V33C-1509
Postcollapse Volcanism in the Valles Caldera, New Mexico: Magma System Dynamics During a Transition Between Large Volume Explosive and Small Volume Effusive Eruptions
The Valles Caldera formed during eruption of the ~250 km3 upper Bandelier Tuff at 1.26 Ma. Following caldera collapse a series of 3 intracaldera rhyolites (Deer Canyon, Redondo Creek and Del Medio) erupted within 54 ka. They are petrographically diverse, ranging from coarsely porphyritic (Deer Canyon type B, Redondo Creek) to nearly aphyric (Deer Canyon type A, Del Medio) and contain distinctive phenocryst assemblages. Disequilibrium textures are common and include resorbed quartz, strongly resorbed plagioclase, antirapikivi feldspars, and zoned sanidine and plagioclase. Plagioclase in Deer Canyon rhyolite ranges widely from labradorite (An60) to oligoclase (An24). Most are andesine in composition and often exhibit oscillatory zonation with overgrowths up to ~200 um wide. Alkali feldspars include both anorthoclase and sanidine with a total variability in Or content of ~50%. Plagioclase and alkali feldspars in Redondo Creek rhyolite show less variability, but similarly large ranges. One phenocryst exhibits an andesine core, oligoclase overgrowth, and sanidine rim. Del Medio rhyolite lacks plagioclase, but contains alkali feldspars (both anorthoclase and sanidine) with variability in Or content of ~30%. Patchy zonation is common, with Or content within phenocrysts varying by ~7%. Redondo Creek rhyolite ranges from ~72-74 wt.% SiO2 whereas Deer Canyon and Del Medio are high silica rhyolites with ~76-78 wt.% SiO2. Trace elements show more significant variability. Redondo Creek samples have low Nb, Rb, Sc, and Lu and high La, Sr, Ba and Hf compared to Del Medio samples. In contrast, Deer Canyon samples exhibit extreme variability in trace element concentrations (e.g., Nb ranging from 32-83 ppm). Incompatible trace element ratios (Th/Nb, Th/Yb, Ta/Yb) versus Nb plots show that Redondo Creek and Del Medio samples have distinctive, limited compositions, whereas Deer Canyon exhibits widely varying values (e.g., Th/Yb ranging from ~3.5-10.5). 238U/206Pb ion microprobe dating of zircons from Redondo Creek rhyolite show crystallization ages from 0.97 Ma to 1.60 Ma, with a mean age of 1.29 ± 0.15 Ma. These ages range from the timing of eruption of Redondo Creek rhyolite to the eruption of the lower Bandelier Tuff (1.61 Ma). In contrast, zircons from Deer Canyon rhyolite (type A) range from 4.8 to 5.6 Ma, with a mean age of 5.3 ± 0.3 Ma. All zircons are >3.6 Ma older than the eruption age of the Deer Canyon rhyolite. No zircons were recovered from Del Medio samples. These data indicate that postcollapse rhyolites erupted within 54 ka of caldera collapse represent distinct magma batches which are not comagmatic with each other, nor with the preceding upper Bandelier Tuff. Deer Canyon rhyolite represents two distinct melt compositions which were mingled and erupted quickly, such that sufficient time for homogenization did not occur. Type A Deer Canyon rhyolite was produced by remelting of a pre-caldera pluton at depth, whereas type B rhyolite originated within the residual Bandelier crystal mush by crystal-liquid separation. Redondo Creek rhyolite was produced by interaction of a hotter, more mafic magma with residual Bandelier crystal mush, production of new rhyolitic melt and crystal transfer, followed by interaction of this magma with a third, hotter, more K-rich rhyolitic melt prior to eruption. Del Medio rhyolite appears to be a single batch of crystal poor rhyolite separated from residual Bandelier crystal mush.
V33C-1510
Advanced fractional crystallisation and homogenization of large-volume rhyolite before the Oraefajokull 1362 AD plinian eruption, SE Iceland
In the 50 km wide Icelandic rift zones rhyolite magma is generated by partial melting of hydrated metabasaltic crust, subsiding under the weight of the growing volcanic pile. This mechanism of silicic melt formation is indicated by the basalt-rhyolite bimodality and rhyolite O-isotope composition. The low 18/16O-isotope ratios of rift zone rhyolites trace the high-latitude meteoric water component of the subsiding hydrated basalts [1]. The rhyolites of the volcanic flank zones (VFZ), however, have generally as heavy oxygen as the associated alkaline to transitional basalts and intermediate volcanics [2,3]. The minor volcanic loading of the older, thicker and stronger VFZ crust is insufficient for significant subsidence, and less pronounced basalt-rhyolite bimodality combined with other geochemical features support silicic melt generation by fractional crystallization. An extreme case in Icelandic, as well as global, perspective is the rhyolite magma of the plinian eruption from the large VFZ-volcano, Oraefajokull, in 1362 AD [4]. Glass, mineral and bulk tephra analyses show no chemical variation exceeding the analytical precision for the entire erupted volume of 2 km3 DRE. This applies even to the glass shards from distant locations in Greenland, Norway and Ireland. The total phenocryst content is 0.5-1 wt percent, with oligoclase (An14 Ab81 Or5.5), fayalite (Fa99.7 Fo0.3) and hedenbergite (Wo44.7 En2.6 Fs52.7) constituting 50- 80, 10-25 and 10-25 percent of the total phenocrysts, respectively. The extreme mineral compositions (especially pure fayalite and hedenbergite) resemble those of the granophyres in the Skaergaard and Bushveld complexes and differ from all other investigated rhyolites. The advanced fractionation and homogenisation to form the erupted 2 km3 DRE rhyolite is petrogenetically challenging, and a parental magma chamber of 20-40 km3 seems like a conservative estimate. The time-scale of the historic magma chamber evolution under Oraefajokull is limited by the large 1362-eruption, followed by a minor benmoritic eruption in 1727 and the lack of geophysical indications of present crustal magma reservoirs. [1] H Nicholson, M Condomines, JG Fitton, AE Fallick, K Gronvold, G Rogers J. Petrol. 32, 1005-1020, 1991. [2] O Sigmarsson, M Condomines, S Fourcade, Earth Planet Sci Lett 110, 149-162, 1992. [3] T Prestvik, S Goldberg, H Karlsson, K Gronvold, Earth Planet Sci Lett 190, 211–220. [4] RS Selbekk, RG Trønnes, J Volc Geotherm Res 160, 42-58, 2007.
V33C-1511
Magma Recharge and Mixing Processes That Triggered the Eruption of Trachytes and Phonolites at Suswa Volcano, Kenya Rift, East Africa
Suswa Volcano, one of a series of Holocene central vent volcanoes located in the central part of the Kenya Rift, is divided into four major stages: 1) pre-caldera; 2) syn-caldera; 3) post-caldera I; and 4) post-caldera II. In addition to central vent volcanoes are basalt and basaltic-trachyandesite (BTA) flows (e.g. Tandamara and Elmenteita) that occur in low-lying areas adjacent to the central volcanoes. Both pre- and syn-caldera rocks include trachyte to phonolite. Syn-caldera rocks also include BTA similar to Tandamara. Matrix glass in pre-caldera samples is similar to whole-rock. However, for syn-caldera samples, light and dark mixed glasses are observed petrographically and compositions range from trachyandesite to trachyte. Pre-caldera samples have a phenocryst assemblage of anorthoclase (An0-5Ab50-60Or30-45), clinopyroxene (En27Fs28Wo45) and Fe-Ti oxide. Syn-caldera trachyte-phonolite contain this same assemblage but also plagioclase (An52Ab45Or3), with a composition identical to BTA samples. Clearly, the syn-caldera event represents magma mixing between BTA and trachyte. With the developing East Africa rifting, introduction of mafic magmas into the Suswa trachytic chamber was possible, similar to the rupture events in Ethiopia in 2005 (Wright et al., 2006). Post-caldera rocks have phenocrysts of alkali feldspar, olivine (Fa70), clinopyroxene and Fe-Ti oxides. Post-caldera II alkali feldspars are zoned and contain two different core compositions, one with low anorthite content similar to pre- and syn-caldera samples (An3Ab64Or33) and the other with higher anorthite content (An17Ab69Or14). They exhibit oscillatory zoning, with compositional variation between Ca2O and K2O and have thin rims with composition similar to the matrix feldspars. The thin rims may represent magma recharge that triggered eruption of the phonolite. Matrix glass in post-caldera rocks includes both trachyandesite and phonolite, indicating that hybridization of the contrasting magmas is still ongoing. Processes in addition to mixing contribute to this dynamic volcanic setting. Post-caldera rocks have compositions of essentially all elements that are intermediate between BTA and trachyte, as appropriate to mixing. However, samples show variable Na20 content at constant K20, Fe0, MgO, and CaO. A possible explanation for this variable Na2O content is assimilation of sodalite-bearing syenite roof and sidewall rocks into the evolving magma chamber. References: Wright, T.J., et al., 2006, Magma-maintained rift segmentation at continental rupture in the 2005 Afar dyking episode. Nature, 442: 291-294.
V33C-1512
Coeval Intrusion and Batholith-Wide Mingling in the Gobi-Tienshan Intrusive Complex, Southern Mongolia
The Gobi-Tienshan Intrusive Complex (GTIC) in southern Mongolia is the southernmost magmatic belt of the Central Asian Orogenic Belt (Geomin, 2003). The GTIC was likely generated as an active continental margin by the closure of the final ocean basin between Mongolia and the Tarim block (Fillipova, 1990), evidenced by its calc- alkaline, magnetite series magmatism and medium to high-K compositions. The system was likely augmented by subsequent continental collision (Fillipova, 1990). Detailed mapping of the GTIC reveals a batholith and associated volcanics constructed by a wide spectrum of magma compositions, which include hbl>bt granodiorites and hbl cumulates, hbl-free granites, high-K red granites, syenogranites and syenites, and andesite to quartz diorite dikes. Field evidence for the coeval nature of granodiorites and granites includes cusbate/lobate margins between units and mutual diking. Dikes are commonly found disaggregating into microgranitoid enclaves and hybridizing with both hbl-rich granodiorites and hbl-free granites. Dikes are likely related to magma mingling seen throughout the GTIC in granodioritic units, including an enclave mega-plume, where enclave percentages increase sharply from 5 to 50% for 15 km2 in aerial extent. These features are commonly on a multi-km scale, thus are unlikely to be the effect of re-heating. Field relationships are reflected in chemical trends, including incongruent REE and trace element patterns for mutually intruded magmas. The transition from chemically distinct to highly hybridized enclaves is also observed both in the field and in REE and other trace element trends. Meanwhile, structural orientations in the GTIC are highly varied, with sheets dipping generally <45o cut by a huge diabase dike swarm that is sub-vertical. Sheets are approximately parallel to an enclave foliation striking 40 degrees and dipping shallowly, while mineral fabrics defined by hbl and bt have a similar strike, but consistently steep dips. The diverse magma types in the GTIC are traditionally interpreted to reflect different tectonic environments, therefore their coeval nature implies rapid changes in tectonic environment or a highly unusual tectonic setting. Thus, the GTIC presents a natural laboratory for such high-flux magmatic systems and will contribute to constraining the terminating continental collision of the Central Asian Orogenic Belt.
V33C-1513
The importance of mixing in the evolution of silicic magmas in northern Costa Rica
Silicic volcanism in the northern Costa Rican segment of the Central American volcanic arc was widespread from the Miocene through the Middle Pleistocene. Ignimbrites in the Bagaces formation are among the earliest high- silica products in this part of the arc (<10 Ma) and are thus important to understanding its chemical, temporal and spatial evolution. The large silicic eruptions are especially interesting in this part of the arc, since Costa Rica is built upon thick oceanic plateau of the Chorotega block, located on the western extent of the Caribbean Large Igneous Province (CLIP). A detailed study of two ignimbrites demonstrates that mixing is an important process in the evolution of large silicic systems in this part of the arc. The Papagayo and Pan de Azucar Tuffs outcrop north of the Nicoya Peninsula in northern Costa Rica. The Papagayo Tuff contains mingled pumice fragments. Petrography, whole-rock chemistry and microprobe data are consistent with the mingling and eruption of rhyolitic and andesitic magma batches. Dacitic pumice fragments from the Pan de Azucar unit are more homogeneous and chemically similar to intermediate bulk-rock compositions of mingled Papagayo pumice samples, supporting a model in which the Pan de Azucar magma was a homogenized part of the Papagayo magmas. However, small differences in composition require modification of the Pan de Azucar magma by some other process (e.g. magma mixing or assimilation). The data supporting mingling and mixing in these units provide an ideal test case for Polytopic Vector Analysis (PVA). Relatively new to igneous petrology, PVA is a multivariate statistical program that can incorporate all available geochemical analytes to simultaneously unmix samples, finding the number and composition of end members required to explain the variation within the population. For the Papagayo samples alone, PVA yields a three end member solution. One end member is andesitic (57 wt.% SiO2) and the other two are both rhyolitic (71 wt.% SiO2), but have different trace element compositions. When the Pan de Azucar samples are included with the Papagayo samples, PVA generates a four end member solution that indicates mixing among two rhyolitic end members (71 and 72 wt.% SiO2), a dacitic end member (66 wt.% SiO2), and a basaltic end member (52 wt.% SiO2). These solutions are mutually consistent and supported by petrographic and chemical data from the rocks.
V33C-1514
Streaked Pumice From the Youngest Toba Tuff: New Constraints on Compositional and Temperature Gradients
Earth's largest Quaternary volcanic eruption (75 ka) expelled more than 2800 km3 of compositionally zoned silicic magma from Toba caldera, Indonesia, and produced the extensive Youngest Toba Tuff (YTT). Past petrologic studies identified that the YTT magma was zoned with respect to composition, temperature, and mineralogy. To understand the dynamics of differentiation leading to eruption of the YTT magma, we analyzed the geochemistry and mineralogy of rare mixed dacitic and rhyolitic pumice that extend the range of YTT compositions to less-evolved and apparently hotter compositions. Mixed pumices are streaked with light and dark domains mostly containing ca. 20-50% crystals of plagioclase, hornblende, quartz, biotite, and pyroxenes. In general, crystal size in the dark domains is smaller than in higher SiO2 pumice. Dark domains range in composition from dacite to low-silica rhyolite (63-72 wt.% SiO2, anhydrous); light domains typically contain higher SiO2 (65-74 wt.% SiO2, anhydrous) than their darker counterparts. SiO2 concentrations of the pumice domains correlate with trace element concentrations, including positive covariation with typically incompatible elements (e.g., Nb, Ta, U, Th) and negative covariation with typically compatible elements (e.g., Sr, Eu, Sc). Domain glasses mimic these same trace element trends. Within both domain types, plagioclase composition varies from An30 to An50. Hornblende composition generally covaries with pumice SiO2. Pressures calculated using the Johnson and Rutherford (1989) barometer range from ca. 2 kb to 5 kb. However, pressures calculated using the Anderson and Smith (1995) barometer range from ca. 0 kb to 2 kb. Hornblende-plagioclase pairs yield temperatures ranging from ca. 800° to 900°C, while equilibrium Fe-Ti oxide pairs yield temperatures ranging from ca. 760° to 800°C. In general, dark pumice domains yield the highest temperatures. The Pb and Nd isotope compositions of domains within single streaked pumice samples are variable, including different isotope compositions for glasses from adjacent streaks. Isotopic heterogeneity suggests mixing of melts that experienced different degrees of open-system evolution. The mixed pumice samples provide a new limit on the mafic end-member of YTT magma composition, and record mixing of magma with contrasting origins. Mixed pumice containing cm-scale compositional heterogeneity suggests mixing of contrasting magmas just prior to, or during, evacuation of the voluminous YTT reservoir, perhaps in response to heating after intrusion of new magma. Differences in hornblende composition between dacitic and rhyolitic pumice may reflect contrasting depths of crystallization for dacitic and rhyolitic end- members, or temperature-induced control of amphibole composition within a shallow reservoir beneath Toba.
V33C-1515
Pre-eruption Thermal Rejuvenation and Stirring of a Partly Crystalline Rhyolite Pluton Revealed by the Earthquake Flat Pyroclastics Deposits, New Zealand
The Earthquake Flat Pyroclastics (EFP) form a 10 km3 rhyolite deposit erupted at 50 ka from the margin of Okataina Volcanic Centre (OVC), immediately following the caldera-forming eruption (100 km3) of the Rotoiti Pyroclastics from vents 20 km to northeast. The EFP deposits display textural and compositional complexity on a crystal-scale consistent with remelting and stirring of a near-crystalline pluton located in the upper crust. Quartz and plagioclase crystals are extensively resorbed, while hornblende and biotite are euhedral. Pre-eruption temperatures estimated from Fe-Ti oxides cover a wide range (702 - 805°C). Differences of up to 70°C within individual pumice lapilli show that crystals were chaotically juxtaposed during magma stirring and evacuation. Al- and Ti- zoning reflects thermally-controlled atomic substitutions within hornblende crystals, with rim-ward increases in temperature of about 50°C estimated from plagioclase-hornblende geothermometry. The EFP deposits contain no evidence for widespread magma-mixing with a mafic intrusion that could have acted as a heat source; this absence implies a convective self-stirring process that was likely driven by mafic underplating. Extensive resorption of crystals deep in the EFP magma may have produced a Ca, Fe and Mg-enriched rhyolite melt that percolated upward allowing the growth of reverse zoned hornblende. Similar textural rejuvenation features are also found in late ejecta of the Rotoiti Pyroclastics, suggesting that the Rotoiti-EFP eruptions disrupted a semi-continuous, partly crystalline pluton that extended at mid-crustal depths across the OVC. Development of this pluton coincided with a period of relative volcanic quiescence lasting >100 kyrs and is consistent with previous isotope studies that demonstrate a long crystallisation history for the magmas. The rejuvenation of the EFP magma, together with microdiorite lithics in the EFP and Rotoiti deposits and a small basaltic eruption that immediately preceded the Rotoiti eruption, suggest that extensive mafic underplating provided a major thermal and volatile pulse to drive these caldera eruptions.
V33C-1516
Geochemical Constraints on Rhyolitic Source Processes, TaupoVolcanic Zone, New Zealand
High-Si rhyolite magmas of Central North Island New Zealand (Taupo Volcanic Zone) occur as discrete physiochemical batches that are produced both within and between eruption episodes. The processes that create these compositional variations are magma mixing and mingling, crystal fractionation and variation in source parameters (melt proportion, source composition, residual mineral assemblage). Here we use a suite of samples selected to exclude magma mixing and mingling and examine the role of fractionation and variable source parameters in the production of magmas erupted from adjacent active caldera complexes (Okataina and Taupo Volcanoes) that have been active during the last 50 ka. Although both volcanoes have been highly active during this period, eruption rates are different as are phenocryst proportions and assemblages, pre-eruption temperatures and oxygen fugacity. REE abundance patterns all show inclined La-Sm and flat Gd-Lu. Magma batches show different total REE abundances and there are also variations within batches; most show small negative Eu anomalies. Modeling indicates that all have essentially the same origin in amphibole dominated middle crustal (0.3-0.5 Gpa) sources. However, HFS element abundances and ratios demonstrate the presence of source compositions that are discrete and indicate that the proportions of melting vary between magma batches even on short time scales (less than 5 ka). The magmatic systems feeding rhyolite volcanoes of the Taupo Volcanic Zone are characterized by continuous melting and rapid assembly prior to eruption rather than by the long term accumulation of magmas in magma chambers. This dynamic and unstable behavior is related to the thin crust (less than 25 km) and very high heat flows that characterize the central North Island of New Zealand and is in contrast with longer lived systems found in mature continental settings such as Yellowstone in the USA.
V33C-1517
Trace Element and Textural Variation in Plagioclase Phenocrysts from Prehistoric Eruptions at Mount St. Helens
Ra-Th dating of major phases in lavas from Mount St. Helens (MSH) has indicated that plagioclase phenocrysts resided for up to several thousands of years before finally being erupted. In most cases, these plagioclase contain significant compositional zoning related to changing magmatic conditions. Hence, individual and populations of plagioclase may preserve an important record of crystallization and thermochemical evolution beneath MSH. In order to quantify the record of magmatic evolution recorded by prehistoric plagioclase at MSH, we have measured major and trace element compositions via LA-ICPMS and electron microprobe in thirty single crystals from dacite and andesite lavas that erupted over the last four millennia (during the Smith Creek, Pine Creek, Sugar Bowl, Kalama, and Goat Rocks episodes of the Spirit Lake stage). In each episode, a significant proportion of the plagioclase contain distinct cores, some with sieved and/or resorbed textures, that are surrounded by rims with oscillatory zoning of variable thickness. Inclusion-free cores have compositions that are similar between plagioclase from the different episodes, with an average of An40 (range of An27 - An47), while sieved cores have a range from An20 to An80. Rim compositions also show significant variability, with a range of An31 to An65. Ba and Sr concentrations in plagioclase across the eruptive episodes contain up to 10-fold variation in concentration (~30-300 ppm, ~600-1800 ppm, respectively). Plagioclase from Pine Creek, Sugar Bowl (East Dome) and Kalama dacites have marked decreases in Sr concentration within the rim-most 50 to 100 micrometers. Near-rim concentrations of Ba are decoupled from Sr, with variation between crystals from the same sample as well as eruptive episodes. Similar isotopic ages, major and trace element compositions, and disequilibrium textures for plagioclase cores between eruptive episodes suggest that recycling of crystal material was significant in the MSH magmatic system over the last 4000 years. Near-rim changes in Sr and Ba concentrations and covariation likely result from the kinetics of ascent-driven crystallization, as is recognized from other studies at Mount St Helens (e.g. Berlo et al., 2007). This rim-ward zoning preserves an important record of magmatic evolution prior to eruption, which may be amenable to geospeedometry.
V33C-1518
Composition of Magmas, Minerals, and Melts of Tuffs and Lavas from the San Luis Caldera Complex, Colorado: Framework for Crystal Recycling and Melt Extraction Processes
The stratigraphy of the Oligocene San Luis Caldera Complex comprises three voluminous tuffs issued from overlapping calderas and numerous, compositionally diverse pre- and post-caldera lavas. The entire complex was erupted over an amazingly short period of 50-100 k.y. From oldest to youngest, the tuffs are the Rat Creek Tuff (>150 km3) that ranges in bulk composition from dacite to rhyolite to high-silica rhyolite, the dacitic Cebolla Creek Tuff (>250 km3), and the Nelson Mountain Tuff (>500 km3) that is again compositionally zoned from dacite to high-silica rhyolite. Pre- and post-caldera lavas range in composition from andesite to high-silica rhyolite. Phenocryst contents of most units range from 15 to 25%; only high-silica rhyolites are notably crystal-poorer (?5%) and tuffs can reach crystal contents of 35-45%. The compositional and lithological framework of the San Luis Caldera complex combined with the tight temporal and spatial control provide an ideal field laboratory for investigating whether minerals in younger volcanic units may be in part derived from recycling crystals of non-erupted magma of older units and for investigating generation processes of crystal-poor high-silica rhyolites. Many eruptive units are distinct on the level of bulk rock in terms of major and trace elemental composition and/or mineral proportions. For example, the dacitic to rhyolitic Rat Creek and Nelson Mountain Tuffs are dominated by pyroxene with only minor amphibole while the intervening dacitic Cebolla Creek Tuff contains abundant amphibole with subordinate pyroxene. When glass and mineral compositions are considered, eruptive units are less distinctive and observed compositions of individual units may cluster tightly or spread over most of the observed range for all units but in either case individual units typically follow broad compositional trends defined by all units. Interstitial melts of andesites to rhyolite are rhyodacitic to high-silca rhyolite.
V33C-1519
Crystallization and Melt Removal at Arenal Volcano, Polytopic Vector Analysis
Tephra sequences ET3 and ET4 from Arenal volcano in Costa Rica have recently been interpreted to be a product of crystal fractionation by Bolge and coworkers in a series of papers (2004, 2006). The two tephra units are part of a sequence of 22 tephra units that represent a 7000 year span of the Arenal volcano activity. The tephro- stratigraphy has been described extensively by Melson (1982; 1994). The ET3 and ET4 tephras were interpreted (based on major- and trace-element, isotopic analyses of whole rocks and microchemical analyses of individual phases) as clear evidence of crystal separation by gravity settling (Bolge et al., 2004, 2006). The lower ET4 tephra sequence (andesitic and crystal poor) and the upper ET3 tephra (basaltic and crystal rich) represent an inverted snapshot of the magma chamber with contrasting geochemical properties. The ET3 sequence (deeper part of the magma chamber) has nearly constant composition with only a few elements varying stratigraphically (best represented by CaO). This is consistent with gradually decreasing amounts of melt in the upper part of ET3. The lower ET4 tephra (upper part of the magma chamber) contains large chemical gradients in both incompatible and compatible elements. In the present study we use whole-rock geochemical data from the recent tephra sequences ET3 and ET4 as inputs to Polytopic Vector Analysis (PVA) (for a review of this method see Vogel and coworkers, in press). With this method we produce a three end member solution that is consistent with crystallization of Olivine, plagioclase and pyroxene from the most mafic end member (EM1) resulting in a crystal rich mush zone. As crystallization progresses the compositions of the liquids are driven towards an intermediate end member (EM3), which has an intermediate composition liquid. At EM3 composition, rapid depletion of FeO, MgO and TiO2 by crystallization of Fe-Ti oxides, rapidly drives the liquid composition towards the silicic EM1 (incompatible element enriched end member). Using PVA we refine the interpretations of Bolge and coworkers and show that melt from the crystalline rich ET3 tephra was removed and ponded in the magma chamber above the crystalline mush (top part of ET4 unit). Thus when the eruption occurred the most evolved tephra (ET4) were deposited first followed by the least evolved tephra (ET3), which resulted in sampling of a chemically zoned magma chamber. Using PVA on stratigraphically controlled whole-rock analyses of tephra samples, we can unambiguously identify processes and end members that are involved in crystal accumulation and liquid separation processes. Thus PVA is a rigorous analytical tool that uses only whole-rock chemical data to produce robust results that can be used with other analytical techniques to test petrological models.
V33C-1520
Complete Chemical Analyses of Amphibole and Biotite: Evidence for Thermal Input and Volatile Loss in Shallow Silicic Magma Chambers by Multiple Mafic Magma Recharge Events at Lassen Volcanic Center
The effects of periodic recharge of mafic magma into or under silicic magma bodies were investigated by acquiring complete chemical compositions, including Fe3+/Fe2+ ratio, water content, and D/H ratio microanalyses, for coexisting biotite (Bt) and amphibole (magnesiohornblende, MHb) separates from silicic volcanic rocks at the Lassen Volcanic Center, California. Eruptive units studied include the ~35 ka rhyolite of Kings Creek lava and pyroclastic flow complex (~70 wt% SiO2), the ~27 ka multi-lobed dacite of Lassen Peak dome complex (70 - 66 wt% SiO2), the ~1.1 ka Chaos Crags dome and pyroclastic flow complex (70 - 68 wt% SiO2), and the 1915 eruptions of Lassen Peak (63.9 - 59.5 wt% SiO2). Key findings to date include the following: (1) Bt and MHb cation chemistry is monotonous. (2) MHb always contains minor amounts of Bt. (3) Chaos Crags samples were least affected by low or high temperature post- eruption alteration. From the oldest to youngest Crags eruptions, Bt water contents dropped dramatically and Fe3+/Fe2+ ratios concomitantly increased. MHb has a weaker, noisier inverse trend in water contents and Fe3+/Fe2+ ratios. Corresponding δD values for Bt range from -75 to -30 ‰, while MHb δD values are confined to a narrower range (-71 to -52 ‰). (4) The most water-enriched Bt (up to 5.6 wt%) and MHb (up to 2.5 wt%) is present in the lithic pyroclastic flow of Kings Creek. (5) Lassen Peak dacitic samples have oxy-Bt (2.5 - 1.7 wt% H2O) and surviving oxy-MHb (1.9 - 1.6 wt% H2O) that are among the driest in rocks of the eruptive sequences. (6) MHb from 1915 units is almost completely reacted to an anhydrous mineral assemblage (pyx, plag, opq), and oxy-Bt have low H2O contents and high Fe3+/Fe2+ ratios for dome (2.1 wt% and 2.0) and lava flow (2.2 wt% and 1.6) separates, respectively. These relations are interpreted to indicate that thermal input from mafic magma recharge events and degassing in perturbed silicic magma chambers are primary driving forces for changes in hydrous mineral H2O contents, δD values, and Fe3+/Fe2+ ratios.
V33C-1521
A New Explanation For Oscillatory Zoning In Plagioclase Phenocrysts
Plagioclase phenocryts in volcanic rocks commonly record various differentiation processes in their magma chambers. A fundamental assumption is that the normal zoning followed by a reverse zoning or oscillatory zoning are either due to the changes in temperature (T) during mixing of two magmas or due to the fluctuations in water pressure (PH2O) and total pressure (PTotal) during decompressional crystallization. However, this assumption is not always true as demonstrated here for a parental basaltic composition from Small Hasandag Volcano, Central Anatolia, Turkey. Our hypothesis is that the changes recorded in the plagioclase zones of the Small Hasandag Volcano can be explained by the differentiation of magma under isochoric conditions. Once the magma fills the magma chamber, system is isochoric between two successive eruptions provided that wall rocks are rigid. Under these conditions, volume (V) and T are independent variables, whereas P is a dependent variable which changes with subtle differences in V of crystals and melt during crystallization. In order to test this hypothesis, a parental composition (Mg#=61.98 with 3% H2O) is allowed to differentiate under isochoric fractional crystallization conditions. First, a rapid decrease in P has been recorded due to the bulk compressibility 10-6bar-1. Later in the crystallization history of the magma, the water in the original magma, exsolves causing a P increase. As denser minerals crystallize in response to the decreasing T, P decreases significantly from 1500 bars to a minimum of 900 bars at about 7% crystallization and then increases to 1500 bars at about 75% crystallization and continues to increase to 1600 bars at 85% of crystallization and then decreases again to 1540 bars at 90% of crystallization. These fluctuations in P affect the anorthite content (XAn) resulting in normal, reverse and oscillatory zoning in plagioclase phenocrysts.
V33C-1522
Dynamic Magma Chamber Processes Reflected in Trace-Element Data From the May 1915 Eruption of Lassen Peak, California
We present new WDXRF trace-element data from products of the May 1915 eruption of Lassen Peak in Lassen Volcanic National Park (LVNP), California. Four main rock types are present in the 1915 deposits: light dacite, black dacite, dark andesite and andesite inclusions in light and black dacites. Trace-element data collected for this study confirm that rocks produced by the 1915 eruption at Lassen Peak are the result of complex magma mixing. Similar to the conclusions of Clynne (1999), we find that dacites present in the 1915 eruption have been variably modified by addition and disaggregation of andesite inclusions. Geochemical evidence shows that dark andesite was modified by mixing with variable amounts of dacitic magma. Andesite inclusions, although the most mafic of the rocks erupted in May 1915, display disequilibrium mineral textures and, thus, cannot represent the ultimate mafic end member. New trace element data also indicate that processes that generate the mafic end member may be more complicated than previously recognized. Although not incompatible with mixing, our trace element data record large variations in some incompatible trace element compositions of andesite inclusions, suggesting two possible mechanisms: 1) varying proportions of incompatible element-bearing phenocrysts or 2) interaction with an aqueous phase. Our conclusions mirror those based on published major-element data, supporting the idea of a multiple-stage mixing process and providing insight into magma chamber dynamics (Clynne, 1999). The 1915 eruption of Lassen Peak involves mixing of basaltic andesite with dacite to generate hybrid andesites that crystallized and vesiculated to form mafic foam. The vesiculated foam layer is unstable and breaks up into fragments (andesite inclusions), which are buoyant and are stirred by convection into the overlying dacite. Continued hybridization and heating of the reservoir dacite through partial disaggregation of andesite inclusions increases the temperature, modifies the composition of dacite and eventually cuts off formation of foam. Increased temperature and buoyancy of black dacite may have triggered the eruption on May 19, 1915, when three types of magma erupted: light dacite, black dacite and andesite inclusions. The final step in the mixing process involves the mingling of black dacite and dark andesite magmas to produce the banded pumice observed only in the May 22 eruption. Our trace element study strengthens conclusions supporting a complex mixing process to generate compositionally diverse products of the 1915 eruption and adds to current knowledge about dynamic magma chamber processes and the chemistry of magma mixing.
V33C-1523
Experimental Determination of Olivine Reaction Rim Growth Rates in the Lassen Peak, CA Magma Chamber
A series of hydrothermal (high-pressure and –temperature) experiments have been performed on starting materials consisting of ~5 wt% of Fo85 olivine separated from a spinel lherzolite xenolith (UM-5) from Kilbourne Hole, NM combined with powdered natural dacite pumice from the 1915 eruption of Lassen Peak, CA. The dacite represents the silicic end member (67.8 wt% SiO2) of mixed dacite and andesite magmas present in the 1915 Lassen Peak eruptive products. The olivine represents xenocrystic grains from the admixed andesite that combined with the dacite magma prior to the eruption. In the natural dacitic eruptive products, xenocrystic olivine grains display morphologically complex reaction rims of orthopyroxene. These rims are interpreted to have grown during magma mixing/mingling prior to the eruption. The growth rate of these rims may be used to constrain the length of time between magma recharge and eruption. Time series experiments were performed to reproduce olivine reaction rims within the dacite. Experiments were performed with run durations of 50, 100, 200, 400, and 600 hours at 50 MPa and 825°C and 875°C. Water-saturated run conditions at the Ni-NiO buffer represent plausible dacite equilibration conditions based on earlier phase equilibria experiments. Preliminary measurements of orthopyroxene rim widths measured on two olivine grains from experiments run at 50 MPa and 825°C for 50 and 600 hours show an increase in rim width from 2.2±1.8 μm to 3.3±2.5 μm, respectively (average and 1σ standard deviation of 490 and 651 linear measurements). In the 50-hour experiment, rim widths range from 0 to 13.4 μm, whereas in the 600 hour run, widths are 0.63-23.1 μm. The 600 hour run also has a larger percentage of width measurements above 5 μm (24.8% of measured widths, compared to 2.8% of measurements in the 50-hour experiment.) In the 50-hour experiment, 21.0% of the rim width measurements are 1 μm or less, whereas only 6.4% of measured olivine reaction rim widths are <1μm in the 600-hour run. Additional measurements on these and other experiments are under way using the linear measurement technique applied here, as well as other methods to estimate rim width and area. Experimentally determined rim growth rates will be compared with rims on the natural samples in order to constrain the timing and duration of magma mixing prior to the eruption.
V33C-1524
Magnetic fabric of the Torres del Paine, Patagonia
The Miocene bimodal Torres del Paine Intrusive Complex (TIPC) consists of the lower Paine-Mafic-Complex (PMC) and the overlaying Paine Granite. Igneous structures (layering, xenolith orientations, and major contacts) are vertical in the westernmost part of the PMC, while they are sub-horizontal in the central and eastern parts. The TPIC is very homogeneous except in the PMC where complex interactions between mafic and felsic magmas were observed. No foliation or lineations are present in the field. Based on field-observations we distinguish at least three different sheet-like units in the Paine Granite, each one several tens to hundreds of meters thick. Younger sheets generally under-plate their precursor. We measured the anisotropy of magnetic susceptibility (AMS) on 145 samples from sites distributed horizontally over the entire surface of the TPIC and along 4 vertical profiles. The high intensity of the magnetic susceptibility for most of the Paine Granite and the PMC samples is due to the presence of magnetite. The magnetic lineations show a general WNW-ESE trend, parallel to the long axis of the intrusion. Lineations in the western part are strongly west-dipping to subvertical, parallel to the magmatic layering in the PMC. This supports the interpretation of a feeder-zone in that area. The lineations are sub-horizontal in the rest of the intrusion. Only minor variations in lineation orientation are observed in several hundred meter long vertical profiles in the central part of the Paine Granite. The magnetic foliations show the structural control exerted by the contact of the intrusion during its assembly: they are mostly horizontal in the central part, and are progressively steeper towards the contact. The low anisotropy is in agreement with the absence of texture observed in the field and thin sections. No sub-solidus deformation took place during the TPIC construction. The internal structures and magnetic fabrics support the interpretation of an intrusion assembled by horizontal transfer of magma in the form of sheet-like pulses stacking them vertically. Our results also show that the regional syn-magmatic deformation played no role in the emplacement of the TPIC.
V33C-1525
Evidence for volcano-pluton connections in the distal volcanic record: Examples from the western United States
The supracrustal cover rocks of batholith belts are often rapidly eroded, leaving at best a fragmentary record of contemporaneous volcanism. A complementary record is often preserved as distal volcanic ash in sedimentary basins. This record is also incomplete and depends on deposition in favorable environments and on a structural history that first buries and preserves and then allows erosion to expose the record. When pieced together with near the source record, a more complete picture of the connection between plutonic and volcanic rocks can be constructed. For example, the Mesozoic and Cenozoic history of the western United States is marked by the development of plutonic belts of different ages and locations that migrated in response to geodynamic changes. For much of this time, favorable sedimentary environments existed in the back arc, hundreds or thousands of kilometers away from the plutonic belt. Analysis of of the existing sedimentary record and its enclosed volcanic components shows that episodes of volcanism and plutonism correspond closely in space and time. Magmatic pulses found in the plutonic record are generally obvious in the volcanic record-with abundant ash in the Triassic, Middle and Late Jurassic, Cretaceous, and late Paleogene and Neogene. However, less ash is found during the Middle Jurassic, even though sedimentary rocks are preserved behind the arc. This is consistent with the observation that much of the Middle Jurassic arc in California was submarine, hindering the eruption and widespread dispersal of volcanic ash behind the arc. Moreover, as the locus of plutonism migrated, so did the deposition of volcanic ash. During the Jurassic and middle Cretaceous ash is common in the Colorado Plateau and adjacent regions to the south. When the Idaho and Boulder batholiths developed, thick ash accumulated in Montana and Wyoming. Although it has been suggested that volcanism and plutonism are decoupled in space and time, we conclude that for most of the Mesozoic and Cenozoic, plutonism and volcanism were closely coupled in the western United States. Finally, the distal volcanic record is strongly biased toward preserving deposits from large eruptions formed when large volumes of molten magma existed in a chamber. Thus, the abundance of these ash deposits implies that batholith-size chambers (>10 km across) filled with magma existed across much of the Cordillera
V33C-1526
Magnetotelluric imaging of the Miocene Kumano-Omine volcano-plutonic complex in the Kii Peninsula, southwest Japan
The Kumano Acidic Rocks and the Omine Granitic Rocks, which are located in the Kii Peninsula, southwest Japan, is representative of the large igneous complexes in the Middle Miocene. They are adjacent each other within several kilometers and generally extend along the Omine Mountains in an N-S direction. A wide-band magnetotelluric survey was carried out with a 50 km long MT profile running across the Omine Mountains in order to image the deep crustal structure of the Kumano Acidic Rocks and the Omine Granitic Rocks. A body with high resistivity (~1000 ohm) is clearly visible in the subsurface shallower than 20 km, which corresponds to the distribution of the Omine Granitic Rocks and the western part of the Kumano Acidic Rocks. The resistive body extends to about 20 km depth and widens with increasing depth. The model suggests that these two igneous complexes are connected in the subsurface rather than discrete resistive body corresponds to either igneous complex. Radiometric ages, geochemical compositions and other geological data also support that the resistive body represents the fossil of large silicic magma system in relation to the Kumano-Omine volcano-plutonic complex. It has been established that relative uplift took place in the central part of Kii Peninsula after the Miocene. Therefore, the top part of the Omine Mountains on which the Omine Granitic Rocks are distributed has likely undergone more exhumation than the coastal area in which the Kumano Acidic Rocks are located. As a result, the Kumano Acidic Rocks and the Omine Granitic Rocks may correspond to the shallower and the deeper parts respectively, of the Kumano-Omine volcano-plutonic complex.
V33C-1527
Incremental formation, differentiation, and explosion of an arc magma chamber: Isotopic and physical study of multi-caldera Ksudach volcano, Kamchatka, Russia
Ksudach volcano is the most explosive Holocene volcano in Kamchatka that had four episodes of caldera formation in Holocene: 8800 BP (2km3), 6300 BP (1km3), 6000 BP (7-8km3), and 1800 BP (18-19 km3, as well as two episodes in Pleisocene (one at ca 160ky and the other younger). While mineral compositional trends suggest fractional crystallization is sufficient to explain magma diversity, oxygen isotopic trends in conjunction with Sr and Nd isotopes suggest progressive formation of a large magma chamber by assimilation of shallow crust prior to the 1800 BP eruption. Lavas and smaller volume pyroclastic units belonging to the pre-1800BP sequence form a temporal array of decreasing d18O values of phenocrysts and increasing Sr isotopic values. Oxygen isotopes in Holocene Ksudach melts decrease from 5.4 to 4.7 permil, while Sr isotopes increase from 0.70331 to 0.70338, and two isotopes anticorrelate. Piston cylinder phase equilibria experiments suggest shallow, 1.0 kbar preeruptive storage conditions for the products of the 1800 BP KS1 caldera-forming eruption. Low-d18O values, shallow pressures of storage, and temporal isotopic trends allows us to suggest that at least 18-19km3 of siliceous magma was stored in the very shallow crust prior to the 1800 BP caldera formation, and that an entire magma reservoir was emptied in the course of that eruption. Older and smaller episodes of caldera-formations did not emptied the magma chamber completely: trend of d18O depletion persisted while Sr and Nd isotopes exhibited quickly recuperating wiggles toward pre-caldera values.Temporal isotopic trends suggest that the dominant petrogenetic process for thousands of years before the 1800 BP eruption was assimilation and melting of low-d18O hydrothermally altered carapace around this magma body. Following the 1800 BP caldera-forming eruption, largely basaltic Shtyubel" stratocone started to form that had produced ~1km3 dacitic differentiate in the course of 1907 AD eruption. These youngest products of the intracaldera Shtyubel cone have more normal, yet diverse, in d18O and 87/86 Sr values, signifying a fundamentally new cycle of magmatic activity. This study demonstrates that tens of cubic kilometers of siliceous magma can be produced, stored for thousands of years, and erupted effusively and explosively from a magma chamber only 3km below the surface. At the same time, roof melting and assimilation proceeds rapidly at such shallow depths.
V33C-1528
HREE Enrichment in Iron-rich Pyroxenes from Felsic Liquids
Anomalous HREE enrichments in iron-rich pyroxenes from felsic liquids have been observed in a number of different systems (e.g., sodic salites in the phonolitic Fasnia Member, Tenerife; hedenbergites in the high-silica rhyolitic Bandelier Tuff, New Mexico, and aegirines and aegirine-augites in the Illimaussaq alkaline intrusion, Greenland). HREE enrichments are not predicted by the simple single-site lattice strain model for crystal-liquid partitioning of REE between pyroxenes and liquids, however HREE-enrichment can be modeled if the REE are present in both eight-fold and six-fold coordination states. Because ionic radii for REE in 8-coordination are very similar to the ionic radius of Ca in 8-coordination (1.16 to 0.977 angstrom and 1.12 angstrom, respectively), REE are typically modeled to substitute for Ca in the pyroxene M2 site. Considering that REE might also substitute for Fe (0.78 angstrom) and Mn (0.83 angstrom) in octahedral coordination, in either M2 or M1, creates a situation that favors the smaller ionic radii of the HREE over other REE (e.g. Lu(VI): 0.860 angstrom). This implies that measured partition coefficients for the REE are the sum of the different partition coefficients for the two coordination states. Observed HREE-enriched patterns of the Fasnia sodic salites are predicted using this modeling approach, as are HREE-enriched hedenbergites from the Bandelier Tuff and aegirines and aegirine- augites from Illimaussaq. Yttrium is interesting because its radius in 8-coordination in larger than that of Ho (1.019 vs. 1.015 angstrom), but in 6-coordination is effectively identical (0.900 vs. 0.901 angstrom, respectively). Predicted Y partition coefficients for iron-rich pyroxenes are most similar to Ho.
V33C-1529
Rare Earth Element (REE)- SiO2 Variations in Intra-oceanic arc Felsic Liquids Generated by Basalt Fractionation and Amphibolite Melting: A Potential test for Discrimination
The origin of felsic (>63 wt. % SiO2) magmas in intra-oceanic arc settings is still in doubt. Two very different processes are currently invoked to explain their origin. These include fractional crystallization of basaltic magma and partial melting of lower crustal amphibolite. Because both fractionation and melting can lead to similar major element, trace element and isotopic characteristics in felsic magmas, such lines of evidence have been generally unsuccessful in discriminating between the two processes. However, two seemingly different lines of reasoning may provide such a means of discrimination. First, an important difference between the two processes is the role played by hornblende. In basalt fractionation, hornblende is usually not an important fractionating phase. In amphibolite melting, hornblende is an important residual phase during much of the melting interval during which felsic magmas are produced. Second, Rare Earth Element (REE) D values increase significantly with increasing SiO2 content of the liquid phase. When these two factors are incorporated into carefully constrained mass-balance models for basalt fractionation and amphibolite melting, the following general predictions emerge. For felsic liquids in intra-oceanic arcs: (1) hornblende-absent fractional crystallization should produce steadily increasing REE abundances with increasing liquid SiO2; (2) hornblende-bearing fractional crystallization should yield constant REE abundances with increasing liquid SiO2; and (3) amphibolite melting should produce either constant and then decreasing, or steadily decreasing REE abundances with liquid SiO2. These predictions constitute a potential means of determining whether or not a given suite of intra-oceanic arc felsic lavas were generated by fractional crystallization or amphibolite melting.
V33C-1530
The role and fate of peritectic garnet in S-type granite Petrogenesis: The example of the Cape Granite Suite (South Africa)
The 560-530 Ma S-type components of the Cape Granite Suite (CGS) vary in composition from granodiorite to leucogranite. In places these rocks contain an abundance of xenoliths, including rare granulite facies metasediments and metabasic rocks. Thermobarometry applied to these assemblages gives consistent results of 850 ± 45 °C and 9 ± 1 kbar. These conditions are interpreted to reflect those of the source area at the time of magma genesis and lie within the experimentally determined interval for biotite fluid-absent melting, which, under these conditions, produces garnet as the dominant peritectic phase. Entrainment of different proportions of this generation of garnet (up to 20wt%) into the experimentally determined leucocratic melt compositions produces magma compositions that match well with the major element bulk composition of the CGS S-type granites. Zr and Fe + Mg are strongly positively correlated in the CGS S-types, and in many rocks occur in concentrations exceeding those possible in pure melts. This indicates the co-entrainment of peritectic garnet, derived from biotite breakdown, and, and zircon, typically hosted as an inclusion within biotite prior to anatexis. A model for the trace element composition of the granites, that is a near perfect fit with the natural rocks, can be created by applying a partial disequilibrium melting model. In this model, elements contained within pre- existing metamorphic garnet and the refractory fraction of zircon are not available for sequestration into the melt and melt trace element abundance is a function of the trace element composition of the accessible portion of the source, the fraction of zircon that dissolves and partitioning between melt and the peritectic garnet and other residual minerals. Magma trace element compositions are a function of the melt composition and the compositions of the entrained garnet, zircon and other accessory minerals such as monazite. Ce concentration in the granites over that which occurs in the compositions identified as close to pure melts is taken to be a proxy for monazite entrainment. Garnet in the CGS rocks is strongly zoned from relatively homogenous Mn-poor interiors to Mn-enriched rims some 100 μm in width. Pseudosections created for the more mafic varieties of S-type granite indicate that the garnet interiors equilibrated at close to 750 °C and 5 kbar and the rims at 730 °C and 3 kbar. This is interpreted to reflect re-equilibration and then partial re-equilibration of the entrained peritectic garnet during the ascent of the magma.