The isotopic and chemical evolution of Mount St. Helens

Earth and Planetary Science Letters, 63 (1983) 241-256

241

Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands [2]

The isotopic and chemical evolution of Mount St. Helens A.N. Halliday 1, A.E. Fallick 1, A.P. Dickin 1, A.B. Mackenzie 1, W.E. Stephens 2 and W. Hildreth 3 1 Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow G75 OQU (U.K.) 2 Department of Geology, University of St. Andrews, St. Andrews, Fife KYI6 9ST(U.K.) 3 U.S. Geological Survey, Menlo Park, CA 94025 (U.S.A.)

(Received November 3, 1982) Revised version received February 1, 1983

Isotopic and major and trace element analysis of nine samples of eruptive products spanning the history of the Mr. St. Helens volcano suggest three different episodes; (1) 40,000-2500 years ago: eruptions of dacite with ~Nd = +5, Csr = --10, variable 8tSo, 2°6pb/2°4pb- 18.76, C a / S r - 6 0 , R b / B a - 0.1, L a / Y b - 18, (2) 2500-1000 years ago: eruptions of basalt, andesite and dacite with ~rq,~= + 4 to + 8, %r = - 7 to -22, variable 8180 (thought to represent melting of differing mantle-crust reservoirs), 2°6pb/204 Pb = 18.81-18.87, variable Ca/Sr, Rb/Ba, La/Yb and high Zr, (3) 1000 years ago to present day: eruptions of andesite and dacite with crqd = +6, Csr = -13, 8 1 8 0 - 6%o, variable 2°6pb/2°4Pb, C a / S r - 77, Rb/Ba = 0.1, L a / Y b - 11. None of the products exhibit Eu anomalies and all are LREE enriched. There is a strong correlation between 87Sr/S6Sr and differentiation indices. These data are interpreted in terms of a mantle heat source melting young crust bearing zircon and garnet, but not feldspar, followed by intrusion of this crustal reservoir by mantle-derived magma which caused further crustal melting and contaminated the crustal magma system with mafic components. Since 1000 years ago all the eruptions have been from the same reservoir which has displayed a much more gradual re-equilibration of Pb isotopic compositions than other components suggesting that Pb is being transported via a fluid phase. The Nd and Sr isotopic compositions lie along the mantle array and suggest that the mantle underneath Mt. St. Helens is not as depleted as MORB sources. There is no indication of seawater involvement in the source region.

1. Introduction The origin of calc-alkaline magmas produced n e a r d e s t r u c t i v e p l a t e m a r g i n s has l o n g b e e n of i n t e r e s t to p e t r o l o g i s t s . D e s p i t e this t h e r e h a v e b e e n v e r y few studies o f the i s o t o p i c a n d c h e m i c a l c h a n g e s t h a t t a k e p l a c e w i t h t i m e in a single v o l c a n o . Mt. St. H e l e n s is i d e a l for s u c h a s t u d y b e c a u s e the s t r a t i g r a p h i c r e l a t i o n s h i p s are r e a s o n a b l y well e s t a b l i s h e d , t h e r e is a g o o d r a d i o c a r b o n c h r o n o l o g y , a n d t h e v o l c a n o has b e e n s t u d i e d int e n s i v e l y in a s s o c i a t i o n w i t h its r e c e n t p r o v o c a t i v e behaviour. W e are p a r t i c u l a r l y i n t e r e s t e d in the f o l l o w i n g questions: 0012-821X/83/$03.00

©1983 Elsevier Science Publishers B.V.

(1) W h a t causes crustal m e l t i n g a n d assimilation and fractionation? (2) H o w q u i c k l y d o m a g m a b o d i e s c h a n g e b y fractionation and assimilation? (3) A r e a s s i m i l a t i o n a n d f r a c t i o n a l crystallisat i o n well c o u p l e d p r o c e s s e s a n d are t h e s e m o r e or less c o n t i n u o u s o r e p i s o d i c in m a g m a c h a m b e r s ? (4) W h a t are the r e l a t i o n s h i p s b e t w e e n m e l t i n g , magma ascent and eruption? (5) H o w d o m a g m a b a t c h e s m i x ? T h i s p a p e r r e p o r t s o n an initial a t t e m p t to answer these questions empirically. S o m e i s o t o p i c (Pb, Sr) a n d c h e m i c a l d a t a f r o m M t . St. H e l e n s h a v e a l r e a d y b e e n p u b l i s h e d . C h u r c h a n d T i l t o n [1] a n d C h u r c h [2] p r e s e n t e d

242

2. Geological background

Pb a n d Sr i s o t o p i c d a t a for the C a s c a d e s as a w h o l e a n d c o n c l u d e d t h a t the m a g m a s w e r e genera t e d via a m u l t i s t a g e r e m e l t i n g process. T h e y s h o w e d t h a t the d a t a w e r e c o m p a t i b l e w i t h m e l t i n g o f n e i t h e r s u b d u c t e d o c e a n i c crust n o r subd u c t e d s e d i m e n t n o r c o n t i n e n t a l crust on their own. They favoured melting of both high-alumina basalt and andesites from a mantle source but with n o direct f r a c t i o n a l c r y s t a l l i s a t i o n r e l a t i o n s h i p bet w e e n the t w o types. L o p e z - E s c o b a r et al. [3], a r g u e d for an e c l o g i t e s o u r c e o n the basis of r a r e e a r t h e l e m e n t ( R E E ) analyses.

D e t a i l e d d e s c r i p t i o n s of the g e o l o g i c a l h i s t o r y o f Mt. St. H e l e n s h a v e b e e n g i v e n e l s e w h e r e [4-7]. F o r this r e a s o n o n l y a b r i e f o u t l i n e is g i v e n here. M t . St. H e l e n s is a r e l a t i v e l y y o u n g C a s c a d e v o l c a n o , the o l d e s t d e p o s i t s h a v i n g b e e n a s s i g n e d a n age of - 35,000 to - 40,000 years o n the basis o f r a d i o c a r b o n dates. I n T a b l e 1 the h i s t o r y o f the v o l c a n o is s u m m a r i s e d ( b a s e d o n r e f e r e n c e s 5 a n d 7), t o g e t h e r w i t h the s t r a t i g r a p h i c r e l a t i o n s of the s a m p l e s c h o s e n for this study. It c a n b e seen t h a t

TABLE 1 Summary of eruptive history of Mt. St. Helens with sample numbers (based on references 5 and 7) Eruptive period

Recent

Approximate age (years before 1982) 2

Eruptive products

dacite dome pyroclastic flows (SH25), pumiceous and lithic tephra, blast deposit

dormant interval of - 130 years Goat Rocks

180-130

dacite dome, andesite lava flow, tephra layer T (SH2)

480-380

dome (SH16), andesite and dacite lava and pyroclastic flows, tephra

dormant interval of - 200 years Kalama dormant interval of - 700 years Sugar Bowl

1150

dacite dome (SH15), blast deposit

dormant interval of - 600 years Castle Creek

> 2200-1700

basaltic, andesitic and dacitic tephra, lava flows (SHI7) and domes (SH20)

dormant interval of - 300 years Pine Creek

3000-2500

dacitic pyroclastic flows and tepbra

4000-3300

dacitic pyroclastic flow and tephra (Loowit Creek Dome (SHI4)?)

dormant interval of - 300 years Smith Creek dormant interval of - 4000 years Swift Creek dormant interval of - 5000 years

13,000- > 8000

dacitic pyroclastic flows and tephra (SH4)

Cougar

20,000-18,000

dacitic pyroclastic flows, domes, and tephra

dormant interval of - 15,000 years Ape Canyon

- 40,000(?) - 35,000

dacitic pyroclastic flows (SHI 1) and tephra

243 the frequency of eruptions appears to have increased with time with initial hiatuses of thousands of years and more recent hiatuses of 100-200 years. The eruptive products have been basaltic, andesitic and dacitic flows and tephra units. Verhoogen [4, p. 294] noted a general time trend from the eruption of more acidic to the eruption of more basic material. Mullineaux and Crandell [7] also noted a change which took place approximately 2500 years ago from dacitic volcanism to products varying in composition from basalt to dacite (Table 1). Lipman et al. [8, pp. 638-640] commented on the similarity in chemistry between most of the products of the 1980 eruptions and those of the 19th and 17th century eruptions. On the basis of this they pose the question of whether or not the magma erupted in 1980 is the product of (slight) differentiation of the same magma body as gave rise to the 19th and 17th century eruptions. (This question is tackled further in this paper.) The country rocks at Mt. St. Helens give no indication of old basement at depth. It is thought that most of the crust is a mixture of older Cascade orogenic volcanics and volcaniclastics (principally andesite) with continental margin scrapings resting on (unexposed) oceanic crust. These older volcanics are several kilometres thick and are mainly of Oligocene or younger age. The most probable maximum age is late Eocene [9,10]. Mt. St. Helens volcanic ejecta from the 1980 eruptions include xenoliths of metavolcanics, granodiorite, tonalite, diorite, hornfels, white quartzite, gabbro and amphibolites [ 11,12].

3. Techniques Most major and trace element analyses were carried out using a Philips PW1212-X-R.F. Major oxides except Mn were analysed on beads prepared with Spectroflux 105 with ammonium nitrate as oxidant. Major elements were analysed using a Cr tube with TAP, PE, Ge, LiF 200 crystals, flow counter and 1 ~m windows. Trace elements and MnO were analysed with W and Au tubes on pressed powder pellets. The rare earth elements (except Nd), Mn, Sc,

Cs, Hf, Ta and Th were analysed by instrumental neutron activation analysis at S.U.R.R.C. Whole-rock oxygen isotopic data were obtained by fluorination using C1Fs following the technique of Clayton and Mayeda [13] as modified by Borthwick and Harmon [14]. Replicate aliquots of a quartz standard show better than 0.1%o (lo) reproducibility; a somewhat greater spread than this is usual for whole-rock powders, and the precision can be gauged from the duplicate and triplicate results presented in Table 4. Oxygen isotope silicate standards analysed were Snowbird Quartz ( + 16.2 + 0.1%0) and African Glass Sand ( + 9.56 _+ 0.06%0). Isotopic compositions were determined on a V.G. Micromass Ltd. 903 triple collecting mass spectrometer, with a working standard CO 2 calibrated against water, silicate and carbonate oxygen reference materials. Hydrogen isotope results were obtained by a method similar to that of Friedman [15]. The gas yield during hydrogen extraction was measured manometrically to provide a quantitative determination of water content. The reproducibility is indicated by the duplicate analyses in Table 4. Hydrogen isotope ratios were measured on a V.G. Micromass Ltd. 602D double collecting mass spectrometer with the working standard H 2 calibrated against the Vienna reference waters V-SMOW, V-SLAP and GISP. All hydrogen and oxygen isotope results are reported in the usual delta permil (8%o) notation relative to V-SMOW [16]. Sm and Nd concentrations were determined by isotope dilution. Sr and the RRE's were separated using a standard cation exchange column with 2.5 N HCI acid as eluent following decomposition in pressurised digestion vessels. The remainder of the separation procedure closely followed that outlined by O'Nions et al. [17]. Total Sr and Nd blanks averaged 5 and 1 ng respectively. All analyses were made in a fully automated V.G. Isomass 54E mass spectrometer. All isotopic data are corrected for machine discrimination using 88Sr/86Sr = 8.37521 and 146Nd/t44Nd = 0.7219. The average 87Sr/86Sr for NBS 987 during the course of these analyses was 0.710275 + 7 (2o m. . . . N = 79). The average 143Nd//144Nd of BCR-1 and GSP-1 at the time of this work were 0.512633 + 12 (2o m. . . . N = 5) and 0.511383 + 12 (2o . . . . . N = 2).

244

Details of the reproducibility of the mass spectrometer and values obtained for other isotopic ratios are given elsewhere [18]. The data presented here are the result of between 600 and 2000 scans of each mass spectrum. Pb was separated on 0.5 ml (primary) and 0.05 ml (clean-up) anion exchange columns using HBr eluent. Total chemistry blanks averaged 4 ng. Pb was loaded onto Re filaments with silica gel and phosphoric acid and analysed on a V.G. Isomass 54E mass spectrometer. Analyses are corrected for

mass fractionation by addition of 0.1% per a.m.u. Within run precision of Pb isotope ratios averaged 0.05% (20) and between run accuracy is estimated to be 0.1% (20).

4. Chemistry In Tabe 3, major and trace element analyses are presented. The nine samples are arranged in order of eruption and range from basalt (one sample) to

TABLE 2 P e t r o g r a p h i c notes on samples used in this study Sample No.

Age (years B.P.)

SH25

- 0

SH2 SHI6

180 350-450

SHI5

1150

SHI7

- 1900

SH20

2000-2400

SHI4

- 4000-3000(?)

SH4 SHI 1

- 13,000 35,000-40,000

Unit

Phenocrysts

P e t r o g r a p h i c notes

p u m i c e o u s pyroclastic flow, 22 July 1980 T e p h r a Set T Summit Dome

plag > hyp > hb > m t > Jim

glassy p u m i c e b l o c k

plag > hyp > hb > m t > (tr)cpx > ilm plag > hb - hyb > cpx - m t > ilm

Sugar Bowl D o m e

plag > hyp > hb > m t > ilm

basalt atop Dogs H e a d Dome

plag > olivine > cpx > m t

Dogs H e a d D o m e

plag > hyp > hb > m t > Jim

Loowit Creek D o m e

plag > hyp > hb > m t > ilm

T e p h r a Set SO p u m i c e o u s pyroclastic flow related to T e p h r a Set C

plag > hb > c m > hyp > bt > ilm plag > h b - c m > m t > bi - qz > ilm

glassy p u m i c e lapilli - 40% p h e n o c r y s t s in nonvesicular, p a r t l y glassy, microcrystalline g r o u n d m a s s - 15% p h e n o c r y s t s in nonvesicular, glassy groundmass with a b u n d a n t plagioclase microlites - 25% p h e n o c r y s t s in fresh microcrystalline groundmass; a few scattered vesicles < 0.5 mm. Plag a n d olivine up to 1 m m - 35% p h e n o c r y s t s in nonvesicular partly glassy g r o u n d m a s s with a b u n d a n t plag microlites - 40% p h e n o c r y s t s in microcrystalline g r o u n d m a s s . Strongly zoned plagioclase up to 5 mm. Mafic phenocrysts p a r t l y altered, o p a q u e margins glassy p u m i c e lapilli glassy p u m i c e lapilli

Ages from H o b l i t t et al. [6] a n d M u l l i n e a u x and Crandell [7]. All samples collected b y H i l d r e t h 1975, 1978, 1981. All samples m a y also contain traces of a p a t i t e a n d sulphide blebs. c m = c u m m i n g t o n i t e ; bi = biotite; mt = t i t a n o m a g n e t i t e ; ilm = ilmenite; qz = quartz; plag = plagioclase; hyp = hypersthene; hb = h o r n b l e n d e ; cpx = clinopyroxene.

245

dacite, petrographic notes including phenocryst phases being detailed in Table 2. Where a comparison can be drawn, the chemical results are in good agreement with data previously reported by Hoblitt et al. [6] and Lipman et al. [8]. An AFM plot (not shown) reveals that the data define a trend exactly along the line of "average Cascades" compositions plotted by Carmichael et al. [19, p. 568]. This suggests that in some respects the St. Helens suite is typical of the Cascades. In a plot of total alkalis against SiO 2 (not shown), it can be seen that all samples except the one basalt (SH17) plot in the high-alumina field of Kuno [20]. In Fig. 1 the data can be seen to plot on the low K-side of the calc-alkaline fields defined by Peccerillo and Taylor [21]~ This is consistent with the data for the Cascades, Aleutians and Alaska collated by Ewart and Le Maitre [22]. Again the basalt SH17 is anomalous with high K 2 0 for its SiO 2 content. SH17 is also anomalous in being exceedingly Ti rich and in having very low B a / L a for calc-alkaline suite rocks (see Ito et al. [23]).

Z.i

Table 2 shows that the glassy and pumiceous rock SH4 is unusual in having > 19 wt.% A 1 2 0 3 and it has > 3% normative corundum. Although this is a low-SiO 2, low-K20 sample a comparison of Fe203, MgO, TiO2, A1203 ratios shows that its main affinity is with the high-SiO 2 ( - 66%) sampies. It appears that this sample has lost SiO 2 as well as varying proportions of the alkalies and alkaline earths. The order of decreasing loss based on a comparison of ratios of some elements in SH4 with S H l l and SH14 is R b - K > Ba > Si > Na>Sr-Ca>A1-Fe-Mn-Mg. There is no evidence for severe alteration of the sample although it displays some iron staining. Furthermore the oxygen isotopic composition is normal (see below). Therefore this is taken to be an example of volatilisation of alkalis and alkaline earths analogous to the vapour phase transport mechanism for generating peraluminous compositions discussed elsewhere [19, p. 265]. The general features of the volcano can be seen from Tables 1 and 3 and Fig. 2. In particular the

I

1

borokit~

[

shoshonite

high K rhyolite high K - d o c i t e

high K a n d e s i t e

lbsarokite K20 (wt %)

dacite

basattic andesite andesite 11

basaltic andesite

i J

rhyolite

............ ...........

07039 0' 70318

.... ~. . . . .

07037~

.__&&

I¸

. . . . . . h,

2. tow K r h y o l i t e

~ low K tho e l i t e 50

Jlow K basattic andesite 55

low K o n d e s i t e

i

low K- d a c i t e

I

I

60

65

70

75

Si 02 (wt %)

Fig. 1. K 2 0 versus SiO 2 plot showing Mt. St. Helen's data with fields as defined in K u n o [20]. Contours are SVSr/S6Sr for > 62% SiO 2 samples. C i r c l e = b a s a l t , s q u a r e s = andesite, t r i a n g l e s = d a c i t e (filled, < 1000 years old; half-filled, 2500-1000 years old; open, > 2500 years old; dashed = sample SH4, see text).

246 TABLE3 Chemicaldata SHI1 3.5-4.0 ×104yr SiO 2 TiO 2 A1203 Fe203 * MnO MgO CaO Na20 K 20 P20~ L,O.I. Total XRF D.I.

65.58 0.50 16.87 3.83 0.07/(0.068) 1.43 4.29 4.45 1.42 0.16 1.70 100.30 69.44

SH4 -1.3 ×104yr 60.32 0.57 19.39 4.44 0.07/(0.076) 1.66 4.39 4.30 1.11 0.17 2.80 99.22 62.61

SH14 -3.0-4.0 ×103yr?

SH20 2.0-2.4 ×103yr

SHI7 -1.9 ×103yr

66.50 0.52 17.26 3.90 0.07/(0.067) 1.55 4.37 4.64 1.42 0.15 0.00 100.38

62.68 0.76 17.52 5.33 0.08/(0.079) 2.18 5.24 4.67 1.46 0.21 0.00 100.13

49.95 1.95 16.97 10.41 0.15/(0.157) 7.00 9.01 3.89 1.28 0.40 0.00 101.01

70.48

64.65

40.46

Sc V Cr Ni Cu Zn Rb Sr Y Zr Nb Cs Ba La Ce Nd Sm Eu Tb Yb Lu Hf Ta Pb Th

6.8 42 3 19 13 53 35 514 12 103 6 2.70 350 13.6 23 13.6 2.81/3.1 0.95 0.35 0.8 0.17 3.4 1. I 13 3.3

8.4 54 10 9 10 51 27 529 12 137 7 1.99 273 12.2 24 2.8 1.05 0.43 1.1 0.18 3.8 0.9 9 3.6

6.6 50 8 10 24 55 36 527 12 115 8 1.68 371 14.6 26 14.0 2.85/3.2 0.96 0.36 0.8 0.12 3.3 0.8 12 2.8

11.2 74 11 9 22 57 38 477 16 150 9 2.35 396 17.9 30 13.9 2.93/3.5 1.18 0.68 1.4 0.22 4.1 1.1 12 3.9

28.2 230 159 82 60 72 20 577 24 174 22 0.70 309 22.6 42 22.7 5.05/5.5 1.84 0.78 2.4 0.36 4.7 2.4 6 3.1

V/Ni Rb/Sr Ca/Sr Ba/La

2.2 0.068 60 26 4.1 17 0.100 337 2.7

6.0 0.051 59 22 3.4 11 0.099 341 3.0

5.0 0.068 59 25 5.2 18 0.097 327 3.0

8.2 0.080 79 22 4.6 13 0.096 319 3.2

2.8 0.035 112 14 7.3 9 0.065 531 3.3

La/Th La/Yb Rb/Ba K/Rb Rb/Pb

Major elements in wt.%; D.I. = Thornton and Tuttle index; trace elements in ppm. Second value for MnO is determined by INAA; first value for Sm is determined by isotope dilution.

247

SH15 1.15 × 103 yr

SHI6 3.5-4.5 × 102 yr

SH2 180 yr

SH25 0 yr

69.22 0.38 16.19 3.32 0.06/(0.060) 0.96 3.17 4.83 1.91 0.14 0.00 100.18

62.52 0.67 18.09 5.24 0.08/(0.088) 2.22 5.54 4.34 1.30 0.15 0.00 100.15

63.30 0.62 17.82 4.92 0.08/(0.086) 2.07 5.16 4.72 1.33 0.17 0.00 100.19

62.59 0.68 17.90 5.10 0.08/(0.082) 2.35 5.45 4.82 1.26 0.17 0.00 100.40

78.14

62.05

65.11

63.79

4.7 22 0 7 17 53 53 388 17 173 10 3.13 452 17.1 25 15.7 3.18/3.1 0.94 0.34 1.2 0.22 4.3 1.4 13 4.1

9.9 78 0 8 15 48 31 531 15 121 6 1.85 319 12.3 21 12.1 2.64/2.7 0.93 0.35 1.0 0.19 3.1 0.8 8 2.9

9.3 63 7 9 33 58 31 481 14 128 7 1.98 307 13.7 26 12.7 2.67/3.0 0.91 0.39 1.2 0.23 3.4 0.8 9 3.0

10.5 83 15 11 45 57 30 488 10 113 7 1.63 299 11.4 19 11.6 2.49/2.5 0.85 0.38 1.0 0.15 3.0 1.7 10 3.1

3.1 0.137 58 26 4.2 14 0.117 299 4.1

9.8 0.058 75 26 4.2 12 0.097 348 3.9

7.0 0.064 77 22 4.6 11 0.101 356 3.4

7.5 0.062 80 26 3.7 11 0.100 349 3.0

248 i

120 100 Ca/St

•

80

•

rl

& "A

A i

6O 012

b

010

I,,

&

i

A

Rb/Bo 0 06 O0E

I

c

iA

I

I

i

Rb/Pb 4 0 30

l,

&

18C Zr p p m

BI

14C •

•

q

•

&

&

10C I

e

Si02 7(

i

A •

(wt %) 60

-

50

rI

&--I

i 10

I

I

& ^ L~

I

102 103 104 Years before present (1982)

105

F i g . 2. Plot s h o w i n g c h e m i c a l changes with time (symbols as in F i g . 1).

fact that the earlier eruptions were generally dacitic (e.g. Shl 1, 14 and probably 4), that between 2500 and 1000 years ago the products were very variable (SH17 basaltic, SH15 dacitic) and that the last three eruptive episodes (17th and 19th century and 1980) have produced uniform magma compositions as noted by Lipman et al. [8]. Both these

LU I-E D z 10( O

I

I

I

I

I

T• >2500 ----:-

years

i

2500-1000 y e a r s oid-

~<1000yeors

~ 60

laid

data and those of Mullineaux and Crandell [7] and Hoblitt et al. [6] indicate a compositional gap between approximately 51 and 57% SiO 2, that is the "field" of basaltic andesite appears to be missing at Mt. St. Helens. Fig. 3 shows the chondrite-normalised REE data for the 9 samples studied here. The results are very similar to those presented in summary form by Lopez-Escobar et al. [3]. The lack of Eu anomaly does suggest that plagioclase was not a constituent of the source and that most of the source constituents had not been through a previous plagioclase fractionation cycle. It can be seen that the one basalt has the highest La and Yb concentrations (and lowest L a / Y b ratio). The basalt is clearly not related to the other samples simply by fractional crystallisation. Fig. 3 also shows that, despite the overall similarities of the profiles, there is a tendency for the older samples (pre-2500 years old) to have higher L a / Y b than the youngest samples (post-1000 years old). In addition, the 2500-1000 year old samples have the highest REE abundances despite including the two samples with highest (SH15) and lowest (SH17) SiO 2 contents. This (surprising) feature is also true of Zr concentration as is shown in Fig. 2 and also in Fig. 4 where the Yb and Zr contents of the samples can be seen to be positively correlated. The small number of samples preclude any detailed discussion of this based on the chemistry alone. However, the interpretation of the REE profiles of Mt. St. Helens lavas in terms of an eclogite source [3] is clearly equivocal since zircon as an important

old

20C

IH O < 30 l.xl

\, 15C Zr

4'

ppm

N o_ Z t~

. - - -~ ";

6

. A''--''I,,I

I LaCe

r

I Nd

J

I

Sm Eu

I Tb

i

I

Yb Lu

10C

I

F i g . 3. Rare earth element profiles chondrite-normalised to C1 average chondrites as determined by Evensen et al. [28]. Sample n u m b e r s ( S H ) n given for 2 5 0 0 - 1 0 0 0 year old samples because of diverse compositions.

-"

--

....

~

2,500 -1,000yrs

.."~l,O00yrs

' z~ /" . . . . "'-"~";2,500 10

j~ OI

yrs(e×dudlng 115

SH4) 2tO

i

25

Yb ppm

Fig. 4. The positive correlation between Y b and Z r (symbols as in F i g . 1).

249

residual source constituent or fractionating phase would explain both the REE profile and the relationship between Zr and Yb. A very important feature of Fig. 2 is that despite the changes in other parameters the R b / B a ratios before and after the basalt extrusion (SH17) are identical ( - 0.1). This strongly implies that the Mr. St. Helens magmas have principally tapped the same reservoir throughout the volcano's history. It also suggests that either the reservoir is a sufficiently large system for the intrusion of basalt such as SH17 not to produce a lasting effect on the R b / B a ratios of the system, or that the main reservoir was somehow isolated from the magma components of the 2500-1000 year period.

5. Stable isotopes 8~80 and 8D (with H 2 O + ) results are given in Table 4. The spread amongst the 8180 analyses of SH 17 is larger than expected from analytical precision, and may be indicative of a small degree of isotopic heterogeneity within this sample. In Fig. 5a 8D is plotted against H2 O+. The dashed vertical line indicates the maximum water content expected for unaltered basalt. It is readily apparent that the two pumiceous samples which are by far the oldest, S H l l and SH4, have experienced hydration to a considerable extent, leading to deuterium depletion. It is conceivable that groundwater circulation has affected the more recent

pumiceous deposits SH2 and SH25 also, though to a much lesser extent. The very low water content and relatively high 8D for SH16 is most likely a result of preferential loss of isotopically light volatiles during degassing. In Fig. 5b 8D is plotted against 8180. As expected, SH 17 falls in the region defined by mantle samples. Despite the evidence for hydration of SH11 and SH4 discussed above, the 8180 values of these samples are thought not to have been disturbed because a feldspar separate from SH14 yields a 8180 value of 6.77%0 identical to the whole rock (6.78). The higher 8180 values of these sampies, and also SH15 are consistent with them containing a component derived from the continental crust. The most surprising feature of Fig. 5b and the data in Table 4 is the very low 8180 measured for SH14. This result is most readily accommodated by a process of high-temperature oxygen exchange with (meteoric?) fluid. Further evidence to support this comes from the low 8D (the lowest value found, apart from the hydrated _,~F) °_oo I

;

-6olin (a/ l,

-120F SD°looI

TABLE 4

H20 + (wt.%)

8DsMow (%o)

818OsMow (%)

SH2 SH4 SH11 SH14 SH15 SH16 SHI7 SH20 SH25

0.28 2.07, 1.82 1.20 0.08, 0.10 0.10 0.02 0.03 0.04 0.22

-

6.35 7.26, 6.78 4.51, 7.42, 6.11 5.62, 6.45 6.37

103 120, - 116 126 110, - 112 -94 -58 -83 - 97 -97

7.33 4.67 7.35 6.22, 5.83

I

I

A

I I

(b)

I

I

05

10

15

I

I

I

-60 -80 I

Sample No.

I

-s°)A ," 0

Stable isotope data

I

•

H ~0÷

(If A

-100 -120 F

A I

5

I

6

I

7

81800100

Fig. 5. (a) 8D%o vs. H 2 0 + %. The dashed vertical line is the m a x i m u m water content expected for fresh, unaltered basalt. (b) 8D%o vs. 81So%o. Both 8 values are relative to V-SMOW and symbols are as in Fig. 2.

250

samples SH4 and S H l l ) and the fact that in contrast to the other domes sampled, SH14 exhibits partial oxidation of its mafic phenocrysts. The groundmass is microcrystalline, not glassy.

6. Strontium and neodymium isotopes In Table 5 the Sr, Nd, and Pb isotopic compositions for all nine samples are reported in stratigraphic order (oldest at top). The 87Sr/86Sr and 143Nd/la4Nd are low and high respectively (0.7029-0.7040 and 0.51285-0.51305) relative to bulk earth. In Fig. 6 the Nd and Sr isotopic compositions can be seen to be negatively correlated. The basalt has by far the highest ~Yd ( + 7.7) and lowest Csr ( - 2 2 ) . In Fig. 7 the data are portrayed relative to present-day values for other western U.S.A. volcanic and crustal types and can be seen to plot in the "depleted mantle" field. The Mt. St. Helens suite fall along the mantle array with the basalt at the margin of the field for MORB data as presented by Ito et al. [23]. The field of data for the Basin and Range Province presented by Menzies et al. [24] overlap with the St. Helens data. It is important to note that although the St. Helens results fall along the mantle array they exhibit a variation in 8180 along the array (Fig. 6) which suggests that the variation is due to interaction between mantle and crustal materials. Nevertheless even the silicic magmas have quite clearly not interacted significantly with old continental

crust because the ~ya values are too high. It is possible that the magmas have interacted with very young (Tertiary) crust however. The isotopic signature for such a crust would be difficult to distinguish from that of the mantle. The data quite clearly indicate that the Cascade magmas were not derived from oceanic crust nor sedimentary detritus that had been altered by seawater, nor from mantle enriched by subducted seawater fluids. Magmas derived from such a source would be expected to yield data points displaced to the right of the mantle array [25]. Fig. 6 shows the 8 7 S r / / 8 6 S r ratios plotted against 8180 and if SHl4 (which has anomalously light oxygen) is excluded, a positive correlation is evident. Such a positive correlation has been discovered in a wide variety of igneous complexes [24,31,32] and suggests incomplete mixing of materials such as by partial assimilation. The overall low 8JSO values suggests a mantle-derived component is dominant but the range in 8180 and correlation with 87Sr/86Sr suggests that some samples, notably SH15 are of magmas which have incorporated small but significant amounts of continental crust. The plutonic ejecta erupted with Mt. St. Helens include alumina-rich (A1203 = 22%) coarse-grained augite-hypersthene gabbros [4]. Verhoogen commented on the fact that A1203 is generally high at Mt. St. Helens compared with other Cascade volcanoes. However, the isotopic data clearly preclude the possibility of a large amount of sediment assimilation and, or, melting (cf. [4, p. 292]).

TABLE 5 Radiogenic isotope data Sample

87Sr/S6Sr

No.

+ 20rnea n

S H 11 SH4 S H 14 SH20 S H 17 S H 15 SH16 SH2 S[-I25

0.703745 + 13 0.703807 _+ 14 0.703745 _ 11 0.703755 ___18 0.702953 + 15 0.703966 + 12 0.703535 + 14 0.703601 __+30 0 . 7 0 3 6 0 7 + 18

~Sr

143Nd/14aNd

~Nd

206pb Z04pb

2OTpb 2o4pb

2ospb 2o4pb

+ 4.7 + 4.8 + 5.4 + 4.1 + 7.7 + 4.6 + 5.9 + 5.7 +5.8

18.761 18.770 18.812 18.839 18.862 18.813 18.831 18.785 18.756

15.542 15.536 15.566 15.555 15.564 15.546 15.567 15.559 15.546

38.361 38.409 38.468 38.477 38.406 38.443 38.473 38.422 38.361

__.~2 time a n

- 10.7 - 9.8 - 10.7 - 10.6 - 22.0 - 7.6 - 13.7 - 12.8 - 12.7

0.512883 + 22 0.512887 + 24 0.512918 _ 42 0 . 5 1 2 8 5 0 _+ 22 0.513035 _+ 22 0.512875 _+ 18 0.512943 + 18 0.512933 + 24 0.512937_+ 18

251

8.0

a

I

I

'ESr

I BE

A

0 5132

&

~~MORB

&

8180 (%0 SMOW)

J

0.5126

~ "

143Nd 144Nd

E1

()

*80 I

*160

420 I

12

BASIN AND R A N G E PROVINCE

~'T

7.0

6.0

*40 I

4

COLUMBIA VER BA SALTS

',~RI x'~k .... ~ ', >,.

0' 51 24

8

BE -4

X

'ENd x

ISLANDS

0 51213

5.0

I

I

05116 0.702

I

I 0 710 87Sr/86Sr

0706

0.5131 (::If 0 5130

//

143Nd

,,'

144Nd 015129

0,512~

/ //

80

/

I

0.7030

z L) a

20

0 718

A _

7C
6O I

I 0 714

-2,500yrs (exc $ H 4 ) ~

o

D] 0.7035

-16 0 736

,', c;\°°

mI

0 basalt [] andesite A dacite

x xx-T,-12 0.766

Fig. 7. Present-day Nd and Sr isotopic data for Mt. St. Helens samples compared with present-day values for MORB [22], Oceanic Islands (O1), Columbia River Basalts ]28], Basin and Range basalts (B) [23], and western U.S.A. crustal xenoliths and country rocks [28,29].

G\°°

%

2:.'%

+MOUNT ST HELENS xW USA CRUSTAL ROCKS

A I

- "---

I-8

I

07040

~l,000yrs

5C 40

Fig. 6. (a) d l 8 0 vs. 87Sr/86Sr. (b) 143Nd/144Nd vs. 87Sr/86Sr with contours for 8180 (excluding SH14). Symbols as in Fig. 1.

@

b

I

I

@

Fig. 8a shows the striking positive correlation of 87Sr/86Sr with Thornton and Tuttle (differentiation) index. A similar result is obtained with SiO2 (not shown). There is a suggestion in Fig. 8b that the more silicic samples erupted during the 2500-1000 year period (SH15 and SH20) have Sr which is slightly radiogenic for the Ca/Sr ratio of the magma. These plots show that the bulk chemistry of the Mt. St. Helens lavas is related to the mixing of material derived from a source with high BTSr/865r, such as the continental crust, with mantle-derived magma. However, the relationship

-

/

87Sr/86Sr

I

2,500 1,O00yrs

4

IO0 .--, / (m I ',am

Co/Sr


,' ~ A'~ A

5O -2,500yr~ k

...

0 7030

L

k

0.7035 B7Sr/

07040

B6 5r

Fig. 8. Correlations of 875r/86Sr with (a) Thornton and Tuttle (differentiation) index and (b) C a / S r ratio. Symbols as in Fig. 1.

252

could be direct, in terms of components added, or indirect in terms of the fractionation of magma which has undergone significant cooling as a result of interaction with colder crust, or, most likely, a combination of both. In Fig. 9 the isotopic data are shown plotted against time. This figure should be compared with u

i

O

0 5130 !43Nd 144Nd

A_N

A

0 5129 []

5128 07040

I

0

I

b rlAq

A

&--

0.7035 875r 865r 0 703C

O I

i

-C

7C

Z~

6~eO (%, $MOW',

[]

&--

6C

O

5.C

the equivalent chemical plot (Fig. 2). It appears from these diagrams that the pre-2500 year old samples were erupted over a very long period ( > 30,000 years) from a uniform reservoir with negligible assimilation of rocks with any isotopic contrast. During the 2500-1000 year old period there was an introduction of mantle-derived basalt (like SH17). This thermal perturbation can be expected to have resulted in further melting of the continental crust. This is a plausible explanation for the succeeding eruption of SH 15--with "high" 3180 and 87Sr/86Sr. Table 4 and Fig. 9 shows the Sr, Nd and O isotopic uniformity of the magma reservoir over the last few hundred years. Lipman et al. [8, pp. 638-640] noted the chemical similarity between products of the recent, 19th century and 17th century eruptions and speculated that these eruptions were all from the same magma reservoir. The isotopic data presented here indicate unequivocally that this is the case, and that negligible change has taken place in terms of assimilation for at least 500 years. One cannot assume that just because SH17 is basaltic with lowest Csr and 6180 and highest Nd and Sr content, it therefore represents parent magma to the earlier SH4, SH 11 and SH14 materials, and that these have been modified by combined assimilation/fractional crystallisation. On the other hand it would be an odd coincidence if the dacitic magma produced for the first 30,000 + years possibly by melting of contintal crust, was not related, at last thermally, to mantle melting if unequivocally mantle-derived magma was erupted subsequently.

A

I

L i

.d

7. Lead isotopes

[

O

18"85 206pb 204pb 1880

•

A

-

18.75

I

1

l

L 103 104 Years before present (1982)

0102

105

Fig. 9. I s o t o p i c c o m p o s i t i o n s versus time, s y m b o l s as in Fig. 1.

The Pb isotope data display a strikingly different kind of variation to that observed for Sr, Nd and O (Tables 4 and 5, Fig. 9). The Pb isotope ratios show a consistent pattern with time. The 2°6pb/2°4pb ratios increase to the "thermal perturbation" period 2500-1000 years ago. They then decrease to present-day values which are close to those of the oldest samples. There appears to be no direct relationship with major element chemistry, unlike for 87Sr/86Sr (Fig. 12). The most striking example of this is that the most basic

253 I

•

Juan de Fuca ridge Juan de Fuco sea mounts v Juan de Fuca pelagic c t a y & Mn nodules • Continental, derived sediments 4, Cascades voLcanics • Mt St Hetens 395

i

I

q

4.2 R~ Pb

3-8 3.4

1

3"C 39.0

02a

v

v ~

•

2.E 2oap___.bb 2°4pb

••V

•~

38.5

v

,IM,~e+

Zr ppm

+ 38.0

b

180

dtlA& 6 +

[]

HA ~'2,500yrs _~._ :m

87Sr 0-7036 Z°Tpb zo4pb 156

v ~

•

+

- - -

865r 0.7032

w ~ " "~v~ vv ~ 'w +~4-~ + • ÷v

0.7028

+.v~,

•

<,~---- ~ & ",, i

. . . . . .

x_


Ell \ \

n~

,, ,/,,

2,500- / - 1,000yrs

C I

~8-72

15 4.

18-0

A

, & "~

O2cr

J ,t

A

IO0 0 7040

15.5

t

140

158

157

I

18.76

I

\\\

",~, I

18.80 18.84 2o6pb/2O4pb

18-88

Fig. 11. Rb/Pb, Zr and STSr/86Sr vs. 2°rpb/Z°4pb. Symbols as in Fig. 1. (a) shows arrows to indicate change with time. [

I

18.5

19.0

195

2O6pb 2o4pb

Fig. 10. Pb isotopic data from this study and elsewhere [2,26].

(SH17) and the most acid (SH15) samples have virtually the same Pb isotope composition. In Fig. 10 the Pb isotope data are plotted with the data published [2,26] for the Cascades volcanics, western U.S.A. continental-derived sediments, and the Juan de Fuca Ridge, seamounts and deep-sea sediments. It was with these published data that Church [2] revised his previous genetic interpretations in favour of model of crustal contamination at the c r u s t / m a n t l e interface. Fig. 10 shows that the Mt. St. Helens data reported here fall in the middle of the field of Cascades volcanics in good agreement with the data reported by Church [2]. Church noted a tendency for more silicic Cascade volcanics to have more radiogenic 2°6pb/Z°4pb. Most importantly, however, the data can be seen to rule out subducted pelagic

sediments and sea mounts as components in the magma. In Fig. 11 the 2°6pb/2°apb ratios are plotted against R b / P b , Zr and 87Sr/86Sr. It can be seen that there is no correlation between 875r/86Sr and 2°6pb/2°apb. Indeed the samples which are most uniform in 87Sr/86Sr (i.e. those which are < 1000 years old) display the largest range in 2°6pb/2°4pb and the 2500-1000 year old samples with variable 87Sr/86Sr all have high 2°6pb/2°4pb. It has already been argued that the high Zr content of the more evolved 2500-1000 year old samples could be related to assimilation of crustal zircon. This is supported by the high 2°6pb/2°4pb ratios (Figs. 9, l lb). We suspect that the low Pb content has resulted in a relatively easy modification of the Pb isotopic composition of SH17, since the Sr, O and Nd data suggest it is relatively unmodified by crustal contamination. Whereas the Sr and Nd isotopic compositions relate closely to the chemistry of the samples the

254 Pb isotopic variations appear totally unrelated and this is most easily explained by transport of Pb via fluids in the deep crust. Notice that the Pb isotopes respond very sensitively to the 2500-1000 year crustal warming, basalt intrusion epipode and that whereas the other isotopic systems and chemistry readjust rapidly to a new post-1000 year regime, the Pb isotopes change gradually (Figs. 9, lla). The change in R b / P b (Figs. 2 and l l a ) is intriguing. Rb, Ba and Pb all increase with SiO 2 content indicating the (anticipated) incompatible behaviour of these trace elements. However, as has already been noted, the R b / B a ratios during the pre-2500 and post-1000 year periods are strikingly uniform ( - 0.1, Fig. 2) whereas the R b / P b is very variable although systematic. The Pb concentration is evidently being effected by processes other than fractionation. The relationships between this and 2°6pb/z°4pb, Zr and Yb leads us to the hypothesis that breakdown of old zircon has dominated the evolution of the Pb isotopes. This in turn suggests a crustal environment. The decoupling of Pb from other systematics suggests that Pb is being transported via a fluid phase in the deep crust.

8. Origins of the magmas From the data already presented we can place strong constraints on the source(s) of the Mt. St. Helens magmas. (1) The lack of a Eu anomaly and the high Sr contents preclude plagioclase as a significant source phase. This suggests the source lies in the lowermost crust or mantle. (2) The Sr, Nd and O isotope data indicate only very small amounts of contamination with old crustal material or metasediment. Altered oceanic crust or slab-derived fluids did not contribute significantly to the magmas. (3) The sequence of eruption (Figs. 2 and 9) and the intimate relationship between chemical composition and isotopic composition suggest that dacitic magma in a deep reservoir was invaded by, and mixed with basaltic magma, to produce lowSiO 2 dacite and andesite observed in the more

recent ( < 1000 year old eruptions). However, the basalt observed (SH17) has REE and other trace element concentrations which require a more complex relationship with the other magmas. (4) The uniformity of R b / B a ratio before and after the 2500-1000 year period indicates that there was one main reservoir for the magmas and that the incompatible trace element concentrations were being buffered by large volumes of material. This is difficult to conceive of at high crustal levels both because of the volume required and because uniform R b / B a ratios are difficult to maintain in an environment in which micas and feldspars are stable phases. For the same reason we doubt that phologopite was an important source phase. (5) The light REE enrichment, and low heavy R E E concentrations are suggestive of garnet or zircon as fractionating or residual source phases. The relationships of Yb concentration with Zr, R b / P b and 2°6pb/2°4pb render the previous interpretation of REE patterns from Mt. St. Helens in terms of an eclogitic source [3] equivocal. The suggested role of zircon and mild 180 enrichments suggest some slight crustal influence on the m a g m a reservoir. (6) The "depleted" isotopic signature of Nd and Sr suggests that the source region had lower R b / S r and higher S m / N d ratios than are observed in the samples. The R b / S r ratios are particularly difficult to explain since the absence of an Eu anomaly and the uniformity in C a / S r (Fig. 2) over long periods of time suggest that plagioclase fractionation was not great. One possible explanation is that the source had undergone a previous but recent fractionation but this merely removes the questions one step further back. (7) The mantle source underneath Mt. St. Helens was apparently not as depleted as the source regions of MORB. We would tentatively suggest that the local sub-continental lithosphere has characteristics intermediate between the source of MORB and that of many oceanic islands. A similar conclusion was reached for the source of the continental flood basalts by All6gre et al. [33]. The chronological information enables us to construct the following as a history of magmatism of Mt. St. Helens.

255

Stage 1:40,000-2500 years ago ( S H l l , SH4 a n d SH14). H e a t f r o m the mantle, s u p p l i e d b y an u p w a r d m o v e m e n t of p a r t i a l l y m o l t e n m a t e r i a l w a r m s the u p p e r m o s t m a n t l e a n d base of a y o u n g c o n t i n e n t a l crust. T h e l o w e r m o s t p a r t of this crust is a l m o s t entirely of mafic to i n t e r m e d i a t e c o m p o s i t i o n with negligible a m o u n t s of m a t e r i a l that h a d h a d an u p p e r crustal history. As the crust warms, p a r t i a l melts collate to form a b u o y a n t m a s s of silicic m a g m a w i t h high e n o u g h l i q u i d / c r y s t a l ratio to rise a n d form the earliest m a g m a at Mt. St. Helens. E m p t y i n g of the reservoir creates a hiatus d u r i n g which time m o r e melt collects until it too forms a mass b u o y a n t e n o u g h to o v e r c o m e frictional a n d regional forces a n d rise as a further mass. This repetitive process o f melt collation, m a g m a f r a c t i o n a t i o n a n d ascent continues with increased frequency as m a n t l e derived heat progressively w a r m s the u p p e r m o s t m a n t l e a n d lower crust.

lower ~Sr a n d slightly higher CNa relatively freq u e n t l y . A s the m a g m a s y s t e m cools the 2 ° 6 p b / 2 ° 4 p b a n d R b / P a ratios decrease p o s s i b l y reflecting the w a n i n g i m p o r t a n c e of b r e a k d o w n of zircon. A p a r t from this the levels of i n c o m p a t i b l e elements have merely suffered d i l u t i o n b y the mafic input.

Acknowledgements W e are grateful to J. Jocelyn, J. H u t c h i n s o n a n d J. Borthwick for technical assistance. A . N . H . a c k n o w l e d g e s Bruce C h a p p e l l for some stimulating discussion. The S.U.R.R.C. is s u p p o r t e d b y the Scottish Universities a n d the I s o t o p e G e o l o g y U n i t is also f u n d e d b y the N a t u r a l E n v i r o n m e n t Research Council. J. Hewitt, J. Bennett a n d D. M a c lean are t h a n k e d for their assistance in p r e p a r i n g the script. P.J. H a m i l t o n a n d K . G . Cox are t h a n k e d for their criticisms of earlier versions of the script.

Stage 2." 2500-1000 years ago (SH20, SH17 a n d SH15) (1) The m a n t l e - d e r i v e d basaltic m a g m a rises i n t o the p a r t i a l l y m o l t e n lower crust with the result that i m m e d i a t e l y there is an increase in t e m p e r a ture in the m a g m a system. This has the effect of increasing the degree of p a r t i a l melting p r o d u c i n g slightly m o r e mafic melts with (for e x a m p l e ) higher Z r a n d 2 ° 6 p b / 2 ° 4 p b (due to b r e a k d o w n of source zircon which then e r u p t (e.g. SH20). (2) This is i m m e d i a t e l y followed b y e r u p t i o n of the basaltic m a g m a itself (SH17). It has suffered s o m e o t h e r c o n t a m i n a t i o n n o t i c e a b l e in the 2°6pb//2°4pb ratios. In i n t r u d i n g the crustal m a g m a system it has (a) h e a t e d the m a g m a b o d y , (b) p a r t i a l l y crystallised, (c) c o n t a m i n a t e d the crustal m a g m a b o d y , (d) heated a n d p a r t i a l l y m e l t e d the crust at higher levels d u r i n g ascent. (3) As a result of melting at higher levels in the crust a new b a t c h of m a g m a with higher 8180, 87Sr/86Sr, R b / S r , R b / B a , etc., collects, rises a n d e r u p t s (SH 15).

Stage 3 : 1 0 0 0 years ago to present .The lower crustal m a g m a system, now elevated in t e m p e r a ture a n d c o n t a m i n a t e d by b a s a l t forms b u o y a n t masses of slightly m o r e mafic m a g m a (with slightly

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