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Cause of chemical zoning in the Bishop (California) and Bandelier (New Mexico) magma chambers
Earth and Planetary Science Letters, 111 (1992) 97-108 Elsevier Science Publishers B.V., Amsterdam
97
[CL]
Cause of chemical zoning in the Bishop (California) and Bandelier (New Mexico) magma chambers Richard L. Hervig
a
and Nelia W. D u n b a r b, ,
a Center for Solid State Science, Arizona State University, Tempe, A Z 85287-1704, USA t, Department of Geosciences, New Mexico Institute of Mining and Technology, Socorro, N M 87801, USA Received April 1, 1991; revision accepted March 3, 1992
ABSTRACT Rhyolitic magma chambers often erupt to form deposits with a wide range of trace element chemistry, inferred to reflect zoning in the magma chamber prior to eruption. Ion probe microanalyses of trapped melt inclusions and matrix glass from the large Lower Bandelier Tuff and Bishop Tuff eruptions shows that much of this compositional variation can be blamed on the intrusion of a second rhyolitic magma into the base of the chambers. The second rhyolite was composed of similar major elements but contained significantly higher Ti, Sr, and Ba in both examples. Microanalyses of sanidine phenocrysts show pronounced trace element zoning profiles in accord with the glass chemistry. Applying the available diffusion coefficients for Sr in sanidine to the zoning suggests residence times on the order of 104 yrs after the mixing event. The source of the second magma is not known, but similarities in chemical zoning patterns in silicic magmas throughout the world point toward a common process. Mixing of less fractionated magma derived from similar source rocks is the simplest mechanism. Detailed isotopic studies may help distinguish different sources. Independent of the second magma, large variations in trace elements are observed in the melt inclusions from the Lower Bandelier and Bishop Turfs which can be modeled by ~ 40% fractional crystallization.
1. I n t r o d u c t i o n
That magma chambers are chemically zoned is commonly accepted among igneous petrologists today. In fact, magma chambers with eruptive deposits that do not show significant chemical zoning have become quite noteworthy [e.g., 1,2]. However, the cause of zoning in silicic magma chambers is usually difficult to identify. In some cases, erupted material provides evidence for mixing of distinct chemical end members [3], but in many other cases, large volumes of silicic material are ejected with fairly small major element
* Present address: Environmental Science Division, MS 6038, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, USA. Correspondence to: R.L. Hervig, Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704, USA.
ranges and quite large trace element variations [4-8]. Several causes for zoning in magma chambers have been put forward by Hildreth [5] who rejected: (1) rising trace element-rich vapor bubbles; (2) partial melting of a progressively depleted/heterogeneous source; and (3) fractionation of crystals as viable mechanisms, suggesting instead that thermogravitational diffusion should be investigated further. Michael [9] concluded that fractional crystallization could explain many chemical features of the Bishop Tuff [but see 10,11]. Later, measurements of thermal diffusion coefficients proved that thermogravitational diffusion was inefficient at generating large chemical gradients in magma chambers [e.g., 12]. The purpose of our paper is to show that the observed chemical zoning of the large Bishop and Lower Bandelier rhyolitic eruptions can be explained entirely by a combination of fractional crystallization and the mixing of a second rhyolite
0012-821X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
98
magma into the base of the chamber. The second magma has major element chemistry virtually indistinguishable from the initial composition of the chamber, but the trace elements are much different. Our approach was to use the ion probe to obtain microanalyses of trace elements in melt inclusions, phenocrysts, and matrix glass throughout the stratigraphic height of the deposits, in order to observe chemical variations of the magma on a fine scale. This microanalytical approach is similar to that of Lu et al. [13,14] who recently analyzed melt inclusions and matrix glass from the Bishop Tuff with electron and ion microprobes and infrared spectroscopy, and concluded that the compositional zonation of the tuff was largely caused by variable addition of less differentiated, CO2-rich melt. We have confirmed their conclusions and expanded on them with the data from the Lower Bandelier Tuff.
2. Analytical conditions
2.1 Ion microprobe Ion microprobe analyses were made on the Cameca IMS 3f instrument at Arizona State University. We used a 1-2 nA mass-analyzed primary beam of 160- ions focussed to a spot 20-25 ~ m in diameter. Secondary ions were accelerated to + 4.5 keV and the transfer optics and field aperture were set to accept secondary ions into the mass spectrometer from a circular area 20/xm in diameter on the sample. The energy bandpass was set at 40 eV. After the secondary ion signal had stabilized (about 7 min) the sample voltage was ramped +__100V from 4500 while the intensity of 3°Si+ ions was monitored. The sample voltage was returned to the centroid of the intensity vs. sample potential curve to correct for the small amount of charging which occurred. Molecular ion interferences were removed by collecting secondary ion intensities at high energies, which were achieved by offsetting the sample voltage - 7 5 V from the centroid position for H, 7Li, 11B, 19F, 31p, 47Ti' SaFe ' 85Rb' 88Sr' 89y, 9 0 Z r ' 9 3 N b ' 138Ba, 14°Ce, and 232Th. In fact, there are no serious interferences on Li or B so it is not necessary to collect ion signals with an offset, but it was found that reproducibility of the analyses could be substantially improved by collecting all
R.L. H E R V I G A N D N.W. D U N B A R
secondary ion intensities at the same offset voltage. Trace elements were calibrated against NBS 610, a sodium- and silica-rich glass doped with 61 trace elements at the 500 ppm level. Comparison of NBS 610 with bulk-analyzed rhyolite glasses indicated that the trace elements studied were within 10% of their assayed (when available) or nominal concentration, except for phosphorus and titanium, which gave concentrations of 350 and 590 ppm, respectively, when compared with rhyolite glasses. Analyses for F used a combination of NBS 610 and KN-18, a comendite glass with 6400 ppm F. Observed reproducibility of secondary standards suggested that most trace elements are precise to + 10% but that this error increases to ~ 30% for elements near 1-5 ppm abundance given the counting intervals used in this study. Analyses of crystals with major element chemistry much different than the standard glasses are subject to matrix effects, but the use of relatively high-energy ions (> 50 eV initial kinetic energy) keeps such effects to a minimum. Early, very conservative estimates of the matrix effects at these conditions were 100% [15], but recent comparisons of secondary ion intensities of trace elements from natural samples of quartz, rhyolite, leucitite, minette, and synthetic glasses and crystals from simple ternary systems show that absolute abundances given should be well within 30% of the true amount [16 and unpublished data from the ASU SIMS laboratory].
2.2 Electron microprobe Major elements and chlorine were analyzed with a JEOL 8600 electron microprobe at Arizona State University. The error for CI determinations was + 100 ppm based on replicate analyses of reference glasses containing 3000 ppm CI.
3. Results
3.1 Lower Bandelier Tuff The Lower Bandelier Tuff erupted at 1.51 Ma [17] and is composed of a plinian tephra and associated ignimbrite. Bulk analyses of pumice lumps show striking variations which has suggested to many researchers that this was indeed a strongly zoned magma chamber [18,19]. The
C A U S E O F C H E M I C A L Z O N I N G IN T H E BISHOP A N D B A N D E L I E R M A G M A C H A M B E R S
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+ +
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~ . 300 Q. J~
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Nb (ppm) Fig. 1. (a) Variation of Nb with Ti from bulk analyses of plinian and ignimbritepumice samples and microanalysesof melt inclusionsfrom the LowerBandelierTuff. (b) Variation of Nb with Rb in the samplesfrom(a).
stratigraphy of the deposit shows weak to no correlation with trace element chemistry leading researchers to suspect that an originally smooth chemical gradient was disrupted during eruption [19]. We analyzed 77 melt inclusions found in quartz, sanidine, and pyroxene phenocrysts and matrix glass from the plinian and ignimbrite with the electron and ion probes and 20 bulk samples by XRF. Trace element covariation plots are shown in Figs. 1-3. The complete tables of analyses will be published in separate, detailed papers on the individual eruptions [20,21]. Bulk and microbeam data are plotted together in Fig. la showing Nb vs. Ti. The agreement between the two analytical techniques indicates that the melt inclusions (regardless of their host
99
phenocryst) represent unaltered magma compared to the bulk. Inclusions and pumice lumps with over ~ 100 ppm Nb show little variation in their Ti contents while Nb-poor inclusions and pumice lumps show a negative correlation with Ti. Rubidium shows a slightly more linear pattern (Fig. lb). Barium, ST, and Zr are correlated with Ti, while Y and Th follow generally linear patterns similar to Rb. Iron, Ce, Li, and P do not show strong correlations with the other trace elements. Boron and C1 are very low in the Ti-rich, Nb-poor inclusions. Bulk analyses of plinian samples show only a small range in Nb (at 185 _+ 10 ppm) and other trace elements [19]. The phenocryst content of most pumice lumps ranges between 15 and 20% (corrected for vesicularity [19l). The concentrations of selected trace elements in melt inclusions from one pumice lump (sample 030) in the ignimbrite collected 5 m from the base of the Cow Section (located at a road cut along route 4 at the base of Otowi Mesa) are plotted separately in Fig. 2 and full analyses are given in Table 1. High-Ti inclusions and one analysis of matrix glass are low in Y, Nb, Th, and Cl and high in Ba. One quartz phenocryst contains separate high- and low-Ti inclusions (connected by a dashed line segment in the figures). We have marked the bulk Y and Th contents (from Table l) in the figures, which are similar to the high-Ti melt inclusions and matrix glass. We have found high-Ti, low-Nb melt inclusions in three out of every four pumice lumps collected from the Cow Section, in one out of two samples from the Guaje (Copar) pumice mine locality stratigraphically above the plinian deposit, but none from three pumice lumps from the Pueblo Mesa locality. No high-Ti, low-Nb inclusions have been found in the plinian samples. Bulk analyses from Kuentz [19] reveal high-Ti, low-Nb compositions in ignimbrite samples from the same sites, as well as Cat Mesa and Wildcat Canyon (not studied by us). We note that all localities showing high-Ti chemistry are from the southern side of the caldera, while Pueblo Mesa is on the north side.
3.2 Bishop Tuff The Bishop Tuff erupted 740,000 years b.p. from the Long Valley Caldera, California, and is
100
R,L. HERVIG AND N.W. DUNBAR
one of the most thoroughly studied rhyolitic deposits in the world [4,9,22-25]. Trace elements show smooth progressions up the stratigraphic sequence from early to late erupted material. The Ti and Ba contents of 175 analyses of melt inclusions in quartz and sanidine phenocrysts from the Bishop Tuff are shown in Fig. 3. Barium varies from 1 to 500 ppm while Ti varies over a > 3-fold range. Most of the low values are from melt inclusions in the plinian and early-erupted ignimbrites (Chidago, Gorges) with higher values concentrated in the last erupted ignimbrites (Tablelands, Adobe Valley, Mono Lobe). These reflect approximately the early/late enrichment factors based on bulk analyses given by Hildreth [5] and are similar to the ion and electron probe
analyses of melt inclusions and matrix glass reported in [13]. The phenocryst content of pumice lumps from the Bishop eruption ranges from 5 to 25% (corrected for vesicularity [22]). Melt inclusion and matrix glass compositions from one pumice lump from the Adobe Valley ignimbrite are shown in Fig. 4 with bulk and microbeam analyses listed in Table 2. This sample was collected near the base of the section and is analogous to sample B-70 of Hildreth [22] which gave an Fe-Ti oxide temperature of 763°C. Comparisons with bulk analyses by us and by Hildreth [22] on matrix glass separates from the same locality are shown in Fig. 4. High-Ti melt inclusions and matrix glass are generally similar in chemistry to the bulk analysis, and are rich in
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Fig. 2. Variation of chemistry in melt inclusions (dots) from sample 030 from a pumice lump in the ignimbrite of the Lower Bandelier Tuff. T h e circle represents matrix glass and the dashed line connects two inclusions in the same quartz phenocryst. (a) Variation of Ti with CI. (b) Variation of Ti with Ba; legend as in (a). (e) Variation of Ti with Y; legend as in (a). (d) Variation of Ti
with Th; legendas in (a).
CAUSE OF CHEMICAL ZONING IN THE BISHOP AND BANDELIER MAGMA CHAMBERS
Ba, Sr, Ce, and Zr and low in Rb, Y, and Th on average compared to low-Ti melt inclusions. No clear difference in Nb, F, and P could be discerned, but hydration of the matrix has occurred, and we note high Li and B in the analyses of Ti-rich matrix glass compared to Ti-rich melt inclusions. Other pumice lumps that showed highand low-Ti melt inclusions include one out of three Tablelands samples (from the base and middle of the section), two out of three Adobe Valley (both from the base of the section), and all three Mono Lobe samples (all near the base of the section). No high-Ti melt inclusions have been identified from the Chidago or Gorges ash flows or the plinian tephra.
101
known to show a smooth variation over a wide range, and Figs. 1 and 3 demonstrate that this is equally true when a microbeam technique is used to characterize the samples. In this contribution, we show that when examined on a smaller scale --within a pumice lump, and sometimes within a single phenocryst--there is extreme local chemical variation indicating that in the case of the Bishop and Lower Bandelier Tufts, magma mixing was an important process in generating zoned magma chambers. 4.1 Processes other than magma mixing. We can reject fractional crystallization as a mechanism for generating all of the chemical variations in Figs. 2 and 4 as the last melt (matrix glass) is rich in Ba. It would be impossible to enrich the Ba-poor Bishop glasses in 095 by fac-
4. Discussion
The distribution of trace element chemistry in the Bishop and Bandelier deposits has long been TABLE 1
Analyses of high and low-Ti melt inclusions from the Bandelier Tuff compared with bulk analyses Element SiO 2 AI20 3 Fe20 3 MgO MnO CaO Na20 K20 CI H20 Li B F P Ti Rb Sr Y Zr Nb Ba Ce Th n
1
2
75.6 + 12.1 + 1.2 + 0.01 + 0.06 + 0.22 + 4.35+ 4.45+ 0.22+ 3.3 + 43 11 300 94 276 264 1.0 67 209 130 4 49 22 6
+ + + + + + + + + + + + +
1.1 0.1 0.1 0.01 0.03 0.05 0.19 0.19 0.02 0.7 18 8 100 16 25 49 0.6 14 42 29 1 7 6
76.0 + 12.1 + 1.4 + 0.03 + 0.04 + 0.24 + 4.24+ 4.51+ 0.12+ 2.7 +
1.2 0.3 0.1 0.01 0.03 0.03 0.29 0.17 0.01 1.1
4 6 + 12 5 + 1 400 + 200 100 + 15 612 -+_ 28 119 + 20 1.5 + 0.6 29 + 3 186 ± 22 52 + 11 8 + 2 65 -I- 8 10 5: 2 8
3
4
5
75.8 12.1 1.3 0.03 0.00 0.24 4.30 4.46 0.13 n.a.
77.2 12.0 1.45 0.03 0.07 0.28 4.11 4.83 n.a. n.a.
77.4 11.7 1.35 0.06 0.05 0.30 3.77 4.74 n.a. n.a.
48 7 150 80 599 99 1.1 31 186 61 6 65 13 1
n.a. n.a. n.a. 35 330 280 5.2 97 213 143 < 10 103 35 1
n.a. n.a. n.a. 52 659 111 5.3 37.6 237 54 18 106 16 1
6
114 5 38 201 52
13 1
1 = Average low-Ti melt inclusion from ignimbrite lump 030. This ~ 11 c m X 7 cm sample was collected from the Cow Section 5 m above the base of the ignimbrite. 2 = Average high-Ti melt inclusion from ignimbrite lump 030. 3 = Microbeam analysis of matrix glass from ignimbrite lump 030. 4 = Bulk analysis of low-Ti pumice lump (sample 17/29) selected on the basis of Ti and Nb contents from Kuentz [19]. 5 = Bulk analysis of high-Ti pumice lump (sample 27/63) selected on the basis of Ti and Nb contents from Kuentz [19]. 6 = Bulk analysis of 030. Errors are 1 standard deviation.
102
R.L. H E R V I G A N D N.W. D U N B A R
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1000
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Ti (ppm) Fig. 3. Variation of Ti with Ba in melt inclusions from the Bishop Tuff. Inclusions from the early plinian eruption ( ~ 720-725°C), the subsequent Chidago and Gorges ash flows ( ~ 722-738°C) the later Tableland ( ~ 730-760°C), and the Adobe Valley and Mono Lobe ( ~ 750-790°C) ash flows are distinguished in the legend. The bulk analyses of early and late eruptions (E and L, respectively) are shown in the figure. These analyses and the Fe-Ti temperature ranges for the respective samples are taken from Hildreth [5].
tors of ~ 15 with sanidine in the precipitating assemblage (Fig. 4a) because measured partition coefficients for Ba between rhyolitic liquids and sanidine are quite large, e.g., 5-24 [26], giving bulk partition coefficients greater than unity for the eutectic crystallizing assemblages involved in these localities. Fractional crystallization is an important process in the evolution of the magmas, which will be discussed below. The extreme depletion of CI in the Lower Bandelier matrix glass and high-Ti melt inclusions is also difficult to explain given the absence of fractionating CIbearing phases. Although C1 would partition into a hydrous vapor, the low water content of these inclusions ( ~ 3 wt.% H20; Table 1) precludes saturation of the magma with water vapor. Strong CI depletions would not be expected if mixed H 2 0 - C O 2 fluids were present because partition coefficients for CI between such fluids and melt are not significantly greater than unity [27].
4.2 Magma mixing The hypothesis that mixing of a separate rhyolitic magma is responsible for the observed chemistry is testable. For example, if mixing oc-
curred, then any phenocrysts growing at this time should have responded to t h e change in the chemistry of the magma. Major element chemistry of inclusion and matrix glass (electron probe) and glass separates (bulk techniques) are not distinguishable (Tables 1 and 2; [21]) and Hildreth [22] found that sanidines in the Bishop Tuff were unzoned. However, sanidine crystals would be expected to show edgeward increases in trace elements that are enriched in the mixed magma. We have tested this by measuring core-to-rim variations of Li, P, Ti, Na (or Fe), Rb, Sr, Ba, and Ce in sanidines from several pumice lumps in the Bishop Tuff and Lower Bandelier Tuff. Some sanidines from Lower Bandelier pumice lumps in which high-Ti melt inclusions were found show trace element enrichments at their edge (Fig. 5). The thickness of these enriched rims ranges from 100 to 400 /zm. In the late-erupted ignimbrites from the Bishop Tuff (Tableland, Adobe Valley, Mono Lobe), all sanidines we have studied show pronounced edgeward enrichments in Ti, Sr, and Ba, in accord with the melt chemistry shown in the earlier figures (Fig. 6). Thicknesses of these rims range from 250 to 600/zm. For all samples, the absolute thicknesses are subject to cutting effects, so that the minimum measured thickness is probably most reliable. In contrast, we have not found sanidine with trace element-rich rims from early erupted plinian and ignimbrite (Gorges, Chidago lobes) pumice lumps. The enrichment is quite variable from one sanidine crystal to another, even within the same pumice lump. Perhaps this can be related to incomplete mixing of the second magma within the chamber, or during syneruptive formation of pumice lumps. One very important line of evidence supporting magma mixing is represented by the quartz phenocryst in sample 030 which contains a high-Ti and a low-Ti melt inclusion. We have found quartz phenocrysts from two other pumice lumps from the Lower Bandelier Tuff with the two different magmas included. Trace element scans in these quartz crystals were not attempted because of the extremely low concentrations of the elements of interest. We have not found phenocrysts from the Bishop Tuff that contain both magma types, but the contrasting chemistry between matrix glass and low-Ti inclusions in quartz supports the argument that the chemistry of the magma surround-
CAUSE OF CHEMICAL ZONING IN THE BISHOP AND BANDELIER MAGMA CHAMBERS
ing these crystals changed dramatically during its evolution. There is another way to test the mixing hypothesis. If a magma with distinct trace element characteristics invaded the chamber at depth, then a subsequent eruption might show the high-Ti, Ba-Sr signature of the second magma In fact, several small eruptions occurred between the Lower and Upper Bandelier caldera-forming events. Analyses by Stix and Gorton [28] and Layne and Stix [29] of the first-erupted rhyolite after the Lower Bandelier Tuff (their sample 15-8) show a Ti-, Zr-, Ba-, Sr-rich and CI-, B-poor chemistry, quite similar to the chemistry of the high-Ti magma reflected in Fig. 2. While complete analyses are not available, post-Bishop rhyolites (K-Ar dates ranging from 730,000 to 630,000
103
years b.p.) are Ba-rich compared to the Bishop Tuff compositions and textural features suggest high magmatic temperatures [R.A. Bailey, U.S. Geological Survey, pers. commun.]. Because only the late-erupted sanidines from the Bishop Tuff show the trace element-rich edges and because the subsequent eruption to the Lower Bandelier Tuff shows similar chemistry to the high-Ti-Ba-Sr magma, we conclude that in both cases the new rhyolite mixed into the base of the chamber. (By base, we mean the deepest part of the chamber that was sampled.)
4.3 Temperature of the second magma In the Bishop Tuff, Lu and Anderson have measured the chemistry of Fe-Ti oxides included
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Fig. 4. Variation of chemistry in melt inclusions (dots) from sample 095 from a pumice lump in the Adobe Valley ash flow of the Bishop Tuff. The circles represent analyses of matrix glass. Bulk analyses from [21] and from Hildreth [22] on similar samples are shown when available. (a) Variation of Ti with Ba. (b) Variation of Ti with Sr; legend as in (a). (c) Variation of Ti with Y; legend as in (a). (d) Variation of Ti with Ce; legend as in (a).
104
R.L. HERVIG AND N.W. DUNBAR
in quartz and in the matrix of the pumice, finding indications that the magma temperature increased subsequent to entrapment [14]. They concluded that the injection of a hot, less evolved rhyolite magma could explain this. Without suitable phenocryst assemblages in the Lower Bandelier Tuff it is not possible to discuss the temperature in a quantitative sense. The addition of sufficiently hot material would presumably rise as a plume in either chamber. This would not lead to a simple relationship between depth in the chamber and chemistry. Below some critical temperature, influx of magma would be less disruptive, preserving pre-existing chemical gradients (if present).
4.4 Volatile content
We do not emphasize volatiles in this work (see Dunbar and Hervig, [20,21] for volatile data). This is because we can find no relationship between water contents and the mixed component (the mean water contents of the high- and low-Ti inclusions in Tables 1 and 2 are indistinguishable) and because we cannot at this time use the ion microprobe to measure the carbon contents of melt inclusions (high instrumental backgrounds at no better than 600 ppm CO 2 at time of writing). However, Lu and Anderson [14] found high Ba and CO 2 contents in some melt inclusions from the last erupted ignimbrites from the Bishop Tuff,
TABLE 2 Analyses of high and low Ti-melt inclusions and matrix glass from the Bishop Tuff compared with bulk data
Element SiP 2 AI20 3 FeO MgO CaP Na20 K20 CI H2 O
1
2 76.5 + 1.1 12.4 5 : 0 . 3 0.66+ 0.07 0.035:0.01 0.47 5 : 0 . 0 5 3.70+ 0.30 4.71+ 0.36 0.07 + 0.02 2.3 5 : 0 . 8
Sum
100.83
Li B F P Ti Rb Sr Y Zr Nb Ba Ce Th n
39 43 180 240 376 136 2 12 67 15 4 38 14 5
75.9 5: 12.3 5: 0.76+ 0.04+ 0.47 + 3.58+ 4.69+ 0.05 + 2.1 5:
0.5 0.1 0.11 0.01 0.02 0.12 0.20 0.01 0.5
3
4
5
75.4 n.a. 1.1 0.16 0.7 3.2 5.1 n.a. n.a.
n.a. 13.1 0.5 n.a. n.a. 3.6 5.9 n.a. n.a.
6
99.79 + 3 5:1 5:40 +30 5:83 +_ 12 5:1 + 3 5:8 __. 5 5:1 5:4 5:1
39 34 300 270 622 123 13 9 81 9 76 57 8 3
5: 5 5: 1 5:170 + 40 5:36 + 25 + 3 5: 1 5: 3 5: 6 5:15 + 5 5: 4
155
84 56 17 2
53+ 8 385: 6 340 5:130 3305:20 583 + 47 109 + 20 12+ 2 95: 2 765: 8 11+ 2 70 5: 6 49 5: 5 105:3 5
143 40 13 112 12
15 1
1 = Average chemistry of low-Ti, Ba melt inclusions from Adobe Valley pumice lump 095 collected near the base of the ash flow. This represents the same locality as Hildreth's sample B-70 [22]. It is taken from the lowest exposure of the basal non-welded zone of this lobe. Of the eight inclusions, six were analyzed for water and trace elements by ion probe (one inclusion has only a partial trace element analysis), while all were analyzed by electron probe. 2 = M e a n of three analyses of high-Ti, Ba inclusions from 095 (all analyzed for water and trace elements by ion probe and for major elements by electron probe). 3 = Average bulk analysis for Adobe Valley pumice from Hildreth [22]. 4 = Average matrix glass separate analysis for Adobe Valley pumice from Hildreth [22]. 5 = Average of five ion probe analyses of matrix glass in 095. 6 = Bulk analysis of 095 from this work. Errors are 1 standard deviation.
CAUSE OF CHEMICAL ZONING IN THE BISHOP AND BANDELIER MAGMA CHAMBERS
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Position (gm) Fig. 6. Step-scan across a sanidine crystal in sample 091, a pumice lump from the Mono Lobe ash flow within the Bishop Tuff. Position axis indicates distance from the edge of the crystal (step size = 25 /zm). (a) Variations in Rb and Sr. (b) Variations in Ba. This pumice lump was collected near the base of the Mono Lobe; the bulk analysis matches with samples from [22] giving Fe-Ti temperatures of 790°C.
10oo
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Position (l~m) Fig. 5. Step-scan across a sanidine crystal in sample 027, a pumice lump from the Lower Bandelier ash flow. Position axis indicates distance from the edge of the crystal (step size = 20 /zm). (a) Variations in Li, Sr, and Ce. (h) Variations in Ti and Rb. (c) Variations in P and Ba. This sample was collected from the Cow Section 3 m stratigraphically above sample 030.
concluding that a high-CO 2 rhyolite had been mixed into the magma chamber. Knowing the CO 2 content is important because: (1) recent experimental work [30] shows that H 2 0 - C O 2 fluid mixtures can lower significantly the solidus of granitic crystalline assemblages, and (2) a high CO 2 content could indicate the presence of a free fluid phase whose influence on the bulk physical properties of the magma could effect the eruption dynamics.
106
4.5 Timing The question of timing of the introduction of the mixed component is interesting, but our discussion is admittedly speculative. Did this mixing of new magma occur suddenly upon removal of shallow-level magma in the early plinian/ignimbrite eruptive events? Or was it a gradual infiltration of material into the base of the chamber? The nearly bimodal distribution of certain elements on the scale of a hand sample (or even phenocryst) argues for a fairly rapid process. However, the mixing was not so rapid as to preclude the growth of several hundred microns on the sanidine crystals before eruption. Giletti [31] measured the diffusion of Sr in orthoclase at 800°C and 1 kbar water pressure. Experiments run shorter than about one day were neglected, because they may be affected by surface reactions with the fluid [31]. The remaining eight measured values range from 2.5 × 10-16 to 1.9 × 10-15 cm2/s. At this pressure and temperature, Sr gradients in sanidines would begin to smooth out over length scales of ~ 100 /zm in 2-13 k.y. There are no measurements of the Ba diffusion coefficient in sanidine, but given the larger ionic radius, we can expect it to be slower than Sr, eventually yielding significantly different profiles for the two elements. The profiles in Figs. 5 and 6 argue that after mixing, the sanidines resided in the magma chamber for a period on the order of ~ 104 years before reaching closure temperature (eruption). This time scale is in good agreement with the calculations by Lu and Anderson [14] based on trapped Fe-Ti oxide inclusions. Note also that there are two distinct changes in the chemistry of the sanidine in Fig. 5: one at 425/~m and one at 175/zm from the edge. Not only did the chemistry of the magma change dramatically during the growth of the crystal in the Lower Bandelier chamber, changes happened repeatedly.
R . L H E R V I G AND N.W. DUNBAR
of the Bishop and Lower Bandelier eruptions. However, the question of where this magma came from is more difficult to address. The simplest model would explain the high-Ti mixed component as a melt which derived from sources similar to those which produced the bulk of the Bishop and Lower Bandelier magmas, but which is the product of much less fractionation. The isotopic signature of this magma might be quite similar to the initial magmas, as is suggested by the available isotopic data for the Bishop [23,32]. However, the strong zoning of Sr in sanidines developing during the mixing event suggests that a detailed isotopic study of cores and rims of sanidines would be useful in learning more about the origin of the high-Ti magma, and may influence earlier conclusions concerning residence times of rhyolitic magmas [33]. There are no detailed Sr isotopic data on the Lower Bandelier Tuff, but we note that the preBandelier San Diego Canyon rhyolites have many of the trace element characteristics (low Nb, high Ti, Ba, and Sr) of the mixed component [17,34,35]. We take this to indicate that sources for melting, mobilization, and intrusion of high-Ti rhyolite into the Bandelier chamber exist in the region. Regardless of the source of the material, a common thread in the Lower Bandelier and Bishop Tufts is the addition of hotter magma after a period of evolution within the chamber. That heat is being periodically supplied to the two regions is obvious, considering the duration of rhyolitic activity. However, the input of heat into the magmatic systems might be expected to be linked to eruption if the models of Sparks et al. [36] and Huppert and Sparks [37] can be applied. The apparent significant residence time after mixing (of the order of 104 years) suggests that the temperature differences between the two magmas were not sufficient to cause directly the eruption.
4. 7 Importance of fractional crystallization 4.6 Origin of the second magma Given our data set, we believe it is easy to become convinced of the importance of magma mixing in generating the trace element chemistry
This paper focusses on the role of magma mixing, but it would be wrong to suggest that fractional crystallization is not an important process in defining the character of magma chambers. For the Lower Bandelier Tuff, if melt inclu-
CAUSE OF CHEMICALZONING IN THE BISHOP AND BANDELIERMAGMACHAMBERS
sions and bulk analyses which show the mixed component are removed (e.g., compositions in Fig. lb with < 120 ppm Nb), trace element variations can be described by fractional crystallization models assuming ~ 40% crystallization of quartz and alkali feldspar (and trace amounts of fractionating chevkinite [20]). Although the Lower Bandelier Tuff contains only ~ 20% crystals, significant side-wall crystallization cannot be ruled out, and may even be required to explain the observed gradient in water content within the deposit [20]. If the high-Ti, -Ba, -Sr component inclusions are subtracted from the Bishop data, variations in incompatible elements such as B, Nb, and Th, can be fit by a fractional crystallization model, and again, about 40% crystallization is required to explain the observed range in chemistry [21], in good agreement with Lu et al.
[38]. 5. Conclusions Late addition of rhyolite magmas with similar major elements but distinct trace elements changed the trace element chemistry of the lower region of the magma chambers that erupted to produce the Bishop and Lower Bandelier Tufts. The similarity between the trace element characteristics of the two added components suggests a common origin in both localities. Melting of source material similar to that which produced the initial magma but which had undergone a lesser degree of fractionation could generate the high-Ti rhyolites. In the case of the Bishop Tuff, the added magma was rich in CO 2 [14], and this may be important in correlating mixing with change of physical parameters of the magma and perhaps ultimately, the initiation of eruption. Many rhyolitic eruptions exhibit similar enrichment factors between early and late erupted material, which may be caused by processes similar to the one described here. The chemical variations observed in the Cerro Toledo eruptives [35] might also be explained most simply by mixing of rhyolites with variable trace element chemistry. In extreme cases, we note that at Obsidian Dome in Long Valley there are instances of high Ba, Sr and low Ba, Sr rhyolites mixing as they erupt [39,40] and similar syneruptive mixing has been documented at Timber Mountain [41]. Thus, this
107
process may be a widespread phenomenon in magmatic systems. While we used the ion microprobe to discover the wide range in trace elements, the abundances of C1, Ti, and sometimes Ba are often in the range of the electron probe. Thus it should be relatively simple for other researchers to measure zoning in sanidines and variability in melt inclusions and determine if a mixing process has occurred in other rhyolites.
Acknowledgements RLH and NWD thank support from NSF (EAR88-14652 and EAR88-03694, respectively), J. Clark for assistance on the ASU electron probe (obtained with the aid of NSF EAR84-08163), and R. Thomas for keeping the ion probe in excellent operating condition. Special thanks to Peter Michael and two anonymous reviewers for pointing out new data and helping to clarify our discussion.
References 1 J.A. Whitney and J.C. Stormer, Mineralogy, petrology, and magmatic conditions from the Fish Canyon Tuff, central San Juan volcanic field, Colorado, J. Petrol. 26, 726-762, 1985. 2 N.W. Dunbar, P.R. Kyle and C.J.N. Wilson, Evidence for limited zonation in silicic magma systems, Taupo Volcanic Zone, New Zealand, Geology 17, 234-236, 1989. 3 A.T. Anderson, Jr., Magma mixing: petrological process and volcanological tool, J. Volcanol. Geotherm. Res. 1, 1-33, 1976. 4 W. Hildreth, The Bishop Tuff: evidence for the origin of compositional zonation in silicic magma chambers, Geol. Soc. Am. Spec. Pap. 180, 43-75, 1979. 5 W. Hildreth, Gradients in silicic magma chambers: implications for lithospheric magmatism, J. Geophys. Res. 86, 10153-10192, 1981. 6 C.R. Bacon, R. Macdonald, R.L. Smith and P.A. Baedeeker, Pleistocene high-silica rhyolites of the Coso volcanic field, Inyo County, California, J. Geophys. Res. 86, 10223-10241, 1981. 7 H.R. Crecraft, W.P. Nash and S.H. Evans, Jr., Late Cenozoic volcanism at Twin Peaks, Utah: geology and petrology, J. Geophys. Res. 86, 10303-10320, 1981. 8 G.A. Mahood, A summary of the geology and petrology o1 the Sierra La Primavera, Jalisco, Mexico, J. Geophys. Res. 86, 10137-10152, 1981. 9 P.J. Michael, Chemical differentiation of the Bishop Tuff and other high-silica magmas through crystallization processes, Geology 11, 31-34, 1983.
108 10 W. Hildreth, Comment on "Chemical differentiation of the Bishop Tuff and other high-silica magmas through crystallization processes", Geology 11,622-623, 1983. 11 P.J. Michael, Reply to comment on "Chemical differentiation of the Bishop Tuff and other high-silica magmas through crystallization processes", Geology 11, 623-624, 1983. 12 C.E. Lesher and D. Walker, Thermal Diffusion in Petrology (Springer-Verlag Series in Advances in Physical Geochemistry, Diffusion, Atomic Ordering, and Mass Transport), Vol. 8, pp. 396-451, 1991. 13 F. Lu, A.T. Anderson, Jr. and A.M. Davis, Implications of glass inclusions for the origin of high silica rhyolite and compositional zonation of the Bishop Tuff, Eos 71, 651, 1990. 14 F. Lu and A.T. Anderson, Mixing origins of volatile and thermal gradients in Bishop magma, Eos 72, 312. 15 V.I. Kovalenko, R.L. Hervig and M.F. Sheridan, Ion microprobe analyses of trace elements in anorthoclase, hedenbergite, aenigmatite, quartz, apatite, and glass in pantellerite: evidence for high water contents in pantellerite melt, Am. Mineral. 73, 1038-1045, 1988. 16 R.L. Hervig, Ion microprobe analysis of Li and B in silicate glasses, Eos 66, 401, 1985. 17 T.L. Spell, T.M. Harrison and J.A. Wolff, 4°Ar/39Ar dating of the Bandelier Tuff and San Diego Canyon ignimbrites, Jemez Mountains, New Mexico: temporal constraints on magmatic evolution, J. Volcanol. Geotherm. Res. 43, 175-193, 1990. 18 R.L. Smith, Ash-flow magmatism, Geol. Soc. Am. Spec. Pap. 180, 5-27, 1979. 19 D.A. Kuentz, The Otowi member of the Bandelier Tuff: a study of the petrology, petrography, and geochemistry of an explosive silicic eruption, Jemez Mountains, New Mexico, M.S. Thesis, Univ. Texas, Arlington. 20 N.W. Dunbar and R.L. Hervig, Volatile and trace element composition of melt inclusions from the Lower Bandelier Tuff: implications for magma chamber processes and eruptive style, J. Geophys. Res., submitted. 21 N.W. Dunbar and R.L. Hervig, Petrogenesis and volatile stratigraphy of the Bishop Tuff: evidence from melt inclusion analysis, J. Geophys. Res., submitted. 22 W. Hildreth, The magma chamber of the Bishop Tuff: gradients in temperature, pressure and composition, Ph.D. Thesis, Univ. Calif., Berkeley, 1977. 23 A. Halliday, A.E. Fallick, J. Hutchinson and W. Hildreth, A N d , Sr, and O isotopic investigation into the causes of chemical and isotopic zonation in the Bishop Tuff, California, Earth Planet. Sci. Lett. 68, 370-391, 1984. 24 D.K. Bailey, G.B. Dalrymple and M.A. Lanphere, Volcanism, structure, and geochronology of Long Valley caldera, Mono County, California, J. Geophys. Res. 81, 725-744, 1976. 25 J.E. Gardner, H. Sigurdsson and S.N. Carey, Eruption dynamics and magma withdrawal during the plinian phase of the Bishop Tuff eruption, Long Valley Caldera, J. Geophys. Res. 96, 8097-8111, 1991. 26 W.P. Nash and H.R. Crecraft, Partition coefficients for trace elements in silicic magmas. Geochim. Cosmochim. Acta 49, 2309-2322, 1985.
R.L. HERVIG AND N.W. DUNBAR 27 J.D. Webster and J.R. Holloway, Experimental constraints on the partitioning of Cl between topaz rhyolite melt and H 2 0 and H 2 0 4-CO 2 fluids: new implications for granitic differentiation and ore deposition, Geochim. Cosmochim. Acta 52, 2091-2105, 1988. 28 J. Stix and M.P. Gorton, Changes in silicic melt structure between the two Bandelier caldera-forming eruptions, New Mexico, USA: evidence from zirconium and light rare earth elements, J. Petrol. 31, 1261-1283, 1990. 29 G.D. Layne and J, Stix, Volatile and light lithophile element evolution of the Cerro Toledo rhyolite, Jemez Mountains, New Mexico: an ion microprobe study, Eos 71,651, 1990. 30 J.W. Petersen and R.C. Newton, Experimental biotitequartz melting in the KMASH-CO 2 system and the role of CO 2 in the petrogenesis of granites and related rocks, Am. Mineral. 75, 1029-1042, 1990. 31 B.J. Giletti, Rb and Sr diffusion in alkali feldspars, with implications for cooling histories of rocks, Geochim. Cosmochim. Acta 55, 1331-1343, 1991. 32 J.N. Christensen and D.J. DePaolo, Isotopic study of processes in a silicic magma chamber: the Bishop Tuff, Long Valley, California, Eos 68, 434, 1987. 33 A. Halliday, G.A. Mahood, P. Holden, J. Metz, T.J. Dempster and J.P. Davidson, Evidence for long residence times of rhyolitic magma in the Long Valley magmatic system: the isotopic record in precaldera lavas of Glass Mountain, Earth Planet. Sci. Lett. 94, 274-290, 1990. 34 B.N. Turbeville and S. Self, San Diego Canyon ignimbrites: Pre-Bandelier Tuff explosive rhyolitic volcanism in the Jemez Mountains, New Mexico, J. Geophys. Res. 93, 6148-6156, 1988. 35 J. Stix, F. Goff, M.P. Gorton, G. Heiken and S. Garcia, Restoration of compositional zonation in the Bandelier silicic magma chamber between two caldera-forming eruptions: geochemistry and origin of the Cerro Toledo Rhyolite, Jemez Mountains, New Mexico, J. Geophys. Res. 93, 6129-6147, 1988. 36 R.S.J. Sparks, S. Sigurdsson and L. Wilson, Magma mixing: a mechanism for triggering acid explosive eruptions, Nature 267, 315-318, 1977. 37 H.E. Huppert and S.J. Sparks, The generation of granitic magmas by intrusion of basalt into continental crust, J. Petrol. 29, 599-624, 1988. 38 F. Lu, A.M. Davis, C.M. Skirius and A.T. Anderson, Jr., Significance of uranium variations in rhyolitic melt inclusions from the Bishop plinian and early ash flow deposits, GSA Abstr. Progr. 21, A271, 1989. 39 H.W. Stockman, H.R. Westrich and C.D. Miller, Geochemistry of Obsidian Dome and the Inyo Dike, Eos 66, 385, 1985. 40 T.A. Vogel, L.W. Younker and B.C. Schuraytz, Constraints on magma ascent and eruption: geochemical and mineralogical data from drill core samples at Obsidian Dome, Inyo Chain, California, Geology 15, 405-408, 1987. 41 T.P. Flood, B.C. Schuraytz and T.A. Vogel, Magma mixing due to disruption of a layered magma body, J. Volcanol. Geotherm. Res. 36, 241-255, 1989.
97
[CL]
Cause of chemical zoning in the Bishop (California) and Bandelier (New Mexico) magma chambers Richard L. Hervig
a
and Nelia W. D u n b a r b, ,
a Center for Solid State Science, Arizona State University, Tempe, A Z 85287-1704, USA t, Department of Geosciences, New Mexico Institute of Mining and Technology, Socorro, N M 87801, USA Received April 1, 1991; revision accepted March 3, 1992
ABSTRACT Rhyolitic magma chambers often erupt to form deposits with a wide range of trace element chemistry, inferred to reflect zoning in the magma chamber prior to eruption. Ion probe microanalyses of trapped melt inclusions and matrix glass from the large Lower Bandelier Tuff and Bishop Tuff eruptions shows that much of this compositional variation can be blamed on the intrusion of a second rhyolitic magma into the base of the chambers. The second rhyolite was composed of similar major elements but contained significantly higher Ti, Sr, and Ba in both examples. Microanalyses of sanidine phenocrysts show pronounced trace element zoning profiles in accord with the glass chemistry. Applying the available diffusion coefficients for Sr in sanidine to the zoning suggests residence times on the order of 104 yrs after the mixing event. The source of the second magma is not known, but similarities in chemical zoning patterns in silicic magmas throughout the world point toward a common process. Mixing of less fractionated magma derived from similar source rocks is the simplest mechanism. Detailed isotopic studies may help distinguish different sources. Independent of the second magma, large variations in trace elements are observed in the melt inclusions from the Lower Bandelier and Bishop Turfs which can be modeled by ~ 40% fractional crystallization.
1. I n t r o d u c t i o n
That magma chambers are chemically zoned is commonly accepted among igneous petrologists today. In fact, magma chambers with eruptive deposits that do not show significant chemical zoning have become quite noteworthy [e.g., 1,2]. However, the cause of zoning in silicic magma chambers is usually difficult to identify. In some cases, erupted material provides evidence for mixing of distinct chemical end members [3], but in many other cases, large volumes of silicic material are ejected with fairly small major element
* Present address: Environmental Science Division, MS 6038, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831, USA. Correspondence to: R.L. Hervig, Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704, USA.
ranges and quite large trace element variations [4-8]. Several causes for zoning in magma chambers have been put forward by Hildreth [5] who rejected: (1) rising trace element-rich vapor bubbles; (2) partial melting of a progressively depleted/heterogeneous source; and (3) fractionation of crystals as viable mechanisms, suggesting instead that thermogravitational diffusion should be investigated further. Michael [9] concluded that fractional crystallization could explain many chemical features of the Bishop Tuff [but see 10,11]. Later, measurements of thermal diffusion coefficients proved that thermogravitational diffusion was inefficient at generating large chemical gradients in magma chambers [e.g., 12]. The purpose of our paper is to show that the observed chemical zoning of the large Bishop and Lower Bandelier rhyolitic eruptions can be explained entirely by a combination of fractional crystallization and the mixing of a second rhyolite
0012-821X/92/$05.00 © 1992 - Elsevier Science Publishers B.V. All rights reserved
98
magma into the base of the chamber. The second magma has major element chemistry virtually indistinguishable from the initial composition of the chamber, but the trace elements are much different. Our approach was to use the ion probe to obtain microanalyses of trace elements in melt inclusions, phenocrysts, and matrix glass throughout the stratigraphic height of the deposits, in order to observe chemical variations of the magma on a fine scale. This microanalytical approach is similar to that of Lu et al. [13,14] who recently analyzed melt inclusions and matrix glass from the Bishop Tuff with electron and ion microprobes and infrared spectroscopy, and concluded that the compositional zonation of the tuff was largely caused by variable addition of less differentiated, CO2-rich melt. We have confirmed their conclusions and expanded on them with the data from the Lower Bandelier Tuff.
2. Analytical conditions
2.1 Ion microprobe Ion microprobe analyses were made on the Cameca IMS 3f instrument at Arizona State University. We used a 1-2 nA mass-analyzed primary beam of 160- ions focussed to a spot 20-25 ~ m in diameter. Secondary ions were accelerated to + 4.5 keV and the transfer optics and field aperture were set to accept secondary ions into the mass spectrometer from a circular area 20/xm in diameter on the sample. The energy bandpass was set at 40 eV. After the secondary ion signal had stabilized (about 7 min) the sample voltage was ramped +__100V from 4500 while the intensity of 3°Si+ ions was monitored. The sample voltage was returned to the centroid of the intensity vs. sample potential curve to correct for the small amount of charging which occurred. Molecular ion interferences were removed by collecting secondary ion intensities at high energies, which were achieved by offsetting the sample voltage - 7 5 V from the centroid position for H, 7Li, 11B, 19F, 31p, 47Ti' SaFe ' 85Rb' 88Sr' 89y, 9 0 Z r ' 9 3 N b ' 138Ba, 14°Ce, and 232Th. In fact, there are no serious interferences on Li or B so it is not necessary to collect ion signals with an offset, but it was found that reproducibility of the analyses could be substantially improved by collecting all
R.L. H E R V I G A N D N.W. D U N B A R
secondary ion intensities at the same offset voltage. Trace elements were calibrated against NBS 610, a sodium- and silica-rich glass doped with 61 trace elements at the 500 ppm level. Comparison of NBS 610 with bulk-analyzed rhyolite glasses indicated that the trace elements studied were within 10% of their assayed (when available) or nominal concentration, except for phosphorus and titanium, which gave concentrations of 350 and 590 ppm, respectively, when compared with rhyolite glasses. Analyses for F used a combination of NBS 610 and KN-18, a comendite glass with 6400 ppm F. Observed reproducibility of secondary standards suggested that most trace elements are precise to + 10% but that this error increases to ~ 30% for elements near 1-5 ppm abundance given the counting intervals used in this study. Analyses of crystals with major element chemistry much different than the standard glasses are subject to matrix effects, but the use of relatively high-energy ions (> 50 eV initial kinetic energy) keeps such effects to a minimum. Early, very conservative estimates of the matrix effects at these conditions were 100% [15], but recent comparisons of secondary ion intensities of trace elements from natural samples of quartz, rhyolite, leucitite, minette, and synthetic glasses and crystals from simple ternary systems show that absolute abundances given should be well within 30% of the true amount [16 and unpublished data from the ASU SIMS laboratory].
2.2 Electron microprobe Major elements and chlorine were analyzed with a JEOL 8600 electron microprobe at Arizona State University. The error for CI determinations was + 100 ppm based on replicate analyses of reference glasses containing 3000 ppm CI.
3. Results
3.1 Lower Bandelier Tuff The Lower Bandelier Tuff erupted at 1.51 Ma [17] and is composed of a plinian tephra and associated ignimbrite. Bulk analyses of pumice lumps show striking variations which has suggested to many researchers that this was indeed a strongly zoned magma chamber [18,19]. The
C A U S E O F C H E M I C A L Z O N I N G IN T H E BISHOP A N D B A N D E L I E R M A G M A C H A M B E R S
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stratigraphy of the deposit shows weak to no correlation with trace element chemistry leading researchers to suspect that an originally smooth chemical gradient was disrupted during eruption [19]. We analyzed 77 melt inclusions found in quartz, sanidine, and pyroxene phenocrysts and matrix glass from the plinian and ignimbrite with the electron and ion probes and 20 bulk samples by XRF. Trace element covariation plots are shown in Figs. 1-3. The complete tables of analyses will be published in separate, detailed papers on the individual eruptions [20,21]. Bulk and microbeam data are plotted together in Fig. la showing Nb vs. Ti. The agreement between the two analytical techniques indicates that the melt inclusions (regardless of their host
99
phenocryst) represent unaltered magma compared to the bulk. Inclusions and pumice lumps with over ~ 100 ppm Nb show little variation in their Ti contents while Nb-poor inclusions and pumice lumps show a negative correlation with Ti. Rubidium shows a slightly more linear pattern (Fig. lb). Barium, ST, and Zr are correlated with Ti, while Y and Th follow generally linear patterns similar to Rb. Iron, Ce, Li, and P do not show strong correlations with the other trace elements. Boron and C1 are very low in the Ti-rich, Nb-poor inclusions. Bulk analyses of plinian samples show only a small range in Nb (at 185 _+ 10 ppm) and other trace elements [19]. The phenocryst content of most pumice lumps ranges between 15 and 20% (corrected for vesicularity [19l). The concentrations of selected trace elements in melt inclusions from one pumice lump (sample 030) in the ignimbrite collected 5 m from the base of the Cow Section (located at a road cut along route 4 at the base of Otowi Mesa) are plotted separately in Fig. 2 and full analyses are given in Table 1. High-Ti inclusions and one analysis of matrix glass are low in Y, Nb, Th, and Cl and high in Ba. One quartz phenocryst contains separate high- and low-Ti inclusions (connected by a dashed line segment in the figures). We have marked the bulk Y and Th contents (from Table l) in the figures, which are similar to the high-Ti melt inclusions and matrix glass. We have found high-Ti, low-Nb melt inclusions in three out of every four pumice lumps collected from the Cow Section, in one out of two samples from the Guaje (Copar) pumice mine locality stratigraphically above the plinian deposit, but none from three pumice lumps from the Pueblo Mesa locality. No high-Ti, low-Nb inclusions have been found in the plinian samples. Bulk analyses from Kuentz [19] reveal high-Ti, low-Nb compositions in ignimbrite samples from the same sites, as well as Cat Mesa and Wildcat Canyon (not studied by us). We note that all localities showing high-Ti chemistry are from the southern side of the caldera, while Pueblo Mesa is on the north side.
3.2 Bishop Tuff The Bishop Tuff erupted 740,000 years b.p. from the Long Valley Caldera, California, and is
100
R,L. HERVIG AND N.W. DUNBAR
one of the most thoroughly studied rhyolitic deposits in the world [4,9,22-25]. Trace elements show smooth progressions up the stratigraphic sequence from early to late erupted material. The Ti and Ba contents of 175 analyses of melt inclusions in quartz and sanidine phenocrysts from the Bishop Tuff are shown in Fig. 3. Barium varies from 1 to 500 ppm while Ti varies over a > 3-fold range. Most of the low values are from melt inclusions in the plinian and early-erupted ignimbrites (Chidago, Gorges) with higher values concentrated in the last erupted ignimbrites (Tablelands, Adobe Valley, Mono Lobe). These reflect approximately the early/late enrichment factors based on bulk analyses given by Hildreth [5] and are similar to the ion and electron probe
analyses of melt inclusions and matrix glass reported in [13]. The phenocryst content of pumice lumps from the Bishop eruption ranges from 5 to 25% (corrected for vesicularity [22]). Melt inclusion and matrix glass compositions from one pumice lump from the Adobe Valley ignimbrite are shown in Fig. 4 with bulk and microbeam analyses listed in Table 2. This sample was collected near the base of the section and is analogous to sample B-70 of Hildreth [22] which gave an Fe-Ti oxide temperature of 763°C. Comparisons with bulk analyses by us and by Hildreth [22] on matrix glass separates from the same locality are shown in Fig. 4. High-Ti melt inclusions and matrix glass are generally similar in chemistry to the bulk analysis, and are rich in
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Fig. 2. Variation of chemistry in melt inclusions (dots) from sample 030 from a pumice lump in the ignimbrite of the Lower Bandelier Tuff. T h e circle represents matrix glass and the dashed line connects two inclusions in the same quartz phenocryst. (a) Variation of Ti with CI. (b) Variation of Ti with Ba; legend as in (a). (e) Variation of Ti with Y; legend as in (a). (d) Variation of Ti
with Th; legendas in (a).
CAUSE OF CHEMICAL ZONING IN THE BISHOP AND BANDELIER MAGMA CHAMBERS
Ba, Sr, Ce, and Zr and low in Rb, Y, and Th on average compared to low-Ti melt inclusions. No clear difference in Nb, F, and P could be discerned, but hydration of the matrix has occurred, and we note high Li and B in the analyses of Ti-rich matrix glass compared to Ti-rich melt inclusions. Other pumice lumps that showed highand low-Ti melt inclusions include one out of three Tablelands samples (from the base and middle of the section), two out of three Adobe Valley (both from the base of the section), and all three Mono Lobe samples (all near the base of the section). No high-Ti melt inclusions have been identified from the Chidago or Gorges ash flows or the plinian tephra.
101
known to show a smooth variation over a wide range, and Figs. 1 and 3 demonstrate that this is equally true when a microbeam technique is used to characterize the samples. In this contribution, we show that when examined on a smaller scale --within a pumice lump, and sometimes within a single phenocryst--there is extreme local chemical variation indicating that in the case of the Bishop and Lower Bandelier Tufts, magma mixing was an important process in generating zoned magma chambers. 4.1 Processes other than magma mixing. We can reject fractional crystallization as a mechanism for generating all of the chemical variations in Figs. 2 and 4 as the last melt (matrix glass) is rich in Ba. It would be impossible to enrich the Ba-poor Bishop glasses in 095 by fac-
4. Discussion
The distribution of trace element chemistry in the Bishop and Bandelier deposits has long been TABLE 1
Analyses of high and low-Ti melt inclusions from the Bandelier Tuff compared with bulk analyses Element SiO 2 AI20 3 Fe20 3 MgO MnO CaO Na20 K20 CI H20 Li B F P Ti Rb Sr Y Zr Nb Ba Ce Th n
1
2
75.6 + 12.1 + 1.2 + 0.01 + 0.06 + 0.22 + 4.35+ 4.45+ 0.22+ 3.3 + 43 11 300 94 276 264 1.0 67 209 130 4 49 22 6
+ + + + + + + + + + + + +
1.1 0.1 0.1 0.01 0.03 0.05 0.19 0.19 0.02 0.7 18 8 100 16 25 49 0.6 14 42 29 1 7 6
76.0 + 12.1 + 1.4 + 0.03 + 0.04 + 0.24 + 4.24+ 4.51+ 0.12+ 2.7 +
1.2 0.3 0.1 0.01 0.03 0.03 0.29 0.17 0.01 1.1
4 6 + 12 5 + 1 400 + 200 100 + 15 612 -+_ 28 119 + 20 1.5 + 0.6 29 + 3 186 ± 22 52 + 11 8 + 2 65 -I- 8 10 5: 2 8
3
4
5
75.8 12.1 1.3 0.03 0.00 0.24 4.30 4.46 0.13 n.a.
77.2 12.0 1.45 0.03 0.07 0.28 4.11 4.83 n.a. n.a.
77.4 11.7 1.35 0.06 0.05 0.30 3.77 4.74 n.a. n.a.
48 7 150 80 599 99 1.1 31 186 61 6 65 13 1
n.a. n.a. n.a. 35 330 280 5.2 97 213 143 < 10 103 35 1
n.a. n.a. n.a. 52 659 111 5.3 37.6 237 54 18 106 16 1
6
114 5 38 201 52
13 1
1 = Average low-Ti melt inclusion from ignimbrite lump 030. This ~ 11 c m X 7 cm sample was collected from the Cow Section 5 m above the base of the ignimbrite. 2 = Average high-Ti melt inclusion from ignimbrite lump 030. 3 = Microbeam analysis of matrix glass from ignimbrite lump 030. 4 = Bulk analysis of low-Ti pumice lump (sample 17/29) selected on the basis of Ti and Nb contents from Kuentz [19]. 5 = Bulk analysis of high-Ti pumice lump (sample 27/63) selected on the basis of Ti and Nb contents from Kuentz [19]. 6 = Bulk analysis of 030. Errors are 1 standard deviation.
102
R.L. H E R V I G A N D N.W. D U N B A R
1000
L
ee• Q
100
E D.
Q.
v
tO m
+ o
10
[] •
100
PIInlan
Chidego, Gorges Tableland A d o b e V., Mono
1000
10000
Ti (ppm) Fig. 3. Variation of Ti with Ba in melt inclusions from the Bishop Tuff. Inclusions from the early plinian eruption ( ~ 720-725°C), the subsequent Chidago and Gorges ash flows ( ~ 722-738°C) the later Tableland ( ~ 730-760°C), and the Adobe Valley and Mono Lobe ( ~ 750-790°C) ash flows are distinguished in the legend. The bulk analyses of early and late eruptions (E and L, respectively) are shown in the figure. These analyses and the Fe-Ti temperature ranges for the respective samples are taken from Hildreth [5].
tors of ~ 15 with sanidine in the precipitating assemblage (Fig. 4a) because measured partition coefficients for Ba between rhyolitic liquids and sanidine are quite large, e.g., 5-24 [26], giving bulk partition coefficients greater than unity for the eutectic crystallizing assemblages involved in these localities. Fractional crystallization is an important process in the evolution of the magmas, which will be discussed below. The extreme depletion of CI in the Lower Bandelier matrix glass and high-Ti melt inclusions is also difficult to explain given the absence of fractionating CIbearing phases. Although C1 would partition into a hydrous vapor, the low water content of these inclusions ( ~ 3 wt.% H20; Table 1) precludes saturation of the magma with water vapor. Strong CI depletions would not be expected if mixed H 2 0 - C O 2 fluids were present because partition coefficients for CI between such fluids and melt are not significantly greater than unity [27].
4.2 Magma mixing The hypothesis that mixing of a separate rhyolitic magma is responsible for the observed chemistry is testable. For example, if mixing oc-
curred, then any phenocrysts growing at this time should have responded to t h e change in the chemistry of the magma. Major element chemistry of inclusion and matrix glass (electron probe) and glass separates (bulk techniques) are not distinguishable (Tables 1 and 2; [21]) and Hildreth [22] found that sanidines in the Bishop Tuff were unzoned. However, sanidine crystals would be expected to show edgeward increases in trace elements that are enriched in the mixed magma. We have tested this by measuring core-to-rim variations of Li, P, Ti, Na (or Fe), Rb, Sr, Ba, and Ce in sanidines from several pumice lumps in the Bishop Tuff and Lower Bandelier Tuff. Some sanidines from Lower Bandelier pumice lumps in which high-Ti melt inclusions were found show trace element enrichments at their edge (Fig. 5). The thickness of these enriched rims ranges from 100 to 400 /zm. In the late-erupted ignimbrites from the Bishop Tuff (Tableland, Adobe Valley, Mono Lobe), all sanidines we have studied show pronounced edgeward enrichments in Ti, Sr, and Ba, in accord with the melt chemistry shown in the earlier figures (Fig. 6). Thicknesses of these rims range from 250 to 600/zm. For all samples, the absolute thicknesses are subject to cutting effects, so that the minimum measured thickness is probably most reliable. In contrast, we have not found sanidine with trace element-rich rims from early erupted plinian and ignimbrite (Gorges, Chidago lobes) pumice lumps. The enrichment is quite variable from one sanidine crystal to another, even within the same pumice lump. Perhaps this can be related to incomplete mixing of the second magma within the chamber, or during syneruptive formation of pumice lumps. One very important line of evidence supporting magma mixing is represented by the quartz phenocryst in sample 030 which contains a high-Ti and a low-Ti melt inclusion. We have found quartz phenocrysts from two other pumice lumps from the Lower Bandelier Tuff with the two different magmas included. Trace element scans in these quartz crystals were not attempted because of the extremely low concentrations of the elements of interest. We have not found phenocrysts from the Bishop Tuff that contain both magma types, but the contrasting chemistry between matrix glass and low-Ti inclusions in quartz supports the argument that the chemistry of the magma surround-
CAUSE OF CHEMICAL ZONING IN THE BISHOP AND BANDELIER MAGMA CHAMBERS
ing these crystals changed dramatically during its evolution. There is another way to test the mixing hypothesis. If a magma with distinct trace element characteristics invaded the chamber at depth, then a subsequent eruption might show the high-Ti, Ba-Sr signature of the second magma In fact, several small eruptions occurred between the Lower and Upper Bandelier caldera-forming events. Analyses by Stix and Gorton [28] and Layne and Stix [29] of the first-erupted rhyolite after the Lower Bandelier Tuff (their sample 15-8) show a Ti-, Zr-, Ba-, Sr-rich and CI-, B-poor chemistry, quite similar to the chemistry of the high-Ti magma reflected in Fig. 2. While complete analyses are not available, post-Bishop rhyolites (K-Ar dates ranging from 730,000 to 630,000
103
years b.p.) are Ba-rich compared to the Bishop Tuff compositions and textural features suggest high magmatic temperatures [R.A. Bailey, U.S. Geological Survey, pers. commun.]. Because only the late-erupted sanidines from the Bishop Tuff show the trace element-rich edges and because the subsequent eruption to the Lower Bandelier Tuff shows similar chemistry to the high-Ti-Ba-Sr magma, we conclude that in both cases the new rhyolite mixed into the base of the chamber. (By base, we mean the deepest part of the chamber that was sampled.)
4.3 Temperature of the second magma In the Bishop Tuff, Lu and Anderson have measured the chemistry of Fe-Ti oxides included
100
20 a
~
Bulk
80 E 4",
15
o M.t,lxo,.s.--I Melt Inclusion I
6O
Bulk A
E Q. r,
Q. 40
10
>-
20 0
I
0
200
400
600
I
800
200 .o
Ti (ppm)
I
I
400
600
Ti (ppm) 80
20
60
Q.
T.T,~ -Bul---~k
E
A
E I~,
800
÷
12. 40 Q. Q
10
o
20
0
I
20O
400
Ti (ppm)
600
800
0
I
I
I
200
400
600
800
Ti (ppm)
Fig. 4. Variation of chemistry in melt inclusions (dots) from sample 095 from a pumice lump in the Adobe Valley ash flow of the Bishop Tuff. The circles represent analyses of matrix glass. Bulk analyses from [21] and from Hildreth [22] on similar samples are shown when available. (a) Variation of Ti with Ba. (b) Variation of Ti with Sr; legend as in (a). (c) Variation of Ti with Y; legend as in (a). (d) Variation of Ti with Ce; legend as in (a).
104
R.L. HERVIG AND N.W. DUNBAR
in quartz and in the matrix of the pumice, finding indications that the magma temperature increased subsequent to entrapment [14]. They concluded that the injection of a hot, less evolved rhyolite magma could explain this. Without suitable phenocryst assemblages in the Lower Bandelier Tuff it is not possible to discuss the temperature in a quantitative sense. The addition of sufficiently hot material would presumably rise as a plume in either chamber. This would not lead to a simple relationship between depth in the chamber and chemistry. Below some critical temperature, influx of magma would be less disruptive, preserving pre-existing chemical gradients (if present).
4.4 Volatile content
We do not emphasize volatiles in this work (see Dunbar and Hervig, [20,21] for volatile data). This is because we can find no relationship between water contents and the mixed component (the mean water contents of the high- and low-Ti inclusions in Tables 1 and 2 are indistinguishable) and because we cannot at this time use the ion microprobe to measure the carbon contents of melt inclusions (high instrumental backgrounds at no better than 600 ppm CO 2 at time of writing). However, Lu and Anderson [14] found high Ba and CO 2 contents in some melt inclusions from the last erupted ignimbrites from the Bishop Tuff,
TABLE 2 Analyses of high and low Ti-melt inclusions and matrix glass from the Bishop Tuff compared with bulk data
Element SiP 2 AI20 3 FeO MgO CaP Na20 K20 CI H2 O
1
2 76.5 + 1.1 12.4 5 : 0 . 3 0.66+ 0.07 0.035:0.01 0.47 5 : 0 . 0 5 3.70+ 0.30 4.71+ 0.36 0.07 + 0.02 2.3 5 : 0 . 8
Sum
100.83
Li B F P Ti Rb Sr Y Zr Nb Ba Ce Th n
39 43 180 240 376 136 2 12 67 15 4 38 14 5
75.9 5: 12.3 5: 0.76+ 0.04+ 0.47 + 3.58+ 4.69+ 0.05 + 2.1 5:
0.5 0.1 0.11 0.01 0.02 0.12 0.20 0.01 0.5
3
4
5
75.4 n.a. 1.1 0.16 0.7 3.2 5.1 n.a. n.a.
n.a. 13.1 0.5 n.a. n.a. 3.6 5.9 n.a. n.a.
6
99.79 + 3 5:1 5:40 +30 5:83 +_ 12 5:1 + 3 5:8 __. 5 5:1 5:4 5:1
39 34 300 270 622 123 13 9 81 9 76 57 8 3
5: 5 5: 1 5:170 + 40 5:36 + 25 + 3 5: 1 5: 3 5: 6 5:15 + 5 5: 4
155
84 56 17 2
53+ 8 385: 6 340 5:130 3305:20 583 + 47 109 + 20 12+ 2 95: 2 765: 8 11+ 2 70 5: 6 49 5: 5 105:3 5
143 40 13 112 12
15 1
1 = Average chemistry of low-Ti, Ba melt inclusions from Adobe Valley pumice lump 095 collected near the base of the ash flow. This represents the same locality as Hildreth's sample B-70 [22]. It is taken from the lowest exposure of the basal non-welded zone of this lobe. Of the eight inclusions, six were analyzed for water and trace elements by ion probe (one inclusion has only a partial trace element analysis), while all were analyzed by electron probe. 2 = M e a n of three analyses of high-Ti, Ba inclusions from 095 (all analyzed for water and trace elements by ion probe and for major elements by electron probe). 3 = Average bulk analysis for Adobe Valley pumice from Hildreth [22]. 4 = Average matrix glass separate analysis for Adobe Valley pumice from Hildreth [22]. 5 = Average of five ion probe analyses of matrix glass in 095. 6 = Bulk analysis of 095 from this work. Errors are 1 standard deviation.
CAUSE OF CHEMICAL ZONING IN THE BISHOP AND BANDELIER MAGMA CHAMBERS
105
30 a
--.-o,i
Li Sr
RbI
Sr
3O0
2O
200
100
8
o 0
0 0
200
400
I
0
800
600
80110
100
E Q.
I
80
'. . . .
I- S -
60
0 P 0
I
800
1000
•
,
.
,
.
,
.
,O 4000
o t--
I
600
A E s~ ¢,j ¢,j ¢-
Q.
m
400
Position (~m)
Position (l~m)
A
I
200
40
0 2000 ¢,, 0
Q
20
o
0
,
0
I
200
i
I
i
400
I
600
200
i
800
Position (~m)
E D.
600
800
10o0
Position (gm) Fig. 6. Step-scan across a sanidine crystal in sample 091, a pumice lump from the Mono Lobe ash flow within the Bishop Tuff. Position axis indicates distance from the edge of the crystal (step size = 25 /zm). (a) Variations in Rb and Sr. (b) Variations in Ba. This pumice lump was collected near the base of the Mono Lobe; the bulk analysis matches with samples from [22] giving Fe-Ti temperatures of 790°C.
10oo
A
400
800
Q. c-
600
.o 400 r, o O O
200
0 0
200
400
600
800
Position (l~m) Fig. 5. Step-scan across a sanidine crystal in sample 027, a pumice lump from the Lower Bandelier ash flow. Position axis indicates distance from the edge of the crystal (step size = 20 /zm). (a) Variations in Li, Sr, and Ce. (h) Variations in Ti and Rb. (c) Variations in P and Ba. This sample was collected from the Cow Section 3 m stratigraphically above sample 030.
concluding that a high-CO 2 rhyolite had been mixed into the magma chamber. Knowing the CO 2 content is important because: (1) recent experimental work [30] shows that H 2 0 - C O 2 fluid mixtures can lower significantly the solidus of granitic crystalline assemblages, and (2) a high CO 2 content could indicate the presence of a free fluid phase whose influence on the bulk physical properties of the magma could effect the eruption dynamics.
106
4.5 Timing The question of timing of the introduction of the mixed component is interesting, but our discussion is admittedly speculative. Did this mixing of new magma occur suddenly upon removal of shallow-level magma in the early plinian/ignimbrite eruptive events? Or was it a gradual infiltration of material into the base of the chamber? The nearly bimodal distribution of certain elements on the scale of a hand sample (or even phenocryst) argues for a fairly rapid process. However, the mixing was not so rapid as to preclude the growth of several hundred microns on the sanidine crystals before eruption. Giletti [31] measured the diffusion of Sr in orthoclase at 800°C and 1 kbar water pressure. Experiments run shorter than about one day were neglected, because they may be affected by surface reactions with the fluid [31]. The remaining eight measured values range from 2.5 × 10-16 to 1.9 × 10-15 cm2/s. At this pressure and temperature, Sr gradients in sanidines would begin to smooth out over length scales of ~ 100 /zm in 2-13 k.y. There are no measurements of the Ba diffusion coefficient in sanidine, but given the larger ionic radius, we can expect it to be slower than Sr, eventually yielding significantly different profiles for the two elements. The profiles in Figs. 5 and 6 argue that after mixing, the sanidines resided in the magma chamber for a period on the order of ~ 104 years before reaching closure temperature (eruption). This time scale is in good agreement with the calculations by Lu and Anderson [14] based on trapped Fe-Ti oxide inclusions. Note also that there are two distinct changes in the chemistry of the sanidine in Fig. 5: one at 425/~m and one at 175/zm from the edge. Not only did the chemistry of the magma change dramatically during the growth of the crystal in the Lower Bandelier chamber, changes happened repeatedly.
R . L H E R V I G AND N.W. DUNBAR
of the Bishop and Lower Bandelier eruptions. However, the question of where this magma came from is more difficult to address. The simplest model would explain the high-Ti mixed component as a melt which derived from sources similar to those which produced the bulk of the Bishop and Lower Bandelier magmas, but which is the product of much less fractionation. The isotopic signature of this magma might be quite similar to the initial magmas, as is suggested by the available isotopic data for the Bishop [23,32]. However, the strong zoning of Sr in sanidines developing during the mixing event suggests that a detailed isotopic study of cores and rims of sanidines would be useful in learning more about the origin of the high-Ti magma, and may influence earlier conclusions concerning residence times of rhyolitic magmas [33]. There are no detailed Sr isotopic data on the Lower Bandelier Tuff, but we note that the preBandelier San Diego Canyon rhyolites have many of the trace element characteristics (low Nb, high Ti, Ba, and Sr) of the mixed component [17,34,35]. We take this to indicate that sources for melting, mobilization, and intrusion of high-Ti rhyolite into the Bandelier chamber exist in the region. Regardless of the source of the material, a common thread in the Lower Bandelier and Bishop Tufts is the addition of hotter magma after a period of evolution within the chamber. That heat is being periodically supplied to the two regions is obvious, considering the duration of rhyolitic activity. However, the input of heat into the magmatic systems might be expected to be linked to eruption if the models of Sparks et al. [36] and Huppert and Sparks [37] can be applied. The apparent significant residence time after mixing (of the order of 104 years) suggests that the temperature differences between the two magmas were not sufficient to cause directly the eruption.
4. 7 Importance of fractional crystallization 4.6 Origin of the second magma Given our data set, we believe it is easy to become convinced of the importance of magma mixing in generating the trace element chemistry
This paper focusses on the role of magma mixing, but it would be wrong to suggest that fractional crystallization is not an important process in defining the character of magma chambers. For the Lower Bandelier Tuff, if melt inclu-
CAUSE OF CHEMICALZONING IN THE BISHOP AND BANDELIERMAGMACHAMBERS
sions and bulk analyses which show the mixed component are removed (e.g., compositions in Fig. lb with < 120 ppm Nb), trace element variations can be described by fractional crystallization models assuming ~ 40% crystallization of quartz and alkali feldspar (and trace amounts of fractionating chevkinite [20]). Although the Lower Bandelier Tuff contains only ~ 20% crystals, significant side-wall crystallization cannot be ruled out, and may even be required to explain the observed gradient in water content within the deposit [20]. If the high-Ti, -Ba, -Sr component inclusions are subtracted from the Bishop data, variations in incompatible elements such as B, Nb, and Th, can be fit by a fractional crystallization model, and again, about 40% crystallization is required to explain the observed range in chemistry [21], in good agreement with Lu et al.
[38]. 5. Conclusions Late addition of rhyolite magmas with similar major elements but distinct trace elements changed the trace element chemistry of the lower region of the magma chambers that erupted to produce the Bishop and Lower Bandelier Tufts. The similarity between the trace element characteristics of the two added components suggests a common origin in both localities. Melting of source material similar to that which produced the initial magma but which had undergone a lesser degree of fractionation could generate the high-Ti rhyolites. In the case of the Bishop Tuff, the added magma was rich in CO 2 [14], and this may be important in correlating mixing with change of physical parameters of the magma and perhaps ultimately, the initiation of eruption. Many rhyolitic eruptions exhibit similar enrichment factors between early and late erupted material, which may be caused by processes similar to the one described here. The chemical variations observed in the Cerro Toledo eruptives [35] might also be explained most simply by mixing of rhyolites with variable trace element chemistry. In extreme cases, we note that at Obsidian Dome in Long Valley there are instances of high Ba, Sr and low Ba, Sr rhyolites mixing as they erupt [39,40] and similar syneruptive mixing has been documented at Timber Mountain [41]. Thus, this
107
process may be a widespread phenomenon in magmatic systems. While we used the ion microprobe to discover the wide range in trace elements, the abundances of C1, Ti, and sometimes Ba are often in the range of the electron probe. Thus it should be relatively simple for other researchers to measure zoning in sanidines and variability in melt inclusions and determine if a mixing process has occurred in other rhyolites.
Acknowledgements RLH and NWD thank support from NSF (EAR88-14652 and EAR88-03694, respectively), J. Clark for assistance on the ASU electron probe (obtained with the aid of NSF EAR84-08163), and R. Thomas for keeping the ion probe in excellent operating condition. Special thanks to Peter Michael and two anonymous reviewers for pointing out new data and helping to clarify our discussion.
References 1 J.A. Whitney and J.C. Stormer, Mineralogy, petrology, and magmatic conditions from the Fish Canyon Tuff, central San Juan volcanic field, Colorado, J. Petrol. 26, 726-762, 1985. 2 N.W. Dunbar, P.R. Kyle and C.J.N. Wilson, Evidence for limited zonation in silicic magma systems, Taupo Volcanic Zone, New Zealand, Geology 17, 234-236, 1989. 3 A.T. Anderson, Jr., Magma mixing: petrological process and volcanological tool, J. Volcanol. Geotherm. Res. 1, 1-33, 1976. 4 W. Hildreth, The Bishop Tuff: evidence for the origin of compositional zonation in silicic magma chambers, Geol. Soc. Am. Spec. Pap. 180, 43-75, 1979. 5 W. Hildreth, Gradients in silicic magma chambers: implications for lithospheric magmatism, J. Geophys. Res. 86, 10153-10192, 1981. 6 C.R. Bacon, R. Macdonald, R.L. Smith and P.A. Baedeeker, Pleistocene high-silica rhyolites of the Coso volcanic field, Inyo County, California, J. Geophys. Res. 86, 10223-10241, 1981. 7 H.R. Crecraft, W.P. Nash and S.H. Evans, Jr., Late Cenozoic volcanism at Twin Peaks, Utah: geology and petrology, J. Geophys. Res. 86, 10303-10320, 1981. 8 G.A. Mahood, A summary of the geology and petrology o1 the Sierra La Primavera, Jalisco, Mexico, J. Geophys. Res. 86, 10137-10152, 1981. 9 P.J. Michael, Chemical differentiation of the Bishop Tuff and other high-silica magmas through crystallization processes, Geology 11, 31-34, 1983.
108 10 W. Hildreth, Comment on "Chemical differentiation of the Bishop Tuff and other high-silica magmas through crystallization processes", Geology 11,622-623, 1983. 11 P.J. Michael, Reply to comment on "Chemical differentiation of the Bishop Tuff and other high-silica magmas through crystallization processes", Geology 11, 623-624, 1983. 12 C.E. Lesher and D. Walker, Thermal Diffusion in Petrology (Springer-Verlag Series in Advances in Physical Geochemistry, Diffusion, Atomic Ordering, and Mass Transport), Vol. 8, pp. 396-451, 1991. 13 F. Lu, A.T. Anderson, Jr. and A.M. Davis, Implications of glass inclusions for the origin of high silica rhyolite and compositional zonation of the Bishop Tuff, Eos 71, 651, 1990. 14 F. Lu and A.T. Anderson, Mixing origins of volatile and thermal gradients in Bishop magma, Eos 72, 312. 15 V.I. Kovalenko, R.L. Hervig and M.F. Sheridan, Ion microprobe analyses of trace elements in anorthoclase, hedenbergite, aenigmatite, quartz, apatite, and glass in pantellerite: evidence for high water contents in pantellerite melt, Am. Mineral. 73, 1038-1045, 1988. 16 R.L. Hervig, Ion microprobe analysis of Li and B in silicate glasses, Eos 66, 401, 1985. 17 T.L. Spell, T.M. Harrison and J.A. Wolff, 4°Ar/39Ar dating of the Bandelier Tuff and San Diego Canyon ignimbrites, Jemez Mountains, New Mexico: temporal constraints on magmatic evolution, J. Volcanol. Geotherm. Res. 43, 175-193, 1990. 18 R.L. Smith, Ash-flow magmatism, Geol. Soc. Am. Spec. Pap. 180, 5-27, 1979. 19 D.A. Kuentz, The Otowi member of the Bandelier Tuff: a study of the petrology, petrography, and geochemistry of an explosive silicic eruption, Jemez Mountains, New Mexico, M.S. Thesis, Univ. Texas, Arlington. 20 N.W. Dunbar and R.L. Hervig, Volatile and trace element composition of melt inclusions from the Lower Bandelier Tuff: implications for magma chamber processes and eruptive style, J. Geophys. Res., submitted. 21 N.W. Dunbar and R.L. Hervig, Petrogenesis and volatile stratigraphy of the Bishop Tuff: evidence from melt inclusion analysis, J. Geophys. Res., submitted. 22 W. Hildreth, The magma chamber of the Bishop Tuff: gradients in temperature, pressure and composition, Ph.D. Thesis, Univ. Calif., Berkeley, 1977. 23 A. Halliday, A.E. Fallick, J. Hutchinson and W. Hildreth, A N d , Sr, and O isotopic investigation into the causes of chemical and isotopic zonation in the Bishop Tuff, California, Earth Planet. Sci. Lett. 68, 370-391, 1984. 24 D.K. Bailey, G.B. Dalrymple and M.A. Lanphere, Volcanism, structure, and geochronology of Long Valley caldera, Mono County, California, J. Geophys. Res. 81, 725-744, 1976. 25 J.E. Gardner, H. Sigurdsson and S.N. Carey, Eruption dynamics and magma withdrawal during the plinian phase of the Bishop Tuff eruption, Long Valley Caldera, J. Geophys. Res. 96, 8097-8111, 1991. 26 W.P. Nash and H.R. Crecraft, Partition coefficients for trace elements in silicic magmas. Geochim. Cosmochim. Acta 49, 2309-2322, 1985.
R.L. HERVIG AND N.W. DUNBAR 27 J.D. Webster and J.R. Holloway, Experimental constraints on the partitioning of Cl between topaz rhyolite melt and H 2 0 and H 2 0 4-CO 2 fluids: new implications for granitic differentiation and ore deposition, Geochim. Cosmochim. Acta 52, 2091-2105, 1988. 28 J. Stix and M.P. Gorton, Changes in silicic melt structure between the two Bandelier caldera-forming eruptions, New Mexico, USA: evidence from zirconium and light rare earth elements, J. Petrol. 31, 1261-1283, 1990. 29 G.D. Layne and J, Stix, Volatile and light lithophile element evolution of the Cerro Toledo rhyolite, Jemez Mountains, New Mexico: an ion microprobe study, Eos 71,651, 1990. 30 J.W. Petersen and R.C. Newton, Experimental biotitequartz melting in the KMASH-CO 2 system and the role of CO 2 in the petrogenesis of granites and related rocks, Am. Mineral. 75, 1029-1042, 1990. 31 B.J. Giletti, Rb and Sr diffusion in alkali feldspars, with implications for cooling histories of rocks, Geochim. Cosmochim. Acta 55, 1331-1343, 1991. 32 J.N. Christensen and D.J. DePaolo, Isotopic study of processes in a silicic magma chamber: the Bishop Tuff, Long Valley, California, Eos 68, 434, 1987. 33 A. Halliday, G.A. Mahood, P. Holden, J. Metz, T.J. Dempster and J.P. Davidson, Evidence for long residence times of rhyolitic magma in the Long Valley magmatic system: the isotopic record in precaldera lavas of Glass Mountain, Earth Planet. Sci. Lett. 94, 274-290, 1990. 34 B.N. Turbeville and S. Self, San Diego Canyon ignimbrites: Pre-Bandelier Tuff explosive rhyolitic volcanism in the Jemez Mountains, New Mexico, J. Geophys. Res. 93, 6148-6156, 1988. 35 J. Stix, F. Goff, M.P. Gorton, G. Heiken and S. Garcia, Restoration of compositional zonation in the Bandelier silicic magma chamber between two caldera-forming eruptions: geochemistry and origin of the Cerro Toledo Rhyolite, Jemez Mountains, New Mexico, J. Geophys. Res. 93, 6129-6147, 1988. 36 R.S.J. Sparks, S. Sigurdsson and L. Wilson, Magma mixing: a mechanism for triggering acid explosive eruptions, Nature 267, 315-318, 1977. 37 H.E. Huppert and S.J. Sparks, The generation of granitic magmas by intrusion of basalt into continental crust, J. Petrol. 29, 599-624, 1988. 38 F. Lu, A.M. Davis, C.M. Skirius and A.T. Anderson, Jr., Significance of uranium variations in rhyolitic melt inclusions from the Bishop plinian and early ash flow deposits, GSA Abstr. Progr. 21, A271, 1989. 39 H.W. Stockman, H.R. Westrich and C.D. Miller, Geochemistry of Obsidian Dome and the Inyo Dike, Eos 66, 385, 1985. 40 T.A. Vogel, L.W. Younker and B.C. Schuraytz, Constraints on magma ascent and eruption: geochemical and mineralogical data from drill core samples at Obsidian Dome, Inyo Chain, California, Geology 15, 405-408, 1987. 41 T.P. Flood, B.C. Schuraytz and T.A. Vogel, Magma mixing due to disruption of a layered magma body, J. Volcanol. Geotherm. Res. 36, 241-255, 1989.