rocks in general-is the mechanism for causing their low TiO2 and heavy REE content relative to oceanic basalts. Further north in Chile (33 °–34 °S and 21 °–22 °S) amphibole-bearing andesites have REE abundances consistent with derivation from a garnet-bearing source such as incompatible, element-rich eclogite (e.g., Franciscan eclogites) or garnet peridotite. The marked petrological and geochemical changes along strike of the Andes are probably related to the varying nature of the subduction process; e.g., dips of the downgoing slab varying from 10 to 30 °.

167 citations


Journal ArticleDOI
TL;DR: Oxygen and hydrogen isotope studies of a number of granite suites and mineral separates from the New England Batholith indicate that δO18 can be used to discriminate the major granite protoliths as mentioned in this paper.
Abstract: Oxygen and hydrogen isotope studies of a number of granite suites and mineral separates from the New England Batholith indicate that δO18 can be used to discriminate the major granite protoliths. The granite suites previously subdivided on the basis of mineralogical and geochemical criteria into S-type (sedimentary) and I-type (igneous) have δO18 values consistently higher in the S-type granites (10.4–12.5) than in the spatially related I-type plutons (7.7–9.9). There appears to be a systematic variation in δO18 from the most S-type to the most I-type granites, the dividing point between the two occuring at δO18 equal to 10. A group of leucocratic granites that form about half of the batholith and difficult to classify mineralogically and geochemically is found to have low δO18 values (6.4–8.1), suggesting an affinity to the most I-type granites. A single leucogranite pluton with minor muscovite has a δO18 of 9.6 which is significantly higher than other leucogranites indicating a different origin perhaps involving amphibole fractionation.

162 citations


Journal ArticleDOI
TL;DR: In this article, it was shown that spessartine-rich garnets are stable in silicic liquids to pressures as low as 3 kb and that the grossular content of near-liquidus garnets crystallizing from a range of compositions increases with increasing pressure.
Abstract: Melting experiments on a model pelitic composition yield low-spessartine garnet as an important residual phase at pressures above 7 kb. The K D values for distribution of iron and magnesium between coexisting garnet and liquid in the pelitic composition are mainly sensitive to temperature, but also have a small pressure dependence. At temperatures above 950 ° C garnet has a higher $$\frac{{Mg}}{{Mg + Fe}}$$ value than coexisting liquid, but below 950 ° C the garnet $$\frac{{Mg}}{{Mg + Fe}}$$ value is lower than that of the coexisting liquid. Thus at temperatures below 950 ° C silicic magmas may fractionate garnet and produce more magnesian derivative liquids. Reconnaissance experiments with added MnO content in the model pelite demonstrate that spessartine-rich garnets are stable in silicic liquids to pressures as low as 3 kb. The MnO and CaO contents of the experimentally crystallized garnets show an antipathetic relation. Also, the grossular content of near-liquidus garnets crystallizing from a range of compositions increases with increasing pressure. The spessartine and grossular contents of most natural garnets in eastern Australian granitic rocks suggest that these garnets formed at pressures greater than 5 kb. Increased spessartine content allows crystallization of garnet in equilibrium with a silicic magma well within the pressure limit of stability of cordierite, provided the garnet contains ≧10 mol.% spessartine. Thus the depth range over which garnet and cordierite may coexist in a silicic melt is broadened, subject to the availability of MnO. The effect of increased Mn content on the low-pressure stability limit of garnet may also explain the lack of resorption of some garnets in granitic magmas, as these magmas rise to shallower levels. These euhedral garnets characteristically show zoning from an Mn-poor core (typically 4 % MnO) and may reflect continued growth of the garnet in a low pressure regime, stabilized by Mn concentrated in the residual liquid fractions of the crystallizing magma.

Journal ArticleDOI
TL;DR: Garnet-pyroxene skarns were formed 90 m.y.p. in the Osgood Mountains at or near contacts of grandiorite with calcareous rocks of the Cambrian Preble Formation as mentioned in this paper.
Abstract: Garnet-pyroxene skarns were formed 90 m.y. B.P. in the Osgood Mountains at or near contacts of grandiorite with calcareous rocks of the Cambrian Preble Formation. The metasomatic replacement followed contact metamorphic recrystallization of the Preble. The sources, temperature, and variation in H2O/CO2 ratios of the metasomatic fluid are interpreted from 269 analyses of oxygen, carbon, hydrogen, and sulfur isotopes in whole rocks, minerals and inclusion fluids. Skarns formed in three mineralogical stages. Oxygen isotope data indicate that temperatures during the crystallization of garnet, pyroxene and wollastonite (Stage I) were least 550 ° C, and that the metasomatic fluid had an $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.035 in the massive skarns, and ≦ 0.12 in vein skarns up to 3 cm thick. Pore fluids in isotopic equilibrium with garnet in calc-silicate metamorphic rocks, on the other hand, had $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.15. The metasomatic fluids of Stage I were derived primarily from the crystallizing magma. The isotopic composition of magmatic water was δ18O =+9.0, δD= −30 to −45. Oxygen isotope temperatures of greater than 620 ° C were determined for the granodiorite. Isotopic and chemical equilibria between mineral surfaces and the metasomatic fluid were approached simultaneously in parts of the skarn several meters or more apart, while isotopic and chemical disequilibria (i.e. zoning) have been preserved between 20 to 40 μm-thick zones in grandite garnet. More Fe-, or andradite-rich garnet crystallized in more H2O-rich C-O-H fluids ( $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.01) than present with grossularite-rich garnet ( $${\text{X}}_{{\text{CO}}_{\text{2}} } $$ ≦ 0.035). Stage II was marked by the replacement of garnet and pyroxene by quartz, amphibole, plagioclase, epidote, magnetite, and calcite. Many of the replacement reactions took place over a relatively narrow range in temperature (480–550 ° C), as indicated by 18O fractionations between quartz and amphibole. Meteoric water comprised 20 to 50% of the metasomatic fluid during Stage II. Calcite was formed along with pyrite, minor pyrrhotite, and chalcopyrite during Stage III, although the crystallization of pyrite and calcite had begun earlier, during Stages I and II, respectively. Carbon and sulfur isotope compositions of calcite and pyrite indicate a magmatic source for most of the C and S in the metasomatic fluids of Stage III. By the end of Stage III, meteoric water constituted as much as 100% of the metasomatic fluid. Minerals from grandiorite and skarn do not show large depletions in 18O because the oxygen isotope composition of the metasomatic fluid was buffered by the calcareous wall rocks and the grandiorite. Meteoric water in the vicinity of the Osgood Mountains during the Late Crectaceous (δ18Ocale. ≃ −14.0, δD = − 107) was slightly enriched in 18O and D relative to present-day meteoric water (δ18O = 15.9, δD = − 117)

Journal ArticleDOI
TL;DR: In this article, the phase relationships of spinel lherzolite-bearing olivine melilitite from Tasmania were investigated over a P, T range with varying MH-buffered conditions.
Abstract: Near-liquidus phase relationships of a spinel lherzolite-bearing olivine melilitite from Tasmania were investigated over a P, T range with varying $${\text{x}}_{{\text{H}}_{\text{2}} {\text{O}}} $$ , $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ , and $${\text{f}}_{{\text{O}}_{\text{2}} } $$ . At 30 kb under MH-buffered conditions, systematic changes of liquidus phases occur with increasing $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ ( $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ = CO2/CO2 +H2O+olivine melilitite). Olivine is the liquidus phase in the presence of H2O alone and is joined by clinopyroxene at low $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ . Increasing $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ eliminates olivine and clinopyroxene becomes the only liquidus phase. Further addition of CO2 brings garnet+orthopyroxene onto the liquidus together with clinopyroxene, which disappears with even higher CO2. The same systematic changes appear to hold at higher and lower pressures also, only that the phase boundaries are shifted to different $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ . The field with olivine- +clinopyroxene becomes stable to higher $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ with lower pressure and approaches most closely the field with garnet+orthopyroxene+clinopyroxene at about 27 kb, 1160 °C, $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ ∼ 0.08 and $${\text{x}}_{{\text{H}}_{\text{2}} {\text{O}}} $$ ∼ 0.2 (i.e., 6–7% CO2+ 7–8% H2O). Olivine does not coexist with garnet+orthopyroxene+clinopyroxene under these MH-buffered conditions. Lower oxygen fugacities do not increase the stability of olivine to higher $${\text{x}}_{{\text{CO}}_{\text{2}} } $$ and do not change the phase relationships and liquidus temperatures drastically. Thus, it is inferred that olivine melilitite 2927 originates as a ∼ 5% melt (inferred from K2 O and P2O5 content) from a pyrolite source at about 27kb, 1160 dg with about 6–7% CO2 and 7–8% H2O dissolved in the melt. The highly undersaturated character of the melt and the inability to find olivine together with garnet and orthopyroxene on the liquidus (in spite of the close approach of the respective liquidus fields) can be explained by reaction relationships of olivine and clinopyroxene with orthopyroxene, garnet and melt in the presence of CO2.

Journal ArticleDOI
TL;DR: In this article, a microprobe analysis of 20 coexisting oligoclase (An18 3o) and microcline perthite (Ors7 87) pairs from charnockites and granitic gneisses gives KD(Na/(Na + Ca + K)plag/(Na/( Na + Ca+ K))or = 2-3 yielding temperatures of 650 ~ to 750~ in comparison to Seck's (1971) experimental and Stormer's (1975) calculated temperatures for inferred pres- sures of 8 kilobars.
Abstract: Thermometry of regionally metamorphosed granulites of the Adi- rondack Highlands has been undertaken using feldspar and iron-titanium- oxide equilibria. Electron microprobe analyses of 20 coexisting oligoclase (An18 3o) and microcline perthite (Ors7 87) pairs from charnockites and granitic gneisses give KD(Na/(Na + Ca + K)plag/(Na/(Na + Ca + K))or = 2-3 yielding temperatures of 650 ~ to 750~ in comparison to Seck's (1971) experimental and Stormer's (1975) calculated temperatures for inferred pres- sures of 8 kilobars. Microprobe analyses of 10 coexisting titaniferous magne- tite (ulv6spinels~ 1645) and ilmenite (hematitess 4.7-6.5) pairs from the Marcy massif anorthosite and related gabbros give temperatures of 620 ~ to 800 ~ C in comparison to Buddington and Lindsley's (1964) experimental data. Oxygen fugacities buffered by this assemblage range between 10-2~ and 10 16 and always lie within 10 +1 of thefo2 buffered by fayalite-magne- tite-quartz. Exsolved albite in alkali feldspar and ilmenite (oxidized ulv6spinel lamellae) must be reintegrated to infer metamorphic temperatures. Both thermometers give internally consistent, reproducible and geologically rea- sonable results. The inferred 750 ~ and 700~ isotherms wrap around the anorthosite massif in roughly concentric circles. Maximum metamorphic temperatures (790_+ 50 ~ C) occur between Saranac Lake and Tupper Lake, New York.

Journal ArticleDOI
TL;DR: In the southern Gregory Rift valley, a series of transitional basalt, ferrobasalt, and benmoreite flows are overlain by flood trachyte lavas.
Abstract: In the southern Gregory Rift valley a series of transitional basalt, ferrobasalt, and benmoreite flows (1.65–1.4 Myr) is overlain by flood trachyte lavas (1.3–0.9 Myr). Mass balance calculations for major element compositions of rocks of this suite and their phenocrysts and microphenocrysts suggest that the ferrobasalts and benmoreites formed from magma resembling the most primitive basalt by closed system fractionation of plagioclase, clinopyroxene, olivine, titanomagnetite, and apatite. The trachytes formed from evolved magmas largely by alkali feldspar fractionation. Estimates of phenocryst and liquid densities and Rayleigh-law modelling of trace element contents support these conclusions. From Rayleigh-law modelling, we derived a set of effective distribution coefficients. Partial melting of crustal rocks or volatile transfer processes had no significant effect on the petrogenesis of this suite. The duration of the eruptive cycle, cooling time calculations, and mass balance calculations suggest that fractionation occurred in a magma reservoir with volume of at least 3 × 104 km3 during an interval of about 0.8 Myr. Temperatures during fractionation probably ranged from about 1200 °C to 900 °C, and pressures may have been roughly 5 to 8 Kb. We suggest that rift development was accompanied by large-scale injection of basaltic magma and dilation of the crust, extensive fractionation, preferential eruption of low-density and fluid trachytic flood lavas, and by several episodes of normal faulting.


Journal ArticleDOI
TL;DR: In this article, detailed chemical and mineralogical data are given for three sequences of basalts and picrite basalts from bore-holes in Western India, and it is suggested that a slow settling rate for plagioclase relative to clinopyroxene and olivine is sufficient to account for this feature.
Abstract: Detailed chemical and mineralogical data are given for three sequences of basalts and picrite basalts from bore-holes in Western India. The picrite basalts show bulk compositional variation generated by the fractionation of olivine and chromite. Evolved picrite basalt magma appears to have given rise to basalt by the fractionation of olivine+clinopyroxene, despite the presence of abundant plagioclase phenocrysts. It is suggested that a slow settling rate for plagioclase relative to clinopyroxene and olivine is sufficient to account for this feature. The high degree of equilibrium crystallisation which many of the lavas have apparently undergone is interpreted in terms of the mechanism of compensated crystal settling (Cox and Bell, 1972). Experimentally determined atmospheric pressure phase relations are used to model dyke-like magma chambers in some detail. Finally volumetric and age relationships are used to argue that the picrite basalts, despite their porphyritic nature, crystallised from ultramafic liquids containing in some cases at least 16% MgO.

Journal ArticleDOI
TL;DR: In this article, it was shown that the settling velocities of residual crystals in partial melts increase by several orders of magnitude as the degree of melting of mantle peridotite increases, the liquids that are formed become more basic and less viscous and the spacing between residual crystals increases.
Abstract: As the degree of melting of mantle peridotite increases, the liquids that are formed become more basic and less viscous, and the spacing between residual crystals increases. The settling velocities of residual crystals in partial melts consequently will increase by several orders of magnitude, from 9.4 × 10−4 cm/s to 4.3 × 10−1 cm/s for a 1 cm olivine grain, as the proportion of liquid increases from 15 to 60%. To produce an ultrabasic komatiitic magma from a source with commonly assumed mantle composition requires 50 to 80% melting. Before this degree of melting can be reached, a highly fluid picritic magma produced by 30 to 50% melting will segregate from the source. Ultrabasic magmas probably form by a sequential melting process and are derived from a residuum composed of refractory minerals and trapped liquid left by previous episodes of partial melting and magma extraction. Trace element concentrations in ultrabasic komatiite lavas are consistent with this theory.

Journal ArticleDOI
TL;DR: The mineralogical and geochemical features of the lower Ordovician Betts Cove ophiolite of northeastern Newfoundland indicate that hydrothermal circulation of seawater near a mid-ocean ridge has been involved in the metamorphism of the complex as mentioned in this paper.
Abstract: The mineralogical and geochemical features of the lower Ordovician Betts Cove ophiolite of northeastern Newfoundland indicate that hydrothermal circulation of seawater near a mid-ocean ridge has been involved in the metamorphism of the complex. The degree of greenschist facies metamorphism increases with stratigraphie depth in the ophioli te. Calcite, hematite and epidote distributions show that the metamorphosing fluid penetrated downward and was reduced with depth. The mobilities of major and trace elements support the hypothesis of the interaction of seawater and basalt: Fe2O3, MgO, Na2O and H2O increase whereas CaO and Cu decrease in the rock after alteration; SiO2, total iron, K2O, Ba and Rb can either be depleted or enhanced in the altered material; TiO2, P2O5, Zr, Y, Cr and Ni remain stable during the metamorphic episode. Finally, the occurrence of massive sulphides and incipient rodingitic gabbro is explicable in a circulatory seawater system.

Journal ArticleDOI
TL;DR: The chemical variations across the volcanic belt reflect systematic changes in the composition of the magmas due to a decreasing degree of partial melting with increasing depth, and probably also due to the heterogeneity of the source materials as discussed by the authors.
Abstract: Mesozoic to Recent volcanic rocks from a transect of the Central Andes between latitudes 26 ° and 28 ° South in northern Chile and Argentina show chemical and temporal zonation with respect to the Peru-Chile trench. Jurassic to Eocene lavas occur closer to the trench and are comparable to calc-alkaline rocks of island arcs. Eastwards they are followed by Miocene to Quaternary sequences of typical continental margin calc-alkaline rocks which have higher contents of K, Rb, Sr, Ba, Zr, and REE and also higher K/Na and La/Yb ratios. The rocks occurring farthest from the trench have shoshonitic affinities. The distribution of major and trace elements is consistent with a model in which magmas were derived by anatexis of an upper mantle source already enriched in LILE and located above the descending oceanic slab. It is suggested that the chemical variations across the volcanic belt reflect systematic changes in the composition of the magmas due to a decreasing degree of partial melting with increasing depth, and probably also due to the heterogeneity of the source materials.

Journal ArticleDOI
TL;DR: In this article, a systematic investigation of the system KAlSi3O8-NaAlSi 3O8, KCl-NaCl-H2O has been undertaken, where experiments have been carried out at temperatures from 300 °C to 660 °C, pressures from 0.2 to 2 kbar and total chloride concentrations ranging from 0 to 14 moles/kg H2O.
Abstract: Because of frequent discrepancies between the available experimental data and the measured composition of alkali chloride aqueous solutions coexisting with two alkali feldspars in high temperatures-low pressures natural systems, a systematic investigation of the system KAlSi3O8-NaAlSi3O8-KCl-NaCl-H2O has been undertaken. Experiments have been carried out at temperatures from 300 °C to 660 °C, pressures from 0.2 to 2 kbar and total chloride concentrations ranging from 0.05 to 14 moles/kg H2O. No effect of pressure on the feldspars solvus could be detected. Smoothing the experimental data on the basis of the regular assymetric solid solution model yields a critical temperature of 661°C and a critical composition of Or0.36Ab0.64. The “equilibrium constant” C = m KCl/m NaCl does not depend on total chloride molality, as long as the aqueous solution is homogeneous. But, in the miscibility gap (liquid+vapour) of the fluid, C is always lower in the vapour than in the liquid. The higher the temperature and the lower the pressure, the more striking this effect. For instance, at 500 ° C∶ C vaqour/C liquid = 1 above 1 kb, 0.9 at 600 bars, 0.8 at 500 bars, 0.7 at 400–450 bars. The effect of pressure can be neglected in homogeneous fluids and in the liquid phase of unmixed fluids, but it is very important in the vapour phase (dilute solutions at low pressure). The selected values of C max are (±0.01)∶ 300 ° C∶0.083; 400 ° C∶0.139; 500 ° C∶0.200; 600 ° C∶0.264; 650 ° C∶0.298 Such a behaviour of the fluid at low pressures explains the abnormally low values of m KCl/m NaCl measured in many natural hydrothermal systems. A new mechanism of “alkali metasomatism” (especially potassic alterations) is also proposed, taking into account the unmixing of alkali chloride aqueous solutions. This model seems particularly interesting in late magmatic hydrothermal processes, such as those occuring in porphyry type deposits.

Journal ArticleDOI
TL;DR: In this paper, major and trace element and modal analyses are presented for unaltered, epidotized, and carbonated tholeiite flows from the Barberton greenstone belt.
Abstract: Major and trace element and modal analyses are presented for unaltered, epidotized, and carbonated tholeiite flows from the Barberton greenstone belt. Au, As, Sb, Sr, Fe+3, Ca, Br, Ga, and U are enriched and H2O, Na, Mg, Fe+2, K, Rb, Ba, Si, Ti, P, Ni, Cs, Zn, Nb, Cu, Zr, and Co are depleted during epidotization. CO2, H2O, Fe+2, Ti, Zn, Y, Nb, Ga, Ta, and light REE are enriched and Na, Sr, Cr, Ba, Fe+3, Ca, Cs, Sb, Au, Mn, and U are depleted during carbonization-chloritization. The elements least affected by epidotization are Hf, Ta, Sc, Cr, Th, and REE; those least affected by carbonization-chloritization are Hf, Ni, Co, Zr, Th, and heavy REE. Both alteration processes can significantly change major element concentrations (and ratios) and hence caution should be used in distinguishing tholeiites from komatiites based on major elements alone. The amount of variation of many of the least mobile trace elements in the altered flows is approximately the same as allowed by magma model calculations. Hence, up to about 10% carbonization and 60% epidotization of tholeiite do not appreciably affect the interpretation of trace-element models for magma generation.

Journal ArticleDOI
TL;DR: The relationship between thermal history and degree of ordering suggests that stratigraphic boundaries are usually parallel to isopleths of d (101) spacings, but do not always coincide with them as mentioned in this paper.
Abstract: Examination of hydrothermally transformed silica from controlled experiments reveals that amorphous silica changes to quartz through an intermediate phase of opal-CT and that the d(101) spacing of cristobalite progressively decreases from 4.10 A to 4.05 A. The rate of spacing decrease is definitely dependent on the reaction temperature, being faster at higher temperatures. This spacing change represents ordering of opal-CT crystals with the passage of time. The relationship between thermal history and degree of ordering suggests that stratigraphic boundaries are usually parallel to isopleths of d (101) spacings, but do not always coincide with them. The isopleths should be more or less discordant to the stratigraphic boundaries where the strata have been folded. This discordancy can be ascribed to the difference of ordering, chiefly controlled by the thermal history during the burial and folding process.

Journal ArticleDOI
TL;DR: In this article, a large-scale shear zone at Yellowknife displays corresponding changes in major element abundances, including grain size reduction from 1200 μm to <20 μm, depletion of SiO2 (≃5%) and Na2O, together with hydration, and a decrease in specific gravity from 2.97 to 2.80.
Abstract: Metabasalts subjected to progressive deformation in large-scale shear zones at Yellowknife display corresponding changes in major element abundances. Deformation, under conditions of greenschist facies metamorphism, has involved grain size reduction from 1200 μm to <20 μm, depletion of SiO2 (≃5%) and Na2O, together with hydration, and a decrease in specific gravity from 2.97 to 2.80. Chemical redistribution by deformation has been accomplished through a decrease in grain diameter of quartz and albite by intercrystalline diffusive mass transport (pressure solution), with concomitant transfer of material into extension veins. The degree of chemical modification is related to the finite strain. Deformation has involved a redistribution of ∼7.1015g of SiO2 over a volume of about 50km3.

Journal ArticleDOI
TL;DR: In this article, the authors analyzed 87 samples of granulitic xenoliths scavenged by the Neogene volcanoes in French Massif Central, and confirmed the presence of rocks with both sedimentary and igneous derivation chemically similar to other granulite rocks, especially those of the Frazer Range in Australia.
Abstract: 87 samples of granulitic xenoliths scavenged by the Neogene volcanoes in French Massif Central, have been analysed for major elements and Li, Rb, Sr, Ba, V, Cr, Co, Ni, Cu and Zn. These data confirm the presence of rocks with both sedimentary and igneous derivation chemically similar to other granulitic rocks, especially those of the Frazer Range in Australia. Relatively high K and Rb concentration of the metasedimentary rocks conflicts with the hypothesis of systematic depletion of lithophile elements in the lower continental crust.

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TL;DR: In this article, a statistical analysis of 136 natural tourmaline compositions from the literature reveals the presence and extent of coupled substitutions involving several cations and structural sites, such as de-hydroxylation and de-alkali-defect substitution.
Abstract: Statistical analysis of 136 natural tourmaline compositions from the literature reveals the presence and extent of coupled substitutions involving several cations and structural sites. In schorls and dravites these are a dehydroxylation type substitution (1) (OH)−+R2+ = R3++O2− and an alkali-defect type substitution (2) R++R2+ = R3++□, Al3+ being the predominant R3+ action. Substitution (1) which represents solid solution towards a proton-deficient end-member, R+ R 3 3+ R 6 3+ (BO3)3 Si6O18O3(OH), accounts for three times as much of the observed compositional variability as does (2) which represents substitution toward a hypothetical alkali-free end-member, □(R 2 2+ R3+) R 6 3+ (BO3)3Si6O18(OH)4. The occurrence of both of these substituions produces intermediates between end-member schorl/ dravite, R+ R 3 2+ R 6 3+ (BO3)3Si6O18(OH)4, and a new series within the tourmaline group, R 1−x + R 3 3+ R 6 3+ (BO3)3Si6O18O3−x (OH)1+x. In addition to dehydroxylation type, 2(OH)−+Li+ = R3++202−, and possibly alkali-defect type, 2R++Li+ = R3++2□, substitutions, a third type Li++O2− = (OH)−+□, occurs in the elbaites giving rise to Li-poor, proton-rich species. All three substitutions serve to reduce the Li-content of natural elbaite which, as a result, does not attain the composition of the ideal end-member, Na(Li1.5Al1.5)Al6(BO3)3Si6O18(OH)4. Substitution from elbaite and schorl/dravite toward R 1−x + R 3 3+ R 6 3+ (BO3)3Si6O18O3−x(OH)1+x is very extensive and may be complete. Substitution toward R 1−x + R 3 3+ R 6 3+ (BO3)3Si6O18O3−x(OH)1+x results in improved local charge balance. The mean deviation $${\Delta \zeta \left( {\text{O}} \right)}$$ from oxygen charge saturation $$\left( {\zeta \left( {\text{O}} \right) = 2.0} \right)$$ is at a maximum in end-member schorl, dravite and elbaite. Substitutions (1) and (2) progressively decrease $${\Delta \zeta \left( {\text{O}} \right)}$$ but substitution (1) does so more effectively, which may explain its predominance in nature. However, alkali-defective end-members appear to be unstable regardless of $${\Delta \zeta \left( {\text{O}} \right)}$$ . Substitution (3) in the elbaites cannot be discussed on the basis of charge balance considerations at present due to the lack of structural information on proton-rich species.

Journal ArticleDOI
TL;DR: Two granite suites from the New England Batholith, N.S.W., have Upper Carboniferous ages, indicating that they predate by 40 m.y.p. as discussed by the authors.
Abstract: Two “S-type” (pelitic) granite suites from the New England Batholith, N.S.W., have Upper Carboniferous ages, indicating that they predate by 40 m.y. the intrusion of hornblende biotite granites, and are the oldest plutons of the batholith. Mineralogically and geochemically both suites have “pelitic” characteristics, one suite containing an Al-rich biotite, muscovite and cordierite, the other an Al-rich biotite and rare pyrope-almandine garnet. Low initial 87Sr/86Sr ratios of 0.706 for both suites probably reflect the volcanoclastic nature and young age of the sedimentary source of these granites at the time of melting. The age of the suites coincides with the last stages of (Andean type?) volcanism along an andesite/dacite volcanic chain to the west, suggesting an origin for the “S-type” granitic magma by partial melting of deformed sediments marginal to a continental region.

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TL;DR: Feldspathic hornblende granulites from Doubtful Sound, New Zealand with the assemblage plagioclase+hornblende+clinopyroxene+orthopy-roxene +oxide+apatite are crisscrossed by a network of garnetiferous anorthosite veins and pegmatites as mentioned in this paper.
Abstract: Feldspathic hornblende granulites from Doubtful Sound, New Zealand with the assemblage plagioclase+hornblende+clinopyroxene+orthopy-roxene +oxide+apatite are criss-crossed by a network of garnetiferous anorthosite veins and pegmatites The feldspathic gneiss in contact with anorthosite has a reaction zone containing the assemblage plagioclase +garnet+clinopyroxene+quartz+rutile+apatite The garnet forms distinctive coronas around clinopyroxene The origin of these rocks is discussed in the light of mineral and whole rock chemical analyses and published experimental work It is thought that under conditions leading up to ∼ 750 °C, ∼ 8 kb load pressure and ∼ 5 kb H2O pressure, partial melting occured in feldspathic hornblende granulites The melt migrated into extensional fractures and eventually crystallised as anorthosite pegmatites and veins The gneisses adjacent to the pegmatites from which the melt was extracted changed composition slightly, by the loss of H2O and Na2O, so that plagioclase reacted simultaneously with hornblende, orthopyroxene, and oxide to form garnet, clinopyroxene, quartz and rutile

Journal ArticleDOI
TL;DR: In this article, the effect of the addition of iron on calcite-dolomite solvus in the system CaCO3-MgCO3 is predicted from an approximate thermodynamic description of the systems CaCO 3-mg CO3 and CaCO 2-FeCO3.
Abstract: The effect of the addition of iron on the calcite-dolomite solvus in the system CaCO3-MgCO3 is predicted from an approximate thermodynamic description of the systems CaCO3-MgCO3 and CaCO3-FeCO3. Coexisting calcites and dolomites in 15 samples from the Glockner area of the Tauern Window have been analysed to estimate metamorphic temperatures. These range from about 410 °C at the top of the Matrei Zone, on the southern margin of the Tauern Window, to about 490 °C at Hochtor in the centre of the Glockner Depression, implying a geothermal gradient of about 25 °C/km during metamorphism.

Journal ArticleDOI
Jack M. Rice1
TL;DR: In this article, a consistent sequence of uniformly distributed zones of low-variance mineral parageneses separated by abrupt and distinctive isograds was observed in the 1.5 to 2.5 km wide contact aureole surrounding the northernmost portion of the boulder batholith.
Abstract: Progressive metamorphism of impure dolomitic limestone in the 1.5 to 2.5 km wide contact aureole surrounding the northernmost portion of the boulder batholith has resulted in a consistent sequence of uniformly distributed zones of low-variance mineral parageneses separated by abrupt and distinctive isograds. In silica-undersaturated, aluminous marbles, the following mineral assemblages occur, in order of increasing grade: calcite-dolomite-calcic amphibole-chlorite, calcite-dolomite-calcic amphibole-chlorite-spinel, calcite-dolomite-calcic amphibole-chlorite-olivine-spinel, calcite-dolomite-chlorite-olivine-spinel, calcite-dolomite-olivine-spinel. The spatial distribution of parageneses and the occurrence of low-variance parageneses indicate buffering of the pore fluid composition by the local mineral assemblages. The observed sequence of mineral reactions and the spacing of isograds is in good agreement with experimental and calculated equilibria in terms of P-T-XCO2and temperatures of equilibration inferred from calcite-dolomite geothermometry, which range from 435 to 607 °C across the aureole.