V42C-01 INVITED
Episodic Growth and Solidification of the Vinalhaven Intrusive Complex, Maine, USA
The Silurian Vinalhaven intrusive complex is about 12 km in diameter and consists mainly of cg granite, a thick section of inward-dipping gabbro-diorite sheets in the SE half of the complex, and a core of fg granite. The lowest exposed part of the complex occurs on its S margin. Along its NW margin, granite intrudes the older, cogenetic Vinalhaven rhyolite, which consists of effusive and pyroclastic units of largely high-silica rhyolite. It is probable that plutonic rocks coeval with the exposed rhyolite units occur at depth and younger rhyolite units, coeval with the plutonic complex, were eroded. The complex was fed by multiple replenishments of isotopically distinct basaltic and granitic magmas. Basaltic replenishments typically produced gabbroic sheets that ponded on granitic crystal mush at the base of a silicic chamber. Where basalt encountered only crystal-rich granitic magma, it locally remelted and mixed with granite, producing bodies of porphyry with corroded phenocrysts and mafic enclaves. CL and Ti zoning in quartz phenocrysts records corrosion followed by growth of high T rims (based on Ti-in-qtz thermometer of Wark and Watson, 2006). Fg granitic dikes and the fg granitic core of the complex represent silicic replenishments. Contact relationships between these dikes and the surrounding granite provide insight into the rheology of the granite at the time of intrusion. Where the resident granitic magma was crystal rich, aphyric magma in the dikes intimately mixed and commingled with the resident cg granite mush. Convective mixing was important in the silicic magma chambers. Where mafic sheets are present in granite, upward gradations through hybrid rocks back into granite and the occurrence of mafic enclaves in granite far above the mafic sheets demonstrate mixing within overlying silicic magma. Complex CL and Ti zoning in the cores of granitic quartz far from the mafic input also records thermal perturbations caused by the mafic input. The oldest plutonic rocks consist of cg granite in the west and interlayered granite and mafic sheets associated with large blocks of country rock in the east. In the west, schlieren structures in cg granite indicate crystal accumulation on a chamber floor of transitional rheology. In the east, granitic pipes and other outcrop-scale features indicate that the mafic layers ponded on granitic crystal mush. At higher levels, mafic sheets and country rock blocks gradually become restricted to the eastern third of the complex and are entirely absent in the upper half of the intrusion. Within cg granite the wide occurrence of schlieren related to magma flow and sinking enclaves indicate continued accumulation on a chamber floor. Later injections of basaltic magmas rejuvenated pockets of mostly crystallized granite in the upper levels of homogeneous granite and suggest that crystallization of the granite also proceeded inward from the sides and roof of the chamber. The complex preserves a stratigraphic record of magma chamber evolution and pluton growth by crystal accumulation on magma chamber floors that was interrupted by episodes of replenishment and rejuvenation. This field-based interpretation is corroborated by high-precision U-Pb zircon ages of granitic rocks distributed throughout the complex that indicate the complex was constructed over a nominal time-span of about 0.7 m.y. (Hawkins and Wiebe, this volume).
V42C-02
Combining major and accessory phase geothermometry and geochronology to delimit the thermochemical evolution of high-silica rhyolite at Yellowstone caldera
Recent advances in zircon geothermometry and microbeam analysis provide an unprecedented opportunity for employing single crystals to establish absolute age limits on the differentiation history of silicic magma reservoirs. When coupled with compositional variations and geothermometry from coexisting major and accessory phases, the results can provide a unique perspective on the thermochemical evolution of silicic magmas. To quantify the thermal and chemical history of high-silica rhyolites that make up the Central Plateau Member of the Plateau Rhyolite at Yellowstone caldera, we have combined zircon and major phase geothermometry with 238U- 230Th and U-Pb of single crystals, and have analyzed associated mineral and glass compositions. The CPM rhyolites erupted in pulses between ca. 165 ka and 70 ka, and thus serve as sequential "snapshots" into the evolution of Yellowstone's postcaldera reservoir. With decreasing eruption age, CPM glasses contain higher Rb, Y, Nb, U, LREE and Th, and lower Eu, Sr, and Ba. Clinopyroxene and sanidine phenocrysts are more evolved with decreasing eruption age. However, some individual phenocrysts contain abrupt normal or reverse zoning. CPM zircons yield average ages that range from ca. 0 to 60 thousand years before their respective eruption ages. In a single rhyolite, the composition of chevkinites (LREE-Th-silicate) correlates with apparent 238U- 230Th ages that fall within the age range of coexisting zircons. In general, zircon trace element concentrations parallel the trace element co-variations of groundmass glasses. Coupled Ti-in-zircon thermometry (with aTiO2 calculated from groundmass and inclusion glasses) and geochronology reveals a general decrease in average crystallization temperature within the range of ca. 850° to 780° C. Zircon rims typically yield temperatures that are ~10-20° lower than their cores. Temperatures calculated from QUILF thermometry using major minerals correlate with the results from zircon thermometry. Ti-in-quartz thermometry from ion probe measurements yields temperatures ranging from ca. 800°-900° C. Although some individual quartz contain irregular Ti zoning, rims generally yield lower temperatures (by ~10-20°) than cores. Quartz crystals from a single ~100 ka rhyolite contain rims that yield higher (up to 40°) temperatures than cores. The combined results from geochronology and geothermometry, as well as the secular variation of mineral and glass compositions, suggest that batches of rhyolite in the CPM reservoir evolved over tens of thousands of years and over a relatively narrow temperature range to generally cooler and more-evolved compositions. Nevertheless, this differentiation was punctuated by periodic episodes of reheating and/or magma mixing, which locally heated melts by up to at least 40° and produced abrupt compositional zoning within some crystals, particularly in major phases.
V42C-03
Rapid Crystallization of the Bishop Magma
Substantial effort has been made to understand the longevity of rhyolitic magmas, and particular attention has been paid to the systems in the Long Valley area (California). Recent geochronological data suggest discrete magma bodies that existed for hundreds of thousands of years. Zircon crystallization ages for the Bishop Tuff span 100-200 ka, and were interpreted to reflect slow crystallization of a liquid-rich magma. Here we use the diffusional relaxation of Ti zoning in quartz to investigate the longevity of the Bishop magma. We have used such an approach to show the short timescales of crystallization of Ti-rich rims on quartz from early- erupted Bishop Tuff. We have now recognized Ti-rich cores in quartz that can be used to derive the timescales of their crystallization. We studied four samples of the early-erupted Bishop. Hand-picked crystals were mounted on glass slides and polished. Cathodoluminescence (CL) images were obtained using the electron microprobe at the University of Chicago. Ti zoning was documented using the GeoSoilEnviroCARS x-ray microprobe at the Advanced Photon Source (Argonne National Lab). Quartz crystals in all 4 samples include up to 3 Ti-bearing zones: a central core (50-100 μm in diameter, ca. 50 ppm Ti), a volumetrically predominant interior (~40 ppm Ti), and in some crystals a 50-100 μm thick rim (50 ppm Ti). Maximum estimates of core residence times were calculated using a 1D diffusion model, as the time needed to smooth an infinitely steep profile to fit the observed profile. Surprisingly, even for the largest crystals studied – ca. 2 mm in diameter – core residence times are less than 1 ka. Calculated growth rates imply that even cm-sized crystals crystallized in less than 10 ka. Crystal size distribution data show that crystals larger than 3 mm are exceedingly rare, such that the important inference is that the bulk of the crystallization of the early-erupted Bishop magma occurred in only a few thousand years. This timescale is 2 orders of magnitude smaller than the shortest durations derived from geochronology. In the current paradigm, this implies that the Bishop magma existed virtually free of crystals for 100-200 ka. Occasional recharge of the system could cause resorption of crystals. The challenge, however, is to explain how a large- volume, liquid- and volatile-rich system, was prevented from erupting for over 100 ka. The trouble is such that it puts into question the whole concept of a long-lived, liquid-rich magma body. Evidence has accumulated to show that the Bishop magma was stratified and did not convect during crystallization, the stratification was established prior to phenocryst crystallization, and crystal migration did not significantly perturb the stratification. All these are simpler to explain if liquid-rich magma only existed for a short period of time, and we estimate the time as being on the order of 1 ka. The geospeedometric timescale inferred can be reconciled with the geochronological evidence if we interpret zircon crystallization ages as reflecting episodic growth in response to waxing and waning of a mushy body, rather than continuous crystallization from liquid-rich magma in a long-lived, large-volume magma body. We speculate that only after 100-200 ka did favorable conditions emerge and allowed for the generation of a large volume of liquid-rich magma. Once such a body of magma was established, it progressed rather quickly towards eruption.
V42C-04
Evidence From a Crystal-Poor, Zoned (Rhyolite-Andesite) Pyroclastic Deposit From Volcan Tepetiltic, Western Mexico for Rapid Generation of Silicic Melt by Partial Melting of Granitoid and not by Segregation From a Long-Lived Crystal-Rich Mush
Volcan Tepetiltic is an intermediate (56-66 wt % SiO2) arc stratovolcano (~42 km3) that is composed primarily of crystal-rich (25-40 vol%), hornblende-absent andesite flows, with an elliptical caldera (5 km x 2.5 km). The Plinian, caldera-forming eruption produced a zoned (60-75 wt % SiO2) pyroclastic deposit (6-9 km3) that is everywhere crystal-poor (0-3 vol%); where crystals are present, hornblende occurs. The caldera wall exposes ~600 m of stratified lava flows. A series of 40Ar/39Ar dates on these caldera-wall flows, along with numerous dates on flank flows, constrain the time scale for the construction of the andesitic, main edifice of V. Tepetiltic. The collection of Ar dates indicates that V. Tepetiltic was formed at ~525 ka, over an interval of ~90 ± 75 kyrs. In other words, within the 2 sigma error on all the Ar dates, the eruption interval for cone construction of V. Tepetiltic may have been as long as 165 kyrs or as short as 15 kyrs. At the 95% confidence interval, cone construction ceased by ~380 ka. Peripheral to the central vent of V. Tepetiltic are a series of basaltic andesite (56-58 wt%) flows, cones, and one shield (total volume is ~9 km3), which all erupted at ~214 ± 64 ka, over an interval that may have been as long as 128 kyrs or as short as 22 kyrs, within 2 sigma error of the Ar dates. The pyroclastic deposit is found both underneath and on top of these basaltic andesite units, which broadly constrains its eruption age and indicates a temporal association with the pulse of basaltic andesite eruptions. The age of the caldera-forming eruption is further constrained by (must be younger than) the age of a small dacite dome on the caldera floor (190 ± 22 ka) and the age of a lithic (113 ± 76 ka) within the pyroclastic deposit. In summary, the best estimate for the eruption age of the pyroclastic deposit is at ~190 ka. Thus, there was a hiatus of ~200 kyrs between the cone- building episode that produced the crystal-rich (25-40 vol%), intermediate (56-66 wt% SiO2), hornblende- free lavas of V. Tepetiltic and the explosive, caldera-forming eruption that produced the crystal-poor (0-3 vol%), zoned (60-75 wt% SiO2), hornblende-bearing pyroclastic deposit. Most models for the formation of high- silica rhyolite, especially when part of an eruption that zones to andesite/dacite, is that it forms by melt segregation from the andesite/dacite crystalline mush. In the case of the erupted products from V. Tepetiltic, however, this oft-cited model is not viable. The crystal-poor andesite cannot be the parent for the rhyolite as too few crystals have formed. Instead, the most plausible scenario is that the magma chamber that fed construction of the main edifice of V. Tepetiltic solidified below its solidus by ~350 ka. Then, at ~214 ± 64 ka, a pulse of basaltic andesite magma was emplaced into the upper crust, which drove partial melting of granitoid beneath V. Tepetiltic, by the transfer of heat and volatiles. The partial melt (ranging from voluminous rhyolite to minor andesite) segregated, ascended, and erupted quickly (at ~190 ka) before extensive degassing- induced crystallization could occur. This model for the formation of rhyolite (by partial melting of granitoid) may be more widespread than currently recognized.
V42C-05
Cryptic young zircon and young plagioclase in the Kaharoa Rhyolite, Tarawera, New Zealand: Implications for crystal recycling in magmatic systems
We measured in-situ 238U-230Th zircon and bulk plagioclase 238U-230Th-226Ra disequilibria in rhyolite lava and tephra from the ~1315 AD Kaharoa eruption of Tarawera Volcano, New Zealand in order to constrain its history of chemical evolution. These data suggest that zircon records a protracted history (10s of kyr) whereas plagioclase is dominantly young (few kyr), but both phases crystallize up to the eruption. The Kaharoa eruptive period at Tarawera consists of ~2.5 km3 of crystal-rich rhyolite (74-75 wt% silica) lava and ~5 km3 of coeval tephra deposits, making it the largest silicic eruption in New Zealand in the last 1,000 years. 238U-230Th disequilibria measurements of zircon determined via SHRIMP-RG analyses produce an array of ages, with three main populations: (1) within error of eruption age; (2) 15-80 k.y.; (3) 100-175 k.y. Very few analyzed zircon fall within error of eruption age and little difference is seen in the age distribution of zircon between the lava and tephra. In contrast, 238U-230Th and 230Th-226Ra plagioclase ages appear to be within error of eruption age however this age is complicated zircon inclusions in the plagioclase. This contamination by zircon is seen in the 238U-230Th disequilibria and trace element data, where the addition of zircon pushes the bulk plagioclase separate towards more U-enriched values and high Zr values. However, the (230Th)/(232Th) ratios for the separates are the same as the whole rock values, indicating that any zircon in the bulk separate must be young (eruption age). This finding is also borne out in 230Th-226Ra disequilibria, where zero-age zircon contamination is reflected in increased (230Th)/[Ba] with no change in (226Ra)/[Ba]. In both cases, as little as 1 ppm of zero-age zircon contamination is needed to create these patterns. This signal of young plagioclase and zircon growth is in contrast to the protracted history seen in the SHRIMP-RG zircon data. This suggests that young zircon growth in the Kaharoa Rhyolite is "masked" in SHRIMP-RG analyses because the young zircon is present either inclusions in plagioclase excluded from zircon separate or as thin rims on older zircons that can't be effectively analyzed. The Kaharoa data implies that zircon records a longer history than plagioclase, and older zircon ages represent the mush/plutonic roots of the volcano. The existence of old zircon with little evidence for old plagioclase suggest that remobilization of the plutonic roots by new heating events destroys most of the old plagioclase crystals. Plagioclase and zircon both crystallize during the short period between remobilization and eruption. However, young zircon growth is cryptic, suggesting that in-situ zircon ages may in general be biased toward older ages. Overall, plagioclase and zircon offer potential to unravel a large time period of the history of a magmatic system, especially silicic magma with long histories.
V42C-06
The 280 ka Matahina Eruption, Okataina Volcanic Centre, New Zealand: A Protracted Rhyolite Magma Assembly by Fractional Crystallization, Melt Extraction, Rhyolite-Rhyolite Mixing, and Magmatic Stratification
The 280 ka Matahina eruption from the Taupo Volcanic Zone (TVZ), New Zealand, produced voluminous pyroclastic deposits representing >220 km3 of andesitic to rhyolitic magma and the climax of magmatic/volcanic activity from ca. 340 ka to 280 ka within the Okataina Volcanic Centre. The Murupara subgroup of eruptions preceding this large, caldera-forming event is used to document the progressive growth of a large volume magma body in the mid- to upper-crust over 103 or 104 years. The dominant magma composition of the caldera-forming eruption was rhyodacite/rhyolite (70.8 to 78.1 wt % SiO2), with a subordinate volume of andesitic to rhyolitic (58.5 to 77.6 wt % SiO2) juvenile pyroclastics erupted following the caldera collapse. Phenocrysts (plagioclase+quartz+opx±amph+Fe-Ti oxides) from the pumice clasts define two distinct compositional populations that evolved independent of one another following rhyolite melt extraction from a fractionating andesitic parent magma body. We interpret these phenocryst populations to represent two phase assemblages that evolved in a slightly stratified rhyolitic magma chamber overlying a less-evolved basal mush layer. Oxide geothermometry and phase equilibria suggest a 740- 783°C magma storage temperature at 7-10 km depth. Most of the bulk-rock, melt, and phenocryst compositional variations are consistent with fractional crystallization (FC) of an andesitic parent magma body. However, trace element variation in pumice clasts, distinct rhyolitic glass major element compositions, and plagioclase disequilibrium accompanied by complex zoning are not consistent with closed-system FC alone, but are consistent with both FC and mixing in characterizing the magmatic diversity. In addition, the application of Polytopic Vector Analysis (PVA), a multivariate statistical treatment of the bulk-rock geochemistry, provides a mixing solution with a robust platform for accurately defining the timing and nature of the mixing event. Hence, the subtle geochemical disparities that occur between the main erupted rhyolitic magma and post-caldera collapse andesite-rhyolite assemblage specifically demonstrate mixing with a compositionally distinct rhyolite that must have followed the extraction of the evolved melt from the host crystal-mush as the magma system evolved. Mingled textures in the least-evolved, crystal-rich (35-40 % crystal content), pumice clasts also indicate a mafic input immediately preceding the eruption; probably acting as a thermal trigger for the main caldera-forming eruptive episode. Therefore, similar to some more recent large, rhyolitic eruptions from the TVZ (50 ka Rotoiti and Earthquake Flat eruptions, OVC; 26.5 ka Oruanui eruption, Taupo Volcano), the Matahina rhyodacite/rhyolite records a protracted, polygenetic, multi-stage magmatic and eruptive history, punctuated by the main caldera-forming eruption.
V42C-07
Dynamics of a Magma Chamber: Insights Into Time and Length Scales of Internal Processes in the Tuolumne Batholith, CA
Recent studies on large, zoned batholiths such as the 95-85 Ma Tuolumne batholith (TB), Sierra Nevada, are slowly advancing our understanding of batholith assembly. However, it is still questioned whether large magma chambers exist at any time during batholith construction and if preserved compositional variations are related to processes at the source, ascent or emplacement level. Moreover, the knowledge about time and length scales of internal processes and their significance in these long lived systems remains challenging. To better understand the importance of different internal processes in the TB, we examined four magmatic lobes composed of the Kuna Crest, equigranular and porphyritic Half Dome and Cathedral Peak granodiorites, three of the four major TB units. Lobes are petrologically less complicated than the main body, freeze quicker and thus preserve snapshots at different stages of batholith evolution. In contrast, the main batholith is more complex and preserved a cumulative compositional imprint and/or the last increments of batholith activity before crystallization. All four lobes are normally zoned with granodioritic units at the margin and leucogranite in the center. Contacts between units are generally gradational and young inward shown by both structural and geochronology data. Linear geochemical trends and relics of cumulates suggest the zonal pattern in the lobes is dominantly due to fractionation crystallization and local mixing, which is supported by Nd and Sr isotopes indicating a homogeneous source for each unit. In comparison to the same units in the main body, Nd and Sr isotopes in the lobes are more primitive in composition and slightly older in age, suggesting that the lobes represent magmas that entered the magma chamber and underwent differentiation, but then crystallized quickly without further interchange. We conclude that compositional pattern in the TB can be explained by short time scale fractionation processes (0.5-1 myrs) that are overprinted by longer term mixing processes (a few myrs) of magmas derived from at least two different sources. While fractionation crystallization pattern dominate in the more quickly crystallized lobes, compositions in the main batholith are hybrids with variable components of different pulses and their differentiates mixing at cm to km scales. Our field observations, geochemistry data and U/Pb geochronology from the TB (Matzel et al., 2007) indicate that sizeable magma chambers form during batholith construction in which pulsing, fractionation and mixing are responsible for compositional variations at the emplacement level.
V42C-08 INVITED
Growth of the Tuolumne Batholith: Zircon Crystallization Temperature, Age and Trace Element Data
Deciphering the intrusive record of magma systems is essential to understanding the links between surface volcanism and the long-term storage and evolution of magma reservoirs. Here we use age and geochemical data from zircon crystals to track mixing between different parts of the Tuolumne Batholith (Sierra Nevada, California). U-Pb zircon TIMS analyses from all locations examined in the batholith exhibit appreciable dispersion of single crystal or crystal fragment ages (several 105 yrs to 1x106 yrs) and, in addition, display distinctly older ages that likely represent zircon crystals entrained from older parts of the Tuolumne magmatic system. Since techniques aimed at eliminating Pb loss (and thus age scatter) have been employed prior to analysis, we interpret the age dispersion to reflect real variation in the timing of zircon crystallization. Two samples that show a high degree of age dispersion (> 1 Myr) were selected for trace element analysis and Ti- in zircon geothermometry by SHRIMP-RG. Crystallization temperatures ranged from 780-640°C and averaged 695°C (aTiO2 0.75 based on presence of titanite). No clear correlation exists between crystal age and temperature, and in most cases, the temperatures from crystal centers are within uncertainty of the temperatures at the rims. Trace element ratios vary systematically with temperature (e.g. decreasing Th/U ratio with decreasing T) and are attributed to fractionation, although neither sample represents strongly fractionated melt. Low total Zr indicates that the magmas were initially undersaturated in zircon when emplaced, which is also consistent with late zircon crystallization. Combined evidence from TIMS age analyses, geothermometry and trace element data suggests that entrainment of zircon from older parts of the magmatic system occurred late in the history of the batholith, and recycling of zircon crystals during successive magmatic injections is compatible with progressive growth of a large, long-lived, crystal mush body.