Showing papers in "Contributions to Mineralogy and Petrology in 1985"

Chemical mass transfer in magmatic processes

Abstract: Thermodynamic and mathematical relations are presented to facilitate the description of an algorithm for the calculation of chemical mass transfer in magmatic systems. This algorithm extends the silicate liquid solution model of Ghiorso et al. (1983) to allow for the quantitative modelling of natural magmatic processes such as crystal fractionation, equilibrium crystallization, magma mixing and solid-phase assimilation. The algorithm incorporates a new method for determining the saturation surface of a non-ideal multicomponent solid-solution crystallizing from a melt. It utilizes a mathematical programming (optimization) approach to determine the stable heterogeneous (solids+liquid) equilibrium phase assemblage at a particular temperature and pressure in magmatic systems both closed and open to oxygen. Closed system equilibria are computed by direct minimization of the Gibbs free energy of the system. Open system equilibria are determined by minimization of the Korzhinskii potential (Thompson 1970), where oxygen is treated as a perfectly mobile component. Magmatic systems undergoing chemical mass transfer processes are modelled in a series of discrete steps in temperature, pressure or bulk composition, with each step characterized by heterogeneous solid-liquid equilibrium. A numerical implementation of the algorithm has been developed (in the form of a FORTRAN 77 computer program) and calculations demonstrating its utility are provided in an accompanying paper (Ghiorso and Carmichael 1985).

A chlorite solid solution geothermometer the Los Azufres (Mexico) geothermal system

Abstract: Chlorite constitutes a major hydrothermal alteration product of metamorphism of andesites, in the active geothermal system of Los Azufres (Mexico). Electron microprobe analyses performed on a set of crystals from each sample show wide variations in composition. Correlation coefficients among chemical constituents were calculated. It is shown that the tetrahedral charge is positively correlated with the octahedral vacancy and negatively with the iron content, and there is almost no correlation with the octahedral aluminium and magnesium content. A procedure is proposed to select end-members and substitution vectors, and to give a general formula for these chlorites. Their formation temperatures are estimated with great accuracy, combining results of microthermometric data on fluid inclusions from gangue minerals of chlorites (quartz, calcite), direct measurements in wells (Kuster equipment), and chemical geothermometers. Correlations between chlorite compositions, range and nature of site occupancy, and temperature are good. Formation temperatures of chlorites range from 130° C to 300° C. As no other thermodynamic parameter varies significantly in the studied field (composition of the host rocks, nature of the geothermal fluids, pressure, ...), these variations of site occupancy (mainly Al(IV) and the octahedral occupancy (6-Al(VI)-(Mg+Fe(2+)) = VAC) are considered mainly as temperature dependent. Molar fractions of each end-member show very different variations with increasing temperature: X-kaolinite decreases, and X-chamosite increases, while X-talc-3 brucite does not show significant change. From these data, activity coefficients and standard state chemical potential of major components, and molar free energy formation of chlorite have been calculated for each temperature of crystallisation.

Dissolution kinetics of plagioclase in the melt of the system diopside-albite-anorthite, and origin of dusty plagioclase in andesites

Abstract: The textures and kinetics of reaction between plagioclase and melts have been investigated experimentally, and origin of dusty plagioclase in andesites has been discussed. In the experiments plagioclase of different compositions (An96, An61, An54, An23, and An22) surrounded by glasses of six different compositions in the system diopside-albite-anorthite was heated at temperatures ranging from 1,200 to 1,410° C for 30 min to 88 h. Textures were closely related to temperature and chemical compositions. A crystal became smaller and rounded above the plagioclase liquidus temperature of the starting melt (glass) and remained its original euhedral shape below the liquidus. Whatever the temperature, the crystal-melt interface became rough and often more complicated (sieve-like texture composed of plagioclase-melt mixture in the scale of a few μm was developed from the surface of the crystal inward; formation of mantled plagioclase) if the crystal is less calcic than the plagioclase in equilibrium with the surrounding melt, and the interface remained smooth if the crystal is more calcic than the equilibrium plagioclase. From these results the following two types of dissolution have been recognized; (1) a crystal simply dissolves in the melt which is undersaturated with respect to the phase (simple dissolution), and a crystal is partially dissolved to form mantled plagioclase by reaction between sodic plagioclase and calcic melt (partial dissolution). The amount of a crystal dissolved and reacted increased proportional to the square root of time. This suggests that these processes are controlled by diffusion, probably in the crystal. Mantled plagioclase produced in the experiments were very similar both texturally and chemically to some of the so-called resorbed plagioclase in igneous rocks. Chemical compositions and textures of plagioclase phenocrysts in island-arc andesites of magma mixing origin have been examined. Cores of clear and dusty plagioclase were clacic (about An90) and sodic (about An50), respectively. This result indicates that dusty plagioclases were formed by the partial melting due to reaction between sodic plagioclase already precipitated in a dacitic magma and a melt of intermediate composition in a mixed magma during the magma mixing.

Heat capacity of minerals in the system Na 2 O-K 2 O-CaO-MgO-FeO-Fe 2 O 3 -Al 2 O 3 -SiO 2 -TiO 2 -H 2 O-CO 2 : representation, estimation, and high temperature extrapolation

Abstract: A revised equation is proposed to represent and extrapolate the heat capacity of minerals as a function of temperature: C P=k0+k1 T −0.5+k2 T −2+k3 T −3 (where k1, k2≤0). This equation reproduces calorimetric data within the estimated precision of the measurements, and results in residuals for most minerals that are randomly distributed as a function of temperature. Regression residuals are generally slightly greater than those calculated with the five parameter equation proposed by Haas and Fisher (1976), but are significantly lower than those calculated with the three parameter equation of Maier and Kelley (1932). The revised equation ensures that heat capacity approaches the high temperature limit predicted by lattice vibrational theory (C P=3R+α2VT/β). For 16 minerals for which α and β have been measured, the average C Pat 3,000 K calculated with the theoretically derived equation ranges from 26.8±0.8 to 29.3±1.9 J/(afu·K) (afu = atoms per formula unit), depending on the assumed temperature dependence of α. For 91 minerals for which calorimetric data above 400 K are available, the average C Pat 3,000 K calculated with our equation is 28.3±2.0 J/(afu·K). This agreement suggests that heat capacity extrapolations should be reliable to considerably higher temperatures than those at which calorimetric data are available, so that thermodynamic calculations can be applied with confidence to a variety of high temperature petrologic problems. Available calorimetric data above 250 K are fit with the revised equation, and derived coefficients are presented for 99 minerals of geologic interest. The heat capacity of other minerals can be estimated (generally within 2%) by summation of tabulated ‘oxide component’ C Pcoefficients which were obtained by least squares regression of this data base.

The geochemistry of potassic lavas from Vulsini, central Italy and implications for mantle enrichment processes beneath the Roman region

Abstract: Major and trace element and 143Nd/144Nd (0.51209–0.51216) and 87Sr/86Sr (0.70879–0.71105) isotope analyses are presented on a representative group of lavas from the Vulsini district of the Roman magmatic province. Three distinct series are identified; the high-K and low-K series are similar to those described from other Italian volcanoes, while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near-primary mantle melts. Major and trace element variations within the high-K series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites. Crustal contamination resulted in an increase in 87Sr/86Sr with increasing fractionation, but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub-continental mantle. These are reviewed using the available results from Vulsini, Roccamonfina and Ernici. Transition element abundances and Ta/Yb ratios indicate that the pre-enrichment mantle was similar to that of E-type MORB, and that these elements were not mobilised by the enrichment process. Mixing calculations suggest that three components were involved in the enrichment process; mantle comparable with the source of MORB, and two other components rich in trace elements. One, the low-K component, had high Sr/Nd, Th/Ta and Ba/Nb and no europium anomaly while the second had lower Sr/Nd, a negative europium anomaly and very high Th/Ta. It was also characterised by low Nb/Ba and high Rb/Ba ratios, similar to those reported from phlogopite-rich peridotite xenoliths. The trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high-K end-member reflecting the subduction of sediments and the stabilisation of mantle phlogopite.

Garnet-orthopyroxene-plagioclase-quartz barometry : refinement and application to the English River subprovince and the Minnesota River valley

Abstract: Recent experimental, theoretical, and thermodynamic studies permit better calibration of two reactions for geobarometry: grossular+pyrope+quartz=anorthite+enstatite grossular+almandine+quartz=anorthite+ferrosilite If both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet-pyroxene-plagioclase-quartz assemblage. Application to a variety of high-grade terrains generally yielded excellent results. However, poor results have been obtained for high-Fe rocks which can be traced to errors in activity models for garnet and/or pyroxene. Either a two-site ideal mixing model for orthopyroxene (cf. Wood and Banno 1973) underestimates enstatite activity for high-Fe orthopyroxenes or the Ganguly and Saxena (1984) model overestimates pyrope activity in low-Mg garnets.

Petrology and origin of primitive lavas from the Troodos ophiolite, Cyprus

Abstract: Parental magmas to the Troodos ophiolite are characterised by low TiO2 and Al2O3 and high SiO2. Extremely fresh and chemically primitive (high MgO) rocks are found within the Upper Pillow Lavas and along the Arakapas Fault Belt of Cyprus and contain forsteritic olivine±enstatite and groundmass clinopyroxene set in glass or plagioclase, with accessory magnesiochromite and sometimes hornblende. They are quartz-normative and may have originally contained up to 3 wt% H2O. Geochemically, there are three distinct groups of primitive lavas, based on TiO2 and Zr contents but also reflected by CaO, Na2O and REE abundances. These groups cannot be related by crystal fractionation and are considered to have been generated by incremental melting of a variably depleted source region. The parental magma to the least depleted group (Group I) was that of the major portion of the Troodos plutonic complex and is similar to those postulated for other “low-Ti” ophiolites. Chemically it has close affinities with komatiitic basalts. The most depleted lavas (Group III) all have U-shaped REE profiles and variable 143Nd/ 144Nd ratios, interpreted in terms of metasomatism of the source region by an incompatible element-enriched component which was probably derived from a subducted slab. These lavas represent an intermediate step in the development of boninite series rocks.

The speciation of carbon dioxide in sodium aluminosilicate glasses

Abstract: Infrared spectroscopy has been used to study the speciation of CO_2 in glasses near the NaAlO_2-SiO_2 join quenched from melts held at high temperatures and pressures. Absorption bands resulting from the antisymmetric stretches of both molecular CO_2 (2,352 cm^(-1)) and CO^(2-)_3 (1,610 cm^(-1) and 1,375 cm^(-1)) are observed in these glasses. The latter are attributed to distorted Na-carbonate ionic-complexes. Molar absorptivities of 945 liters/mole-cm for the molecular CO_2 band, 200 liters/mole-cm for the 1,610 cm^(-1) band, and 235 liters/mole-cm for the 1,375 cm^(-1) band have been determined. These molar absorptivities allow the quantitative determination of species concentrations in the glasses with a precision on the order of several percent of the amount present. The accuracy of the method is estimated to be ±15-20% at present. The ratio of molecular CO_2 to CO^(2-)_3 in sodium aluminosilicate glasses varies little for each silicate composition over the range of total dissolved CO_2 content (0-2%), pressure (15-33 kbar) and temperature (1,400-1,560° C) that we have studied. This ratio is, however, a strong function of silicate composition, increasing both with decreasing Na_2O content along the NaAlO_2-SiO_2 join and with decreasing Na_2O content in peraluminous compositions off the join. Infrared spectroscopic measurements of species concentrations in glasses provide insights into the molecular level processes accompanying CO_2 solution in melts and can be used to test and constrain thermodynamic models of CO_(2-) bearing melts. CO_2 speciation in silicate melts can be modelled by equilibria between molecular CO_2 , CO^(2-)_3, and oxygen species in the melts. Consideration of the thermodynamics of such equilibria can account for the observed linear relationship between molecular CO_2 and carbonate concentrations in glasses, the proposed linear relationship between total dissolved CO_2 content and the activity of CO_2 in melts, and observed variations in CO_2 solubility in melts.

Composition of liquids coexisting with spinel lherzolite at 10 kbar and the genesis of MORBs

Abstract: Because of the controversy over the nature of the parental magma for MORBs, experiments have been performed at 10 kbar in order to assess the effect of modal variations in the source peridotite and the effect of temperature (degree of partial melting) on the composition of partial melts. A peridotite-basalt sandwich method was used and a run duration of 72 h was found to be necessary to equilibrate basalt and peridotite. A range of melt compositions, coexisting with olivine, orthopyroxene, clinopyroxene and spinel, was produced at 10 kbar, indicating that partial melting of peridotite cannot be regarded as isobarically pseudoinvariant. On projections in the normative tetrahedron OL-PL-CPX-SIL, the liquids obtained in this study define an area, rather than a point or narrow band. The compositions of some liquids in this study are similar to magnesian MORBs (MgO>9.5 wt%), providing evidence in support of the derivation of magnesian MORBs by partial melting of mantle lherzolite at about 10 kbar.

Jorullo Volcano, Michoacán, Mexico (1759–1774): The earliest stages of fractionation in calc-alkaline magmas

Abstract: Between 1759 and 1774, Jorullo Volcano and four associated cinder cones erupted an estimated 2 km3 of magma which evolved progressively with time from early, hypersthene-normative, primitive basalts to late-stage, quartz-normative, basaltic andesites. All lavas contain <6 vol% phenocrysts of magnesian olivine (Fo90-70) with Cr-Al-Mg-spinel inclusions, and microphenocrysts of plagioclase and augite; late-stage basaltic andesites also carry phenocrysts of plagioclase, augite, and rare orthopyroxene, hornblende pseudomorphs, and microphenocrysts of titanomagnetite. Olivine-melt compositions indicate liquidus temperatures ranging from 1,230° C to 1,070° C in the early- and late-stage lavas, respectively; $$f_{{\text{O}}_{\text{2}} } $$ was about 0.6 log units above the Ni-NiO buffer in the early lavas but increased to 2.5 log units above Ni-NiO in the late lavas, perhaps through groundwater-magma interaction. Smooth major and trace element compositional trends in the lavas can be largely modeled by simple crystal fractionation of olivine, augite, plagioclase, and minor spinel. La, Ce, and other incompatible elements (Rb, Sr, Ba, Hf, Th, Ta), however, are anomalously enriched in the latestage lavas, whereas the heavy rare earth elements (Dy, Yb, Lu) are anomalously depleted. The modeled crystal fractionation event must have occurred at lower-crustal to upper-mantle pressures (8–15 kb), although the crystals actually present in the Jorullo lavas appear to have formed at low pressures. Thus, a two-stage crystallization history is implied. Despite the presence of granitic xenoliths in middle-stage lavas from Jorullo, bulk crustal assimilation appears to have played an insignificant role in generating the compositional trends among the lavas. As MgO decreases from 9.3 to 4.3 wt% through the suite, Al2O3 increases from 16.4 to 19.1 wt%. Most highalumina basalts reported in the literature have 18 to 21 wt% Al2O3, but are too depleted in MgO, Ni, and Cr to have been generated directly through mantle partial melting. These high-alumina basalts have probably undergone significant fractionation of olivine, augite, plagioclase, and spinel from primitive parental basalts similar to the early Jorullo lavas. Such primitive basalts are rarely erupted in mature arcs and may be completely absent from mature stratovolcanoes. Cerro La Pilita is a late-Quaternary cinder and lava cone centered just 3 km south of Jorullo. The primitive trachybasalts of Cerro La Pilita, however, are radically different from the Jorullo basalts. They are nepheline normative with high concentrations of K2O (>2.5 wt%), P2O5 (>0.9 wt%), Ba (1,200 ppm), Sr (>2,000 ppm), and many other incompatible elements, and contain crystals of hornblende and apatite in addition to olivine, spinel, augite, and plagioclase. The magmas of these two neighboring volcanoes cannot be related to one another by any simple mechanism, and must represent fundamentally different partial melting events in the mantle. The contrasts between Jorullo and Cerro La Pilita demonstrate the difficulty in defining simple relationships between magma type and distance from the trench in the Mexican Volcanic Belt.

Polybaric differentiation of alkali basaltic magmas: evidence from green-core clinopyroxenes (Eifel, FRG)

Abstract: The Quaternary foidites and basanites of the West Eifel (Germany) contain optically and chemically heterogeneous clinopyroxenes, some of which occur as discrete zones within individual crystals: Most clinopyroxene phenocrysts are made up of a core and a normally zoned comagmatic titanaugite mantle Most cores are greenish pleochroic and moderately resorbed (fassaitic augite) Some are pale green and strongly resorbed (acmitic augite) Cores of Al-augite composition and of Cr-diopside derived from peridotite xenoliths are rare The fassaitic augites are similar in trace element distribution pattern to the titanaugites, but are more enriched in incompatible elements The acmitic augites, in contrast, are clearly different in their trace element composition and are enriched in Na, Mn, Fe and depleted in Al, Ti, Sr, Zr A model for polybaric magma evolution in the West Eifel is proposed: Primitive alkali basaltic magma rises through the upper mantle precipitating Al-augite en route It stagnates and differentiates near the crust/mantle boundary crystallizing Fe-rich fassaitic augites The magma differentiated at high pressure is subsequently mixed with new pulses of primitive magma from which the rims of pyroxene are crystallized Sporadic alkali pyroxenite xenoliths are interpreted to represent cumulates of cognate phases formed within the crust and not metasomatized upper mantle material (Lloyd and Bailey 1975)

Partial melting of a phlogopite-clinopyroxenite nodule from south-west Uganda: an experimental study bearing on the origin of highly potassic continental rift volcanics

Abstract: Melting experiments on a mantle-derived nodule assemblage consisting of clinopyroxene, phlogopite and minor titanomagnetite, sphene and apatite have been done at 20 and 30 kbar between 1,175 and 1,300° C. The nodule composition was selected on the basis of modal and chemical analyses of 84 mantle derived nodules with metasomatic textures from the Katwe-Kikorongo and Bunyaruguru volcanic fields of south-west Uganda. At 30 kbar, 1,225 and 1,250° C, representing 20–30% partial melting, the compositions of glasses compare favourably to those of the average composition of 26 high potassic mafic lavas from the same region. Glasses produced by sufficiently low degrees of partial melting at 20 kbar could not be analysed. Glass compositions obtained for 20–30% melting at 30 kbar have high K2O (3.07–5.05 wt.%), low SiO2 (35.0–39.2 wt.%), high K/K + Na (0.54–0.71), K + Na/Al (0.99–1.08) and Mg/ Mg + FeT of 0.59–0.62. These results support the suggestion of Lloyd and Bailey (1975) that the nodules represent the source material for the high K-rich lavas of south-west Uganda. If this conclusion is correct it implies that anomalous mantle source of phlogopite clinopyroxenite composition could produced the Ugandan lavas by relatively higher degrees of partial melting than that normally considered for highly alkaline mafic magmas derived from a pyrolitic mantle source. Higher degrees of melting are considered likely from such a different source region, rich in alkalis, water and radioactive elements. Steeper geotherms and increased fluxing of sub-rift mantle by degassing would also produce higher degrees of partial melting.

Petrology and geochemistry of basalts from the American-Antarctic Ridge, Southern Ocean: implications for the westward influence of the Bouvet mantle plume

Abstract: Ridge segments and fracture zones from the American-Antarctic Ridge have been systematically dredge sampled from ∼4° W to ∼18° W. Petrographic studies of the dredged basalts show that the dominant basalt variety is olivine-plagioclase basalt, although olivine-plagioclase-clinopyroxene basalt is relatively common at some localities. Selected samples have been analysed for major and trace elements, rare earth elements and Sr and Nd isotopes. These data show that the majority of samples are slightly evolved (Mg#=69-35) N-type MORB, although a small group of samples from a number of localities have ‘enriched’ geochemical characteristics (T- and P-type MORB).

Strontium and oxygen isotopic variations in Mesozoic and Tertiary plutons of central Idaho

Abstract: Regional variations in initial 87Sr/86Sr ratios (r i) of Mesozoic plutons in central Idaho locate the edge of Precambrian continental crust at the boundary between the late Paleozoic-Mesozoic accreted terranes and Precambrian sialic crust in western Idaho. The r i values increase abruptly but continuously from less than 0.704 in the accreted terranes to greater than 0.708 across a narrow, 5 to 15 km zone, characterized by elongate, lens-shaped, highly deformed plutons and schistose metasedimentary and metavolcanic units. The chemical and petrologic character of the plutons changes concomitantly from ocean-arc-type, diorite-tonalite-trondhjemite units to a weakly peraluminous, calcic to calcalkalic tonalite-granodiorite-granite suite (the Idaho batholith). Plutons in both suites yield Late Cretaceous ages, but Permian through Early Cretaceous bodies are confined to the accreted terranes and early Tertiary intrusions are restricted to areas underlain by Precambrian crust. The two major terranes were juxtaposed between 75 and 130 m.y. ago, probably between 80 and 95 m.y. Oxygen and strontium isotopic ratios and Rb and Sr concentrations of the plutonic rocks document a significant upper-crustal contribution to the magmas that intrude Precambrian crust. Magmas intruding the arc terranes were derived from the upper mantle/subducted oceanic lithosphere and may have been modified by anatexis of earlier island-arc volcanic and sedimentary units. Plutons near the edge of Precambrian sialic crust represent simple mixtures of the Precambrian wall-rocks with melts derived from the upper mantle or subducted oceanic lithosphere with r i of 0.7035. Rb/Sr varies linearly with r i, producing “pseudoisochrons” with apparent “ages” close to the age of the wall rocks. Measured δ 18O values of the wall rocks are less than those required for the assimilated end-member by Sr-O covariation in the plutons, however, indicating that wall-rock δ 18O was reduced significantly by exchange with circulating fluids. Metasedimentary rocks of the Belt Supergroup are similarly affected near the batholith, documenting a systematic depletion in 18O as much as 50 km from the margin of the batholith. Plutons of the Bitterroot lobe of the Idaho batholith are remote from the accreted terranes and represent mixtures of Precambrian wall-rocks with melts dominated by continental lower crust (r i>0.708) rather than mantle. “Pseudoisochrons” resulting from these data are actually mixing lines that yield apparent “ages” less than the true age of the wall rocks and meaningless “ri”. Assimilation/ fractional-crystallization models permit only insignificant amounts of crystal fractionation during anatexis and mixing for the majority of plutons of the region.

A model for mixing basaltic and dacitic magmas as deduced from experimental data

Abstract: It has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours. The presence of phenocrysts larger than 0.5 mm increased considerably the efficiency of mixing. Flow patterns in the experimental system were visualized using Pt spheres, which indicated that convection occurs in basalt melt, but not in dacite melt. The Reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10−3 and 10−6, respectively. Mixing proceeds initially by mechanical mixing of the two magmas in a large scale, but later by coupling interfacial convection and mutual diffusion. Thus, depending on the depth where vesiculation and following disruption of the magma occurs, banded pumice, homogeneous pumice and homogeneous andesite lava are erupted. The observed textures of mixed rocks of Plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model.

Stable isotopic evidence for large-scale seawater infiltration in a regional metamorphic terrane; the Trois Seigneurs Massif, Pyrenees, France

Abstract: Oxygen isotopic analyses of 95 metamorphic and igneous rocks and minerals from a Hercynian metamorphic sequence in the Trois Seigneurs Massif, Pyrenees, France, indicate that all lithologies at higher metamorphic grades than the “andalusite in” isograd have relatively homogeneous δ18O values. The extent of homogenization is shown by the similarity of δ18O values in metacarbonates, metapelites and granitic rocks (+11 to +13), and by the narrow range of oxygen isotopic composition shown by quartz from these lithologies. These values contrast with the δ18O values of metapelites of lower metamorphic grade (δ18O about +15). Homogenization was caused by a pervasive influx of hydrous fluid. Mass-balance calculations imply that the fluid influx was so large that its source was probably high-level groundwaters or connate formation water. Hydrogen isotopic analyses of muscovite from various lithologies are uniform and exceptionally heavy at δD=−25 to −30, suggesting a seawater origin. Many lines of petrological evidence from the area independently suggest that metamorphism and anatexis of pelitic metasediment occurred at depths of 6–12 km in the presence of this water-rich fluid, the composition of which was externally buffered. Deep penetration of surface waters in such environments has been hitherto unrecognized, and may be a key factor in promoting major anatexis of the continental crust at shallow depth. Three types of granitoid are exposed in the area. The leucogranites and the biotite granite-quartz diorite are both mainly derived from fusion of local Paleozoic pelitic metasediment, because all these rocks have similar whole-rock δ18O values (+11 to +13). The post-metamorphic biotite granodiorite has a distinctly different δ18O (+9.5 to +10.0) and was probably derived from a deeper level in the crust. Rare mafic xenoliths within the deeper parts of the biotite granite-quartz diorite also have different δ18O (+8.0 to +8.5) and possibly represent input of mantle derived magma, which may have provided a heat source for the metamorphism.

Igneous Petrogenesis of the Yakuno Ophiolite (Japan) in the context of the diversity of ophiolites

Abstract: An ophiolite complex includes three major members: basaltic volcanics, mafic-ultramafic cumulates, and residual peridotite. From the aspect of igneous petrology, three distinct types are recognized among the associations of the three members: (Liguria type) alkalic basalt, plagioclase-type cumulates, and lherzolite; (Yakuno type) high-alumina tholeiite, clinopyroxene-type cumulates, and clinopyroxene-bearing harzburgite; (Papua type) low-alumina tholeiite, orthopyroxene-type cumulates, and clinopyroxene-free harzburgite. In the light of recent experimental studies, the three types represent cogenetic, complementary products of low ( 30%) degrees of partial melting in the lherzolitic source mantle, respectively. The cumulates of the Yakuno ophiolite show structural and chemical continuity to the underlying residual peridotite, and were recrystallized at high pressures (5–10 kb). They originated in a deep, “soft-floored” magma chamber directly overlying the partially melted residual harzburgite, from which the magma was extracted. The three members of the Yakuno ophiolite were cogenetically formed through a magmatic event induced by a moderate degree of partial melting in the mantle.

Experimental determination of cation diffusivities in aluminosilicate garnets

Abstract: Data from experimentally-induced diffusion profiles at approximately 40 Kbar, 1,300–1,500° C in spessartine-almandine couples and a pyrope-almandine couple at ∼ 40 Kbar, 1,440° C, described in Part I, were used to derive tracer diffusion coefficients (D*) of Fe, Mn and Mg in garnet. The experimental data were fitted by numerical simulations that model multicomponent, compositionally-dependent difussion, including the effects of nonideal thermodynamic mixing. The simulations use the formalism of irreversible thermodynamics and an eigenvector technique of solution. We were able to fit the asymmetrical spessartine-almandine profiles using constant D* and either the Darken/Hartley-Crank or Manning-Lasaga models relating D* and interdiffusion coefficients, and both models yielded DMg*consistent with the direct measurement of DMg*in by Cygan and Lasaga (1985) at lower temperatures (750–900° C). The results (equations 4.1–4.3 and Table 1) indicate that DFe*≅DMg* QMn, where Q is the activation energy. In contrast, the asymmetry of pyrope-almandine profiles is too great to fit with either tracer model assuming constant D* and indicates that DMg*is similar to its value in spessartine-almandine couples but DFe*is an order of magnitude less. The fit also suggests that DCa*< DFe*

Aleutian tholeiitic and calc-alkaline magma series I: The mafic phenocrysts

Abstract: Diagnostic mafic silicate assemblages in a continuous spectrum of Aleutian volcanic rocks provide evidence for contrasts in magmatic processes in the Aleutian arc crust. Tectonic segmentation of the arc exerts a primary control on the variable mixing, fractional crystallization and possible assimilation undergone by the magmas. End members of the continuum are termed calc-alkaline (CA) and tholeiitic (TH). CA volcanic rocks (e.g., Buldir and Moffett volcanoes) have low FeO/MgO ratios and contain compositionally diverse phenocryst populations, indicating magma mixing. Their Ni and Cr-rich magnesian olivine and clinopyroxene come from mantle-derived mafic olivine basalts that have mixed with more fractionated magmas at mid-to lower-crustal levels immediately preceding eruption. High-Al amphibole is associated with the mafic end member. In contrast, TH lavas (e.g., Okmok and Westdahl volcanoes) have high FeO/MgO ratios and contain little evidence for mixing. Evolved lavas represent advanced stages of low pressure crystallization from a basaltic magma. These lavas contain groundmass olivine (FO 40–50) and lack Ca-poor pyroxene. Aleutian volcanic rocks with intermediate FeO/MgO ratios are termed transitional tholeiitic (TTH) and calc-alkaline (TCA). TCA magmas are common (e.g., Moffett, Adagdak, Great Sitkin, and Kasatochi volcanoes) and have resulted from mixing of high-Al basalt with more evolved magmas. They contain amphibole (high and low-Al) or orthopyroxene or both and are similar to the Japanese hypersthene-series. TTH magmas (e.g., Okmok and Westdahl) contain orthopyroxene or pigeonite or both, and show some indication of upper crustal mixing. They are mineralogically similar to the Japanese pigeonite-series. High-Al basalt lacks Mg-rich mafic phases and is a derivative magma produced by high pressure fractionation of an olivine tholeiite. The low pressure mineral assemblage of high-Al basalt results from crystallization at higher crustal levels.

Rare earth element partitioning between clinopyroxene and silicate liquid at moderate to high pressure

Abstract: Experimental determination of over seventy sets of clinopyroxene/silicate liquid (glass) partitition coefficients (D) for four rare earth elements (REE — La, Sm, Ho, Lu) in a range of REE-enriched natural rock compositions (basalt, basaltic andesite, andesite and rhyodacite) demonstrate a convex upward pattern, favouring the heavy REE (Ho, Lu) and markedly discriminating against the light REE (La). These patterns are consistent with previously documented clinopyroxene D values reported from natural phenocryst/matrix pairs and from experimental work using either REE-enriched compositions and electron microprobe analytical techniques (as in the present study) or natural or synthetic undoped compositions and mass spectrometric, ion probe or X-ray autoradiographic analytical techniques. However, the large data base in the present study allows evaluation of the effect of compositional and physical parameters on REE partitioning relationships. Considering DHo, it is shown that (1) D increases 6-fold with increasing SiO2 content of the coexisting liquid from ∼ 50 to ∼ 70 wt% SiO2 (2) D increases 4-fold with decreasing temperature from 1,120°C to 900° C (3) D increases 2-fold with increasing pressure from ∼ 2.5 to 20 kb. (4) D increases ∼ 2-fold fO2 increases from approximately that of the MW buffer to the HM buffer (5) D remains unchanged within experimental error as the water content of the melt changes from ∼ 0.3 to ≳ 10% by weight H2O. The absolute REE content of the clinopyroxene shows no consistent trend with temperature, but decreases slightly with increasing pressure, paralleling an increase in the jadeite component of the pyroxene. Thus the increase in D with increasing pressure is attributed to changes in the silicate liquid structure, which discriminate against accommodation of REE with increasing pressure. The clinopyroxene REE content increases with increasing fO2, and in this case the increase in D with increasing fO2 may be attributed mainly to this change in the clinopyroxene composition. Application of the present results to geochemical modelling allows a more appropriate choice of D values, according to the liquid composition and physical conditions applicable in the modelled system. They may also be used to evaluate cognate or xenocrystic relationships between clinopyroxene megacrysts and their host matrix.

Strontium isotopic and selected trace element variations between two Aleutian volcanic centers (Adak and Atka): implications for the development of arc volcanic plumbing systems

Abstract: Major and trace element concentrations and initial 87Sr/86Sr ratios of lavas from the Aleutian volcanic centers of Adak and Atka have been used to study the evolution of their respective lithospheric plumbing systems. The centers are within 150 km of one another and show similar overall silica ranges (47–67%), but Adak (∼40 km3) is smaller than Atka (∼200 km3). Adak's lavas are chemically and isotopically heterogeneous (87Sr/86Sr:0.70285–0.70330) and two units contain lithospheric xenoliths. The lavas of the much larger Atka, on the other hand, have much less variability in major and trace elements as well as 87Sr/86Sr (0.70320–0.70345). We suggest that these characteristics are a measure of the relative maturity and “cleanliness” of the lithospheric plumbing systems that supply magma to these centers. Because Aleutian volcanic centers often remain fixed for relatively long periods of time (∼5 m.y.), once established, magmatic passageways are repeatedly used. Young plumbing systems are relatively cool and contain large amounts of wallrock contaminant, and ascending magmas undergo contamination as well as concurrent crystallization and fractionation. With time, however, heat and mass transfer between ascending magmas and wallrock produce thermal and chemical boundary layers that insulate subsequent magmas. In effect, the plumbing system matures. The chemical heterogeneity displayed by young, “dirty” systems (like Adak) reflects not only the magma source but also the wallrock encountered during ascent and possibly the effects of extensive crystal fractionation. Thus, it is the petrologic data of mature, clean systems, like Atka, that yield the most direct and unambiguous information on the ultimate origin of the lavas and their near surface evolution.

Mafic potassic lavas of the Quaternary West Eifel volcanic field

Abstract: Major and trace element analyses for 103 volcanoes of the Quaternary West Eifel volcanic field show the lavas to be dominantly primitive (MgO>7 wt.%) and potassic (Na2O/K2O∼1). The rocks are divided into (1) a foidite (F)-suite, volumetrically dominant and consisting of four types: leucitites and nephelinites, melilite-bearing foidites, olivine-free foidites, sodalite-bearing melilite-free foidites, and (2) a younger olivine-nephelinite and basanite (ONB)-suite, concentrated in the southeastern part of the field. Dominantly cpx-phyric F-suite magmas differ from the dominantly ol-phyric ONB-suite mainly in higher K2O/ Na2O and CaO/Al2O3-ratios, higher Rb, Cu, H2O, CO2 and LREE concentrations and slightly lower Sr, Ni and Y contents. Most magmas have fractionated small amounts of olivine, clinopyroxene, and minor phlogopite. Systematic compositional variations within volcanoes or volcano groups are rare. Five more differentiated volcanoes (2 tephrites, 3 phonolites) occur in the center of the field. Their magmas are interpreted to have formed by fractionation within crustal magma chambers.

Structural controls and mechanisms of diffusion in natural silicate melts

Abstract: The diffusion properties of Na, Cs, Ba, Fe and Eu ions have been determined experimentally for a pantellerite melt and of these ions plus Li, Mn and Co in pitchstone melt, using the radiotracer residual-activity method, and narrow platinum capillaries, over the temperature range 1,200–1,400° C. In addition, Eu diffusion in a basaltic and an andesitic melt was determined. Diffusion of all cations follows an Arrhenius relationship, activation energy values being high for diffusion in the pantellerite melt (e.g. Eu: 100 kcal mol−1) except in the case of Na (24.3 kcal mol−1). Activation energies of diffusion in the pitchstone melt are similar to values recorded earlier for andesitic and basaltic melts. The new data are used, along with previously published data for diffusion in other composition melts, to examine the compositional and structural controls on diffusion. The range of diffusivities shows a marked change with melt composition; over two orders of magnitude for a basaltic melt, and nearly four orders for a pantellerite melt (both at 1,300° C). Diffusivity of all cations (except Li and Na) correlates positively with the proportion of network modifying cations. In the case of Li and Na the correlation is negative but the diffusivity of these ions correlates positively with the proportion of Na or of Na + K ions in the bulk melt. Diffusion behaviour in the pantellerite melt departs from the relationships shown by the data for other melt compositions, which could be partly explained by trivalent ions (such as Fe) occupying network forming positions. The diffusivity of alkali metal ions is strongly dependent on ionic radius, but this is not the case with the divalent and trivalent ions; diffusivity of these ions remains relatively constant with change in radius but decreases with increase in ionic charge. A compensation diagram shows four distinct but parallel trends for the majority of the cations in four melt types but the data for Li and Na plot on a separate trend. This and the other relationships are used to elucidate possible mechanisms of diffusion. Exchange mechanisms appear to be common, with the preservation of local charge balance. Li and Na diffuse by a distinct mechanism which involves exchange of similar or identical ions. The diffusion behaviour of the smaller alkali metal ions is sufficiently distinct from all other cations to indicate that diffusion could be an important factor in the geochemical fractionation of the alkali elements.s

Hydrogen and oxygen isotope geochemistry of Ascension Island lavas and granites: variation with crystal fractionation and interaction with sea water

Abstract: Lavas and pyroclastics on Ascension Island contain plutonic blocks that include fluid-inclusion-bearing peralkaline-granite. 18O/16O ratios, F and Cl have been analysed on whole rocks and/or minerals for lavas and granites, and D/H ratios and H2O+ for comenditic obsidians and granites. Whole rock 18O/16O ratios of fresh alkali-basalt, hawaiite, trachyandesite, trachyte and comendite range from 6.0 to 6.9‰ with 18O tending to increase with increase in SiO2. The δ 18O values of the granites are from 0.0 to 0.3‰ depleted in 18O relative to the comendites. Comenditic obsidians have δD= −80±4‰ and H2O+ ∼0.3 wt.% while amphiboles from the granites have δD= −56±2‰ The O-isotope trend of the lavas is consistent with a crystal fractionation model. Fresh igneous rocks with δ 18O values greater than 7‰ involve processes in addition to crystal fractionation of a basaltic magma. The D/H ratios and Cl contents (∼ 3,000 ppm) of the H2O-poor comenditic obsidians represent undegassed primary magmatic values. The H-isotope compositions and low H2O and Cl (167 ppm) contents of the granites are consistent with the major degassing (loss of >90% of initial H2O) of an H2Osaturated magma derived from the interaction of sea (or possibly meteoric) water with the H2O-undersaturated comenditic melt. It is proposed that, associated with caldera subsidence and stoping, water was sucked in around the residual magma before the system had time to be sealed up. The H2O-undersaturated magma consumed this H2O with possibly some minor partial dehydration and dewatering of the hydrated volcanic roof blocks, at a pressure of about 1.5 kb. The granites are the plutonic equivalents of rhyolitic pyroclastics and not directly of the comendites. Granites from oceanic islands may, in general, be a result of generating an H2O-saturated acid melt by such direct or indirect crustal water-magma interaction processes.

The difference between oceanic and continental tholeiites: a fluid dynamic explanation

Abstract: Continental tholeiites have higher SiO2, K2O and light rare earth element contents and more evolved isotopic characteristics than their oceanic counterparts. These differences can be explained if the compositions of the parent magmas to both types of tholeiites are similar but if continental magmas assimilate significant amounts of continental crust en route to the surface. Although there is little doubt that most continental tholeiites have assimilated crustal material, the lcoation and mechanism of assimilation remain uncertain. Longhi (1981) has argued that magmas derived directly from the mantle should crystallize little orthopyroxene. The abundance of orthopyroxene in most continental layered intrusions suggests that they have crystallized from magmas which have assimilated continental crust. Since orthopyroxene is an early crystallizing phase in layered intrusions, this assimilation must occur early, before the magma enters the chamber. Assimilation can occur at the margins of the dykes which feed magma chambers, depending on the nature of the flow. If the flow is turbulent the high temperatures at the centre of the dyke will extend to the margins and the magma will erode the dyke walls. If the flow is laminar, a conductive profile develops at the margin and the flowing magma chills against the walls, protecting them from thermal erosion. The nature of flow in a dyke can be predicted from the Reynolds number, the criteria for turbulence. Reynolds number calculations suggest that the flow of primitive magmas in continental dykes will be fully turbulent and, if this is the case, assimilation of low melting point components in the walls of the dyke is inevitable. It is therefore suggested that many of the geochemical characteristics of continental tholeiites result from melting at the walls of dykes as primitive magmas ascend through the crust.

The Baffin Bay lavas and the value of picrites as analogues of primary magmas

Abstract: The Baffin Bay picrites have been the focal point of a controversy concerning the MgO content of primary magmas derived from the upper mantle. A sample population of 48 lava chilled margins collected across the Baffin Bay volcanic succession at the northeastern tip of Padloping Island exhibits a prominent compositional mode between 14 and 16 weight percent MgO (19–22 Mg, cation units = Mg/100 cations). The petrography of these samples, however, requires that the Padloping magmas were mixtures of olivine crystals and liquid at their eruption. Olivine phenocrysts constituted 15 to 30 volume percent of these magmas and retain compositions requiring coexisting liquid compositions with only 10 to 13.5 weight percent MgO (14–18.5 Mg). However, highly magnesian, olivine xenocrysts (up to Fo 93) found in the most magnesian lavas require the former existence of liquids with at least 18 weight percent MgO (24 Mg). If these xenocrysts represent early cumulates, then the primary liquids of the Padloping suite must have been at least this MgO rich with temperatures greater than 1,425° C. Such primary liquids could have evolved by olivine crystallization to a ‘steady state’, equilibrated crystal — liquid mixtures in a shallow reservoir system prior to eruption. The compositions of the liquids of these mixtures appear to have been perched at the point of plagioclase saturation at approximately 1,275° C.

Petrology of the mafic rocks of the Xigaze ophiolite, Tibet

Abstract: The Xigaze ophiolite (Yarlung-Zangbo suture zone, Southern Tibet, China) shows an unusual crustal sequence characterized by a lack of large masses of cumulate gabbros, by dolerites intrusive throughout the whole ophiolite sequence, and by the injection of dolerites in already serpentinized peridotites. The abyssal tholeiitic nature of all the mafic rocks indicates that they have been generated at an oceanic ridge. All the geological arguments and petrological and textural data on the mafic rocks point to very low heat production and large heat losses through widespread intensive sea-water circulation, for the spreading centre in which they have been formed, in good agreement with a slow-spreading ridge origin.

Experimental determination of cation diffusivities in aluminosilicate garnets - II. Multicomponent simulation and tracer diffusion coefficients

Abstract: Data from experimentally-induced diffusion profiles at approximately 40 Kbar, 1,300–1,500° C in spessartine-almandine couples and a pyrope-almandine couple at ∼ 40 Kbar, 1,440° C, described in Part I, were used to derive tracer diffusion coefficients (D*) of Fe, Mn and Mg in garnet. The experimental data were fitted by numerical simulations that model multicomponent, compositionally-dependent difussion, including the effects of nonideal thermodynamic mixing. The simulations use the formalism of irreversible thermodynamics and an eigenvector technique of solution. We were able to fit the asymmetrical spessartine-almandine profiles using constant D* and either the Darken/Hartley-Crank or Manning-Lasaga models relating D* and interdiffusion coefficients, and both models yielded DMg*consistent with the direct measurement of DMg*in by Cygan and Lasaga (1985) at lower temperatures (750–900° C). The results (equations 4.1–4.3 and Table 1) indicate that DFe*≅DMg*QMn, where Q is the activation energy. In contrast, the asymmetry of pyrope-almandine profiles is too great to fit with either tracer model assuming constant D* and indicates that DMg*is similar to its value in spessartine-almandine couples but DFe*is an order of magnitude less. The fit also suggests that DCa*< DFe*

Thermodynamic analysis of quadrilateral pyroxenes

Abstract: A thermodynamic model for quadrilateral pyroxene solutions, is developed from the following assumptions: 1) Non-convergent cation-disorder of Fe and Mg occurs over the Ml and M2 sites; 2) Ca is partitioned completely into the M2 site; and 3) Clinopyroxenes unmix to a high-Ca plus low-Ca phase. This ternary model is an extension of Thompson's (1969, 1970) binary model. Model expressions are presented that describe cation distributions, exchange equilibria and chemical potentials of both ternary and quadrilateral components.