Showing papers on "Phenocryst published in 1999"

Structure and petrology of Tauride ophiolites and mafic dike intrusions (Turkey): Implications for the Neotethyan ocean

Abstract: Cretaceous Neotethyan ophiolites occur in four east-west–trending subparallel zones within the Tauride tectonic belt in southern Turkey The ophiolites of the Inner, Intermediate, and Outer zones tectonically overlie the Mesozoic platform carbonates of the Tauride belt and are commonly underlain by a Cenomanian ophiolitic melange These ophiolites consist mainly of tectonized mantle rocks, mafic-ultramafic cumulates, and gabbros, and commonly lack sheeted dike complexes and extrusive rocks of a complete ophiolite sequence Metamorphic soles that are several hundred meters thick occur as thrust-faulted slices beneath these ophiolites and show well-developed metamorphic field gradients Ophiolitic units and the metamorphic soles are intruded by mafic dike swarms that are truncated at the contact with the underlying melange unit Dike rocks are made of subalkalic basalt to andesite typical of evolved island-arc tholeiites; they display large compositional variations, with SiO 2 content between 50 and 60 wt% and MgO between 8 and 4 wt%, and contain higher Ti augite phenocrysts and significantly less calcic plagioclase than their host cumulates The majority of the analyzed dike rocks show a slight depletion in light rare earth elements (REE) with low La/SmN ratios and are depleted in both high-field strength (HFS) and heavy REEs, while enriched in large-ion-lithophile elements (LILE) relative to normal mid-ocean ridge basalt (MORB) These characteristics suggest a mantle source that underwent previous melt extractions and subsequent metasomatism by LILE- and light REE-enriched fluids Geochemical modeling of trace elements shows that melting occurred at relatively low pressures under hydrous conditions and that it may have required the existence of an asthenospheric window, in which the dike magmas developed through tapping and mixing of melts generated within a rising melting column starting slightly within the garnet stability field, or in a transitional zone between the garnet and spinel stability fields at about 60 km depth This asthenospheric window was probably created during subduction of a Neotethyan ridge system; magmas ascending from the melt column within this window generated dikes that crosscut the metamorphic soles and were injected into the overlying mantle wedge and oceanic lithosphere The new 40 Ar/ 39 Ar hornblende dates of 92–90 Ma and 90–91 Ma from the metamorphic soles and dike swarms, respectively, show that evolution of these two geologic units was closely related in time and space and that they formed at the same intraoceanic subduction zone within the Inner Tauride seaway These data suggest that the Tauride ophiolites within the three zones to the north originated from the same root zone situated north of the Tauride carbonate platform, and that they constitute remnants of a single ophiolitic nappe sheet derived from the Inner Tauride seaway within the Neotethyan ocean

A Complex Magma Mixing Origin for Rocks Erupted in 1915, Lassen Peak, California

Abstract: The eruption of Lassen Peak in May 1915 produced four volcanic INTRODUCTION rock types within 3 days, and in the following order: (1) hybrid Magma mixing is an important process among interblack dacite lava containing (2) undercooled andesitic inclusions, mediate magmas in volcanic arcs (e.g. Turner & (3) compositionally banded pumice with dark andesite and light Campbell, 1986; Philpotts, 1990, and references therein). dacite bands, and (4) unbanded light dacite. All types represent Laboratory studies of synthetic analogs of silicate magma stages of a complex mixing process between basaltic andesite systems and mathematical modeling of mixing viscous and dacite that was interrupted by the eruption. They contain liquids have suggested a range of possible mixing mechdisequilibrium phenocryst assemblages characterized by the coanisms (e.g. Cashman & Bergantz, 1991). However, use existence of magnesian olivine and quartz and by reacted and of these results to interpret magmatic systems is limited unreacted phenocrysts derived from the dacite. The petrography and by the applicability of synthetic analogs to magmatic crystal chemistry of the phenocrysts and the variation in rock conditions and by the imprecise knowledge of the effective compositions indicate that basaltic andesite intruded dacite magma viscosity of silicate liquid–crystal mixtures at magmatic and partially hybridized with it. Phenocrysts from the dacite magma temperatures and pressures. If magmas mix incompletely, were reacted. Cooling, crystallization, and vesiculation of the hybrid i.e. mingle, features such as compositional banding or andesite magma converted it to a layer of mafic foam. The decreased undercooled inclusions are usually apparent, but if they density of the andesite magma destabilized and disrupted the foam. mix completely, mineralogical disequilibrium may be the Blobs of foam rose into and were further cooled by the overlying dacite magma, forming the andesitic inclusions. Disaggregation of only direct evidence for a mixing origin. andesitic inclusions in the host dacite produced the black dacite and If the thermal and compositional contrasts between light dacite magmas. Formation of foam was a dynamic process. two magmas are great and the ratio of silicic to mafic Removal of foam propagated the foam layer downward into the magma is large, there is little interaction between them, hybrid andesite magma. Eventually the thermal and compositional and the mafic magma is undercooled (e.g. Bacon, 1986). contrasts between the hybrid andesite and black dacite magmas were Many undercooled inclusions contain reacted phereduced. Then, they mixed directly, forming the dark andesite magma. nocrysts inherited from their host silicic magma (Heiken About 40–50% andesitic inclusions were disaggregated into the & Eichelberger, 1980), and Bacon (1986) pointed out host dacite to produce the hybrid black dacite. Thus, disaggregation that undercooled inclusions are typically formed from of inclusions into small fragments and individual crystals can be hybrid magmas. In general, the formation of undercooled an efficient magma-mixing process. Disaggregation of undercooled inclusions retards further mixing (Sakuyama, 1984; inclusions carrying reacted host-magma phenocrysts produces coThompson & Dungan, 1985; Sparks & Marshall, 1986; existing reacted and unreacted phenocryst populations. Koyaguchi & Blake, 1991). However, fragmentation and/ or disaggregation of undercooled inclusions plays an important role in hybridization in some magma systems (Thompson & Dungan, 1985; Coulon et al., 1986; Clynne & Christiansen, 1987; Clynne, 1989; Linneman & Myers,

Gradients in H2O, CO2, and exsolved gas in a large-volume silicic magma system: Interpreting the record preserved in melt inclusions from the Bishop Tuff

Abstract: Infrared spectroscopic analyses of ∼140 melt inclusions in quartz phenocrysts from the zoned Bishop rhyolitic tuff demonstrate that systematic gradients in dissolved magmatic H2O and CO2 concentrations were present during preemptive crystallization of the magma body. Melt inclusions from the earliest erupted samples contain lower H2O (5.3±0.4 wt %) and CO2 (62±37 ppm) than inclusions from the middle of the eruption (5.7±0.2 wt % H2O; 120±60 ppm CO2). Melt inclusions from late erupted samples have much lower H2O (4.1±0.3 wt %) and higher and variable CO2 (150–1085 ppm). Trace element analyses of melt inclusions by ion microprobe show that inclusions within single pumice clasts from the early and middle Bishop Tuff have an inverse correlation between CO2 and incompatible elements. This pattern indicates that the magma was gas-saturated during crystallization, with CO2 partitioning into a coexisting gas phase. Quantitative modeling using H2O-CO2 solubility relations reveals a preeruptive gradient in exsolved gas, with gas contents varying from ∼1 wt % in the deeper regions of the magma body to nearly 6 wt % near the top. Dissolved Cl, B, Li, and Be in melt inclusions correlate negatively with CO2. Mass balance modeling of Cl loss to exsolving H2O-rich gas during crystallization provides strong corroborating evidence for the mass fractions of exsolved gas estimated from H2O, CO2, and trace element data. Pressures of quartz crystallization and melt inclusion entrapment calculated from inclusion H2O-CO2 data are consistent with progressive downward tapping of a zoned magma body during the eruption. Melt inclusion gas saturation pressures, magma volume estimates, and time-stratigraphic-compositional relations suggest that early erupted magma was stored at the top of a downward widening magma body. Melt inclusion data and the inferred gradients in dissolved H2O, CO2 and exsolved gas in the Bishop magma body suggest that gas saturation plays an important role in the formation and subsequent preservation of compositional gradients in silicic magma reservoirs.

Magmatic interactions as recorded in plagioclase phenocrysts of Chaos Crags, Lassen Volcanic Center, California

Abstract: The silicic lava domes of Chaos Crags in Lassen Volcanic National and mingling of this material into the silicic host. These processes Park contain a suite of variably quenched, hybrid basaltic andesite are commonplace in some orogenic magma systems and may be magmatic inclusions. The inclusions represent thorough mixing elucidated by isotopic microsampling and analysis of the plagioclases between rhyodacite and basalt recharge liquids accompanied by some crystallizing from them. mechanical disaggregation of the inclusions resulting in crystals mixing into the rhyodacite host preserved by quenching on dome emplacement. Sr/Sr ratios (~0·7037–0·7038) of the inclusions are distinctly lower than those of the host rhyodacite

Plagioclase residence times at two island arc volcanoes (Kameni islands, Santorini, and Soufriere, St. Vincent) determined by Sr diffusion systematics

Abstract: The diffusive relaxation of trace element profiles in plagioclase phenocrysts may provide important constraints on magma residence times in crustal magma chambers. Initial trace element profiles in plagioclase phenocrysts are governed by variations in the concentration of a trace element in the melt and by the plagioclase-melt partition coefficient. Trace element diffusion will subsequently act to modify this initial profile and – given enough time – produce a profile that is in equilibrium with the anorthite variations within the crystal. We argue that the trace element partition coefficient Da/b between two parts a and b of a plagioclase crystal of variable anorthite content is equal to the ratio of their crystal-liquid partition coefficients, and that the equilibrium profile of the crystal can be calculated. The time required to establish diffusive equilibrium is dependent on the wavelength and amplitude of the initial trace element concentration range and on the diffusivity of the trace element in plagioclase. Strontium plagioclase-melt partition coefficients and diffusivities are calculated for a range of magmatic temperatures and plagioclase compositions. A one-dimensional diffusion model is developed that describes the diffusive destruction of oscillatory trace element zoning with time and allows the calculation of upper limits for plagioclase crystal residence times in a magma reservoir. The model is tested using major and trace element concentrations measured along crystal traverses of plagioclase phenocrysts from the Kameni Island dacites, Santorini, and from the 1979 Soufriere andesite, St. Vincent. Three out of eight plagioclase phenocrysts have Sr concentration profiles that are not in diffusive equilibrium. For these, the diffusion model is employed to calculate maximum crystal residence times from incomplete diffusive equilibration of trace element zoning in plagioclase. Maximum crystal residence times range from 100 to 450 years. This is in good agreement with estimates from crystal size distribution and from Ra-Th disequilibrium studies for the Kameni Islands. For Soufriere, however, such short residence times are incompatible with U-Th mineral errorchron data that suggest residence times of >40 ka in a thermally buffered magma reservoir. To reconcile these apparently different ages, we invoke a more complicated magmatic history for Soufriere where an initially buffered magma reservoir is disturbed by magma mixing and suffers limited additional crystal fractionation prior to eruption.

Evidence for Fractional Crystallization of Periodically Refilled Magma Chambers in Tenerife, Canary Islands

Abstract: Major and trace element data on basanitic to phonolitic lavas of INTRODUCTION different ages and from different parts of Tenerife (Canary Islands), Melts in equilibrium with Mg-rich olivine (Fo[ 88) are and their mafic silicates have been used to obtain more detailed generally assumed to have suffered little or no modiinformation about processes taking place in crustal magma chambers fication by fractionation processes and to have chemical associated with ocean island magmatism in Tenerife. Clinopyroxene characteristics close to those of primary magmas. Ratios phenocrysts in basanitic to phonolitic lavas consist of diopside–salite of incompatible trace elements in such magmas are (referred to as Al-salite) with alternating normal and reverse zoning, assumed to be inherited from their mantle sources and and commonly contain a rounded or corroded core of homogeneous Naare frequently used to shed light on the trace element rich diopside–salite (referred to as Na-salite). In general Al-salite characteristics of these sources (e.g. Weaver, 1991). Until contains lower amounts of rare earth elements (REE) and Y, and more recently much of the attention given to ocean island Mg, Al, Ti, Cr, Sc and Ni than Na-salite. Variations in trace element basalts was focused on their mantle sources (e.g. Zindler concentrations and ratios are only weakly related to variations in mg& Hart, 1986; Weaver, 1991; Chauvel et al., 1992; number. Petrographic and compositional relations among the lavas and Hofmann, 1997; Sims & DePaolo, 1997). mafic silicates are interpreted as the results of fractional crystallization However, a number of recent papers have shown that in periodically refilled magma chambers (FCM processes). The FCM ocean island magmas may undergo complex processes processes took place at temperatures of 1040–1260°C and pressures such as fractional crystallization at different depths, crystal of about 0·2–0·5 GPa; that is, partly within the old oceanic crust and partly within the overlying sequence of Canary Islands lavas. FCM accumulation, magma mixing, contamination through processes may lead to significant fractionation between incompatible trace assimilation of hydrothermally altered basement, conelements whose ratios in mafic magmas are used to characterize their tamination by oceanic sediments, and contamination by mantle source(s). Melts subjected to FCM processes will, furthermore, melts generated in mantle lithosphere (e.g. Ablay et al., produce significantly larger masses of cumulates than melts of similar 1998; Class et al., 1998; Davis et al., 1998; Garcia et al., mg-number that have only been subjected to simple fractional crys1998; Gee et al., 1998; Hoernle, 1998). Before conclusions tallization. are drawn about the mantle sources of a given series of volcanic rocks, it is important to have detailed information about the shallow-level processes to which the magmas have been subjected, and the degree to which these processes have affected the trace element characteristics

Petrologic investigations of the 1995 and 1996 eruptions of Ruapehu volcano, New Zealand: formation of discrete and small magma pockets and their intermittent discharge

Abstract: Ruapehu volcano erupted intermittently between September and November 1995, and June and July 1996, producing juvenile andesitic scoria and bombs. The volcanic activity was characterized by small, sequential phreatomagmatic and strombolian eruptions. The petrography and geochemistry of dated samples from 1995 (initial magmatic eruption of 18 September 1995, and two larger events on 23 September and 11 October), and from 1996 (initial and larger eruptions on 17–18 June) suggest that episodes of magma mixing occurred in separate magma pockets within the upper part of the magma plumbing system, producing juvenile andesitic magma by mixing between relatively high (1000–1200 °C)- and low (∼1000 °C)- temperature (T) end members. Oscillatory zoning in pyroxene phenocrysts suggests that repeated mixing events occurred prior to and during the 1995 and 1996 eruptions. Although the 1995 and 1996 andesitic magmas are products of similar mixing processes, they display chronological variations in phenocryst clinopyroxene, matrix glass, and whole-rock compositions. A comparison of the chemistry of magnesian clinopyroxene in the four tephras indicates that, from 18 September through June 1996, the tephras were derived from at least two discrete high-temperature (high-T) batches of magma. Crystals of magnesian clinopyroxene in the 23 September and 11 October tephras appear to be derived from different high-T magma batches. Whole-rock and matrix-glass compositions of all tephras are consistent with their derivation from distinct mixed melts. We propose that, prior to 1995 there was a shallow low-temperature (low-T) magma storage system comprising crystal-rich mush and remnant magma from preceding eruptive episodes. Crystal clots and gabbroic inclusions in the tephras attest to the existence of relict crystal mush. At least two discrete high-T magmas were then repeatedly injected into the mush zone, forming discrete and mixed magma pockets within the shallow system. The intermittent 1995 and 1996 eruptions sequentially tapped these magma pockets.

Pliocene Potassic Magmas from the Kings River Region, Sierra Nevada, California: Evidence for Melting of a Subduction- Modified Mantle

Abstract: During the Late Pliocene, absarokite and minette magmas (43–57 INTRODUCTION wt % SiO2) erupted along the western slope of the Sierra Nevada, A genetic relationship between potassic magmas and California, within the Kings River drainage. The absarokites contain subduction zones has long been recognized (e.g. Johnson phenocrysts of olivine ± augite, whereas the minettes contain et al., 1978; Peccerillo, 1985; Rogers & Hawkesworth, phlogopite + augite ± olivine; both are distinguished by a lack 1985). Trace element enrichments in large ion lithophile of feldspar phenocrysts. Pre-eruptive magmatic temperatures and elements (LILE) and light rare earth elements (LREE) pressures for a felsic and mafic minette are 1138 and 1144 combined with relative depletions in the high field (±50)°C, and 12 and 16 (±4) kbar, respectively. These magmas strength elements (HFSE) are common to arc lavas (e.g. are characterized by extreme enrichments in the large ion lithophile Gill, 1981). The eruption of potassic and ultrapotassic elements (e.g. 1·9–8·1 wt % K2O, 1380–3719 ppm Ba), magmas with an extreme arc trace element signature depletions in high field strength elements (Ba/NbPM of 7–33), and often post-dates active subduction and occurs synhigh oxygen fugacities (1–3 log units above the Ni–NiO buffer). chronously with uplift, extension or strike-slip motion (e.g. Trace element ratios (e.g. Ba/Rb 20–100) are distinct from those Sloman, 1989). This relationship indicates that chemical observed for mid-ocean ridge basalt and ocean island basalt. heterogeneities, produced in the mantle during subVariations in K and Ba with respect to other incompatible elements duction via modal (the formation of new minerals) and/ require that phlogopite ± potassic amphibole was an important or cryptic (enrichment of pre-existing minerals in inresidual phase during magma generation. The buoyant ascent of compatible elements) metasomatism, can exist for subthe Kings River magmas through ~40 km of sialic crust requires stantial periods of time after subduction has ceased; pre-eruptive volatile concentrations (H2O and F) >2 wt %. subsequent tectonic or thermal events may then trigger Volcanism probably was triggered as part of the regional response further magma generation ( Johnson et al., 1978; Roden, to Basin and Range extension, which resulted in asthenospheric 1981; Rogers et al., 1987; Mauger, 1988; Sloman, 1989). upwelling and therefore higher heat flow to the subduction-modified On the western slope of the Sierra Nevada, small lithosphere. volume eruptions of potassic and ultrapotassic lavas have

Sr isotope disequilibrium during differentiation of the Bandelier Tuff: Constraints on the crystallization of a large rhyolitic magma chamber

Abstract: The 1.61 Ma Otowi Member of the Bandelier Tuff (Jemez Mountains volcanic field, New Mexico) is a caldera-forming high-silica rhyolitic ignimbrite with a precursor fallout deposit. The Otowi Member has high Rb/Sr ratios; the 87 Rb/ 86 Sr ratios from sanidines and glasses range from 14 to 570. Most sanidines from glomerocrysts in the ignimbrite have 87 Sr/ 86 Sr I = 0.7052–0.7056, whereas the ratios from glasses from glomerocrysts range from 0.7052 to 0.7079. Quartz phenocrysts containing glass inclusions from both the initial fallout and the ignimbrite are markedly more radiogenic, at 87 Sr/ 86 Sr I = 0.7105–0.7113, despite having much lower Rb/Sr ratios than glomerocryst glasses. These relations require that the inclusion glasses are more contaminated with Proterozoic country rock than are glomerocryst glasses. Textural, isotopic, and trace element data support a model in which crystals grow in a boundary layer with the most inclusion-rich quartz grains closest to the magma–country rock contact. Phenocrysts and glomerocrysts represent fragments from different zones of the chamber9s crystalline carapace, disseminated throughout the magma prior to eruption. An important implication of these results is that glass inclusions do not necessarily represent precursor magma compositions; hence extrapolation of measured volatile contents of inclusion glasses to the entire volume of an erupted magma should be approached with caution.

Local and regional variation of MORB parent magmas: evidence from melt inclusions from the Endeavour Segment of the Juan de Fuca Ridge

Abstract: The development of petrogenetic models of igneous processes in the mantle is dependent on a detailed knowledge of the diversity of magmas produced in the melting regime. These primary magmas, however, undergo significant mixing and fractionation during transport to the surface, destroying much of the evidence of their primary diversity. To circumvent this problem and to determine the diversity of melts produced in the mantle, we used melt inclusions hosted in primitive plagioclase phenocrysts from eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour Segment, Juan de Fuca Ridge. This area was selected for study because of the demonstrated close association of enriched (E-MORB) lavas and incompatible element enriched depleted (N-MORB) lavas. Rehomogenized melt inclusions from E-MORB, T-MORB, and N-MORB lavas have been analyzed by electron and ion microprobe for major and trace elements. The depleted and enriched lavas, as well as their melt inclusions, have very similar compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion compositions are more primitive than, yet collinear with, the host lava suites. In contrast, the minor and trace element characteristics of melt inclusions from depleted and enriched lavas are different both in range and absolute concentration. N-MORB lavas contain both depleted and enriched melt inclusions, and therefore exhibit the largest compositional range (K2O: 0.01 to 0.4 oxide wt%, P2O5: <0.01 to 0.2 oxide wt%, LaN: 7 to 35, YbN: 1 to 13, and Ti/Zr: <100 to 1300). E-MORB lavas contain only enriched inclusions, and are therefore relatively homogeneous (K2O: 0.32 to 0.9 oxide wt %, P2O5: 0.02 to 0.35 oxide wt%, LaN: 11 to 60, YbN: 4 to 21, and Ti/Zr: ∼100). In addition, the most primitive E-32 inclusions are similar in composition to the most enriched inclusions from the depleted hosts. Major element data for melt inclusions from both N-MORB and E-MORB lavas suggest that the magmas lie on a low pressure cotectic, consistent with a petrogenesis including fractional crystallization. However, the minor and trace element compositions in melt inclusions vary independently of the major element composition implying an alternative history. When fractionation-corrected, inclusion compositions correlate with their host glass composition. Hence, the degree of enrichment of the lavas is a function of the composition of aggregated melts, not of processing in the upper mantle or lower crust. Based on this fact, the lava suites are not produced from a single parent magma, but from a suite of primary magmas. The chemistry of the melt inclusions from the enriched lavas is consistent with a derivation from variable percentages of partial melting within the spinel stability field by a process of open system (continuous or critical) melting assuming a depleted lherzolite source veined with clinopyroxenite. The low percentage melts are dominantly enriched melts of the clinopyroxenite. In contrast, the depleted lavas were created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba and the LREE. Such a source was likely melted up to or past the point at which all of its clinopyroxene was consumed. This set of characteristics is consistent with a scenario by which diverse melts produced at different depths travel through the melting regime to the base of the crust without homogenizing en route. The homogeneous major element characteristics are created in the lower crust by fractional crystallization and reaction with lower crustal gabbros. Therefore, the degree of decoupling between major and trace element characteristics of the melt inclusions (and lavas) is dictated by the reaction rate of the melts with the materials in the conduit walls, as well as the residence times and flux rate, in the upper mantle and lower crust.

Chemical zoning and crystallization mechanisms in the magma chamber of the Pomici di Base plinian eruption of Somma-Vesuvius (Italy)

Abstract: Products of the Pomici di Base plinian eruption of Somma-Vesuvius consist of pumice and scoria fall deposits overlain by lithic-rich phreatomagmatic deposits. The plinian fall, which represents most of the magma volume involved in the eruption, ranges in composition from trachyte (SiO2 = 62.5 wt%) to latite (SiO2 ≈ 58 wt%) in the lower one-third of the deposit, whereas the upper two-thirds of the total thickness consists of latitic scoriae with fairly uniform composition (SiO2 ≈ 55–56 wt%). All the products have very low content of phenocrysts (from 4 wt% in trachyte pumice to 1 wt% in the latite scoriae), most of which are not in equilibrium with the host rock. Minerals not in equilibrium, both in trachytic and latitic rocks, consist of discrete crystals of sanidine and plagioclase wetted by trachytic glass and felsic aggregates with interstital trachytic glass. Trends of major and trace elements are consistent with crystal-liquid fractionation processes and rule out syn-eruptive mixing processes between latitic and trachytic magmas. We suggest that discrete crystals and crystal aggregates not in equilibrium with the host rock represent fragments of the crystallising boundary layer at the upper walls of the magma chamber, which were wrenched and admixed into the magma during the ascent. This process diversifies the mineral assemblage and increases the crystal content of the rocks. We propose that diffusive crystallization processes operating at the wall of the chamber allowed the formation of a two-fold layered reservoir with a more mafic, homogeneous lower body and a more evolved, compositionally graded upper body. Around one-quarter of crystals adhering to the upper part of the magma chamber were admixed into the magma during the eruption. The absence of significant syn-eruptive mixing processes and the major role played by diffusive crystallization are consistent with a low aspect ratio magma chamber (width/height <1).

Mingling in the magmatic system of ischia (Italy) in the past 5 ka

Abstract: In the past 5 ka volcanism has been very intense at Ischia, a densely populated and still active volcanic island in the Gulf of Napoli. Mineralogical, geochemical and isotopic investigations have been carried out on the pyroclastic deposits and lava flows of the historical eruptions of Fiaiano (530–960 A. D.) and Arso (1302 A. D.). These products are characterized by strong petro-chemical disequilibria. Mineralogical disequilibria are shown by the occurrence of resorbed and reversely zoned phenocrysts, and by the coexistence of phenocrysts and microlites with variable composition. Chemical inhomogeneity characterizes the glass of the juvenile fragments. Isotopic disequilibria have been detected among all separated mineral phases and whole-host rock. The detected disequilibria are interpreted as the result of a physical mixing process between variable fractionated and isotopically distinct batches of magmas. Petrochemical data suggest that mingling occurred between a highly crystallized alkali-trachytic and an isotopically distinct latitic to trachytic magmas, for both the eruptions. The pre-eruptive scenario proposed is the uprising of a latitic-to-trachytic magma (characterized by a Sr-isotope ratio of about 0.7068) from a deep reservoir and successive mingling with batches of more viscous, highly crystallized alkali-trachytic magma (with a Sr-isotope composition of about 0.7061) at shallow depth.

Ascent and eruption of a lunar high-titanium magma as inferred from the petrology of the 74001/2 drill core

Abstract: — An analysis of the orange glasses and crystallized beads from the 68 cm deep 74001/2 core has been conducted to understand the processes occurring during ascent and eruption of the Apollo 17 orange glass magma. Equilibrium between melt and metal blebs (Fe85Ni14Co1) within the core, along with Cr contents in olivine phenocrysts, suggest there was an oxidation of C and a reduction of the melt at an O fugacity of IW-1.3 and 1320 °C to form CO gas at 200 bars or ∼4 km depth. This was followed by development of more oxidized conditions during ascent. Also during ascent, there was formation of euhedral, homogeneous Fo81 olivine crystals and spinel crystals with higher Al and Mg contents than the smaller spinels in the crystallized beads. Both the metal blebs and Al-rich spinels were trapped inside the Fo81 olivine phenocrysts as they grew prior to eruption. The composition of the orange glasses are homogeneous throughout the core, except for a few distinct glasses at the top that appear to have been mixed in by micrometeorite reworking. A few glassy melt inclusions of orange glass composition trapped in the Fo81 phenocrysts contain 600 ± 100 ppm S and ∼50 ppm Cl compared to the 200 ppm S and 50 ppm Cl in the orange glass melt when quenched. These inclusions therefore document the addition of 400 ppm S to the CO-rich volcanic gas during the eruption. The size and distribution of different volcanic beads in the Apollo 17 deposit indicate a mode of eruption in which the orange glasses and partially crystallized beads formed further away from the volcanic vent where cooling rates were faster. Progressively larger and more numerous crystals in the black beads reflect slower cooling rates at higher optical densities in the volcanic plume. The development of a brown texture in the orange glasses at the bottom of the core, where the black beads dominate, is interpreted to result from devitrification by subsolidus heating either as the orange glasses fell back through the hot plume or after deposition on the surface. The change from domination by orange glasses to black beads in the core probably reflects a decrease in gas content over time, which consequently would increase the plume optical density and favor slower cooling rates.

Volcanism in a late Variscan intramontane trough: the petrology and geochemistry of the Carboniferous and Permian volcanic rocks of the Intra-Sudetic Basin, SW Poland

Abstract: The Carbonifeorus-Permian volcanic rocks of the Intra-Sudetic Basin represent products of late- to post-collisional volcanism associated with extension within the eastern part of the Variscan belt of Europe. The volcanic succession is subdivided into the older, calc-alkaline suite (the early and late Carboniferous) and the younger, mildly alkaline suite (the late Carboniferous and early Permian). The rhyodacites with subordinate basaltic andesites and andesites of the older suite show convergent plate margin affinities. The rhyolitic tuffs, rhyolites with less widespread trachyandesites and basaltic trachyandesites of the younger suite are largely characterised by within-plate affinities, with some gradations towards convergent plate margin affinities. This geochemical variation compares well with that found in some Tertiary-Recent extensional settings adjacent to former active continental margins (e.g. the Basin and Range province of the SW USA). The parental magmas for each suite of the Intra-Sudetic Basin possibly originated from similar, garnet free mantle sources at relatively shallow depths (within the subcontinental mantle?), but at variable degrees of partial melting (lower for the mildly alkaline rocks). The convergent plate margin-like geochemical signatures of the volcanic rocks may either have been inherited from their mantle sources, or be related to the assimilation of crustal rocks by the ascending and fractionating primary magmas. The intermediate-acidic rocks within each suite mainly originated due to fractional crystallisation of variable mineral assemblages equivalent to the observed phenocrysts (mainly plagioclase and pyroxenes, with hornblende and biotite in the calc-alkaline suite, and K-feldspar in the mildly alkaline suite). The trace element patterns of the volcanic rocks were also strongly influenced by fractionation of accessory minerals, such as spinels, ilmenite, zircon, apatite and others. The petrographic evidence (e.g. quartz phenocrysts with reaction rims, complexly zoned or sieve-textured feldspar phenocrysts) suggests that assimilation and/or magma mixing processes might also have taken place during the evolution of the magmas.

Mineral chemistry of submarine lavas from Hilo Ridge, Hawaii: implications for magmatic processes within Hawaiian rift zones

Abstract: The crustal history of volcanic rocks can be inferred from the mineralogy and compositions of their phenocrysts which record episodes of magma mixing as well as the pressures and temperatures when magmas cooled. Submarine lavas erupted on the Hilo Ridge, a rift zone directly east of Mauna Kea volcano, contain olivine, plagioclase, augite ±orthopyroxene phenocrysts. The compositions of these phenocryst phases provide constraints on the magmatic processes beneath Hawaiian rift zones. In these samples, olivine phenocrysts are normally zoned with homogeneous cores ranging from ∼ Fo81 to Fo91. In contrast, plagioclase, augite and orthopyroxene phenocrysts display more than one episode of reverse zoning. Within each sample, plagioclase, augite and orthopyroxene phenocrysts have similar zoning profiles. However, there are significant differences between samples. In three samples these phases exhibit large compositional contrasts, e.g., Mg# [100 × Mg/(Mg+Fe+2)] of augite varies from 71 in cores to 82 in rims. Some submarine lavas from the Puna Ridge (Kilauea volcano) contain phenocrysts with similar reverse zonation.

Quartz and feldspar microstructures in metamorphic rocks

Abstract: The shapes of quartz and feldspar grains in metamorphic rocks are among the most reliable criteria for determining parental rock-types. Rational faces and elongate crystals of feldspar, especially with oscillatory zoning, indicate an igneous precursor, and residual faces and embayments in quarlz indicate a volcanic precursor Simple twinning in K-feldspar indicates a magmatic origin, and aligned crystals of feldspar indicate magmatic flow. Quartz and plagioclase inciusions are useful for distinguishing between phenocrysts and porphyroblasts of K-feldspar in metamorphic terranes. K-feldspar phenocrysts are characterized by zonally arranged inclusions, whereas K-feldspar porphyroblasts are characterized by spherical inclusions ofquartz and plagioclase, either at random or arranged in ffails that reflect an overgrown foliation Inclusions of quartz and feldspar tend to be spherical in metamorphic porphyroblasts (e.g., staurolite), even where the boundary between the porphyroblast and quarlz or feldspar in the matrix is a rational face, which may be due to absence of fluid along the hosrinclusion boundary, compared with its accumulation along the advancing porphyroblast-matrix boundary The following microstructural criteria, preserved best in less deformed migmatites,indicateanatecticleucosome (1)CrystalfacesofK-feldsparorplagioclasemayoccuragainstquarlz (2)Inclusion trails are absent, in contrast to grains of the same minerals in the mesosome (3) Overgrowths free of inclusion trails may occur on minerals with inclusion trails (a.g., K-feldspar, cordierite). (4) Simple twinning may occur in K-feldspar, which appears to be diagnostic of crystallization of K-feldspar in a melt, rather than in the solid state.

Enrichment of basalt and mixing of dacite in the rootzone of a large rhyolite chamber: inclusions and pumices from the Rattlesnake Tuff, Oregon

Abstract: A variety of cognate basalt to basaltic andesite inclusions and dacite pumices occur in the 7-Ma Rattlesnake Tuff of eastern Oregon. The tuff represents ∼280 km3 of high-silica rhyolite magma zoned from highly differentiated rhyolite near the roof to less evolved rhyolite at deeper levels. The mafic inclusions provide a window into the processes acting beneath a large silicic chamber. Quenched basaltic andesite inclusions are substantially enriched in incompatible trace elements compared to regional primitive high-alumina olivine tholeiite (HAOT) lavas, but continuous chemical and mineralogical trends indicate a genetic relationship between them. Basaltic andesite evolved from primitive basalt mainly through protracted crystal fractionation and multiple cycles (≥10) of mafic recharge, which enriched incompatible elements while maintaining a mafic bulk composition. The crystal fractionation history is partially preserved in the mineralogy of crystal-rich inclusions (olivine, plagioclase ± clinopyroxene) and the recharge history is supported by the presence of mafic inclusions containing olivines of Fo80. Small amounts of assimilation (∼2%) of high-silica rhyolite magma improves the calculated fit between observed and modeled enrichments in basaltic andesite and reduces the number of fractionation and recharge cycles needed. The composition of dacite pumices is consistent with mixing of equal proportions of basaltic andesite and least-evolved, high-silica rhyolite. In support of the mixing model, most dacite pumices have a bimodal mineral assemblage with crystals of rhyolitic and basaltic parentage. Equilibrium dacite phenocrysts are rare. Dacites are mainly the product of mingling of basaltic andesite and rhyolite before or during eruption and to a lesser extent of equilibration between the two. The Rattlesnake magma column illustrates the feedback between mafic and silicic magmas that drives differentiation in both. Low-density rhyolite traps basalts and induces extensive fractionation and recharge that causes incompatible element enrichment relative to the primitive input. The basaltic root zone, in turn, thermally maintains the rhyolitic magma chamber and promotes compositional zonation.

Systematics of Ni and Co in olivine from planetary melt systems; lunar mare basalts

Abstract: The systematics of Co and Ni in olivine from six Apollo 12 olivine basalts were studied by SIMS techniques and correlated with major and minor element data obtained with the electron microprobe. Our results, together with previous studies, demonstrate that one of these basalts (12009) was extruded, as a liquid, onto the lunar surface and was parental to five other cumulates (12075, 12020, 12018, 12040, and 12035). The concentrations of Ni in zoned olivine phenocrysts behave as expected for a highly compatible element with initial estimated D Ni = 9.9. On the other hand. Co concentrations in olivine vary hardly at all and show flat patterns across crystals that retain normal zoning trends for Mg, Fe, Mn, and Ni. The explanation for this behavior is not that D Co nearly equal 1 (our estimated D Co = 4) or that Co zoning has been erased by rapid diffusion of Co compared to the other elements (e.g., Mg and Fe) that still show normal zoning. The explanation for the behavior, as originally suggested by Kohn et al. (1989), is that the increase in D Co with crystallization exactly balances the depletion of Co in the melt. This decoupling of the behavior of Ni and Co in olivine results in significant increases in Co/Ni with crystallization.