Showing papers on "Phenocryst published in 1984"

Partition coefficients of Hf, Zr, and ree between phenocrysts and groundmasses

Abstract: Partition coefficients of Hf, Zr, and REE between olivine, orthopyroxene, clinopyroxene, plagioclase, garnet, amphibole, ilmenite, phlogopite, and liquid are presented. Samples consist of megacrysts in kimberlite, phenocrysts in alkaline basalts, tholeiitic basalts and andesitic to dacitic rocks, and synthetic garnet and clinopyroxene in Hawaiian tholeiites. The Hf-Lu and Zr-Lu elemental fractionations are as large as the Lu-Sm or Lu-Nd fractionation. The Hf and Zr partition coefficients between mafic phenocrysts and liquids are smaller than the Lu partition coefficients, but are similar to the Nd or Sm partition coefficients. The Hf and Zr partition coefficients between ilmenite, phlogopite, and liquid are larger than the Lu partition coefficients for these minerals and their corresponding liquids. The Hf-Zr elemental fractionation does not occur except for extreme fractionation involving Zr-minerals and extremely low fO2. These data have an important bearing on chronological and petrogenetic tracer studies involving the Lu-Hf isotopic system.

Catastrophic isotopic modification of rhyolitic magma at times of caldera subsidence, Yellowstone Plateau Volcanic Field

Abstract: The Yellowstone Plateau volcanic field has undergone repeated eruption of rhyolitic magma strongly depleted in 18O. Large calderas subsided 2.0, 1.3, and 0.6 Ma ago, on eruption of ash flow sheets that represent at least 2500, 280, and 1000 km3 of zoned magma. More than 60 other rhyolite lavas and tuffs permit reconstruction of the long-term chemical and isotopic evolution of the silicic system. Narrow δ18O ranges in the ash flow sheets contrast with wide δ18O variations in postcaldera lavas of the first and third caldera cycles. Earliest postcollapse lavas are 3 to 6‰ lighter than the preceding ash flow sheets. The O18 depletions were short-lived events that immediately followed caldera subsidence; hundreds of cubic kilometers of magma were drastically 18O depleted and thousands were depleted by 1–2‰. Sequences of postcaldera lavas record partial recovery toward precaldera δ18O values; secular trends between collapse events thus reflect gradual reenrichment of the roofmost magma in δ18O. Much of the subcaldera reservoir was affected, because lavas that erupted as far apart as 115 km reflect the same pattern of depletion and partial recovery. Contemporaneous extracaldera rhyolites have the highest δ18O values in the volcanic field and show no effects of the repeated depletions. Sr and Pb isotope ratios of intracaldera rhyolites jump to more radiogenic values at times of caldera formation and show a longterm zigzag pattern like that of δ18O. Although some contamination by foundering roof rocks seenis to be required, water was probably the predominant contaminant. Even if roof rocks had been strongly depleted in O18 before engulfment, their assimilation would have been far from sufficient to account for the large O18 shift. The low- O18 lavas contain no xenocrysts and show no trace element or phenocryst evidence of massive contamination. Their Fe-Ti-oxide temperatures indicate no cooling relative to the caldera-forming ash flow magma, and their whole-rock, glass, and phenoeryst chemistry suggests compositional continuity with the ash flow sequence. Oxygen exchange between the magma and a mass of low-O18 water greatly exceeding solubility limits may require (1) recurrent explosive activity to sustain access and mixing of water with the magma and (2) convection of the magma reservoir to prevent local saturation.

Caldera Volcanoes of the Taupo Volcanic Zone, New Zealand

Abstract: The Taupo volcanic zone (TVZ) has been active since 2 Ma and has erupted >104 km3 of dominantly rhyolitic magma during the last 1 m.y. Most of the volcanism is concentrated in a 125×60 km area forming the central TVZ and is expressed largely as six major caldera volcanoes, Rotorua, Okataina, Kapenga, Mangakino, Maroa, and Taupo, marked by localized collapse of the underlying basement and clustering of known or inferred vent sites. These centers have activity spans from 150 to 600 ka and have each erupted at least 300 to 1000 km3 of magma. All centers except Rotorua are known or inferred to have had complex histories of multiple caldera collapse, which have occurred alongside general basement collapse within the TVZ accompanying regional extension. Deep-seated NE trending basement lineations and/or faults have influenced vent sites at Okataina, Maroa, and Taupo. Welded ignimbrites are prominent in the pre-140 ka record; their absence since then is attributed to the effects of surface water on eruption styles rather than to a change in eruptive behavior. Volcanism from the centers has been overwhelmingly rhyolitic (>97% SiO2 69–77 wt%) with minor high-A1 basalt and dacite and traces of andesite, mostly as lithic fragments in ignimbrites from Okataina and Mangakino. Although insignificant in volume, the basalt is important as a low-Si end-member in mixing relationships with the rhyolite (at one extreme generating the dacites) and occasionally as a trigger for the rhyolitic eruptions. The current average rhyolite magma eruption rate from the central TVZ is ∼0.27 m3 s−1, equally divided between Okataina and Taupo, a figure close to the long-term average for the last 1.1 Ma. However, geothermal heat flow data imply that a further 1.4–1.8 m3 s−1 of magma may be intruded within the crust. The ratio of inferred intruded material to erupted material is higher at centers where lava extrusions are volumetrically significant (Okataina, Maroa), and this is correlated with lower phenocryst equilibration temperatures in the eruptives. Evidence for resurgent doming and long-term (>105 years) magma cycles documented at similarly sized rhyolitic calderas in the western United States is absent from the TVZ centers; this is attributed to the young faulted crust of the region, preventing the formation of sufficiently large high-level magma chambers. In overall terms, the central TVZ is comparable in size and longevity to the Yellowstone system, but its individual eruptions have very much shorter recurrence intervals and smaller volumes.

The volcanic‐tectonic cycle of the FAMOUS and AMAR Valleys, Mid‐Atlantic Ridge (36°47′N): Evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections, AMAR 3

Abstract: The 1978 AMAR expedition extended the investigation of the Mid-Atlantic Ridge that was begun by the 1973–1974 FAMOUS expedition from the northern end of the FAMOUS rift (the original FAMOUS area) to the narrow central and southern end of the FAMOUS rift (Narrowgate region) and into the broad AMAR valley south of Fracture Zone B. Available field and geochemical data allow us to characterize in detail the volcanic-tectonic cycle for these two segments of the mid-ocean ridge system. A dynamic model of the crust-forming processes within these two rift valley segments is deduced from (1) variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins, (2) composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution, and (3) detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor. Each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition. The decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits. Intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes, the common evidence of crystal resorption, and the apparent disequilibrium between megacrysts, phenocrysts, and their enclosing glass. This indicates control by a crustal magma chamber. Liquid and crystal compositions and textural relations can best be explained by a three-stage crystallization history: (1) crystallization of olivine, plagioclase, and clinopyroxene onto the floor of a magma chamber, (2) partial resorption of mineral phases by a superheated, undersaturated liquid above the floor, (3) intratelluric crystallization of olivine and plagioclase (± clinopyroxene) during rise of melt to the surface through the conduit system above the magma chamber. A magma chamber model is suggested for the AMAR-FAMOUS rift valleys that satisfies both seismic and geological constraints. We propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment (AMAR and Narrowgate regions) but that these thin and terminate near the intersecting fracture zones. Expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization. The FAMOUS-Narrowgate rift is currently in a contraction period so that in the northern FAMOUS region, primitive liquid has erupted at Mount Venus and Mount Pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the Narrowgate region.

Magmatic inclusions in rhyolites, contaminated basalts, and compositional zonation beneath the Coso volcanic field, California

Abstract: Basaltic lava flows and high-silica rhyolite domes form the Pleistocene part of the Coso volcanic field in southeastern California. The distribution of vents maps the areal zonation inferred for the upper parts of the Coso magmatic system. Subalkalic basalts (<50% SiO2) were erupted well away from the rhyolite field at any given time. Compositional variation among these basalts can be ascribed to crystal fractionation. Erupted volumes of these basalts decrease with increasing differentiation. Mafic lavas containing up to 58% SiO2, erupted adjacent to the rhyolite field, formed by mixing of basaltic and silicic magma. Basaltic magma interacted with crustal rocks to form other SiO2-rich mafic lavas erupted near the Sierra Nevada fault zone. Several rhyolite domes in the Coso volcanic field contain sparse andesitic inclusions (55–61% SiO2). Pillow-like forms, intricate commingling and local diffusive mixing of andesite and rhyolite at contacts, concentric vesicle distribution, and crystal morphologies indicative of undercooling show that inclusions were incorporated in their rhyolitic hosts as blobs of magma. Inclusions were probably dispersed throughout small volumes of rhyolitic magma by convective (mechanical) mixing. Inclusion magma was formed by mixing (hybridization) at the interface between basaltic and rhyolitic magmas that coexisted in vertically zoned igneous systems. Relict phenocrysts and the bulk compositions of inclusions suggest that silicic endmembers were less differentiated than erupted high-silica rhyolite. Changes in inferred endmembers of magma mixtures with time suggest that the steepness of chemical gradients near the silicic/mafic interface in the zoned reservoir may have decreased as the system matured, although a high-silica rhyolitic cap persisted. The Coso example is an extreme case of large thermal and compositional contrast between inclusion and host magmas; lesser differences between intermediate composition magmas and inclusions lead to undercooling phenomena that suggest smaller ΔT. Vertical compositional zonation in magma chambers has been documented through study of products of voluminous pyroclastic eruptions. Magmatic inclusions in volcanic rocks provide evidence for compositional zonation and mixing processes in igneous systems when only lava is erupted.

Origin and evolution of a peraluminous silicic ignimbrite suite: The Violet Town Volcanics

Abstract: The Violet Town Volcanics are a 373 Ma old, comagmatic, S-type volcanic sequence mainly comprising crystal-rich intracaldera ignimbrites. Rock types vary from rhyolites to rhyodacites, all containing magmatic cordierite and garnet phenocrysts. Variation in the suite is primarily due to fractionation of early-crystallized quartz, plagioclase and biotite (plus minor accessory phases) in a high-level magma chamber prior to eruption. Early magmatic crystallization occurred at around 4 kb and 850° C with melt water contents between 2.8 and 4 wt.%. This high-temperature, markedly water-undersaturated, restite-poor, granitic magma was generated by partial melting reactions involving biotite breakdown in a dominantly quartzofeldspathic source terrain, leaving a granulite facies residue.

Zoning in highly alkaline magma bodies

Abstract: We present chemical data on magmatically heterogeneous pyroclastic deposits of late Quaternary age erupted from zoned magma systems underlying Tenerife (Canary Islands), Sao Miguel and Faial (Azores), and Vesuvius. The most fractionated magmas present at each centre are respectively Na-rich phonolite, trachyte, and K-rich phonolite. Within any one deposit, chemical variation is either accompanied by changes in the phenocryst assemblage (petrographic zonation) or is largely manifested in trace element abundances, unaccompanied by any petrographic change (occult zonation). Zoning is analogous to that in calc-alkaline systems where the most fractionated products are high-silica rhyolites. When a range of magma types are considered, a correlation emerges between roofward depletion of trace elements (especially REE) in the zoned system and compatability of those same trace elements in the accessory phenocryst phases present. Thus, allanite- or chevkinite-bearing rhyolitic systems are light-REE depleted roofwards, the sphene-bearing Tenerife system is middle-REE depleted roofwards, the melanite-bearing Vesuvius system is heavy-REE depleted roofwards, while the Azores systems, which lack these phases, display roofward REE enrichment. Therefore, the behaviour of trace elements may in each case be explained by fractionation of observed phenocryst assemblages. The resemblance between features of zoned magma systems and published work on the dynamic consequences of cooling saturated aqueous solutions prompts us to suggest that sidewall crystallization and consequent boundary-layer uprise to form a capping layer at top of the system may be a plausible mechanism for the generation of both petrographic and occult zonation. Reverse zoning occurs among the first-erupted tephra of some deposits, demonstrating that the most highly differentiated magma available is not always the first to be tapped during an eruption from a zoned system.

Petrology of basalts from the Mohns-Knipovich Ridge; the Norwegian-Greenland Sea

Abstract: Major element compositions of submarine basalts, quenched glasses, and contained phenocrysts are reported for samples from 25 dredge stations along the Mohns-Knipovich Ridge between the Jan Mayen fracture zone and 77°30′N. Most of the basalts collected on the Jan Mayen platform have a subaerial appearance, are nepheline normative, rich in incompatible elements, and have REE-patterns strongly enriched in light-REE. The other basalts (with one exception) are tholeiitic pillow basalts, many of which have fresh quenched glass rims. From the Jan Mayen platform northeastwards the phenocryst assemblage changes from olivine±plagioclase±clinopyroxene±magnetite to olivine +plagioclase±chrome-spinel. This change is accompanied by a progressive decrease in the content of incompatible elements, light-REE enrichments and elevation of the ridge that are similar to those observed south of the Azores and Iceland hotspots. Pillow basalts and glasses collected along the esternmost part of the Mohns Ridge (450 to 675 km east of Jan Mayen) have low K2O, TiO2, and P2O5 contents, light-REE depleted patterns relative to chondrites, and Mg/(Mg+Fe2+) ratios between 0.64 and 0.60. Pillow basalts and glasses from the Knipovich Ridge have similar (Mg/Mg+Fe2+) ratios, but along the entire ridge have slightly higher concentrations of incompatible elements and chondritic to slightly light-REE enriched patterns. The incompatible element enrichment increases slightly northward. Plagioclase phenocrysts show normal and reverse zoning on all parts of the ridge whereas olivines are unzoned or show only weak normal zoning. Olivine-liquid equilibrium temperatures are calculated to be in the range of 1,060–1,206° C with a mean around 1,180° C. Rocks and glasses collected on the Jan Mayen Platform are compositionally similar to Jan Mayen volcanic products, suggesting that off-ridge alkali volcanism on the Jan Mayen Platform is more widespread than so far suspected. There is also evidence to suggest that the alkali basalts from the Jan Mayen Platform are derived from deeper levels and by smaller degrees of partial melting of a mantle significantly more enriched in light-REE and other incompatible elements than are the tholeiitic basalts from the Eastern Mohns and Knipovich Ridge. The possibility of the presence of another hitherto unsuspected enriched mantle region north of 77° 30′ N is also briefly considered. It remains uncertain whether geochemical gradients revealed in this study reflect: (1) the dynamics of mixing during mantle advection and magma emplacement into the crust along the Mid-Atlantic Ridge (MAR) spreading axis, (e.g. such as in the mantle plume — large-ion-lithophile element depleted asthenosphere mixing model previously proposed); or (2) a horizontal gradation of the mantle beneath the MAR axis similar to that observed in the overlying crust; or (3) a vertical gradation of the mantle in incompatible elements with their contents increasing with depth and derivations of melts from progressively greater depth towards the Jan Mayen Platform.

The petrology and mineralogy of Mt. Popa Volcano and the nature of the late-Cenozoic Burma Volcanic Arc

Abstract: Mt. Popa is situated on a line of widely-spaced, calc-alkaline volcanic centres in the central lowlands of Burma. Latites, rhyodacites, and ignimbrites are interbedded with strongly-folded arenaceous sediments of topmost Miocene and Pliocene age. These are overlain unconformably by a composite cone of basalt and basaltic andesite lavas and pyroclastic deposits of Pleistocene or early Recent age. The two groups of lavas are regarded as a single petrographic suite in which several continuous and discontinuous mineral reaction series are recognized. Phenocryst phases, analysed by microprobe, are: olivine, bronzite-hypersthene, biotite, diopsidic augite-salite, magnesio-hastingsite-magnesio-hornblende, titaniferous magnetite, bytownite-oligoclase, and sanidine. Whole-rock analyses are characterized by relatively high K, Ba, Sr, Rb, and Zr. High Mg/(Mg + Fe") ratios of 0.85 to 0.65 in both phenocrysts and whole-rock, high K/Rb (767) and low Sr 87 /Sr 86 (0.70431) and Rb/Sr (0.0155) ratios suggest that the basalts are relatively primitive mantle-derived compositions, modified slightly by fractionation, with negligible crustal contamination. The more siliceous lavas may have been derived by fractional crystallization of such magmas at crustal levels. This high-K calc-alkaline suite is typical of a continental margin orogenic environment. Analyses of contemporaneous lavas from elsewhere in Burma define three N-S volcanic lines which become more alkaline from W to E. The magmatism is attributed to the waning stages of orogeny as eastward subduction of the Indian plate beneath China gave way to strike-slip movement, associated with the Andaman Sea spreading-centre. Within this tectonic model, magmas were probably generated by hydrous mantle melting above dehydrating oceanic crust of the subduction zone.

A Review of the Kimberlitic Rocks of Western Australia

Abstract: As the result of a planned exploration programme for diamonds spanning 10 years, CRA Exploration Pty. Limited and the Ashton Joint Venture have discovered four diamond bearing kimberlitic provinces in Western Australia. Three of these provinces are marginal to the Kimberley craton in the north of the State (Fig. 2) and one lies in the Carnarvon Basin, adjacent to the Yilgarn Block, 1300 km to the southwest. The distribution of the kimberlitic rocks suggests that emplacement was controlled by major fractures associated both with early Proterozoic mobile zones and with later Phanerozoic rifting and continental break-up. Radiometric dating suggests intrusion of kimberlitic rocks took place 160 m.y. ago in the Carnarvon Basin and 25 m.y. ago in the West Kimberley, but other occurrences in the Kimberley region could be older. The bodies range in size from dyke-like features less than one metre wide to volcanic crater deposits with surface areas up to 128 hectares. These craters are champagne-glass shaped in cross-section, the narrow stem corresponding to the pipe feeder. The craters are infilled with volcanoclastic and epiclastic sediments. A late stage phase in many of the West Kimberley bodies was the emplacement of massive, igneous-textured, magmatic lamproite, rising to the surface and intruding into the crater. The exploration discoveries were facilitated by early recognition that in petrography, mineralogy and chemistry the kimberlitic rocks in Western Australia vary from classical kimberlites resembling those of Kimberley, South Africa, to the soda-rich rocks of Wandagee in the Carnarvon Basin, to the olivine lamproites (terminology of Jaques et al, this volume) of the Kimberley region of Western Australia which are composed essentially of phenocrysts of olivine ± clinopyroxene ± phlogopite ± potassic richterite ± glassy groundmass. Mantle nodules recovered range from dunite to lherzolite; graphic-textured intergrowths of picroilmenite and silica (after diopside?) occur at the Skerring pipe. Heavy mineral concentrates from the kimberlitic rocks yield pyrope, picroilmenite, chrome-diopside, chromite and zircon, the former two minerals being more abundant in the classical types of kimberlite. Recognition of the diamond-bearing potential of the olivine lamproites strongly influenced exploration techniques with use being made of such minerals as chromite, andradite and zircon as indicator minerals during heavy mineral gravel sampling. In regions where host rocks display low magnetic responses it was found that the kimberlitic bodies produced recognisable magnetic anomalies from detailed aeromagnetic surveys and this technique has been an important aid to exploration. To date only rare diamond has been found in the classical kimberlites of Western Australia, and in the more sodic varieties at Wandagee. Western Australia's rich diamond deposits have been found in the highly potassic olivine lamproites and many of the associated olivine-poor leucite lamproites contain rare diamond. Feasibility studies of the Argyle AK1 lamproite are based on a 2.9 million tonnes per annum operation to come into production in 1985, producing some 25 million carats per year, while limited commercial production from associated alluvial deposits commenced in January 1983. Lamproites thus constitute a new terrestrial primary source for diamond, hitherto thought solely restricted to kimberlite.

Bishop tuff revisited: new rare Earth element data consistent with crystal fractionation.

Abstract: The Bishop Tuff of eastern California is the type example of a high-silica rhyolite that, according to Hildreth, supposedly evolved by liquid-state differentiation. New analyses establish that the Bishop Tuff "earlyllate" rare earth element trend reported by Hildreth mimics the relations between groundmass glasses and whole rocks for allanite-bearing pumice. Differences in elemental concentrations between whole rock and groundmass are the result of phenocryst precipitation; thus the data of Hildreth are precisely those expected to result from crystal fractionation.

Rare-element–enriched, S-type ash-flow tuffs containing phenocrysts of muscovite, andalusite, and sillimanite, southeastern Peru

Abstract: Ash-flow tuffs of Neogene age exposed over 2,500 km 2 in the Macusani region of southeastern Peru are the volcanic equivalent of S-type granites. The strongly peraluminous tuffs contain phenocrysts of andalusite, sillimanite, and muscovite and have high 87 Sr/ 86 Sr i (0.7258 and 0.7226) and δ 18 O (+11‰). Elevated concentrations of Li, Cs, Be, Sn, B, and other minor elements compare with those in “tin granites.” Mineral phase relations and composition are indicative of low magmatic temperatures and oxygen fugacities and high a HF/ a H 2 O. The chemical, isotopic, and mineralogical features and regional geologic relations are consistent with models of magma generation involving the incorporation of large amounts of pelitic rock.

Petrology and Chemistry of Volcanic Rocks of the Southern Andes

Abstract: Pliocene to Quaternary volcanic rocks of andesitic to dacitic composition predominate in the northern (33–37°S; Province I), and southern (48–56°S; Province III) sectors of the SVZ, whereas rocks of basaltic composition predominate in the central sector (37–46°S; Province II). Many stratovolcanoes in the latter sector have precaldera lavas of basaltic composition and postcaldera lavas of dacitic composition (e.g. Quetrupillan, Lanin and Puyehue volcanoes). Hornblende is a common phenocryst phase in lavas of Provinces I and III (e.g. Marmolejo-San Jose volcanic group and Mt. Burney volcano), but are uncommon in Recent lavas from Province II, particularly between 37–42°S (e.g. Antuco, Callaqui, Lonquimay, Villarrica, Puyehue, Osorno, Puntiagudo and Calbuco volcanoes). Nevados de Chillan (36°50′S) located near the boundary of Provinces I and II is transitional, with lavas of andesitic composition lacking hornblende. Lavas from Province II generally have relatively flat chondrite-normalized REE patterns with low La/Sm ratios and HREE contents greater than 8 × chondrites; lavas from Province I have variable La/Yb ratios with HREE ranging from about 15 × chondrites to less than 5 × chondrites. Most samples from stratovolcanoes such as Puyehue and Nevados de Chillan, have very subparallel REE patterns; other volcanic suites have REE patterns of varying slope with poor correlations of La/Sm, La/Yb and total REE with major elements, especially SiO2 content. Miocene volcanism in the SVZ is represented by subvolcanic rocks of basaltic to dacitic composition distributed along the Longitudinal Depression (33–42°S) and High Andes Cordillera (33–38°S). Manifestations of Miocene igneous activity associated with porphyry copper deposits are also observed between 28–33°S where Quaternary volcanism is absent.

The Etnean Lavas, 1977-1983: Petrology and Mineralogy

Abstract: The products emitted by Mount Etna in the 1977–1983 period are porphyric, sodic trachybasalts («etnaites») that, though substantially higher in K2O, resemble most of the historic lavas. The 1983 flows, for instance, are composed of 50–55% glassy groundmass and abundant phenocrysts or microphenocrysts: plagioclase (25–30%), clinopyroxene (12–15%), olivine (3–5%), titanomagnetite (2%). Scarce olivine Fo 83-78 and diopsidic pyroxene are present in the cores of some phenocrysts. Outer zones and/or more numerous crystals are olivine Fo 75-68, salite En 43-37 Fs 10-15, plagioclase An 83-55 an Al-Mg rich magnetite Usp 43-33. The chemical composition of glass inclusions interpreted as trapped liquids suggests that olivine and magnetite crystallize first and are followed by pyroxene and plagioclase. The early appearance of olivine has been determined by optical thermometry to about 1170°C, soon followed by plagioclase and pyroxene (1160–1140°C). Pre-eruptive crystallization further progresses until 1073°C which is the field-measured temperature at the lava vent. Then, the appearance and composition of microlites (and outer rims of phenocrysts) depend upon the cooling rate of the samples (water quenching or natural cooling). Electron microprobe analyses of chlorine and sulphur have been performed on crystal trapped and groundmass glasses. Sulphur ranges from an initial content of 1500 ppm to 400 ppm in residual glass, leading to an average sulphur emission of 1500 tons/day (3000 t/d SO2), as estimated from the volume of erupted lava. The most striking chemical feature of the 1977–1983 lavas lies in the abnormal potassium behaviour, especially in the earliest flows of the 1978–1979 fissure eruptions. Although these lavas are amongst the most femic of the period, they are relatively high in K2 O, leading to an aberrant K/Na ratio (i.e. increasing with increasing basicity). Furthermore, a less pronounced but steady increase of K2 O/Na2 O is evidenced in all the lavas of the past 12 years. This ratio ranges from 0.42 in the 1971 lavas to 0.51 in the 1983 ones, for about the same differentiation index. Such an abnormal potassium behaviour first appeared during the 1974 eruption on the west slope of the volcano. It is unknown, however, in any other historic lava, nor in any lava of entire Etna so far analyzed.

Crustal contamination in northern Andean volcanics

Abstract: Late Cenozoic andesitic rocks of the Colombian Andes are characterized by small but systematic and well-correlated variations in 87 Sr/ 86 Sr, 143 Nd/ 144 Nd, and δ 18 O that reflect significant crustal contamination. The range in isotopic ratios measured in northern Andean andesites is most pronounced for oxygen, relatively much more subtle for Sr and Nd. Pb isotopic data, so diagnostic of crustal contamination in central Andean lavas, exhibit minimal variation in the northern Andean volcanics. Trace element trends and phenocryst populations suggest dominantly mafic mineral fractionation, although plagioclase fractionation may be significant in later-stage differentiation. An assimilation-fractional crystallization model is constructed to match measured isotopic data. The results of that modelling indicate that the andesitic magmas of Galeras and Ruiz volcanoes may have assimilated up to 10–20% of crustal material. Correlated variations in measured isotopic ratios can be modelled satisfactorily by assuming a crustal contaminant of composition δ 18 O = 10 per mil, 87 Sr/ 86 Sr = 0.710–0.712 (Sr ≈ 90ppm), and 143 Nd/ 144 Nd=0.5120–0.5122 (Nd ≈ 20–30 ppm). We estimate that the composition of the primary magma (prior to crustal contamination) was in the range δ 18 O = 6.5 per mil, 87 Sr/ 86 Sr = 0.704, and 143 Nd/ 144 Nd = 0.51285, values that may suggest an additional crustal component was present in the primary magma. If we assume the added crustal component was derived from partial melting of subducted continental sedimentary material and/or oceanic crust (source contamination), then model calculations suggest that crustal material comprises less than 10 to 15 wt% of the primary magma. This model-dependent effect of source contamination is to shift isotopic ratios to the right on the Sr–Nd plot (see also Hawkesworth et al. 1977), whereas crustal contamination of the magma during ascent through the Andean crust appears to have produced isotopic trajectories that cut downward and through the mantle array.

Petrogenesis of the Kirkpatrick Basalt, Solo Nunatak, northern Victoria Land, Antarctica, based on isotopic compositions of strontium, oxygen and sulfur

Abstract: Chemical and isotopic compositions of Jurassic tholeiites of the Kirkpatrick Basalt Group from Solo Nunatak, northern Victoria Land, indicate that these rocks are contaminated with crustal material. The basalts are fine grained and contain phenocrysts of augite, pigeonite, hypersthene and plagioclase. The flows on Solo Nunatak are chemically more similar to average tholeiite than flows from Mt. Falla and Storm Peak in the Central Transantarctic Mountains (TAM) which appear to be more highly differentiated. Initial 87Sr/86Sr ratios of the flows on Solo Nunatak are high (>0.710) and are similar to those reported for the Kirkpatrick Basalt in the Central TAM. Whole-rock δ18O values are also high, ranging from +6.0 to +9.3‰ and correlate positively with initial 87Sr/86Sr ratios, similar to the Kirkpatrick Basalt in the Central TAM. The correlation between initial 87Sr/86Sr ratios and δ18O values is explained as the result of simultaneous fractional crystallization and assimilation of a crustal contaminant. Sulfur isotope compositions vary between limits of δ34S= -4.01 to +3.41‰ Variations in (δ34S probably resulted from outgassing of SO2 under varying oxygen fugacities.

Ti-phlogopites of the Shaw's Cove minette: a comparison with micas of other lamprophyres, potassic rocks, kimberlites, and mantle xenoliths

Abstract: The two generations of micas in the Shaw's Cove minette (mica-potash feldspar lamprophyre) of northern New Brunswick, Canada, are TiO2-rich phlogopites with variable but low silica and potash contents, low soda, and efectively all their aluminum in tetrahedral sites. A majority of the micas have an excess of Y cations and a deficiency of X cations; there are positive correlations between excess Y ions and number of Al ions in Z sites, between K2O and SiO2 contents, and between K2O and volatile-free totals; yet the micas optically appear fresh, do not respond to hydration or glycolation, and X-ray diffraction patterns show none of the peaks of talc, montmorillonites, vermiculite, or 7- or l4-A chlorites. This suggests that some normally octahedrally coordinated cations and water or hydronium ions are located in the interlayer X sites. Phenocryst micas are sharply zoned. Cores are paler colored and have high mg [atomic ratio Mg/(Mg + Fe{2)] (0.810.88), Cr2O3 (i : 0.88 wt.Vo), and NiO (i : 0.08 wt.%), but relatively low TiO2 $ = 2.93 wt.%), and negligible BaO. Phenocryst rims and groundmass micas are darker reddish brown and have lower mg (0.68-0.79), nil CrzOr and NiO, but high TiO2 (X = 5.40 wt.%) and BaO (X = 0.38 wt.%). Micas in minettes from other localities (62 analyses) are frequently richer in silica and potash than the Shaw's Cove samples but otherwise are generally chemically comparable. Chemical zoning or bimodalism is nearly universal in minette micas, though the boundary between high-zg-low-TiOz (phenocryst) and low-mghigh-TiOz (groundmass) groups varies from province to province and even from minette to minette within a swarm. Comparison of analyses of minette micas with 353 analyses of micas from other mafic (largely potassic) rocks indicates that micas chemically indistinguishable from those of minettes can be found within some other lamprophyres, (ultra)potassic rocks, kimberlites, and high-pressure xenoliths in alkaline volcanic rocks and kimberlites, though each ofthese groups also contains micas unlike those in minettes. Those kimberlite phlogopites that match minette micas are predominantly secondary rims of phenocrysts (or megacrysts) or groundmass crystals (mainly "Type II"), but some are unzoned pre-fluidization phenocrysts. While primary, primary-metasomatic, and uaruo-suite phlogopites of mantle xenoliths are unlike minette micas (higher mg and SiOz, lower TiO2 and AlzOr), some of the secondary-metasomatic phlogopites in sheared garnetiferous mantle xenoliths are wholly minettelike. The chemical identity of minette-mica phenocryst cores and some phenocrysts of diverse mantle-derived rocks implies crystallization under similar conditions. By analogy with the chemistry of phlogopites produced experimentally in potassic systems at high pressures and studies of equilibration conditions of natural phlogopites, it is concluded that phenocrystic phlogopites in minettes may form at temperatures and pressures up to at least -1250"C and -40 kbar and probably under /o. conditions between the NNO and HM buffers: higher /o, than for primary, primary-metasomatic, and uanto-suite micas in kimberlites and their included xenoliths.

The Oxide Minerals of the Wesselton Mine Kimberlite, Kimberley, South Africa

Abstract: The compositions of the constituent spinels, ilmenites, rutiles and perovskites have been determined in six kimberlite intrusions at Wesselton. Groundmass spinels show a normal magmatic trend of Cr-rich, Ti-poor chromite cores and Ti-rich, Cr-poor titanomagnetite rims. Spinels from peridotite xenoliths are compositionally distinct from groundmass spinels. Ilmenites occur as (a) macrocrysts (b) in metasomatised peridotites (c) as inclusions in olivine phenocrysts and (d) as groundmass grains in the kimberlite. Despite some overlap these groups can be distinguished compositionally. The origin and crystallisation sequence of oxide minerals at Wesselton is : PREINTRUSION : Chromites (xenocrysts), rutile macrocrysts (xenocrysts), ilmenite macrocrysts (phenocrysts or xenocrysts ?), groundmass chromite, rutile and ilmenite inclusions in olivine phenocrysts, groundmass ilmenite (microphenocrysts). POSTINTRUSION : groundmass titanomagnetites, perovskites and magnetites (microphenocrysts).

Anion and cation partitioning between olivine, plagioclase phenocrysts and the host magma: A new application of ion microprobe study

Abstract: Partition coefficients for -1, -2, -3, +1, +2, +3, +4 and +5 valent ions between the groundmass of tholeiite basalt and coexisting olivine and plagioclase phenocrysts from the Mid-Atlantic Ridge have been determined by secondary ion mass spectrometry. The present cation partitioning strongly supports the “crystal structure control” mechanism. The partition coefficient for an anion is also under control of the crystal structure, so that each of the cation and anion positions in the crystal structure gives rise to a parabola-shaped peak on the partition coefficient vs. ionic radius diagram.

Magma mixing in alkaline series: an example from Sancy volcano (Mont-Dore, Massif Central, France)

Abstract: Three magmatic units (Grande Cascade pyroclastic deposits, Grande Cascade lava flow, Durbise nuee ardente deposits) from the Quaternary volcano Sancy (Mont-Dore area, Auvergne, France) show textural evidences of magma mixing between a silica undersaturated basic magma (alkali basalt and hawaiite) and an acid magma (quartz-bearing trachyte). Three kinds of mixed rock types are described: basic inclusions within an acid host, « emulsified rocks » showing infracentimetric basic globules disseminated within an acid groundmass, and « banded rocks » in which elongated acid and basic zones alternate. The chemical compositions of mixed rocks plot systematically onto linear trends in petrographic diagrams. Microprobe analyses of the groundmass show similar linear variations between basic and acid end-members. The mineralogical associations of these mixed rocks are highly complex and present many disequilibrium features. Olivine is stable in the basic component and becomes rimmed by orthopyroxene in the acid one. Zoning patterns of feldspars are complex. Clinopyroxene, kaersutite and phlogopite phenocrysts have increasing component Mg contents from core to rim both in the basic and the acid. Titanomagnetite and hemoilmenite phenocrysts were equilibrated at 900-800° C under high oxygen fugacities. Mixing results primarily from a mechanical disintegration of partly liquid basic inclusions within their acid host, and also from a mechanical transfer of phenocrysts from one component to the other, in which they often remain surrounded by a coating of their original groundmass. Chemical data on the groundmass indicates that some « true » hybridization between coexisting acid and basic liquids may also have occurred. The extent of mixing is controlled by the type of emplacement. For pyroclastic deposits a chemical gap exists between basic inclusions and their acid hosts; in contrast, mechanical mixing was enhanced during the emplacement of the viscous Grande Cascade lava flow, and complete transitions occur between basic and acid components. The two end-members are genetically associated, the latter deriving from the former by crystal fractionation. Mixing appears as a late-stage phenomenon in the petrogenetic history of the Mont-Dore series; in the case of the Grande Cascade lava flow, its extent is primarily dependent on emplacement modalities.