Showing papers on "Phenocryst published in 2003"

High 3He/4He ratios in picritic basalts from Baffin Island and the role of a mixed reservoir in mantle plumes.

Abstract: The high 3He/4He ratio of volcanic rocks thought to be derived from mantle plumes is taken as evidence for the existence of a mantle reservoir that has remained largely undegassed since the Earth's accretion. The helium isotope composition of this reservoir places constraints on the origin of volatiles within the Earth and on the evolution and structure of the Earth's mantle. Here we show that olivine phenocrysts in picritic basalts presumably derived from the proto-Iceland plume at Baffin Island, Canada, have the highest magmatic 3He/4He ratios yet recorded. A strong correlation between 3He/4He and 87Sr/86Sr, 143Nd/144Nd and trace element ratios demonstrate that the 3He-rich end-member is present in basalts that are derived from large-volume melts of depleted upper-mantle rocks. This reservoir is consistent with the recharging of depleted upper-mantle rocks by small volumes of primordial volatile-rich lower-mantle material at a thermal boundary layer between convectively isolated reservoirs. The highest 3He/4He basalts from Hawaii and Iceland plot on the observed mixing trend. This indicates that a 3He-recharged depleted mantle (HRDM) reservoir may be the principal source of high 3He/4He in mantle plumes, and may explain why the helium concentration of the 'plume' component in ocean island basalts is lower than that predicted for a two-layer, steady-state model of mantle structure.

Magmatic Conditions and Magma Ascent as Indicated by Hornblende Phase Equilibria and Reactions in the 1995–2002 Soufrière Hills Magma

Abstract: A study of amphiboles and associated minerals in samples of Soufri ere Hills andesite erupted from 1995 to 2002 shows significant compositional variations within hornblende phenocrysts, a separate set of small pargasitic crystals in the groundmass, and two types of reaction rims on the phenocrysts. The composition of the amphiboles and coexisting phases defines the thermal history of the erupting magma. As many as seven zones (5200 mm wide) in the hornblende phenocrysts begin with a sharp increase in Mg and Si, and then change gradually to a more Feand Al-rich hornblende, a transition that is consistent with a temperature rise. Analyses of the hornblende phenocrysts and associated Fe±Ti oxides verify previous conclusions that the pre-eruption magma was at 130MPa and 830 10 C, but was variably heated before eruption. The heating occurred within 30 days of eruption for all magmas erupted, based on the width of Ti-rich rims on titanomagnetite phenocrysts. Experimental phase equilibria for the andesite confirm that the natural hornblende phenocrysts would be stable between 825 and 855 C at a PH2O of 130MPa, and would be even more Al rich if crystallized at higher pressure. Pargasite is not stable in the andesite, and its presence, along with high-An plagioclase microphenocrysts, requires mafic magma mingling and hybridization with pre-existing andesite. Experimental melts of the andesite at 130MPa and 830 and 860 C compare well with melt inclusions in quartz and plagioclase, respectively. Reaction rims on a few hornblende crystals in each andesite sample are rich in high-Ca pyroxene and are produced experimentally by heating the andesite above the stability limit for hornblende. Decompression-induced breakdown rims occur in some samples, and the rate of this reaction has been experimentally calibrated for isothermal andesite magma ascent at 830±860 C. The average ascent rate of magma during much of the 1995±2002 eruption has been40 02 m/s, the rate that allows hornblende to erupt free of decompression-induced reaction rims.

Experimental Constraints on the Origin of the 1991 Pinatubo Dacite

Abstract: Crystallisation and mantle-interaction experiments have been performed on the 1991 Pinatubo dacite to constrain its petrogenesis. In the dacite- H2O system at 1000 MPa, magnetite and either clinopyroxene or amphibole are the liquidus phases. No garnet is observed at this pressure. Dacite-mantle interaction at 920 MPa produces massive orthopyroxene crystallisation, in addition to amphibole ± phologopite. Amphibole crystallising in dacite at 1000 MPa faithfully reproduces aluminium-rich hornblende preserved in the cores of amphibole phenocrysts of the 1991 dacite, suggesting a high pressure stage of dacite crystallisation under high melt H2O contents (> 10 wt%) and low temperatures (< 950C). Altogether, plagioclase, amphibole and melt inclusion compositions suggest that the Pinatubo dacite was water-rich, oxidized and not much hotter than 900C, when emplaced in the shallow reservoir, in which most phenocrysts precipitated until the onset of the 1991 eruptions. The strongly fractionated REE pattern demands garnet somewhere during dacite genesis, which in turn requires high pressure derivation. Partial melting of subducted oceanic crust gives melts unlike the Pinatubo dacite. Interaction of these slab melts with sub-arc peridotite is unable to produce such type of dacite, nor is a direct mantle origin conceivable on the basis of mantle-dacite interaction experimental results. Dehydration melting of underplated basalts requires temperatures too high to match with petrological constraints and does not allow production of dacite melts having the low FeO/MgO, high H2O, Ni and Cr contents typical of Pinatubo-like magmas. On the face of chemical and phase equilibrium constraints, the most plausible origin of the Pinatubo dacite is via high pressure fractionation in the upper mantle of an hydrous, oxidised, primitive basalt that crystallises amphibole and garnet upon cooling, as shown by recent phase equilibrium work. Dacite melts so produced are directly expelled from the uppermost mantle or lower crust to shallow level storage reservoirs from which they erupt occasionally, following injection of H2O-poor mantle-derived basalts that can ascent without massive crystallisation unlike their hydrous counterparts, parental to dacite magmas, that rest at mantle depths. Dacites such as the Pinatubo one may thus witness particular H2O-rich conditions of the sub-arc mantle rather than the melting of a young and hot subducting oceanic plate.

Quantifying the Building Blocks of Igneous Rocks: Are Clustered Crystal Frameworks the Foundation?

Abstract: Most phenocryst populations in volcanic rocks, and those preserved in shallow-level igneous intrusions, are clustered (variously referred to as clots, clumps or glomerocrysts). These clusters of crystals are the building blocks that accumulate to form the high-porosity, touching crystal frameworks from which igneous cumulates form. Examination of touching crystal frameworks in olivine- (komatiite cumulates and experimental charges) and plagioclase-dominant crystal populations (Holyoke flood basalt, Connecticut, USA) reveal complex, high-porosity, clustered crystal arrangements. Olivine touching frameworks in komatiite flows are interpreted to form in hundreds of days. Plagioclase frameworks are calculated to have formed in less than 17 years for a crystal growth rate of 1 x 10-10 mm/s to less than 3 years for a growth rate of 5 x 10-10 mm/s based on crystal size distributions. The origin of crystal clusters is likely to involve either (or a combination of) heterogeneous nucleation, remobilization of cumulate mushes or crystals sticking together during settling and/or flow. The spatial distribution pattern of clustered crystal frameworks from both natural and experimental examples constrains fields on spatial packing diagrams that allow the identification of touching and non-touching crystal populations, and further improve our understanding of crystal packing arrangements and cluster size distributions.

Magma Emplacement and Remobilization Timescales Beneath Montserrat: Insights from Sr and Ba Zonation in Plagioclase Phenocrysts

Abstract: RECEIVED NOVEMBER 22, 2001; ACCEPTED DECEMBER 12, 2002Since 1995, the Soufriere Hills volcano on Montserrat, LesserAntilles island arc, has erupted crystal-rich andesite magmathat is geochemically very similar to the volcanic products of atleast the last 174 3kyr (1s). Nomarski images of plagio-clase phenocrysts from six andesites erupted between 151 4ka(1s)and

The 1999 and 2000 eruptions of Mount Cameroon: eruption behaviour and petrochemistry of lava

Abstract: Mount Cameroon (4,095 m high and with a volume of ∼1,200 km3) is one of the most active volcanoes in Africa, having erupted seven times in the last 100 years. This stratovolcano of basanite and hawaiite lavas has an elliptical shape, with over a hundred cones around its flanks and summit region aligned parallel to its NE-SW-trending long axis. The 1999 (28 March–22 April) eruption was restricted to two sites: ∼2,650 m (site 1) and ∼1,500 m (site 2). Similarly, in the eruption in 2000 (28 May–19 June), activity occurred at two sites: ∼4,095 m (site 1) and ∼3,300 m (site 2). During both eruptions, the higher vents were more explosive, with strombolian activity, while the lower vents were more effusive. Accordingly, most of the lava (∼8×107 m3 in 1999 and ∼6×106 m3 in 2000) was emitted from the lower sites. The 1999–2000 lavas are predominantly basanites with low Ni (5–79 ppm), Cr (40–161 ppm) and mg numbers (34–40). Olivine (Fo77–85, phenocrysts and Fo68–72, microlites), clinopyroxene (Wo47En41Fs10 to Wo5lEn34FS15), plagioclase (An49–67) and titanomagnetite are the principal phenocryst and groundmass phases. The lavas contain xenocrysts of olivine and clinopyroxene, which are interpreted as fragments of intrusive rocks disrupted by magma ascent. Major and trace element characteristics point to early fractionation of olivine. The clinopyroxenes (Al2O3 1.36–7.83 wt%) have high Aliv/Alvi ratios (1.3–1.8) and are rich in TiO2, characteristics typical of low pressure clinopyroxenes. Geochemical differences between the 1999–2000 lavas and those from previous eruptions, such as higher Nb/Zr of the former, suggest that different eruptions discharged magmas that evolved differently in space and time. Geophysical and petrological data indicate that these fractionated magmas originated just below the geophysical Moho (at 20–22 km) in the lithospheric mantle. During ascent, the magmas disrupted intrusions and earlier magma pockets. The main ascent path is below the summit, where newly arrived magma degasses. Degassed magma simultaneously intrudes the flank rift zones where most lava is extruded.

Geology of the Bajo de la Alumbrera Porphyry Copper-Gold Deposit, Argentina

Abstract: The Bajo de la Alumbrera porphyry Cu-Au deposit, Argentina, is in the eastern Andes, near the north edge of a region of reverse fault-bound basement uplifts that overlie a low-angle segment of the subduction zone. Alumbrera, now above the transition from steep to flat subduction, formed at ~7 Ma in the Farallon Negro volcanic field, which was active as volcanism was waning regionally above the flattening subduction zone. Reconstruction of volcanic structure suggests that the top of the exposed orebody was emplaced beneath about 2.5 km of andesite and dacite but not directly beneath the vent of a stratovolcano. Production plus remaining resources are 605 million metric (Mt) tons of ore that averages 0.54 percent Cu and 0.64 g/t Au. The deposit is centered on a closely spaced cluster of small felsic porphyry stocks and dikes, emplaced into andesites during seven phases of intrusion. Dikes of several phases define a radial pattern. Most of the porphyries are very similar to one another, with phenocrysts of plagioclase, hornblende, biotite, and quartz, in a matrix of fine-grained quartz, K-feldspar, and minor plagioclase, biotite, and magnetite. Individual porphyries are distinguished mainly on the basis of intrusive contact relationships. Highest Cu-Au grades are associated with abundant quartz veins, secondary K-feldspar, ±magnetite, ±biotite, ±anhydrite, in the earliest porphyry (P2), and adjacent andesite. P2-related mineralization is truncated by porphyries of the second phase of ore-related intrusions (Early P3 and Quartz-eye porphyry), which contain similar but generally less intense mineralization and alteration. Porphyries of the next phase (Late P3) truncate mineralization associated with earlier phases and are weakly mineralized with Cu-Au, sparse quartz veins, and secondary biotite. The still later Northwest porphyries truncate most Cu-Au, quartz veins, and potassic alteration, and themselves contain only traces of such mineralization and partially biotitized hornblende. Postmineral porphyries, the youngest, truncate all such mineralization and alteration, and none of their hornblende is biotitized. Los Amarillos porphyry and igneous breccia, along the western periphery of the porphyry cluster, is between P2 and Early P3 in age but shows little relationship to mineralization. Zones of secondary K-feldspar associated with the earlier porphyries are surrounded by a larger zone of secondary biotite. All significant Cu-Au lies within these potassic zones. The biotite zone is surrounded by epidote-chlorite alteration lacking significant sulfides. Like potassic alteration, epidote-chlorite alteration is also truncated by Postmineral porphyries. Strong feldspar destructive alteration, consisting mostly of veinlet-controlled sericite-quartz-pyrite, is younger than all secondary K-feldspar, biotite, and epidote-chlorite and occurs in a shell in the outer parts of the biotite zone. Weaker feldspar destructive alteration occurs inside and outside this shell. Pyrite veins with sericite-quartz-pyrite alteration cut Postmineral porphyries. In the earliest secondary K-feldspar assemblage, which is usually barren of Cu sulfides, biotite is altered to magnetite plus K-feldspar. Most Cu sulfides are associated with slightly later K-feldspar-biotite ± magnetite assemblages. Where feldspars and biotite are not overprinted by later feldspar destructive or chloritic alteration, Cu minerals are bornite and chalcopyrite, coexisting with magnetite. Barren as well as Cu sulfide-bearing assemblages are associated with early veinlets, including A-type quartz, which are truncated by the next later porphyry. Deposition of Cu-Au during or between emplacement of closely related porphyries suggests high temperatures and magmatic fluids, and the assemblage bornite-chalcopyrite-magnetite indicates a relatively low sulfidation state, and along with the assemblage K-feldspar-biotite ± magnetite ± anhydrite a relatively high oxidation state. Cu-Au distribution is not related to feldspar destructive zones nor to the interface between sericitic and potassic zones. Much Cu-Au mineralization, however, has been overprinted by late alteration, resulting in partial destruction of feldspars, chloritization of mafics, and sulfidation of bornite-chalcopyrite-magnetite to chalcopyrite-pyrite ± relict magnetite. This probably took place at significantly lower temperature. A low-grade core zone consists in large part of barren K-feldspar-magnetite alteration and quartz veins in Early P3 porphyry, and in part consists of later barren porphyry, so is mostly younger than the Cu-Au deposited with P2 porphyry. The youngest features at Alumbrera include small postore normal faults, gypsum veins due to hydration and mobilization of anhydrite, local dissolution of gypsum veins, and locally developed thin zones of near-surface oxidation, leaching, and secondary enrichment.

Melt mixing and crystallization under Theistareykir, northeast Iceland

Abstract: Analysis of the compositions of crystals and melt inclusions from a suite of 40 gabbroic and wehrlitic nodules in a single eruptive body provides a record of concurrent mixing and crystallization of melts under NE Iceland. The crystals in the nodules have a similar range of compositions to those found as phenocrysts in the flow, and many of the nodules may have been generated by crystallization of a magma with a similar composition to that of the host flow. While plagioclase is only present in nodules where the average forsterite content of olivines is 0.8 GPa and is in agreement with estimates of crystallization pressures for the host basalt. The relationship between the compositional variability of melt inclusions and the forsterite content of the host olivine is revealed by REE analyses of over 120 melt inclusions. The degree of variability in REE concentrations and REE/Yb ratios decreases with falling forsterite content of the host olivine, as expected if melt mixing and fractional crystallization are operating together. The standard deviation of the REEs falls by a factor of ~4 between Fo90 and Fo87. This change in olivine composition can be produced by crystallization of 20% which occurs on cooling of ~50°C. The relative rates of mixing, cooling and crystallization may provide constraints upon the dynamics of magma bodies. The oxygen isotopic composition of olivines from the nodules and phenocrysts is highly variable (δ18O from 3.3–5.2 per mil) and shows little correlation with the forsterite content of the olivine. The full range of oxygen isotope variation is present in olivines with Fo89–90, and the low δ18O signal is associated with melts of high Mg# and La/Yb. Such geochemical relationships cannot be produced by assimilation of low Mg# crustal materials alone, and may reflect oxygen isotopic variation within the mantle source. The geochemistry of the melt inclusions and their host crystals can be accounted for by fractional melting of a mantle source with variable composition, followed by concurrent mixing and crystallization beneath the Moho.

Crustal origin for coupled 'ultra-depleted' and 'plagioclase' signatures in MORB olivine-hosted melt inclusions: evidence from the Siqueiros Transform Fault, East Pacific Rise

Abstract: Geochemical data from melt inclusions in olivine phenocrysts in a picritic basalt from the Siqueiros Transform Fault on the northern East Pacific Rise provide insights into the petrogenesis of mid-ocean ridge basalts (MORB). The fresh lava contains approximately 10% of olivine phenocrysts (Fo89.3 - 91.2) and rare, small (<1 mm) plagioclase phenocrysts with subhedral to irregular shapes with a range of compositions (An80-90, An57-63). Melt inclusions in olivine phenocrysts are glassy, generally rounded in shape and vary in size from a few to approximately 200 lm. Although most of the inclusions have compositions that are generally consistent with being representative of parental melts for the pillow-rim glasses, several inclusions are clearly different. One inclusion, which contains a euhedral grain of high-Al, low-Ti spinel, has a composition unlike any melt inclusions previously described from primitive phenocrysts in MORB. It has a very high Al2O3 (approximately 20 wt%), very low TiO2 (approximately 0.04 wt%) and Na2O (approximately 1 wt%) contents, and a very high CaO/Na2O value (approximately 14). The glass inclusion is strongly depleted in all incompatible elements (La =0.052 ppm; Yb =0.34; La/Sm(n) approximately 0.27), but it has large positive Sr and Eu anomalies (Sr/Sr* approximately 30; Eu/Eu* approximately 3) and a negative Zr anomaly. It also has low S (0.015 wt%) and relatively high Cl (180 ppm). We suggest that this unusual composition is a consequence of olivine trapping plagioclase in a hot, strongly plagioclase-undersaturated magma and subsequent reaction between plagioclase and the host olivine producing melt and residual spinel. Two other melt inclusions in a different olivine phenocryst have compositions that are generally intermediate between 'normal' inclusions and the aluminous inclusion, but have even higher CaO and Sr contents. They are also depleted in incompatible elements, but to a lesser degree than the aluminous inclusion, and have smaller Sr and Eu anomalies. Similar inclusions have also been described in high-Fo olivine phenocrysts from Iceland and northern Mid-Atlantic Ridge. We suggest that the compositions of these inclusions represent assimilation of gabbroic material into the hot primitive magma. The localised nature of this assimilation is consistent with it occurring within a crystal mush zone where the porosity is high as primitive magmas pass through earlier formed gabbroic 'cumulates'. In such an environment the contaminants are expected to have quite diverse compositions. Although the interaction of primitive melts with gabbroic material may not affect the compositions of erupted MORB melts on a large scale, this process may be important in some MORB suites and should be accounted for in petrogenetic models. Another important implication is that the observed variability in melt inclusion compositions in primitive MORB phenocrysts need not always to reflect processes occurring in the mantle. In particular, inferences on fractional melting processes based on geochemistry of ultra-depleted melt inclusions may not always be valid.

The Petrology and Geochemistry of High-Magnesium Andesites at the Western Tip of the Setouchi Volcanic Belt, SW Japan

Abstract: K±Ar ages, and petrographical and geochemical characteristics of high-magnesium andesites and plagioclase-phyric andesites from the NE Kyushu region, Japan, are presented. K±Ar ages range from 10 7 0 3 to 14 4 0 4Ma, overlapping those reported for lavas of the Setouchi Volcanic Belt in other regions (11±16Ma). This, together with major and incompatible trace element, and Sr±Nd±Pb isotopic characteristics, confirms that the Setouchi Volcanic Belt, which is characterized by the occurrence of high-magnesium andesites, extends for 600 km along the SW Japan arc. Thus, it may be suggested that the unusual tectonic setting required for high-magnesium andesite magma generation was attained only for this limited region, possibly in association with subduction of the young, hence hot, lithosphere of the Shikoku Basin. Two types of high-magnesium andesite, possessing identical bulk-rock compositions, are recognized: one contains olivine phenocrysts with chromian spinel inclusions showing compositional characteristics consistent with their crystallization as liquidus phases, whereas the other contains Niand Fe-rich olivine and Fe3‡-rich spinel. One of the possible causes for such unusual mineral compositions could be effective elemental diffusion within and through olivine crystals associated with a long residence time in a crustal magma chamber and the slow rate of cooling of the host magma. The compositions of liquidus chromite and olivine, as well as whole-rock major element compositions, suggest that the NE Kyushu highmagnesium andesite magmas were produced by higher degrees of partial melting than those in other regions of the Setouchi Volcanic Belt.

Formation of tonalite from basaltic magma at the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge in the Philippine Sea, and growth of Izu-Ogasawara (Bonin)-Mariana arc crust

Abstract: Tonalitic rocks dredged from the Komahashi-Daini Seamount, northern Kyushu-Palau Ridge are classified as biotite–hornblende tonalites and hornblende tonalites. These rocks have radiometric ages of 37–38 Ma, indicating that felsic plutonic activity occurred during the early stages of Izu-Ogasawara (Bonin)-Mariana (IBM) arc volcanism. Therefore, this tonalite complex has great importance for understanding the initial processes of island arc and continental crust formation. These tonalitic rocks exhibit the following petrological and geochemical characteristics: (1) common lamellar twins and oscillatory zoning patterns in plagioclase phenocrysts throughout the compositional range; (2) hornblende tonalite shows parallel REE patterns and increasing total REE content with increasing SiO2, except for an increasingly strong negative Eu anomaly at higher SiO2 levels; and (3) isotopic composition remains constant over a wide silica variation. We compare this tonalite with younger tonalities of the same arc from the Tanzawa Complex (10–5 Ma), central Japan, considered to represent the lower–middle crust of the IBM arc, and find the following differences: (1) cumulate textures found in Tanzawa tonalites are not observed in samples from the Komahashi-Daini Seamount; and (2) Komahashi-Daini Seamount tonalites, unlike those from Tanzawa, exhibit linear variations of Zr and REEs vs. SiO2 plots. These data and other observations support the interpretation that tonalite in the Komahashi-Daini Seamount was produced by crystal fractionation from basaltic magma. We suggest that fractional crystallization operated during the early stage of oceanic island arc formation to produce tonalite, whereas tonalities in later stages formed largely by partial melting of basaltic lower crust, as represented by the tonalites in the Tanzawa Complex.

Stratigraphy, distribution and geochemistry of widespread felsic volcanic units in the Mesoproterozoic Gawler Range Volcanics, South Australia*

Abstract: Three widespread felsic volcanic units, the Eucarro Rhyolite, Pondanna Dacite Member and Moonaree Dacite Member, have been distinguished in the Mesoproterozoic Gawler Range Volcanics. These three units are the largest in the Gawler Range Volcanics, each in excess of 500 km3. Each unit is ~ 300 m thick and includes a black, formerly glassy base, a granophyric columnar-jointed interior, and an amygdaloidal outer margin. The units are very gently dipping and locally separated by thin (<20 m) lenses of either ignimbrite (Mt Double Ignimbrite), tuffaceous sandstone or faults. The youngest unit, the Moonaree Dacite Member, covers a central area with a diameter greater than 80 km. The southern two units have east-west extents in the order of 180 km, but are much less extensive from south to north (5-60 km). All three units are dominated by euhedral phenocrysts and are relatively crystal rich. Both the Eucarro Rhyolite and Moonaree Dacite Member contain clasts of basement granitoid and other lithologies and are locally heterogeneous in texture and composition. Some granitoid clasts have disintegrated, liberating feldspar and quartz crystals into the surrounding host. These liberated crystals cause textural variations, but can be identified on the basis of shape (amoeboid or skeletal) and/or size (megacrysts). Textural and lithofacies characteristics are consistent with the interpretation that these units are lavas; the strongly elongate distribution and wide extent of the Eucarro Rhyolite and Pondanna Dacite Member could indicate that vents were aligned along an extensive east-west-trending fissure system. Stratigraphic nomenclature has been revised to better reflect the presence of the three emplacement units. The oldest unit, the Eucarro Rhyolite, is dominated by plagioclase-phyric rhyolite that locally includes granitoid clasts and megacrysts. Along the northern margin, the rhyolite is amygdaloidal and has mingled with a quartz-rich rhyolite (Paney Rhyolite Member). The Eucarro Rhyolite and Paney Rhyolite Member replace the formerly defined Eucarro Dacite, Nonning Rhyodacite, Yannabie Rhyodacite and Paney Rhyolite. The two younger units, Pondanna Dacite Member and Moonaree Dacite Member, are compositionally and spatially distinct, newly defined members of the Yardea Dacite.

Chemistry of grain boundaries in mantle rocks

Abstract: The compositions of olivine grain boundaries have been analyzed with scanning transmission electron microscopy (STEM) via energy dispersive X-ray (EDX) spectrum profiling in three specimens: a peridotite ultramylonite, olivine phenocrysts in a basaltic rock, and synthesized compacts of olivine + diopside. Composition profiles across grain boundaries in both natural and synthetic samples exhibit a characteristic width of 5 nm and a deplel ion of Mg and concomitant enrichments of Ca, Al, Ti, and Cr. Chemical segregation is known to affect grain boundary processes such as grain boundary diffusion sliding, fracture, and migration, all of which influence the rheological properties of polycrystalline aggregates. Also, because grain boundaries are enriched in trace elements, the boundaries can be important storage sites for such elements in mantle rocks. Mantle-derived melts with unusual compositions, such as those rich in Ca and/or Ti, might be explained by preferential melting of olivine grain boundaries enriched in these elements. The common chemical signatures at grain boundaries in all samples indicate that chemical segregation is an energetically favorable phenomenon and thus should occur elsewhere in Earth's mantle. Segregation of trace elements to grain boundaries may play an important role in dynamical and geochemical processes in Earth's mantle.

Crystal sizes in evolving silicic magma chambers

Abstract: Crystal size distributions (CSDs) of quartz and zircon phenocrysts in individual pumice clasts from several voluminous ash-flow tuffs provide a quenched snapshot view of conditions in preclimactic magma chambers. A common feature of these CSDs is a concave-down, lognormal shape, in contrast to the reported linear CSDs in more mafic systems. This feature is interpreted to be a general result of surface-controlled, size-dependent growth by a layer nucleation in silicic magmas at low supersaturation. Specific CSDs may be important for interpreting nucleation and crystal-growth conditions and mechanisms in magmas erupted as large ash-flow tuffs and smaller-volume volcanic units, and for fingerprinting different magma batches (layers) in products of the same eruption.

Time vs. composition trends of magmatism at Sunlight volcano, Absaroka volcanic province, Wyoming

Abstract: Sunlight volcano is a dissected, but well-preserved eruptive center in the eastern, high-K belt of the Absaroka volcanic province. On the basis of high-precision 40Ar/39Ar ages, magmatism at the volcano commenced prior to 49.6 Ma and continued until ca. 48.1 Ma. Stratigraphic relationships along with geochemical and 40Ar/39Ar data distinguish three principal stages of effusive volcanism that are interpreted to reflect the inception, climax, and decay of a crustal magmatic system related to three major pulses of basalt and high eruptive rates. During the first two stages, shoshonitic and latitic lavas with small but distinct compositional ranges accumulated. The third stage included eruption of picritic basaltic lavas and intrusion and solidification of magmas in the volcanic edifice. As this stage produced magmas compositionally more evolved and diverse (basalts to trachytes) than in previous stages, it likely represents the terminal pulse of basalt to the system, followed by cooling, protracted differentiation, and cessation of eruptive activity. Petrographic, geochemical, and Sr and Nd isotope data for Sunlight igneous rocks indicate that crystallization differentiation was the dominant magma-evolution process, although phenocryst accumulation and crustal contamination also occurred. Evidence for phenocryst accumulation is most pronounced in final-stage rocks. This relationship is interpreted to reflect recharge of late magmas into mature chambers laden with crystals deposited during earlier stages. The importance of crystallization differentiation at Sunlight volcano contrasts with recent petrogenetic models for calc-alkaline magmas in the Absaroka volcanic province, which require a more important role for early magma mixing. The results presented here indicate that across-strike increases in K2O contents of rocks in the field derive from variations in the conditions of crustal differentiation, rather than from mantle processes involving a subducting slab.

Trace elements and cathodoluminescence of igneous quartz in topaz granites from the Hub Stock (Slavkovský Les Mts., Czech Republic)

Abstract: ¶Igneous quartz of the late-Variscan topaz-bearing granites from the Hub Stock (Slavkovský Les, Czech Republic) was investigated by cathodoluminescence (CL) and electron probe micro-analysis (EPMA) to demonstrate the intra-granular heterogeneity of growth patterns and trace element distribution in quartz. We show that EPMA is well suited for the in situ study of Al and Ti in zoned quartz, because of its high spatial resolution down to 5 µm in conjunction with the ability to combine spot analyses with CL imaging. In the quartz phenocrysts of the topaz granites high Ti is associated with blue luminescent growth zones. High Ti (>40 ppm) in quartz indicates a high crystallisation temperature and pressure. The groundmass quartz of the granites which is almost free of Ti, has higher Al than the phenocrysts which may reflect an increase of lithophile elements and water content in melt during the late magmatic stage. The occurrence of similar quartz phenocrysts in most of the late-Variscan granites and rhyolites of the Krusne Hory/Erzgebirge which intruded over a period of about 40 Ma points to a similar crystallisation environment and origin of the quartz phenocrysts in the lower to middle crust.

Magmatic zoning in apatite: a monitor of porosity and permeability change in granites

Abstract: Apatites from the Shap Granite, northern England, are strongly zoned, reflecting multiple generations of growth and dissolution. Such chemical zoning is most readily displayed in cathodoluminescence images and correlates well with trace element variation determined using LA-ICP-MS analyses. The zoned apatites provide a detailed record of the changing scales of permeability during progressive crystallisation within the magma chamber. Early periods of apatite growth are preserved within cores and represent both early growth within a magma chamber dominated by vigorous mixing processes and inherited grains with significantly different chemistries. The main phase of apatite growth within the magma was strongly controlled by the presence of adjacent biotite phenocrysts and is characterised by fine scale oscillatory zoning, followed by the growth of a thin rim of relatively uniform composition. The chemical evolution of the later phases of apatite growth and the stratigraphy of the zoning appear to record late stage crystallisation within progressively more isolated interstitial melt pockets.

The Origin of Basic-Ultrabasic Sills with S-, D-, and I-shaped Compositional Profiles by in Situ Crystallization of a Single Input of Phenocryst-poor Parental Magma

Abstract: An attempt is made to develop an in situ crystallization model based on the concept of Soret fractionation to explain the origin of commonly observed S-, D-, and I-shaped compositional profiles in sills formed from a single pulse of phenocryst-poor parental magma. The model envisages that the various compositional profiles observed in sills can be interpreted in terms of different combinations of four principal units—Basal Zone and Layered Series forming the floor sequence, and Top Zone and Upper Border Series constituting the roof sequence. The Basal and Top Zones represent mirror images of the Layered and Upper Border Series, respectively, and therefore are referred to as basal and top reversals. It is proposed that the formation of basal and top reversals takes place through the non-equilibrium Soret differentiation of liquid boundary layers which form as a consequence of the temperature gradient imposed by the cold country rock. In contrast, the Layered and Upper Border Series originate predominantly through the crystal‐liquid boundary layers developing in equilibrium conditions. The model permits the production of S-, D-, and I-shaped compositional profiles from the same magma composition. All that is necessary to produce a specific shape of compositional profile is an appropriate temperature gradient imposed by the cold country rock on the liquid boundary layers of a parental magma of a given composition.

Using Quantitative Textural Analysis to Understand the Emplacement of Shallow- Level Rhyolitic Laccoliths—a Case Study from the Halle Volcanic Complex, Germany

Abstract: RECEIVED FEBRUARY 1, 2002; ACCEPTED NOVEMBER 8, 2002In qualitatively homogeneous magmatic bodies, quantitativetextural analysis—such as crystal size distribution, modalabundance, and spatial distribution pattern analyses—allowstheir internal heterogeneity to be measured and interpreted. Inthis study, these methods are applied to samples from a 300mdrill core through one of the porphyritic rhyolitic laccoliths(Petersberg unit) of the ˘300Ma Halle Volcanic Complex,Germany.Qualitatively,thegeochemicallyhomogeneousPetersbergunit does not show much textural variation. Quantitatively,however, the crystal size distributions of the three most commonphenocryst phases (orthoclase, plagioclase and quartz) suggestcontinuouscrystalgrowthduringmagmaascentandemplacement,but different growth histories of the phenocryst phases throughoutthe genesis of the laccolith. In situ cooling did not affect thephenocryst population. Size distributions of the phenocrysts varyon a centimetre to decimetre scale, but are similar on the scale ofthe laccolith. The modal abundance of the phenocryst phases isvery similar throughout the drill core. Quantification of thespatial distribution of phenocrysts, however, reveals a trend forclusteringtowardstheinteriororupperpartofthelaccolith,whichis attributed to flow and shear processes during emplacement anddiscontinuities in the interior relating to the intrusion of differentmagmapulses.Circularstatisticsoftheorientationoflongaxesofcrystals reveala weakalignment oftheorthoclaseand plagioclasephenocrystsonthesamplescaleasaresultofflowinthemagmainspite of little acicularity. In general, laccoliths can be fedby several pulses of magma without major cooling betweenbatches.

Genovesa Submarine Ridge: A manifestation of plume‐ridge interaction in the northern Galápagos Islands

Abstract: [1] Despite its circular coastline and calderas, Genovesa Island, located between the central Galapagos Platform and the Galapagos Spreading Center, is crosscut by both eruptive and noneruptive fissures trending NE-SW. The 075° bearing of the fissures parallels that of Genovesa Ridge, a 55 km long volcanic rift zone that is the most prominent submarine rift in the Galapagos and constitutes the majority of the volume of the Genovesa magmatic complex. Genovesa Ridge was the focus of detailed multibeam and side-scan sonar surveys during the Revelle/Drift04 cruise in 2001. The ridge consists of three left stepping en echelon segments; the abundances of lava flows, volcanic terraces, and eruptive cones are all consistent with constructive volcanic processes. The nonlinear arrangement of eruptive vents and the ridge's en echelon structure indicate that it did not form over a single dike. Major and trace element compositions of Genovesa Ridge glasses are modeled by fractional crystallization along the same liquid line of descent as the island lavas, but some of the glasses exhibit higher Mg # than material sampled from the island. Most of the submarine and the subaerial lavas have accumulated plagioclase. Incompatible trace element abundances of dredged Genovesa Ridge rocks are lower than the island's lavas, but ratios of the elements are similar in the two settings, which suggests that the island and ridge lavas are derived from nearly identical mantle sources. Glass inclusions in plagioclase phenocrysts from the ridge are compositionally diverse, with both higher and lower MgO than the matrix glass, indicative of homogenization at shallow levels. The structural and geochemical observations are best reconciled if Genovesa Ridge did not form in response to injection of magma laterally from a hot spot-supplied central volcano, like Kilauea's Puna Ridge. Instead, Genovesa Ridge and its western extension are the result of passive upwelling directed by far-field tectonic stresses that are generated by tension across the 91°W transform. The proximity of the plume causes magmatism in the extensional zones where it would not ordinarily occur.

Pb isotope variations among Bandelier Tuff feldspars: No evidence for a long-lived silicic magma chamber

Abstract: We report, for the first time, high-precision Pb isotope data from a high-silica rhyolite. Prior work on Sr isotopes in the 1.6 Ma Otowi Member of the Bandelier Tuff (Valles caldera, New Mexico) established that large 8 7 Sr/ 8 6 Sr variations exist among Otowi glasses and sanidine phenocrysts. While the glasses display unequivocal evidence for wall-rock contamination of the Otowi magma following sanidine growth, a positive correlation between 8 7 Sr/ 8 6 Sr i and 8 7 Rb/ 8 6 Sr among the feldspars could be interpreted as either a mixing line or an in situ magmatic isochron dating a differentiation event ∼270 k.y. prior to eruption. The 2 0 6 Pb/ 2 0 4 Pb and 8 7 Sr/ 8 6 Sr ranges for Otowi sanidines are 17.790 ′ 0.002 to 17.831 ′ 0.002 and 0.7074-0.7052, respectively. This Pb isotope range cannot be produced by radiogenic ingrowth at the U/Pb ratios of the host magma on any geologically reasonable time scale, and hence is unequivocal evidence for open-system behavior of the Otowi magma prior to and/or concurrent with feldspar growth. Open-system behavior is predicted to control Sr isotope variations due to much higher concentrations of Sr, relative to Pb, in the country rock than in the magma. These observations therefore undermine any age significance of the Rb-Sr isotope variations. In the absence of supporting data, Rb-Sr relations alone do not impart any information about residence times of high-silica rhyolite magmas with subchondritic concentrations of Sr.

Low-pressure phase equilibria of anhydrous anorthite-bearing mafic magmas

Abstract: [1] One of the most persistent questions regarding the phase equilibria of mid-ocean ridge basalts (MORB) pertains to the petrogenesis of the anorthitic plagioclase phenocrysts (>An90) that are characteristic of the more primitive members of such suites. Anorthitic phenocrysts are present in many if not most MORB suites in spite of the fact that no naturally occurring MORB glasses have ever been discovered to be in equilibrium with plagioclase more calcic than An85. We have addressed this paradox by attempting to saturate natural basalts with anorthite in a series of 1 atm experiments using three different natural basaltic starting compositions: an N-MORB, an E-MORB, and a continental high-alumina basalt. To ensure duplication of the olivine and anorthite saturation observed in natural anorthite-bearing basalt, the experiments were run in An93-6 capsules with Fo92 olivine added to the starting glass. The compositions of experimental liquids are generally colinear with the trends observed in the lava suites used as the source material for the starting glasses. Significantly, aluminous spinel (Al2O3 contents of 61–68 wt%) was produced at 1290°C in all compositions and chromites (Al2O3 contents of 33–42 wt%) at lower temperatures in N-MORB-derived liquids despite no spinel having been added to the starting mixture. In addition, the experiments produced basaltic liquid in equilibrium with both >Fo89 olivine and >An85 feldspar at temperatures of 1230° and 1210°. These liquids have compositions with Mg# (at% Mg/Mg + FeT*100) that range from 63 to >85. The TiO2-MgO correlation indicates large (∼16–23%) amounts of crystallization for each percent decrease in MgO. These results suggest the possibility that dry, anorthite-bearing basaltic magmas are the product of the interaction between primary melt and Al-spinel-bearing upper mantle. In addition, the results indicate that MORB magmas can undergo a large amount (>50%) of crystallization prior to reaching 8% MgO. Further, although anorthite-bearing magmas have characteristics consistent with their being a significant volumetric component of MORB “parent” magmas, the reaction mechanism suggested for their petrogenesis indicates that they are not necessarily primary magmas.

Alkali Picrites Formed by Melting of Old Metasomatized Lithospheric Mantle: Manîtdlat Member, Vaigat Formation, Palaeocene of West Greenland

Abstract: are required in the melting assemblage and dominate the Pb Alkaline picrites and basalts constitute 20–200 m of lava flows isotope compositions. The various magma types and the partly and hyaloclastites in the middle part of an >2 km thick succession complementary relation between them suggest that the lithospheric of tholeiitic picrites and basalts formed during continental rifting of mantle had an ordered structure, possibly with old metasomatic West Greenland around 60 Ma. The alkaline rocks, found only zones formed by successive trapping of elements in migrating fluids. in northern Disko, have phenocrysts of olivine + chromite ± clinopyroxene; lava flows contain abundant groundmass clinopyroxene and plagioclase, whereas pillow breccias contain abundant fresh alkali basaltic glass. Six compositional types are present; all