Showing papers in "Contributions to Mineralogy and Petrology in 1968"

The oxygen isotope geochemistry of igneous rocks

Abstract: Oxygen isotope analyses have been obtained for 443 igneous rock and mineral samples from various localities throughout the world. Detailed studies were made on the Medicine Lake, Newberry, Lassen, Clear Lake, S. E. Guatemala, Hawaii and Easter I. volcanic complexes and on the Bushveld, Muskox, Kiglapait, Guadalupe, Duluth, Nain, Egersund, Lac St. Jean, Laramie, Skaergaard, Mull, Skye, Ardnamurchan and Alta, Utah plutonic complexes, as well as upon several of the zoned ultramafic intrusions of S. E. Alaska. Basalts, gabbros, syenites and andesites are very uniform in O18/O16, commonly with δ-values of 5.5 to 7.0 per mil. Many rhyolite obsidians, particularly those from oceanic areas and the Pacific Coast of the United States, also lie in this range; this indicates that such obsidians are differentiates of basaltic or andesitic magma at high temperatures (about 1,000° C). They cannot represent melted sialic crust. The only plutonic granites with such low δ-values are some of the hypersolvus variety, suggesting that these also might form by fractional crystallization. Obsidians from the continental interior, east of the quartz-diorite line, have higher δ-values. This is compatible with their having assimilated O18-rich sialic crust. A correlation generally exists between the O18/O16 ratios of SiO2-rich differentiates and the chemical trends in volcanic complexes. High O18/O16 ratios accompany those trends having the lower Fe/Mg ratios, while ferrogabbro trends are associated with depletion in O18. Variations in oxygen fugacity may be responsible for these effects, as abundant early precipitation of magnetite should lead to both O18-enrichment and Fe-depletion in later differentiates. Plutonic granites have higher O18/O16 ratios than their volcanic equivalents, because (a) their differentiation occurred at much lower temperatures, or (b) they are in large part derived from O18-rich sialic crust by partial melting or assimilation. Also, the oxygen isotope fractionations among coexisting minerals are distinctly larger in plutonic rocks than in volcanic rocks. This is in keeping with their lower crystallization temperatures and their longer cooling history, which promotes post-crystallization oxygen isotope exchange. Hydrated obsidians and perlites have δO18-values that are much different from their primary, magmatic values. A correlation exists between D/H and O18/O16 ratios in hydrated volcanic glass from the western U.S.A., proving that the isotopic compositions are a result of exchange with meteoric waters. The O18 contents of the glasses appear to be about 25 per mil higher than their associated waters; hence, these hydrated glasses have not simply absorbed H2O, but they have exchanged with large quantities of it. The igneous rocks from Mull, Skye, Ardnamurchan and the Skaergaard intrusion are all abnormally depleted in O18 relative to “normal” igneous rocks. This is a result of their having exchanged at high temperatures with meteoric water that was apparently abundant in the highly jointed plateau lavas into which these igneous rocks were intruded. In part, this exchange occurred with liquid magma and in part with the crystalline rock; in the latter case the feldspar was more easily exchanged and has become much more depleted in O18 than has coexisting quartz or pyroxene. The later differentiates of the Muskox intrusion are markedly O18-rich, but this is not a result of fractional crystallization. It is in large part a result of deuteric exchange between feldspars and an oxygen-bearing fluid (H2O ?) that was either O18-rich or had a relatively low temperature. This phenomenon was also observed in a number of granophyres from other localities, particularly those containing brick-red alkali feldspar. The exchanged feldspars in all these examples are turbid or cloudy, and may be filled with hematite dust. It is concluded that most such feldspar in nature is the result of deuteric exchange and is probably drastically out of oxygen isotopic equilibrium with its coexisting quartz.

Genesis of the calc-alkaline igneous rock suite

Abstract: A high pressure experimental study of the partial melting fields of synthetic high-alumina olivine tholeiite, high-alumina quartz tholeiite, basaltic andesite, andesite, dacite and rhyodacite under dry and wet $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} < P_{{\text{LOAD }}} } \right)$$ conditions has been conducted in order to investigate possible origins of the calc-alkaline series from the upper mantle Detailed analyses of crystallizing phases using the electron microprobe has enabled calculation of the liquid line of descent in these compositions at various pressures At 27–36 kb garnet and clinopyroxene are the liquidus or near-liquidus phases in dry tholeiite, basaltic andesite and andesite, while quartz is the liquidus phases in dry dacite and rhyodacite Under wet conditions at 27 kb garnet, not quartz, is the liquidus phase in the dacite Qualitatively these results show that the low melting fraction of a quartz eclogite at 27–36 kb under dry conditions is of andesitic composition whereas under wet conditions it is rhyodacitic or granodioritic At these pressures under dry conditions the andesite liquidus lies in a marked low temperature trough between the more basic and more acid compositions Quantitatively, the calculated compositions of liquid fractionates for varying degrees of melting of the quartz eclogite bulk composition broadly follow the calc-alkaline trend At 9–10 kb under wet conditions $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} < P_{{\text{LOAD }}} } \right)$$ sub-silicic amphibole and pyroxenes are the near-liquidus phases in tholeiite and basaltic andesite compositions Calcic plagioclase and garnet occur nearer the solidus The calculated liquid fractionates follow the calc-alkaline trend and demonstrate that the calc-alkaline series may be derived by the partial melting of amphibolite at lower crustal depths under wet conditions $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} < P_{{\text{LOAD }}} } \right)$$ , Or by the fractional crystallization of a hydrous basalt magma at similar depths These experimental results support two complementary hypotheses for the derivation of the calc-alkaline igneous rock suite from the mantle by a two stage igneous process In the first stage of both hypotheses large piles of basalt are extruded on the earth's surface Subsequently this pile of basalt may, under dry conditions, transform to quartz eclogite, sink into the mantle and finally undergo partial melting at 100–150 kms depth This partial melting gives rise to the calc-alkaline magma series leaving a residuum of clinopyroxene and garnet Alternatively, if wet conditions prevail in the basalt pile and the geotherms remain high, partial melting of the basalt may take place near the base of the pile, at about 10 kb pressure $$\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} < P_{{\text{LOAD }}} } \right)$$ The liquids so formed constitute the calc-alkaline suite and the residuum consists of amphibole, pyroxenes and possibly minor garnet and calcic plagioclase Both models may be directly linked to the hypothesis of sea-floor spreading

A contribution to the theory of variation diagrams

Abstract: Harker diagrams and other ternary weight per-cent diagrams distort genetic trends by changing their slopes. They can not be readily used to discriminate between rival hypotheses. Two theorems have been proven which show the desirability of using certain types of ratios, instead of per-cent values, for comparisons. It is suggested that, when it is desired to discriminate between rival hypotheses, or to determine the exact variation between components, weight per-cent comparisons be discontinued (wherever possible) in favour of molar ratio comparisons in which one of the components (the divisor) is a constant. Variation diagrams of this type offer reliable discrimination between rival hypotheses, and yield the true relationship between variables in every case. An example involving crystal fractionation of olivine and pyroxene from a basic sill is developed in detail, and other potential uses are mentioned. This technique is completely general and is not restricted to chemical components, or to any scale of system, or to any field of study, providing certain essential conditions are fulfilled.

Some clinopyroxenes from ultramafic inclusions in Dreiser Weiher, Eifel

Abstract: Eight clinopyroxenes from wehrlites and clinopyroxenites and three clinopyroxenes of crystal lapilli in tuff of Dreiser Weiher in Eifel, Germany, have been separated and chemically analysed. One hornblende and two phlogopites from a wehrlite and clinopyroxenites have also been analysed. The rocks enclosing these inclusions are alkali basalts of basanite composition. The analysed clinopyroxenes contain considerable amounts of Al2O3 (3.87–10.84 wt%). The calculated Tschermak's component ranges from 5.9 to 18.4 mol per cent. All of the analysed clinopyroxenes are clearly different from chromian diopsides in lherzolite inclusions in basaltic rocks in Dreiser Weiher and other localities; the former has higher contents of total FeO, CaO and TiO2 and lower contents of MgO and Cr2O3 than the latter. Two clinopyroxenes separated from apatite-bearing clinopyroxenites show high contents of Fe2O3 with about 2 per cent of Na2O, indicating the presence of considerable amounts of acmite component in addition to Tschermak's component. The relative proportions of Al in the tetrahedral site and that in the octahedral site in the analysed clinopyroxenes are clearly different from those of the common igneous clinopyroxenes and eclogites, and similar to those of the clinopyroxenes from other inclusions in basaltic rocks and granulites. It is suggested that all the analysed clinopyroxenes and their host inclusions have crystallized from alkali basalt magmas in relatively deep levels of the continental crust.

Mineralogy as a function of depth in the prehistoric Makaopuhi tholeiitic lava lake, Hawaii

Abstract: The electron probe X-ray microanalyzer has been used to determine the compositional variability of the groundmass minerals and glass in 10 specimens from a complete 225-foot section of the prehistoric tholeiitic lava lake of Makaopuhi Crater, Hawaii. The order of beginning of crystallization was: (1) chromite, (2) olivine, (3) augite, (4) plagioclase, (5) pigeonite, (6) iron-titanium oxides and orthopyroxene, (7) alkali feldspar and apatite, and (8) glass. Although the lake is chemically tholeiitic throughout, the occurrence of ferromagnesian minerals is as though there were a gradation from alkali olivine basalt in the upper chill downwards to olivine tholeiite. Groundmass olivine decreases downwards and disappears at about 20 feet. Pigeonite is absent in the uppermost 5±2 feet, then increases in amount down to 20 feet, below which augite and pigeonite coexist in constant 2∶1 proportions. Strong zoning and metastable compositions characterize the pyroxenes of the chilled zones, but these features gradually disappear towards the interior of the lake to give way to equilibrium pyroxenes. Relatively homogeneous poikilitic orthopyroxene (≈ Ca4Mg70Fe26) occurs in the olivine cumulate zone, having formed partly at the expense of pre-existing olivine, augite, and pigeonite (≈ Ca8Mg66Fe26). The growth of orthopyroxene is believed to have been facilitated by the slower cooling rate and higher volatile pressure at depth, and by the rise in Mg/Fe ratio of the liquid due to the partial dissolution of settled olivine. Unlike olivine and pyroxene, feldspar is least zoned in the upper and lower chilled regions. The greatest range of compositional zoning in feldspar occurs at 160 to 190 feet, where it extends continuously from Or1.0Ab22An77 to Or64Ab33An3. The feldspar fractionation trend in the An-Ab-Or triangle gradually shifts with depth toward more “equilibrium” trends, even though the zoning becomes more extreme. The variation with depth in the initial (core) composition of the plagioclase suggests the influence of either slow nucleation and growth (undercooling) or slow diffusion in the liquid, relative to the rate of cooling. Idiomorphic opaque inclusions in olivine phenocrysts are chrome-spinels showing continuous variation from 60 percent chromite to 85 percent ulvospinel and to magnetite-rich spinel. A pre-eruption trend of increasing Al with decreasing Cr can be recognized in chromites from the upper chill. Most of the inclusions show a trend of falling Cr and Al, toward an ulvospinelmagnetite solid solution which is progressively poorer in Usp with depth. This trend was produced by solid state alteration of the chromite inclusions during cooling in the lava lake. Ilmenite (average Ilm91Hm9) coexists with variably oxidized titaniferous magnetite in the basalt groundmass. Estimated oxygen fugacities agree well with other independent determinations in tholeiitic basalt. No sulfide phase has been detected. Fractional crystallization produced a groundmass glass of granitic composition. Average, in percent, is: SiO2, 75.5; Al2O3, 12.5; K2O, 5.7; Na2O, 3.1; CaO, 0.3; MgO, 0.05; total FeO, 1.2; and TiO2, 0.8. Normative Or> Ab. Minor changes in glass composition with depth are consistent with a greater approach towards the granite minimum. Incipient devitrification precluded reliable analysis of glass from the lower half of the section. The SiO2-phase associated with devitrification contains alkalis and Al and is believed to be cristobalite. Needle-like apatite crystals in the groundmass glass are Siand Fe-bearing fluorapatites containing appreciable rare earths (predominantly Ce) and variable Cl. The grain-size and maximum An content of the cores of plagioclase grains were controlled by cooling rate and are at a maximum at the center of the section. The most homogeneous pyroxene (and olivine, Moore and Evans, 1967), most equilibrium pyroxene trends, most abundant alkali feldspar, and most equilibrium feldspar trends are found at 160 to 190 feet, which is appreciably below that part of the lake which was slowest to crystallize. Volatile pressure, increasing with depth, possibly controlled the degree of attainment of equilibrium more than cooling rate. Since they are dependent on cooling history, some of the modal criteria commonly used for recognizing basalt types, such as the absence of Ca-poor pyroxene, presence of groundmass olivine, and the presence of alkali feldspar, should be applied with caution. Petrographic comparison of basalts from one flow, volcano, or province, with another, should recognize the possible variations due to cooling history alone.

On recasting analyses of garnet into end-member molecules

Abstract: Most published analyses of garnet deviate from structural ideality. Consequently, compositions expressed as molecular percentages of end-member molecules may differ if different re-calculation sequences have been used. A suitable standard calculation procedure is presented, and is demonstrated to be satisfactory by its application to 69 published analyses of garnets both common and rare.

Melting and phase relations in the plane tholeiite-lherzolite-nepheline basanite to 40 kilobars with geological implications

Abstract: Six crystalline mixtures, picrite, olivine-rich tholeiite, nepheline basanite, alkali picrite, olivine-rich basanite, and olivine-rich alkali basalt were recrystallized at pressures to 40 kb, and the phase equilibria and sequences of phases in natural basaltic and peridotitic rocks were investigated. The picrite was recrystallized along the solidus to the assemblages (1) olivine+orthopyroxene+ clinopyroxene +plagioclase+spinel below 13 kb, (2) olivine+orthopyroxene+clinopyroxene+spinel between 13 kb and 18 kb, (3) olivine+orthopyroxene+clinopyroxene+ garnet+spinel between 18 kb and 26 kb, and (4) olivine+clinopyroxene+garnet above 26 kb. The solidus temperature at 1 atm is slightly below 1,100° and rises to 1,320° at 20 kb and 1,570° at 40 kb. Olivine is the primary phase crystallizing from the melt at all pressures to 40 kb. The olivine-rich tholeiite was recrystallized along the solidus into the assemblages (1) olivine+ clinopyroxene+plagioclase+spinel below 13 kb, (2) clinopyroxene+orthopyroxene+ spinel between 13 kb and 18 kb, (3) clinopyroxene+garnet+spinel above 18 kb. The solidus temperature is slightly below 1,100° at 1 atm, 1,370° at 20 kb, and 1,590° at 40 kb. The primary phase is olivine below 20 kb but is orthopyroxene at 40 kb. In the nepheline basanite, olivine is the primary phase below 14 kb, but clinopyroxene is the first phase to appear above 14 kb. In the alkali-picrite the primary phase is olivine to 40 kb. In the olivine-rich basanite, olivine is the primary phase below 35 kb and garnet is the primary phase above 35 kb. In the olivine-rich alkali basalt the primary phase is olivine below 20 kb and is garnet at 40 kb. Mineral assemblages in a granite-basalt-peridotite join are summarized according to reported experimental data on natural rocks. The solidus of mafic rock is approximately given by T=12.5 P Kb+1,050°. With increasing pressure along the solidus, olivine disappears by reaction with plagioclase at 9 kb in mafic rocks and plagioclase disappears by reaction with olivine at 13 kb in ultramafic rocks. Plagioclase disappears at around 22 kb in mafic rocks, but it persists to higher pressure in acidic rocks. Garnet appears at somewhat above 18 kb in acidic rocks, at 17 kb in mafic rocks, and at 22 kb in ultramafic rocks. The subsolidus equilibrium curves of the reactions are extrapolated according to equilibrium curves of related reactions in simple systems. The pyroxene-hornfels and sanidinite facies is the lowest pressure mineral facies. The pyroxene-granulite facies is an intermediate low pressure mineral facies in which olivine and plagioclase are incompatible and garnet is absent in mafic rocks. The low pressure boundary is at 7.5 kb at 750° C and at 9.5 kb at 1,150° C. The high pressure boundary is 8.0 kb at 750° C and 15.0 kb at 1,150° C. The garnet-granulite facies is an intermediate high pressure facies and is characterized by coexisting garnet and plagioclase in mafic rocks. The upper boundary is at 10.3 kb at 750° C and 18.0 kb at 1,150° C. The eclogite facies is the highest pressure mineral facies, in which jadeite-rich clinopyroxene is stable. Compositions of minerals in natural rocks of the granulite facies and the eclogite facies are considered. Clinopyroxenes in the granulite-facies rocks have smaller jadeite-Tschermak's molecule ratios and higher amounts of Tschermak's molecule than clinopyroxenes in the eclogite-facies rocks. The distribution coefficients of Mg between orthopyroxene and clinopyroxene are normally in the range of 0.5–0.6 in metamorphic rocks in the granulite facies. The distribution coefficients of Mg between garnet and clinopyroxene suggest increasing crystallization temperature of the rocks in the following order: eclogite in glaucophane schist, eclogite and granulite in gneissic terrain, garnet peridotite, and peridotite nodules in kimberlite. Temperatures near the bottom of the crust in orogenic zones characterized by kyanitesillimanite metamorpbism are estimated from the mineral assemblages of metamorphic rocks in Precambrian shields to be about 700° C at 7 kb and 800° C at 9 kb, although heat-flow data suggest that the bottom of Precambrian shield areas is about 400° C and the eclogite facies is stable. The composition of liquid which is in equilibrium with peridotite is estimated to be close to tholeiite basalt at the surface pressure and to be picrite at around 30 kb. The liquid composition becomes poorer in normative olivine with decreasing pressure and temperature. During crystallization at high pressure, olivine and orthopyroxene react with liquid to form clinopyroxene, and a discontinuous reaction series, olivine → orthopyroxene → clinopyroxene is suggested. By fractional crystallization of pyroxenes the liquid will become poorer in SiO2. Therefore, if liquid formed by partial melting of peridotite in the mantle slowly rises maintaining equilibrium with the surrounding peridotite, the liquid will become poorer in MgO by crystallization of olivine, and tholeiite basalt magma will arrive at the surface. On the other hand, if the liquid undergoes fractional crystallization in the mantle, the liquid may change in composition to alkali-basalt magma and alkali-basalt volcanism may be seen at a late stage of volcanic activity.

The variation in garnet, biotite and chlorite composition in medium grade pelitic rocks from the Dalradian, Scotland, with particular reference to the zonation in garnet

Abstract: Garnet, biotite and host rock have been analysed along a traverse from the garnet isograd to the kyanite zone in the Dalradian of Central Perthshire, Scotland. FeO and MgO increase and MnO and CaO decrease in the garnet with increasing grade. Microprobe analyses of the garnets reveal zoning, which indicates that a garnet crystal as a whole does not equilibrate with the matrix during growth. Coexisting biotite varies in composition as a result of the abstraction of MnO, FeO etc. from the rook by the growing garnet, i.e. the mg/mg + fe ratio increases with grade. The microprobe analyses also reveal the size of the system from which garnet abstracted material varied from 0.100 to 2.000 g and the nucleation was frequently “instantaneous”. It also reveals the equilibrium or non-equilibrium nature of the assemblage, and explains the variation in garnet composition with grade in terms of a segregation model with a changing distribution coefficient. Primary chlorite was analysed from rocks near to the garnet isograd containing garnet and biotite. It has a similar mg/mg + fe value to the coexisting biotite. The results show that the three phase field defining the garnet isograd moves towards the mg corner with increasing grade. The higher grade fields lie to the mg rich side of the three phase field so that the sequence of mineral assemblages across the Barrovian zones in Perthshire, from the garnet isograd to the kyanite zone, can be summarized and displayed on a phase diagram.

Chert and its sodium-silicate precursors in sodium-carbonate lakes of East Africa

Abstract: Chert has formed from two sodium-silicate minerals, magadiite (NaSi7,O13(OH)3·3H2O) and kenyaite (NaSi11O20.5(OH)4·3H2O), in uppermost Pleistocene deposits of lakes Magadi and Natron in Kenya and Tanzania. The chert consists of finely crystalline quartz and characteristically forms nodules of irregular shape with white coatings having reticulate surface patterns. Similar nodules are widespread in lower and middle Pleistocene lacustrine deposits in the vicinity of Lake Magadi, Lake Natron, and Olduvai Gorge. Although magadiite and kenyaite are absent in the lower and middle Pleistocene deposits, the chert in these beds probably formed from a sodium-silicate precursor. All of the chert-bearing sediments were deposited in saline, alkaline lakes rich in dissolved sodium carbonate-bicarbonate. Magadiite (and chert) may form either thin, widespread deposits or localized masses which may be cross-cutting. Thin, widespread layers of magadiite have been precipitated by mixing of silica-rich brine with fresh water in a chemically stratified lake; localized masses may have been formed by interaction of brine with fresher water entering the floor or margin of the lake. Magadiite and kenyaite can alter to chert in contact with sodium-carbonate brine and possibly by leaching with relatively fresh water over a period of 20,000 years or less. The siliceous zeolites clinoptilolite and erionite predominate in trachyte tuffs associated with magadiite and chert; less-siliceous phillipsite predominates in trachyte tuffs of chert-free sequences.

Origin of garnet phenocrysts in calc-alkaline rocks

Abstract: A large number of garnet phenocrysts from Palaeozoic rhyodacites and granodiorite porphyrites from Central and Northeastern Victoria have been analyzed using the electron microprobe. These garnets, from an area of several thousand square miles, are very uniform in composition (dominantly almandine, with subordinate pyrope and minor grossular and spessartine). They show minor zoning with a very thin outer rim slightly richer in almandine and spessartine than the remainder of the phenocryst. They are surrounded by a complex intergrowth of cordierite and hypersthene forming a reaction rim. Resorbed quartz phenocrysts are typically associated with the garnet phenocrysts. The uniform composition, the conspicuous size and the subhedral-euhedral form of the garnet phenocrysts indicate that they crystallized directly from the acid calc-alkaline magma at an early stage of its crystallization. High pressure experimental work on a natural garnet-bearing rhyodacite glass demonstrates that almandine-rich garnet and quartz are near-liquidus phases at 18 and 27 kb \(\left( {P_{{\text{H}}_{\text{2}} {\text{O}}} < P_{{\text{LOAD }}} } \right)\), but garnet does not appear until well below the liquidus at 9 kb. A comparison of the composition of the experimentally crystallized garnets with the natural garnets suggests that these acid calc-alkaline magmas began to crystallize at pressures between 9 and 18 kb, i.e. at depths corresponding to the lower crust or upper mantle.

Trace and minor element geochemistry of the rhyolitic volcanic rocks, Central North Island, New Zealand

Abstract: Analytical data are presented for the following elements: Cs, Rb, Ba, K, Sr, Ca, Na, Fe, Mg, Cu, Co, Ni, Li, Sc, V, Cr, Ga, Al, Si, La, Y, and Zr. Eight samples were analysed by the spark source method for rare earths, Tl, Pb, Hf, Sn, Nb, Mo, Bi, and In. In addition to data on rhyolitic volcanics, a small number of intermediate volcanics and eugeosynclinal sediments were analysed for comparative purposes.

Experimental investigation of the reaction forsterite + H2O ⇌ serpentine + brucite

Abstract: A new determination of the equilibrium reaction: $$\begin{gathered} 2{\text{ Mg}}_{\text{2}} [{\text{SiO}}_{\text{4}} ] + 3{\text{ H}}_{\text{2}} {\text{O}} \rightleftharpoons {\text{1 Mg}}_{\text{3}} [({\text{OH)}}_{\text{4}} |{\text{Si}}_{\text{2}} {\text{O}}_{\text{5}} ] + 1{\text{ Mg(OH)}}_{\text{2}} \hfill \\ \hfill \\ {\text{ forsterite serpentine brucite}} \hfill \\ \end{gathered} $$ yielded equilibrium temperatures which lie (at identical H2O-pressures) about 60° C lower than all previously published data (Bowen and Tuttle, 1949; Yoder, 1952; Kitahara et al., 1966; Kitahara and Kennedy, 1967). It has been shown that the above authors have determined not the stable equilibrium curve but instead a metastable “synthesis boundary”. The actual (stable) equilibrium curve is located at 0,5 kb and 350° C 2,0 kb and 380° C 3,5 kb and 400° C 5,0 kb and 420° C 6,5 kb and 430° C.

Quaternary lavas from the southern Cascades, western U.S.A.

Abstract: A series of basaltic and andesitic lavas from three centers in the Cascades (Lassen, Medicine Lake, Mt. Shasta) have been investigated. The lavas are weakly porphyritic, containing phenocrysts of plagioclase, augite, and olivine or orthopyroxene; these phases are also found in the groundmass. Titanomagnetite is a groundmass phase in most lavas but it appears to be absent in some. A sub-calcic augite is found in the groundmass in some of the basic lavas. Orthopyroxenes are present only in the salic lavas and show an increase in calcium with increasing iron. The range in composition shown by both phenocryst and groundmass plagioclase is very similar except that the phenocrysts extend to slightly more calcic compositions. The residual glasses in many of the lavas have a rhyolitic composition. However, only those from the Shasta andesites have normative salic constituents that plot near the ternary minimum in the Ab-Or-Qtz system at 500 bars. Both chemical and mineralogical data allow the lavas of the different centers to be distinguished from one another. The most likely origin for the orogenic lavas of the Cascades is by partial melting of the upper mantle.

Petrology of palagonite tuffs of Koko Craters, Oahu, Hawaii

Abstract: Tuff deposits of the Koko Crater group consist largely of alkali basalt glass, either fresh or palagonitized. Most of the deposits are progressively palagonitized at depth, and topographic relations of palagonite on Koko Crater indicate that the palagonite was formed after the cone had been deeply eroded. The principal authigenic minerals in the palagonite tuffs were deposited in following sequence: phillipsite, chabazite, analcime, montmorillonite together with opal, and calcite. The amount of authigenic minerals in a given sample is generally proportional to the amount of palagonite, indicating that the authigenic minerals are produced in palagonitization of glass. Chemical analyses of sideromelane and associated palagonite by the electron microprobe show that about a quarter of the SiO2, half of the Al2O3 and MgO, and three quarters or more of the CaO, Na2O, and K2O are lost in converting sideromelane to an equal volume of palagonite. A substantial proportion of these components lost from the sideromelane are precipitated nearby in zeolites, montmorillonite, opal, and calcite. Reaction of sideromelane with percolating ground water at low temperatures accounts for the vertical zoning from relatively fresh tuffs down into palagonite tuffs. The pH and ionic strength of percolating water probably increased with depth by solution and hydrolysis of glass, and where the pH and ionic strength became sufficiently high, the glass reacted to form palagonite and zeolites. Palagonite was formed by a microsolution-precipitation mechanism rather than by hydration or devitrification.

Structure and products of subaquatic volcanoes in Iceland

Abstract: Iceland provides a unique opportunity to study the mechanism of subaquatic eruptions. The Neovolcanic zone dissecting the country from NE to SW has been volcanically active since the end of the Tertiary, and a large number of eruptions have occurred under glaciers where melt water provided a subaquatic environment.

Geochemistry of the pantellerites of Mayor Island, New Zealand

Abstract: Recent pantelleritic lavas comprise the whole of the isolated and outlying volcano of Mayor Island. Mineralogically, they are characterised by phenocrystic anorthoclase-sodic27 sanidine, quartz, sodic ferrohedenbergite, and cossyrite. Nine new chemical analyses of the lavas are presented (including one residual glass), confirming their strongly sodic and peralkaline nature. One analysis is also given of trachybasalt, which occurs as common inclusions in the mantling pumice deposits. These inclusions are characterised by abundant feldspar phenocrysts. Detailed trace element data is presented for five of the lava samples, representing the mam volcanic phases and the trachybasalt inclusions. The following conclusions are presented: The data is interpreted to indicate that the pantellerites were derived by crystal differentiation from a postulated mildly alkali olivine basalt parent — feldspar fractionation is considered to have been extremely important in this process. It is shown that the element enrichment occurring in the younger lavas may not be wholely explained by crystallisation differentiation alone — it is possible that some additional process is required. It is also shown that the observed enrichment of sodium in the youngest lavas can only occur during crystal fractionation if quartz, as well as anorthoclase, separate from the magma. This is due to the higher alkali abundances of the anorthoclase phenocrysts, relative to the pantellerite compositions. There is limited evidence that post-eruptive devitrification of some of the lavas has resulted in some modification of the lava chemistry, notably sodium loss.

Plagioclase compositions and non-anatectic origin of migmatitic gneisses in Northern Cascade mountains of Washington State

Abstract: Regionally developed migmatitic gneisses make up most of the Skagit Gneiss, though there are some orthogneisses also, derived from pre- to late-metamorphic intrusives. The migmatites contain countless, though quantitatively subordinate, remnants of biotite schists, less abundant amphibolitic rocks, and minor varieties of metasediments. Biotite schists (fine-grained paragneisses) predominantly are plagioclase-rich, having the compositions of highly immature graywackes; quartz-rich varieties are minor, and alumina-excess rocks very rare. There are para- and ortho-amphibolites, the latter of basaltic-gabbroic parentage. Leucocratic hornblende schists are largely meta-sedimentary. Apart from subordinate varieties in which K-feldspar is a major constituent, the leucocratic migmatitic gneisses have (leuco-) trondhjemitic and, to a lesser extent, quartz-dioritic compositions.

Stabilitätsbeziehungen zwischen Chlorit, Cordierit und Almandin bei der Metamorphose

Abstract: The stability relations between cordierite and almandite in rocks, having a composition of CaO poor argillaceous rocks, were experimentally investigated. The starting material consisted of a mixture of chlorite, muscovite, and quartz. Systems with widely varying Fe2+/Fe2++Mg ratios were investigated by using two different chlorites, thuringite or ripidolite, in the starting mixture.

On phase equilibria in metamorphosed siliceous dolomites

Abstract: Basing on calculations and experiments the equilibrium curves of 15 reactions occuring during metamorphism of siliceous dolomites are presented. In a T—XCO2 diagram these curves intersect at four isobaric invariant points. This is consistent with the fact, that different reaction sequences are known from the nature.

Terrestrial metamorphic clinoenstatite in kinks of bronzite crystals

Abstract: Bronzite in a highly deformed and partially metamorphosed gabbro from Central Australia shows intensive bending and kinking. The optical properties of some kinked and deformed areas differ significantly from those of the original bronzite. Single crystal X-ray and optical investigations show that some highly strained areas of bronzite have inverted to clinoenstatite (2V=50±6°; sign +;ZΛc=32±2°). Mixed phases of both also occur. The new clinoenstatite lattice is untwinned.

Picritic magma — residual dunite relationships in garnet peridotite at Kalskaret near Tafjord, South Norway

Abstract: Field and textural relationships have indicated the tectonic emplacement of the Norwegian garnet peridotites as relatively cold intrusions into their present environment. Mineralogical data demonstrate considerable heterogeneity. Olivines and orthopyroxenes in garnet rich peridotites are significantly more ferriferous than those in garnet free peridotites. Mineralogical features indicate that the mineral assemblages have been equilibrated at subsolidus temperatures. However, the hypothesis that these garnet peridotites have resulted from the eclogite facies metamorphism in deep levels of the crust of other peridotite mineral facies assemblages is considered and rejected.

The geometry of kink bands in crystals — a simple model

Abstract: A crystallographic explanation for the geometry of kink bands in crystals is proposed and three possible types of kinking are discussed, 1. simple kinking, 2. kinking associated with a phase change and 3. kinking associated with twin formation. The angular relationships at kink band boundaries have been calculated for a number of minerals and the results compared with observations in the literature. It is proposed that the crystallography of a mineral may be the dominant control of kink band geometry rather than external stress conditions as is generally assumed, this is probably particularly true of kink bands formed under natural conditons.

Deutung diskordanter Zirkonalter der Silvrettadecke und des Gotthardmassivs (Schweizer Alpen)

Abstract: U-Pb isotopic analyses are reported for zircons of nine igneous rocks and high grade metamorphic paragneisses from the Silvretta nappe and the Gotthard massif, Switzerland.

Variation in ferrous iron-magnesium distribution coefficients of metamorphic pyroxenes from Quairading, Western Australia

Abstract: Distribution coefficients (KD·Fe++-Mg) calculated for orthopyroxene-clinopyroxene pairs from 12 basic granulites of the Quairading district, Western Australia, range from 1.87 for magnesian pyroxenes (Opx Mg value=78.1) to 1.70 for iron-rich varieties (Opx Mg value 37.7). Field and petrographic evidence indicates that these pyroxenes have probably reached equilibrium within a narrow temperature range. In order to account for the observed variation of KD values it is suggested that one (or both) of the pyroxene structures is not the ideal Fe++-Mg "solution" proposed in the thermodynamic model of the pyroxene equilibrium exchange. After consideration of the geometry of the pyroxene cation sites, the relative bond energies of each site (especially crystal field stabilization energy) and structural ordering a model is proposed to explain the non-ideal behaviour of Fe++-Mg in the pyroxene system. The distribution pattern in low-iron pyroxenes will probably show Fe++ favouring the M2·Opx site; competing unfavourably with Mg++ for the M1·Cpxsite; and probably excluded by Mg++ from the M1·Opxsite. As the iron content of the system increases the M2·Opxsite will begin to become saturated with Fe++ and this ion will enter the M1·Opx site. Further increases in the iron content of the system will cause the Fe++-Mg distribution to depend increasingly on the relative attraction of the M1 sites of both pyroxenes. Of these sites Fe++ should show preference to the more distorted M1·Cpxsite. The distribution coefficient reflects this swing towards a relative enrichment of Fe++ in the clinopyroxene by decreasing regularly with increasing iron content. It is likely that this downward trend will not become evident until the iron content of the M2·Opx site reaches saturation. This would explain why the KD values for the magnesian pyroxenes remain practically unchanged until the orthopyroxene Mg value is approximately 60; from here on the iron-rich pyroxenes show a rapid decrease in KD value with increasing iron content. The Ca content of the pyroxenes is also significant since the Quairading pyroxenes show a marked increase in mutual solubility with increasing iron content. Calcium taken into the orthopyroxene structure will enter the M2 site ahead of Fe++ so that this site will reach Fe++ saturation at a lower iron content than when the orthopyroxene is Ca-free. The application of KD values to the regional study of basic granulites, particularly when establishing relative temperature zones on the basis of KD variation, should only be attempted when pyroxenes which extend over a wide range of Fe-Mg content are available.

Strontium isotope studies on young volcanic rocks from Germany and Italy

Abstract: 87Sr/86Sr ratios of Tertiary tholeiitic, basalts alkali olivine basalts and olivine nephelinites from Lower Saxony and Hessia and Quaternary leucite-nepheline tephrites from the Laacher See area are similar to those obtained from Hawaii and range from 0.7031 to 0.7054. Three trachytes and one phonolite from the Westerwald and one phonolite from the Laacher See area have higher values (0.7063 to 0.7093). Three Vesuvian lavas, three Somma lavas and two trachytes of the Phlegraic Fields show substantially higher ratios than the comparable basaltic rocks from N.W. Germany (0.7071 to 0.7102). Three peridotite nodules vary between 0.7048 and 0.7081. Three limestone composites of Paleozoic and Mesozoic age show values between 0.7129 and 0.7174. The 87Sr/86Sr ratios of the trachytes and phonolites from the Westerwald and Laacher See area are probably influenced by crustal material. Assimilation processes of limestones producing the olivine-nephelinites from N.W. Germany seem to be unlikely. A discussion of the origin of the Sr ratios in Vesuvian rocks and of mantle homogeneity is included.

The problem of rock assimilation by Somma-Vesuvius magma

Abstract: Mineralogical and chemical compositions of ejected carbonate inclusions of Vesuvian gaseous phase are compared with those of carbonate sediments of the Somma-Vesuvius area. The basement of Vesuvius mainly consists of Mesozoic limestones. Most of these are characterized by extremely low insoluble residues. In this area thick dolomite beds occur in the Triassic system only. A calcareous layer of approximately 100 to 150 meters thickness characterized by high Sr contents (0.19% Sr in the average) is to be found within the Triassic dolostones. Several carbonate ejecta also show high Sr contents (0.1% Sr) but chemical composition of some of these ejecta differs somewhat from that of the Triassic layer high in Sr. Contact with the volcanic volatile phase and melt has produced some alterations in the composition of many ejecta. Magnesian calcites are abundant and periclase, brucite, tremolite, phlogopite and magnesite were found in the ejecta examined. Silicon, manganese, iron, zinc and, to a smaller extent, potassium and copper have been most probably transported from the volcanic gas phase into ejected carbonate inclusions. Owing to metasomatic actions of the volcanic volatile components, rearrangement and alteration of Ca and Mg contents occurred in the carbonate minerals of several ejecta. Under the influence of volcanic pressure and temperature, magnesium content originally in dolomite might be transformed into the calcite structure. \(\frac{{{\text{MgO}}}}{{{\text{CaO + MgO}}}}\) molar ratios of several carbonate ejecta do not correspond with those found in sedimentary limestones and dolostones. It cannot be proved whether the Mg of the mixed calcite-dolomite ejecta has been partially introduced from the volatiles or lost from the carbonate phases.

A study of garnets from eclogite and peridotite xenoliths found in kimberlite

Abstract: The chemical compositions of garnets from 58 eclogite, 72 peridotite and 4 pyroxenite xenoliths in kimberlites have been estimated from their unit cell edge length and refractive indices. The samples studied were obtained from 17 kimberlite occurrences and include all those of known source which remain in the famous Williams (1932) collection which is stored at the University of Cape Town. Every suitable sample available to the authors has been examined. A gap in the range of garnet volume percentages occurs in the samples studied between approximately 15 and 30%. Garnet peridotites characteristically have 30% garnet. Very rare exceptions occur. Our collection contains no eclogites with olivine and only one with orthopyroxene. All but two of the peridotite-pyroxenite group contain orthopyroxene. The garnets from the peridotites and pyroxenites plot on a pyrope-almandine-uvarovite triangle in a narrow band with a remarkably constant almandine/uvarovite ratio. Garnets from the eclogites are plotted on a pyrope-almandine-grossularite triangle and have a wide spread of compositions. These fall into 4 groups viz. eclogite I, eclogite II, kyanite eclogite and corundum eclogite. The reasons for the differences in garnet chemistry are considered and a tentative evolutionary scheme suggested by partial melting of the garnet peridotite which is assumed to occur in the upper mantle. Recent models of upper mantle composition and the genesis of garnet-bearing xenoliths in kimberlite are briefly and critically examined.

Origin and differentiation of Triassic dolerite magmas, North Carolina, USA

Abstract: Ten dolerite dikes intruded into Triassic fault troughs in the Piedmont area of North Carolina have been analyzed for the contents of major elements plus selected trace elements. The average composition of the initial magma, as indicated by four chill margins for major elements and three for trace elements, is: SiO2, 48.6%; Al2O3, 16.9%; TiO2, 0.57%; Fe2O3, 3.30%; FeO, 6.72%; MgO, 10.59; CaO, 10.42%; Na2O, 2.03%; K2O, 0.20%; MnO, 0.20%; Rb, 2.6 ppm; Sr, 133 ppm; Zr, 46 ppm; Th, 0.4 ppm; and U, below detection limit of approximately 0.1 ppm. One large dike (BP) exhibits a Palisades-type of differentiation by crystal settling of olivine, and the comparatively thick JY dike shows development of micropegmatite toward the center; the smaller dikes, however, are relatively homogeneous across their width. Study of the relationship between SiO2 content and the ratio FeO+Fe2O3/MgO+ FeO+Fe2O3 indicates that most dikes crystallize under conditions of decreasing oxygen pressure, but the differentiation trend of the JY, RD, and RS dikes indicates either constant or increasing oxygen pressure during their evolution.

Distribution of Mg, Fe, Al, Ca and Na in coexisting olivine, orthopyroxene and clinopyroxene in the Totalp serpentinite (Davos, Switzerland) and in the Alpine metamorphosed Malenco serpentinite (N. Italy)

Abstract: The primary minerals of a partly serpentinized Alpine type ultrabasic mass, lying in a zone of low Alpine metamorphism, were analysed with an electron microprobe. The distribution coefficients of Mg and Fe in coexisting orthopyroxenes, clinopyroxenes and olivines indicates an equilibrium temperature of around 1400°C. The Ca/Ca+Mg ratios in the clinopyroxenes indicate temperatures between 800° and 1000°C. This discrepancy in temperature estimations can partly be explained by the high Al2O3-contents of the clinopyroxenes, since a good correlation between Al2O3-contents and Ca/Ca+Mg ratio was found. The Na2O-content of the clinopyroxenes decreases in the successive pyroxenites that differentiated from the main lherzolite. From the mineralogical composition of the different rock types and the chemical compositions of the minerals it is concluded that the Totalp peridotite originated in the Upper Mantle.

Trace element analyses of magnetites from andesitic and dacitic lavas from Bay of Plenty, New Zealand

Abstract: Five magnetites from andesitic and dacitic lavas from the volcanoes Edgecumbe, Whale Island, and White Island were separated, and analysed for Mg, Ti, Cu, V, Ni, Co, Mn, and Cr. The vanadium contents provide difficulties for Osborn's (1962) hypothesis for the origin of calc-alkaline volcanic rocks.