Showing papers on "Phenocryst published in 1998"
TL;DR: In this paper, high pressure (to 3 kbar), water saturated melting experiments were conducted on an andesite (62 wt% SiO2) and a basaltic andesitic (55 wt%) from western Mexico, showing that the CO2 content was minimal in the fluid phase.
Abstract: We have conducted high pressure (to 3 kbar), water saturated melting experiments on an andesite (62 wt% SiO2) and a basaltic andesite (55 wt% SiO2) from western Mexico. A close comparison between the experimental phase assemblages and their compositions, and the phenocryst assemblages of the lavas, is found in water saturated liquids, suggesting that the CO2 content was minimal in the fluid phase. Thus the historic lavas from Volcan Colima (with phenocrysts of orthopyroxene, augite, plagioclase, and hornblende) were stored at a temperature between 950–975 °C, at a pressure between 700–1500 bars, and with a water content of 3.0–5.0 wt%. A hornblende andesite (spessartite) from Mascota, of nearly identical composition but with only amphibole phenocrysts, had a similar temperature but equilibrated at a minimum of 2000 bars pressure with a dissolved water content of at least 5.5 wt% in the liquid. Experiments on the basaltic andesite show that the most common natural phenocryst assemblages (olivine, ±augite, ±plagioclase) could have precipitated at temperatures from 1000–1150 °C, in liquids with a wide range of dissolved water content (∼2.0–6.0 wt%) and a corresponding pressure range. A lava of the same bulk composition with phenocrysts of hornblende, olivine, plagioclase, and augite is restricted to temperatures below 1000 °C and pressures below 2500 bars, corresponding to <5.5 wt% water in the residual liquid. Although there is some evidence for mixing in the andesites (sporadic olivine phenocrysts), the broad theme of the history of both lava types is that the phenocryst assemblages for both the andesitic magmas and basaltic andesitic magmas are generated from degassing and reequilibration on ascent of initially hydrous parents containing greater than 6 wt% water. Indeed andesitic magmas could be related to a basaltic andesite parent by hornblende-plagioclase fractionation under the same hydrous conditions.
437 citations
TL;DR: An ion probe study of trace elements in Mg-rich clinopyroxene phenocrysts in primitive Aleutian lavas provides constraints on the genesis of Aleutians adakites, and possible insights into the source of common aleutian magmas as discussed by the authors.
201 citations
TL;DR: The andesite lava currently erupting at the Soufriere Hills volcano, Montserrat, contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesites.
Abstract: The andesite lava currently erupting at the Soufriere Hills volcano, Montserrat, contains ubiquitous mafic inclusions which show evidence of having been molten when incorporated into the andesite. The andesite phenocrysts have a range of textures and zonation patterns which suggest that non-uniform reheating of the magma occurred shortly before the current eruption. Reheating resulted in remobilisation of the resident magma and may have induced eruption.
187 citations
TL;DR: Ewart et al. as discussed by the authors inferred that the mafic lavas are derived from the Goboboseb-Messum Centre and the Messum Carter Basalts (MCB) and showed evidence for crystal fractionation, have 'arc-like' trace element signatures, correlated e-SiO, e-Ti/γ and -Ti/Zr.
Abstract: The Goboboseb Mountains and Messum Complex represent a major Cretaceous (132 Ma) bimodal eruptive centre in the southern Etendeka continental flood basalt (CFB) province. The eruptives compris the Awahab Formation and are represented by a lower sequence of mafic lavas, followed by the Goboboseb quartz latite members, the Messum Mountain Basalts, and finally the Springbok quartz latite. The sequence is cut by numerous dolerite dykes, sills and plugs, rare rhyolite, and carbonatite. The mafic lavas comprise two distinct series, which although corresponding broadly to the Etendeka regional low Ti and Zr basalts (LTZ type), are distinguished by Ti/Zr ratios into the LTZ.H (higher Ti/Zr) and LTZ.L (lower Ti/Zr) series. The LTZ.H basalts have no previously described extrusive equivalent in the Etendeka (or Parana) CFB, and consist of magnesian, mildly alkaline to tholeiitic lavas, dominated by oliv + cpx phenocryst assemblages which fractionate (near the surface) to phono-tephrite. They are identified as predominantly mantle plume melts (Tristan-Walvis plume). The LTZ.L lavas are less magnesian, extending to icelandites, are tholeiitic, with cpx ± oliv + pl + Fe-Ti oxide phenocryst assemblages and groundmass pigeonite and subcalcic augite. Stratigraphically, the LTZ.H lavas are interbedded with LTZ.L types in the lower part of the sequence and also occur as dykes. Within the Messum Complex, a remnant early sequence of basalts (Messum Carter Basalts) are in part transitional between the LTZ.L and LTZ.H series. The LTZ.H, and at least some of the LTZ.L lavas are inferred to have been erupted from the Goboboseb-Messum Centre. Chemically, the LTZ.H melts are broadly intermediate between E-MORB and OIB magmas, with higher Ti/Zr Sm/ γb and Ti/γ ratios than the LTZ.L types, which suggest segregation depths between the garnet and spinel peridotite stability fields. The Pb-Nd-Sr isotopic compositions of the LTZ.H eruptives are similar to, but not identical with the modern Tristan plume composition, and the observed variability is attributable to limited lower-crust assimilation and/or Atlantic MORB source mixing. The LTZ.L lavas show evidence for crystal fractionation, have 'arc-like' trace element signatures, correlated e-SiO, e-Ti/γ and -Ti/Zr, e-1/Sr and 1/Nd-e variations, and relatively radiogenic Pb, evolved Sr(e, 58-174) and low Nd(e -6·1 to -9·5) isotopic compositions. Their geochemistry is inferred to be AFC (assimilation-fractional crystallization) controlled, and is modelled by three-component mixing involving mantle plume derived melt, mafic lower crust and silicic mid-upper crust. The voluminous quartz latites (Part II, Ewart et al., 1978) extend these geochemical trends.
178 citations
TL;DR: The Torfajokull central volcano in south-central Iceland contains the largest volume of exposed silicic extrusives in Iceland (∼225 km^3) as mentioned in this paper.
160 citations
TL;DR: In this paper, phase equilibrium studies allow pre-eruptive conditions to be precisely determined and demonstrate that recent eruptions, either Plinian or Pelean, tapped magmas with melt H2O contents of 5.3-6.5 wt, stored at 2 ± 0.5 kbar, 875-900°C and ΔNNO = + 0.4-0.8.
159 citations
TL;DR: In this article, the authors reproduce melt inclusions in plagioclase in a series of partial dissolution experiments, and the compositional differences from the surrounding matrix melt were evaluated, showing that the molar ratio of Ca/(Ca + Na) and the MgO concentration in the synthesized melt inclusion are up to 17 and 61% lower than those in the matrix melt, respectively.
147 citations
TL;DR: In this article, the authors focus on sparsely crystalline felsic that have amphibole-rich phenocrysts, relatively high silica (SiO2 > 68 wt %) volcanic rocks erupted in the south-most Cascades, and pronounced middle rare earth element depletions.
Abstract: The majority of felsic rocks from composite centers in the southernmost INTRODUCTION Cascades have geochemical and Sr, Nd and Pb isotopic ratios Despite the importance of felsic magmas in calc-alkaline that suggest derivation by partial melting of lower crust that is rock suites, as both the end of the compositional spectrum compositionally similar to calc-alkaline basalts observed in the region. of lavas and as a potential assimilant or mixing endOnly a few felsic rocks have d 18 O and Pb isotopic compositions member, most petrogenetic studies have focused on bathat indicate interaction with the upper crust. Mineralogical and saltic magmas. This stems from the fact that sparsely geochemical diVerences among the felsic magmas result primarily crystalline basaltic lavas are usually the most primitive from melting under variable f(H2O) and temperature conditions. lavas within a given suite of rocks, and therefore have Partial melting under low f(H2O) and high temperature conditions compositions that are most indicative of magma sources. leaves an amphibole-poor residuum, and produces magmas that In contrast, felsic rocks have high incompatible element have orthopyroxene as the most abundant ferromagnesian phenocryst, abundances and are often highly crystalline, and may relatively low silica contents, and straight rare earth element patterns. therefore have been extensively modified by fractional Partial melting under higher f(H2O) and lower temperature con- crystallization and assimilation fractional crystallization ditions leaves an amphibole-rich residuum, and produces magmas (AFC). This study focuses on sparsely crystalline felsic that have amphibole ‐ biotite phenocrysts, relatively high silica (SiO2 > 68 wt %) volcanic rocks erupted in the southcontents, and pronounced middle rare earth element depletions. These ernmost Cascades that demonstrate evidence for only a conclusions are consistent with published thermal models that suggest minor amount of diVerentiation. We use mineralogy, that reasonable volumes of basaltic magma emplaced beneath large major and trace element geochemistry, and isotopic compositions of felsic rocks erupted from composite cencomposite centers in the southernmost Cascades can serve as the ters to constrain their petrogenesis. These felsic rocks heat source for melting of the lower crust. Melting of the lower provide new insights into petrogenetic processes that crust under variable f(H2O) conditions is likely to result from generate felsic arc magmas.
130 citations
TL;DR: The Ordovician Betts Cove ophiolite of Newfoundland has a well-developed cumulate sequence, in which is rooted a sheeted dyke complex that grades up into pillow lavas as discussed by the authors.
126 citations
TL;DR: The West Greenland picrites from the 61 million year old Vaigat Formation of the Nuussuaq Peninsula in West Greenland have 3 He= 4 He ratios trapped in olivine phenocrysts which range up to 30 times the atmospheric ratio as discussed by the authors.
125 citations
TL;DR: In this paper, a large number of clinopyroxene (cpx) phenocrysts and hosted melt inclusions (MI) in pyroclasts from several Vesuvius eruptions were analyzed.
Abstract: A large number of clinopyroxene (cpx) phenocrysts and hosted melt inclusions (MI) in pyroclasts from several Vesuvius eruptions were analyzed. Assuming that their temperature of homogenization (Thom) reflects that of crystallization of the host mineral and that after homogenization and quenching, composition of the MI represents the melt from which the host crystallized, each cpx-MI pair records equilibrium conditions at Thom. These data were used to discuss the magma evolution within shallow magma chamber. They formed by means of the periodic arrival of mafic batches, recorded by the MI in diopside (Fs4–7) crystallized at 1150°–1200°C. Under open conduit conditions the chambers are small, and each magma pulse induces thermal and compositional variations recorded by oscillatory zoned pyroxenes (Fs4–15). Their MI and Thom summarize the evolution of the chamber resulting from cycles of magma injection, crystal settling, and magma extraction. The deposits of the explosive eruptions which reopen the conduit reflect variable mixing during magma withdrawal from zoned reservoirs, whose layering was deciphered through MI and Thom in salite (Fs15–30). We suggest that the Vesuvius magma chambers evolve from prolate toward subequant, changing their layering with increasing volume and age: (1) initial stage, high aspect ratio chamber, homogeneous mafic melt (T°C∼1100) crystal enriched downward; (2) young stage, medium aspect ratio, continuous gradation from mildly evolved (T°C∼1050) to felsic melt (T°C∼850–900); and (3) mature stage, low aspect ratio, twofold chamber with stepwise gradient separating lower, convective, mildly evolved portion (T°C∼1050) from upper, stratified, felsic portion (T°C∼800–950).
TL;DR: Olivine-hosted homogenized melt inclusions in a primitive basalt AII32-12-7 from 43oN Mid-Atlantic Ridge have been observed in this paper.
TL;DR: The Whakamaru group ignimbrites are widespread voluminous welded ignimrites which crop out along the eastern and western margins of the Taupo Volcanic Zone (TVZ), New Zealand as discussed by the authors.
TL;DR: In the case of the Canary Islands, the authors of as discussed by the authors showed that the highest density fluid inclusions in many Canary Island mantle xenoliths have probably survived in-situ near-isobaric heating at the depth of xenolith entrainment.
Abstract: Gabbroic and ultramafic xenoliths and olivine and clinopyroxene phenocrysts in basaltic rocks from Gran Canaria, La Palma, El Hierro, Lanzarote and La Gomera (Canary Islands) contain abundant CO2-dominated fluid inclusions. Inclusion densities are strikingly similar on a regional scale. Histogram maxima correspond to one or more of the following pressures: (1) minimum 0.55 to 1.0 GPa (within the upper mantle); (2) between 0.2 and 0.4 GPa (the Moho or the lower crust); (3) at about 0.1 GPa (upper crust). Fluid inclusions in several rocks show a bimodal density distribution, the lower-density maximum comprising both texturally early and late inclusions. This is taken as evidence for an incomplete resetting of inclusion densities, and simultaneous formation of young inclusions, at well-defined magma stagnation levels. For Gran Canaria, pressure estimates for early inclusions in harzburgite and dunite xenoliths and olivine phenocrysts in the host basanites overlap at 0.9 to 1.0 GPa, indicating that such magma reservoir depths coincide with levels of xenolith entrainment into the magmas. Magma chamber pressures within the mantle, inferred to represent levels of mantle xenolith entrainment, are 0.65–0.95 GPa for El Hierro, 0.60–0.68 GPa for La Palma, and 0.55–0.75 GPa for Lanzarote. The highest-density fluid inclusions in many Canary Island mantle xenoliths have probably survived in-situ near-isobaric heating at the depth of xenolith entrainment. Inclusion data from all islands indicate ponding of basaltic magmas at Moho or lower crustal depths, and possibly at an additional higher level, strongly suggestive of two main crustal accumulation levels beneath each island. We emphasize that repeated magmatic underplating of primitive magmas, and therefore intrusive accretion, are important growth mechanisms for the Canary Islands, and by analogy, for other ocean islands. Comparable fluid inclusion data from primitive rocks in other tectonic settings, including Iceland, Etna and continental rift systems (Hungary, South Norway), indicate that magma accumulation close to Moho depths shortly before eruption is not, however, restricted to oceanic intraplate volcanoes. Lower crustal ponding and crystallization prior to eruption may be the rule rather than the exception, independent of the tectonic setting.
TL;DR: The results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy) are presented in this paper.
Abstract: This work presents the results of a microthermometric and EPMA-SIMS study of melt inclusions in phenocrysts of rocks of the shoshonitic eruptive complex of Vulcano (Aeolian Islands, Italy). Different primitive magmas related to two different evolutionary series, an older one (50–25 ka) and a younger one (15 ka to 1890 A.D.), were identified as melt inclusions in olivine Fo88–91 crystals. Both are characterized by high Ca/Al ratio and present very similar Rb/Sr, B/Be and patterns of trace elements, with Nb and Ti anomalies typical of a subduction zone. The two basalts present the same temperature of crystallization (1180±20 °C) and similar volatile abundances. The H2O, S and Cl contents are relatively high, whereas magmatic CO2 concentrations are very low, probably due to CO2 loss before low-pressure crystallization and entrapment of melt inclusions. The mineral chemistry of the basaltic assemblages and the high Ca/Al ratio of melt inclusions indicate an origin from a depleted, metasomatized clinopyroxene-rich peridotitic mantle. The younger primitive melt is characterized with respect to the older one by higher K2O and incompatible element abundances, by lower Zr/Nb and La/Nb, and by higher Ba/Rb and LREE enrichment. A different degree of partial melting of the same source can explain the chemical differences between the two magmas. However, some anomalies in Sr, Rb and K contents suggest either a slightly different source for the two magmas or differing extents of crustal contamination. Low-pressure degassing and cooling of the basaltic magmas produce shoshonitic liquids. The melt inclusions indicate evolutionary paths via fractional crystallization, leading to trachytic compositions during the older activity and to rhyolitic compositions during the recent one. The bulk-rock compositions record a more complex history than do the melt inclusions, due to the syneruptive mixing processes commonly affecting the magmas erupted at Vulcano. The composition and temperature data on melt inclusions suggest that in the older period of activity several shallow magmatic reservoirs existed; in the younger one a relatively homogeneous feeding system is active. The shallow magmatic reservoir feeding the recent eruptive activity probably has a vertical configuration, with basaltic magma in the deeper zones and differentiated magmas in shallower, low-volume, dike-like reservoirs.
TL;DR: In this article, the authors used a model of Fe-Mg interdiffusion to estimate the age of peridotitic mantle xenoliths from the 1949 La Palma eruption.
Abstract: Spinel-bearing peridotitic mantle xenoliths from the 1949 eruption on La Palma were modified mineralogically and chemically during prolonged reaction with their host magma. The magmatism that brought the peridotites to the surface caused two distinct generations of xenolith fractures: (1) Old fractures are characterized by crystalline selvages with cumulus textures towards the host magma, or by polymineralic veins. They are accompanied by 0.9–2 mm wide diffusion zones where peridotite olivine became less forsteritic through diffusive exchange with the host magma. Old fractures represent most of each xenolith's surface. (2) Young fractures show no selvages and only narrow diffusion zones of <0.02 mm width. Calculations based on a model of Fe-Mg interdiffusion give an age of 6 to 83 years and <4 days for old and young fractures, respectively. A combination of these data with fluid inclusion barometry indicates that selvages and veins formed during xenolith transport rather than representing wall-rock reactions or mantle metasomatism. The results provide ample evidence for prolonged storage of the xenoliths in the crust, constraining a multi-stage magma ascent: Years to decades prior to eruption, ascending magma ruptured peridotitic wall-rock possibly through hydraulic fracturing and stoping around magma reservoirs. Magma batches transported the peridotite xenoliths to the crust at ascent rates exceeding 0.2 ms−1. The xenoliths and their host magma stagnated during at least 6 years in possibly sill-like reservoirs at 7–11 km depth. The xenoliths became deposited and subsequently embedded in a mush of settled phenocrysts, while selvages and veins crystallized until the eruption commenced. At the end of the eruption, the xenoliths were finally transported to the surface within hours to days. Decompression during the rapid ascent induced internal stresses and caused renewed fragmentation of the xenoliths, producing the young fractures.
TL;DR: The recent eruption of the Soufriere Hills Volcano in Montserrat (July, 1995, to present; October, 1997) has produced a hornblende-bearing, andesitic lava dome.
Abstract: The recent eruption of the Soufriere Hills Volcano in Montserrat (July, 1995, to present; October, 1997) has produced a hornblende-bearing, andesitic lava dome. It is possible to petrologically estimate changes in ascent rates of amphibole-bearing magmas. For certain rates of decompression, a breakdown rim of fine-grained, anhydrous reaction products forms where amphibole is in contact with melt. The thickness of the rim varies with ascent rate. Most of the amphibole phenocrysts in the magma storage region lack breakdown rims. About 10% have 200–400 µm-thick, coarse-grained breakdown rims that are interpreted to be relicts of a past heating event. Study of a time series of new dome andesites showed that ascent rate increased from December, 1995 (∼0.001 m/s), to July, 1996 (∼0.008 m/s), while eruptive style remained extrusive. Ascent rate increased to >0.012 m/s in August, 1996, and the first major explosive eruption occurred on 17–18 September, 1996.
TL;DR: In this paper, the Goboboseb-Messum volcanic centre is the source of two voluminous silicic eruptive sequences, the Springbok quartz latiites (Units I-III).
Abstract: The Goboboseb-Messum volcanic centre is the source of two voluminous silicic eruptive sequences, the Goboboseb quartz latiites (Units I-III), and the Springbok quartz latite unit (both within the Awahab Formation). Intrusive equivalents exist as plugs and a laccolith peripheral to the Messum Complex. The recognition of correlatives of these quartz latite units in the southeastern Parana suggests eruptive volumes of ∼2320 km (Goboboseb units) and 6340 km (Springbok unit). The latter is thought to be a single eruptive event. Phenocryst assemblages are plagioclase (An), pyroxene, titanomagnetite and apatite. Pyroxene assemblages range from augite, to augite + pigeonite, to pigeonite, to pigeonite ± hypersthene, the assemblages changing progressively from the Goboboseb unit through to the Springbok unit. Although pyroxene phenocrysts from individual samples are compositionally very uniform there is a small increase in Fe through the sequence, attributed to decreasing temperature (± pressure). Thermometry suggests melt temperatures >1000°C. Many plagioclases contain abundant glass inclusions of three compositional types, thought to result from active disequilibrium melting at magma chamber walls. Relatively small, but systematic, changes in whole-rock composition occur stratigraphically from the lowest Goboboseb unit through to the Springbok unit and to their correlatives in the Parana, best shown by SiO (67-71%) and FeO* (5·4-7·4%) which increase and decrease, respectively, in the progressively younger eruptive phases. P, Ti, γ, Zr, Nb and Cu are positively correlated with FeO*, whereas e and Pb isotope compositions correlate inversely with FeO*. Crust-normalized spidergram plots indicate strong negative Sr anomalies, accompanied by significant Eu/Eu* anomalies (0·62-0·67). The quartz latite melts can be interpreted in terms of large-scale assimilation-fractional crystallization (AFC)-style processes, involving high degrees of lower- and upper-crustal melting, with thermal and material input from hybridized LTZ.L-type basaltic magmas (Part I). Thermal source is inferred to be the Tristan plume. The crustal end-member is thought to be the mid-Proterozoic restite source of the Damara granites, although some shallower crustal input is also likely. Modelling suggests the source may be similar to A-type granites and charnockites (i.e. relatively REE and HFSE enriched). Available seismic data suggest a simple velocity crustal profile, possibly the result of the massive crustal and uppermost mantle melting that accompanied the evolution of the Awahab magma system.
TL;DR: Tan and gray clasts have average densities between 1.5 and 1.7 g/cc and vesicularities (phenocryst free) of approximately 42% as mentioned in this paper.
Abstract: The 1992 eruption of Crater Peak, Mount Spurr, Alaska, involved three subplinian tephra-producing events of similar volume and duration. The tephra consists of two dense juvenile clast types that are identified by color, one tan and one gray, of similar chemistry, mineral assemblage, and glass composition. In two of the eruptive events, the clast types are strongly stratified with tan clasts dominating the basal two thirds of the deposits and gray clasts the upper one third. Tan clasts have average densities between 1.5 and 1.7 g/cc and vesicularities (phenocryst free) of approximately 42%. Gray clasts have average densities between 2.1 and 2.3 g/cc, and vesicularities of approximately 20%; both contain abundant microlites. Average maximum plagioclase microlite lengths (13–15 μm) in gray clasts in the upper layer are similar regardless of eruptive event (and therefore the repose time between them) and are larger than average maximum plagioclase microlite lengths (9–11 μm) in the tan clasts in the lower layer. This suggests that microlite growth is a response to eruptive processes and not to magma reservoir heterogeneity or dynamics. Furthermore, we suggest that the low vesicularities of the clasts are due to syneruptive magmatic degassing resulting in microlitic growth prior to fragmentation and not to quenching of clasts by external groundwater.
TL;DR: In this article, the ages of sanidine grains from the Huckleberry Ridge Tuff, Mesa Falls Tuff and member B of the Lava Creek Tuff were determined by laser-fusion 40Ar/39Ar dating.
Abstract: Ash beds associated with the three climactic Yellowstone ignimbrites form important Quaternary chronostratigraphic markers over much of the continental United States. Previous K-Ar ages determined on crystal concentrates from these ashes varied by as much as 60–100 k.y. Laser-fusion 40Ar/39Ar dating of single sanidine grains from these units reveals a small number of grains with anomalously old ages. Eliminating these from the weighted averages results in highly precise refined ages of 2.003 ± 0.014, 1.293 ± 0.012, and 0.602 ± 0.004 Ma (2σ errors) for the Huckleberry Ridge Tuff, Mesa Falls Tuff, and member B of the Lava Creek Tuff, respectively. Individual single-grain ages that are slightly too old could result from incomplete degassing of xenocrysts in the magma. Electron-microprobe analyses of sanidine splits reveal no obvious xenocrystic compositions, suggesting another possibility—that phenocrysts from the crystallized rind of the magma chamber were re-entrained into the magma prior to eruption. Contamination and natural variation in phenocryst age may create larger uncertainty in bulk-crystal dating of young silicic volcanic rocks than incomplete extraction of Ar from sanidine.
TL;DR: In this paper, mesoscopic flow lineations and anisotropy of magnetic susceptibility (AMS) have been measured for dikes within the Cretaceous-age Troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth.
Abstract: Mesoscopic flow lineations and anisotropy of magnetic susceptibility (AMS) have been measured for dikes within the Cretaceous-age Troodos ophiolite with the goal of comparing the direction of initial magma flow through dike conduits immediately following crack propagation with that of flow of subsequent magma emplaced during later stages of dike growth. Dike margin indicators of flow include cusp axes and elongate vesicles found high in the ophiolite pseudostratigraphy and ridge-and-groove structures termed hot slickenlines found throughout the complex. A unique flow direction is determined where elongate vesicles near dike margins display imbrication with respect to the margin. Significant changes in vesicle elongation directions across dikes likely indicate either changes in magma flow direction after dike propagation or back-flow of magma during the waning stages of intrusion. Surface lineations generally lie subparallel to the direction of flow inferred from AMS determinations on cores within 5 cm of dike margins. Surface lineations also lie subparallel to the long axis (e1) of the orientation ellipsoid defined by long axes of groundmass plagioclase phenocrysts measured in sections from AMS cores. Correlation of surface lineations with interior indicators of flow (AMS, plagioclase trachytic texture) indicate that the surface features are good proxies for grain-scale magma flow directions during dike propagation in Troodos dikes. Orientations of surface flow features in the dikes of the Troodos ophiolite indicate an approximately equal mix of subhorizontal to near-vertical magma flow, contradicting the paradigm of primarily vertical flow of magma beneath continuous axial magma chambers at oceanic spreading centers. Our data are consistent with a model of magma emplacement both vertically and horizontally away from isolated magma chambers beneath axial volcanoes spaced along a ridge crest.
TL;DR: In this article, a small cluster of Pleistocene andesite cones and flows are classified into two distinct groups: high-magnesium andesites (>6% MgO, 57-59% SiO2), conveniently called basalt-andesites, with phenocrysts of orthopyroxene and augite, or augite and olivine, at pressures of about 1 kbar.
Abstract: Approximately 150 km west of Mexico City in the central part of the Mexican Volcanic Belt (MVB) near Zitacuaro, Mexico, young volcanism has produced shield volcanoes, large volume silicic deposits, and fault-related basalt and andesite lava flows and cinder cones. This paper concerns a small cluster of Pleistocene andesite cones and flows which can be separated into two distinct groups: high-magnesium andesites (>6% MgO, 57–59% SiO2), conveniently called basaltic andesites, with phenocrysts of orthopyroxene and augite, or augite and olivine; and andesites (60–62% SiO2, 3 wt%) at pressures of about 1 kbar. Compared to basaltic andesites of western Mexico, the Zitacuaro basaltic andesites have ∼2 wt% lower Al2O3 concentrations, which causes plagioclase to precipitate at significantly lower temperatures, and it therefore follows the crystallization sequence: olivine, augite, and orthopyroxene. Based on ubiquitous quartz xenocrysts, with glassy rhyolitic inclusions, a reasonable conclusion is that substantial mixing of a quartz-bearing rhyolitic magma with a parental basaltic andesite has occurred at low pressure (shallow depth), and this would account for the low Al2O3 concentrations in the Zitacuaro basaltic andesites. Whatever the mechanism of incorporation, the quartz xenocrysts are evidence of contamination of basaltic magma with more siliceous material, thus making it difficult to use these magmas as indicators of mantle melting processes.
TL;DR: In this paper, the evolution of a magma system from initiation at 2.1 Ma to eruption at 0.76 Ma is described in the context of pre-caldera high-silica rhyolites and the voluminous, zoned rhyolitic Bishop Tuff.
Journal Article•
TL;DR: A rhyolitic dyke at Richemont, Haute-Vienne, France, is shown to be the subvolcanic counterpart of rare-metal-bearing granites and pegmatites as mentioned in this paper.
Abstract: A rhyolitic dyke at Richemont, Haute-Vienne, France, is shown to be the subvolcanic counterpart of rare-metal-bearing granites and pegmatites. Textural relationships provide evidence for a quenched silicate melt with less than 5% phenocrysts, consisting of albite (50%), quartz (20%), K-feldspar (20%), and muscovite (10%). The affinity with pegmatites arises from the ore mineralogy, with "uran-euxenite" and "wolframo-ixiolite" as the main rare-metal carriers. An affinity with rare-metal granites arises from the geochemistry. The composition of the rhyolite is quite similar to that of the Beauvoir Ta-Li-bearing granite, corresponding to the high-phosphorus, high-fluorine class of strongly peraluminous leucogranites, enriched in Ta, Nb, Sn, Li, and Be. The melt belongs to the family that typically crystallizes as LCT granitic pegmatites. Analyses of muscovite phenocrysts provide estimates of muscovite-melt partition coefficients, allowing an explanation of some geochemical characteristics of this type of magmatism, e.g., Li, Cs, Ta and Sn increase during differentiation, and patterns of fractionation involve the ore elements W, Nb and Ta. Isolated aggregates of phosphate with inclusions of sulfosalts and of Nb, W, and Sn oxides are interpreted as signaling the onset of silicate-phosphate melt unmixing processes. Microcrystalline facies of the rhyolite are tentatively identified as the result of complete fluid unmixing, allowing a qualitative assessment of element extraction by fluids escaping from high-F melts. Although Sn, Ti, Th, Nb, and Ta are not depleted relative to the melt, other elements are removed, mildly in the case of Rb, Mn, S, Be, and Zn, strongly in the case of U, Li, B, As, F, and Ba, and very strongly in the case of W, Sb, Sr, P, and Ca.
TL;DR: The lower pliocene volcanic rocks of the Golcuk area of SW Turkey exhibit alkaline major element trends with a general potassic character as mentioned in this paper, and they contain high amounts of LILE (Ba, Sr, Rb and K), LREE, and Zr.
TL;DR: The six eruption episodes of the ∼10 ka Pahoka-mangamate (PM) sequence (see companion paper) occurred over a?200-400-year period from a 15km-long zone of multiple vents within the Tongariro Volcanic Centre (TgVC), located at the southern end of the TVZ as mentioned in this paper.
TL;DR: In this paper, a Wannienta basalt from Kuanyinshan,northern Taiwan, was studied using a high temperature furnace at atmospheric pressure, and fifteen runs were made to locate the liquidus temperature, the solidustemperature, and the melting interval of the Wannientsa baseltic magma.
Abstract: A Wannienta basalt from Kuanyinshan,northern Taiwan,is studiedusing a high temperature furnace at atmospheric pressure.The purpose ofthis study is to investigate the crystallization sequence of the phenocrysts,to analyze the chemical compositions of the phenocrysts,and to estimatethe evolution trend of the magma in the Kuanyinshan area.Fifteen runs are made to locate the liquidus temperature,the solidustemperature,and the melting interval of the Wannienta basaltic magma.The experimental temperatures range from 1087℃ to 1281℃.The durationisfrom five hours to nineteen hours.Compositions of glasses and phenocrystsare analyzed with an electron microprobe.The experimental results show that the liquidus temperature is locatedat 1270℃ and the solidus temperature is estimated to be close to 1080℃.The melting interval is about 190℃.The liquidus minerals are oxides withcompositions in spinel series.The following crystallized phase is olivine atabout 1217℃.Plagioclase and clinopyroxene appear,respectively,at about1178℃ and 1137℃.The evolution of the residual magma shows the chemicaltrend as:enrichment in silicon,aluminum,and alkalis;and depletion incalcium,iron,and magnesium with the decreasing temperature.The fractionationtrend of the Kuanyinshan volcanic series is similar to the variationtrend of the residual magmas.It implies that the depth for the fractionalcrystallization of the Wannienta basaltic magma to produce andesitescould be modeled at low pressure.The fractionates to produce biotitehornblende andesitic magma could be oxides,olivine,plagioclase,andclinopyroxene.
TL;DR: The lamprophyres are typically alkaline as shown by high contents of P2O5, TiO2, alkalies and incompatible trace elements such as light REE, Zr, Nb, Y, Ba and Sr, and by high Ti/V (>50) and chondrite-normalized (La/Yb)n (8−25) ratios as discussed by the authors.
Abstract: The northeast part of the Czech Republic (Moravia) and the adjoining part of Poland host a 100-km-long and 15- to 25-km-wide belt containing numerous isolated bodies (mainly sills) of lamprophyre of Lower Cretaceous age. The lamprophyres range from mafic (melanocratic) to evolved, feldspar-rich (mesocratic) variants. Mineralogically, they are characterized by compositionally zoned kaersutite phenocrysts, biotite and high Al–Ti salitic to diopsidic clinopyroxene. The lamprophyres are typically alkaline as shown by high contents of P2O5, TiO2, alkalies and incompatible trace elements such as light REE, Zr, Nb, Y, Ba and Sr, and by high Ti/V (>50) and chondrite-normalized (La/Yb)n (8–25) ratios. They resemble alkali basalts, basanites and nephelinites. Major element composition and trace element patterns and Nd–Sr isotopic values (eNd ca. +5.5 to +6.6 and eSr ca. –9.5 to –24.0) indicate that the lamprophyric magma was derived from a mantle source that was compositionally similar to the source of ocean island basalts with HIMU affinities and some continental extension-related alkali basaltic suites. The lamprophyres do not show any subduction imprint. They were generated in the garnet stability field by a variable degree of melting. Evolved lamprophyres were formed by clinopyroxene-dominated fractional crystallization of mafic lamprophyric magma. The lamprophyres are interpreted to have been emplaced along conduits formed during the formation of a basin/graben structure in the Early Cretaceous.
TL;DR: A set of phenocryst/matrix partition coefficients was obtained for up to 29 trace elements (ICP-MS analyses) in hawaiite to trachyte lavas from the Ellittico volcanic sequence (Mt. Etna system, Sicily) as mentioned in this paper.
Abstract: A set of phenocryst/matrix partition coefficients was obtained for up to 29 trace elements (ICP-MS analyses) in hawaiite to trachyte lavas from the Ellittico volcanic sequence (Mt. Etna system, Sicily). Partition coefficients were determined for plagioclase, clinopyroxene, olivine, kaersutite and Ti-magnetite. These phases, along with apatite (not analysed in this work), constitute the common fractionating solid assemblage of alkaline magmas feeding Mt. Etna volcanic system. The obtained data set forms the first attempt to characterise the solid/melt trace-element partitioning for Etnean magmas, and can be usefully applied in other sites of alkaline volcanism. The partition coefficients are here used to define the scale of incompatibility of 29 trace elements and to asses the extent of differentiation processes and the prevailing oxygen fugacity of Ellittico magmas.
TL;DR: The evolution of basaltic magma by mushy boundary layer processes is documented for an alkali basalt (Kutsugata) from Rishiri Volcano, northern Japan, on the basis of zoning interest and has been studied extensively.
Abstract: The evolution of basaltic magma by mushy boundary layer processes INTRODUCTION in a magma chamber is documented for an alkali basalt (Kutsugata The evolution of magma chambers is of major petrologic lava) from Rishiri Volcano, northern Japan, on the basis of zoning interest and has been studied extensively. Experimental patterns of plagioclase phenocrysts and their spatial distribution in and theoretical studies that have investigated the cooling the lava. Plagioclase phenocrysts with complex zoning patterns can of magma chambers have demonstrated that a magma be divided into four types. Type 1 is characterized by an extremely chamber solidifies through formation of a mushy boundAn-rich core (An71–90), which is commonly corroded and filled with ary layer composed of crystals and interstitial melt along sodic plagioclase. Type 2 has an Ab-rich core surrounded by a calcic the chamber walls (e.g. Brandeis & Jaupart, 1986; Turner mantle characteristic of partial dissolution. Type 3 is characterized by et al., 1986; Marsh, 1988; Tait & Jaupart, 1992). Layered igneous complexes (e.g. Morse, 1969; Shirley, 1987; an Ab-rich core that is commonly surrounded by a reversely zoned McBirney, 1996) provide useful information on how slightly calcic mantle. Type 4 has an Ab-rich core without a calcic mafic magma chambers evolve and consolidate, but they mantle. The An-rich cores of the Type 1 plagioclase are too calcic do not offer much insight into the instantaneous state of to have crystallized from a liquid represented by any whole-rock a chamber, because those intrusions integrate processes composition at the estimated temperatures of the Kutsugata magma that occurred over long solidification times (McBirney & system. The An-rich cores are inferred to have been formed in a Noyes, 1979; Tait, 1988). Volcanic products, such as mushy boundary layer along the wall of a magma chamber, where lava flows and tephra deposits, can sample the inthe magma was relatively cool and rich in water because of significant stantaneous state of a magma chamber. There are a few olivine fractionation and possibly because of addition of water studies along this line of approach (Tait, 1988; de Silva, expelled from the chilled margin and/or groundwater from the 1989; Nakada et al., 1994). These studies focused on surrounding crust. The Ab-rich cores of Types 2–4 plagioclase crystal aggregates in volcanic products and concluded phenocrysts are interpreted to have crystallized in the main magma that the crystal formation in a mushy boundary layer body of the reservoir. The Types 2 and 3 Ab-rich cores were brought plays a dominant role in magmatic evolution in a magma to the mushy layer by convection. The temporal and spatial variations reservoir. However, these studies were mostly on silicic of zoning patterns of plagioclase phenocrysts in basaltic lava flows magmas and there are significant differences in physical give useful information on the evolution of mafic magma chambers. and chemical features between silicic and mafic magma chambers. In this paper, the magmatic evolution of an alkali basalt (Kutsugata lava) from Rishiri Volcano, northern Japan, is studied. The Kutsugata lava is divided into three lava flows on the basis of whole-rock chemical