Showing papers in "Contributions to Mineralogy and Petrology in 1976"

Geochemistry of eocene calc-alkaline volcanic rocks from the Kastamonu area, Northern Turkey

Abstract: Analytical data for Sr, Rb, Cs, Ba, Pb, rare earth elements, Y, Th, U, Zr, Hf, Sn, Nb, Mo, Ni, Co, V, Cr, Sc, Cu and major elements are reported for eocene volcanic rocks cropping out in the Kastamonu area, Pontic chain of Northern Turkey. SiO2% versus K2O% relationship shows that the analyzed samples belong to two major groups: the basaltic andesitic and the andesitic ones. High-K basaltic andesites and low-K andesites occur too. Although emplaced on continental type basement (the North Anatolian Crystalline Swell), the Pontic eocene volcanics show elemental abundances closely comparable with typical island arc calc-alkaline suites, e.g. low SiO2% range, low to moderate K2O% and large cations (Cs, Rb, Sr, Ba, Pb) contents and REE patterns with fractionated light and almost flat heavy REE patterns. ΣREE and highly charged cations (Th, U, Hf, Sn, Zr) are slightly higher than typical calc-alkaline values. Ferromagnesian elements show variable values. Within the basaltic andesite group the increase of K%, large cations, ΣREE, La/Yb ratio and high valency cations and the decrease of ferromagnesian element abundances with increasing SiO2% content indicate that the rock types making up this group developed by crystalliquid fractionation of olivine and clinopyroxene from a basic parent magma. Trace element concentration suggest that the andesite group was not derived by crystal-liquid fractionation processes from the basaltic andesites, but could represent a distinct group of rocks derived from a different parent magma.

Two-liquid partition coefficients: Experimental data and geochemical implications

Abstract: Partition coefficients for Cs, Ba, Sr, Ca, Mg, La, Sm, Lu, Mn, Ti, Cr, Ta, Zr, and P between immiscible basic and acidic liquids in the system K2O-Al2O3-FeO-SiO2 were experimentally determined at 1,180 °C and 1 atm. Phosphorus is most strongly enriched in the basic melt (by a factor of 10), followed by rare earth elements, Ta, Ca, Cr, Ti, Mn, Zr, Mg, Sr, and Ba (enriched by a factor of 1.5). Of the elements studied, only Cs is enriched in the acidic melt. The two-liquid partition coefficients of Zr, Ta, Sm, and Mn are constant for concentrations ranging from <0.1% to as high as 1 wt.-%, suggesting that Henry's law is applicable in silicate melts (at least for these elements) to concentrations well above typical trace element levels in rocks. The strong relative preference of many elements for the basic melt implies that the structural characteristics of basic melts more readily permit stable coordination of cations by oxygen. Partitioning of elements between crystal and liquid in a magma must therefore be influenced by the composition (and consequent structure) of the liquid. Application of the two-liquid partition coefficients to possible occurrences of liquid immiscibility in magmas reveals that typical basalt-rhyolite associations are probably not generated by two-liquid phase separation. However, liquid immiscibility cannot be discounted as a possible origin for lamprophyric rocks containing felsic segregations.

An experimental investigation of olivine morphology

Abstract: Olivine crystals can adopt ten types of shape. Experimental crystallization of eight rock melts shows that there is a systematic change from polyhedral or granular olivines → hopper olivines → branching olivines → randomly oriented chain olivines → parallel-growth chain olivines → chain+lattice olivines → plate or feather olivines, with increase in cooling rate and with increase in degree of supercooling. This sequence involves changes from complete to progressively less complete crystals and from equant habit to elongate bladed habit (c>a≫b) to tabular habit (a≃c ≫ b). The sequence is not affected by the phase relations of the melt. The larger the olivine content of a melt the slower the cooling rate at which a particular olivine shape grows, whereas the lower the melt viscosity, the greater the cooling rate. Irrespective of the melt composition, comparable crystal shapes grow at the same degrees of supercooling. By comparison of the shapes of olivine crystals in experiments with those in rocks of similar composition, it is possible to deduce the cooling rate through the olivine crystallization interval and the approximate degree of supercooling at which the olivine crystals nucleated and grew in the rocks. The various shapes of skeletal olivines in many picrites, olivine-rich basalts and the Archaean “spinifex” rocks are not due to rapid cooling, but to rapid olivine growth caused by the high normative olivine content of the magma.

Carbonation and melting reactions in the system CaO–MgO–SiO2–CO2 at mantle pressures with geophysical and petrological applications

Abstract: Bowen's petrogenetic grid was based initially on a series of decarbonation reactions in the system CaO-MgO-SiO2-CO2 with starting assemblages including calcite, dolomite, magnesite and quartz, and products including enstatite, forsterite, diopside and wollastonite. We review the positions of 14 decarbonation reactions, experimentally determined or estimated, extending the grid to mantle pressures to evaluate the effect of CO2 on model mantle peridotite composed of forsterite(Fo)+orthopyroxene(Opx)+clinopyroxene(Cpx). Each reaction terminates at an invariant point involving a liquid, CO2, carbonates, and silicates. The fusion curves for the mantle mineral assemblages in the presence of excess CO2 also terminate at these invariant points. The points are connected by a series of reactions involving liquidus relationships among the carbonates and mantle silicates, at temperatures lower (1,100–1,300° C) than the silicate-CO2 melting reactions (1,400–1,600° C). Review of experimental data in the bounding ternary systems together with preliminary data for the system CaO-MgO-SiO2-CO2 permits construction of a partly schematic framework for decarbonation and melting reactions at upper mantle pressures. The key to several problems in the peridotite-CO2 subsystem is the intersection of a subsolidus carbonation reaction with a melting reaction at an invariant point near 24 kb and 1,200°C. There is an intricate series of reactions between 25 kb and 35 kb involving changes in silicate and carbonate phase fields on the CO2-saturated liquidus surfaces. Conclusions include the following: (1) Peridotite Fo+Opx+Cpx can be carbonated with increasing pressure, or decreasing temperature, to yield Fo+Opx+Cpx+Cd (Cd=calcic dolomite), Fo+Opx+Cd, Fo+Opx+Cm (Cm=calcic magnesite), and finally Qz+Cm. (2) Free CO2 cannot exist in subsolidus mantle peridotite with normal temperature distributions; it is stored as carbonate, Cd. (3) The CO2 bubbles in peridotite nodules do not represent free CO2 in mantle peridotite along normal geotherms. (4) CO2 is as effective as H2O in causing incipient melting, our preferred explanation for the low-velocity zone. (5) Fusion of peridotite with CO2 at depths shallower than 80 km produces basic magmas, becoming more SiO2-undersaturated with depth. (6) The solubility of CO2 in mantle magmas is less than about 5 wt% at depths to 80 km, increasing abruptly to about 40 wt% at 80 km and deeper. (7) Deeper than 80 km, the first liquids produced are carbonatitic, changing towards kimberlitic and eventually, at considerably higher temperatures, to basic magmas. (8) Kimberlite and carbonatite magmas rising from the asthenosphere must evolve CO2 at depths 100-80 km, which contributes to their explosive emplacement. (9) Fractional crystallization of CO2-bearing SiO2-undersaturated basic magmas at most pressures can yield residual kimberlite and carbonatite magmas.

Elemental mobility during zeolite facies metamorphism of the Tertiary basalts of eastern Iceland

Abstract: Major and trace element analyses are reported for 70 lavas from the Tertiary succession of eastern Iceland The low grade regional metamorphism responsible for the zeolite zones appears to have caused significant mobilization of some elements, particularly Si, Mg, K, Rb, Sr and light rare earth elements (LREE) In contrast, values for Ti, P, Zr, Y, Nb, Ta, Hf and some of the rare earth elements show a high degree of correlation and this is taken to imply that these elements have been relatively unaffected by metasomatic transport The demonstrated mobility of Sr and LREE suggest that Sr isotopic and rare earth data obtained from the eastern Iceland lavas must be interpreted with caution

Shock processes in porous quartzite: Transmission electron microscope observations and theory

Abstract: A high-resolution study of shocked Coconino Sandstone from Meteor Crater, Arizona, was undertaken using transmission electron microscopy to investigate the textural relations of high-pressure phases produced by meteorite impact In weakly shocked rocks (estimated average pressure, P < 100 kb), quartz in the interiors of grains retains its initial microstructure, but near original grain boundaries, quartz is altered by fractures and planar features resembling Brazil twins, and is partially transformed to coesite and glass In moderately shocked rocks (estimated average pressure, 100 < P <250 kb), as much as 50% of the residual quartz is fractured but otherwise undeformed Near grain boundaries relatively undamaged quartz exists in direct contact with coesite and stishovite Filamentary, microvesicular “froth” fills cracks and fractures in the regions containing high-pressure phases Coesite present in regions which are collapsed pores has a unique texture, not previously reported for a shock-formed phase: the grains are equidimensional and form a mosaic pattern characteristic of products of high-temperature recrystallization In strongly shocked rocks (estimated pressure P <250 kb) quartz contains abundant glass lamellae, identical to optical “planar features” except that they are so closely spaced that they would not be resolved optically Vesicular glassy regions in strongly shocked rocks contain remnants of large (∼5 μm) coesite crystals, indicating that the shock-formed glass in these regions formed by melting of coesite rather than quartz

Oxygen isotope studies of potassic volcanic rocks of the Roman Province, Central Italy

Abstract: The 18O/16O ratios of rocks and coexisting minerals were measured for 93 samples of leucite-bearing lavas, pyroclastics, and related volcanic rocks from the Quaternary Roman Co-Magmatic Province, Italy. The δ18O values were found to generally increase northward in the sequence: Ischia (5.8 to 7.0); Somma-Vesuvius and Phlegrean Fields (7.3 to 8.3); Alban Hills (7.3 to 8.7); M. Sabatini (7.3 to 9.7); Vico Volcano (7.4 to 10.2); and M. Vulsini (8.1 to 11.7). The northward increase in δ18O parallels a similar increase in 87Sr/86Sr, and these data indicate that the Roman magmas have interacted strongly with high-18O continental crust. A marked increase in δ18O occurs just north of Rome where the Roman Province begins to overlap the calc-alkaline, oversaturated Tuscan Magmatic Province. Therefore, some of the observed 18O/16O and 87Sr/86Sr enrichments in the Roman magmas may have been facilitated by direct mixing with the high-18O Tuscan magmas or because the high-18O country rocks underwent widespread heating during a couple of million years of Tuscan igneous activity. Although many of the Roman magmas underwent fractional crystallization without appreciable change in δ18O, contamination has produced a correlation between δ18O and SiO2 content at several of the volcanic centers; thus the trachytes are typically higher in 18O than the undersaturated rocks. The major features of the oxygen isotope data can be explained in terms of a simple two-component mixing model in which one end-member was a primary, strongly undersaturated magma derived from the upper mantle, with δ18O≈+6, 87Sr/86Sr≈0.704 to 0.705, and SiO2<44wt.%. However, none of the analyzed samples have these values, as they have all been contaminated to some extent. The closest approach is found in some of the leucitepyroxenite ejecta from the Alban Hills. The second end-member, derived from the continental crust, had a variable composition with δ18O≈+12 to +20, 87Sr/86Sr≧0.712 to 0.720, and SiO2≧65wt.%, and it mixed in much greater proportions in the volcanoes north of Rome than in those of the Alban Hills or the Naples area. The widespread interactions between the Roman magmas and the continental crust are probably due to (1) the fact that such low-SiO2 magmas always have a very strong tendency to interact with quartz-bearing rocks of the continental crust, and (2) in Italy, these magmas were emplaced into a tectonically very active area containing poorly consolidated sedimentary rocks, and in the northern part of the belt there had been a prior history of extensive calc-alkaline igneous activity.

The significance of incompatible elements in mid-Atlantic ridge basalts from 45°N with particular reference to Zr/Nb

Abstract: K, Rb, Ba, Sr, Y, Zr and Nb have been determined in samples of MORB (mid-ocean ridge basalt) from the mid-Atlantic ridge at 45 °N by X-ray fluorescence spectrometry. This suite is characterised by higher concentrations of K, Rb, Ba and Nb (and lower K/Rb, K/Ba and Zr/Nb) than low-K tholeiites (typical MORB) previously described in the literature. Available data from other sources also shows that the 45 °N suite has higher Cs and U contents, rare earth-patterns enriched in the light REE, and higher 87Sr/86Sr ratios relative to typical MORB. The inter-element and isotopic ratios of typical MORB have been previously interpreted to imply that these samples have been derived from source areas which have undergone earlier differentiation and/or melting episodes. The 45 °N MORB samples are considered to be derived from more primitive or relatively undepleted mantle. It is noteworthy that these samples have inter-element ratios very similar to those obtained for oceanic island basalts associated with the mid-ocean ridges. The significance of the relative enrichment or depletion of Nb in MORB has not been previously noted and the use of the Zr/Nb ratio to illustrate these effects is emphasised. The abundances of Zr and Nb are apparently unaffected by sea-water alteration and thus the Zr/Nb ratio is potentially a more useful measure of depletion than ratios involving K and Rb, which are very sensitive to sea-water alteration. Brief mention is made of the variation of Zr/Nb in other MORB samples to demonstrate the existence of varying degrees of depletion in the respective source areas of these samples.

Rare Earth data and petrogenesis of andesite from the North Chilean Andes

Abstract: Abundances of 9 Rare-Earth Elements are reported for 21 andesite and dacite samples from a small group of volcanoes in the North Chilean segment of the Andean plate margin. The data are correlated with major and trace element data (Roobol et al., in press) and are used to critically assess several models proposed to account for the origin of andesite. A three-stage model for andesite petrogenesis is proposed. This involves: (i) initiation of melting at the subduction zone and rise of magmas into the overlying mantle-wedge, which thereby becomes richer in garnet-pyroxene components and large-ion lithophile (LIL) elements, (ii) partial melting of this LIL-enriched garnet pyroxenite material to form andesitic magmas within the mantle wedge, (iii) minor crystal fractionation during rise through the crust.

Phase relationships in granulitic metapelites from the Ivrea-Verbano zone (Northern Italy)

Abstract: Paragneisses of the Ivrea-Verbano zone exhibit over a horizontal distance of 5 km mineralogical changes indicative of the transition from amphibolite to granulite facies metamorphism. The most obvious change is the progressive replacement of biotite by garnet via the reaction: a $${\text{Biotite + sillimanite + quartz }} \to {\text{ Garnet + K - feldspar + H}}_{\text{2}} {\text{O}}$$ which results in a systematic increase in the modal ratio g = (garnet)/(garnet + biotite) with increasing grade. The systematic variations in garnet and biotite contents of metapelites are also reflected by the compositions of these phases, both of which become more magnesian with increasing metamorphic grade. The pressure of metamorphism has been estimated from the Ca3Al2Si3O12 contents of garnets coexisting with plagioclase, sillimanite and quartz. These phases are related by the equilibrium: b $$\begin{gathered} 3 CaAl_2 {\text{Si}}_{\text{2}} {\text{O}}_{\text{8}} \rightleftharpoons Ca_3 Al_2 {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} + 2 Al_2 {\text{SiO}}_{\text{5}} + {\text{SiO}}_{\text{2}} \hfill \\ plagioclase garnet sillimanite quartz \hfill \\ \end{gathered} $$ which has been applied to these rocks using the available data on the mixing properties of plagioclase and garnet solid solutions. Temperature and f H 2O estimates have been made in a similar way using thermodynamic data on the biotite-garnet reaction (a) and the approximate solidus temperatures of paragneisses. Amphibolite to granulite facies metamorphism in the Ivrea-Verbano zone took place in the P-T ranges 9–11 kb and 700–820 °C. The differences in temperature and pressure of metamorphism between g= 0 and g = 1 (5 kms horizontal distance) were less than 50° C and approximately 1 kb. Retrogression and re-equilibration of garnets and biotites in the metapelites extended to temperatures more than 50° C below and pressures more than 1.5 kb below the peak of metamorphism, the degree of retrogression increasing with decreasing grade of the metamorphic “peak”. The pressure and temperature of the peak of metamorphism are not inconsistent with the hypothesis that the Ivrea-Verbano zone is a slice of upthrusted lower crust from the crust-mantle transition region, although it appears that the thermal gradient was too low for the zone to represent a near-vertical section through the crust. The most reasonable explanation of the granulite facies metamorphism is that it arose through intrusion of mafic rocks into a region already undergoing recrystallisation under amphibolite facies conditions.

Some problems of the projection of chemical data into mineralogical classifications

Abstract: AbstractTo enable chemical data to be used in mineralogical classifications it is recommended that a)Or, Ab and An are recalculated to alkali feldspar and plagioclase using the relationshipalkali feldspar=Or × Tplagioclase =An × T where T=(Or + Ab + An)/(Or + An)b)any adjustment to the ratio of FeO to Fe2O3 is done according to the relationshipOx=0.93−0.0042 SiO2−0.022(Na2O+K2O) for volcanic rocks andOx=0.88−0.0016 SiO2−0.027 (Na2O+K2O) for plutonic rocks where Ox=FeO/(FeO+Fe2O3.

Evolved lavas from the Snake River Plain: Craters of the Moon National Monument, Idaho

Abstract: Holocene lavas from Craters of the Moon (COM) National Monument are representative of “differentiated” lavas which occur around the margins of the Snake River Plains (SRP) and they range serially in composition from alkali- and phosphorous-rich ferrobasalts to ferrolatites. Petrographic study indicates that these lavas evolved primarily by cotectic crystallization of olivine, plagioclase, magnetite and apatite in the mafic members of the suite (ferrobasalts), and by cotectic crystallization of plagioclase, magnetite, clinopyroxene and minor olivine in the salic members. Quantitative phase relations in the COM lavas, calculated by means of a leastsquares mixing program, indicate that the observed range in composition among these lavas corresponds to at least 70 percent crystallization of a magma similar to the most mafic COM lavas. Anhydrous one-atmosphere experimental crystallization studies fail to reproduce exactly the inferred phase relations; the discrepancy is attributed to the presence of water in the naturally crystallized magmas. The origin of COM parental magma cannot be unequivocably resolved. Available evidence suggests that COM lavas do not represent melts derived directly from the mantle: (1) high Sr87/Sr86 ratios (0.708 to 0.712), (2) relatively high Fe/(Fe+Mg) and excluded-element content in even the most mafic COM lavas, (3) occurrence of megacrysts of inferred high-pressure origin in the Lava Creek flow. Megacrysts occur in the Lava Creek flow as clusters of labradorite, aluminous clinopyroxene, and olivine. Analogy with the experiments of Thompson [1] and least squares mixing calculations indicate that intermediate (ca. 8 to 10 Kbar) pressure fractionation of such megacrysts from olivine tholeiite magma may yield derivative COM-type liquids.

Equilibrium-disequilibrium relations in the Monte Rosa Granite, Western Alps: Petrological, Rb-Sr and stable isotope data

Abstract: Nine samples from the Monte Rosa Granite have been investigated by microscopic, X-ray, wet chemical, electron microprobe, stable isotope and Rb-Sr and K-Ar methods. Two mineral assemblages have been distinguished by optical methods and dated as Permian and mid-Tertiary by means of Rb-Sr age determinations. The Permian assemblage comprises quartz, orthoclase, oligoclase, biotite, and muscovite whereas the Alpine assemblage comprises quartz, microcline, albite+epidote or oligoclase, biotite, and phengite.

Solubility of CO2 in olivine melilitite at high pressures and role of CO2 in the earth's upper mantle

Abstract: The positions of the liquidi and the near-liquidus phases of olivine-melilitite+CO2 have been determined under MH-buffered and ‘furnace-buffered’ conditions up to 40 kb. It is found that CO2 alone lowers the liquidus compared to ‘dry’ conditions, yet its influence is minor compared to H2O. The major role of CO2 is to favour the growth of orthopyroxene and garnet over that of olivine at least at high pressures. CO2-contents of glasses from experiments just above the liquidus (MH-buffered) were determined as 5.1 % at 10kb; 7.5 % at 20kb, 9.3 % at 30kb and 10–11 % (estimated) at 40 kb. Experiments on (pyrolite −40 % olivine)+H2O+CO2 show that CO2 occurs under mantle conditions as carbonate under subsolidus conditions and dissolved in a melt above the solidus. At 30kb, the solidus lies between 1,000 ° C and 1,050 ° C for vapour-saturated conditions, at \(X_{{\text{H}}_{\text{2}} {\text{O}}} = 0.5 = X_{{\text{CO}}_{\text{2}} } \) and at \(X_{{\text{H}}_{\text{2}} {\text{O}}} = 0.9,{\text{ }}X_{{\text{CO}}_{\text{2}} } = 0.1\).

Rare earth element evidence for differentiation of McMurdo volcanics, Ross Island, Antarctica

Abstract: Eighteen samples of the McMurdo volcanics on Ross Island, Antarctica consisting of basanitoid, trachybasalt and phonolite have been analyzed for rare earth elements (REE) in order to determine the details of differentiation using quantitative trace element modeling. The basanitoids have REE patterns similar to those for alkali basalts or nephelinites from ocean islands. Since there is no correlation between REE and silica contents among five basanitoids, some of the variability in the REE contents must be related to the extent of partial melting, variation in the residual mineralogies of the mantle during melting, or to inhomogeneities in the REE composition of the mantle.

Some effects of tectonic recrystallisation on fluid inclusions in vein quartz

Abstract: Filling temperature data obtained from tectonic vein quartz varies according to the state of intracrystalline deformation. Strain free domains within grains exhibit abundant primary fluid inclusions, from which internally consistent temperatures are recorded. The onset of optical features associated with intracrystalline deformation by dislocation creep is accompanied by a decrease in the number of fluid inclusions and an increase of filling temperatures. At higher states of strain tectonic recrystallisation, evidenced by the formation of new subgrains, leads to the complete destruction of inclusion arrays. Empty cavities are swept out of the new grains during recrystallisation, into subgrain walls. Heterogeneous deformation of vein quartz at the intracrystalline level may be due in part to selective hydrolytic weakening in areas where fluid inclusions have leaked and thereby increased the structural water content.

High-18O igneous rocks from the Tuscan Magmatic Province, Italy

Abstract: The 18O/16O ratios were measured for 60 rocks and coexisting minerals from the Plio-Pleistocene, calc-alkaline, Tuscan Magmatic Province, Italy. The δ18O values of these magmas were as follows: Elba, Giglio, and Montecristo granodiorites (11.4 to 12.1); M. Cimini rhyolites and trachytes (11.2 to 11.7); Roccastrada, S. Vincenzo, and M. Amiata rhyolites (12.3 to 13.4); and the Tolfa rhyolites and quartz latites (15.3 to 16.4). The latter are by far the highest δ18O values yet reported for primary volcanic rocks. The extremely high δ18O values in the Tuscan Province indicate that these magmas all formed by melting or large-scale assimilation of high-18O argillaceous sedimentary rocks, in agreement with previous evidence based on the high 87Sr/86Sr ratios and ubiquitous presence of cordierite. Low δ18O values (+5 to +6) were found in the feldspar, but not the quartz, of the small Porto Azzuro stock on Elba, as well as in the marginal facies of the M. Capanne stock, indicating that these bodies interacted with low-18O meteoric-hydrothermal fluids subsequent to solidification. The M. Cimini lavas apparently formed about 1 m.y. ago by mixing of high-18O Tuscan magmas containing large K feldspar crystals with potassic, undersaturated, low-18O magmas of the adjacent Roman Co-Magmatic Province. These large sanidine crystals did not completely equilibrate 18O/16O with the coexisting magma.

U-Pb zircon and Rb-Sr whole-rock dating of low-grade metasediments example: Montagne Noire (Southern France)

Abstract: Three detrital, Proterozoic zircon suites extracted from siltstones progressively metamorphosed between chlorite- and staurolite-grade independently date the major Caledonian metamorphism within the gneiss dome of the Montagne Noire (Southern France). From this, the following conclusions concerning U-Pb systematics of zircons in low-, medium- and highgrade metamorphic rocks can be drawn: 1) Temperatures of at most 350–400 °C are sufficient to open U-Pb systems of metamict zircons or domains within zircons. 2) The observed open U-Pb system behaviour during metamorphism of the host rocks was found to be due to a low-temperature recrystallisation of highly radiation damaged zircon lattices, probably enhanced by high concentrations of fluid phases in the dehydrating rock volumes. 3) Recrystallisation of metamict zircons under low temperatures causes maximum U-Pb ages for the thermal climax of metamorphism of medium-and high-grade metamorphic rocks, as annealing and accompaning closing of U-Pb systems took place before the maximum temperatures of metamorphism were reached. 4) Low-temperature recrystallisation of old — generally Proterozoic—zircons can readily help to explain the fact that the numerous zircon suites from ancient shield areas yield “lower intercept ages” which are not correlated to any known geological event. Thus, either a weak thermal pulse, not necessarily registered by the mineral assemblage of the host rock, and/or elevated temperatures during burial in the crust might supply enough energy for a structural reordering and simultaneous lead loss of at least the most disordered lattice domains.

Garnet and pyroxene compositions in some diamondiferous eclogites

Abstract: Analyses are reported for garnet and pyroxene from 17 eclogites that contain diamond. The garnets contain small but significant contents of Na, Ti and P and the pyroxenes contain traces of K. The diamond-bearing eclogites do not constitute a unique compositional group but show a range of mineral compositions consistent with a very high P-T environment.

Volcanic rocks and processes of the Mid-Atlantic Ridge rift valley near 36 ° 49′ N

Abstract: Eighty samples of submarine basaltic lava were sampled from an 8 km segment of the floor and walls of the inner rift valley of the Mid-Atlantic Ridge during the French American Mid-Ocean Undersea Study (project Famous). The samples were collected from outcrops and talus slopes by the three submersibles: Alvin, Archimede, and Cyana at water depths of about 2600 meters. The early formed mineral content of the pillow lavas' glassy margins enables classification of the rocks into 5 types: (1) olivine basalt, (2) picritic basalt, (3) plagioclase-olivine-pyroxene basalt, (4) aphyric basalt, and (5) plagioclase-rich basalt. Chemical and mineralogical study indicates that at least 4 types are directly interrelated and that types (1) and (2) are higher-temperature, primitive lavas, and types (3) and (4) are lower-temperature, differentiated lavas derived from the primitive ones by crystal-liquid differentiation. The plagioclase-rich basalts also have a chemical composition of their glass comparable to that of the most differentiated basalts (types 3 and 4) but they differ in their greater amount of early formed plagioclase (12–35%). In general, the mineralogical variation across the rift valley shows an assymetrical distribution of the major basalt types. Despite the mineralogical diversity of the early formed crystals, the chemistry of the basalt glasses indicates a symmetrical and a gradual compositional change across the rift valley. Based primarily on their chemistry, the rock types 1 and 2 occupy an axial zone 1.1 km wide and make up the central volcanic hills. Differentiated lavas (types 3, 4) occupy the margins and walls of the inner rift valley and also occur near the center of the rift valley between the central hills. FeO/MgO ratios of olivine and coexisting melt indicate that the average temperature of eruption was 40 ° C higher for the primitive melts (types 1 and 2). Aside from major elements trends, the higher temperature character of the primitive basalts is shown by their common content of chrome spinel. The thickness of manganese oxide and palagonite on glassy lava provide an estimate of age. In a general fashion the relative age of the various volcanic events follow the compositional zoning observed in the explored area. Most of the youngest samples are olivine basalt of the axial hills. Most older samples occur in the margins of the rift valley (West and N.E. part of explored area) but are significantly younger than the spreading age of the crust on which they are erupted. Intermediate lava types occur mainly east of the rift valley axis and in other areas where plagioclase—olivine—pyroxene basalt and aphyric basalt are present. The above relations indicate that the diverse lava types were erupted from a shallow, zoned magma chamber from fissures distributed over the width of the inner rift valley and elongate parallel to it. Differentiation was accomplished by cooling and crystallization of plagioclase, olivine, and clinopyroxene toward the margins of the chamber. The centrally located hills were built by the piling up of frequent eruption of mainly primitive lavas which also are the youngest flows. In contrast smaller and less frequent eruptions of more differentiated lavas were exposed on both sides of the rift valley axis.

Rare-Earth Abundances in Basalts and Metabasalts from the Troodos Massif, Cyprus

Abstract: Abundances of some rare-earth elements (REE) in twelve pillow lavas from the Upper Pillow Lavas and Axis Sequence of the Troodos Massif are reported. The samples consist of three fresh basalts and nine zeolite facies metabasalts, metamorphosed at temperatures between 0 and 200° C. All give similar light rare-earth element (LREE) depleted patterns, indicating that hydrothermal metamorphism within this temperature range does not appreciably affect these REE patterns. The LREE depletion is consistent with a petrogenetic model in which Troodos formed at a spreading axis. Variations in profile shape indicate that mantle melting beneath the axis may have taken place during a series of discrete episodes.

Isotopic evidence for the derivation of some Roman region volcanics from anomalously enriched mantle

Abstract: Strontium isotope data are reported for primitive lavas (leucitites, tephritic leucitites, K-rich basalts, and related types) from the Roccamonfina volcano. A strong positive correlation is found between 87Sr/86Sr and the abundances of K, Rb, Sr, Ba and Zr. It is argued that the present contents of these elements in the lavas are not far removed from their concentrations in their parental primary magmas. Models involving disequilibrium and equilibrium melting of respectively homogeneous and heterogeneous source rocks are discussed. It is concluded that a heterogeneous source highly enriched in incompatible elements at some stage in the past (? 300 m.y.) is indicated.

The Major Petrological and Geochemical Characters of the 3,600 m.y. Uivak Gneisses from Labrador

Abstract: The 3,622±72 m.y. Uivak gneisses of northern Labrador consist of two major rock suites: regionally developed layered granodioritic gneisses (Uivak I suite) interpreted as derived from earlier tonalitic igneous parents by the massive introduction of potassium and rubidium, and a less extensive group of iron-rich porphyritic granodiorites and ferro-diorites (Uivak II suite) emplaced after at least one major period of deformation and migmatisation had affected the Uivak I gneisses. Samples from both suites fall on the 3,622 m.y. Rb/Sr whole-rock isochron.

Titanochondrodite and titanoclinohumite derived from the upper mantle in the Buell Park Kimberlite, Arizona, USA

Abstract: Kimberlite from Buell Park, Arizona, which was intruded into Permian sediments about 30 m.y. ago, is characterized by the hydrous silicates titanochondrodite and titanoclinohumite. Titanochondrodite is the first finding in kimberlites. Optical properties, chemical compositions and cell dimensions of these two minerals are determined.

Flowage differentiation: Limitation of the “Bagnold effect” to the narrow intrusions

Abstract: The “Bagnold effect”, the most important in the flowage differentiation, is practically inoperative in porphyritic rocks in the case of large dykes and pipes with a width or a diameter over 100 meters. For these large channels, the magmatic differentiation between the core and the walls is not related to the forced magmatic flow, but should be ascribed to other mechanisms: crystal settling, natural convection, synchronous injection of already differentiated materials.

Ammonium Silicate Stability Relations

Abstract: A method has been developed to control ammonium fugacity, $$f_{{\text{NH}}_{3}}$$ , at elevated temperatures and pressures. The method uses an internal nitrogen buffer, the assemblage Cr + CrN, in conjunction with a traditional external hydrogen buffer. In this manner, all gas fugacities in the system N-O-H can be calculated. The Cr + CrN buffer has been applied to study equilibria between buddingtonite (ammonium feldspar), ammonium muscovite, sillimanite, and quartz at a constant gas pressure of 2,000 bars. Two of the five relevant reactions were measured experimentally; from these data, it is possible to calculate isothermal sections at 500, 600, and 700° C. Below 600° C, ammonium muscovite is stable even at extremely low levels of $$f_{{\text{NH}}_{3}}$$ , while buddingtonite requires $$f_{{\text{NH}}_{3}}\;\geqq\;10^4$$ bars. Release of NH3 during progressive metamorphism can be achieved by three processes: thermal decomposition, dehydration, and cation exchange. Within the crust, $$f_{{\text{NH}}_{3}}$$ predominates over $$f_{{\text{N}}_{2}}$$ by several orders of magnitude; but on the surface, nitrogen released as NH3 by metamorphism will be oxidized to N2. Biological materials provide important intermediate storage for nitrogen compounds during the nitrogen cycle.

Late Archean metamorphism in the Buksefjorden region, Southwest Greenland

Abstract: Standard state thermodynamic data extracted from experimental studies and applied to mineral assemblages in orthogneisses, metasedimentary gneisses and metabasites show that conditions of late Archean (2,850 m.y.) upper amphibolite facies were P solid≈7.0 kb, T≈630° C, and rose to P solid≈10.5 kb, T≈810° C in adjacent granulite facies. The estimates of solid pressure for the granulite facies suggest a late Archean crustal thickness of ca. 35 km, comparable to present day continental crust. Upper amphibolite facies assemblages were in equilibrium with $$P_{H_2 O}$$ about one half P solid, while granulite assemblages equilibrated at much lower $$P_{H_2 O}$$ , varying from about one tenth P solid in quartzofeldspathic gneisses to one third P solid in more basic layers.

CO 2 solubility and solubility mechanisms in silicate melts at high pressures

Abstract: CO2 solubility has a slight negative temperature dependence in olivine melilitite at 30 kb with 9% CO2 dissolved at 1,450 °C, 8.5% at 1,550 °C and 8.3% at 1,650° C. CO2 is dissolved as the carbonate molecule (CO32−) only. Feldspar melts (albite-anorthite) dissolve much less CO2 at 30 kb (around 2%) with a slight increase with increasing anorthite content. A CO2 absorption peak in infrared spectra of albite-rich glasses diappears in favour of the CO32−peak with increasing anorthite content. It is inferred that CO2 was present as CO32−in albite-rich melts also, but reverts to CO2 during quenching because of bonding differences related to Ca2+ and Na+ in the melts.

High pressure cognate inclusions in the Newer Volcanics of Victoria

Abstract: High pressure pyroxene- and amphibole-rich inclusions are found in a number of Victorian Newer Volcanics volcanoes. The host lavas range from nepheline basanite to nepheline hawaiite and nepheline mugearite. The wide variation in chemistry and mineralogy of the inclusions is explained by crystallization from basaltic magmas under varying P-T and PH2O conditions at depth. At moderate pressure wehrlite inclusions (ol+cpx) form, whereas at higher pressures pyroxenites (opx+cpx) and genetically related megacrysts form. Under relatively anhydrous conditions the clinopyroxene megacrysts show a trend of Ca enrichment whereas under hydrous conditions, when amphibole is also stable, the pyroxene shows a trend to greater iron enrichment. The trend nepheline basanite to nepheline mugearite has developed by extensive fractionation of amphibole at elevated pressures under hydrous conditions. Under less hydrous conditions where clinopyroxene assumes the dominant role during crystal fractionation, derivative liquids display a trend of increasing K2O/Na2O ratio, with little modification of their level of undersaturation. Olivine plays a decreasing role in crystal fractionation processes with increasing pressure. The available evidence indicates that the only magma which could have been parental to the observed basanites was a more picritic basanite.

Variations in shock deformation at the Slate Islands impact structure, Lake Superior, Canada

Abstract: A progressive change in the level of shock deformation is documented in autochthonous rocks from the central uplift of the Slate Islands impact structure, Lake Superior. Correlation of these observations, which are based mainly on the relative frequency of planar features of specific crystallographic orientation in quartz, with experimental data is used to estimate the average shock pressures recorded in the samples studied. Recorded pressures range from 5.8 to 15.3 GPa and generally increase towards the proposed shock centre. Variations in the shock response of quartz of different grain size and texture are observed within and between samples. It is apparent that large interlocking quartz grains in “eyes” record approximately 15–20% higher levels of shock deformation than small grains in mosaics or large isolated phenocrysts. These variations in shock deformation are attributed to the effect of shock wave reverberations between grains and length of shock pulse duration within grains. Comparison of the Slate Islands data with similar observations at the larger Charlevoix impact structure indicates that the rate of change of recorded shock pressure with distance is greater at the Slate Islands structure. This is interpreted as due to variations in the strain rates and/or the rate of shock wave attenuation with radial distance between impact structures of different size.