Eiszeitalter u. Gegenwart
33
1—7 4 fig.
Hannover 1983
Distribution of the Heavy Minerals in the Downwind Tephra Lobe of the May 1 8 , 1 9 8 0 Eruption of the Mount St. Helens (Washington, USA) E T I E N N E J U V I G N E & S U S A N SHIPLEY * )
Correlation, marker bed, volcanic ash, heavy mineral, hornblende, hypersthene, quantitative special distribution, analyse, eruption, Upper Holocene. Washington, Mount St. Helens A b s t r a c t : Quantitative analysis of heavy minerals from tephra samples of the May 18 eruption of Mount St. Helens shows variations in the total content of heavy minerals as well as in the ratio hypersthene/hornblende. Both decrease along the lobe with increasing distance from the volcano and from the lobe margins to the axis. The basal dark-grey layer contains more hypersthene than the overlying light-grey layer. The observed sorting is a function of the specific gravity of the minerals, and grain shape. Variation across the lobe is attributed to the relative thicknesses of the dark- and light-colored layers. [Schwermineralverteilung i m Verbreitungsgebiet der Aschen v o m Ausbruch d e s Mount Saint Helens (Washington-U.S.A.) a m 18. 5.1980] K u r z f a s s u n g : Die stratigraphische Korrelation von vulkanischen Leithorizonten auf grund ihrer Schwermineralzusammensetzung bringt Probleme mit sich, da die Frage, ob die Zu sammensetzung im Verbreitungsgebiet konstant oder wechselnd ist, noch nicht eindeutig geklärt werden konnte. In Europa stellt sich dieses Problem im wesentlichen bei unterschiedlichen tuffitischen Lagen (Laachersee Tuff, Eltviller Tuff, Tuf de Rocourt). Dieser Beitrag betrifft die Aschen, die vom Mount Saint Helens-Vulkan am 18. 5. 1980 gefördert wurden. Davon wurden quan titative Analysen von Schwermineralien in der Absicht durchgeführt, tephrostratigraphische Pro bleme zu lösen. Es wurde nachgewiesen, daß der Gehalt an Schwermineralen mit der Entfernung vom Vulkan und zum Rand des Verbreitungsgebietes hin abnimmt. Die transparenten Schwer minerale bestehen im wesentlichen aus Hypersthen und basaltischer Hornblende. Das Verhältnis Hypersthen/Hornblende ändert sich schrittweise mit der Entfernung vom Vulkan und der Ent fernung von der Hauptachse zum Rand des Verbreitungsgebietes. Die Ursachen liegen in der unterschiedlichen Dichte und Form der einzelnen Minerale. Änderungen innerhalb des Verbrei tungsgebietes hängen ferner von der wechselnden Dicke der dunklen und hellen Schicht ab. Es ist auch nachgewiesen, daß die Prozentsätze der Schwermineralspektren innerhalb eines kleinen Ge bietes, unter Berücksichtigung der Fehlergrenzen (95 °/o Niveau), übereinstimmen. Introduction In the last 30 years t e p h r a layers, a n d especially their content of h e a v y minerals, h a v e been used as a basis for correlation of Q u a t e r n a r y deposits. I n some cases, differences in t h e mineralogical content a d d t o difficulties of correlating ash l a y e r s such as "Laachersee Tuff" ( J U V I G N E 1977, 1983), "Eltviller Tuff" (BIBUS 1 9 7 3 ; J U V I G N E & SEMMEL 1981), "tuf de R o c o u r t " ( J U V I G N E 1977b) a n d " r e t o m b e superieure" ( M A R T I N I 1971). H e n c e a n i m p o r t a n t question is t h e v a r i a t i o n in c o m p o s i t i o n of a single ash-fall w i t h distance from the source vent. ::
") Addresses of the authors: D r . E. J u v i g n e , University de Liege, Laboratoire de Geomor phologie et de Geologie du Quaternaire, Place du X X Aoüt, 7 - 4000 Liege, Belgium. — S u s a n S h i p l e y , Quaternary Research Center, University of Washington, Seattle, WA 98195. ISSN-Nr. 0424 - 7116 / 83 / 0033-XXX $ XXX © 1983 Schweizerbart'sche Verlagsbuchhandlung, D-7000 Stuttgart 1
2
Etienne Juvigne & Susan Shipley
I n this p a p e r , t h e example of t h e M a y 18 e r u p t i o n of M o u n t St. Helens is used t o demonstrate t h e possibility of correlating ash layers whose h e a v y mineral contents a r e q u a n t i t a t i v e l y different. T r a n s p a r e n t h e a v y m i n e r a l suites of t h e dark a n d t h e light l a y e r s P r e l i m i n a r y analyses of t h e composition of t h e d o w n w i n d basal d a r k - g r e y a n d o v e r lying light-colored ash layer w e r e reported b y S A R N A - W O J C I C K I et al. (1981). Since this w o r k , t w o layers h a v e been distinguished w i t h i n t h e light-colored l a y e r : a light-grey u n i t overlain by a t a n u n i t . F o r this p a p e r , samples from each layer w e r e collected in J u l y , 1982 at 6 localities along the axis of t h e lobe (Fig. 2 ) . T h e t a n layer w a s n o t discernible at the t w o w e s t e r n most localities, so analyses are l i m i t e d here to b u l k light-colored ash a n d elsewhere t o t h e d a r k - and l i g h t - g r e y layers. C a r e was taken t o r e m o v e any r e w o r k e d or u n d e r l y i n g m a terial from t h e samples. Method 1. 2. 3. 4. 5.
All the samples were t r e a t e d b y t h e following m e t h o d : W e t sieve t h r o u g h screens of 500 /im a n d 63 /urn; avoid abrasion of samples d u r i n g sieving. D r y 6 3 — 5 0 0 jum fraction in o v e n . Separate in purified b r o m o f o r m (density: 2.89) using a centrifuge (JUVIGNE 1979). M o u n t h e a v y minerals in C a n a d a balsam. C o u n t 200 t r a n s p a r e n t h e a v y minerals from each slide using t h e ribbon m e t h o d . Results
The most c o m m o n h e a v y minerals are o r t h o p y r o x e n e s a n d amphiboles. Investigations b y K U N T Z et al. (1981) of deposits of the M a y 18 e r u p t i o n h a v e s h o w n t h a t all a m p h i b o l e can be considered t o be h o r n b l e n d e a n d all o r t h o p y r o x e n e t o be hypersthene. A m p h i b o l e s can be essentially considered b r o w n hornblende. H y p e r s t h e n e plus amphiboles represent a t least 95 % of t h e total t r a n s p a r e n t h e a v y minerals of our samples. T h e o t h e r 5 °/o consists of traces of clinopyroxenes a n d u n k n o w n minerals. T h e percentages of amphiboles a n d o r t h o p y r o x e n e s from t h e 6 sites are s h o w n in figure 2. A t 5 of t h e 6 localities ( 1 , 3, 4, 5, a n d 6) t h e d a r k - c o l o r e d l a y e r contains a h i g h e r a m o u n t of h y p e r s t h e n e and a l o w e r a m o u n t of h o r n b l e n d e t h a n t h e light-colored l a y e r . After application of error at t h e 95.4 level of confidence o n l y 1 of 6 values in each set does not o v e r l a p , b o t h within t h e d a r k - and light-colored layers.
V a r i a t i o n of t r a n s p a r e n t m i n e r a l s u i t e s i n t h e l o b e Bulk samples of t e p h r a collected within 3 d a y s after t h e M a y 18 eruption were a n a lyzed. These samples were p r e p a r e d using methods described earlier. I n figure 1, t h e values of t h e r a t i o h y p e r s t h e n e / h o r n b l e n d e h a v e been plotted a t t h e corresponding s a m p l i n g sites. Isolines of this r a t i o h a v e been a d d e d t o suggest t h e m a i n t r e n d of v a r i a t i o n s . Isopach lines a r e also shown in figure 1 after S A R N A - W O J C I C K I et al. (1981, Fig. 336) t o h e l p position o u r results w i t h i n t h e lobe. T h e r a t i o h y p e r s t h e n e / h o r n blende clearly decreases w i t h distance from t h e v o l c a n o as well as from the m a r g i n s of t h e lobe to its axis.
Distribution of the Heavy Minerals in the Downwind Tephra Lobe
3
EXPLANATION
0
0
PERCENTAGES RELATING TO
Âť
PERCENTAGES R E L A T I N G TO THE DARK
1
ERROR IN PERCENT AT THE
1
1
0
50
1 100 DISTANCE
THE LIGHT GREY
1 150 FROM
95,4
LEVEL
1
LAYER
LAYER OF CONFIDENCE
1
200 2 5 0 KM M O U N T ST. H E L E N S
Fig. 1: Percentages of hypersthene and hornblende to all transparent heavy minerals from the dark and the light-grey layers from 6 localities along the axis of the lobe of the May 18 eruption of Mount St. Helens. Localities are shown in fig. 2.
Looking a t t h e separate percentages, 9 . 5 % t o 1 7 % h o r n b l e n d e a n d 8 1 . 5 % t o 8 9 % hypersthene are present in t h e transect closest t o source (about 130 k m from t h e volcano) a n d 3 4 % t o 45 % h o r n b l e n d e a n d 5 1 % t o 6 2 . 5 % hypersthene occur in t h e far distal transect ( a b o u t 600 k m from t h e volcano). A similar trend can be recognized in figure 2 w i t h respect t o t h e percentages from the 12 samples of b o t h t h e d a r k a n d light layers. O u r observations indicate sorting of t h e t r a n s p a r e n t h e a v y minerals in t h e p l u m e . Sorting along t h e lobe axis m a y be explained b y a difference in specific g r a v i t y which is l o w e r for h o r n b l e n d e (3 t o 3.3) t h a n for h y p e r s t h e n e (3.3 t o 3.5). Differences in grain shape m a y also influence p a r t i c l e t r a n s p o r t , t h e flatter, t a b u l a r h o r n b l e n d e t r a v e l l i n g f a r t h e r t h a n t h e prismatic h y p e r s t h e n e . W e a t t r i b u t e t h e symmetrical v a r i a t i o n of t h e h y p e r s t h e n e / h o r n b l e n d e ratios a b o u t t h e lobe axis t o changes in t h e p r o p o r t i o n of light-colored ash t o d a r k t h r o u g h o u t t h e lobe. T h e thickness of t h e lobe is controlled l a r g e l y b y variation in t h e thickness of t h e lightcolored (light-grey plus t a n ) l a y e r ( S A R N A - W O J C I C K I et al. 1 9 8 1 , Fig. 339). T h e d a r k -
Etienne Juvigne & Susan Shipley
4
grey ash remains relatively u n i f o r m in thickness across t h e lobe ( S A R N A - W O J C I C K I et a l . 1 9 8 1 , Fig. 3 4 0 ; D . D Z U R I S I N , w r i t t e n commun., 1 9 8 0 ; S . SHIPLEY, u n p u b . d a t a ) . C o m p o nents of b o t h d a r k - a n d light-colored layers w e r e p r o b a b l y deposited as a mixed l a y e r a t the lobe m a r g i n s . T h e higher p e r c e n t of h o r n b l e n d e relative t o hyperstene in t h e lightcolored layer corresponds well w i t h t h e isopach m a p . A comparison of o u r results w i t h those of K U N T Z et al. ( 1 9 8 1 ) t o look for a t r e n d from our w e s t e r n m o s t transect t o t h e source v e n t is n o t possible because the technical methods w e r e n o t t h e same ( J U V I G N E 1 9 8 3 ) a n d t h e turbulence w i t h i n the plume in t h e vicinity of t h e v o l c a n o was quite different t h a n t h a t d o w n w i n d . EXPLANATION "5-
- ISOPACH
•2,4
VALUES
- 8 —
ISOLINES
AFTER OF
LOCATION »2,7
OF
A.M. SARNA-WOJCICKI
RATIO RATIO
HYPERSTHENE HYPERSTHENE
OF E A C H PAIR
RATIOS F R O M
EACH
SET
OF OF
11981,
TO TO
SAMPLES
336);VALUES
IN
MILLIMETERS
HORNBLENDE
S A M P L E S : DARK 16
FIGURE
HORNBLENDE
AND LIGHT
FROM
SHORT
GREY
LAYERS
TRANSECTS
Fig. 2: Variation of ratio hypersthene/hornblende in the lobe of the May 18 eruption of Mount St. Helens. V a r i a t i o n of t h e h e a v y m i n e r a l s u i t e s a l o n g s h o r t t r a n s e c t s This investigation is of p a r t i c u l a r i m p o r t a n c e t o c u r r e n t E u r o p e a n t e p h r o s t r a t i g r a p h i c research. A t t e m p t s t o correlate t h i n , distal t e p h r a layers w i t h i n small areas (for example, J U V I G N E 1 9 8 3 ) h a v e been h i n d e r e d b y high v a r i a b i l i t y a m o n g h e a v y m i n e r a l suites. Three sets of 1 6 bulk samples w e r e collected in September, 1 9 8 2 to determine local v a r i a t i o n w i t h i n t h e heavy m i n e r a l suites. Each set w a s collected a l o n g transects a b o u t 1.5 k m l o n g ; samples were t a k e n a b o u t 1 0 0 m a p a r t . Set 1 w a s collected a p p r o x i m a t e l y 6 0 k m south of Wenatchee; set 2 , a b o u t 7 5 k m west of R i t z v i l l e ; a n d set 3 , about 1 0 0 k m southwest of S p o k a n e (Fig. 2 ) . T h e p r e p a r a t i o n technique described earlier was a p p l i e d to all samples. T h e greatest deviations are 1 1 % for h y p e r t h e n e a n d 1 0 % for hornblende, b o t h in set 3 (Fig. 3 ) . After application of s t a n d a r d error a t t h e 9 5 . 4 level of confidence t h e 1 6 values o v e r l a p in each set b o t h for hypersthene a n d h o r n b l e n d e .
Distribution of the Heavy Minerals in the Downwind Tephra Lobe
SET
SET 2
SET
5
3
X
CL >X
o X
100
200 DISTANCE
300 FROM MOUNT
4 0 0 KM ST. HELENS
Fig. 3: Percentages of hypersthene and hornblende to all transparent heavy minerals for 3 sets of 16 bulk samples collected along short transects. D a t a points represent minimum and maximum values at each site; remaining values lie along the dashed lines. Vertical bars represent probable error at 95.4 confidence level.
In spite of the variations along each s h o r t transect, the m e a n values for each set of 16 samples a r e consistent w i t h the general t r e n d of variation of h e a v y mineral suites dis足 cussed earlier; mean values are p l o t t e d in figure 1. V a r i a t i o n of t h e a m o u n t o f h e a v y m i n e r a l s i n t h e l o b e T h e a b u n d a n c e of h e a v y minerals is h i g h l y variable t h r o u g h o u t the lobe. H e n c e q u a n t i t a t i v e analyses were m a d e to determine t h e exact a m o u n t of h e a v y minerals ( t r a n s 足 p a r e n t plus o p a q u e ) present. Method 1. D r y b u l k sample in oven. 2. Weigh 100 m g from each sample. 3. S h a k e s a m p l e in 100 m l purified b r o m o f o r m using centrifuge tube. T h e density of b r o m o f o r m was checked t o be 2.89 w h e n used. 4. Centrifuge 5 minutes a t 4 0 0 0 r p m . 5. Freeze t h e b o t t o m of t h e centrifuge t u b e in nitrogen. 6. S h a k e t h e tube. 7. R e p e a t steps 4, 5, a n d 6.
Etienne Juvigne & Susan Shipley
6
8.
R e p e a t S t e p s 4 a n d 5.
9. R e m o v e t h e floating light fraction and rinse t u b e w i t h acetone. 10. W a r m frozen b r o m o f o r m , p o u r o n t o p a p e r filter a n d rinse w i t h acetone. 1 1 . Weigh h e a v y minerals w i t h a balance precise t o 1 0 g. — 5
Results T h e results a r e p l o t t e d as percentages in figure 4a. I n figure 4b t h e lowest, t h e m e a n , a n d the highest percentages of each transect are represented w i t h respect to distance f r o m t h e volcano in o r d e r t o sketch t h e t r e n d of the v a r i a t i o n s . T h e r e is a clear decrease in t h e a m o u n t of h e a v y minerals d o w n w i n d in the lobe. T h e a m o u n t in t h e Missoula transect is m o r e t h a n 100 times lower t h a n t h a t in the westernmost transect. Variation a m o n g the values a l o n g each transect is n o t as great; t h e ratios between t h e highest and lowest values v a r y f r o m about 2 (transect 75 k m west of Ritzville) to 6 ( M i s soula transect). M o r e o v e r , t h e highest a m o u n t of h e a v y minerals is never at the m a r g i n of the lobe b u t does n o t correspond w i t h the lobe axis. Conclusions Sorting of h e a v y minerals w i t h i n the p l u m e of t h e M a y 18 e r u p t i o n of M o u n t St. Helens has been s h o w n to be based on their specific g r a v i t y a n d grain shape. The a m o u n t EXPLANATION 4A
•••30•••
•7,4
ISOPACH
AFTER
PERCENTAGES
A.M. SARNA-WOJCICKI
O F HEAVY
MINERALS
ET
AL.C1981, FIGURE
336)
1 .0,05 •3,7
•2,4
P
•0,03
•2.5
SPOKANE
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WALLA WALLA
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, /
I ' I
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HELENS
114"
4B
EXPLANATION
«
HIGHEST RATIO
o
MEAN
v
LOWEST
RATIO RATIO
Fig. 4: Amount of heavy minerals (opaque and transparent) in bulk samples from the lobe of the May 18 eruption of Mount St. Helens: 4a. Percentages at different localities within the lobe 4b. Lowest, mean and highest percentages for each transect.
Distribution of the Heavy Minerals in the Downwind Tephra Lobe
7
of the heaviest particles decreases d o w n w i n d from t h e volcano. This sorting agrees w i t h other v a r i a t i o n s within t h e l o b e as described b y S A R N A - W O J C I C K I et al. ( 1 9 8 1 ) for grain size and b y F R U C H T E R et al. ( 1 9 8 1 ) for chemical composition. O n e of t h e most i m p o r t a n t consequences of t h e sorting is t h e q u a n t i t a t i v e v a r i a t i o n of t h e mineral assemblages w i t h i n t h e lobe. M o r e o v e r , great v a r i a t i o n s of the h e a v y mineral suites occur locally, but after application of s t a n d a r d error, all values o v e r l a p . O u r results m a y be used f o r correlation of o t h e r ash layers using mineral assemblages, b u t further w o r k is needed t o evaluate v a r i a t i o n in other minerals. Acknowledgments This research was c o n d u c t e d at the Q u a t e r n a r y Research C e n t e r , U n i v e r s i t y of W a s hington, a n d w a s supported b y a grant to o n e of us (E. J U V I G N E ) from the N o r t h A t l a n t i c T r e a t y O r g a n i z a t i o n . T h e U . S . Geological S u r v e y p r o v i d e d m a n y of t h e b u l k samples. References BIBUS, E. (1973): Ausbildung und Lagerungsverhältnisse quartärer Tuff vorkommen in der Wetter au. — Notizbl. hess. L.-Amt Bodenforsch., 101: 346—361; Wiesbaden. FRUCHTER, J. S., ROBERTSON, D . E., EVANS, J. C , OLSEN, K . B., LEPEL, E. A., LAUL, J . C , ABEL, K . H , SANDERS, R . W., JACKSON, P. O., WOGMAN, N . S., PERKINS, R. W., VAN TUYL, H . H , BEAUCHAMP, R. H., SHADE, J. W., DANIEL, J. L., ERIKSON, R. L., SEHMEL, G. A., L E E , R. N.,
ROBINSON, A V . , Moss, O. R., BRIANT, J. K . & CANNON, W. C . (1980): Mount St. Helens ash characterizations, May 18, 1980, eruptionchemical, physical, mineralogical and biological properties. — Science, 209: 1116—1125; Washington. JUVIGNE, E. (1977a): La zone de dispersion des poussieres emises par une des dernieres eruptions du volcan du Laachersee (Eifel). — Z. Geomorph. N . F., 21: 323—342; Berlin-Stuttgart. — (1977b): Zone de dispersion et äge des poussieres volcaniques du tuf de Rocourt. — Ann. Soc. Giol. Belg., 100: 13—22; Liege. — (1979): Scheidetrichtermethode oder Zentrifugaltrennung zur quantitativen Gewinnung von Schwermineralen. — Senckenbergiana marit., 11: 171—174; Frankfurt a. Main. — (1983): Two different volcanic ash-falls of Alleröd age in High Belgium. — Geologie enMijnbouw; s'Gravenhage. — [In press.] — & SEMMEL (1981): Un tuf volcanique semblable a l'Eltviller Tuff dans les loess de Hesbaye (Belgique) et du Limbourg neerlandais. — Eiszeitalter u. Gegenwart, 31: 83—90; Hannover. KUNTZ, M. A., ROWLEY, P. D . , MACLEOD, N . S., REYNOLDS, R. L., MCBROOME, L. A., KAPLAN,
A. M. & LIDKE, D . F. (1981): Petrography and particle-size distribution of pyroclasticflow, ash-cloud, and surge deposits. In: P. W. LIPMAN & D . R. MULLINEAUX (Eds.): The 1980 Erup tions of Mount St. Helens, Washington. — U.S.G.S. Prof. Paper, 1250: 525—539; Washington. MARTINI, J. (1971): Recherche de retombees volcaniques quaternaires dans le sud-est de la France et la Suisse occidentale. — Arch. Sei. Gen., 23: 641—674; Geneve. SARNA-WOJCICKI, A. M., SHIPLEY, S., WAITT, R. B., Jr., DZURISIN, D . & WOOD, S. H .
(1981):
Areal distribution, thickness, mass, volume, and grain size of air-fall ash from the six major eruptions of 1980. — In: P . W. LIPMAN & D . R. MULLINEAUX (Eds.): The 1980 Eruptions of Mount St. Helens, Washington. — U.S.G.S. Prof. Paper 1250: 577—600; Washington. Manuscript accepted on 25. 1. 1983.
Etienne Juvigne & Susan Shipley