Journal of Petrology Advance Access originally published online on March 29, 2007
Journal of Petrology 2007 48(5):951-999; doi:10.1093/petrology/egm007
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Published by Oxford University Press (2007).
Compositional Zoning of the Bishop Tuff
1US Geological Survey, MS-910, Menlo park, CA 94025, USA
2School of Geography, Geology and Environmental Science, University of Auckland, PB 92019 Auckland Mail centre, Auckland 1142, New Zealand
RECEIVED JANUARY 7, 2006; ACCEPTED FEBRUARY 13, 2007
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONCompositional data for >400 pumice clasts, organized according to eruptive sequence, crystal content, and texture, provide new perspectives on eruption and pre-eruptive evolution of the >600 km3 of zoned rhyolitic magma ejected as the Bishop Tuff during formation of Long Valley caldera. Proportions and compositions of different pumice types are given for each ignimbrite package and for the intercalated plinian pumice-fall layers that erupted synchronously. Although withdrawal of the zoned magma was less systematic than previously realized, the overall sequence displays trends toward greater proportions of less evolved pumice, more crystals (0·5–24 wt %), and higher FeTi-oxide temperatures (714–818°C). No significant hiatus took place during the 6 day eruption of the Bishop Tuff, nearly all of which issued from an integrated, zoned, unitary reservoir. Shortly before eruption, however, the zoned melt-dominant portion of the chamber was invaded by batches of disparate lower-silica rhyolite magma, poorer in crystals than most of the resident magma but slightly hotter and richer in Ba, Sr, and Ti. Interaction with resident magma at the deepest levels tapped promoted growth of Ti-rich rims on quartz, Ba-rich rims on sanidine, and entrapment of near-rim melt inclusions relatively enriched in Ba and CO2. Varied amounts of mingling, even in higher parts of the chamber, led to the dark gray and swirly crystal-poor pumices sparsely present in all ash-flow packages. As shown by FeTi-oxide geothermometry, the zoned rhyolitic chamber was hottest where crystal-richest, rendering any model of solidification fronts at the walls or roof unlikely. The main compositional gradient (75–195 ppm Rb; 0·8–2·2 ppm Ta; 71–154 ppm Zr; 0·40–1·73% FeO*) existed in the melt, prior to crystallization of the phenocryst suite observed, which included zircon as much as 100 kyr older than the eruption. The compositions of crystals, though themselves largely unzoned, generally reflect magma temperature and the bulk compositional gradient, implying both that few crystals settled or were transported far and that the observed crystals contributed little to establishing that gradient. Upward increases in aqueous gas and dissolved water, combined with the adiabatic gradient (for the
5 km depth range tapped) and the roofward decline in liquidus temperature of the zoned melt, prevented significant crystallization against the roof, consistent with dominance of crystal-poor magma early in the eruption and lack of any roof-rind fragments among the Bishop ejecta, before or after onset of caldera collapse. A model of secular incremental zoning is advanced wherein numerous batches of crystal-poor melt were released from a mush zone (many kilometers thick) that floored the accumulating rhyolitic melt-rich body. Each batch rose to its own appropriate level in the melt-buoyancy gradient, which was self-sustaining against wholesale convective re-homogenization, while the thick mush zone below buffered it against disruption by the deeper (non-rhyolitic) recharge that augmented the mush zone and thermally sustained the whole magma chamber. Crystal–melt fractionation was the dominant zoning process, but it took place not principally in the shallow melt-rich body but mostly in the pluton-scale mush zone before and during batchwise melt extraction. KEY WORDS: Bishop Tuff; ignimbrite; magma zonation; mush model; rhyolite
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INTRODUCTION |
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The Bishop Tuff, product of one of the world's greatest Quaternary eruptions, was released at 760 ka during an episode about 6 days long from the Long Valley magma chamber in eastern California. Fallout remnants are preserved from the Pacific Ocean to Nebraska, over an area of >2·5 x 106 km2. Concurrent with the fallout, ash flows spread >70 km SE down Owens Valley, 40–50 km east to bank against the White Mountains, 40–50 km north into Mono Basin and Adobe Valley, and tens of kilometers SW down the San Joaquin River canyon (Fig. 1). Long Valley caldera collapsed along a 12 km x 22 km elliptical ring-fault zone that became active only after half or more of the erupting magma had escaped from the chamber. The caldera was enlarged by syneruptive slumping and secular erosive recession of the walls to form the modern 17 km x 32 km depression, which has been the site of many postcaldera eruptions, resurgent structural uplift, hydrothermal activity, and current unrest (Bailey et al., 1976
; Bailey, 1989; Sorey et al., 1991; Hill et al., 2002; Hildreth, 2004).
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The Bishop Tuff consists predominantly of ash and pumice clasts of biotite–plagioclase–quartz–sanidine high-silica rhyolite (mainly 74–77·7% SiO2), though scarcer pumice types (reported here) extend the compositional range to dacite. Proximal to medial fall deposits are plinian pumice lapilli and crystal-rich ash, but beyond about 200 km from source the fallout is largely vitric ash that includes both plinian and coignimbrite contributions. Outflow ignimbrite sheets are as thick as 170 m and range from nonwelded to eutaxitic, and from unconsolidated, sintered, or variably welded vitric zones to fully devitrified zones and zones of intense vapor-phase crystallization. Intracaldera Bishop Tuff, not exposed but sampled in many drillholes (Suemnicht & Varga, 1988
; Bailey, 1989; McConnell et al., 1995), is devitrified, densely welded, and as thick as 1500 m.
Eruptive volume is not well known owing to (1) erosion, transport, and resedimentation of distal ash, (2) erosion or burial of distal outflow sheets, (3) thick postcaldera fill that conceals the intracaldera tuff, and (4) irregular faulting of the caldera floor beneath it. McConnell et al. (1995) estimated 340 km3 of intracaldera Bishop Tuff; we estimate 200 km3 initially for the outflow sheets; and application of the method of Fierstein & Nathenson (1992) to sparse data for unreworked downwind ashfall (Izett et al., 1988) gives roughly 250 km3 of fallout. Recalculating to a density of 2·2 g/cm3 for hydrous rhyolite magma yields a crude estimate of 600–650 km3 for the magma that erupted to produce the Bishop Tuff. The two northerly ignimbrite lobes (Fig. 1), emplaced late in the eruptive sequence, account for 10–15% of the total volume erupted.
For simplicity, we restrict use of the term Bishop Tuff to the deposit, and we refer to Bishop magma, crystals, and melt, none of which became tuff until they had ceased being magma. The melt-dominant volume from which the eruption issued is referred to here as the magma body, and what constituted the Bishop magma chamber as a whole and what complex reservoir may have continued beneath it are discussed in the section ‘Definition of magma chamber’.
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LONG VALLEY MAGMATISM |
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The Bishop Tuff was by far the largest of more than 200 eruptions in the Long Valley volcanic field during the past 4·5 Myr. Numerous mafic and intermediate magma batches that erupted during the interval 4·5–2·5 Ma were followed by incremental construction of the rhyolitic Glass Mountain complex of lava domes and pyroclastic deposits between 2·2 and 0·79 Ma (Bailey et al., 1976
; Metz & Mahood, 1985, 1991; Bailey, 1989, 2004; Metz & Bailey, 1993). Taking into account dispersed pyroclastics (Sarna-Wojcicki et al., 2005), the 60 or more precaldera vents of Glass Mountain erupted 100 ± 20 km3 (Hildreth, 2004) of high-silica-rhyolite magma, most of it similar to or even more evolved than the most differentiated pumice in the Bishop Tuff (Metz & Mahood, 1991). Early postcaldera phenocryst-poor rhyolites (760–650 ka) include lavas, tuffs, and intrusions that likewise add up to 100 km3. In large part coextensive and contemporaneous with growth of a 10 km wide resurgent uplift (Bailey, 1989), these early intracaldera rhyolites (74–75% SiO2) are less evolved than the precaldera rhyolites, and in most respects they overlap compositionally with the less evolved pumice emplaced late in the zoned Bishop Tuff sequence (McConnell et al., 1995; Hildreth, 2004). Younger rhyolitic, intermediate, and mafic volcanism within and west of the caldera has continued, intermittently and far less voluminously, from 525 ka to the present (Bailey, 1989; Hildreth, 2004). This activity is not dealt with here, as it reflects events and processes subsequent to those that built and organized the magma reservoir that released the Bishop Tuff. |
PREVIOUS WORK ON THE BISHOP TUFF |
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The Bishop Tuff was initially mapped, described, and named by Gilbert (1938
), who characterized the pyroclastic-flow origin and welding of its outflow sheets. Bateman (1965) described the basal fall deposit as an integral part of the Bishop Tuff, and Izett et al. (1988) documented surviving remnants of the regionally dispersed ashfall. Sheridan (1965, 1968, 1970) recognized the multi-lobate distribution of the ignimbrite, studied its mineralogy, and described its fossil fumaroles. Contrasting suites of basement lithic fragments in sequentially emplaced subunits allowed Hildreth & Mahood (1986) to locate the initial eruption site and to document ring-fault-controlled opening of subsequent vents that influenced sectorial emplacement of outflow sheets.
Compositional zonation of the Bishop Tuff was first recognized and reconnoitered by Hildreth (1977, 1979, 1981, 1983, 1985) and Halliday et al. (1984), and the processes that brought about that zoning have remained topics of lively discussion and investigation ever since. Confirmation of zoning in volatiles by analysis of melt inclusions in Bishop Tuff phenocrysts has provided important insights about the pre-eruptive magma body (Anderson et al., 1989, 2000; Skirius, 1990; Anderson, 1991; Lu, 1991; Dunbar & Hervig, 1992; Lu et al., 1992; Wallace et al., 1995, 1999). Investigations of the growth, differentiation, and physical and thermal condition of the pre-Bishop magma reservoir (Metz & Mahood, 1985, 1991; Halliday et al., 1989; Mahood, 1990; Sparks et al., 1990: Davies et al., 1994) and of pre-eruptive residence times of magmas that led to the Bishop Tuff (Christensen & DePaolo, 1993; van den Bogaard & Schirnick, 1995; Christensen & Halliday, 1996; Davies & Halliday, 1998; Reid & Coath, 2000; Winick et al., 2001; Simon et al., 2005) have provoked spirited debate.
In this paper we review these contributions and present a revised and expanded database of Bishop analytical data, supplementing and superseding that of Hildreth (1977, 1979). These data are tied to our revisions of the tuff stratigraphy and eruptive sequence (Wilson & Hildreth, 1997). We extend previous studies by (1) documenting the full compositional range of major and minor types of pumice, (2) recording the complexities of magma withdrawal reflected in fluctuating proportions of varied pumice types that were concurrently emplaced, and (3) drawing revised inferences concerning pre-eruptive processes in the magma reservoir.
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ERUPTIVE STRATIGRAPHY |
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The eruptive sequence (Fig. 2; Wilson & Hildreth, 1997
) includes a fall deposit (F) divided into nine widely recognizable units (F1–F9) and some 13 packages (or subpackages) of ignimbrite (Ig), which are chronologically or sectorially distinctive bodies of tuff. Most, possibly all (see below) of the ignimbrite packages are demonstrably syn-plinian, rather than post-plinian as previously thought by Hildreth (1977, 1979). Within the fall sequence exposed east of vent (Fig. 1) there is no sign of a time break, except a short one between F8 and F9 when a few centimeters of ash (Fig. 3) had time to settle. Each ignimbrite package contains distinctive lithic and pumice populations, has consistent lithologic characteristics (independent of welding zonation), and was emplaced as a rapid succession of pulses or flow units (Wilson & Hildreth, 1997, 2003).
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The sequence of Wilson & Hildreth (1997
) is updated and summarized (Fig. 2) as follows. The eruption began in the south–central part of what later became the caldera, feeding a plume that gradually grew from about 18 km to 45 km in height and was largely blown eastward. East and SE of vent, several ignimbrite packages successively offlapped, each one shingling progressively farther away from source. The earlier Ig1Ea and b were coeval with fall units F2–F8; they lack pyroxene phenocrysts and generally lack the rhyolite lithics (derived from Glass Mountain and its volcaniclastic apron) characteristic of the overlying Ig2E packages, except where they flowed eastward across the Glass Mountain apron. Later eastern packages (Ig2Ea, b, c) were coeval with uppermost F8 and F9, and like those fall layers they do contain pyroxene-bearing pumice and vent-derived rhyolite lithics, the latter indicating northeastward propagation of vents along the ring-fault zone during caldera subsidence. Because the plinian plume was driven strongly eastward, proximal to medial fall deposits are not present in northern and southwestern outflow sectors (Fig. 1), ruling out direct correlation with deposits in the (today noncontiguous) eastern sector. Nonetheless, the northerly packages (Ig2N, Ig2NW) are rich in pyroxene-bearing pumice, overlie a remnant of pyroxene-free ignimbrite (Ig1NW) near the north margin of the caldera, and have suites of pumice types that partly overlap with that of Ig2E. In the San Joaquin canyon to the SW, lithic suites were in large part picked up locally, but pumice suites and mineral chemistry link the lower of two packages (Ig1SW) to the Ig1E eruptive interval and the upper (Ig2SW) to the Ig2 interval.
Recognition that fall and flow deposits were emplaced synchronously permitted estimates of accumulation time for each plinian layer to be applied for the intercalated synplinian ignimbrite packages as well (Wilson & Hildreth, 1997, table 4). Accumulation times so estimated are: 90 h for fall units F1–F8, inclusive; 25 h for Ig1Ea; 36 h for Ig1Eb. Accumulation of fall unit F9 was originally estimated to occupy at least 8 h, but new data from the west side of the White Mountains, where F9 is 207 cm thick, imply an accumulation time of roughly 26 h. For the northern Ig2 packages, their generally massive nature was taken, by analogy with deposition patterns in Ig1E, to indicate that their thickest parts (120–140 m) accumulated in no more than 35 h. Evidence discussed by Wilson & Hildreth (1997) and data gathered since imply that F9, Ig2E, and the northern packages erupted synchronously and that the whole eruption thus took less than 6 days.
The importance of this chronostratigraphic framework is threefold. (1) Because much of the Bishop Tuff underwent devitrification and vapor-phase crystallization, many phenocrysts are exsolved, oxidized, or otherwise altered. Detailed understanding of the emplacement sequence allows sampling of pumice from fresh glassy parts of every emplacement package. (2) Proportions of pumice clasts of different characteristics or composition can be estimated by clast counts of each eruptive subunit, permitting assessment of the time–volume–compositional progress of the eruption. (3) Understanding the opening and migration of successive vents around the caldera and the changing proportions of different compositions that erupted from each vent segment provides evidence important for attempting to reconstruct the distribution of magma in the pre-eruptive reservoir or to model dynamic processes of magma withdrawal.
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TYPES AND PROPORTIONS OF PUMICE |
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Pumice clasts in the Bishop Tuff (Figs 4 and 5) range continuously in phenocryst content from <1% to 25 wt %, as determined by mineral separations in heavy liquids (Electronic Appendix 1, available for downloading at http://www.petrology.oxfordjournals.org). There is a variety of primary textures in glassy nonwelded pumice that, in combination with differences in phenocryst (hereafter, crystal) contents, is used to divide the pumices into a dominant ‘normal’ spectrum (which ranges widely in crystal content), plus a number of variant types (summarized in Table 1). Banded and composite pumice are sparsely present (Table 1) but were not analyzed. In addition to >400 pumices, a few dense vitrophyric blobs and glassy fiamme were sampled. Proportions of the pumice types were determined by clast counts at
110 outcrops (Fig. 5), supplemented by laboratory counts of material bagged in the field. Within a chosen outcrop area, we tried to count all pumice clasts larger than 4 cm (Fig. 5), resorting to counting 2–4 cm pumices only when required to achieve adequate numbers. For chemical analysis, most pumices taken from ignimbrite were 10–25 cm in diameter; those from fall deposits mostly 4–7 cm. There is no evidence in our observational or chemical data for any correlation between composition and size of pumices. Ash and pumice granules are created by fragmentation and comminution of all pumice types.
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There are systematic variations in proportions of the different pumice types through the tuff, as summarized in Fig. 6 and detailed in Electronic Appendix 2. The main continuum of crystal-poor (xp: 0–6%) through medium (xm: 6–12%) to crystal-rich (xr: >12%) normal pumice (Table 1) generally makes up 93–99% of the juvenile clasts in all deposits, except where diluted by a few spatially restricted influxes of swirly pumice in Ig1Eb (Watterson subunit—see below). The proportion of xr pumice, however, increased drastically from <5% in Ig1Ea to
30% at the Ig1–Ig2 transition, and to as much as 90% in the latest ignimbrite packages (Fig. 6). Conversely, xp pumice was predominant in Ig1, remained abundant throughout much of Ig2E, but dropped to about 5% of the final packages (Electronic Appendix 2). Of the variant pumice types, the glistening variety of xp pumice is sparsely present throughout Ig1 and Ig2E but is rare north of the caldera (Electronic Appendix 2). Swirly pumice occurs in the earliest ignimbrite (Ig1Ea) and throughout the eruptive sequence, typically at 1–5% of the clast count, but is especially enriched in the Watterson subunit (Fig. 6; see below) and locally elsewhere in Ig1Eb and Ig2Ea. Dark gray pumice is also represented in Ig1Ea and irregularly throughout the sequence of emplacement units. Local spikes in the count fractions of low-density dark, swirly, and glistening pumice (Electronic Appendix 2) are interpreted to reflect buoyant concentration during outflow.
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12% crystals; unornamented white, normal pumice with 0·5–12% crystals (xp + xm lumped). Diagonal lines indicate ranges of within-unit variability.New data collected since Wilson & Hildreth (1997
) lead us here to define two distinctive local lithofacies of Ig1Eb, the Watterson and Sherwin subunits (Electronic Appendix 2; Fig. 6). The Watterson subunit is (like most of Ig1) poor in xr pumice (2–12%) but is atypically rich in swirly pumice (5–28%), and much of it further contrasts with the rest of Ig1E in containing significant amounts of rhyolite lithics. The subunit lies in the middle of Ig1Eb, below the upper zone of dense welding that is prominent along Owens Gorge [welding zone c of Wilson & Hildreth (2003)] and in Chidago Canyon, and it is sandwiched between ignimbrite that is virtually devoid of rhyolite lithics. The subunit is poorly represented in Owens Gorge but is well expressed in an east–west strip about 20 km long and a few kilometers wide (between Lake Crowley and Chidago Canyon) of non-welded to poorly welded ignimbrite that forms the northern edge of the ignimbrite plateau where it laps onto the Glass Mountain debris fan along Watterson Canyon. Available data do not discriminate between deposition by flows directed eastward from the initial vent site that scoured that fan (Fig. 1) vis-à-vis flows representing a short-lived precursory outburst from a vent farther east that directly penetrated the Glass Mountain debris fan. In contrast, the Sherwin subunit is more typical of Ig1E in containing few rhyolite lithics and only modest amounts of swirly pumice, but it is exceptional in having large fractions of xr pumice (27–64%; Electronic Appendix 2), including Adobe-type textures. This subunit forms the topmost part of Ig1Eb, above welding zone c, principally near the rim of Rock Creek gorge in the area of Sherwin Hill, though scattered outliers occur farther east. It is absent from Chidago Canyon, and, as such, its distribution is distinct from that of the Watterson subunit. |
COMPOSITIONAL RANGES OF PUMICE TYPES |
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Each pumice type identified has a significant range of composition (Table 2; Figs 7–10
). The range is least conspicuous for SiO2—only 73·4–77·9% for the main array of normal pumice that makes up >90% of the Bishop Tuff. However, the ranges of FeO* (0·40–1·73%), MgO (<0·01–0·48%), CaO (0·42–1· 61%), TiO2 (0·08–0·25%), Zr (71–154 ppm), and Rb (75–195 ppm) for the main array are proportionately far more extensive, and the ranges for Ba (2–614 ppm) and Sr (9–185 ppm) are extraordinary. All such ranges are continuous, not bimodal or otherwise clustered. At the evolved ends of their ranges, the main xp, xm, and xr arrays are all compositionally similar (Table 2; Figs 9 and 10): 77·6–77·9% SiO2, 0·08% TiO2, 12·2% Al2O3, 
0·01% MgO, 75 ppm Zr, and 10 ppm each Ba and Sr. At the less evolved ends, however, the crystal-rich arrays extend to less differentiated compositions than the crystal-poor arrays; in this respect, the xm arrays consistently end at intermediate values for all elements (Table 2).
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Such extensive overlap of the compositional ranges for main-suite pumices signifies that major- and trace-element compositions are to some degree independent of crystal content (Electronic Appendix 1), there being at best a crude correlation for a few elements (Fig. 7). This, in turn, suggests (1) that the main body of high-silica rhyolitic magma that erupted to form the Bishop Tuff had been compositionally zoned before the observed spectrum of crystal contents developed, and (2) that crystal accumulation within the magma erupted was not a leading compositional control.
Data for the dense vitrophyric clasts and fiamme extensively overlap the compositional ranges of the main xp–xm–xr arrays described above (Table 2), consistent with these being dense equivalents of normal Bishop pumice. Exceptions are two or three dense fiamme that may once have been Ba-rich swirly pumice. Whole-rock welded-tuff vitrophyre has generally been avoided owing to the obvious potential for contamination and crystal–glass sorting during eruption and emplacement. We note, however, that compositions of the four such samples in our data set are monotonously normal: 76·1–76·4% SiO2, 12·7–12·9% Al2O3, 0·94–1· 04% FeO*, 3·48–3·55% Na2O, 4·97–5·06% K2O, 0·5–2·1% weight loss on ignition (LOI), 116–129 ppm Rb, 48–65 ppm Sr, 179–255 ppm Ba, and 105–127 ppm Zr. Because the four were lithic-free and from low in thick welded emplacement packages, they may have lost little glass-enriched elutriate and suffered little sorting. Their lesser hydration and limited alkali variability suggest that carefully selected samples of bulk ignimbrite vitrophyre can (in such special circumstances) offer some advantages over badly hydrated pumice.
Of the variant pumice types, glistening pumice shows a limited compositional range, consistently at or near the extreme end of Bishop arrays, and it includes the highest SiO2 (78·2%) and lowest Al2O3 (12·1%) values recorded for fresh clasts in the Bishop Tuff (Table 2). Swirly pumice is by far the most abundant Bishop component not belonging to the main xp–xm–xr continuum. It differs in texture, color, and in combining low phenocryst content with generally elevated Ba and Sr abundances. For most elements determined, the swirly pumice spans the same compositional ranges as the normal xr (including Adobe-type) pumice of the main array, but it extends to still higher values of FeO*, MnO, MgO, CaO, Ba, and Sr (Table 2), and to lower SiO2. Dark gray pumice is likewise crystal-poor and rich in Ba and Sr; most elements span ranges similar to those of the swirly pumice (to which it may be magmatically related), but a few of the dark gray clasts extend the ranges to extreme (non-rhyolitic) values: 67% SiO2, 4·14% FeO*, 3·85% CaO, 1350 ppm Ba, and 460 ppm Sr.
Hydration and alteration
Nearly all samples analyzed were single clasts or fiamme, unaffected by the crystal–glass fractionation and lithic contamination intrinsic to emplacement of bulk ignimbrite. To minimize effects of post-emplacement vapor transport and secular alteration, we avoided taking clasts from devitrified or vapor-phase zones, or from exposures displaying case-hardening (i.e. potentially leached and/or silicified). Although the samples thus approach the compositions of magma erupted, all consist of 75–99% glass, which has been variably hydrated. Water content was not measured directly, but LOI at 900°C provides a fair approximation to the degree of hydration of the (FeO-, CO2- and halogen-poor) glass. The hydrous phenocrysts, biotite and allanite, together make up <1 wt % of the pumice, so their water contributions are negligible.
Most of the vitrophyre clasts and fiamme give LOI values of 1–3 wt % and most pumice clasts yield 2–5 wt % (Fig. 8). (The two low-LOI samples of Fig. 8 are partly devitrified phenocryst-rich fiamme.) Because hydration of volcanic glass usually results in Na loss and, also commonly, in K gain (Lipman, 1965; Noble, 1967), probably in part by ion exchange between groundwater and glass (Truesdell, 1966), the Na–K data for the Bishop Tuff sample suite have been scrutinized. The wide scatter in Na2O and K2O contents (Fig. 8) certainly in part reflects alkali mobility, both groundwater leaching and ion exchange, but much of the spread also reflects magmatic zoning and real differences among the types of pumice coerupted. Glassy melt inclusions sealed in unbroken quartz crystals contain 3·0–3·8 wt % Na2O (n = 55; Wallace et al., 1999). Main-suite (xp–xm–xr) pumices having Na2O <2·8 wt % and K/Na > 2·3 have probably lost some Na, whereas those with Na2O contents greater than 4 wt % may have gained Na. Notably, however, there is no simple relationship between Na loss (or K/Na ratio) and LOI (Fig. 8). Many of the highest LOI values are for samples within the main range of K/Na ratios.
A few additional points can be observed in Fig. 8. (1) For the normal pumice suite, LOI tends to be lower for xr samples and higher for xp ones, despite much overlap. (2) Few samples have patently excess K2O (>6 wt %) or Na2O (>3·8 wt %). (3) There is little evidence for K loss. (4) For the normal pumice suite, there is a rough tendency toward lower K2O abundances with increasing SiO2, presumably related to separation (at some stage) of melt from sanidine (± biotite). A negative K–Si correlation is opposite to expectation if K gain by ion exchange were linked with greater hydration of higher-silica samples. (5) If any of the low-Na pumice also lost silica, the loss was limited to 1–2 wt % SiO2 and affected few samples. There is no correlation between SiO2 and LOI (plot not shown). (6) The suites of dark and swirly pumice have ranges of Na2O similar to that of the main xp–xm–xr suite, but both of the subordinate suites extend to lower K2O and SiO2 abundances. All three suites overlap at the high-silica ends of the arrays.
Plotting LOI vs every other element determined revealed no evidence for preferential mobilization in high-LOI samples. In particular, no systematic gains or losses of TiO2, FeO*, MgO, CaO, Y, Zr, Nb, Ba, or even Rb are discernible with progressive hydration for any clast type in the data set. Scrutiny of LOI and inter-element plots identified five samples that appear to have lost SiO2 and five more that have gained Ca and Sr; these 10 samples were omitted from the data set discussed below. Two samples of the dark gray pumice that may have lost SiO2 and another that may have gained SiO2 have been retained, owing to uncertainties about the primary compositional range of this highly varied suite.
Discussion of compositional ranges
A general feature of major- and trace-element plots of Bishop Tuff pumice data is an absence of narrow compositional arrays. Instead, all x–y plots show broadly scattered trends for the main suite of normal xp–xm–xr pumices and generally different trends (typically even more scattered) for the subordinate suites of variant swirly and dark pumices. Processes that might have contributed to such scatter could include: (1) heterogeneous incremental assembly and growth of the laterally and vertically extensive zoned magma body; (2) varied degrees of mixing among three or more magma domains that had evolved, at least for a time, separately; (3) spatially inhomogeneous crystal–melt–vapor fractionation within the zoned magma reservoir; (4) meter-scale (or smaller) melt–crystal segregation and/or crystal accumulation by convective sorting, either preeruptive or in syneruptive magma flow toward or within conduits; (5) posteruptive alteration.
Variation vs SiO2 of key major and trace element contents are shown in Figs 9 and 10. Data for the normal xp pumices cluster at the high-silica low-Ca–Fe–Ti ends of the arrays (Fig. 9), whereas the xm and xr pumices range from near the same high-silica terminations to as low as 73·4% SiO2, where their CaO and TiO2 contents are three times greater and their FeO* twice that of the most evolved pumices. The scattered arrays of dark and swirly pumice extend to still less silicic compositions, but both overlap extensively the main xp–xm–xr array in CaO and TiO2. At high silica, the swirly pumice array has FeO* similar to the main array, though dark pumice tends to have higher FeO* at given values of SiO2.
Comparable relationships for Ba, Zr, and Rb are illustrated in Fig. 10. Data for the high-silica xp normal pumice are concentrated at the high-Rb (175 ± 25 ppm), low-Zr (75–100 ppm), low-Ba (<50 ppm) ends of the main arrays, whereas crystal-richer normal pumice trends toward far less evolved compositions, filling broad fields that overlap the fields of the subordinate dark and swirly pumice. Despite considerable overlap, the dark and swirly pumice arrays tend to have higher Rb and lower Zr, Ba, and Ti (at equivalent SiO2) than the xr segments of the main arrays (Figs 9 and 10).
Data arrays for swirly pumice converge with the main xp–xm–xr array at 76–77% SiO2 for all elements determined (Figs 9 and 10). Of the dark gray pumice, only five of 25 samples analyzed have >75% SiO2, and because those data are scattered, specifying the high-silica evolutionary path of this variant is more equivocal. At the low-silica end, the array of xp swirly pumice extends to 71·5% SiO2 and that of xp dark pumice also to 71·5% (with a single homogeneous dark outlier at 67% and two strongly hydrated suspect samples at 69·5 and 70·6%). In the FeO* and Ba panels (Figs 9 and 10), the SiO2 scale is extended down to 56%, to show the dark outlier, plus two dacite and two trachyandesite clasts (Table 1), which appear to be roughly collinear with the dark and swirly pumice arrays.
Many trace-element plots exhibit fairly coherent arrays for the main suite but broadly scattered fields for the swirly and dark pumices (Figs 11–13). At equivalent Ba contents (Fig. 11), the main suite has lower Rb, generally lower Sr, and (despite more overlap) a tendency toward higher Zr than the swirly and dark pumices. It should be noted, however, that no dark and few swirly pumices have <100 ppm Ba, whereas two-thirds of the normal pumices have <100 ppm and, of the 50 main-suite samples with >250 ppm Ba, nearly all are crystal-rich (Fig. 11).
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Figure 12 shows that, at any given FeO* content, the swirly and dark pumices tend to have lower Zr and Ti but similar or higher Nb abundances than the crystal-richer parts of the normal pumice arrays. Conversely, however, the evolved (low-Fe) ends of the main arrays extend to somewhat lower Zr and Ti and slightly higher Nb than the swirly and dark pumices. Apart from clusters of highly evolved samples (predominantly but not exclusively xp pumice) that terminate the main-suite arrays, the scattered fields of swirly and dark pumice nonetheless exhibit essentially the same ranges in Nb, Ti, and Zr abundances as the main suite (Fig. 12).
Trace-element ratios plotted against FeO* (Fig. 13) further illustrate relationships among the suites, presumably reflecting fractionations dominated by feldspars, Fe–Ti oxides, and zircon. The extended field of Rb/Zr values for swirly and dark pumice spans almost the same fivefold range as the narrowly curved main array. For Zr/Nb ratios, discrete arrays for the crystal-richer normal pumice and for the swirly and dark xp pumices converge upon the evolved cluster of low-FeO* (predominantly crystal-poor) samples; the scattering of some data between arrays may reflect mutual mixing. For Ba/Sr ratios, the two principal arrays are nearly perpendicular; the low-FeO* end of the main array is essentially vertical, dropping to values of Ba/Sr < 0·5, whereas the dark and swirly pumice array is nearly horizontal, with most Ba/Sr ratios in the range 2–4. There is a kink in the main array of normal pumice near Ba/Sr 3, above which a subset of predominantly xr pumice trends with increasing FeO* to Ba/Sr 6. The observation that a dozen or more samples of dark and swirly pumice also follow this (positively sloping) trend (Fig. 13) is unlikely to reflect crystal accumulation because those pumices are all crystal-poor.
Compositional arrays of successive emplacement packages
Each successive emplacement package (Fig. 2) had a variety of pumice types (Fig. 6; Electronic Appendix 2) and a range of pumice compositions (Figs 8–13
), but proportions of the various pumice types coerupting changed markedly during the eruption. In Fig. 14, we show the range of pumice compositions sampled within each of 14 emplacement units.
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Considering first the main suite of (xp–xm–xr) normal pumice (which makes up >90% of the Bishop Tuff), the plinian deposit and Ig1 represent only the more evolved half of the compositional spectrum erupted. For example, these deposits have normal pumices that range only from 0·08 to 0·12% TiO2, from 73 to 106 ppm Zr, from 9 to 69 ppm Sr, and from 2 to 126 ppm Ba. In contrast, the Ig2 packages, though likewise containing the highly evolved pumices, have much wider ranges of these and other elements, extending to 0·24% TiO2, 163 ppm Zr, 124 ppm Sr, and 579 ppm Ba (Fig. 14). Comparable partial overlaps exist for plots of Y, Nb, and other major and trace elements not shown. For SiO2 contents, normal (plus glistening) pumice in Ig1 ranges from 78·2 to 74·8%, though only seven of 120 samples have <76%. On the other hand, normal pumice in Ig2 ranges from 77·8% down to 73·4% but 50 of 200 samples have <76% SiO2, and for the Ig2NW packages alone, 17 of 27 normal pumices have <76%. Most of the least evolved pumices are in Ig2N and Ig2NW, though several are also in Ig2E. It is important to observe (Fig. 14) that Ig2E [the Tableland sheet of Hildreth (1977
, 1979)] contains pumice clasts that span most of the compositional range of the entire Bishop Tuff. Considering next the crystal-poor suite of swirly and dark pumices (Fig. 14), there are few tendencies toward systematic compositional change with eruption progress. These pumices tend to have higher Rb and lower Zr in Ig1 than later in the eruptive sequence, but there are exceptions. There are also tendencies for swirly and dark pumices in Ig1 to have lower Ba/Sr and higher Fe/Ti than in Ig2 (Fig. 14). Many (but not all) of the swirly and dark pumices with low Rb and high Zr and Ba are from the northern outflow sheets (Ig2N, Ig2NW). Figure 14 also shows that the swirly and dark pumices extend to lower SiO2 and to higher Fe, Ti, Ba, and Sr contents than the main suite. However, the main and subordinate suites exhibit similar ranges of Rb and Zr as well as of Y (both suites 8–33 ppm) and Nb (7–31 vs 8–25 ppm, respectively). Overall, the wide compositional range of the subordinate magma that produced the swirly and dark pumices remained available throughout the eruption, having been released unsystematically as a small but fluctuating fraction (Fig. 6) that accompanied eruption of the zoned rhyolite that produced the main suite.
Fluctuating proportions of pumice types throughout the eruptive sequence—especially marked in Ig2E (Fig. 6)—might reflect any of several processes: (1) coeruption of varied magmas drawn concurrently into conduits from different parts of a zoned chamber; (2) coeruption from separate nearby chambers, each contributing to a common conduit system; (3) intrusion of the subcaldera chamber by one or more new magma batches from deeper in the crust, soon enough before eruption that intra-chamber distribution remained inhomogeneous; (4) circum-vent convective entrainment of earlier-emplaced pumice swept back up into the eruption column; (5) surficial incorporation of earlier pumice by ash flows sweeping the caldera floor or scouring previously emplaced fallout and outflow sheets. The concurrent emplacement of most kinds of pumice virtually from start to finish strongly suggests the importance of processes (1) and (3); and, in a later section below, we provide abundant evidence for a unitary chamber, thus rejecting process (2). Processes (4) and (5) are unavoidable at some scale during an eruption of this magnitude and duration, and the welded-tuff clasts in Ig2 (Fig. 2; Table 1) clearly demonstrate the reality of syneruptive recycling.
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CRYSTALS AND LITHICS |
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Although the main purpose of this paper is to document the extent and continuity of the Bishop pumice (magmatic) compositional zoning, crystals contribute to bulk magma composition and inevitably warrant discussion. Conceptually, it is useful to distinguish among crystals in a magma that are (1) xenocrysts, accidentally entrained from contrasting magmas or older rocks during intrusion, storage, eruption, or outflow; (2) phenocrysts, which grew in the magma containing them and may have had long or short residence times; and (3) antecrysts, which were inherited by the magma now containing them but had grown as phenocrysts in a discrete but kindred magmatic precursor, known or inferred to have been an earlier component of a waxing–waning multi-stage system. Crystals introduced by recharge batches of similar but not identical magma, crystals re-entrained from floor cumulates or mushy enveloping rinds, and the crystals liberated from precursor batches that had nearly but temporarily solidified provide examples of antecryst-vs-xenocryst ambiguity that require microbeam data and thoughtful interpretation.
Crystal suites
Even though the Bishop crystals are largely unzoned, reflect the bulk zonation, and thus appear to have crystallized after the chamber became zoned (Hildreth, 1977, 1979), attributes of the crystals (and their melt inclusions) nonetheless bear significantly on interpretation of the origin of the zoning.
Total crystal contents of pumices range from <1 to 24 wt % (Electronic Appendix 1; Fig. 7). Across the main xp–xm–xr continuum, sanidine and quartz always predominate in roughly equal proportions, and plagioclase is consistently subordinate, making up only 10–15% of the three tectosilicate species, which together constitute 98–99% of the assemblage. Titanomagnetite and biotite are next in abundance, each increasing from trace amounts (<500 ppm) in crystal-poor pumice to 5000 ppm in crystal-rich pumice. Zircon is ubiquitous, both as free crystals and as inclusions in most other species, and reaches a maximum abundance of 50 ppm. Likewise ubiquitous, but sparser still, is apatite (10 ppm), which forms inclusions in most others species (especially Fe–Ti oxides and py