238U-230Th-226Ra disequilibrium in young Mt. Shasta andesites and dacites

Journal of Volcanology and Geothermal Research, 53 (1992) 227-238 Elsevier Science Publishers B.V., Amsterdam

227

238U-23°Th-226Ra disequilibrium in young Mt. Shasta andesites and dacites

Alan M. Volpe L-232, Lawrence Livermore National Laboratory, Liverrnore, CA 94550, USA

(Received September 23, 1991; revised version accepted February 14, 1992 )

ABSTRACT

Volpe, A.M., 1992. 23aU-23°Th-226Ra disequilibrium in young Mt. Shasta andesites and dacites. J. Volcanol. Geotherm. Res., 53: 227-238.

The paper describes 23SU-series nuclides and 23°Th/232Th ratios measured by mass spectrometry in mineral separates of young Mr. Shasta andesites and dacites. The results constrain the timing of recent calc-alkaline magma fractionation at this volcano. Hotlum, Misery Hill and Black Butte rocks show small, < 13% 23°Th-238U and < 6% 226Ra-23°Th, disequilibria. Plagioclase have 7-26% 226Ra excesses, magnetite and groundmass have 4-5% 226Ra deficits, and pyroxenes have equilibrium (226Ra/23°Th) activity ratios.

Internal ( 23°Th)-(23sU) and Ba-normalized ( 226Ra)-(23°Th) isotope diagrams for Hotlum and Black Butte dacites suggest that closed-system Th-U and Ra-Th fractionation occurred less than 10,000 years ago. Significant 226Ra-2a°Th disequilibria in the Black Butte dacite strongly suggests that this rock erupted more recently than 9400 years ago. Results for Hotlum andesites suggest a longer pre-eruption crystal residence time compared to the dacites. There may also have been recent open Ra-Th system changes in the melt composition. Initial Th/U ratios for the rocks are low (2.43-2.57), similar to those in mid-ocean ridge basalts (MORB), and preclude significant assimilation of crust with markedly different Th-U composition.

Introduction

Disequilibrium between nuclides in the 238U decay series provides an important constraint for understanding processes and time scales of magma genesis. However, the duration and complexity of magmatism may limit the extent to which closed-system dating with short-lived nuclides like 23°Th (t=75,380 a) and 226Ra

(t= 1600 a) is possible. Criteria for absolute age dating of young rocks, especially initial isotopic homogeneity may not always be satisfied (Capaldi et al., 1982, 1985; Hemond and Con- domines, 1985; Pyle et al., 1988). Also, many studies describe petrographic and chemical

Correspondence to: A.M. Volpe, L-232, Lawrence Liver- more National Laboratory, Livermore, CA 94550, USA.

evidence for open-system magma-chamber processes during calc-alkaline magma genesis. These include magma mixing, assimilation, and recycling of crystals in replenished magma chambers (Pearce et al., 1987; Nixon and Pearce, 1987; Bacon and Druitt, 1988; Pyle et al., 1988; Stamatelopoulou-Seymour et al., 1990; Blundy and Shimizu, 1991 ).

Newman ( 1983 ) and Newman et al. (1986) concluded from petrologic, geochemical and 23°Th-238U disequilibrium data that mixing of magmas and assimilation of crust dominated magma genesis at Mt. Shasta. Yet, it was not possible with mineral 23°Th-238U data, ob- tained by alpha spectrometry, in these previous studies to quantify time scales for magma fractionation. This study describes 23SU-series disequilibrium and 23°Th/232Th ratios mea-

0377-0273/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

228 A.M. VOLPE

sured by mass spectrometry for young Mt. Shasta andesites and dacites. The purpose of measuring low nuclide concentrations in these rocks and minerals, particularly 226Ra at sub- picogram levels, is to gain insight into time scales and processes of recent calc-alkaline magma genesis at Mt. Shasta.

Geologic setting

Mount Shasta is in northern California near the southern end of the High Cascade Range. Several studies described the geology and geologic activity (Williams, 1932; Christiansen et al., 1977; Miller, 1980; CrandeU et al., 1984; Christiansen, 1985 ). Mount Shasta is a com- posite volcano comprised of four major over- lapping cones built on the remnants of an older volcano. The four chronologic units corresponding to episodes during which each cone formed are: Sargents Ridge (<250 ka); Mis- ery Hill ( < 130 ka); Shastina ( < 9-10 ka); and Hotlum (<3-4 .5 ka) (Christiansen et al., 1977).

The Hotlum cone forms the present summit of Mount Shasta. Shastina cone is on the west- ern flank, and Black Butte is a flank vent 12 km west of Shastina beyond the map boundary (Fig. 1 ). Eruptions at Mt. Shasta occurred about once every 800 years over the last 10,000 years (Miller, 1980). The most recent eruption was about 200 years ago at the Hotlum summit vent (Christiansen et al., 1977; Miller, 1980).

Previous studies described the petrology and chemistry of rocks erupted at Mount Shasta (Peterman et al., 1970; Church and Tilton, 1973; Condie and Swenson, 1973; Anderson, 1974; Church, 1976; Christiansen et al., 1977; Newman, 1983; Newman et al., 1986). Most of the rocks forming the cones are two-pyroxene andesites that erupted as lava flows from the central vents. Basalts and basaltic andesites erupted early in the evolution of the cones and are at lower elevations. Dacite domes and pyroclastic flows erupted from the summits

and flanks toward the end of each cone-build- ing cycle. It is the consensus of the previous studies that the chemistry of the basalts and andesites at Mount Shasta reflect a mantle source. While there is little indication that sub- ducted ocean crust was a major component, assimilation of crust during magma ascent was an important factor.

The samples, used in this study, are (Fig. 1 ): dacite from the Hotlum summit (80-58); andesites from the Military Pass (80-66) and Fall Springs Creek (80-63) lava flows that origi- nated from the Hotlum summit vent; hornblende dacite from Black Butte west of Shas- tina (78-10), and Misery Hill andesite (80- 57). The samples are from the collection of J.D. Macdougall at Scripps Institution of Oceanography. Newman (1983) and New- man et al., (1986) described the petrography, geochemistry and 238U-E3°Th disequilibrium of these samples.

Analytical methods

Mineral separates were prepared using stan- dard magnetic and density procedures. Ap- proximately 0.5-1.5 gr of the minerals and rock powders were dissolved using an HF/HNO3 acid mixture. During evaporation the samples were treated with HNO3 several times, dried and then dissolved in 4 M HC1. The sample solutions were centrifuged to ensure complete dissolution. Any visible residue was treated with small amounts of HC104, fumed and then dissolved in 4 M HC1.

Details of the chemical separation and mass spectrometry procedures for Ra-Ba (Volpe and Hammond, 1991; Volpe et al., 1991 ) and Th-U (Goldstein et al., 1989) measurements are described elsewhere. Briefly, Ra and Ba were separated using cation exchange resin (H + form), and HC1 as the eluant. Ra was purified using a pressurized capillary column, cation exchange resin ( N H ~ form), and diammonium EDTA as the eluant. Th and U

23sU-23°Th-226Ra DISEQUILIBRIUM IN YOUNG MT. SHASTA ANDESITES AND DACITES 229

$80-66

S78-10 i i

Black Butte

• . .

• • . . . . . • • . . ' " " ' - .

N

S80-63

[ ] Hotlum dacite 77;I Hotlum andesite [ ] Shastina dacite [ ] Misery Hill andesite

0 1 2 miles !

Fig. 1. Geologic map of Mt. Shasta showing sample location. (After Christiansen et al., 1977. )

were separated using anion exchange resin (NO~- form), and HNO3, H20 and HBr as eluant.

An enriched 228Ra spike was prepared for the determination of 226Ra concentrations by isotope dilution. The spike concentration was cal- ibrated with NIST SRM 4953D 226Ra solu- tion. The calibration measurements agree to 0.4%. Total procedural blanks for 226Ra and 228Ra a re below the mass spectrometer detec- tion level for Ra, which is currently < 0.1 fg

(3 X 105 atoms). Sample solutions were also spiked with 229Th, 238U-236U, and 132Ba. Pro- cedural blanks were less than 100 pg for Ba, 10-15 pg for U, and 25-40 pg for Th.

Uncertainties of concentration measurements at the 95% confidence level are less than 0.5% for Ba, Th and U, and better than 1% for 226Ra in whole-rock and groundmass values, and better than 1.5% for 226Ra in minerals. Uncertainty for 23°Th/E32Th isotopic ratios are 0.5-1.0%, and better than 1.0% for 234U/23aU

230 A.M. VOLPE

isotopic ratios. Finally, activities (dpm/g) were calculated using measured abundances and atomic ratios and the following decay con- stants: ,~(238U) = 1.5513X 10 -1° a - l ; ,~(234U) = 2 . 8 2 9 2 × 10 -6 a - l ; ) ,(232Th ) ___

4.9475)< 10 -11 a - l ; 2(23°Th) = 9.1952)< 10 -6

a- l ; and 2(226Ra) = 4.332)< 10-4a -1.

Resnlts

Table 1 lists concentration data for Mt. Shasta rocks and minerals. Silica concentrations of these andesites and dacites show mi- nor variation from 61.5 to 64.4% (Newman, 1983 ). Whole-rock Ba, Th and U abundances are positively correlated. But, increasing Ba

TABLE1

(226-395 ~ug/g) and Th (1.72-4.24 /tg/g) contents are not correlated with SiO2, MgO, or K20 (see Newman, 1983). The Black Butte dacite has low incompatible trace-element concentrations compared with the andesites. Newman (1983) and Newman et al. (1986) explained the trace-element composition in these evolved rocks by variable degree partial melting, crystal fractionation and assimilation.

Mineral phases have lower 226Ra, Ba, Th and U abundances than the finely crystalline groundmass (Table 1 ). Calculated partition coefficients for minerals are low ( < 0.04), except Ba in plagioclase and Th-U in magnetite and clinopyroxene. Relative orders of trace- element compatibility in minerals are: plagio-

Concentration data for Mt. Shasta rocks and minerals

226Ra Ba Th U 23°Th/232Th ( X 10-15g/g) (/zg/g) (/zg/g) (#g/g) ( X 10 -6 )

$80-58 Hotlam daeite Whole rock 566.5

Plagioclase 41.4 Magnetite 66.5 Groundmass 712.4

$80-63 Hotlum andesite Whole rock 374.8

Plagioclase 12.4 Orthopyroxene 11.1 Clinopyroxene 75.4 Groundmass 492.1

$80-66 Hotlum anflesite Whole rock 279.2

Plagioclase 10.8 Pyroxene 27.5 Groundmass 314.2

$78-10 Black Butte daeite Whole rock 253.3

Plagioclase 25.9 Hornblende 8.1 Groundmass 232.9

80-57 Misery Hill andesite Whole rock 483.2

394.8 4.236 1.475 6.413 (396) (4.48) (1.61) 208.6 0.249 0.094 6.427

25.0 0.528 0.181 6.412 466.2 5.337 1.934 6.454

335.8 2.825 1.104 6.538 (322) (2.85) (0.99)

196.3 0.085 0.035 6.611 11.7 0.083 0.031 6.468 54.3 0.546 0.225 6.597

433.9 3.877 1.538 6.496

263.8 2.167 0.757 6.279 (263) (2.35) (0.71 ) 210.4 0.073 0.031 6.565

22.4 0.207 0.076 6.371 307.4 2.536 0.871 6.240

225.5 1.721 0.646 6.712 (240) (1.71) (0.72)

78.1 0.177 0.059 6.595 25.1 0.051 0.016 6.758

279.1 1.663 0.683 6.756

322.1 3.300 1.437 7.136

Numbers in parentheses are from Newman (1983) and Newman et al. ( 1986 ).

23BU-23°Th-226Ra DISEQUILIBRIUM IN YOUNG MT. SHASTA ANDESITES AND DACITES 231

clase, Ba> > Th ~ U; hornblende, Ba > Th > U; orthopyroxene Ba~ Th~ U; clinopyroxene, U > Th > Ba; and magnetite, U,~ Th > > Ba. These results are similar to mineral-groundmass trace-element partitioning in other calc- alkaline rocks (Allegre and Condomines, 1976; Condomines et al., 1982; Volpe and Ham- mond, 1991 ).

Table 2 lists radionuclide activity and activity ratio data. All samples have equilibrium (234U/23au) activity ratios within analytical uncertainty, except hornblende in the Black Butte dacite (78-10). In thin section, euhedral hornblende shows well-developed reaction rims of iron oxides and pyroxene. Therefore, the 3% (234U) excess in hornblende is perhaps the re-

suit of vapor-phase alteration during or im- mediately after emplacement. The andesites and dacites show 3-13% (23°Th) excess relative to (23SU) (Fig. 2) similar to values for calc-alkaline rocks from Mount St. Helens (Bennett et al., 1982; Krishnaswami et al., 1984; Volpe and Hammond, 1991), and to MORB (Condomines et al., 1981; Newman et al., 1983; and others). The Misery Hill andesite (80-57) has 2 3 8 U - 2 3 4 U - 2 3 ° T h - a 2 6 R a in secular equilibrium.

The 23°Th/232Th isotopic ratios measured by mass spectrometry (Tables 1 and 2) agree within 0.5-1.2% with alpha spectrometric measurements of the same whole-rock samples (Newman, 1983; Newman et al., 1986). In

TABLE 2

Activity data for Mt. Shasta rocks and minerals

(226Ra) (23°Th) (226Ra)/ (23°Th) / (238U) / (23°Th) / (234U)/ (dpm/g) (dpm/g) (23°Th) (232Th) (232Th) (23Su) (23su)

$80-58 Hotlum dacite Whole rock 1.243

Plagioclase 0.091 Magnetite 0.146 Groundmass 1.563

$80-63 Hotlum andesite Whole rock 0.823

Plagioclase 0.027 OPX 0.024 CPX 0.165 Groundmass 1.079

$80-66 Hotlum anflesite Whole rock 0.612

Plagioclase 0.024 Pyroxene 0.060 Groundmass 0.690

$78-10 Black Butte dacite Whole rock 0.555

Plagioclase 0.057 Hornblende 0.018 Groundmass 0.511

$80-57 Misery Hill anflesite Whole rock 1.060

1.232 1.009 1.191 1.056 1.128 1.003 (1.285) (1.195) (1.11) (1.075) (1.01) 0.072 1.264 1.193 1.140 1.046 0.998 0.154 0.948 1.191 1.042 1.143 1.008 1.562 1.001 1.194 1.099 1.086 1.003

0.838 0.982 1.214 1.185 1.025 0.996 (0.82) (1.20) (1.07) (1.12) (1.01) 0.025 1.080 1.227 1.258 0.975 1.009 0.024 1.002 1.201 1.141 1.053 1.004 0.163 1.012 1.225 1.248 0.982 1.005 1.142 0.945 1.206 1.203 1.002 1.001

0.617 0.992 1.166 1.059 1.100 1.001 (0.67) (1.18) (0.92) (1.28) (1.03) 0.022 1.092 1.219 1.298 0.939 0.997 0.060 1.008 1.183 1.107 1.066 1.004 0.718 0.960 1.159 1.042 1.113 1.002

0.524 1.059 1.246 1.138 1.095 1.005 (0.51) (1.24) (1.32) (0.94) (0.94) 0.053 1.074 1.224 1.022 1.198 0.994 0.016 1.141 1.255 0.959 1.309 1.032 0.510 1.002 1.254 1.245 1.008 0.998

1.068 0.993 1.330 1.321 1.007 0.999

Numbers in parentheses are from Newman et al., (1986).

232 A.M. VOLPE

¢-

I--

c- I.-- o

1.50

1.40

1.30

1.20

1.10

1.00

0.90

0.80 0.80

Mr. S h ~ Helens

f equiline I I ! I m I

0.90 1 .00 1.10 1 .20 1 .30 1.40 1.50

('38U/ruTh) Fig. 2. (23°Th/232Tb) activity ratio versus (238U/232Th) activity ratio for Mt. Shasta rocks. Filled circles are Hot- lure andesites and dacite; open diamond is Black Butte Dacite; and open square is the Misery Hill andesite. (Field for Mount St. Helens rocks is from Volpe and Hammond, 1991; field for Etna is from Condomines et al., 1982. )

contrast, 238U/232Th ratios listed in Table 2 differ by 5-16% with those of the previous studies. U concentrations, which disagree by 6- 12%, account for most of the variation between studies. For comparison, Th and Ba concentrations agree within < 1-8% and < 1- 5% (Table 1).

Except the Black Butte dacite (78-10), the Mount Shasta rocks have equilibrium (226Ra/ 23°Th) activity ratios within analytical uncertainty (Fig. 3 ). Large 226Ra excesses relative to 23°Th like those observed in young volcanic rocks from Mounts Etna and Stromboli (Ca- paldi et al., 1976, 1983; Condomines et al., 1982), Vesuvius (Oversby and Gast, 1968; Capaldi et al., 1982), St. Helens (Bennett et al., 1982; Krishnaswami et al., 1984; Volpe and Hammond, 1991 ), Batur (Rubin et al, 1989), and MORB (Rubin and MacdougaU, 1988; Reinitz and Turekian, 1989; Volpe and Gold- stein, in press) are not evident.

Plagioclase and hornblende have 7-26% excess 226Ra, and magnetite in dacite and the groundmass in the andesites have 4-5% deficit of 226Ra relative to 23°Th. The pyroxenes have equilibrated 226Ra-23°Th (Table 2). The ex-

C" F--

13 n,-

e~

1.70

1.60

1.50

1.40

1.30

1.20

1.10

1.00

0.90 0.80

Ro>Mt Sh ° o>U>Th

I~ ~ITJ[~ equiline

0.90 1.00 1.10 1.20

(238U/'3°Th) Fig. 3. (226Ra/23°Th) activity ratio versus (23su/23°Th) activity ratio for Mt. Shasta rocks. Symbols and fields are similar to previous diagram.

tent of 226Ra-23°Th disequilibrium is much less than that observed in similar minerals in young calc-alkaline rocks at Mount St. Helens (Volpe and Hammond, 1991 ).

Discussion

Allegre and Condomines (1976) proposed t h e 23°Th-23su internal isochron diagram for dating young volcanic rocks. Condomines et al. ( 1982, 1988), Ivanovich and Harmon (1982), and Geyh and Schleicher (1990) also provide thorough reviews of the principles underlying 238U-series disequilibria. In addition, a similar internal isochron diagram, using Ba as a homologue for a stable Ra isotope, with (226Ra) and (23°Th) activities normalized to the Ba con- tent is possible for dating young rocks (Wil- liams et al., 1986; Rubin and Macdougall, 1990).

23o Th_238 U disequilibrium

The andesites and dacites show 3-13% excess 23°Th relative t o 2 3 8 U , except the Misery Hill andesite that is in equilibrium. Since these rocks are very young, the extent of Th-U fractionation during formation of these young

23aU-23°Th-226Ra DISEQUILIBRIUM IN YOUNG MT. SHASTA ANDESITES AND DACITES 233

rocks is minor. Yet, 23°Th enrichment relative to 238U in all samples suggests that a variable degree of partial melting probably controls the observed fractionation between these highly incompatible trace elements (see Newman et al., 1986). In contrast, the preferential 23sU enrichment characteristic of volcanic rocks from subduction zones, and attributed to U mobility via vapor or fluid phases (e.g., Gill and Williams, 1990) is not evident in these samples.

Figure 4 shows internal (23°Th/E32Th)- (238U/E32Th) isochron diagrams for the young dacites and andesites. Th isotopic homogeneity in the Hotlum dacite suggests that closed- system Th-U fractionation occurred recently (i.e., < 10,000 years ago). The calculated age agrees with ages for eruptions at the Hotlum summit vent (Table 3 ). The error is large be- cause the slope of the regression approaches

zero and there is less than 10% Th-U fractionation between the minerals and groundmass.

Activity ratios in the Black Butte dacite ex- cluding hornblende also show closed-system Th-U evolution. The hornblende (23°Th/ 237Th) value plots on the linear regression for the other separates after correcting (23°Th) activity for 3% (234U) excess (see Fig. 4). The 13 ka age agrees within analytical uncertainty with eruption ages for the Shastina vent and Black Butte flank vent (Table 3). Mineral separates in both young dacites show 226Ra-23°Th disequilibrium that also suggests magma fractionation occurred less than 10,000 years ago.

Mineral separates for the two Hotlum andesites plot along linear trends corresponding to ages (27-28 ka) that are significantly greater than ages for summit lava eruptions (Table 3). The 23°Th-238U ages could be interpreted as closed-system residence time of crystal-melt in

]40

1.30

I.-.-

~ 1.2o r-

1.10

1.00 1.00

1,40

1.30

1.20

1.10

t-

c" l--

v

1.00 I00

Z I.I0 1.20 1.30 1.40

Hot lum Andesi te I J s8o-63 I /

Plog j . _ ~ p.~ Cpx yyo

/ ~0'~ , ~ I ,+- 0.37

1.10 1.20 1.30 1.40

~.4o B-- Butte DaciteJ

1.30 Hbl WR O n d ~ " Plag

1.10 IT = 13 + - 7ke

lOO , I + - o 1 4 0.90 1.00 1.10 1.20 1.30

1.4-0 Hot lum Andesi te I /

1 . 3 0 S80--66 1

1.20 ( )o ~ ~ , Plog

1.1o .... / I T = 28 +- 1o ko

/e 0'~ I " ' ~ + - 0.20 I I [ + - 0.20 1.00

1.00 1.10 1.20 1.30 1.40

(238U/2~2T h ) (2~sU/23~T h)

Fig. 4. Internal 23°Th-238U isotope diagrams for Mt. Shasta volcanic rocks and separates. PL: plagioclase; PX: pyroxene; MGT: magnetite; HBL: hornblende; GND: groundmass or finely crystalline matrix. Equiline is shown on each plot for reference. Errors for ages and initial Th /U ratios are 2am. Open symbol for hornblende in the Black Butte dacite is (23°Th) value corrected for 3% excess (234U).

234

TABLE 3

Eruptive periods and ages for Mt. Shasta rocks.

A.M. VOLPE

Sample Eruptive periods a 23°Th-238U ( 23°Th/232Th ) 226Ra-23°ThC (years ago ) age b ageo

Hotlum dacite Hotlum cone 3 _ 18ka 1.19 _ 0.16 7300a 190(?)-4,500 summit dome

Hotlum andesite lava flow 27 _ 18 ka 1.22 _+ 0.22 > 10,000a (6400a)

Hotlum andesite lava flow 28 + 10 ka 1.20_ 0.09 > 10,000a (7100a)

Black Butte dacite Shastina flank 13 _+ 7 ka 1.25 +_ 0.10 8200a vent 9400-12,000

~Age ranges are from Christiansen et al. (1977). bAges and initial ratios are calculated from the slopes of weighted linear regressions for the data. Errors are 2try. CAges are calculated from the slopes of weighted regressions. Ages in parentheses are calculated for plagioclase-groundmass data.

a magma chamber. Alternatively, linear arrays could be interpreted as magma mixing (New- man et al., 1986), or recycling of old crystals in young replenished melt. Though zoned, plagioclase separates in these andesites lack optical evidence for multi-stage resorption and crystallization that would suggest crystal recycling (Nixon and Pearce, 1987; Pearce et al., 1987; Blundy and Shimizu, 1991 ). Therefore, long residence times or magma mixing are possible explanations.

Constant initial Th /U ratios (e.g., (Th/ U) o) in the Hotlum andesites and dacite suggest derivation from a similar source (Fig. 4 ). The Black Butte dacite has a slightly lower Th/ U ratio (2.43). Black Butte cone is about 13 km from the Hotlum summit vent at Mt. Shasta. Therefore, lavas erupted from the Black Butte vent may be derived from a different, more primitive (i.e., lower Th /U ) source than Hotlum lavas. (Th/U)o ratios for young Mt. Shasta rocks are similar to those for young ( < 2000 a) calc-alkaline volcanic rocks at Mt. St. Helens (2.3-2.6; Volpe and Hammond, 1991 ). The low (Th/U)o ratios are MORB- like and preclude significant assimilation of crust with different Th-U composition. These conclusions are similar to those in the broader study by Newman et al. ( 1986 ). They also are

consistent with conclusions based on Sr-Pb isotopic compositions (Church, 1976; Church and Tilton, 1973; Peterman et al., 1970 ).

226Ra-23°Th disequilibrium

Figure 5 shows Ba-normalized 2 2 6 R a - 2 3 ° T h

internal isochron diagrams for the rocks. Us- ing Ba as a homologue for a stable Ra isotope assumes that magmatic processes do not frac- tionate Ba and Ra. If minerals like plagioclase partition Ra and Ba differently, o r 226Ra excesses in melts reflect non-equilibrium processes, then Ba-normalized internal isochron diagrams do not provide age information. De- termining whether alkaline earth-rich minerals preferentially partition large ion elements like Ra and Ba requires controlled experimentation on crystal-melt systems of known age (see Williams et al., 1986; Volpe and Hammond, 1991; Reagan et al., 1992 ).

Mineral separates in the Hotlum dacite suggest closed-system 226Ra-Ea°Th fractionation (Fig. 5). The calculated age of 7,300 years is consistent with Th-U results (Table 3). It is likely that this rock erupted recently (i.e., < 1000 a), since the Hotlum dacite forms the present summit of Mr. Shasta (Christiansen et al., 1977; Miller, 1980). Assuming a recent

nSU-n°Th-22+Ra DISEQUILIBRIUM IN YOUNG MT. SHASTA ANDESITES AND DAC1TES 23 5

co

CY

e 4

13

v

7000

6000

5000

4000

3000

2000

1000

0

3500

3000

2500

2000

1500

1000

500

0

Hotlu___._mm Docite I , , ' J .~Cnd

,,o

I --;- ~,~'~'+ I T = 7300 o o^^ , ,I , , -,+oo

0 1000 2000 3000 4000 5000 6000 7000

Hotlum Andeste ^ . / 1

P I 0 g x~'~.06 /:++, , o o o

500 1000 1500 2000 2500 3000 3500

(23°Th)/Ba

3000

2500

2000

1500

1000

500

0

3500

3000

2500

2000

1500

1000

5OO

0

B_ But,___t Doci,____tJ /

Gnd

P l a g ~ Hbl ~ " . - - _ _

. I. , .21oo 500 1 0 0 0 1500 2000 2500 3000

Hotlum Andesi te I / S 8 0 - 6 6 I / w ,/Px

Gnd

Plag ./,.,~ e" I . . . . o.)~\ T >= 10,000 o i ~ °" ~'~x~ ' , I ,T > = . 1 0 , 0 7 a

500 1000 1500 2000 2500 3000 3500

('3°Th)/Bo

Fig. 5. Ba-normalized internal 226Ra-23°Th isotope diagrams. Symbols are the same as the previous figure. Radionuclide activities (dpm/g) are normalized to barium concentration (g/g). This normalization uses barium as an analogue for a stable isotope of radium, and assumes equal Ba-Ra partitioning during crystallization.

emplacement time, the 226Ra-23°Th age suggests that the average magma-chamber residence time for this dacitic magma is about 6- 7000 years. This period is considerably longer than that suggested by 226Ra-23°Th disequilibrium in the 1980-1986 dome dacite at Mount St. Helens (i.e., 500-1000 a; Volpe and Ham- mond, 1991 ). Different magma-chamber residence times may be due to differences in the frequency of eruptions at these two volcanoes during the Holocene (see Crandell, 1987; Christiansen et al., 1977 ).

Plagioclase and whole-rock 226Ra-23°Th disequilibrium suggests that the Black Butte dacite erupted more recently than 9400-12,000 years ago. This age range is based on mC dating of pyroclastic eruptions at Black Butte and Shastina vents (Christiansen et al., 1977; Miller, 1980). Based on optical petrographic evidence, plagioclase in this dacite is free of

vapor phase or secondary alteration. Mag- matic fractionation during crystal growth most likely generated the 226Ra-E3°Th disequilibrium in plagioclase. Hornblende, like plagioclase, retains alkaline earth elements relative to actinides (Table 1 ), and 226Ra-E3°Th fractionation is also expected during crystal formation. But, it is not possible to determine the extent to which the 14% 226Ra excess in hornblende is due solely to vapor phase alteration. Presumably vapor phase alteration and formation of reaction rims on hornblende occurs during lava emplacement. Therefore, whether or not Ra-Th fractionation in hornblende is due to primary or secondary processes does not change the conclusion that the dacite erupted less than 10,000 years.

Mineral separates for the Hotlum andesites plot on or near the equiline on the Ba-normalized diagrams (Fig. 5). Equilibrium 226Ra-

236 A.M. VOLPE

23°Th activity ratios suggest that either there was small Ra-Th fractionation, or that fractionation occurred more than 10,000 years ago. The latter possibility agrees with 23°Th-238U ages of about 28 ka. But, plagioclase show 8- 9% 226Ra excesses and groundmass values show 4-5% 226Ra deficits. While the magnitude of the 226Ra-23°Th disequilibria is small, it is significant compared with analytical uncertainty. This is evident on Fig. 6 that graphically shows both Ra-Th and Ba-Th fractionation ages calculated for plagioclase and groundmass pairs in the andesites are 6400 and 7,100 years (Fig. 6).

There are several explanations for discrep- ancy between 23SU-series ages. There may be open-system Ra-Th behavior due to Ra mobility relative to Th in surficial (oxidizing)

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Ba/Th Fig. 6. (226Ra/23°Th) activity versus Ba/Th element ratios for mineral separates and groundmass in the Hotlum andesites. Ages are calculated for plagioclase and groundmass values. Errors are 2G~.

aqueous environments (e.g., Ivanovich and Harmon, 1982). During secondary aqueous alteration, Ra would presumably be leached from high abundance, easily altered material. This could account for the small 226Ra deficit in the groundmass, but not the excesses in plagioclase and equilibrium ratios in the pyroxenes. All minerals in these andesites are free of secondary alteration based on optical petrography. In addition all separates have equilibrium (234U/238U) ratios within analytical uncertainty ( ~< 1%). Therefore, the petrographic and chemical evidence suggests that Ra mobility due to secondary processes is not a likely explanation of 226Ra-23°Th disequilibrium.

Absolute dating of very young rocks by 23°Th-23su disequilibrium can be geologically unreliable and 226Ra-23°Th dating can trace recent events better for several reasons. First, the 226Ra half-life ( 1600 a) is commensurate with the timing of magmatic processes. Second, Ra- Th fractionation during magma extraction (i.e., MORB 3-4 fold) and mineral partitioning is large. The Th-U age for andesite S80- (Fig. 4) may be unreliable, since (23°Th/ 232Th) values overlap within uncertainty and the extent of Th-U fractionation is only 10%. However, ages and initial ratios for the two Hotlum andesites are identical (Fig 4. ).

Finally, it is possible that the Th-U age rep- resents an average magma-chamber residence time of 28 ka for these andesitic magmas. Pla- gioclase and groundmass 226Ra-23°Th disequilibrium may then record the latest period of closed-system crystallization about 7000 years ago. Alternatively, since the plagioclase shows zoning, 226Ra excess in the mineral and n6Ra deficit in the groundmass may reflect a recent influx of replenished magma. The data for mineral separates are averages of the bulk composition. All the "excess" 226Ra in plagioclase may be in the outer rim. Periods of 6400- 7100 a would then be unrelated to either the long crystal residence time, or recent open-system changes in melt composition.

23sU-23°Th-226Ra DISEQUILIBRIUM IN YOUNG MT. SHASTA ANDESITES AND DACITES 237

Conclusions

In summary, internal 23°Th-238U and 226Ra-

23°Th diagrams for the Hot lum and Black Butte dacites suggest closed-system T h - U and R a - Th fractionation occurred less than 10,000 years ago. Initial T h / U ratios are low, MORB- like, and clearly preclude significant assimilation of crust with different T h - U composi t ion (e.g., high T h / U ) . The 226Ra-Ea°Th disequili- br ium in mineral separates in the Black Butte dacite suggests that the total durat ion of t ime from magma fractionation through emplacement at the surface is 8-10 ka. Therefore, it is likely that this rock erupted more recently than 9 .4-12 ka. If so, recent eruption activity at the Black Butte flank vent may correlate with the Ho t lum vent ( < 3-4.5 ka) .

Internal 23°Th-E38U isochrons for the Hot- lum andesites suggest either magma mixing or a long pre-eruption residence and transport t ime compared to the evolved dacites. 226Ra- 23°Th disequil ibrium in plagioclase and groundmass in these andesites suggests that either closed-system crystallization ceased about 7ka, or that there has been recent open system changes in melt composi t ion affecting Ra more than Th. Clearly, the utility of 238U- series dating to determine eruption ages of other calc-alkaline rocks is questionable, especially where corroborat ing field and age evidence is lacking.

Acknowledgement

The author thanks J. Olivares, D. Perrin and D. Rokop for their advice and efforts in mass spectroscopy at Los Alamos, and G. Capaldi for critical review of the manuscript. This work was done while the author was a postdoctoral fellow, and supported by a grant from Basic Energy Sciences, USDOE.

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238U-230Th-226Ra disequilibrium in young Mt. Shasta andesites and dacites