Carl Thornber - Academia.edu (original) (raw)

Papers by Carl Thornber

Rates of olivine dissolution in synthetic lunar basalt 77LI5 and a silica-enriched 77115 composit... more Rates of olivine dissolution in synthetic lunar basalt 77LI5 and a silica-enriched 77115 composition (Sil-77115) at superliquidus temperatures have been determined. Polished olivine plates of known composition, orientation, and size were placed in basalt mixes contained in high-purity iron crucibles and heated in evacuated silica tubes. Olivine plates dissolve in the 77115 melt at rates of-02 to 616 pmlhr over a temperature range of 1265" to 1450"C; these rates are consistently faster in the more siliceous melt. Olivine resorption rates are independent of crystallographic orientation and run duration. Dissolution-rate data have been applied to the problem of the thermal history of fragment-laden impact-melt rocks of the lunar highlands. On the basis of estimates of the survival time of angular olivine xenocrysts in lunar basalt 77ll5,we conclude that the impact melt cooled to near-liquidus temperatures (-1259"91 within I hour and that temperatures did not exceed 1450'C for more than several seconds or remain above 1315'C for longer than a few minutes. In addition, textural and chemical criteria for the recognition of olivine resorption (and growth) phenomena in igneous rocks are discussed.

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From 1994 through 1998, the eruption of K¯ ılauea, in Hawai'i, was dominated by steady-state effu... more From 1994 through 1998, the eruption of K¯ ılauea, in Hawai'i, was dominated by steady-state effusion at Pu'u ' ¯ O'¯ o that was briefly disrupted by an eruption 4 km uprift at N¯ apau Crater on January 30, 1997. In this paper, I describe the systematic relations of whole-rock, glass, olivine, and olivine-inclusion compositions of lava samples collected throughout this interval. This suite comprises vent samples and tube-contained flows collected at variable distances from the vent. The glass composition of tube lava varies systematically with distance and allows for the " vent-correction " of glass thermometry and olivine–liquid K D as a function of tube-transport distance. Combined olivine–liquid data for vent samples and " vent-corrected " lava-tube samples are used to document pre-eruptive magmatic conditions. K D values determined for matrix glasses and forsterite cores define three types of olivine phenocrysts: type A (in equilibrium with host glass), type B (Mg-rich relative to host glass) and type C (Mg-poor relative to host glass). All three types of olivine have a cognate association with melts that are present within the shallow mag-matic plumbing system during this interval. During steady-state eruptive activity, the compositions of whole-rock, glass and most olivine phenocrysts (type A) all vary sympathetically over time and as influenced by changes of magmatic pressure within the summit-rift-zone plumbing system. Type-A olivine is interpreted as having grown during passage from the summit magma-chamber along the east-rift-zone conduit. Type-B olivine (high Fo) is consistent with equilibrium crystallization from bulk-rock compositions and is likely to have grown within the summit magma-chamber. Lower-temperature, fractionated lava was erupted during non-steady-state activity of the N¯ apau Crater eruption. Type-A and type-B olivine–liquid relations indicate that this lava is a mixture of rift-stored and summit-derived magmas. Post-N¯ apau lava (at Pu'u ' ¯ O'¯ o) gradually increases in temperature and MgO content, and contains type-C olivine with complex zoning, indicating magma hybridization associated with the flushing of rift-stored components through the eruption conduit. SOMMAIRE A partir de 1994 jusqu'à la fin de 1998, l'éruption de K¯ ılauea, île de Hawai'i, était dominée par une effusion stable et continue à Pu'u ' ¯ O'¯ o; celle-ci s'est vue interrompue brièvement par une éruption dans le cratère N¯ apau, à 4 km de là en remontant le rift, le 30 janvier, 1997. C'est mon but dans cet article de décrire les relations systématiques de la composition des roches globales, du verre, de l'olivine et des inclusions piégées dans l'olivine des échantillons de lave prélevés au cours de cet intervalle. La suite inclut des échantillons pris des orifices et des coulées confinées aux tubes de lave, prélevés à distance variable de l'orifice. La composition du verre des laves prélevées dans les tubes varie systématiquement avec la distance et permet une correction des températures fondées sur la composition du verre et du rapport K D pour le couple olivine–liquide en fonction de la distance parcourue. Une combinaison de données sur les couples olivine–liquide pour les échantillons des orifices et de données corrigées pour les échantillons pris dans les tubes sert à déterminer les conditions magmatiques pré-éruptives. Les valeurs de K D déterminées sur le verre de la matrice et le coeur des phénocristaux d'olivine définissent trois types de phénocristaux: ceux de type A sont en équilibre avec le verre hôte, tandis que ceux de type B sont enrichis en Mg par rapport au verre, et ceux de type C sont appauvris en Mg, toujours par rapport au verre coexistant. Les trois types d'olivine partagent une association génétique avec les liquides en circulation durant cet intervalle dans le système de " plomberie " magmatique à faible profondeur. Au cours du stade éruptif soutenu, les compositions de roche globale, de verre et la plupart des phénocristaux d'olivine (de type A) varient toutes de façon uniforme avec le temps et selon les changements de pression magmatique au sein du système de rifts près du sommet. La croissance de l'olivine de type A aurait eu lieu lors du passage du magma de la chambre magmatique sommitale le long du conduit menant vers la zone de rift de l'est. L'olivine de type B (Fo élevé) aurait cristallisé à l'équilibre avec la composition globale des échantillons qui la renferment, et probablement à l'intérieur de la chambre magmatique sommitale. L'épanchement de laves de plus basse température, plus évoluées, a eu lieu lors de l'interruption des éruptions soutenues à partir du cratère de N¯ apau. Les relations du §

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Petrologic monitoring of Kīlauea Volcano from January 1983 to October 2013 has yielded an extensi... more Petrologic monitoring of Kīlauea Volcano from January 1983 to October 2013 has yielded an extensive record of glass, phenocryst, melt inclusion, and bulk‐lava chemistry from well‐quenched lava. When correlated with 30+ years of geophysical and geologic monitoring, petrologic details testify to physical maturation of summit‐ to‐rift magma plumbing associated with sporadic intrusion and prolonged magmatic overpressurization. Changes through time in bulk‐lava major‐ and trace‐element compositions, along with glass thermometry, record shifts in the dynamic balance of fractionation, mixing, and assimilation processes inherent to magma storage and transport during near‐continuous recharge and eruption. Phenocryst composition, morphology, and texture, along with the sulfur content of melt inclusions, constrain coupled changes in eruption behavior and geochemistry to processes occurring in the shallow magmatic system. For the first 17 years of eruption, magma was steadily tapped from a summit reservoir at 1–4 km depth and circulating between 1180 and 1200°C. Furthermore, magma cooled another 30°C while flowing through the 18 km long rift conduit, before erupting olivine‐spinel‐phyric lava at temperatures of 1150–1170°C in a pattern linked with edifice deformation, vent formation, eruptive vigor, and presumably the flux of magma into and out of the summit reservoir. During 2000–2001, a fundamental change in steady state eruption petrology to that of relatively low‐temperature, low‐MgO, olivine(‐spinel)‐clinopyroxene‐plagioclase‐phryic lava points to a physical transformation of the shallow volcano plumbing uprift of the vent. Preeruptive comagmatic mixing between hotter and cooler magma is documented by resorption, overgrowth, and compositional zonation in a mixed population of phenocrysts grown at higher and lower temperatures. Large variations of sulfur (50 to >1000 ppm) in melt inclusions within individual phenocrysts and among phenocrysts in most samples provide an unequivocal glimpse of rapid crystal growth amid sulfur degassing at <30 MPa in a turbulent preeruptive environment. We speculate that, during the last decade, one or more shallow open‐system reservoirs developed along the conduit between the summit and Pu'u 'Ō'ō and now serve to buffer the magmatic throughput associated with ongoing recharge and eruption. Lava with identical trace‐element signatures erupted simultaneously at the summit and at Pu'u 'Ō'ō from 2008 to 2013 confirms magmatic continuity between the vents. Complementary changes in compositions of matrix glasses, phenocrysts, and melt inclusions of summit tephra are mirrored by similar changes in contempo-raneous rift lava at eruption temperatures 20–35°C lower than those at the summit. Petrologic parameters measured at opposite ends of the shallow magmatic plumbing system are both correlated with summit deformation, demonstrating that effects of summit magma chamber pressurization are translated throughout interconnected magma pathways in the shallow edifice.

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Professional Paper, 2014

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Twenty six ulttamafic rock types are distinguished among 711 xenoliths and megacrysts from Harrat... more Twenty six ulttamafic rock types are distinguished among 711 xenoliths and megacrysts from Harrat Hutaymah, Saudi Arabia. Distinctive aspects of texture, modal mineralogy and mineral chemistry allow division of this suite into categories of Mg-Cr-Group and AI-Fe-Ti-Group igneous pyroxenite, Mg-Cr-Group and AI-Fe-Ti-Group metamorphic pyroxenite, and associated Mg-Cr-Group mantle peridotites, with hydrous equivalents of each category. Igneous pyroxenite and partial-melt-bearing peridotite inclusions have relatively high pyroxene equilibration temperatures, and textures, modal mineralogy, and mineral compositions that reflect primary igneous crystallization, high temperature deformation, (X' partial melting. These samples show the effects of injection and polybaric crystallization of volcanic-host-. related mafic alkaline magma and the complementary. effects of heating, fluid infdttation and partial melting of surrounding mantle lherzolite. Metaml>rphic pyroxenites and non-melted peridotites have relatively low pyroxene equilibration temperatures, and textures, modal mineralogy and mineral chemistry that reflect varying degrees of deformation, recrystallization and metam(X'phic annealing. Similar bulk rock and mineral compositional trends and gradational textural and modal characteristics between ulttamafic-rocks of igneous affinity and their metamorphic counterparts reflect a similar episode of either Proterozoic or precursory CenQzoic alkaline magmatism within the deep crust and upper mantle. The maximum depth of origin of Mg-Cr-Group xenoliths is broadly restricted by that of pristine spinellhenolite (protollth mantle) at-70 km. Experimental data and thermobarimetric calculations indicate that all AI-Fe-Ti-Group inclusions may have been sampled within ± 10 km of the base of the 40-km crust.

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Bulletin of Volcanology, 1987

... Springer-Verlag 1987 Geothermometry of Kilauea Iki lava lake, Hawaii ... Abstract. Data on th... more ... Springer-Verlag 1987 Geothermometry of Kilauea Iki lava lake, Hawaii ... Abstract. Data on the variation of temperature with time and in space are essential to a complete understanding of the crystallization history of basaltic magma in Kilauea Iki lava lake. ...

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A significant change in the petrology of Pu`u `O`o -Kupaianaha lava occurred in April 2001 (3 yea... more A significant change in the petrology of Pu`u `O`o -Kupaianaha lava occurred in April 2001 (3 years after the onset of the decade-long episode 55). Prior to that time all steady-state eruption products were olivine phryic. After that time and until the Kane Nui o Hamo eruption of June 19, 2007 (episode 56), all magma erupted from vents in and around Pu`u `O`o was olivine and pyroxene-phyric containing <1mm, isolated or clustered clinopyroxene (±olivine, ±plagioclase), usually with resorbed edges. Textures, phase chemistry and low-pressure phase relations define a pre-eruptive mixing environment that is driven by continuous recharge of a stagnant, near-cotectic shallow magma body. The comagmatic nature of the cooler component in the post-2001 hybrid magma is verified by low concentrations of incompatible elements relative to MgO. Since the eruption began in 1983, the olivine-saturated liquid-line-of-descent has progressively shifted toward the present-day low concentrations of incompatible elements. Superimposed on this long-term trend are shorter chemical cycles (months to years) which track fractionation and recharge between comagmatic endmembers of ~10 and ~7 wt% MgO. These shorter cycles correspond to heating and cooling events and imply magmatic recharge of a sustained shallow magma reservoir within the eruptive plumbing system. The longest cooling cycle of the entire eruption began in April 1998 after effusion of the hottest and most- primitive lava erupted since 1985 (episodes 30 and 31). Glass temperatures up to 1168°C and bulk MgO of 9.5 wt% steadily declined for 6 years until late 2004 when they bottomed-out at 1140°C and 6.8 wt%. This signaled a stable near-cotectic magma condition. MgO contents and glass temperatures stailized at ~7.1 wt% and 1146°C for the remainder of episode 55, as the hybrid magmas were stirred by a steady influx of summit-derived magma beneath a complex of intermittently active Pu`u O`o vents. During the June 19, 2007 Kane Nui O Hamo eruption (episode 56), as with the January 1997 Napua Crater event (episode 54), the summit deflated and Pu`u O`o collapsed as magma was drawn from either end of the active rift conduit toward a zone of extension. In both cases, magma returned to the Pu`u `O`o vent area after the conduit repressurized. However, in contrast to cool and porphyritic hybrid magma erupted through isolated and chemically evolved rift magma reservoirs at Napau Crater, the episode 56 lava is relatively primitive (8.7 wt% MgO) and 30 to 50°C hotter at 1160°C. This is likely to be summit-derived magma from within the active rift conduit beneath Kane Nui o Hamo. The episode 56 lava is ~15°C hotter than the late episode 55 hybrid magmas with consistently low incompatible elements and likely represents the recharge component that maintained a shallow reservoir at near-cotectic conditions beneath the vicinity of the Pu`u `O`o vents for the last several years. Both lava erupted from Pu`u `O`o in early June, 2007(episode 57), and lava the from the July 21-24 sequence of fissure eruptions down-rift of Pu`u `O`o (early episode 58) contain a distinctly hybrid phenocryst and glomerocryst assemblage, suggesting a flushing of cooler crystal-laden magma from the conduit.

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Geochmica Et Cosmochimica Acta, Jun 1, 2009

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Monitoring of the current eruption of Mount St. Helens has posed fundamental questions about the ... more Monitoring of the current eruption of Mount St. Helens has posed fundamental questions about the magmatic plumbing system and the source of new dome magma. At the time of this writing (10-24-04) a new lava dome is forming south of the 1980-1986 dome. A primary question, with important implications for volcano hazards, is whether shallow (6 km) gas-rich magma. Geophysical

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The dome-building eruption at Mount St. Helens has occurred through glacial ice and snow that wou... more The dome-building eruption at Mount St. Helens has occurred through glacial ice and snow that would be expected to substantially affect the character of the eruption. Nevertheless, the role of water in the eruption to date has not always been clear. For example, on March 8, 2005, a half-hour-long tephra blast sent a plume to a maximum of ~9 km

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ABSTRACT

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Variable depth and temperatures within the Mount St. Helens (MSH) 2004-5 eruptive plumbing system... more Variable depth and temperatures within the Mount St. Helens (MSH) 2004-5 eruptive plumbing system are recorded by a diverse population of amphiboles. Three compositional arrays among amphiboles in dome dacite are most clearly distinguished by Al and Fe variations. Magnesiohornblende to edenite compositions (Leake classification) define a low-Al group ranging from 6 to 9.5 wt% Al2O3 with increasing FeOT from 12 to 16.5 wt%. The predominant medium-Al group of tschermakite-magnesiohastingite-pargasite amphiboles ranges from 9.5 to 12 wt% Al2O3 with FeOT decreasing from 16.5 to 10 wt%. The high-Al group of magnesiohastingsite-pargasite amphiboles has 12 to 14 wt% Al2O3 with FeOT increasing from 10 to 16 wt%. (The FeO{T trends are consistent with Mg-number variations). Most discrete low-Al amphiboles are broken crystal fragments, often with resorbed edges and some with broad reaction rims. Low-Al compositions also form rims on mid-Al and high-Al amphiboles and are present as cores within some oscillatory- or sector-zoned crystals. Within the low-Al group, higher Al and Fe compositions overlap those in gabbro xenoliths from MSH 1980-86 dome rocks. The medium-Al group also includes phenocrysts with uniform cores, hopper-growth-like characteristics and negligible reaction rims in the most glass-rich 2004 samples. In typical 2004-05 dome dacite, medium- and high-Al groups have amphibole breakdown rims with uniform thickness of ~5 microns. A qualitative assessment of amphibole source depth is provided by the Anderson-Smith (1995) Al-in-hornblende barometer, with the caveats that this barometer requires independent knowledge of temperature and bulk composition. Because the 2004-05 amphibole assemblage is complex and oxide temperatures record only the latest magmatic equilibration, we cannot establish crystallization temperatures for individual amphibole crystals. Consequently, the pressures reported here should only be used in a relative sense and not as a measure of actual depths. The following examples are illustrative: at 850°C, high-and mid-Al amphiboles yield a pressure range of 430 to 270 MPa and 270 to 80 MPa, respectively, with lowest Al compositions yielding barometric results below the ~100 MPa limit of amphibole stability for MSH dacites, and suggesting that the low-Al group crystallized at considerably lower temperatures. At higher temperatures, progressively more of the medium-Al amphiboles yield untenable low pressures, e.g., at 900°C, only high-Al group amphiboles yield pressures >100 MPa. These barometry calculations and preservation of a range of medium- to high-Al amphiboles in each sample suggest: (1) entrainment of high-Al amphiboles formed at relatively low temperature and great depth followed by continued crystallization of medium-Al phenocrysts during nearly isothermal ascent, (2) crystallization during cooling in shallow crustal magma chambers, or a combination of (1) and (2). The inability of experiments to reproduce amphibole with Al2O3 >11 wt% at pressures up to 300 MPa in MSH dacite (Rutherford and Devine, this session) favors a high-pressure origin for the high-Al amphiboles, or crystallization from a more mafic bulk composition. Low-Al amphiboles (Al2O3<7.4 wt%) require <800°C temperatures to yield minimum pressures for amphibole stability (~100 MPa), and suggests remobilization and entrainment of older near-solidus dacite magma or gabbro in hotter 2004-05 magma.

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The 2004-05 eruption of Mount St. Helens began with seismic activity and uplift of the crater flo... more The 2004-05 eruption of Mount St. Helens began with seismic activity and uplift of the crater floor in late September 2004, followed by phreatic explosions and extrusion of a lava dome starting on 11 October 2004 and continuing to this time (September, 2005). Since shortly after the first spine of lava appeared, samples have been collected using a steel box dredge (``JAWS '') suspended 60 or 110 feet below a helicopter. This method was developed to acquire samples from the hot and steep-sided dome, which, because of occasional explosions and frequent collapses, has been unsafe to approach on the ground. To date, 21 samples have been collected from the six spines of the new lava dome and petrologic studies of these samples are reported in this session. The lava dome is composed of dacite (65 wt% SiO2) that is geochemically uniform and slightly more evolved than the 1980-86 dacite. The typical lava is crystal-rich with ~50% phenocrysts of plagioclase, amphibole, hypersthene, and Fe-Ti oxides in a microcrystalline matrix that contains ~13% microlites, ~13% glass and ~25% vesicles. Oxide thermometer data for early spine samples cluster at 840-850° C and NNO+1 log unit. In contrast, samples erupted during the winter of 2004-05 have zoned oxides with apparent temperatures that range to >950° C. Such late-stage heating is likely due to latent heat evolved during rapid groundmass crystallization, or possibly to heating by new magma. Low volatile contents, presence of trydimite and quartz microlites and decreasing H2O with increasing SiO2 in the rhyolite matrix glass indicate extensive shallow (<1 km) crystallization, driven by degassing of water. Major and trace elements of the 1980-86 and 2004-2005 magma batches are similar, which led us to the initial interpretation that the dacite was magma "left over" from the 1980-1986 activity. However, new petrologic and geochemical data suggest instead that the 2004-05 eruption may be fueled by a new batch of dacite magma derived from depth. Geochemically, both Pb and Th isotopes indicate that the 2004 dacite is different from the 1985 dome (Kent et al, this session; Cooper et al., this session). The recent dacite also contains amphibole cores with Al2O3 contents (12-15 wt%) that are too high to be in equilibrium with the MSH dacite at pressures up to 300 MPa (Rutherford and Devine, this session) and suggest incorporation of the amphibole from a higher pressure or from a more mafic magma. Low concentrations of incompatible-elements in MSH dacites indicate derivation of the dacite by melting of lower crustal metabasaltic rocks. Consequently, the isotopic distinctiveness of the 2004-05 dacite and presence of high-Al amphiboles suggest that Mount St. Helens is now being fed by small batches of dacite coming from deeper crustal levels, perhaps in response to unloading of the magmatic system in 1980. These new magmas would have ascended through the remains of the 1980-86-conduit system where they likely mingled with 1980's vintage dacite. The 2004 dacite last equilibrated at P(H2O) of about 140 MPa (Rutherford and Devine, this session), equivalent to depths of about 5 km. Subsequent rise to the surface caused the dacite to degas water and other volatiles, and to crystallize extensively at shallow (<1 km) depths to the point that it erupted as a rheological solid, forming fault-gouge mantled spines. These data have significant implications for the long-term eruptive behavior of Mount St. Helens, as arrival of a new batch of dacitic magma from the deep crust could herald the beginning of a new long-term cycle of eruptive activity.

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Professional Paper, 2014

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Bulletin of Volcanology, 1987

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Geochmica Et Cosmochimica Acta, Jun 1, 2009

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ABSTRACT

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