V32C-01 INVITED
Geochemistry of Apatite in Climactic and Pre-Climactic Tephra from Mt. Mazama, Crater Lake, Oregon
Apatite is a common accessory mineral in arc volcanic rocks that potentially records information about the dissolved volatile (S,Cl,F,OH) and trace-element concentrations (Sr, Ba, REEs) of the melt from which it crystallized. In a previous study of apatite from arc and convergent margin volcanic rocks, Peng et al. (1997) reported 0.63 wt.% SO3 in Mazama apatite grains with a corresponding SrO content of 0.18 wt.%, comprising some of the highest SO3 and SrO values in their data. Our electron microprobe study of apatite in climactic and pre-climactic Mazama tephra was done in order to assess possible correlation of apatite SO3 with Sr content of low-Sr and high-Sr recharge magmas identified based on whole-rock and matrix glass data (Bacon and Druitt, 1988) and Sr content of plagioclase (Druitt and Bacon 1989). Samples chosen represent all magmatic components erupted during the ca. 7700 year before present climactic eruption and precursor Llao Rock and Cleetwood eruptions. We compare the S, Cl, and F content of Mazama apatites with recent experimental data for S, Cl, and F partitioning between apatite and melt and with dissolved volatiles previously measured in melt inclusions from corresponding or similar Mazama samples. Our electron microprobe data confirm the presence of rare Mazama apatites with up to 0.78 wt.% SO3 and 0.12 wt.% SrO in Llao Rock, Cleetwood, and climactic scoria and pumice samples. However, high SO3 and SrO apatites are not restricted to high-Sr scoria hosts, but have been observed in low-Sr scoria, in Llao Rock rhyodacitic pumices and in Cleetwood rhyodacitic pumices, thus indicating significant magma mixing prior to the Llao Rock, Cleetwood and climactic eruptions. Most apatite SO3 and SrO data falls within the 0.06 to 0.36 wt.% and 0.04 to 0.12 wt.% range, respectively. Experimental data on SO3 partitioning between apatite and melt and maximum sulfur contents of 300 to 350 ppm measured in climactic and Cleetwood rhyodacitic melt inclusions predict that most Mazama apatites in Cleetwood and climactic pumices should contain 0.4 wt.% or more SO3. Most apatites in Cleetwood and climactic rhyodacites contain less than 0.20 wt.% SO3 indicating crystallization from rhyodacitic melt that exsolved a sulfur-rich vapor prior to eruption that drastically reduced melt sulfur concentration. Apatites with SO3 greater than 0.60 wt.% most probably were derived from more mafic oxidized magmas with dissolved sulfur contents of 2000 ppm or more that have been measured in high-Sr andesitic scoria melt inclusions. Mole fraction fluorapatite in Mazama apatites ranges from 0.20 to 0.96 and based on comparison to recent experimental data predicts melt F concentrations of 200 to 1800 ppm that agrees with measured F in melt inclusions of 200 to 1300 ppm. Mole fraction chlorapatite ranges from 0.11 to 0.19 and based on recent experimental data predicts Cl concentrations in rhyodacitic melt of 0.3 to 0.4 wt.% in reasonable agreement with Cl concentrations in melt inclusions ranging from 0.18 to 0.39 wt.%. References Bacon C.R., and Druitt T.H. (1988) Compositional evolution of the zoned calcalkaline magma chamber of Mount Mazama, Crater Lake, Oregon. Contrib. Mineral. Petrol. 98:224-256. Druitt T.H., and Bacon C.R. (1989) Petrology of the zoned calcalkaline magma chamber of Mount Mazama, Crater Lake, Oregon. Contrib. Mineral. Petrol. 101:245-259. Peng G., Luhr J.F. and McGee J.J. (1997) Factors controlling sulfur concentrations in volcanic apatite. Am. Mineral. 82:1210-1224.
V32C-02
Shallow mantle melting beneath Newberry Volcano, central Oregon, USA
Newberry Volcano lies ~60km behind the main Cascade arc in central Oregon and at the western terminus of a trend of volcanic centers that originate from the same location as the eastward trending volcanic series now traced to the Yellowstone hot spot. Alkali basalts of Luna Butte represent one of the primitive end members of the spectrum of lavas erupted from Newberry Volcano and possibly represent a primary mantle melt input into the Newberry system. Phase equilibrium experiments have been carried out on the Luna Butte composition from 0.8 to 1.2 GPa. We find a near-liquidus multiple-saturation with clinopyroxene, olivine, and plagioclase at 1.0 GPa and 1300°C. The liquids produced in these experiments are silica-undersaturated, but appear to be only slightly offset from the predicted multiple-saturation that would include orthopyroxene. Based on the major element composition of the lava, we expect that the Luna butte composition will be multiply saturated with a spinel lherzolite residual assemblage at 1.0 to 1.2 GPa. The lava's composition and its near aphyric character precludes the possibility of significant crustal level fractionation of olivine, and the high-pressure multiple saturation point at 1.0 GPa places the depth of the anhydrous multiple-saturation point right at the seismic Moho beneath Newberry. The phase equilibria and multiple saturation conditions are similar to those of a primitive lava from Jordan Valley volcanic center in eastern Oregon, along the trace of the purported split in the Yellowstone Hot Spot trend. High Sr and Ba contents of these two lavas suggest there may be a fluid component involved in the generation process, which would serve to increase the depth of the multiple-saturation point. This could allow the compositions to saturate with the spinel lherzolite assemblage predicted for the Luna Butte alkali-basalt. However, to produce the silica-rich lavas found elsewhere at Newberry volcano, e.g. high-silica rhyolites, there must be additional magmatic inputs into the sub-volcanic system that may not be present in the eastern High Lava Plains.
V32C-03
Sr Isotope and Compositional Zoning of Plagioclase in the Active Mount St. Helens Lava Dome
Isotopic and compositional patterns preserved in zoned plagioclase provide a record of processes occurring within a magma during phenocryst growth. Such information is most valuable when the analyses are paired from individual grains and placed in a chronological framework. In this study, we present new paired Sr-isotope and trace element data on plagioclase crystals from samples collected from the actively growing dacite lava dome at Mt. St. Helens. Plagioclase crystals from sample SH321C, collected Aug 19, 2005, fall into four broad textural and compositional groups defined primarily by their An contents, resorption features and the presence of pyroxene inclusions. The first group has fairly uniform core compositions of An55-60 that monotonically decline toward the rim to An30-35. The second group displays oscillatory zoning from An45 to An55 within a 30-50 micron spacing separated by dissolution boundaries. The third group has unusually sodic (An25-30) core compositions overgrown by oscillatory rims that are similar to the second group. The fourth group includes all grains that are large, sieved, or xenocrystic in appearance. Sr-isotope variations within single plagioclase grains in these four groups were obtained by LA-MC-ICPMS. The first group of feldspars have nearly homogenous Sr isotope compositions of ~0.7032-0.7034, with the majority at 0.7034, which matches the average whole-rock Sr isotope composition. The second group has slightly more variable Sr compositions in the oscillatory-zoned sections with values extending up to 0.7036. The oscillatory zones most likely record larger Sr isotope variations, but at a scale less than the resolution of the laser ablation sampling technique. The third group of feldspars shows the most variability with values reaching ~0.7050 in the cores, decreasing to ~0.7035 at the rims. The forth group displays intermediate Sr isotope compositions of ~0.7037-0.7039. Sr isotope compositions of the plagioclase rims from all groups have values ranging from ~0.7032-0.7034, suggesting overgrowth formation from dominant magma of similar Sr isotope composition to the group one plagioclase and the whole rock. Nonetheless the `crystal cargo` carried by the dome lavas was assembled from a variety of sources and incorporated into the magma at different times. These isotopic and compositional variations suggest a complex magmatic history that involved periods of crystal growth, resorption, magma injection, and growth of plagioclase rims. The crystals with high- 87Sr/86Sr cores record an earlier period of crystal growth from a distinct magma, or, alternatively, the incorporation and assimilation of an older rock into a younger magma chamber.
V32C-04
The Origin and Evolution of a Diverse Suite of Late Pleistocene Andesitic to Dacitic Lavas From the Northern Cascade arc at Mt. Baker, Washington.
Mt. Baker, a dominantly andesitic stratovolcano, is located in the northernmost segment of the Cascade magmatic arc. The origin and evolution of andesites in the northern Cascades has thus far not been explored in any systematic way. This study highlights the geochemical diversity of andesites erupted from Mt. Baker and describes processes that are responsible for the generation of this dominant intermediate lava type. Presented here are petrographic observations, mineral chemistry, major oxide concentrations, and the largest trace and REE data set to date for three Late Pleistocene and Holocene lava flows from Mt. Baker: the basaltic andesite of Sulphur Creek (52.5-57.6 wt.% SiO2, 4.7-5.5 wt.% MgO), the andesite of Glacier Creek (59.5-63.3 wt.% SiO2, 4.7-5.0 wt.% MgO), and the andesite and dacite of Boulder Glacier (60.5-64.1 wt.% SiO2, 2.1-3.5 wt.% MgO). The data are used to characterize and asses the relationship between the three flows. The three lava flows are classified as medium K, and to a lesser degree, high K, calc-alkaline basalts through dacites. Major oxide concentrations for Sulphur Creek and Boulder Glacier lavas form curvilinear trends with increasing SiO2. The andesite of Glacier Creek has distinct major oxide chemistry, with elevated concentrations of MgO and CaO and lower concentrations of Na2O and K2O for a given wt.% SiO2 relative to the major oxide trends of the other lavas. Glacier Creek lavas also show enrichment of Ni, Cr, and Sr and depletion in La, Nb, Ta, Zr and Y. REE patterns and slopes are distinct for each flow, but are not correlated with degree of differentiation. The mafic lavas of Sulphur Creek have the highest REE abundances relative to the other lavas, with the lowest La/Yb (~4.5). The Glacier Creek andesites have the lowest REE abundances and the largest La/Yb (~6.7). The Boulder Glacier andesites and dacites have intermediate REE abundances relative to the other lavas with intermediate La/Yb (~6.4). All lavas display disequilibrium textures such as magmatic reaction and resorption textures, sieved textures and complicated chemical zoning patterns typical of magma mixing and complicated fractionation processes. Major and trace element fractionation modeling of the mafic lavas of Sulphur Creek fail to produce the higher MgO values in the more differentiated Glacier Creek lavas, and the steeper and relatively depleted REE abundances in both the Glacier Creek and Boulder Glacier lavas. The above data and petrographic observations, coupled with major and trace element fractionation modeling, suggest that the generation of these chemically distinct andesites requires multiple mantle sources that have been modified by crustal processes. These processes include magma mixing and/or complicated crystal fractionation processes.
V32C-05
Back arc basalts from Patagonia: sediment input in a distal subduction domain
Cinder cones and lava flows from the Loncopue graben in N Patagonia (37 S) were sampled over a 180 km N-S transect. These mainly basaltic and trachybasaltic lava flows carry olivine with Cr-Al-rich spinel inclusions, while some more evolved flows carry clinopyroxene and plagioclase. Most of these rocks have between 5-8 percent MgO, and show highly variable K and LIL trace element concentrations. The rocks have up to 180 ppm Ni and 250 ppm Cr. Relative trace element abundance diagrams show negative Ta-Nb anomalies in most rocks, although their depths vary strongly. The REE patterns show LREE enrichment and most rocks have no Eu anomalies, indicating the absence of significant plagioclase fractionation. The basalts have constant U/Th values (~0.25) that are similar to those found in the nearby Copahue-Caviahue arc volcanics. Microprobe analyses of the main phases show olivine with Mg # of 80-87 and up to 2600 ppm Ni. Simulations with the Melts-pMelts programs and application of mineral-melt geothermometers suggest that most olivine phenocrysts crystallized at ~8-10 kbar pressure at temperatures of 1170-1220 oC and with 1-3 percent H2O in the melt. The Sr isotope compositions of 9 samples show a range from 0.7033 – 0.7043, which are negatively correlated with Nd isotope ratios (0.51273- 0.51292). Surprisingly, the most MgO-rich basalt has the most radiogenic Sr isotope ratio. The Pb isotope ratios, well outside the DMM range, correlate very poorly with either Sr isotope ratios or in Pb-Pb isotope graphs. The lack of correlation between degree of evolution and Sr isotope ratios as well as the primitive nature of the rocks and crystals suggest that crustal assimilation was not a major process impacting the composition of these small magma volumes. Incompatible trace element patterns of several samples resemble those of detrital sediment samples from the Pacific, which together with the isotopic data suggest that these magmas may carry a subducted sediment component. Trace element concentrations and ratios do not correlate with radiogenic isotope ratios, suggesting that more than one contaminant source impacted the mantle domain, e.g., fluids as well as sediment melts. Isotope and trace element ratios vary considerably between cinder cones that are only 10-15 km apart, suggesting that the underlying mantle is highly heterogeneous, enriched in subducted components in thin veins and patches. The Loncopue basalts seem to be transitional in composition between the very voluminous S-Patagonia back-arc basalts and the Copahue-Caviahue arc rocks to the north.
V32C-06
Identifying Source Heterogeneity of Italian K-rich Volcanism With Melt Inclusions: Roccamonfina Revisited
Volcanism along the Tyrrhenian border of peninsular Italy is characterized by a wide compositional variety of potassic and ultrapotassic rocks, inferred to be related to metasomatic enrichment of mantle sources. Assessing the diversity of their source components and processes from erupted products remains challenging, since the bulk lavas rarely represent primary melts. Roccamonfina stratovolcano has erupted lavas and pyroclastics (e.g., Luhr and Giannetti, 1987; Giannetti and Luhr, 1983, 1990) that have traditionally been distinguished into a qtz- normative K-series (KS), and a Si-undersaturated high-K series (HKS). We have studied olivine-hosted (Fo=93- 87) melt inclusions of five representative mafic lavas (46-52 wt.% SiO2, 5-8 wt.% MgO, 1.4-6.3 wt.% K2O) from both series, aiming to assess the compositional diversity of primary melts. Relatively high CaO contents of HKS olivines (0.4-0.6 wt.%) are clearly distinct from those of KS olivines (0.03- 0.35 wt.%), irrespective of Fo contents, and appear to correlate with the degree of alkali enrichment. Heating- stage-homogenized MI (n=150) show strong compositional variations in each series. No systematic variations exist between the CaO contents of MI and host olivine, but highest K2O, P2O5, F concentrations were found in MI of HKS, and highest CaO (up to 19 wt.%), CaO/Al2O3, (up to 1.5), S and Cl in those of KS. Nevertheless, potassium contents of the primitive melts (MgO>7 wt.%) show a strong diversity in samples from both series (0.5-5 wt.% K2O in KS; 0.5-9 wt.% in HKS). Collectively, the MI data show that most of the bulk lava compositions reported so far do not represent primary melts, tending to overestimate SiO2, K2O, Na2O, and Al2O3, and underestimate MgO, CaO and TiO2. Importantly, bulk samples appear to be composed of multiple melts, suggesting contributions from heterogeneous mantle domains. MI in HKS olivines have the highest incompatible trace element contents. Both series show subduction-type LILE/HFSE ratios, but there is considerable variation in the degree of overall enrichment, particularly in the KS, which includes some anomalous depleted melts. Pb isotopic signatures obtained on MI show an extreme diversity (e.g., 207Pb/206Pb=0.8-0.86, 206Pb/204Pb=18.0-19.6) encompassing much of the range seen in the entire spectrum of Italian magmas. Despite some scatter, KS and HKS melts trend towards isotopically different crustal/sedimentary source components. Our findings are consistent with the hypothesis that distinct mantle domains below Roccamonfina were metasomatized by multiple agents with different properties. Similarities with signatures of the Roman Province suggests that the source of the HKS melts was affected by input from the Adria slab, whereas that of the KS melts was modified by the Ionian subduction in view of a greater affinity with Campanian magmas. We speculate that virtually simultaneous melting of a heterogeneous mantle column below Roccamonfina was promoted by upwelling hot asthenosphere in response to slab detachment, inferred from seismic tomography. References Luhr and Giannetti, 1987, CMP 95, 420-436; Giannetti and Luhr, 1983, CMP 84, 235-252; Giannetti and Luhr 1990, EPSL 101, 404-424.
V32C-07
Mineralogical and Geochemical Characteristics of Magmatic Series Evolution at Gorely Volcano (Southern Kamchatka)
Gorely volcano is the largest eruptive center in Southern Kamchatka. It is comprised of three structural units; (1) Pra-Gorely volcano (2) a thick ignimbrite complex, associated with a caldera forming eruption (3) modern edifice named ‘Young Gorely'. Geochemical studies have been conducted on all structural units of the Gorely volcanic edifice to determine their genetic conditions. After geochemical analysis two evolution series were found. First, Pra-Gorely volcano is represented by a suite of compositions ranging from basalt to rhyolite, with in this series, high-Mg basalts (MgO – 12,2 wt %) were discovered. Second, Young Gorely edifice is composed of only basalt, andesite and dacite. The reconstruction of chemical evolution trends shows that both volcanic series of Gorely volcano share the same genetic history with similar evolutionary stages. We suggest fractionation of an upper mantle peridotite as a common means to produce both volcanic series as a result of which the evolution of all rocks (from basic to acidic) was generated. It is necessary to add, that the discovery of high-Mg basalts at Gorely volcano demonstrates that eruptive centers of Southern Kamchatka are being feed by a mantle source like those of Central Kamchatka. The magmatic series of Pra-Gorely and Young Gorely volcanoes were formed under different geodynamic conditions. Between these two series was a powerful stage of caldera formation, during which 100 km3 of ignimbrites were emplaced. The 12-km diameter caldera collapse was the catalyst for a large-scale reorganization of the volcanic feeding system. Following caldera collapse, Young Gorely volcano was formed by activity inside the caldera and shows very similar evolutionary trends to that of Pra-Gorely volcano. Therefore, it can be confidently stated that crustal components are practically absent in the evolution of the series, and the compositional range is attributed directly to the evolution of the magmatic melts of Gorely volcano. Microprobe analyses conducted on olivine and pyroxene phenocrysts of Gorely volcano lavas, show that there were at least two stages of crystallization during the evolution of magmatic melt. The first stage corresponds to a crystallization of high-Mg and middle-Mg olivines, Mg# 88-77. The second crystallization stage is characterized by pyroxene phenocrysts with core compositions of Mg# 73 67. The two-stage character of initial magmatic melt evolution is confirmed by results of the computer simulation (COMAGMAT software, by A. Ariskin). The first stage is characterized by comparatively high pressures (6-8 kbar), which corresponds to formation at depth and low rates of oxygen fugacity (1% Fe3+ in total Fe). In contrast, the magmatic evolution of the second stage occurred in near-surface conditions (1-1.5 kbar) with high rates of oxygen fugacity (Ni-NiO buffer).