Showing papers on "Phenocryst published in 2002"

Physical conditions, structure, and dynamics of a zoned magma chamber: Mount Pelée (Martinique, Lesser Antilles Arc)

Abstract: [1] Experimental results and petrologic observations of the eruptive history of Mount Pelee are integrated, and a model for the magma storage system is presented. Recent (stage 3) Plinian and Pelean activity (P1, 650 years B.P.; 1902, 1929) erupted relatively homogeneous andesites (average 62 wt % SiO2). They are porphyritic (35–58 vol % crystals) and contain phenocrysts of plagioclase (Plag) (An50–90), orthopyroxene (Opx) (En52–60), and magnetite (Mt) (∼Mt70). Glasses (both interstitial and trapped) are rhyolitic (74–77 wt % SiO2). Clinopyroxene (Cpx), ilmenite (Ilm), amphibole (Amph) (mostly resorbed pargasitic hornblendes), and olivine (Ol) are present as minor phases. Products of 1902 and 1929 contain mafic enclaves (51–59 wt % SiO2) with compositions similar to basaltic andesite lavas erupted during stage 2 (40,000–19,500 years B.P.). Conditions in the andesitic part of the magma chamber, as determined from experimental phase equilibria, do not differ between the P1, 1902, and 1929 eruptions (875–900°C, 2 ± 0.5 kbar, ΔNNO = +0.4–0.8, melt H2O content of 5.3–6.3 wt %). New experimental data on a basaltic andesite composition (53 wt % SiO2) from stage 2, at 4 kbar, 950–1025°C, for melt H2O concentrations from 8.3 to 2.6 wt %, and fO2 between NNO and NNO + 4 simulate crystallization in the mafic part of the chamber. Liquidus or near-liquidus Ol, An-rich Plag, Al- and Fe3+-rich salite and augite, pargasitic hornblende, and Al- and Mg-rich Mt have compositions close to phenocrysts in mafic products from stages 2 and 3. Experimental liquids range from basaltic andesite to dacite. Application of experimentally derived mineral-melt Al/Si and Fe/Mg partition coefficients to mineral compositions from mafic lavas and cumulates from stage 2 shows that the chamber is fed by relatively evolved parental basaltic liquids (Mg # ∼ 55–60)(Mg # = Mg/(Mg + FeT). They have low temperatures (≤1050°C), high melt H2O contents (>5–6 wt %), and fO2 (ΔNNO mostly between +1 and +2) and crystallize an Ol + Cpx + Mt assemblage followed by Plag + Amph, although Amph may have started to crystallize with Ol and Cpx. Compositions of natural glasses and amphibole in mafic cumulates and lavas record a continuous evolution from basaltic-basaltic andesite to basaltic andesite-dacite liquids. Crystal fractionation of basaltic magmas is the main process controlling the chemical diversity at Mount Pelee. Crystallization in the mafic part produces an andesitic-dacitic residual liquid which subsequently evolves to produce the andesitic part. The present-day situation is typical of low fluxes of mafic magmas in comparison with stage 2.

Minor- and trace-element zoning in plagioclase: implications for magma chamber processes at Parinacota volcano, northern Chile

Abstract: Textural and compositional zoning in plagioclase phenocrysts in a sample from Parinacota volcano (Chile) was investigated using backscattered electron images and electron microprobe analysis of major and trace elements. Large (2 mm) oscillatory zoned crystals (type I) with resorption surfaces of moderate An discontinuities (⩽10% An) and decreasing trace-element contents (Sr, Mg, Ti) towards the rim reflect melt differentiation and turbulent convection in the main magma body. Early recharge with a low-Sr mafic magma is seen in the core. Small-scale Sr variations in the core indicate limited diffusion and thus residence and differentiation times of the magma shorter than a few thousand years. Smaller crystals (type II) with low trace-element/An ratio reflect the influence of an H2O-rich melt probably from a differentiated boundary layer. Closed-system in-situ crystallisation, mafic magma recharge and the role of a water-rich differentiated boundary layer can be distinguished from the An–trace element relationships. Crystals apparently move relatively freely between different parts and regimes in the magma chamber, evidence for "convective crystal dispersion". High-Sr type II crystals indicate an earlier input of Sr-rich mafic magma. Recharge of two distinct mafic magma types is thus identified (high-Sr and low-Sr), which must have been present – at increasing recharge rates with time – in the plumbing system throughout the volcano's history.

The andesite aqueduct: perspectives on the evolution of intermediate magmatism in west-central (105–99°W) Mexico

Abstract: Intermediate calc-alkaline magma (52–65% SiO2) in western-central Mexico is the focus of this paper, and the typically porphyritic andesites (57–65% SiO2) form large central volcanoes, whereas basaltic andesites (52–57% SiO2) are less porphyritic, and they are found as cones and flows but are absent from central volcanoes. Several studies of experimental phase equilibria on these lavas relate water concentration to the phenocryst assemblages and to the degree of crystallinity, so that the abundance, composition and variety of phenocrysts can be used to constrain the amount of water dissolved in the magmas. Thus, the plagioclase-rich andesites of Volcan Colima, Mexico, become so as a result of decompressional crystallisation at ~950 °C (the pyroxene phenocryst temperature), and lose their dissolved water (2.5 to 4.5 wt% H2O) which is inversely proportional to the modal abundance of plagioclase. The feeding magma to V. Colima, North America's most productive central volcano, is represented by hornblende lamprophyre, a lava type without plagioclase phenocrysts which requires at least 6 wt% water to reproduce the phenocryst assemblage. Thus, degassing of the V. Colima magmas, and of those of the other central volcanoes in the western-central Mexican volcanic belt, contributes essentially all their dissolved water to the conduit or to the atmosphere. The source of this magmatic water is related to the source of the intermediate magmas. For some this must lie in the mantle, as the incorporation of hornblende-lherzolite nodules in a hydrous andesite with hornblende phenocrysts could only have occurred while ascending through the mantle. Consistent with a mantle source is the composition of the olivine phenocrysts in Mexican lavas with 10 to 5% MgO, which is in the mantle range of Fo88–92. Accordingly, basaltic andesites and andesites with >5% MgO are candidates for a mantle source. The equilibration of intermediate magmas with the mantle, as illustrated by the experiments of various workers, requires that the magmas be hydrous at pressure. An additional constraint is that the activity of silica in the mantle must be equal to that in the hydrous magma at equilibrium. Using published and new experiments to define RTlnγSiO2 in hydrous liquids, this quantity is shown to vary as a function of liquid composition (H2O, MgO, Na2O+K2O), and it approaches zero for quartz-saturated hydrous liquids. Using appropriate values of RTlnγSiO2 for three intermediate lavas, the amount of water required to equilibrate with an olivine-orthopyroxene mantle source is calculated, and within error indicates that only the most silica-rich magma is at water saturation in the mantle, in agreement with published experimental work. Hydrous intermediate magmas, ascending from their hornblende-lherzolite source regions (~1 to 1.5 GPa) along the hydrous adiabat, may not encounter any phase boundaries until 0.2–0.4 GPa because of the increase in the thermal stability of hornblende in water-undersaturated magmas. Therefore, the phenocryst assemblages of hornblende-free andesites equilibrate at low pressures. The virtual absence of basalt in west-central Mexico ( 50% crystallinity, evidently also an eruptible limit for V. Colima andesitic lavas. If the lower limit of water dissolved in Mexican intermediate magmas is accepted as that required for phenocryst equilibration (~6 wt% water), and the upper limit as saturation in the mantle source at 1 GPa (~16 wt%) then, with an estimate of the volcanic and plutonic magma delivery rate (km3/106 year) per km of volcanic arc, the flux of water returned from the mantle along the 35,000-km, global subduction-related arc system can be estimated. Measurements of the volcanic flux are woefully few, and estimates from Mexico, the Lesser Antilles and central America show a range from 4 to 20 km3/106 year×km which, if subtracted from the isotopically constrained continental growth rate, gives the plutonic flux rate. This suggests that, of the magma flux ascending to the continental crust, only about a fifth reaches the surface. If the dissolved magmatic water limits are coupled with the volcanic and plutonic emplacement rates, then the amount of water returned by magmatism to the crust is crudely in balance with that subducted.

Temperature-induced Al -zoning in hornblendes of the Fish Canyon magma, Colorado

Abstract: An extensive electron microprobe survey of amphibole compositions in the Fish Canyon magma (2146 analyses), more than 80% of which are from high-resolution (<10 μm steps) core-to-rim traverses across large euhedral phenocrysts, provides: (1) temporal constraints on the immediately pre-eruptive P - T - f H2O evolution of the magma, and (2) a means of evaluating recent calibrations of the Al-in-hornblende barometer (Anderson and Smith 1995; hereafter AS1995) and thermometers (Blundy and Holland 1990; thermometers A and B of Holland and Blundy 1994; hereafter BH1990, HB1994TA, and HB1994TB). Hornblende phenocrysts are variable for most major elements (e.g., 5–9 wt% Al2O3 and 44–50 wt% SiO2). This compositional range is controlled by two major temperature-sensitive coupled substitutions. Approximately 50% of the total Al variation (~0.8 atoms per formula unit = apfu) is due to the edenite exchange [TSi + A□ = TAl + A(Na + K)] and another 25–30% is the consequence of a Ti-Tschermak exchange (TSi + M1–M3 Mn = TAl + M1–M3Ti). In contrast, the pressure-sensitive Al-Tschermak substitution (TSi + M1–M3 Mg = TAl + M1–M3Al) did not play a significant role, as M1–M3Al does not correlate with TAl and is always <0.2 apfu. In order to constrain the ranges of absolute P and T over which these hornblendes crystallized and to assess the sensitivity of the recent thermo-barometric algorithms of BH1990, HB1994TA (requiring silica saturation), HB1994TB (not requiring silica saturation) and AS1995, we have calculated pressures and temperatures for two selected populations of analyses wherein Al2O3 contents are within analytical error (5.95 to 6.05 wt% Al2O3, N = 78 and 7.7 to 7.8 wt% Al2O3, N = 40). The barometric formulation of AS1995 gives a mean pressure of 2.24 ± 0.05 for the high-Al population at 760 °C, which is indistinguishable from the 2.4 ± 0.5 kbar estimate of Johnson and Rutherford (1989a). A high sensitivity to temperature at low P is suggested by the geologically implausibly shallow depths calculated for the low-Al population (<1 kbar at 760 °C). The three thermometric formulations give reasonable results between 706 and 814 °C, but the HB1994TA calibration gives a mean temperature higher by ~50 °C and is more sensitive to small analytical differences (~100 °C spread for each population). HB1994TB is considered the most reliable calibration of the Al-in-hornblende thermometer as it most precisely reproduces T estimates determined by independent methods. Nine out of 14 traverses across large phenocrysts from the Fish Canyon magma display rimward increases in TAl, A(Na + K), and M1–M3Ti, compensated by decreases in TSi, and M1–M3Mn. Using the HB1994TB algorithm, the low-Al population, typical of near-core compositions, gives a mean temperature of ~715 °C, which is ~35–45 °C above the water-saturated granite solidus at 2–2.5 kbar. The high-Al population, representing the average rim composition, gives a value around 760 °C, which is indistinguishable from independent T determinations using coexisting Fe-Ti oxides and Qtz-Mag oxygen isotope thermometry. These profiles suggest that Fish Canyon hornblendes crystallized during near-isobaric reheating over a temperature range of ~40 °C, which is consistent with our model of rejuvenation and remobilization of a pre-existing near-solidus crystal mush of batholithic dimensions via shallow intrusion of more mafic magma (Bachmann et al. 2002). Crystallization of hornblende from a high-SiO2, low-MgO melt during reheating requires an open system, in which both heat and mass, in particular volatiles, are transferred from the underlying mafic magma.

Melt Inclusions in Olivine Phenocrysts: Using Diffusive Re-equilibration to Determine the Cooling History of a Crystal, with Implications for the Origin of Olivine-phyric Volcanic Rocks

Abstract: A technique is described for determining the cooling history of olivine phenocrysts. The technique is based on the analysis of the diffusive re-equilibration of melt inclusions trapped by olivine phenocrysts during crystallization. The mechanism of re-equilibration involves diffusion of Fe from and Mg into the initial volume of the inclusion. The technique applies to a single crystal, and thus the cooling history of different phenocrysts in a single erupted magma can be established. We show that melt inclusions in high-Fo olivine phenocrysts from mantle-derived magmas are typically partially re-equilibrated with their hosts at temperatures below trapping. Our analysis demonstrates that at a reasonable combination of factors such as (1) cooling interval before eruption ( 1000°C), and (3) inclusion size (<70 µm in radius), partial re-equilibration of up to 85% occurs within 3–5 months, corresponding to cooling rates faster than 1–2°/day. Short residence times of high-Fo phenocrysts suggest that if eruption does not happen within a few months after a primitive magma begins cooling and crystallization, olivines that crystallize from it are unlikely to be erupted as phenocrysts. This can be explained by efficient separation of olivine crystals from the melt, and their rapid incorporation into the cumulate layer of the chamber. These results also suggest that in most cases erupted high-Fo olivine phenocrysts retain their original composition, and thus compositions of melt inclusions in erupted high-Fo olivine phenocrysts do not suffer changes that cannot be reversed. Short residence times also imply that large unzoned cores of high-Fo phenocrysts cannot reflect diffusive re-equilibration of originally zoned phenocrysts. The unzoned cores are a result of fast efficient accumulation of olivines from the crystallizing magma, i.e. olivines are separated from the magma faster than melt changes its composition. Thus, the main source of high-Fo crystals in the erupted magmas is the cumulate layers of the magmatic system. In other words, olivine-phyric rocks represent mixtures of an evolved transporting magma (which forms the groundmass of the rock) with crystals that were formed during crystallization of more primitive melt(s). Unlike high-Fo olivine phenocrysts, the evolved magma may reside in the magmatic system for a long time. This reconciles long magma residence times estimated from the compositions of rocks with short residence times of high-Fo olivine phenocrysts.

Magma Mixing and Crustal Recycling Recorded in Ternary Feldspar from Compositionally Zoned Peralkaline Ignimbrite 'A', Gran Canaria, Canary Islands

Abstract: Miocene Ignimbrite ‘A’ on Gran Canaria contains three compositional endmember fiamme types(two rhyolites and one trachyte) each of which crystallized distinct feldspar. Various textural and compositional criteria are interpreted as reflecting a complex scenario within the magma chamber in which the crystals formed. About 25–30% of the feldspar phenocrysts contain evidence for magma mixing in the form of (1) partial to severe dissolution–resorption rims, (2) distinct zones of drastically different compositions and (3) overgrowth textures on formerly resorbed crystals. Four major types of zoning in the oligoclase to anorthoclase feldspars of ignimbrite ‘A’ include a normal and a reversely zoned type and two complexly zoned types. The feldspars with normal and reverse zonation show only minor compositional amplitudes between individual zones (ΔAb, Or ∼4%), whereas the complexly zoned types show compositional differences between zones of up to 18 mol % Ab and 20 mol % Or and are commonly associated with an internal dissolution surface. Complex zoning with large compositional amplitudes and dissolution textures is taken as evidence of crystal movements within the magma and across compositional boundaries between magma batches. A multiple ‘step-cycle’ model, involving growth and transport of a crystal into another magma batch and its return to the original host magma, is suggested by the data. Moreover, feldspars from one rhyolite compositional group are found to be substantially elevated in δ18O, suggesting an input of a high δ18O component to this rhyolite. The other endmember rhyolite appears to be related to the endmember trachyte by mainly crystal fractionation of anorthoclase feldspar. This observation is consistent with trace element and rare earth element concentrations for the magma endmembers and their feldspars, where contamination led to a depletion in incompatible trace elements and light rare earth elements in the contaminated rhyolite and its feldspar phenocrysts. We suggest that the combination of textural and compositional variation in ternary feldspar of peralkaline rhyolitic systems is well suited to reconstruct dynamic processes such as magma mixing and contamination in evolving rhyolitic magma chambers.

Mixed Magmas, Mush Chambers and Eruption Triggers: Evidence from Zoned Clinopyroxene Phenocrysts in Andesitic Scoria from the 1995 Eruptions of Ruapehu Volcano, New Zealand

Abstract: Juvenile ejecta from the September and October 1995 eruptions of mush network. Hence, eroded crystals from the mush were captured Ruapehu volcano, New Zealand, indicate that mixing occurred by the high-T magma. Repeated reverse zonations in the crystal between relatively higherand lower-temperature (high-T and lowmantles, and variable mantle widths, indicate that injection of highT) andesitic magmas. Compositional zonations in clinopyroxene T magma intermittently continued, both to form and to enlarge phenocrysts provide direct evidence for a pre-eruption crystal–melt magma pockets in the mush. In the process, melt mixing occurred mush chamber containing low-T magma, and elucidate the processes owing to convection and/or conjunction and coalescence of the of magma mixing and eruption, following the injection of high-T magma pockets. The mixing between the magma pockets and magma. Many phenocrysts with Fe-rich cores derived from low-T interstitial melt of the mush gave rise to normal zoning in the magma have extremely reverse zoned mantles around slightly resorbed mantles of contained phenocrysts. Zoning in the outermost rims of cores. Mg-value [100Mg/(Mg + Fe)] increases from 65–70 to pyroxenes differs between the September and October scorias. Almost >85 over a short width ( 85) is the same as or coalescence of magma pockets. These differences in presumed the core compositions of phenocrysts derived from the high-T magma, triggering process are consistent with geophysical observations before suggesting that these extremely zoned phenocrysts were surrounded and during the 1995 eruptions. immediately by invading high-T magma, as opposed to mixed melt. This does not indicate a simple melt-mixing process between crystalrich magmas, but is interpreted as evidence for mixing in a mush chamber. It is suggested that high-T magma was injected into a mushy low-T magma, then the denser and hotter high-T magma

Oxygen isotope study of the Long Valley magma system, California: isotope thermometry and convection in large silicic magma bodies

Abstract: Products of voluminous pyroclastic eruptions with eruptive draw-down of several kilometers provide a snap-shot view of batholith-scale magma chambers, and quench pre-eruptive isotopic fractionations (i.e., temperatures) between minerals. We report analyses of oxygen isotope ratio in individual quartz phenocrysts and concentrates of magnetite, pyroxene, and zircon from individual pumice clasts of ignimbrite and fall units of caldera-forming 0.76 Ma Bishop Tuff (BT), pre-caldera Glass Mountain (2.1–0.78 Ma), and post-caldera rhyolites (0.65–0.04 Ma) to characterize the long-lived, batholith-scale magma chamber beneath Long Valley Caldera in California. Values of δ18O show a subtle 1‰ decrease from the oldest Glass Mountain lavas to the youngest post-caldera rhyolites. Older Glass Mountain lavas exhibit larger (~1‰) variability of δ18O(quartz). The youngest domes of Glass Mountain are similar to BT in δ18O(quartz) values and reflect convective homogenization during formation of BT magma chamber surrounded by extremely heterogeneous country rocks (ranging from 2 to +29‰). Oxygen isotope thermometry of BT confirms a temperature gradient between "Late" (815 °C) and "Early" (715 °C) BT. The δ18O(quartz) values of "Early" and "Late" BT are +8.33 and 8.21‰, consistent with a constant δ18O(melt)=7.8±0.1‰ and 100 °C temperature difference. Zircon-melt saturation equilibria gives a similar temperature range. Values of δ18O(quartz) for different stratigraphic units of BT, and in pumice clasts ranging in pre-eruptive depths from 6 to 11 km (based on melt inclusions), and document vertical and lateral homogeneity of δ18O(melt). Worldwide, five other large-volume rhyolites, Lava Creek, Lower Bandelier, Fish Canyon, Cerro Galan, and Toba, exhibit equal δ18O(melt) values of earlier and later erupted portions in each of the these climactic caldera-forming eruptions. We interpret the large-scale δ18O homogeneity of BT and other large magma chambers as evidence of their longevity (>105 years) and convection. However, remaining isotopic zoning in some quartz phenocrysts, trace element gradients in feldspars, and quartz and zircon crystal size distributions are more consistent with far shorter timescales (102–104 years). We propose a sidewall-crystallization model that promotes convective homogenization, roofward accumulation of more evolved and stagnant, volatile-rich liquid, and develops compositional and temperature gradients in pre-climactic magma chamber. Crystal + melt + gas bubbles mush near chamber walls of variable δ18O gets periodically remobilized in response to chamber refill by new hotter magmas. One such episode of chamber refill by high-Ti, Sr, Ba, Zr, and volatile-richer magma happened 103–104 years prior to the 0.76-Ma caldera collapse that caused magma mixing at the base, mush thawing near the roof and walls, and downward settling of phenocrysts into this hybrid melt.

Magmatic Degassing of Volatiles and Ore Metals into a Hydrothermal System on the Modern Sea Floor of the Eastern Manus Back-Arc Basin, Western Pacific

Abstract: Magmatic fluids were degassed before and during the eruption of vesicular volcanic rocks that host the actively forming massive sulfides at the PACMANUS hydrothermal field in the eastern Manus back-arc basin, western Pacific The dredged samples of fresh lavas, ranging in composition from basalt to rhyolite, define a calc-alkalic trend that is thought to have resulted from fractionation in a common magma chamber These rocks have variable vesicularity and vesicle size distributions that record the degassing history of the magma The highly vesicular basalt and basaltic andesite are the least fractionated melt and experienced preeruptive, syneruptive, and posteruptive degassing The weakly vesicular felsic rocks formed from an evolved magma that was largely degassed before its eruption Vesicularity tends to decrease with Si, K, Ba, and Zr and to increase with Ca, Mg, Fe, and Sc in bulk samples, suggesting that the degassing of volatiles was linked to crystal fractionation of the magma A volatile-rich magma is indicated by high concentrations of H 2 O (09–25%) and Cl (to 045%) in mafic melt inclusions in phenocrysts of the basaltic andesite A fluid phase in the melt inclusions indicates that the magma was saturated with volatiles in the magma chamber Volatiles exsolve as an immiscible fluid with increasing crystal fractionation, and the composition of the degassed magmatic fluid changes with the evolving magma The fluid is CO 2 -dominated during the degassing of weakly fractionated mafic magma and becomes a mixture of CO 2 and H 2 O as H 2 O is increasingly exsolved from the highly fractionated felsic magma The ore metals in the degassed fluid, as inferred from the compositions of the metallic precipitates found in the vesicles of melt inclusions and matrix glass, progressively change from Ni + Cu + Zn + Fe in basalt and basaltic andesite, to Cu + Zn + Fe in andesite, to Cu + Fe in dacite, to Fe in rhyodacite, and to Fe + Zn (+ Pb?) in rhyolite This trend suggests that magmatic fluids, released from a fractionating magma, could be a source of metal for various types of ore deposits and mineral occurrences Significant amounts of magmatic fluid can be degassed during the fractionation of a shallow magma chamber At least 17 percent H 2 O is estimated to have exsolved from the magma at PACMANUS If a fluid is concentrated in a shallow magma chamber and is discharged along a favorable structural zone that extends to the sea floor, it will contribute large quantities of volatiles and ore metals directly to a sea-floor hydrothermal system, a process demonstrated for the geothermal systems of subareal volcanoes The focused discharge of a magmatic fluid as a result of preeruptive degassing, particularly in a fractionated felsic magma, could be responsible for the Fe, Cu, Zn, and Pb metals in the sulfide chimneys at PACMANUS By analogy, a magmatic fluid can provide a major source of ore metals for large or super-large volcanogenic massive sulfide deposits in the geologic record

Northwest Africa 032: Product of lunar volcanism

Abstract: Mineralogy, major element compositions of minerals, and elemental and oxygen isotopic compositions of the whole rock attest to a lunar origin of the meteorite Northwest Africa 032 (NWA 032), an unbrecciated basalt found in October 1999. The rock consists predominantly of olivine, pyroxene and chromite phenocrysts, set in a crystalline groundmass of feldspar, pyroxene, ilmenite, troilite and trace metal. Whole-rock shock veins comprise a minor, but ubiquitous portion of the rock. Undulatory to mosaic extinction in olivine and pyroxene phenocrysts and micro-faults in groundmass and phenocrysts also are attributed to shock. Several geochemical signatures taken together indicate unambiguously that NWA 032 originated from the Moon. The most diagnostic criteria include whole-rock oxygen isotopic composition and ratios of Fe:Mn in the whole rock, olivine, and pyroxene. A lunar origin is documented further by the presence of Fe-metal, troilite, and ilmenite; zoning to extremely Fe-rich compositions in pyroxene; the ferrous oxidation state of all Fe in pyroxene; and the rare-earth element pattern with a well-defined negative europium anomaly. This rock is similar in major element chemistry to basalts from Apollo 12 and 15, but is enriched in light rare-earth elements and has an unusually high Th/Sm ratio. Some Apollo 14 basalts yield a closer match to NWA 032 in rare-earth element patterns, but have higher concentrations of Al2O3. Ar-Ar step release results are complex, but yield a whole-rock age of ca. 2.8 Ga, suggesting that NWA 032 was extruded at 2.8 Ga or earlier. This rock may be the youngest sample of mare basalt collected to date. Noble gas concentrations combined with previously collected radionuclide data indicate that the meteorite exposure history is distinct from currently recognized lunar meteorites. In short, the geochemical and petrographic features of NWA 032 are not matched by Apollo or Luna samples, nor by previously identified lunar meteorites, indicating that it originates from a previously unsampled mare deposit. Detailed assessment of petrographic features, olivine zoning, and thermodynamic modelling indicate a relatively simple cooling and crystallization history for NWA 032. Chromite-spinel, olivine, and pyroxene crystallized as phenocrysts while the magma cooled no faster than 2 °C/hr based on the polyhedral morphology of olivine. Comparison of olivine size with crystal growth rates and preserved Fe-Mg diffusion profiles in olivine phenocrysts suggest that olivine was immersed in the melt for no more than 40 days. Plumose textures in groundmass pyroxene, feldspar, and ilmenite, and Fe-rich rims on the phenocrysts formed during rapid crystallization (cooling rates ~ 20 to 60 °C/hr) after eruption.

Size-frequency distributions of chondrules and chondrule fragments in LL3 chondrites: Implications for parent-body fragmentation of chondrules

Abstract: We measured the sizes and textural types of 719 intact chondrules and 1322 chondrule fragments in thin sections of Semarkona (LL3.0), Bishunpur (LL3.1), Krymka (LL3.1), Piancaldoli (LL3.4) and Lewis Cliff 88175 (LL3.8). The mean apparent diameter of chondrules in these LL3 chondrites is 0.80 Φ units or 570 um, much smaller than the previous rough estimate of ~900 μm. Chondrule fragments in the five LL3 chondrites have a mean apparent cross-section of 1.60 phi units or 330 μm. The smallest fragments are isolated olivine and pyroxene grains; these are probably phenocrysts liberated from disrupted porphyritic chondrules. All five LL3 chondrites have fragment/chondrule number ratios exceeding unity, suggesting that substantial numbers of the chondrules in these rocks were shattered. Most fragmentation probably occurred on the parent asteroid. Porphyritic chondrules (porphyritic olivine + porphyritic pyroxene + porphyritic olivine-pyroxene) are more readily broken than droplet chondrules (barred olivine + radial pyroxene + cryptocrystalline). The porphyritic fragment/chondrule number ratio (2.0) appreciably exceeds that of droplet-textured objects (0.9). Intact droplet chondrules have a larger mean size than intact porphyritic chondrules, implying that large porphyritic chondrules are fragmented preferentially. This is consistent with the relatively low percentage of porphyritic chondrules within the set of the largest chondrules (57%) compared to that within the set of the smallest chondrules (81%). Differences in mean size among chondrule textural types may be due mainly to parent-body chondrule-fragmentation events and not to chondrule-formation processes in the solar nebula.

U–Th Disequilibrium and Rb–Sr Age Constraints on the Magmatic Evolution of Peralkaline Rhyolites from Kenya

Abstract: Mildly peralkaline rhyolites of the Olkaria Volcanic Complex, minor, continental magma system of mildly peralkaline located in the Kenyan sector of the East African rift valley, have composition [molar (Na2O + K2O)/Al2O3 or agpaitic low Sr concentrations and elevated Rb/Sr ratios (Sr 1·3–2 ppm; index, AI >1)]. The work was undertaken as a contrast Rb/Sr = 748–1769) that potentially allow the resolution of to far larger, prominent, caldera-forming peralkaline (e.g. time differences on the order of 1 ka by conventional Sr isotope Mahood, 1984) or metaluminous volcanic systems such determination. Because of their young eruption ages (Ζ20 ka), a as Long Valley (e.g. Halliday et al., 1989; Davies et al., chemically independent assessment of the Sr isotope results has been 1994). The goal is to assess how the rates of magmatic obtained by U-series dating. Rb–Sr isochron ages of pristine glasses processes vary with the size or composition of the system. and phenocrysts from the most chemically evolved rhyolites pre-date We utilize both the Rb–Sr and U–Th geochronometers the eruption ages and are best defined by a mineral isochron of 24 because they have distinct chemical behaviour and re± 1 ka. The glasses are in secular U–Th equilibrium so that no spond in different ways to magmatic processes. Comage information can be obtained. In contrast, glasses and minerals parison of the two isotope systems therefore may provide yield U–Th isochrons of 25 ± 10 ka and are probably controlled additional insights into the time scales of processes that by the Th-enriched accessory phase chevkinite. We therefore ascribe control the evolution of strongly differentiated silicic the pre-eruptive age information to crystallization of the observed magmas. phenocryst phases. Inferred high magma fractionation rates of up Peralkaline felsic magmas are most widespread in reto 2·5 × 10 km/yr are comparable with those for much gions of continental upwelling and/or rifting (Fitton & larger metaluminous silicic magma systems. Magma storage times Upton, 1987), but actually occur in a wide variety of (>22 ky), however, are much shorter and may best be accounted geodynamic settings, including oceanic islands (Caroff et for by the specific size, longevity and thermal gradient of the silicic al., 1993; Mungall & Martin, 1995), island arcs (Bevier magma system. et al., 1979) and extensional continental areas (Mahood, 1981; Civetta et al., 1984; Macdonald, 1987). There are various petrogenetic explanations for the generation of

The role of basalt replenishment in the generation of basaltic andesites of the ongoing activity at Arenal volcano, Costa Rica: evidence from clinopyroxene and spinel

Abstract: The bulk composition of magma erupted from Volcan Arenal has remained nearly constant (SiO2 = 53.6–54.9 wt%; MgO = 5.0–4.5 wt%) during almost 30 years of continuous activity (1969–1996). None the less, clinopyroxene (cpx) phenocrysts and their spinel inclusions record a much more complex open-system evolution in which steady-state production of the erupted basaltic andesitic magma is linked to episodic injections of basalt into Arenal's magma conduit/reservoir system. High-resolution major element zoning profiles (electron microprobe) on a large number of phenocrysts (>14,000 analyses), tied to back-scattered electron (BSE) images, have been used to assess the compositional characteristics of the magmatic end members as well as the timing and dynamics of magma replenishment events. No two cpx phenocrysts have exactly the same zoning profile. The vast majority of our analyses record the crystallization of cpx (Cr2O3 <0.12 wt%; Mg# = 65–79; Al/Ti = 2–7) from a liquid comparable to or more evolved than erupted magma compositions. However, half of all cpx grains are cored by high-Cr cpx (Cr2O3 = 0.2–0.72 wt%) or contain similar basaltic compositions as abrupt growth bands in phenocrysts with and without high-Cr cores; phenocrysts with high-Cr cpx occur throughout the ongoing activity. In a few cases, high-Cr cpx occurs very near the outer margin of the grain without an apparent growth hiatus, particularly in 1968/69 and 1992/93. The main conclusions are: (1) all basaltic andesitic lavas erupted at Arenal during the ongoing activity that began in July, 1968, are the products of magma mixing, (2) clinopyroxenes record multiple replenishment events of basaltic magma in contrast to the near constancy of erupted bulk compositions, (3) some phenocrysts preserve records of multiple interactions with basaltic magmas requiring magmatic processes to operate on timescales shorter than residence times of some phenocrysts, (4) multiple occurrences of clinopyroxene with high-Cr rims suggest that basalt replenishment events have occurred with sub-decadal frequency and may predate eruption by months or less. From this we infer that Arenal volcano is underlain by a continuously active, small-volume magmatic reservoir maintained in quasi-steady state by basalt recharge over several decades. The monotony of erupting Arenal magmas implies that fractionation, recharge, ascent, and eruption are well balanced in order for magmas to be essentially uniform while containing phenocrysts with vastly different growth histories at the time of eruption.

Phenocrysts versus xenocrysts in the youngest Toba Tuff: Implications for the petrogenesis of 2800 km3 of magma

Abstract: The petrogenesis of the 2800 km 3 of magma erupted as the youngest Toba Tuff has been investigated using experimental petrology and 40 Ar/ 39 Ar dating of biotite, sanidine, hornblende, and plagioclase from the tuff. We find that hornblende does not crystallize experimentally from the magma at temperatures and pressures indicated by the natural mineral assemblage. Hornblende is also not in isotopic equilibrium with biotite and sanidine, both of which grew experimentally. Hornblende thus appears xenocrystic, despite being a major phase in the tuff. Some plagioclase is also xenocrystic, on the basis of Ar isotopes, but others are probably phenocrystic, because plagioclase grew experimentally. Crystal clots of hornblende + plagioclase observed in the tuff suggest that the xenocrysts came from a common source, which was at least 1.5 Ma (the oldest hornblende 40 Ar/ 39 Ar age). Our results suggest that the Toba Tuff magma resided at nearly water-saturated pressures of 100–150 MPa and that xenocrysts were entrained as recently as 10 yr before the eruption. The ubiquitous presence of hornblende in the tuff indicates that entrainment occurred throughout the 2800 km 3 of magma.

Calcic cores of plagioclase phenocrysts in andesite from Karymsky volcano: Evidence for rapid introduction by basaltic replenishment

Abstract: Calcic cores in plagioclase of Karymsky andesite of the 1996–2000 eruptive cycle texturally and compositionally (both trace and major elements) mimic the plagioclase phenocrysts of basalt erupted 6 km away at the onset of the cycle. These observations support the view that simultaneous eruption of andesite and basalt at Karymsky in the beginning of the cycle represents an example of replenishment and eruption triggering of an andesitic reservoir. Homogeneity of andesitic output occurred within two months. This suggests to us that blending of injected basalt into reservoir magma was thorough and rapid.

Petrology and geochemistry of the Bandas del Sur Formation, Las Canadas Edifice, Tenerife (Canary Islands)

Abstract: The Bandas del Sur Formation preserves a Quaternary extra-caldera record of central phonolitic explosive volcanism of the Las Canadas volcano at Tenerife. Volcanic rocks are bimodal in composition, being predominantly phonolitic pyroclastic deposits, several eruptions of which resulted in summit caldera collapse, alkali basaltic lavas erupted from many fissures around the flanks. For the pyroclastic deposits, there is a broad range of pumice glass compositions from phonotephrite to phonolite. The phonolite pyroclastic deposits are also characterized by a diverse, 7-8-phase phenocryst assemblage (alkali feldspar + biotite + sodian diopside + titanomagnetite + ilmenite + nosean-hauyne + titanite + apatite) with alkali feldspar dominant, in contrast to interbedded phonolite lavas that typically have lower phenocryst contents and lack hydrous phases. Petrological and geochemical data are consistent with fractional crystallization (involving the observed phenocryst assemblages) as the dominant process in the development of phonolite magmas. New stratigraphically constrained data indicate that petrological and geochemical differences exist between pyroclastic deposits of the last two explosive cycles of phonolitic volcanism. Cycle 2 (0.85-0.57 Ma) pyroclastic fall deposits commonly show a cryptic compositional zonation indicating that several eruptions tapped chemically, and probably thermally stratified magma systems. Evidence for magma mixing is most widespread in the pyroclastic deposits of Cycle 3 (0.37-0.17 Ma), which includes the presence of reversely and normally zoned phenocrysts, quenched mafic glass blebs in pumice, banded pumice, and bimodal to polymodal phenocryst compositional populations. Syn-eruptive mixing events involved mostly phonolite and tephriphonolite magmas, whereas a pre-eruptive mixing event involving basaltic magma is recorded in several banded pumice-bearing ignimbrites of Cycle 3. The periodic addition and mixing of basaltic magma ultimately may have triggered several eruptions. Recharge and underplating by basaltic magma is interpreted to have elevated sulphur contents (occurring as an exsolved gas phase) in the capping phonolitic magma reservoir. This promoted nosean-hauyne crystallization over nepheline, elevated SO3 contents in apatite, and possibly resulted in large, climatologically important SO2 emissions.