scispace - formally typeset
Search or ask a question

Showing papers in "Contributions to Mineralogy and Petrology in 1979"


Journal ArticleDOI
TL;DR: In this paper, a table of mineral-liquid distribution coefficients for Ti, Zr, Y, and Nb for basic, intermediate and acid melt compositions were used to interpret variations of these elements, first in basalts and second during fractional crystallization from basic to acid magmas.
Abstract: Data from experimental runs, coexisting phases in ultramafic rocks and phenocryst-matrix pairs in volcanic rocks have been used to compile a table of mineral-liquid distribution coefficients for Ti, Zr, Y, and Nb for basic, intermediate and acid melt compositions. These values have then been used to interpret variations of these elements, first in basalts and second, during fractional crystallization from basic to acid magmas. For basalts, petrogenetic modelling of Zr/Y, Zr/Ti, and Zr/Nb ratios, when used in conjunction with REE, Cr and isotopic variations, suggests that: (1) the increase in Zr/Y ratio from mid-ocean ridge to within plate basalts and the low Zr/Nb ratios of alkalic basalts are due to (fluid controlled) source heterogeneities; (2) the low Zr and Zr/Y ratio of volcanic arc basalts results from high degree of partial melting of a depleted source; and (3) the high Zr and similar Zr/Y ratio of basalts from fast spreading relative to slow spreading ridges results from open-system fractional crystallization. Modelling of fractionation trends in more evolved rocks using Y-Zr, Ti-Zr and Nb-Zr diagrams highlights in particular the change in crystallizing mafic phases from island arcs (clinopyroxene-dominated) to Andean-type arcs (amphibole±biotite-dominated). These methods can be applied to altered lavas of unknown affinities to provide additional information on their genesis and eruptive environment.

2,680 citations


Journal ArticleDOI
TL;DR: In this paper, a series of basaltic compositions and compositions within the simple system CaO-MgO-FeO-Al2O3-SiO2 have been crystallized to garnet-clinopyroxene bearing mineral assemblages in the range 24-30 kb pressure, 750°-1,300° C temperature.
Abstract: A series of basaltic compositions and compositions within the simple system CaO-MgO-FeO-Al2O3-SiO2 have been crystallized to garnetclinopyroxene bearing mineral assemblages in the range 24–30 kb pressure, 750°–1,300° C temperature. Microprobe analyses of coexisting garnet and clinopyroxene show that K D(Fe2+/MgG+/Fe2+/MgCpx) for the Fe-Mg exchange reaction between coexisting garnet and clinopyroxene is obviously dependent upon the Ca-content and apparently independent of the Mg/(Mg+Fe) content of the clinopyroxene and garnet. The Ca-effect is believed to be due to a combination of non-ideal Ca-Mg substitutions in the garnet and clinopyroxene. Our data and interpretation reconciles previous inconsistencies in the temperature dependence of K D − values determined in experimental studies of simple systems, complex basalt, grospydite and garnet peridotite compositions. Previous differences between the effect of pressure upon K Das predicted from simple system theory (Banno, 1970), and that observed in experiments on multicomponent natural rock compositions (Raheim and Green, 1974a) can now be resolved. We have determined K Das a function of P, T, and X Gt Ca (grossular) and derived the empirical relation $$T\left( {^\circ {\text{K}}} \right) = \frac{{3104X_{{\text{Ca}}}^{{\text{Gt}}} + 3030 + 10.86P\left( {{\text{kb}}} \right)}}{{\ln K_{\text{D}} + 1.9034}}$$ . This empirical relationship has been applied to garnet-clinopyroxene bearing rocks from a wide range of geological environments. The geothermometer yields similar estimates for garnet-clinopyroxene equilibration for neighbouring rocks of different composition and different K Dvalues. In addition, temperature estimates using the above relationship are more consistent with independent temperature estimates based on other geothermometers than previous estimates which did not correct for the Ca-effect. An alternative approach to the above empirical geothermometer was attempted using regular solution models to derive Margules parameters for various solid solutions in garnets and clinopyroxenes. The derived Margules parameters are broadly consistent with those determined from binary solution studies, but caution must be exercised in interpreting them in terms of actual thermodynamic properties of the relevant crystalline solid solutions because of the assumptions which necessarily have to be made in this approach.

1,494 citations


Journal ArticleDOI
TL;DR: In this article, an account of the experimental deformation of partially-melted granite with melt fractions up to 25% at 800 °C and 300 MPa confining pressure in constant strainrate tests between 10−3 and 10−6 S−1, creep tests and cycling tests is given.
Abstract: An account is given of the experimental deformation of partially-melted granite with melt fractions up to 25% at 800 °C and 300 MPa confining pressure in constant strainrate tests between 10−3 and 10−6 S−1, creep tests and cycling tests Microscopic study reveals that under these conditions most of the uniform deformation prior to macroscopic shear failure is accomplished by melt redistribution into films perpendicular to the least compressive stress, and by axial fracturing of grains, the latter occurring even at low macroscopic differential stress The strenght of the partially-melted rock at 10−5 S−1 is found to decrease gradually from about 250 MPa at 5 vol% melt to about 60 MPa at 15 % melt, and then to drop rapidly to less than 1 MPa at 24% melt The critical melt fraction separating granular-framework-controlled flow behaviour from suspension-like behaviour is deduced to be approximately 30 to 35 vol% At low melt fractions the deforming rock tends to take up externally available melt by a mechanism of dilatancy pumping The relevance of these results to natural conditions involving partially-melted rocks is discussed

617 citations


Journal ArticleDOI
TL;DR: For peraluminous melts and melts lying in the quartz-orthoclase-albite composition plane, less than 100 ppm Zr is required for zircon saturation as mentioned in this paper.
Abstract: Hydrothermal experiments were carried out at 2 kbar water pressure, 700 °–800 ° C, with the objective of determining the level of dissolved Zr required for precipitation of zircon from melts in the system SiO2-Al2O3-Na2O-K2O. The saturation level depends strongly upon molar (Na2O + K2O)/Al2O3 of the melts, with remarkably little sensitivity to temperature, SiO2 concentration, or melt Na2O/ K2O. For peraluminous melts and melts lying in the quartz-orthoclase-albite composition plane, less than 100 ppm Zr is required for zircon saturation. In peralkaline melts, however, zircon solubility shows pronounced, apparently linear, dependence upon (Na2O + K2O)/Al2O3, with the amount of dissolvable Zr ranging up to 3.9 wt.% at (Na2O + K2O)/Al2O3 = 2.0. Small amounts (1 wt.% each) of dissolved CaO and Fe2O3 cause a 25% relative reduction of zircon solubility in peralkaline melts. The main conclusion regarding zirconium/zircon behavior in nature is that any felsic, non-peralkaline magma is likely to contain zircon crystals, because the saturation level is so low for these compositions. Zircon fractionation, and its consequences to REE, Th, and Ta abundances must, therefore, be considered in modelling the evolution of these magmas. Partial melting in any region of the Earth's crust that contains more than ∼ 100 ppm Zr will produce granitic magmas whose Zr contents are buffered at constant low (< 100 ppm) values; unmelted zircon in the residual rock of such a melting event will impart to the residue a characteristic U- or V-shaped REE abundance pattern. In peralkaline, felsic magmas such as those that form pantellerites and comendites, extreme Zr (and REE, Ta) enrichment is possible because the feldspar fractionation that produces these magmas from non-peralkaline predecessors does not drive the melt toward saturation in zircon. Zircon solubility in felsic melts appears to be controlled by the formation of alkali-zirconosilicate complexes of simple (2:1) alkali oxide: ZrO2 stoichiometry.

564 citations


Journal ArticleDOI
TL;DR: A comparison of approximately 2,000 abyssal tholeiite compositions to the experimental liquid-line-of-descent equilibria reveals that erupted differentiates which would be expected from advanced fractionation are scarce.
Abstract: Unusual effects have been discovered in the major element phase relations of basalts from the Oceanographer Fracture Zone (OFZ) which suggest that magma mixing of primitive basalt with the differentiation residue of a previous batch of primitive magma has occurred. These effects include a reversal in mineral crystallization sequence which cannot happen during normal differentiation processes or be explained by any plausible change in physical conditions. This unusual effect is encountered as a result of curvature in composition space of the liquid-line-of-descent equilibria involving olivine, plagioclase, and high-calcium clinopyroxene. Mixing of magmas at different stages of their evolution produces mixtures that do not lie on the curved liquid-line-of-descent. Observation of such anomalous compositions in the OFZ suite supplements accumulating petrographic and trace-element geochemistry evidence that magma mixing is an important petrogenetic process.Mixing of fractionated residual liquids can produce mixtures which are either superheated or supercooled depending on the sense of ‘thermal curvature’ of the liquid-line-of-descent. Both senses are encountered in the tholeiitic system, and this effect may exert a qualitative control on the crystallization texture of the mixture.A comparison of approximately 2,000 abyssal tholeiite compositions to the experimental liquid-line-of-descent reveals that erupted differentiates which would be expected from advanced fractionation are scarce. Just this sort of phenomenon (the ‘perched’ steady state) was proposed by O'Hara (1977) as an earmark of the operation of a continuously fractionating magma chamber into which fresh magma is periodically remixed.

549 citations


Journal ArticleDOI
TL;DR: In this paper, major and trace element data are used to establish the nature and extent of spatial and temporal chemical variations in basalts erupted in the Iceland region of the North Atlantic Ocean.
Abstract: Major and trace element data are used to establish the nature and extent of spatial and temporal chemical variations in basalts erupted in the Iceland region of the North Atlantic Ocean. The ocean floor samples are those recovered by legs 38 and 49 of the Deep Sea Drilling Project. Within each of the active zones on Iceland there are small scale variations in the light rare earth elements and ratios such as K/Y: several central complexes and their associated fissure swarms erupt basalts with values of K/Y distinct from those erupted at adjacent centres; also basalts showing a wide range of immobile trace element ratios occur together within single vertical sections and ocean floor drill holes. Although such variations can be explained in terms of the magmatic processes operating on Iceland they make extrapolations from single basalt samples to mantle sources underlying the outcrop of the sample highly tenuous.

508 citations


Journal ArticleDOI
TL;DR: In this article, the partitioning of Fe and Mg between coexisting garnet and olivine has been studied at 30 kb pressure and temperatures of 900 ° to 1,400 °C.
Abstract: The partitioning of Fe and Mg between coexisting garnet and olivine has been studied at 30 kb pressure and temperatures of 900 ° to 1,400 °C. The results of both synthesis and reversal experiments demonstrate that K D (= (Fe/Mg)gt/(Fe/Mg)OI) is strongly dependent on Fe/Mg ratio and on the calcium content of the garnet. For example, at 1,000 °C/30 kb, K D varies from about 1.2 in very iron-rich compositions to 1.9 at the magnesium end of the series. Increasing the mole fraction of calcium in the garnet from 0 to 0.3 at 1,000 ° C increases K D in magnesian compositions from 1.9 to about 2.5. The observed temperature and composition dependence of K D has been formulated into an equation suitable for geothermometry by considering the solid solution properties of the olivine and garnet phases. It was found that, within experimental error, the simplest kind of nonideal solution model (Regular Solution) fits the experimental data adequately. The use of more complex models did not markedly improve the fit to the data, so the model with the least number of variables was adopted. Multiple linear regression of the experimental data (72 points) yielded, for the exchange reaction: 3Fe2SiO4+2Mg3Al2Si3O12 olivine garnet ⇌ 2Fe2Al2Si3O12+3Mg2SiO4 garnet olivine ΔH ° (30kb) of −10,750 cal and ΔS ° of −4.26 cal deg−1 mol−1. Absolute magnitudes of interaction parameters (W ij ) derived from the regression are subject to considerable uncertainty. The partition coefficient is, however, strongly dependent on the following differences between solution parameters and these differences are fairly well constrained: W FeMg ol -W FeMg gt ≃ 800 cal W CaMg gt -W CaFe gt ≃ 2,670 cal. The geothermometer is most sensitive in the temperature and composition regions where K D is substantially greater than 1. Thus, for example, peridotitic compositions at temperatures less than about 1,300 ° C should yield calculated temperatures within 60 °C of the true value. Iron rich compositions (at any temperature) and magnesian compositions at temperatures well above 1,300 °C could not be expected to yield accurate calculated temperatures. For a fixed K D the influence of pressure is to raise the calculated temperature by between 3 and 6 °C per kbar.

450 citations


Journal ArticleDOI
TL;DR: In this paper, an empirical calibration of the spinel-olivine geothermometer (Evans and Frost, 1975) is attempted and applied to parageneses of lherzolitic rocks.
Abstract: 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.

351 citations


Journal ArticleDOI
TL;DR: In this article, a wide variety of Pliocene-Recent volcanic rocks from central Italy are presented, and they are tentatively subdivided into three zones: (1) N. Tuscany where the magmas are believed to reflect crustal anatexis.
Abstract: 143Nd/144Nd, 87Sr/86Sr, and REE analyses are presented on a wide variety of Pliocene-Recent volcanic rocks from central Italy. 143Nd/144Nd varies from 0.51214–0.51289 and 87Sr/86Sr from 0.7255-0.7036; while the rare earth elements are characterised by light RE enrichment and a significant negative Eu anomaly. These Italian volcanics are tentatively subdivided into three zones: (1) N. Tuscany where the magmas are believed to reflect crustal anatexis. (2) A central zone in which hybrid (crust/ mantle) rocks have been recognised. (3) A southern zone, south of Rome, where mantle-derived magmas are identified which have been largely unaffected by interaction with continental crust. At Roccamonfina, in zone 3, Rb/Sr and Sm/Nd pseudo isochrons are observed but since the calculated ages are 0.5 and 2.0 b.y. respectively it is argued that a simple isochron model is not applicable and that the data are most easily explained by a recent mixing event within the upper mantle. It is envisaged that this occurred during metasomatism of the upper mantle source region by a fluid that had high 87Sr/86Sr and low 143Nd/144Nd and was enriched in K, Rb, and LREE's but relatively depleted in Sr2+ and Eu2+.

328 citations


Journal ArticleDOI
TL;DR: In the Oceanographer Fracture Zone, fresh basalts from the seafloor are petrographically and chemically similar to typical abyssal tholeiites, but are somewhat enriched in large ion lithophile elements, with consistent differences among separate dredge hauls as mentioned in this paper.
Abstract: Fresh basalts from the Oceanographer Fracture Zone are petrographically and chemically similar to typical abyssal tholeiites, but are somewhat enriched in large ion lithophile elements, with consistent differences among separate dredge hauls. Olivine compositions are in equilibrium with host basalt for reasonable KDvalues, but some plagioclases are anomalously calcic (e.g., a rock highly differentiated in Mg/Mg+Fe bears some plagioclase of An86). Ti/Al in clinopyroxene is approximately constant at 1/6, but Ti+Al abundance decreases in the sequence basalt groundmass cpx, basalt phenocryst+microphenocryst cpx, gabbro cpx (samples from adjacent dredges), an effect that may be related to decrease in cooling rate. Least-squares calculations indicate that 45% of magma with the composition of the more primitive sample must be removed as plagioclase, clinopyroxene and olivine (in the ratio 4.4∶2.7∶1, respectively) to obtain the composition of the more differentiated sample. Both samples have excess plagioclase on the liquidus, however, which should not be the case for the differentiated composition if it is produced by fractionation of all three phases. The excess of plagioclase and the anomalous plagioclase compositions indicate that the samples cannot be related solely by fractional crystallization. Additional processes such as magma mixing or plagioclase accumulation (or both) must have effected these differences.

286 citations


Journal ArticleDOI
TL;DR: In this paper, the authors show that at 5 and 20 kbar on the composition join of sanidine-potassium carbonate are dominated by a two-liquid region that covers over 60% of the join at 1,300 ° C. Partition coefficients for Ce, Sm, and Tm between the immiscible carbonate and silicate melts at 1:200 ° and 1:300 C at 5 kbar are in favor of the carbonate melt by a factor of 2-3 for light REE and 5-8 for heavy REE.
Abstract: Melting relations at 5 and 20 kbar on the composition join sanidine-potassium carbonate are dominated by a two-liquid region that covers over 60% of the join at 1,300 ° C. At this temperature, the silicate melt contains approximately 19 wt% carbonate component at 5 kbar and 32 wt% carbonate component at 20 kbar. The conjugate carbonate melt contains less than 5 wt% silicate component, and it varies less as a function of temperature than does the silicate melt. Partition coefficients for Ce, Sm, and Tm between the immiscible carbonate and silicate melts at 1,200 ° and 1,300 ° C at 5 and 20 kbar are in favor of the carbonate melt by a factor of 2–3 for light REE and 5–8 for heavy REE. The effect of pressure on partitioning cannot be evaluated independently because of complementary changes in melt compositions. Minimum REE partition coefficients for CO2 vapor/carbonate melt and CO2 vapor/silicate melt can be calculated from the carbonate melt/silicate melt partition coefficients, the known proportions of melt, and maximum estimates of the proportion of CO2 vapor. The vapor phase is enriched in light REE relative to both melts at 20 kbar and enriched in all REE, especially the light elements, at 5 kbar. The enrichment of REE in CO2 vapor relative to both melts is 3–4 orders of magnitude in excess of that in water vapor (Mysen, 1979) at 5 kbar and is approximately the same as that in water vapor at 20 kbar. Mantle metasomatism by a CO2-rich vapor enriched in light REE, occurring as a precursor to magma genesis, may explain the enhanced REE contents and light REE enrichment of carbonatites, alkali-rich silicate melts, and kimberlites. Light REE enrichment in fenites and the granular suite of nodules from kimberlites attests to the mobility of REE in CO2-rich fluids under both mantle and crustal conditions.

Journal ArticleDOI
TL;DR: A re-evaluation of the geothermometer has shown that more realistic temperatures can be obtained for volcanic rocks by using a different free energy value of FeCr2O4 in the formulation.
Abstract: The Irvine olivine-spinel geothermometer, as formulated by Jackson (1969), appears to yield magmatic temperatures when applied to plutonic rocks such as the Stillwater Complex but Evans and Wright (1972) have demonstrated that it gives temperatures in excess of 2,000 ° C when applied to volcanic assemblages. A re-evaluation of the geothermometer has shown that more realistic temperatures can be obtained for volcanic rocks by using a different free energy value of FeCr2O4 in the formulation. The revised geothermometer gives temperatures in the range 1,100–1,300 ° C for samples from Kilauea and 500–800 ° C for basic plutonic rocks from layered intrusions, indicating that Mg and Fe2+ have re-equilibrated at subsolidus temperatures in these intrusions as suggested by Irvine (1965). This theory was tested by heating uncrushed natural samples from layered intrusions to magmatic temperatures for periods ranging from two days to four weeks. The result was a marked increase in the Mg/Fe2+ ratio in the spinels and a decrease in the Mg/Fe2+ ratio in the olivines, confirming that considerable subsolidus re-equilibration had taken place in the unheated samples.

Journal ArticleDOI
TL;DR: In this article, the upper pressure stability of paragonite in the temperature range 550-740 ° C has been investigated and the experimental brackets are thermodynamically consistent with the lower pressure reversals.
Abstract: Hydrothermal reversal experiments have been performed on the upper pressure stability of paragonite in the temperature range 550–740 ° C. The reaction $$\begin{gathered} {\text{NaAl}}_{\text{3}} {\text{Si}}_{\text{3}} {\text{O}}_{{\text{1 0}}} ({\text{OH)}}_{\text{2}} \hfill \\ {\text{ paragonite}} \hfill \\ {\text{ = NaAlSi}}_{\text{2}} {\text{O}}_{\text{6}} + {\text{Al}}_{\text{2}} {\text{SiO}}_{\text{5}} + {\text{H}}_{\text{2}} {\text{O}} \hfill \\ {\text{ jadeite kyanite vapour}} \hfill \\ \end{gathered}$$ has been bracketed at 550 ° C, 600 ° C, 650 ° C, and 700 ° C, at pressures 24–26 kb, 24–25.5 kb, 24–25 kb, and 23–24.5 kb respectively. The reaction has a shallow negative slope (− 10 bar °C−1) and is of geobarometric significance to the stability of the eclogite assemblage, omphacite+kyanite. The experimental brackets are thermodynamically consistent with the lower pressure reversals of Chatterjee (1970, 1972), and a set of thermodynamic data is presented which satisfies all the reversal brackets for six reactions in the system Na2O-Al2O3-SiO2-H2O. The Modified Redlich Kwong equation for H2O (Holloway, 1977) predicts fugacities which are too high to satisfy the reversals of this study. The P-T stabilities of important eclogite and blueschist assemblages involving omphacite, kyanite, lawsonite, Jadeite, albite, chloritoid, and almandine with paragonite have been calculated using thermodynamic data derived from this study.

Journal ArticleDOI
TL;DR: In this article, an isotopic dating investigation (66 K-Ar and 34 Rb-Sr analyses) provided the geochronological framework for the Alpine events of metamorphism and granitic magmatism on Naxos.
Abstract: An isotopic dating investigation (66 K-Ar and 34 Rb-Sr analyses) provided the geochronological framework for the Alpine events of metamorphism and granitic magmatism on Naxos. The oldest phase of high-pressure/medium-temperature metamorphism, M1, was dated by Rb-Sr and K-Ar analyses of paragonites, phengitic muscovites and muscovites at 45±5 Ma (Middle Eocene). Most of the record of the M1 phase has been eraded by a second phase of medium-pressure/high-temperature metamorphism, M2, which induced a metamorphic zonation with anatectic melting in the highest-grade part, the migmatite dome. Most K-Ar dates of M2 hornblendes, muscovites, biotites and tourmalines range from about 21 Ma in the lower-grade part (biotite-chloritoid zone) to about 11 Ma in the migmatite dome. From the pattern of K-Ar mineral dates it is concluded that the M2 phase took place 25±5 Ma ago (Late Oligocene/Early Miocene) and was followed by a prolonged cooling history until about 11 Ma ago (Late Miocene), when the ambient temperature in the migmatite dome had decreased to below 400−360 °C. A Rb-Sr whole-rock isochron analysis of a granodioritic mass dated the intrusion (and the associated M3 phase of contact-metamorphism) at 11.1±0.7 Ma (Late Miocene), with an initial 87Sr/86Sr ratio of 0.7112 ±0.0001. A local phase of low-grade retrograde metamorphism, M4, probably related to Late Alpine overthrusting, was dated at about 10 Ma (Late Miocene).

Journal ArticleDOI
TL;DR: The authors found high-K calc-alkaline lavas over Benioff zone depths in excess of 300 km, where tholeiites were found in the normal island arc association.
Abstract: Since Mesozoic time, Java and Bali have formed part of an evolving system of island arcs comprising the Sunda arc of Indonesia. The present tectonic setting is relatively simple with subduction occurring at the Java Trench to the south. A north-dipping Benioff seismic zone delineates an underthrust lithospheric slab to depths of approximately 600 km beneath the Java Sea. Quaternary lavas of the ‘normal island arc association’ range from tholeiites to high-K calc-alkaline lavas over Benioff zone depths from 120–250 km, respectively. More abundant calc-alkaline lavas lie between these extremes. High-K alkaline lavas are found over Benioff zone depths in excess of 300 km.

Journal ArticleDOI
TL;DR: K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y..
Abstract: Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3T phengite, and omphacite+garnet +3T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2M 1 phengite. K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y., indicating a Late Lutetian age for the high P/T metamorphism. Phengites (2M 1+3T) of less preserved high P/T assemblages yielded K-Ar dates between 48 and 41 m.y. but generally lower Rb-Sr dates. The higher K-Ar dates are interpreted as being elevated by excess argon. K-Ar and Rb-Sr ages on 2M 1 phengites from central Sifnos vary between 24 and 21 m.y. These ages date a second, greenschist-facies metamorphism which overprinted the earlier high-pressure metamorphic rocks.

Journal ArticleDOI
TL;DR: Deception and Bridgeman lavas have been studied in order to provide information on the nature of magmatism associated with the initial stages of back-arc spreading as mentioned in this paper.
Abstract: Bransfield Strait is a narrow basin separating the South Shetland Islands from the Antarctic Peninsula and is attributed to recent back-arc extension behind the South Shetland volcanic arc. The volcanic islands of Deception and Bridgeman are situated close to the axis of spreading, whereas Penguin Island lies slightly to the north of this axis. The mineralogy, petrology and geochemistry of the lavas of the three volcanoes have been studied in order to provide information on the nature of magmatism associated with the initial stages of back-arc spreading. Deception Island lavas range from olivine basalt to dacite, and all are highly sodic, with high Na/K, K/Rb, Ba/Rb and Zr/Nb ratios and with CeN/YbN = 2. Incompatible elements increase systematically between basalt and rhyodacite, while Sr decreases, suggesting that fractional crystallisation is the dominant process relating lava compositions. The rhyodacites have high concentrations of Zr, Y and the REE and negative Eu anomalies and are compositionally similar to oceanic plagiogranite. Bridgeman Island lavas are mostly basaltic andesites, but the levels of many incompatible elements, including REE, are significantly lower than those of Deception lavas, although CeN/YbN ratios and 87Sr/86Sr ratios (0.7035) are the same. Penguin Island lavas are magnesian, mildly alkaline olivine basalts with a small range of composition that can be accommodated by fractional crystallisation of olivine, clinopyroxene and/or chromite. Penguin lavas have higher 87Sr/86Sr (0.7039) and CeN/ YbN (4) ratios than Deception and Bridgeman lavas. The Rb/Sr ratios of Deception and Penguin basalts (ca. 0.01) are much too low to account for their present 87Sr/86Sr ratios. Modelling suggests that the source regions of the lavas of the three volcanoes share many geochemical features, but there are also some significant differences, which probably reflects the complex nature of the mantle under an active island arc combined with complex melting relationships attending the initial stages of back-arc spreading. Favoured models suggest that Bridgeman lavas represent 10–20% melting and the more primitive Deception lavas 5–10% melting of spinel-peridotite, whereas Penguin lavas represent less then 5% melting of a garnet-peridotite source. The mantle source for Bridgeman lavas seems to have undergone short-term enrichment in K, Rb and Ba, possibly resulting from dewatering of the subducted slab. Hydrous melting conditions may also account for the more siliceous, high-alumina nature and low trace element contents of Bridgeman lavas.

Journal ArticleDOI
TL;DR: Petrologic and chemical data are presented for samples from five volcanically active islands in the northern Marianas group, an intra-oceanic island arc as discussed by the authors, including microprobe analyses of phenocryst and xenolith assemblages, whole rock major and trace element chemistry including REE, and Sr isotope determinations.
Abstract: Petrologic and chemical data are presented for samples from five volcanically active islands in the northern Marianas group, an intra-oceanic island arc. The data include microprobe analyses of phenocryst and xenolith assemblages, whole rock major and trace element chemistry including REE, and Sr isotope determinations (87Sr/86Sr=0.7034±0.0001). Quartz-normative basalt and basaltic andesite are the most abundant lava types. These are mineralogically and chemically similar to the mafic products of other intra-oceanic islands arcs. It is suggested, however, that they are not typical of the ‘island arc tholeiitic’ series, having Fe enrichment trends and K/Rb, for example, more typical of calc-alkaline suits. Major and trace element characteristics, and the presence of cumulate xenoliths, indicate that extensive near surface (< 3 Kb) fractionation has occurred. Thus, even least fractionated basalts have low abundances of Mg, Ni and Cr, and high abundances of K and other large cation, imcompatible elements, relative to ocean ridge tholeiites. However, abundances of REE and small cation lithophile elements, such as Ti, Zr, Nb, and Hf are lower than typical ocean ridge tholeiites. The REE data and Sr isotope compositions suggest a purely mantle origin for the Marianas island arc basalts, with negligible input from subducted crustal material. Thus, subduction of oceanic lithosphere may not be a sufficient condition for initiation of island arc magmatism. Intersection of the Benioff zone with an asthenosphere under appropriate conditions may be requisite. Element ratios and abundances, combined with isotopic data, suggest that the source for the Marianas island arc basalts is more chondritic in some respects, and less depleted in large cations than the shallow (?) mantle source for ocean ridge tholeiites.

Journal ArticleDOI
TL;DR: In this paper, the authors determined initial 87Sr/86Sr rations for more than 80 plutonic rocks in Japan and found that the ratios of the granitic rocks vary from 0.7037 to 0.7124.
Abstract: Initial 87Sr/86Sr rations were determined for more than 80 plutonic rocks in Japan. The 87Sr/86Sr ratios of gabbroic and granitic rocks show no significant difference in plutonic terranes where both rocks occur closely associated, implying a genetic relationship between them (e.g., Green Tuff belt) or reequilibration at deep level (e.g., Ryoke belt). Wherever granitic rocks occur independently from gabbroic rocks, the granites have higher ratios than the gabbros. Initial 87Sr/86Sr ratios of the granitic rocks are low (<0.706) in Northeast Japan but high (<0.706) in Southwest Japan, the boundary being the Tanakura Tectonic Line. Within Southwest Japan, the ratios are low along the Japan Sea side of the southernmost area. This regional variation is generally correlated with thickness of the continental crust as deduced from the Bouguer anomaly. Initial 87Sr/86Sr ratios of the granitic rocks vary from 0.7037 to 0.7124. The low group (<0.706) is considered to consist of essentially mantle-derived magmas contaminated by crustal material in lesser but varying degree, because of its geological setting and initial 87Sr/86Sr values. The high group may have been formed by contamination of a deep-seated magmas by crustal material or by generation of the main part of the magmas within the continental crust. The ratios of individual belts reflect their own history depending upon age and Rb/Sr ratio of the crustal material. Initial 87Sr/86Sr ratios of granitic rocks are generally low for the magnetite-series but high for the ilmenite-series. Thus, a negative correlation is observed between initial ratios and δ34S for most Cretaceous-Paleogene granites. However, Neogene ilmenite-series granites are low in both initial 87Sr/86Sr and δ34S indicating interaction of the granitic magma with young sedimentary rocks enriched in 32S.

Journal ArticleDOI
TL;DR: In this article, the mobility of rare earth element (REE) during hydrous burial metamorphism is described from three localities, including the Maddina Volcanics, Fortescue Group, Western Australia and the Canadian Keweenawan Series.
Abstract: The mobility of the rare earth element (REE) during hydrous burial metamorphism is described from three localities. Comparison of REE abundances in relict domains and metadomains from flood lavas in the Maddina Volcanics, Fortescue Group, Western Australia shows that, relative to the relict domains, REE may be strongly depleted in certain metadomains. Strong variations in La/Yb, La/Sm and Eu may also occur due to postcrystallization secondary processes. Similar comparisons in flood lavas from Mamainse Point in the Canadian Keweenawan Series show that REE are mobile and increase in abundance in metadomains. Spilites from U.S. Virgin Islands also show evidence for REE mobility during low grade burial metamorphism. In this case light REE (LREE) have been preferentially mobilized with heavy REE (HREE) remaining parallel or sub-parallel on chondrite normalized plots. REE analyses from these locations together with an evaluation of published work suggests that the mobility of REE can be described by: 1. gross REE and selective LREE enrichment; 2. REE movement around a primary mean; 3. gross REE depletion; 4. selective REE mobility. Failure to take into account REE mobility may lead to incorrect conclusions concerning petrogenetic models based on altered basic sequences. The coherence of REE with other key trace elements during alteration is also discussed.

Journal ArticleDOI
TL;DR: Sulfur isotope composition has been measured on 30 granitoids and 11 gabbroids from the Cretaceous and Tertiary granitic terranes of Japan as mentioned in this paper.
Abstract: Sulfur isotopic composition has been measured on 30 granitoids and 11 gabbroids from the Cretaceous and Tertiary granitic terranes of Japan The two series of granitoids, the magnetite-series and ilmenite-series, defined by Ishihara (1977), show two specific isotope trends The magnetite-series granioids all have positive δ (su34)S (CDT) values from +1 to +9‰, while the ilmenite-series rocks are dominated by negative values between −11 and −1‰ The trend in the ilmenite-series is consistent with the thesis that the magma has been influenced by light biogenic sulfur from the continental crust The inferred large scale magma-crust interaction in the ilmenite-series granitoids indicates that the emplacement of this series of magma has been governed by a stoping mechanism In contrast, the magnetite-series granitoids have little if any evidence for significant magma-crust interaction, indicating that the intrusion of this series of magma may have been more or less of fissure-filling type Their trend towards positive δ (su3 4)S values (average ≈ +5‰) argues for the introduction of some heavy sulfur, probably of seawater origin, into the mantle derived sulfur This is most likely to occur in an arctrench system by the subduction of an oceanic plate which accompanies the sulfate-bearing pelagic sediments The isotopic data of gabbroids, mostly between −1 and +3‰, are close to the commonly assumed value for mantle sulfur Nevertheless, the gabbroids from the magnetite-series granitic terranes tend to have higher δ (su34)S value than those from the ilmenite-series belts It is inferred that the factors controlling the isotope characteristics of the granitoid sulfur have also been operative in these grabbroids at least to some extent

Journal ArticleDOI
TL;DR: In this paper, a suite of fresh volcanic rocks from the New Britain island arc was analyzed for 143Nd/144Nd, 87Sr/86Sr, major and trace elements to investigate relationships between isotopes, trace elements and petrology, and depth to the underlying Benioff zone.
Abstract: A selected suite of fresh volcanic rocks from the New Britain island arc has been analyzed for 143Nd/144Nd, 87Sr/86Sr, major and trace elements to investigate relationships between isotopes, trace elements and petrology, and depth to the underlying Benioff zone. From these relationships inferences about magma generation are made utilizing Nd and Sr isotope systematics in possible source materials. Lavas ranging in composition from basalt to rhyolite show minimal variation of 143Nd/144Nd. Small variations in 87Sr/86Sr do not correlate with depth to the Benioff zone, but are related to magma type. Nd-Sr isotopes suggest that island arc lavas in general are derived from a mixture of suboceanic mantle and hydrothermally altered mid-ocean ridge-type basalt, but the New Britain magma source appears homogeneous with little indication of either the involvement of oceanic crust or mantle inhomogeneity. Trace element patterns in New Britain lavas are not consistent with Nd isotope data for currently accepted petrologic and trace element models of magma genesis. Mafic lavas from New Britain and other island arcs have anomalously high Sr/Nd, possibly due to components derived from subducted oceanic crust.

Journal ArticleDOI
TL;DR: Fission track ages have been measured for 12 sphenes, 18 zircons and 25 apatites separated largely from Lower Tertiary magmatic rocks of East Greenland, with a few examples from Caledonian rocks.
Abstract: Fission track ages have been measured for 12 sphenes, 18 zircons and 25 apatites separated largely from Lower Tertiary magmatic rocks of East Greenland, with a few examples from Caledonian rocks. The sphene and zircon ages of Caledonian rocks agree with other radiometric ages but apatite is strongly discordant indicating that these rocks cooled very slowly over a 200 m.y. period. It was not until the Permian/Lower Jurassic that they finally cooled below 100 ° C, possibly as a consequence of uplift and erosion at this time in connection with extensive rifting. No evidence of a Tertiary imprint has been found in these rocks. Layered gabbros, such as Skaergaard, were emplaced at about the same time (ca. 54 m.y.) as the latest plateau basalts. Some evidence of syenitic activity from this period occurs in the Angmagssalik area ca. 400 km to the south but the syenites of Kangerdlugssuaq cluster around 50 m.y. The Gardiner ultramafic alkaline complex and some of the offshore gabbros apparently also were emplaced at about 50 m.y. Late dykes in the Kangerdlugssuaq area were emplaced over a considerable time span (43-34 m.y.) in keeping with their variable petrographic character, and the Kialineq centre was formed at 36.2±0.4 m.y. Intrusions of the Masters Vig area differ in age. Kap Simpson and Kap Parry to the northeast were emplaced around 40 m.y. whereas the Werner Bjerge complex is the youngest igneous activity so far identified in Greenland with an age of 30.3±1.3 m.y. Many apatites give strongly discordant ages of about 36 m.y. and these are concentrated in the area of a major domal uplift centred on Kangerdlugssuaq. The uplift is older than these ages but on field evidence post-dates the basalts. It probably formed in conjunction with alkaline magmatism at ca. 50 m.y. Cooling below ca. 200 ° was slow for these intrusions and was probably controlled by a number of factors including erosion of the dome, high heat flow caused by continuing dyke injection and regional plateau uplift. The last is believed to have taken place about 35 m.y. ago at the time of emplacement of the Kialineq plutons and last dykes. Renewed rapid erosion and declining heat flow at this time led to rapid cooling of the rocks now at the surface to below 100 °.

Journal ArticleDOI
TL;DR: Forty lavas from the Azores Islands have been analyzed for 87Sr/86Sr ratios, major elements, first transition series metals, and LIL elements as discussed by the authors.
Abstract: Forty lavas from the Azores Islands have been analyzed for 87Sr/86Sr ratios, major elements, first transition series metals, and LIL elements The samples belong to the alkali basalt magma series but range from transitional hy-normative basalts from Terceira to basanitoids from Santa Maria Differentiated lavas include both typical trachytes and comenditic trachytes and comendites Major and trace element concentrations define smooth trends on variation diagrams, and these trends can be related to phases crystallizing in the rocks Systematic interisland differences are also apparent in these variation diagrams LIL element concentrations in island basalts are roughly twice as high as those in tholeiites from the adjacent Mid-Atlantic Ridge which transects the Azores Plateau 87Sr/86Sr ratios in lavas from 6 of the 9 islands range from 070332 to 070354, a range similar to that found in tholeiites from the Mid-Atlantic Ridge transect of the Azores Plateau This suggests that lavas from these islands and this portion of the Mid-Atlantic Ridge may be derived from a similar source However, lavas from the islands of Faial and Pico have 87Sr/86Sr ratios up to 070394 and ratios in Sao Miguel lavas range up to 070525, suggesting basalts from these islands are derived from a chemically distinct source Differences in the average LIL element concentrations of the least fractionated ridge tholeiites from the Azores Plateau and alkali basalts from the islands result from differences in extent of partial melting and residual mineralogy The alkali basalts are derived by roughly half as much melting as are the tholeiites Trace element concentrations in Azores peralkaline lavas preclude their derivation by partial melting of peridotitic mantle or basaltic crust; rather the data suggest they are produced by fractional crystallization of a basaltic parent

Journal ArticleDOI
S. White1
TL;DR: In this article, a combined optical and electron microscopy study of microstructural variations across a quartz mylonite zone with increasing shear strain was performed, and the results indicated that estimates based on grain size are unreliable if phyllosilicates inhibit the growth of grains during recrystallization, and that the dislocation densities are altered during uplift and are unlikely to give meaningful estimates.
Abstract: The results are reported of a combined optical and electron microscopy study of microstructural variations across a quartz mylonite zone with increasing shear strain. The mylonite developed by recrystallization of the deformed quartz grains with increasing shear strain. It was found in a given specimen that the size of recrystallized grains and of sub-grains were always smaller in electron micrographs. The possible reasons for this are discussed. The size of both features decreased with increasing shear strain irrespective of the microscope used. However the density of unbound dislocations remained constant. A marked grain size reduction occurred in phyllosilicate rich layers. Variations in sub-grain size were observed within the relict old grains which remained at low shear strains. These are thought to reflect stress intensification adjacent to grain boundaries during deformation. The relict grains recrystallized at higher strains. Stresses were estimated from grain and sub-grain sizes and from the dislocation density. The results indicate that estimates based on grain size are unreliable if phyllosilicates inhibit the growth of grains during recrystallization, and that the dislocation densities are altered during uplift and are unlikely to give meaningful estimates. It is also concluded that the microstructures reflect stress gradients present during the formation of the mylonite, that is the initiation and propagation of the shear zone and that these were subsequently replaced by strain rate gradients.

Journal ArticleDOI
TL;DR: In this article, the maximum pressure-temperature overlap between muscovite dehydration and initial melting reactions occurs in the pressure range of 4-6 kbar between about 640 ° and 720 ° C, which provides optimum conditions for anatectic melt generation in felsic rocks of appropriate compositions progressively metamorphosed in kyanite-sillimanite facies series.
Abstract: Subsolidus and melting reactions involving calcic plagioclase in pelitic assemblages in the K-Na-Ca model system occur at higher temperatures than their K-Na counterparts. For the most calcic plagioclase compositions observed in high-grade pelitic rocks (An25-An40) the equilibria are rarely extended by more than 30 ° C above those in KA1O2-NaAlO2-Al2O3-SiO2-H2O, although all discontinuities in facies inferred for the K-Na system are continuously displaced when they involve Ca-bearing plagioclase. The maximum pressure-temperature overlap between muscovite dehydration and initial melting reactions occurs in the pressure range of 4–6 kbar between about 640 ° and 720 ° C. This provides optimum conditions for anatectic melt generation in felsic rocks of the appropriate compositions progressively metamorphosed in kyanite-sillimanite facies series. Progressive regional metamorphism at pressures of 2–4 kbar, corresponding to andalusite-sillimanite facies series, shows little overlap between muscovite dehydration and initial melting reactions. Consequently anatectic melt generation in andalusite-sillimanite facies series would require the participation of biotite in dehydration-melting reactions. Felsic intrusive rock in andalusite-sillimanite terranes could have risen upward from their anatectic sites in high grade kyanite-sillimanite facies series at depth. Many andalusite-sillimanite facies series terranes culminating in migmatites could represent upward movement of kyanite-sillimanite facies series rocks to shallower depths with uplift rates faster than cooling rates.

Journal ArticleDOI
TL;DR: In this article, the authors interpreted volcanic evolution on the active volcano of Agrigan in the northern Mariana Island Arc is interpreted as resulting in the production of calc-alkaline andesites by the fractional crystallization of high-alumina basalt.
Abstract: Magmatic evolution on the active volcano of Agrigan in the northern Mariana Island Arc is interpreted as resulting in the production of calc-alkaline andesites by the fractional crystallization of high-alumina basalt. Basaltic products predominate, but the ratio of andesites to basalts increases with time up to an event of voluminous andesitic pyroclastic ejection accompanied by caldera-collapse; post-collapse lavas are entirely basaltic. Moderate iron-enrichment is demonstrated for the volcanic suite, with indications of a progressive, pre-caldera decrease in iron-enrichment; post-caldera lavas display a return to moderate Fe-enrichment. Overall, the lavas are enriched in the LIL elements (K, Rb, Ba, Sr) and depleted in Ti, Mg, Cr, and Ni. From the oldest to the youngest pre-caldera volcanic sequence, the LIL elements increase 3-6X while Ca and Mg decrease by 50% or more. Approximately constant K/ Rb (430±60) and 87Sr/86Sr (0.7032–0.7034) indicate consanguinity of the basalts and the andesites. Cumulate plutonic xenoliths, common in the lavas, are composed of mineral phases also encountered as phenocrysts. The following order of crystallization is indicated: olivine; anorthite-bytownite; clinopyroxene; orthopyroxene and titanomagnetite. Co-existing xenolithic olivines (Fo74–83) and plagioclase (An88–96) are typical of calc-alkaline island-arc assemblages and contrast with assemblages in the tholeiites from the Mariana Trough to the west. The relatively fayalitic composition and low abundances of Ni in olivines and Cr in clinopyroxenes indicate equilibrium with an already-fractionated liquid. These data, along with structural evidence, high Ca in the olivines, and comparison of the observed assemblages with experimental studies, suggests that these xenoliths formed as crystal cumulates at the floor of a shallow (≦ 7 km) crustal magma chamber.

Journal ArticleDOI
TL;DR: In this article, it was shown that the molecular water content of cordierite has profound effects on many equilibria involving this phase, including the stability of the reaction in a petrogenetic grid, and that the minimum pressure required to form this assemblage can be considerably less than the 10 kb required under conditions of 0.87 cal/K.
Abstract: The H2O content of cordierite is often regarded as incidental to its stability, probably because cordierite has substantial fields of stability at low pressures both in wet and dry experimental systems. In this paper we show that, in contrast, the molecular water content of cordierite has profound effects on many equilibria involving this phase. Mg-cordierite has been modelled as an ideal solid solution of the hydrous and anhydrous end-members Mg2Al4Si5O18·1.2H2O and Mg2Al4Si5O18 respectively. The H2O-solubility data of Mirwald and Schreyer (1977) fit this model within experimental uncertainty and yield 1 bar enthalpy and entropy changes for the reaction: $$\begin{gathered} Mg_2 Al_4 Si_5 O_{18} + 1.2H_2 O = Mg_2 Al_4 Si_5 O_{18} \cdot 1.2H_2 O \hfill \\ cordierite fluid codierite \hfill \\ \end{gathered} $$ of −12,300 cal and −32.87 cal/K. This implies that the partial molal entropy of H2O in cordierite at 298 K/l bar is almost exactly the same as the molar entropy of liquid water (16.9 cal/K as opposed to 16.7 cal/K) and that the interaction energy of liquid water with cordierite is only of the order of a few hundred calories per mole. Application of the model to the hydrous experiments of Fawcett and Yoder (1966) and Chernosky (1974) yields a value for ΔG f,298 0 of anhydrous Mgcordierite of between −2,062.71 and −2,074.21 Kcal per mole. This in in good agreement with the calorimetric data of Charlu, Newton and Kleppa (1975) which yield ΔG f,298 0 of −2,067.03±1.18 Kcal. Water pressure has a considerable influence on the (Mg, Fe) isopleths of coexisting cordierite and garnet, and hence, their use as geobarometric curves. Pressures estimated from the Mg/Fe ratios in the high-Mg range can vary by two kilobars or more, depending on the assumed $$P_{H_2 O} $$ , with highest estimates for $$P_{H_2 O} = P_{total} $$ . The stability field of the talc-kyanite “white-schist” assemblage (Schreyer, 1973) is found to expand appreciably as $$P_{H_2 O} $$ is lowered. Thus the minimum pressure required to form this assemblage can be considerably less than the 10 kb required under conditions of $$P_{H_2 O} = P_{total} $$ =P total, as anticipated by Schreyer (1977). The high partial molal entropy of H2O in cordierite results in small entropy changes coupled with large volume changes in dehydration reactions forming cordierite. This greatly influences the slopes and positions of univariant reactions involving cordierite. The stability of cordierite is promoted to higher pressures in H2O-bearing systems where none of the cordierite breakdown products is a hydrate. Cordierite-forming reactions from hydrates can have the H2O released on the relatively low-temperature sides of the reaction curves, an anomalous situation known only in zeolite stability curves. These considerations can have profound effects on model “petrogenetic grids” involving cordierite.

Journal ArticleDOI
TL;DR: In this paper, the P-T relations using isochores in unary system CO2 are estimated using theoretical and experimental data, and the homogenization temperatures of CO2-CH4 inclusions can be expressed in terms of equivalent CO 2-densities.
Abstract: Addition of CH4 to CO2 lowers the temperatures at which phase changes occur with respect to those in the unary system CO2. At high density and high $$X_{{\text{CH}}_{\text{4}} } $$ a melting interval of solid CO2 can be expected. Rearrangement of currently available theoretical and experimental data permits bulk compositions of carbonic fluid inclusions to be determined from the final melting temperature of CO2 and the degree of filling at that temperature. Homogenization temperatures of CO2-CH4 inclusions can be expressed in terms of equivalent CO2-densities, permitting estimates of P-T relations using isochores in the unary system CO2.

Journal ArticleDOI
TL;DR: In this article, the densities of 21 silicate liquids have been determined from 1,000 ° to 1,600 ° C. The compositions studied contain from two to eight oxide components and have the following ranges in composition (mole %): SiO2, 35−79%; TiO2 4−36; Al2O3, 5−25%; FeO, 11−41%; MgO, 7−28%; CaO,7−35%; Na2O, 5-50%; and K2O 4−20%.
Abstract: Densities of 21 silicate liquids have been determined from 1,000 ° to 1,600 ° C. The compositions studied contain from two to eight oxide components and have the following ranges in composition (mole %): SiO2, 35–79%; TiO2, 4–36%; Al2O3, 5–25%; FeO, 11–41%; MgO, 7–28%; CaO, 7–35%; Na2O, 5–50%; and K2O, 4–20%. The compositions thus cover the upper range observed in magmas for each oxide. Precision for each determination of liquid density is always better than ±1%.