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Contrasting fractionation patterns for sequential magmas from two calc-alkaline volcanoes in Central America
Journal of Volcanology and Geothermal Research, 6(1979)217--240
217
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
CONTRASTING FRACTIONATION PATTERNS FOR SEQUENTIAL MAGMAS FROM TWO CALC-ALKALINE VOLCANOES IN CENTRAL AMERICA
L.G. WOODRUFF I *, W.I. ROSE, Jr. 1 and W. RIGOT 2
~Michigan technological University, Houghton, M149931 (U.S.A.) 2Phoenix Memorial Laboratory, University of Michigan, Ann Arbor, M148104 (U.S.A.) (Received January 30, 1978; revised and accepted February 8, 1979)
ABSTRACT Woodruff, L.G., Rose, W.I., Jr. and Rigot, W., 1979. Contrasting fractionation patterns for sequential magmas from two calc-alkaline volcanoes in Central America. J. Volcanol. Geotherm. Res., 6: 217--240. Two sequences of alternating units of lavas and pyroclastic rocks from two volcanoes in Central America were sampled and analysed. The rocks from Izalco, the smaller, more mafic cone, show a continuous increase in SiO~, K20 , Zr and La, and a decrease in CaO, MgO, Co and Cr with time. These trends are shown to be consistent with progressive crystal fractionation. Atitl~n, a larger and more silicic volcano, displays a discontinuous trend in which geochemical reversals are thought to reflect the development of a series of small bodies of zoned magma. The lavas of each zoned section show decreases in SiO2, K20, Hf and Ba, and increases in CaO, MgO, Co and V from the bottom of the section to its top. The different trends at Izalco and Atitl~n are believed to reflect the restrictions cone height places on the effects of crystal fractionation. As cone height increases, the intervals between eruptions progressively lengthen, allowing more time for crystal fractionation to proceed. The two volcanoes may represent an early and a later stage in the general evolutionary development of large composite cones. Despite the remarkable regularity of the alternating flows and pyroclastic eruptions, no systematic chemical variations were recognized between the two types of lava.
INTRODUCTION
The Quaternary volcanic chain of Central America stretches from Mexico to Costa Rica along a converging plate margin. The high composite volcanoes of the chain are typically composed of high-A1203 basalt or andesite. Recent work on one of these cones, Santa Marfa volcano, Guatemala, has demonstrated a general stratigraphic progression to more silicic lavas. Increasingly long periods o f crystal fractionation may cause this variation with time (Rose et al., 1977). *Current address: Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, U.S.A.
218
Katsui et al. (1975) explained the large-scale chronological trend from mafic to felsic compositions of Mashu volcano, East Hokkaido, Japan, as continuous fractional crystallization of a low-alkali tholeiitic parent over a time span of 13,000 years. In the early stages of the Mashu activity, eruptions of mafic lavas followed shortly one after another, while in the latter stages, eruptions of more silicic lavas were separated by longer and longer repose times. The same type of progressive changes with time of eruptive activity is inferred at Santa Marfa. There the height of the cone was suggested to be the controlling factor in eruptive frequency (Rose et al., 1977). As cone height increases, the increase in lithostatic pressure versus hydrostatic pressure makes it increasingly difficult for a magma to reach the surface; repose periods become longer, allowing more time for crystal fractionation to take place {Rose et al., 1977}. The over-all composition of a volcano would become less mafic with time. Small reversals of this general trend are noted in both studies and are assumed to reflect the development and subsequent eruption of zoned magma chambers (Katsui et ai., 1975; Rose et al., 1977). Work on the 1974 eruption of Volc~n Fuego, Guatemala, demonstrated that a zoned magma chamber may develop in high-A1203 basalt by the process of crystal fractionation over a relatively short repose time of one to two years (Rose et al., 1978). To test the application of crystal fractionation to chemical trends of composite cones, t w o interlayered lava/pyroclastic sequences from two Quaternary volcanoes in Central America were sampled and analyzed (Fig. 1).
16"
(~
,,'--
GUATEMALA
jJ i
-
~
f
~. ," Izalce
HONDURAS
0
80kin
EL " \ - - ' - " ' - ' ~ i
Fig. 1. Location map of Atitl~n and Izalco volcanoes. SAMPLING S T R A T E G Y AND SAMPLING SITES
McBirney {1976) has shown that the mineralogy of andesitic lavas correlates statistically with the height of volcanic cones in island arcs and continental margins. Hornblende and biotite are more c o m m o n in the lavas of
219
volcanoes with high summit elevations than they are in lavas of comparable composition of lower elevations. This relation suggests that lavas from higher vent elevations may reflect a greater degree of crystal fractionation or possibly a higher H:O content. To attempt to locate more highly fractionated samples, sampling for this study was confined to the top of the volcanoes. In addition, this strategy takes advantage of good exposures and is less affected by local variation in depositional area, as would occur on the flanks of the cones. The t w o selected cones are of different height, volume, activity pattern and lava composition. Izalco volcano is a small, recently active, basaltic cone in western E1 Salvador. It had nearly continuous eruptions of flows and pyroclastic debris from its beginning in 1770 through 1958 (Mooser et al., 1958). Since 1958 Izalco has been dormant, with the exception of a small flank eruption in 1966 (Rose and Stoiber, 1969). Only fumarolic activity persists at present (Stoiber et al., 1975). Samples were collected from an interlayered flow/pyroclastic sequence (three flows and three pyroclastic units). The exposure that was sampled is on the south wall of the explosion crater that occupies the top of the cone. The units vary in thickness from one to three meters and apparently form a continuous sequence with no large time breaks between units. The samples probably represent the latest activity of Izalco prior to 1958 and are believed to have been erupted sometime in the 1950's. Volc~n Atitl~n is one of the large andesitic composite cones that make up the volcanic chain in Guatemala. Its historic activity consists of few widely-spaced (80--100 years) eruptions. Atitl~n has been dormant since 1853 with only minor fumarolic activity since that time (Mooser et al., 1958). A series of interlayered lava flow and pyroclastic layers are exposed in a scarp on the south side of the volcano. Three flow and three pyroclastic layers that crop o u t a b o u t 70 m below the summit were sampled. The two lowermost units in the sampled sequence are separated from those above by an interval of a b o u t nine meters in which no rocks are exposed. This break in sequence may represent several unexposed units. There is no obvious evidence of long hiatus between the other layers although subtle breaks may occur. A N A L Y T I C A L METHODS
The major element and trace element analyses of six Atitlfin and eight Izalco samples are given in Table 1. The major oxides were determined using a Perkin--Elmer 303 atomic absorption spectrophotometer. P2Os was determined using a p h o s p h o m o l y b a t e complex procedure with a Bausch and Lamb colorimeter (Maxwell, 1968). H20 + was determined using a modified Penfield m e t h o d after Shapiro and Brannock (1955). A Perkin--Elmer 360 atomic absorption s p e c t r o p h o t o m e t e r equipped with a graphite furnace (HGA 2100) was used to determine Ni and V. Strontium and Zr were determined with a Philips electron diffractometer-spectrometer with m o l y b d e n u m radia-
8.23
-403 123
17.91 14.86 2.75 3,18 1.06 1.06 0.30 0.32 1.53 1.70 1.71 1.85 15.0 16.0 25.4 24.8 8.24 6.38 258 238 29.3 30.6
14.49
471 402 134
24.1
1.09
97.97
53.2 18.50 8.42 3.75 8.62 3.30 1.04 0.75 0.17 0.22
IZ-22
14.75 3,07 1.08 0.32 1.54 1.82 11.0 24.5 6.35 239 30.2
8.3
443 378 117
25.8
1.22
100.34
--
54.7 19.11 8.35 3.92 8.76 3.39 1.09 0.84 0.18
IZ-3 ~
* T o t a l Fe as F e O 2 . i f l o w unit; 2p y r o c l a s t i c u n i t .
Zr La Ce Sm Eu Lu Th Hf Ni Co Cr V 8e
Ba
23.7
1.2
98.42
Total
Cs Rb Sr
H20"
P20&
CaO Na20 K20 TiO 2
MgO
54.0 18.19 8.26 3.79 8.61 3.34 1.08 0.74 0.17 0.24
SiO~ AI203 FeO*
IZ-I l
15.74 3.0 1.07 0.31 1.69 1.64 10.0 30,1 9.33 263 30.8
7.19
465 346 118
26.6
1.18
98.49
--
53.1 18.66 8.41 4.44 8.96 3.01 0.91 0.82 0.18
IZ-4a:
15.08 2.97 1.03 0.27 1.45 1.74 14.0 26.7 II.I 248 28.1
7.28
474 395 --
26.4
1.14
98.24
--
52.6 18.82 8.51 4.30 8.89 3.20 0.95 0.80 0.17
IZ-4b ~
15.02 2.43 1.02 0.31 1.67 1.71 15.0 28.3 11.29 281 28.7
7.55
461 373 114
22.5
--
99.08
53.6 19.08 8.40 4.21 8.49 3.40 0.95 0.77 0.18
IZ-51
16.79 2.68 1.07 0.30 1.81 1.68 14.0 29.3 9.87 258 30.9
8.76
472 398 119
22.9
1.15
99.69
53.1 19.16 8.77 4.37 9.17 3.26 0.99 0.70 0.17
IZ-6a ~
14.32 2.65 1.01 0.28 1.38 1.70 16.0 26.6 8.74 256 30.2
7.46
461 414 --
27.6
1.10
101.99
53.1 20.35 8.88 4.58 9.68 3.44 1.01 0.78 0.17
IZ-6b 2
22.81 3.34 1.26 0.30 1.63 3.29 17.5 24.9 13.29 212 24.4
10.66
504 475 151
27.3
1.17
101.39
54.9 19.18 8.60 4.29 8.19 3.87 1.10 1.01 0.25
AT-I 2
25.62 4.30 1.27 0.31 2.38 3.80 16.0 19.4 9.78 165 20.4
13.34
517 543 175
36.8
1.34
99.9
55.8 19.51 7.16 3.38 7.35 4.09 1.31 1.04 0.26
AT.21
C h e m i c a l a n a l y s e s o f e i g h t I z a l c o a n d six A t i t l ~ n s u m m i t s a m p l e s ( o x i d e s i n w t . %; t r a c e e l e m e n t s in p p m )
TABLE 1
19.47 3.35 1.19 0.27 2.86 3.82 21.0 20.4 10.49 168 24.0
11.87
472 607 174
33.3
1.43
100.06
57.6 17.95 7.33 3.38 7.25 4.00 1.34 1.02 0.19
AT-3 s
24.40 3.73 1.26 0.30 2.96 3.61 22.0 21.3 12.17 188 21.6
12.87
520 564 175
32.8
1.46
99.0
--
56.1 17.28 7.77 3.86 7.44 3.96 1.31 1.04 0.24
AT-41
20.78 2.94 1.12 0.25 2.33 2.97 17.0 23.7 7.79 211 23.1
10.13
1.16 26.7 517 499 148
98.56
53.8 18.17 8.34 4,07 8.21 3.60 1.03 0.94 0.20 0.2
AT-52
27.47 3.85 1.41 0.27 3.46 4.13 12.5 16.5 12.33 165 22.5
13.32
1.49 45.7 543 601 189
98.17
56.4 17.54 6.87 3.31 6.16 3.95 1.52 0,99 0.23 1.2
AT-61
t~
221
tion (Reynolds, 1963). All other trace elements were determined by instrumental neutron activation at the Phoenix Memorial Laboratory at the University of Michigan. The samples were irradiated at a flux of 1.5 × 1013 neutrons cm -2 s-1 for ten hours. Two counts were made after decay periods of seven and twenty-one days. U.S.G.S. reference rock standard GSP-1 and an artificial liquid standard prepared by Ward Rigot of the Phoenix Memorial Laboratory were used. U.S.G.S. reference rock standards GH, G2, BCR-1 and W-1 were used in the other chemical analyses. Values from Flanagan (1973) were used for the standards. COMPARISONS AND CONTRASTS
Analyses of the lavas sampled for this study, along with analyses of additional summit samples, are compared with data from previous work on these volcanoes. Fig. 2 shows that the summit analyses are the most silicic for each cone. This tendency is not as clearly demonstrated at Atitl~m as it is at Izalco, perhaps because there is little control (e.g. time of deposition, location of vent area) for the other Atitl~n samples.
O0 0
o
-
Izalco % MgO
I OSUMMITANALYSLS
] • AOOITIONALSUMMITANALYSES 0 OTHERANALYSES I
56
52
is
s'o
%SiO2
0
4
0
Atitl~n
0
0A
0
% MgO D
3 i
• SUUilT ANALYSES / • AOOITIOtlALSUMMITANALYSES 00THLN ANAL~$[U
]
I
5'0
52
s6
% SiO2 Fig. 2. Plot o f MgO vs. SiO 2 for both volcanoes, showing the comparmon o f summit analyses with analyses from previous studies.
222
Fig. 3 shows that the analyses for both volcanoes fall well within the calcalkaline trend defined by Central American volcanic rocks. The Atitl~n group, averaging 55.8% SiO2 (range 53.8--57.6%)is more silicic than the Izalco group, averaging 53.4% (range of 52.6--54.7%}. Physical differences between the two volcanoes are emphasized in Fig. 4 F
• Tertiary
Al
\M
Fig. 3. AFM plot of the Quaternary and Tertiary Central American volcanic rocks showing the different fields occupied by Izalco and Atitl~n. IZALCO 13 48'9"N 89°38'1°W
ATITLAN 14 35'3"N 91°10'9"W 3525 m
Nearly continuous Strombolian activity producing basaltic lava
S
N
1965 m
.." Widelyspaced[80-100 .....--"'" years)Vulcanianactivity •
producingandesitic lava
Fig. 4. Schematic representation of the physical contrasts between Izalco and Atithin. Diff e r e n c e s include height, volume, eruptive type and frequency, and rock type.
223 Atitl~n's more silicic composition is inferred to be a direct reflection of its significantly greater height, volume, age and its lack of recent activity. Izalco, which was formed in 1770, is much younger than Atitl{m. Santa Mafia volcano, which has been tentatively assigned an age of 30,000 years (Rose et al., 1977) is similar to Atitl~n in height, location and composition. PETROGRAPHY
The Izalco lavas are basaltic-andesites with plagioclase (An~5, ranging from Anss to AnTs), olivine, augite and an opaque phase as phenocrysts. Atitl~m's lavas are similar, b u t contain hypersthene and have a slightly more sodic plagioclase (Ane0, ranging from Ans5 to ANT0). The pyroxenes of Izalco and Atithin plot at the high-temperature end of the Skaergaard trend and represent typical pyroxenes for basaltic rocks. The augites in the Izalco lavas have an average composition of En35FslgWo46. The two pyroxenes found at Atitl~n have average compositions of En3sFslTWo48 for the augites, and En66Fs30Wo4 for the hypersthenes. STRATIGRAPHIC PLOTS OF THE PETROGRAPHIC MODES Modal analyses and descriptions of the analyzed samples are given in Table 2. The modes were determined using a point counter and represent only the phenocrysts. An arbitrary size limit of less than 0.1 mm was used to delineate crystallites from phenocrysts. The petrographic modes for the lavas of the t w o volcanoes have slightly different minerals and proportions. The Izalco groundmass percentages range between 52 and 58%. For most of the samples a simplified relationship of relative phenocryst abundance may be expressed by: plagioclase > clinopyroxene = olivine > opaque phase Hypersthene is n o t present in most of the rocks. Fig. 5 is a stratigraphic plot of the phenocryst percentages. A continuous trend with plagioclase decreasing and opaques increasing from b o t t o m to top is indicated. Olivine and clinopyroxene do not vary systematically. There are no suggestions of breaks or reversals. The Atitl~n modes present a pattern that differs from that of Izalco. The lavas have 5 3 - 6 4 % groundmass and show a slight increase in percentage from the b o t t o m of the section to the top. The phenocrysts are present in slightly different proportions than at Izalco: plagioclase > clinopyroxene > olivine > opaque phase > orthopyroxene Fig. 6 is a stratigraphic plot of the observed modes. Changes in slope suggest that the sequence may not be continuous. Breaks in the trend are inferred between AT-3 and AT-4, and AT-4 and AT-5. The break observed in the stratigraphic sequence between AT-4 and AT-5 is marked and is presumed to rep-
Groundmass % remarks
Opaques % average size
Olivine % average size remarks
C lin op yr oxen e % average size remakrs
Plagioclase % average size remarks*
Phenocrysts
56.0 microlites of plag, cpx, ol and op interstitial with glass
2.3 0.2 m m
3.2 0.5 m m some alteration to iddingsite
4.3 2.5 m m inclusions of gm and op
33.5 2 mm An6s ; zoned glass and cpx inclusions
IZ-1
58.4 microlites of m o s t l y plag and some cpx, ol and op interstitial with glass
2.1 0.2 m m
4.9 0.3 m m resorption rims
2.8 0.8 m m few glass inclusions
31.5 1 mm An6~; zoned few glass inclusions
IZ-2
52.9 a b u n d a n t microlites of plag and cpx with some ol and op, interstitial with glass, a b u n d a n t apatite inclusions
1.9 0.3 m m
4.9 0.5 m m some resorption rims
6.8 1.3 m m
33.5 1.3 m m An75; zoned with mi nor glass inclusions
IZ-3
Petrographic mode s and p h e n o c r y s t descriptions for Izalco and Atithln
TABLE 2
57.0 dark brow n glass with few microlites
1.3 0.2 m m
6.2 0.6 m m
4.7 0.8 m m
30.6 1.3 m m An~0; zoned s ome inclusions t h a t follow zones
IZ-4
42.5 large microlites of plag with cpx and op interstitial with glass
1.7 0.3 m m
3.4 0.3 m m reaction rims of cpx?
4.1 1.1 m m inclusions of plag and op
36.3 1.3 m m An~0; zoned; glass inclusions t ha t follow zones
IZ-5
48.6 few microlites in dark b r o w n glass
1.6 0.15 m m
3.6 0.4 m m
4.4 1.5 m m
38.6 1.2 m m Anss; zoned, few inclusions of glass
IZ-6
b~ b~
64.0 dark brown glass w i t h few microlites
0.2 0.1 m m
3.6 0.8 mm
5.4 0.7 mm
27.0 1.0 m m An~0; z o n e d w i t h glass inclusions
AT-1
59.0 microlites of plag and cpx interstitial w i t h glass
1.5 0.5 mm
1.0 0.5 mm
0.2 1.5 m m
4.5 1 mm
33.5 2.5 m m ANss; zoned w i t h glass inclusions
AT-2
54.0 abundant microlites of plag, cpx, and op interstitial w i t h glass, abundant apatite inclusions
2.2 0.3 mm
1.0 0.5 mm
0.3 0.7 mm
4.5 0.8 mm
38.0 2.5 mm AN~0; z o n e d w i t h inclus i o n s o f glass
AT-3
55.1 dark brown glass w i t h f e w microlites
2.5 0.2 mm
2.0 0.2 mm
1.2 0.8 mm
8.2 1.1 m m some resorption rims
31.0 2.5 mm An55; z o n e d with inclusion o f glass
AT-4
* a p = a p a t i t e , c p x = c l i n o p y r o x e n e , ol = olivine, o p = o p a q u e p h a s e , o p x = o r t h o p y r o x e n e ,
Groundmass % remarks
a v e r a g e size
%
Opaques
a v e r a g e size
Olivine %
Orthopyroxene % a v e r a g e size remarks
Clinopyroxene % a v e r a g e size remarks
Plagioclase % a v e r a g e size remarks
Phenocrysts
59.5 dark brown glass w i t h few microlites
2.5 0.15 mm
2.0 0.8 mm some resorption rims
6.0 0.8 mm
30.0 1.8 m m A n s s ; glass inclusions following zone
AT-6
p l a g = p l a g i o c l a s e , g m ffi g r o u n d m a ~ .
52.3 large microlites o f p l a g with cpx and op interstitial w i t h glass, a b u n d a n t apatite inclusions
1.0 0.15 mm
2.0 0.2 mm
0.1 0.5 mm
6.0 0.5 mm
38.0 0.8 mm An55; s u b h i d ial, z o n e d w i t h glass inclusions
AT-5
t~
f~
226 100
100 Covered interval
?-~"
PIeD
10
!
Cpx
i
tD.
0.1 bottom
top IZ,5
IZ'4
IZ'3
IZ,2
IZ'I
Plsg ~ ' - ~ - ~ . . . . ~
10 c~ Cpx
? i"-,
1
IZ'6
Probable Reversal
0.1 bottom AT.6
top AT'5
AT,4
AT.3
AT,2
Fig. 5. Stratigraphic plot of the petrographic modes determined for Izalco (plag = plagioclase, cpx = clinopyroxene, ol = olivine and op = opaque minerals). Fig. 6. Stratigraphic plot of the petrographic modes determined for Atitl~n (plag = plagioclase, cpx = clinopyroxene, op = opaque minerals, ol = olivine and opx = orthopyroxene). Missing units are inferred between AT-4 and AT-5 and the trends of the phenocrysts are extrapolated into the covered interval. r e s e n t u n e x p o s e d o r missing u n i t s in t h e s e q u e n c e . T h e r e is n o o b v i o u s surface e x p r e s s i o n o f t h e p r o p o s e d b r e a k b e t w e e n AT-3 a n d AT-4, b u t it is s t r o n g l y suggested b y t h e m o d a l data. T h r e e s e p a r a t e g r o u p s are also suggested b y t h e p l o t o f c l i n o p y r o x e n e versus t h e o t h e r m a f i c m i n e r a l s in Fig. 7. I t m a y be possible t h a t t h e differe n c e s in t h e p r o p o r t i o n s o f m a f i c p h e n o c r y s t s m a y be used t o i d e n t i f y differe n t b a t c h e s o f m a g m a . H e r e it is p r o p o s e d as criteria f o r b a t c h b r e a k b e t w e e n AT-3 a n d AT-4. MAJOR
AND TRACE ELEMENT
COMPOSITION STRATIGRAPHIC TRENDS
T h e s t r a t i g r a p h i c c h a n g e s o f t h e p e t r o g r a p h i c m o d e s are e l u c i d a t e d b y the c h e m i c a l v a r i a t i o n s o b s e r v e d in the lavas. S t r a t i g r a p h i c p l o t s o f selected m a j o r o x i d e s a n d t r a c e e l e m e n t s f o r t h e Izalco s e q u e n c e are given in Fig. 8. SiO2 a n d K : O increase slightly a n d CaO a n d MgO decrease f r o m t h e b o t t o m (IZ-6) t o t h e t o p (IZ-1) o f t h e s e q u e n c e . T h e t r a c e e l e m e n t s have less c o n s p i c u o u s trends, b u t t h e r e is an increase in Zr a n d La, a n d a decrease in Co a n d Cr
AT'I
227
• I ,AT._4/
7 CPX
%
,~--~-. . . . . . . ,-T.~'~ I 0
6
OJ
~ r~ AT'2 ",e
AT'l/ I AT'3 / e I
OL+OPX+ OP %
Fig. 7. Plot of the observed percentages of clinopyroxene cs. the sum of the observed percentages of olivine, orthopyroxene and opaque minerals for Atitl~n.
::[i
CoO9.0
8.5 I
-
-
MoO4.0
'"-..-_~._~,~
3.5
10
Cr
~
::0[
\ \
I
\\,
13 La11
=
Co
bottom
IZ.6
IZ.5
IZ.4
IZ.3
IZ.2
top
IZ.1
,Z.5
,Z'4
,Z.3
,z.2
Fig. 8. Stratigraphic plot of selected major oxides and trace elements for Izalco. Bars give the measure (+ lo) of the precision for each element. f r o m the b o t t o m t o the top. As with the p e t r o g r a p h i c m o d e s , the s e q u e n c e appears t o be c o n t i n u o u s with n o m a j o r breaks or reversals. T h e stratigraphic plots o f selected m a j o r oxides a n d trace elements for t h e Atitl~n s e q u e n c e are p r e s e n t e d in Fig. 9. The reversals suggested b y the p e t r o g r a p h i c m o d e s are c o r r o b o r a t e d by changes in slope at the same posi-
228 Covered interval
'°t o,o,.5I/
,.o~/
PrDDblo Re~rsal
i
?/
/.~¢' ?/
no[ [
.
V 190 t ~ / ~
170~ /
?/
20
7/./~"
Co
15 bottom AT.6
top
AT.5
AT'4 Cmred Interval
57 SiO2 56
]
AT.3
AT.2
AT,1
Pr|bakie Rove sal
?\\,~
54
K20
1.4
7-.
1.2 1.0
\~., 500 I
[
~"
Hf 3.5 3.0 bottom AT'6
i
AT'5
AT'4
-2
~.~
~.
I~.~
Fig. 9. Stratigraphic plot of selected major oxides and trace elements for Atitl~n. Bars give the measure (+ lo) of the precision for each element.
229 tions. The break between AT-4 and AT-5 is conspicuous. It should also be noted t h a t there is a slight tendency for a decrease in SiO2 and K20 from bott o m to top in each of the two defined groups -- from AT-3 to AT-1 and from AT-6 to AT-5. There is also a corresponding increase in CaO and MgO. Trace elements that are partitioned into the residual glass, such as Hf and Ba, suggest the same pattern as SiO~ and K20. Vanadium and Co show the same trends as CaO and MgO. The two volcanoes display different chemical trends that are consistent with their respective modal variations. The major oxide and trace element trends described for both volcanoes above suggest that shallow crystal fractionation by plagioclase, pyroxene and olivine may be the principal factor responsible for the chemical variations. The reversals in the Atitl~n sequence indicate that a simple, continuous process did n o t operate for that volcano, but rather suggests fractionation of successive batches of magma. Izalco appears to represent a more continuous fractionation trend. As discussed above, differences in summit elevation of the two volcanoes may account for the different fractionation patterns. GENERAL COMMENTS ON ADEQUACY
OF D A T A
Before discussion of chemical and petrographic results, two general comments on the adequacy of data collected herein are in order. One relates to whether the sections examined are long enough to really be representative of the behavior of the cores. The other deals with whether the mainly small variations seen in the data might n o t be just related to statistical sampling error. Short sections are simply what one usually has to deal with at most young volcanoes. You either decide to sample them, inadequate as they seem, or to ignore them. In this paper we did not rigorously examine whether the trends of chemical and petrographic data shown might be merely due to sampling error. Comparison of paired chemical and petrographic data on two sets of samples (IZ-4a, 4b and IZ-6a, 6b) however, do seem to show that in this case, the statistical variations among samples of similar composition are small. This might be due to the relatively small sizes of phenocrysts in most samples. Also we think it unlikely, particularly in the case of the Izalco sequence, that a consistent trend of data points could be statistical in origin. COMPUTER MODELING The hypothesis t h a t the lavas of each stratigraphic sequence can be explained by shallow crystal fractionation may be tested using a least squares c o m p u t e r program (Wright and Doherty, 1970). Compositions of observed phases are introduced with an assumed parental composition and the observed rock composition. The percentages of fractionated phases given by the program may be used in Rayleigh calculations to test the computer fit (Shaw, 1970). Partition coefficients used in the calculations are given in Table 3.
230 TABLE 3 Crystal-liquid partition coefficients used in the Rayleigh calculations (plag = plagioclase, cpx = clinopyroxene, opx -- orthopyroxene, ol = olivine, op -- opaque phase and ap = apatite)
Rb Sr Ba La Ce Sm Eu Lu Th Hf Ni Co Cr V Sc
plag
cpx
opx
ol
op
ap
0.05 1.83 0.3 0.14 0.2 0.15 0.34 0.2 0.01 0.01 0.26 0.1 0.04 0 0.01
0.003 0.12 0.002 0.1 0.175 0.6 0.65 0.8 0.001 0.001 1.8 1.5 15.0 2.31 1.6
0.022 0.017 0.013 0.02 0.05 0.075 0.06 0.75 0.001 0.001 3.8 1.4 2.0 0.5 0.7
0 0.014 0 0.01 0.007 0.01 0.01 0.02 0 0 8.7 3.8 2.4 0.09 0.02
0 0 0 0.002 0 0 0 0 0 0 6.6 8.0 2.8 24.0 2.0
0 2.0 0.01 30.0 30.0 50.0 20.0 25.0 0 0 0 0 0 0 22.0
Data from All6gre et al. (1977), Arth (1976). Leeman and Vitaliano (1976), Duke (1976), Ewart et al. (1973) and Zielinski and Frey (1970).
Choice o f parent If h e i g h t is an inhibiting f a c t o r in c o n e g r o w t h , t h e n it is a s s u m e d t h a t the early, m o r e m a f i c lavas e r u p t e d f r o m a v o l c a n o are c o n t i n u o u s l y being covered b y less m a f i c material a n d are never seen o n the surface o f an old cone. A v o l c a n o like Atitl~n, w h i c h m a y r e p r e s e n t an a d v a n c e d stage in volcanic c o n e d e v e l o p m e n t , m a y have o n l y andesitic lavas o n the surface a l t h o u g h the bulk o f t h e v o l u m e is c o m p o s e d o f m o r e m a f i c lavas. Because o n l y surficial lavas are available, n o s a m p l e d c o m p o s i t i o n s for Atitl~n p r o v e d suitable as a p a r e n t a l c o m p o s i t i o n . T h e r e f o r e , t h e inferred p a r e n t a l c o m p o s i t i o n f o r S a n t a Marfa v o l c a n o ( R o s e et al., 1 9 7 7 ) was used f o r the Atitl~n modeling. As p r e v i o u s l y m e n t i o n e d , the t w o v o l c a n o e s are similar in their g e o g r a p h i c l o c a t i o n , height, a n d c o m p o s i t i o n , s u p p o r t i n g the use o f this parent. Izalco m a y r e p r e s e n t an early stage in volcanic c o n e d e v e l o p m e n t . Conseq u e n t l y , r o c k analyses o f early basaltic lavas are available. A n average o f f o u r analyses o f ashes t h a t fell in 1931 was f o u n d t o be a suitable p a r e n t for t h e s u m m i t lavas (Deger, 1932). Trace e l e m e n t a b u n d a n c e s for b o t h p a r e n t s were d e t e r m i n e d f r o m H a r k e r plots a n d b y c o m p a r i s o n with analyses f o r lavas o f similar c o m p o s i t i o n .
Results o f calculations Izalco. T h e m o d e l i n g results f o r I z a l c o are given in Table 4. T h e close m a t c h b e t w e e n t h e o b s e r v e d a n d c a l c u l a t e d c o m p o s i t i o n s and t h e small errors indi-
231 cate that a good fit is obtained. There is a definite trend in total fractionation, showing a pronounced increase from 18.7% at the b o t t o m of the sequence to 39.1% at the top. The percentages of fractionated minerals show the following relative abundances: plagioclase > clinopyroxene :> opaque phase > olivine Fig. 10 shows a plot of the required percentages of mineral phases fractionated in order to account for the stratigraphic chemical changes. All the phases demonstrate a continuous, unbroken trend towards increasing effect of crystal fractionation from b o t t o m to top. 100
10 Cpx
r,
0,1 bottom IZ'6
top iZ'5
IZ'4
IZ'3
tZ'2
IZ'1
Fig. 10. Stratigraphic plot of fractionating phase percentages calculated by the least-squares c o m p u t e r program for the Izalco sequence (plag = plagioclase, cpx = clinopyroxene, op = opaque minerals and ol = olivine).
A titldn. Table 5 shows the results of the least-squares and Rayleigh calculations for the Atitl~n sequence. The total fractionation ranges between 41 and 60.7%. Again, the close match between calculated and observed (oxide percentages), and the small error indicate a good fit. The following relationship of the mineral phases fractionates is established: plagioclase > olivine > clinopyroxene > opaque phase = o r t h o p y r o x e n e A stratigraphic plot of the percentages of fractionation required to account for the chemistry is presented in Fig. 11. The break between AT-4 snd AT-5 is emphasized, and the other break is less well-defined. The t w o groups
232 TABLE 4 Model of crystal fractionation of Izalco parent producing the compositions observed in the stratigraphic sequence Parent*
IZ-1
IZ-2
obs.*
calc.
IZ-3
obs.*
calc
obs.*
calc.
I. Least-squares mixing calculations (Wright and Doherty, 1970) 51.80 0.98 19.30 9.80 4.40 9.59 3.35 0.10
SiO 2 TiO~ Al203 FeO* MgO CaO Na20 K20
55.10 0.76 18.56 8.43 3.89 8.79 3.41 1.10
55.00 0.76 18.54 8.42 3.89 8.79 3.44 1.17
54.52 0.77 18.96 8.63 3.84 8.83 3.38 1.07
0.0164"* Phases fractionated : plagioclase clinopyroxene olivine Fe oxides apatite***
54.45 0.77 18.94 8.63 3.84 8.83 3.40 1.13
54.61 0.84 19.08 8.34 3.91 8.75 3.39 1.09
0.015
26.5 5.4 3.0 4.1 0.4
54.55 0.84 19.07 8.34 3.91 8.75 3.41 1.13 0.0196
24.2 5.9 2.7 3.8 0.4
24.2 6.5 2.4 4.0 0.4
H. Rayleigh calculations (Shaw, 1970) 15.0 280.0 7.5 14.25 2.5 0.9 0.25 1.0 1.2 20.0 30.0 15.0 350.0 27.0 500.0
Rb Ba La Ce Sm Eu Lu Th Hf Ni Co Cr V Se Sr
23.7 402.0 14.5 17.9 2.8 1.06 0.30 1.53 1.71 15.0 25.4 8.2 258.0 29.3 471.0
24.2 415.0 10.1 19.0 2.9 1.14 0.32 1.63 1.96 13.5 24.5 6.9 140.0 29.9 440.0
25.8 378.0 8.2 14.9 3.18 1.06 0.32 1.70 1.85 16.0 24.8 6.4 238.0 30.6 --
23.5 406.0 9.9 18.6 2.88 1.12 0.31 1.57 1.89 14.0 24.8 6.3 149.0 28.8 --
*Recalculated to 100% after FeO adjustment. **Determined by summing the squares of calculated residuals. * * * N o t determined by least-squares method (see text).
25.8 378.0 8.4 14.8 3.1 1.08 0.32 1.54 1.82 11.0 24.5 6.4 239.0 30.2 443.0
23.5 406.0 9.3 17.6 2.7 1.06 0.30 1.58 1.90 14.1 24.5 5.6 139.0 28.2 452.0
0 C.0
.o
0
0 0
*
234 TABLE 5 Model of crystal fractionation of Atitl~n parent producing the compositions observed in the stratigraphic sequence Parent*
AT-1
AT-2
obs.*
calc.
AT-3
obs.*
calc.
obs.*
calc.
I. Least-squares calculations (Wright and Doherty, 1970) 50.90 1.10 19.30 9.30 6.30 9.40 3.10 0.66
SiO 2 TiO 2 Al~O s FeO* MgO CaO Na~O K~O
54.28 1.00 18.96 8.50 4.24 8.10 3.83 1.09
54.27 1.00 18.96 8.50 4.25 8.11 3.83 1.09
55.75 1.04 19.49 7.15 3.83 7.34 4.09 1.31
0.0315"* Phases fractionated plagioclase clinopyroxene olivine orthopyroxene Fe oxides apatite***
55.71 1.04 19.49 7.17 3.85 7.36 4,08 1.30
57.68 1.02 17.97 7.34 3.38 7.26 4.01 1.34
0.0652
25.5 4.8 5.0 3.6 2.9 0.4
57.68 1.02 17.96 7.34 3.40 7.27 4.03 1.40 0.0296
30.9 7.5 4.3 5.2 4.3 0.5
35.7 6.2 9.7 -4.0 0.6
II. Ray leigh calculations (Shaw 1970) 17.5 300.0 7.0 14.2 2.4 0.85 0.20 1.1 2.0 40.0 30.0 30.0 315.0 20.0 550.0
Rb Ba La Ce Sm Eu Lu Th Hf Ni Co Cr V Sc Sr
27.3 475.0 10.7 22.8 3.34 1.26 0.30 1.63 3.29 17.5 24.9 13.3 212.0 24.4 504.0
29.5 467.0 10.4 19.7 3.01 1.19 0.32 1.88 3.42 21.0 24.6 14.1 184.0 23.7 511.0
36.8 543.0 13.3 25.6 4.30 1.27 0.31 2.38 3.80 16.0 19.4 9.8 165.0 20.4 517.0
35.8 550.0 11.8 19.7 3.2 1.31 0.35 2.29 4.17 18.4 22.6 7.9 115.0 23.0 509.0
*Recalculated to 100% after FeO* adjustment. **Determined by assuming the square of calculated residuals. ***Not determined by least-square method (see text).
33.3 607.0 11.9 19.5 3.35 1.19 0.27 2.86 3.82 21.0 20.4 10.5 168.0 24.0 472.0
38.4 578.0 12.3 22.9 3.3 1.37 0.39 2.46 4.48 13.3 20.5 10.3 140.0 26.8 471.0
O~
m
~
~MN~dN4M
~0 aO 0
t-t 0 0
O~ 0
c~
236 100 Covered Interval
e Reversal :i Proaabl
._~
?-_:_.
10
-
OI
I
Opx
0.1 ~ bottom
i
AT.6 AT'5
Fig. 1 i. Stratigraphic plot of fractionating phase percentages calculated by the least-squares computer program for the Atitl~n sequence (plag = plagioclase, ol = olivine, cpx = clinopyroxene, op = opaque minerals and opx = orthopyroxene).
(AT-3 to AT-1 and AT-6 to AT-5) are less fractionated towards the top of each group.
Other aspects of modeling The modal abundances of olivine in the Izalco and Atitl~n lavas contrast with the percentages of olivine calculated in the fractionation modeling. This is probably a result of the compositions chosen for the parents. The Izalco parent is a real composition that was very likely fractionated by plagioclase and olivine before eruption. Thus, the parent may have been relatively depleted in olivine, which would lower the fractionating phase percentage. For Atitl~n, the parent is an inferred composition that presumably reflects little of no differentiation. Calculations involving this parent could show the entire effect of the crystal fractionation process. The Atitl~n summit lavas may have been derived from a parent much like the Santa Marfa parent, b u t through a series of intermediate steps.
Apatite effect The least-squares and Rayleigh calculations are considered to be permissive of crystal fractionation as the cause of the chemical trends for both volcanoes.
237 However, one of the difficulties encountered in the modeling was the depletion of the rare earth elements relative to the predicted values. Fractionation at depth of pargasitic hornblende, the traces of which may be preserved as crystal clots (Stewart, 1975), would cause enrichment of the light relative to the heavy rare earth elements. This type of enrichment pattern may be present for the lavas of this study, b u t the lack of good analyses for the heavy rare earth elements makes it difficult to evaluate the trend. In any event, the few crystal clots observed in the Izalco and Atitl~n lavas are different from those described by Stewart (1975). In addition, it was found that the amounts of amphibole fractionating at depth necessary to account for the effect was over 25%, which the trace element modeling suggested was unreasonable. Another possible explanation for the depletion is fractionation of plagioclase phenocrysts that contain abundant apatite inclusions. Apatite has very high partition coefficients for all the rare earth elements and if only a small a m o u n t was involved in the fractionation, all the rare earth elements would be significantly affected. Since plagioclase, pyroxene, olivine and opaque minerals exclude P205, it is strongly partitioned into the residual glass as crystallization of a liquid proceeds, unless apatite is crystallizing. Anderson and Greenland (1969) have stated that apatite crystallization will only occur after a b o u t 70% total crystallization. However, their studies were of Hawaiian basalts which are relatively depleted in Cl and other volatiles (Anderson, 1974). Glass inclusion analyses from ashes erupted during the 1974 Fuego eruption show high concentrations of Cl and other volatiles (Rose et al., 1978). Increased volatile pressure, especially F and C1, might p r o m o t e early apatite crystallization. If a rock has an initial P2Os content of 0.2% and is 50% crystallized, the P2Os content is doubled; b u t 0.5% apatite crystallization will deplete the P:Os content by about the same amount, maintaining an almost constant P2Os content. It is thus generalized that for every 10% crystallization, 0.1% apatite crystallization will maintain a constant P2Os content. The Izalco lavas show no systematic variation in P2Os throughout the sequence despite the variation in K20. The Atitl~n lavas do display some changes, but it is noted that for the most part, the P2Os content does not follow the trend of K20, Cs, Th and Hf as would be expected if it were being partitioned solely into the residual glass (Fig. 12). Since the lavas of both volcanoes contain plagioclase phenocrysts with abundant apatite inclusions, it is assumed that apatite was an early crystallizing phase and apatite was therefore used in the Rayleigh calculations. The apatite proportions for the calculations were determined by the percent apatite crystallization percent total crystallization ratio explained above. The uncertainties in the partition coefficients allow a great deal of flexibility, but perhaps define a minimum and a maximum for the apatite effect.
238 0.3
Covered Interval
~ :
Probable Reversal
Atitl:in
71 / 0.2
bottom AT.6 0,2
AT,5
AT,4
AT,3
AT.2
]
-
P2O5 0.1
Izalco
bottom IZ.6
IZ.5
IZ.4
IZ.3
IZ'2
tnn i~"1
Fig. 12. Stratigraphic plots of P2Os for Izalco and Atitl~n. INTERPRETATIONS
The two volcanoes examined have different heights, volumes, eruptive frequencies and compositions. They also display different crystal fractionation trends. The general trend towards a less mafic composition with time that has been established for other volcanoes is well developed for Izalco. The stratigraphic sequence shows a trend that can be explained by successive eruptions from a continuously fractionating mafic parent, producing successively more silicic lavas towards the top of the sequence. At Atitl~n it is suggested that the observed trend may be explained by shallow crystal fractionation cycles. Successive batches of magma are fractionated by plagioclase, pyroxene and olivine, forming a series of small batches of zoned magma that are subsequently erupted and preserved as inverted sequences. The entire stratigraphic sequence may represent three separate zoned magmas, defined by AT-3 to AT-I; AT-4, from which several units appear to be missing; and AT-6 and AT-5. The different trends seen in the t w o stratigraphic sequences, one continuous and the other broken by reversals, may be explained by the physical differences between the t w o cones. At Izalco, the small structure has no strong inhibiting effect on eruptive frequency and allows nearly continuous eruptions of magma. The short repose times mean that crystal fractionation will take place for only a short time before eruption. The greater height of Atitl~n restricts eruptive activity because of the greater lithostatic versus hydrostatic pressure. Longer repose times allow extensive crystal fractionation to zoned magmas. These magmas are erupted in sequence, the top of the chamber first,
239
followed by deeper, more mafic lavas. Reversals in stratigraphic trends occur when repose allows advanced differentiation. Since periods of differentiation lengthen progressively as the cone grows, an oscillatory trend to gradually more silicic lava develops. CONCLUSIONS
The two volcanoes have different chemical trends for the stratigraphic sequences studied. The sequence at Izalco becomes continuously richer in SIO2, K20, Zr and La as CaO, MgO, Co and Cr are depleted from b o t t o m to top. In contrast, at Atithh~ the sequence is incomplete and shows reversals. In each of the sections separated by the reversals, SIO2, K20, Ba and Hf decrease and CaO, MgO, V and Co increase from the b o t t o m of the section to the top. The different groups may be distinguished from each other by the proportions of their mafic minerals. Both chemical trends can be explained by shallow crystal fractionation of the observed phenocryst phases. Because of the large partition coefficients of apatite for the rare earth elements, the effect of apatite crystallization is important in crystal fractionation trends, if apatite is included in the large fractionating phenocrysts. Early apatite crystallization can account for the depletion of all the rare earth elements relative to the other trace elements. ACKNOWLEDGEMENTS
The field work and the analytical work for this study was supported by NSF grant DES 74 19025. A.P. Ruotsala, N.K. Grant and D.C. Noble reviewed the original version of this work and provided many useful suggestions.
REFERENCES
All~gre, C.J., Treuill, M., Minster, J., Minster, B. and Albar~de, F., 1977. Systematic use of trace element in igneous process. Contrib. Mineral. Petrol., 60: 57--75. Anderson, A.T., Jr., 1974. Chlorine, sulfur, and water in magmas and oceans. Geol. Soc. Am. Bull., 85: 1485--1492. Anderson, A.T., Jr. and Greenland, L.P., 1969. Phosphorus fractionation diagram as a quantitative indicator of crystallization differentiation of basaltic liquids. Geochim. Cosmochim. Acta, 33: 493--505. Arth, J.G., 1976. Behavior of trace elements during magmatic processes -- a summary of theoretical models and their applications. J. Res. U.S. Geol. Surv., 4: 41--47. Deger, E., 1932. Zur Kenntnis der mittelamerikanischen vulkanischen Aschen. Chem. Erde, 7: 51--55. Duke, J.M., 1976. Distribution of the period four transition elements among olivine, calcic clinopyroxene and mafic silicate liquid: experimental results. J. Petrol., 17: 499-521. Ewart, A., Bryan, W.B. and Gill, J.B., 1973. Mineralogy and geochemistry of the younger volcanic islands of Tonga. J. Petrol., 14: 429--465.
240
Flanagan, F.J., 1973. Values for International Geochemical Reference Samples. Geochim. Cosmochim. Acta, 37: 1181--1200. Katsui, Y., Ando, S. and Inaba, K., 1975. Formation and magmatic evolution of Mashu Volcano, East Hokkaido, Japan. J. Fac. Sci. Hokkaido Univ., Set. IV, 16: 533--552. Larson, P.B., 1973. The volcanoes Atithln and Toliman and the dome Cerro de Oro, Guatemala. Unpublished Report, Michigan Technological University, Houghton, Mich., 33 pp. Leeman, P.B. and Vitaliano, C.J., 1976. Petrology of McKinney basalt, Snake River Plain, Idaho, Geol. Soc. Am. Bull., 87: 1777--1792. McBirney, A.R., 1976. Some geologic constraints on models for magma generation in orogenic environments. Can. Mineral., 14: 245--254. Maxwell, J.A., 1968. Rock and mineral analysis. Chem. Anal. Set., 27: 393--394. Mooser, F., Meyer-Abich, H. and McBirney, A.R., 1958. Catalogue of the Active Volcanoes of the World, VI. International Volcanological Association, Naples, 146 pp. Reynolds, R.C., 1963. Matrix corrections in trace element analyses by X-ray fluorescence. Am. Mineral., 48: 1133--1143. Rose, W.I., Jr. and Stoiber, R.E., 1969. The 1966 eruption of Izalco volcano, El Salvador. J. Geophys. Res., 74: 3119--3130. Rose, W.I., Jr., Grant, N.K., Hahn, G.A., Lange, I.M., Powell, J.L., Easter J. and DeGraff, J.M., 1977. The evolution of Santa Marfa volcano, Guatemala. J. Geol., 85: 63--87. Rose, W.I., Jr., Anderson, A.T., Jr., Bonis, S. and Woodruff, L.G., 1978. The October 1974 basaltic tephra from Fuego volcano, Guatemala: description and history of the magma body. J. Volcanol. Geotherm. Res., 4: 3--53. Shapiro, L. and Brannock, W.W., 1955. Rapid determination of water in silicate rocks. Anal. Chem., 27: 560--562. Shaw, D.M., 1970. Trace element fractionation during anatexis. Geochim. Cosmochim. Acta, 34: 237--243. Stewart, D.C., 1975. Crystal clots in calc-alkaline andesites as breakdown product of highA1 amphiboles. Contrib. Mineral. Petrol., 53: 195--204. Stoiber, R.E., Rose, W.I., Jr., Lange, I.M. and Birnie, R.W., 1975. The cooling of Izalco volcano (El Salvador) 1964--1974. Geol. Jahrb., 13: 193--205. Wright, T.L. and Doherty, P.C., 1970. A linear programming and least-squares computer method for solving petrologic mixing problems. Geol. Soc. Am. Bull., 81: 1995--2008.
217
© Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands
CONTRASTING FRACTIONATION PATTERNS FOR SEQUENTIAL MAGMAS FROM TWO CALC-ALKALINE VOLCANOES IN CENTRAL AMERICA
L.G. WOODRUFF I *, W.I. ROSE, Jr. 1 and W. RIGOT 2
~Michigan technological University, Houghton, M149931 (U.S.A.) 2Phoenix Memorial Laboratory, University of Michigan, Ann Arbor, M148104 (U.S.A.) (Received January 30, 1978; revised and accepted February 8, 1979)
ABSTRACT Woodruff, L.G., Rose, W.I., Jr. and Rigot, W., 1979. Contrasting fractionation patterns for sequential magmas from two calc-alkaline volcanoes in Central America. J. Volcanol. Geotherm. Res., 6: 217--240. Two sequences of alternating units of lavas and pyroclastic rocks from two volcanoes in Central America were sampled and analysed. The rocks from Izalco, the smaller, more mafic cone, show a continuous increase in SiO~, K20 , Zr and La, and a decrease in CaO, MgO, Co and Cr with time. These trends are shown to be consistent with progressive crystal fractionation. Atitl~n, a larger and more silicic volcano, displays a discontinuous trend in which geochemical reversals are thought to reflect the development of a series of small bodies of zoned magma. The lavas of each zoned section show decreases in SiO2, K20, Hf and Ba, and increases in CaO, MgO, Co and V from the bottom of the section to its top. The different trends at Izalco and Atitl~n are believed to reflect the restrictions cone height places on the effects of crystal fractionation. As cone height increases, the intervals between eruptions progressively lengthen, allowing more time for crystal fractionation to proceed. The two volcanoes may represent an early and a later stage in the general evolutionary development of large composite cones. Despite the remarkable regularity of the alternating flows and pyroclastic eruptions, no systematic chemical variations were recognized between the two types of lava.
INTRODUCTION
The Quaternary volcanic chain of Central America stretches from Mexico to Costa Rica along a converging plate margin. The high composite volcanoes of the chain are typically composed of high-A1203 basalt or andesite. Recent work on one of these cones, Santa Marfa volcano, Guatemala, has demonstrated a general stratigraphic progression to more silicic lavas. Increasingly long periods o f crystal fractionation may cause this variation with time (Rose et al., 1977). *Current address: Department of Geophysical Sciences, University of Chicago, Chicago, IL 60637, U.S.A.
218
Katsui et al. (1975) explained the large-scale chronological trend from mafic to felsic compositions of Mashu volcano, East Hokkaido, Japan, as continuous fractional crystallization of a low-alkali tholeiitic parent over a time span of 13,000 years. In the early stages of the Mashu activity, eruptions of mafic lavas followed shortly one after another, while in the latter stages, eruptions of more silicic lavas were separated by longer and longer repose times. The same type of progressive changes with time of eruptive activity is inferred at Santa Marfa. There the height of the cone was suggested to be the controlling factor in eruptive frequency (Rose et al., 1977). As cone height increases, the increase in lithostatic pressure versus hydrostatic pressure makes it increasingly difficult for a magma to reach the surface; repose periods become longer, allowing more time for crystal fractionation to take place {Rose et al., 1977}. The over-all composition of a volcano would become less mafic with time. Small reversals of this general trend are noted in both studies and are assumed to reflect the development and subsequent eruption of zoned magma chambers (Katsui et ai., 1975; Rose et al., 1977). Work on the 1974 eruption of Volc~n Fuego, Guatemala, demonstrated that a zoned magma chamber may develop in high-A1203 basalt by the process of crystal fractionation over a relatively short repose time of one to two years (Rose et al., 1978). To test the application of crystal fractionation to chemical trends of composite cones, t w o interlayered lava/pyroclastic sequences from two Quaternary volcanoes in Central America were sampled and analyzed (Fig. 1).
16"
(~
,,'--
GUATEMALA
jJ i
-
~
f
~. ," Izalce
HONDURAS
0
80kin
EL " \ - - ' - " ' - ' ~ i
Fig. 1. Location map of Atitl~n and Izalco volcanoes. SAMPLING S T R A T E G Y AND SAMPLING SITES
McBirney {1976) has shown that the mineralogy of andesitic lavas correlates statistically with the height of volcanic cones in island arcs and continental margins. Hornblende and biotite are more c o m m o n in the lavas of
219
volcanoes with high summit elevations than they are in lavas of comparable composition of lower elevations. This relation suggests that lavas from higher vent elevations may reflect a greater degree of crystal fractionation or possibly a higher H:O content. To attempt to locate more highly fractionated samples, sampling for this study was confined to the top of the volcanoes. In addition, this strategy takes advantage of good exposures and is less affected by local variation in depositional area, as would occur on the flanks of the cones. The t w o selected cones are of different height, volume, activity pattern and lava composition. Izalco volcano is a small, recently active, basaltic cone in western E1 Salvador. It had nearly continuous eruptions of flows and pyroclastic debris from its beginning in 1770 through 1958 (Mooser et al., 1958). Since 1958 Izalco has been dormant, with the exception of a small flank eruption in 1966 (Rose and Stoiber, 1969). Only fumarolic activity persists at present (Stoiber et al., 1975). Samples were collected from an interlayered flow/pyroclastic sequence (three flows and three pyroclastic units). The exposure that was sampled is on the south wall of the explosion crater that occupies the top of the cone. The units vary in thickness from one to three meters and apparently form a continuous sequence with no large time breaks between units. The samples probably represent the latest activity of Izalco prior to 1958 and are believed to have been erupted sometime in the 1950's. Volc~n Atitl~n is one of the large andesitic composite cones that make up the volcanic chain in Guatemala. Its historic activity consists of few widely-spaced (80--100 years) eruptions. Atitl~n has been dormant since 1853 with only minor fumarolic activity since that time (Mooser et al., 1958). A series of interlayered lava flow and pyroclastic layers are exposed in a scarp on the south side of the volcano. Three flow and three pyroclastic layers that crop o u t a b o u t 70 m below the summit were sampled. The two lowermost units in the sampled sequence are separated from those above by an interval of a b o u t nine meters in which no rocks are exposed. This break in sequence may represent several unexposed units. There is no obvious evidence of long hiatus between the other layers although subtle breaks may occur. A N A L Y T I C A L METHODS
The major element and trace element analyses of six Atitlfin and eight Izalco samples are given in Table 1. The major oxides were determined using a Perkin--Elmer 303 atomic absorption spectrophotometer. P2Os was determined using a p h o s p h o m o l y b a t e complex procedure with a Bausch and Lamb colorimeter (Maxwell, 1968). H20 + was determined using a modified Penfield m e t h o d after Shapiro and Brannock (1955). A Perkin--Elmer 360 atomic absorption s p e c t r o p h o t o m e t e r equipped with a graphite furnace (HGA 2100) was used to determine Ni and V. Strontium and Zr were determined with a Philips electron diffractometer-spectrometer with m o l y b d e n u m radia-
8.23
-403 123
17.91 14.86 2.75 3,18 1.06 1.06 0.30 0.32 1.53 1.70 1.71 1.85 15.0 16.0 25.4 24.8 8.24 6.38 258 238 29.3 30.6
14.49
471 402 134
24.1
1.09
97.97
53.2 18.50 8.42 3.75 8.62 3.30 1.04 0.75 0.17 0.22
IZ-22
14.75 3,07 1.08 0.32 1.54 1.82 11.0 24.5 6.35 239 30.2
8.3
443 378 117
25.8
1.22
100.34
--
54.7 19.11 8.35 3.92 8.76 3.39 1.09 0.84 0.18
IZ-3 ~
* T o t a l Fe as F e O 2 . i f l o w unit; 2p y r o c l a s t i c u n i t .
Zr La Ce Sm Eu Lu Th Hf Ni Co Cr V 8e
Ba
23.7
1.2
98.42
Total
Cs Rb Sr
H20"
P20&
CaO Na20 K20 TiO 2
MgO
54.0 18.19 8.26 3.79 8.61 3.34 1.08 0.74 0.17 0.24
SiO~ AI203 FeO*
IZ-I l
15.74 3.0 1.07 0.31 1.69 1.64 10.0 30,1 9.33 263 30.8
7.19
465 346 118
26.6
1.18
98.49
--
53.1 18.66 8.41 4.44 8.96 3.01 0.91 0.82 0.18
IZ-4a:
15.08 2.97 1.03 0.27 1.45 1.74 14.0 26.7 II.I 248 28.1
7.28
474 395 --
26.4
1.14
98.24
--
52.6 18.82 8.51 4.30 8.89 3.20 0.95 0.80 0.17
IZ-4b ~
15.02 2.43 1.02 0.31 1.67 1.71 15.0 28.3 11.29 281 28.7
7.55
461 373 114
22.5
--
99.08
53.6 19.08 8.40 4.21 8.49 3.40 0.95 0.77 0.18
IZ-51
16.79 2.68 1.07 0.30 1.81 1.68 14.0 29.3 9.87 258 30.9
8.76
472 398 119
22.9
1.15
99.69
53.1 19.16 8.77 4.37 9.17 3.26 0.99 0.70 0.17
IZ-6a ~
14.32 2.65 1.01 0.28 1.38 1.70 16.0 26.6 8.74 256 30.2
7.46
461 414 --
27.6
1.10
101.99
53.1 20.35 8.88 4.58 9.68 3.44 1.01 0.78 0.17
IZ-6b 2
22.81 3.34 1.26 0.30 1.63 3.29 17.5 24.9 13.29 212 24.4
10.66
504 475 151
27.3
1.17
101.39
54.9 19.18 8.60 4.29 8.19 3.87 1.10 1.01 0.25
AT-I 2
25.62 4.30 1.27 0.31 2.38 3.80 16.0 19.4 9.78 165 20.4
13.34
517 543 175
36.8
1.34
99.9
55.8 19.51 7.16 3.38 7.35 4.09 1.31 1.04 0.26
AT.21
C h e m i c a l a n a l y s e s o f e i g h t I z a l c o a n d six A t i t l ~ n s u m m i t s a m p l e s ( o x i d e s i n w t . %; t r a c e e l e m e n t s in p p m )
TABLE 1
19.47 3.35 1.19 0.27 2.86 3.82 21.0 20.4 10.49 168 24.0
11.87
472 607 174
33.3
1.43
100.06
57.6 17.95 7.33 3.38 7.25 4.00 1.34 1.02 0.19
AT-3 s
24.40 3.73 1.26 0.30 2.96 3.61 22.0 21.3 12.17 188 21.6
12.87
520 564 175
32.8
1.46
99.0
--
56.1 17.28 7.77 3.86 7.44 3.96 1.31 1.04 0.24
AT-41
20.78 2.94 1.12 0.25 2.33 2.97 17.0 23.7 7.79 211 23.1
10.13
1.16 26.7 517 499 148
98.56
53.8 18.17 8.34 4,07 8.21 3.60 1.03 0.94 0.20 0.2
AT-52
27.47 3.85 1.41 0.27 3.46 4.13 12.5 16.5 12.33 165 22.5
13.32
1.49 45.7 543 601 189
98.17
56.4 17.54 6.87 3.31 6.16 3.95 1.52 0,99 0.23 1.2
AT-61
t~
221
tion (Reynolds, 1963). All other trace elements were determined by instrumental neutron activation at the Phoenix Memorial Laboratory at the University of Michigan. The samples were irradiated at a flux of 1.5 × 1013 neutrons cm -2 s-1 for ten hours. Two counts were made after decay periods of seven and twenty-one days. U.S.G.S. reference rock standard GSP-1 and an artificial liquid standard prepared by Ward Rigot of the Phoenix Memorial Laboratory were used. U.S.G.S. reference rock standards GH, G2, BCR-1 and W-1 were used in the other chemical analyses. Values from Flanagan (1973) were used for the standards. COMPARISONS AND CONTRASTS
Analyses of the lavas sampled for this study, along with analyses of additional summit samples, are compared with data from previous work on these volcanoes. Fig. 2 shows that the summit analyses are the most silicic for each cone. This tendency is not as clearly demonstrated at Atitl~m as it is at Izalco, perhaps because there is little control (e.g. time of deposition, location of vent area) for the other Atitl~n samples.
O0 0
o
-
Izalco % MgO
I OSUMMITANALYSLS
] • AOOITIONALSUMMITANALYSES 0 OTHERANALYSES I
56
52
is
s'o
%SiO2
0
4
0
Atitl~n
0
0A
0
% MgO D
3 i
• SUUilT ANALYSES / • AOOITIOtlALSUMMITANALYSES 00THLN ANAL~$[U
]
I
5'0
52
s6
% SiO2 Fig. 2. Plot o f MgO vs. SiO 2 for both volcanoes, showing the comparmon o f summit analyses with analyses from previous studies.
222
Fig. 3 shows that the analyses for both volcanoes fall well within the calcalkaline trend defined by Central American volcanic rocks. The Atitl~n group, averaging 55.8% SiO2 (range 53.8--57.6%)is more silicic than the Izalco group, averaging 53.4% (range of 52.6--54.7%}. Physical differences between the two volcanoes are emphasized in Fig. 4 F
• Tertiary
Al
\M
Fig. 3. AFM plot of the Quaternary and Tertiary Central American volcanic rocks showing the different fields occupied by Izalco and Atitl~n. IZALCO 13 48'9"N 89°38'1°W
ATITLAN 14 35'3"N 91°10'9"W 3525 m
Nearly continuous Strombolian activity producing basaltic lava
S
N
1965 m
.." Widelyspaced[80-100 .....--"'" years)Vulcanianactivity •
producingandesitic lava
Fig. 4. Schematic representation of the physical contrasts between Izalco and Atithin. Diff e r e n c e s include height, volume, eruptive type and frequency, and rock type.
223 Atitl~n's more silicic composition is inferred to be a direct reflection of its significantly greater height, volume, age and its lack of recent activity. Izalco, which was formed in 1770, is much younger than Atitl{m. Santa Mafia volcano, which has been tentatively assigned an age of 30,000 years (Rose et al., 1977) is similar to Atitl~n in height, location and composition. PETROGRAPHY
The Izalco lavas are basaltic-andesites with plagioclase (An~5, ranging from Anss to AnTs), olivine, augite and an opaque phase as phenocrysts. Atitl~m's lavas are similar, b u t contain hypersthene and have a slightly more sodic plagioclase (Ane0, ranging from Ans5 to ANT0). The pyroxenes of Izalco and Atithin plot at the high-temperature end of the Skaergaard trend and represent typical pyroxenes for basaltic rocks. The augites in the Izalco lavas have an average composition of En35FslgWo46. The two pyroxenes found at Atitl~n have average compositions of En3sFslTWo48 for the augites, and En66Fs30Wo4 for the hypersthenes. STRATIGRAPHIC PLOTS OF THE PETROGRAPHIC MODES Modal analyses and descriptions of the analyzed samples are given in Table 2. The modes were determined using a point counter and represent only the phenocrysts. An arbitrary size limit of less than 0.1 mm was used to delineate crystallites from phenocrysts. The petrographic modes for the lavas of the t w o volcanoes have slightly different minerals and proportions. The Izalco groundmass percentages range between 52 and 58%. For most of the samples a simplified relationship of relative phenocryst abundance may be expressed by: plagioclase > clinopyroxene = olivine > opaque phase Hypersthene is n o t present in most of the rocks. Fig. 5 is a stratigraphic plot of the phenocryst percentages. A continuous trend with plagioclase decreasing and opaques increasing from b o t t o m to top is indicated. Olivine and clinopyroxene do not vary systematically. There are no suggestions of breaks or reversals. The Atitl~n modes present a pattern that differs from that of Izalco. The lavas have 5 3 - 6 4 % groundmass and show a slight increase in percentage from the b o t t o m of the section to the top. The phenocrysts are present in slightly different proportions than at Izalco: plagioclase > clinopyroxene > olivine > opaque phase > orthopyroxene Fig. 6 is a stratigraphic plot of the observed modes. Changes in slope suggest that the sequence may not be continuous. Breaks in the trend are inferred between AT-3 and AT-4, and AT-4 and AT-5. The break observed in the stratigraphic sequence between AT-4 and AT-5 is marked and is presumed to rep-
Groundmass % remarks
Opaques % average size
Olivine % average size remarks
C lin op yr oxen e % average size remakrs
Plagioclase % average size remarks*
Phenocrysts
56.0 microlites of plag, cpx, ol and op interstitial with glass
2.3 0.2 m m
3.2 0.5 m m some alteration to iddingsite
4.3 2.5 m m inclusions of gm and op
33.5 2 mm An6s ; zoned glass and cpx inclusions
IZ-1
58.4 microlites of m o s t l y plag and some cpx, ol and op interstitial with glass
2.1 0.2 m m
4.9 0.3 m m resorption rims
2.8 0.8 m m few glass inclusions
31.5 1 mm An6~; zoned few glass inclusions
IZ-2
52.9 a b u n d a n t microlites of plag and cpx with some ol and op, interstitial with glass, a b u n d a n t apatite inclusions
1.9 0.3 m m
4.9 0.5 m m some resorption rims
6.8 1.3 m m
33.5 1.3 m m An75; zoned with mi nor glass inclusions
IZ-3
Petrographic mode s and p h e n o c r y s t descriptions for Izalco and Atithln
TABLE 2
57.0 dark brow n glass with few microlites
1.3 0.2 m m
6.2 0.6 m m
4.7 0.8 m m
30.6 1.3 m m An~0; zoned s ome inclusions t h a t follow zones
IZ-4
42.5 large microlites of plag with cpx and op interstitial with glass
1.7 0.3 m m
3.4 0.3 m m reaction rims of cpx?
4.1 1.1 m m inclusions of plag and op
36.3 1.3 m m An~0; zoned; glass inclusions t ha t follow zones
IZ-5
48.6 few microlites in dark b r o w n glass
1.6 0.15 m m
3.6 0.4 m m
4.4 1.5 m m
38.6 1.2 m m Anss; zoned, few inclusions of glass
IZ-6
b~ b~
64.0 dark brown glass w i t h few microlites
0.2 0.1 m m
3.6 0.8 mm
5.4 0.7 mm
27.0 1.0 m m An~0; z o n e d w i t h glass inclusions
AT-1
59.0 microlites of plag and cpx interstitial w i t h glass
1.5 0.5 mm
1.0 0.5 mm
0.2 1.5 m m
4.5 1 mm
33.5 2.5 m m ANss; zoned w i t h glass inclusions
AT-2
54.0 abundant microlites of plag, cpx, and op interstitial w i t h glass, abundant apatite inclusions
2.2 0.3 mm
1.0 0.5 mm
0.3 0.7 mm
4.5 0.8 mm
38.0 2.5 mm AN~0; z o n e d w i t h inclus i o n s o f glass
AT-3
55.1 dark brown glass w i t h f e w microlites
2.5 0.2 mm
2.0 0.2 mm
1.2 0.8 mm
8.2 1.1 m m some resorption rims
31.0 2.5 mm An55; z o n e d with inclusion o f glass
AT-4
* a p = a p a t i t e , c p x = c l i n o p y r o x e n e , ol = olivine, o p = o p a q u e p h a s e , o p x = o r t h o p y r o x e n e ,
Groundmass % remarks
a v e r a g e size
%
Opaques
a v e r a g e size
Olivine %
Orthopyroxene % a v e r a g e size remarks
Clinopyroxene % a v e r a g e size remarks
Plagioclase % a v e r a g e size remarks
Phenocrysts
59.5 dark brown glass w i t h few microlites
2.5 0.15 mm
2.0 0.8 mm some resorption rims
6.0 0.8 mm
30.0 1.8 m m A n s s ; glass inclusions following zone
AT-6
p l a g = p l a g i o c l a s e , g m ffi g r o u n d m a ~ .
52.3 large microlites o f p l a g with cpx and op interstitial w i t h glass, a b u n d a n t apatite inclusions
1.0 0.15 mm
2.0 0.2 mm
0.1 0.5 mm
6.0 0.5 mm
38.0 0.8 mm An55; s u b h i d ial, z o n e d w i t h glass inclusions
AT-5
t~
f~
226 100
100 Covered interval
?-~"
PIeD
10
!
Cpx
i
tD.
0.1 bottom
top IZ,5
IZ'4
IZ'3
IZ,2
IZ'I
Plsg ~ ' - ~ - ~ . . . . ~
10 c~ Cpx
? i"-,
1
IZ'6
Probable Reversal
0.1 bottom AT.6
top AT'5
AT,4
AT.3
AT,2
Fig. 5. Stratigraphic plot of the petrographic modes determined for Izalco (plag = plagioclase, cpx = clinopyroxene, ol = olivine and op = opaque minerals). Fig. 6. Stratigraphic plot of the petrographic modes determined for Atitl~n (plag = plagioclase, cpx = clinopyroxene, op = opaque minerals, ol = olivine and opx = orthopyroxene). Missing units are inferred between AT-4 and AT-5 and the trends of the phenocrysts are extrapolated into the covered interval. r e s e n t u n e x p o s e d o r missing u n i t s in t h e s e q u e n c e . T h e r e is n o o b v i o u s surface e x p r e s s i o n o f t h e p r o p o s e d b r e a k b e t w e e n AT-3 a n d AT-4, b u t it is s t r o n g l y suggested b y t h e m o d a l data. T h r e e s e p a r a t e g r o u p s are also suggested b y t h e p l o t o f c l i n o p y r o x e n e versus t h e o t h e r m a f i c m i n e r a l s in Fig. 7. I t m a y be possible t h a t t h e differe n c e s in t h e p r o p o r t i o n s o f m a f i c p h e n o c r y s t s m a y be used t o i d e n t i f y differe n t b a t c h e s o f m a g m a . H e r e it is p r o p o s e d as criteria f o r b a t c h b r e a k b e t w e e n AT-3 a n d AT-4. MAJOR
AND TRACE ELEMENT
COMPOSITION STRATIGRAPHIC TRENDS
T h e s t r a t i g r a p h i c c h a n g e s o f t h e p e t r o g r a p h i c m o d e s are e l u c i d a t e d b y the c h e m i c a l v a r i a t i o n s o b s e r v e d in the lavas. S t r a t i g r a p h i c p l o t s o f selected m a j o r o x i d e s a n d t r a c e e l e m e n t s f o r t h e Izalco s e q u e n c e are given in Fig. 8. SiO2 a n d K : O increase slightly a n d CaO a n d MgO decrease f r o m t h e b o t t o m (IZ-6) t o t h e t o p (IZ-1) o f t h e s e q u e n c e . T h e t r a c e e l e m e n t s have less c o n s p i c u o u s trends, b u t t h e r e is an increase in Zr a n d La, a n d a decrease in Co a n d Cr
AT'I
227
• I ,AT._4/
7 CPX
%
,~--~-. . . . . . . ,-T.~'~ I 0
6
OJ
~ r~ AT'2 ",e
AT'l/ I AT'3 / e I
OL+OPX+ OP %
Fig. 7. Plot of the observed percentages of clinopyroxene cs. the sum of the observed percentages of olivine, orthopyroxene and opaque minerals for Atitl~n.
::[i
CoO9.0
8.5 I
-
-
MoO4.0
'"-..-_~._~,~
3.5
10
Cr
~
::0[
\ \
I
\\,
13 La11
=
Co
bottom
IZ.6
IZ.5
IZ.4
IZ.3
IZ.2
top
IZ.1
,Z.5
,Z'4
,Z.3
,z.2
Fig. 8. Stratigraphic plot of selected major oxides and trace elements for Izalco. Bars give the measure (+ lo) of the precision for each element. f r o m the b o t t o m t o the top. As with the p e t r o g r a p h i c m o d e s , the s e q u e n c e appears t o be c o n t i n u o u s with n o m a j o r breaks or reversals. T h e stratigraphic plots o f selected m a j o r oxides a n d trace elements for t h e Atitl~n s e q u e n c e are p r e s e n t e d in Fig. 9. The reversals suggested b y the p e t r o g r a p h i c m o d e s are c o r r o b o r a t e d by changes in slope at the same posi-
228 Covered interval
'°t o,o,.5I/
,.o~/
PrDDblo Re~rsal
i
?/
/.~¢' ?/
no[ [
.
V 190 t ~ / ~
170~ /
?/
20
7/./~"
Co
15 bottom AT.6
top
AT.5
AT'4 Cmred Interval
57 SiO2 56
]
AT.3
AT.2
AT,1
Pr|bakie Rove sal
?\\,~
54
K20
1.4
7-.
1.2 1.0
\~., 500 I
[
~"
Hf 3.5 3.0 bottom AT'6
i
AT'5
AT'4
-2
~.~
~.
I~.~
Fig. 9. Stratigraphic plot of selected major oxides and trace elements for Atitl~n. Bars give the measure (+ lo) of the precision for each element.
229 tions. The break between AT-4 and AT-5 is conspicuous. It should also be noted t h a t there is a slight tendency for a decrease in SiO2 and K20 from bott o m to top in each of the two defined groups -- from AT-3 to AT-1 and from AT-6 to AT-5. There is also a corresponding increase in CaO and MgO. Trace elements that are partitioned into the residual glass, such as Hf and Ba, suggest the same pattern as SiO~ and K20. Vanadium and Co show the same trends as CaO and MgO. The two volcanoes display different chemical trends that are consistent with their respective modal variations. The major oxide and trace element trends described for both volcanoes above suggest that shallow crystal fractionation by plagioclase, pyroxene and olivine may be the principal factor responsible for the chemical variations. The reversals in the Atitl~n sequence indicate that a simple, continuous process did n o t operate for that volcano, but rather suggests fractionation of successive batches of magma. Izalco appears to represent a more continuous fractionation trend. As discussed above, differences in summit elevation of the two volcanoes may account for the different fractionation patterns. GENERAL COMMENTS ON ADEQUACY
OF D A T A
Before discussion of chemical and petrographic results, two general comments on the adequacy of data collected herein are in order. One relates to whether the sections examined are long enough to really be representative of the behavior of the cores. The other deals with whether the mainly small variations seen in the data might n o t be just related to statistical sampling error. Short sections are simply what one usually has to deal with at most young volcanoes. You either decide to sample them, inadequate as they seem, or to ignore them. In this paper we did not rigorously examine whether the trends of chemical and petrographic data shown might be merely due to sampling error. Comparison of paired chemical and petrographic data on two sets of samples (IZ-4a, 4b and IZ-6a, 6b) however, do seem to show that in this case, the statistical variations among samples of similar composition are small. This might be due to the relatively small sizes of phenocrysts in most samples. Also we think it unlikely, particularly in the case of the Izalco sequence, that a consistent trend of data points could be statistical in origin. COMPUTER MODELING The hypothesis t h a t the lavas of each stratigraphic sequence can be explained by shallow crystal fractionation may be tested using a least squares c o m p u t e r program (Wright and Doherty, 1970). Compositions of observed phases are introduced with an assumed parental composition and the observed rock composition. The percentages of fractionated phases given by the program may be used in Rayleigh calculations to test the computer fit (Shaw, 1970). Partition coefficients used in the calculations are given in Table 3.
230 TABLE 3 Crystal-liquid partition coefficients used in the Rayleigh calculations (plag = plagioclase, cpx = clinopyroxene, opx -- orthopyroxene, ol = olivine, op -- opaque phase and ap = apatite)
Rb Sr Ba La Ce Sm Eu Lu Th Hf Ni Co Cr V Sc
plag
cpx
opx
ol
op
ap
0.05 1.83 0.3 0.14 0.2 0.15 0.34 0.2 0.01 0.01 0.26 0.1 0.04 0 0.01
0.003 0.12 0.002 0.1 0.175 0.6 0.65 0.8 0.001 0.001 1.8 1.5 15.0 2.31 1.6
0.022 0.017 0.013 0.02 0.05 0.075 0.06 0.75 0.001 0.001 3.8 1.4 2.0 0.5 0.7
0 0.014 0 0.01 0.007 0.01 0.01 0.02 0 0 8.7 3.8 2.4 0.09 0.02
0 0 0 0.002 0 0 0 0 0 0 6.6 8.0 2.8 24.0 2.0
0 2.0 0.01 30.0 30.0 50.0 20.0 25.0 0 0 0 0 0 0 22.0
Data from All6gre et al. (1977), Arth (1976). Leeman and Vitaliano (1976), Duke (1976), Ewart et al. (1973) and Zielinski and Frey (1970).
Choice o f parent If h e i g h t is an inhibiting f a c t o r in c o n e g r o w t h , t h e n it is a s s u m e d t h a t the early, m o r e m a f i c lavas e r u p t e d f r o m a v o l c a n o are c o n t i n u o u s l y being covered b y less m a f i c material a n d are never seen o n the surface o f an old cone. A v o l c a n o like Atitl~n, w h i c h m a y r e p r e s e n t an a d v a n c e d stage in volcanic c o n e d e v e l o p m e n t , m a y have o n l y andesitic lavas o n the surface a l t h o u g h the bulk o f t h e v o l u m e is c o m p o s e d o f m o r e m a f i c lavas. Because o n l y surficial lavas are available, n o s a m p l e d c o m p o s i t i o n s for Atitl~n p r o v e d suitable as a p a r e n t a l c o m p o s i t i o n . T h e r e f o r e , t h e inferred p a r e n t a l c o m p o s i t i o n f o r S a n t a Marfa v o l c a n o ( R o s e et al., 1 9 7 7 ) was used f o r the Atitl~n modeling. As p r e v i o u s l y m e n t i o n e d , the t w o v o l c a n o e s are similar in their g e o g r a p h i c l o c a t i o n , height, a n d c o m p o s i t i o n , s u p p o r t i n g the use o f this parent. Izalco m a y r e p r e s e n t an early stage in volcanic c o n e d e v e l o p m e n t . Conseq u e n t l y , r o c k analyses o f early basaltic lavas are available. A n average o f f o u r analyses o f ashes t h a t fell in 1931 was f o u n d t o be a suitable p a r e n t for t h e s u m m i t lavas (Deger, 1932). Trace e l e m e n t a b u n d a n c e s for b o t h p a r e n t s were d e t e r m i n e d f r o m H a r k e r plots a n d b y c o m p a r i s o n with analyses f o r lavas o f similar c o m p o s i t i o n .
Results o f calculations Izalco. T h e m o d e l i n g results f o r I z a l c o are given in Table 4. T h e close m a t c h b e t w e e n t h e o b s e r v e d a n d c a l c u l a t e d c o m p o s i t i o n s and t h e small errors indi-
231 cate that a good fit is obtained. There is a definite trend in total fractionation, showing a pronounced increase from 18.7% at the b o t t o m of the sequence to 39.1% at the top. The percentages of fractionated minerals show the following relative abundances: plagioclase > clinopyroxene :> opaque phase > olivine Fig. 10 shows a plot of the required percentages of mineral phases fractionated in order to account for the stratigraphic chemical changes. All the phases demonstrate a continuous, unbroken trend towards increasing effect of crystal fractionation from b o t t o m to top. 100
10 Cpx
r,
0,1 bottom IZ'6
top iZ'5
IZ'4
IZ'3
tZ'2
IZ'1
Fig. 10. Stratigraphic plot of fractionating phase percentages calculated by the least-squares c o m p u t e r program for the Izalco sequence (plag = plagioclase, cpx = clinopyroxene, op = opaque minerals and ol = olivine).
A titldn. Table 5 shows the results of the least-squares and Rayleigh calculations for the Atitl~n sequence. The total fractionation ranges between 41 and 60.7%. Again, the close match between calculated and observed (oxide percentages), and the small error indicate a good fit. The following relationship of the mineral phases fractionates is established: plagioclase > olivine > clinopyroxene > opaque phase = o r t h o p y r o x e n e A stratigraphic plot of the percentages of fractionation required to account for the chemistry is presented in Fig. 11. The break between AT-4 snd AT-5 is emphasized, and the other break is less well-defined. The t w o groups
232 TABLE 4 Model of crystal fractionation of Izalco parent producing the compositions observed in the stratigraphic sequence Parent*
IZ-1
IZ-2
obs.*
calc.
IZ-3
obs.*
calc
obs.*
calc.
I. Least-squares mixing calculations (Wright and Doherty, 1970) 51.80 0.98 19.30 9.80 4.40 9.59 3.35 0.10
SiO 2 TiO~ Al203 FeO* MgO CaO Na20 K20
55.10 0.76 18.56 8.43 3.89 8.79 3.41 1.10
55.00 0.76 18.54 8.42 3.89 8.79 3.44 1.17
54.52 0.77 18.96 8.63 3.84 8.83 3.38 1.07
0.0164"* Phases fractionated : plagioclase clinopyroxene olivine Fe oxides apatite***
54.45 0.77 18.94 8.63 3.84 8.83 3.40 1.13
54.61 0.84 19.08 8.34 3.91 8.75 3.39 1.09
0.015
26.5 5.4 3.0 4.1 0.4
54.55 0.84 19.07 8.34 3.91 8.75 3.41 1.13 0.0196
24.2 5.9 2.7 3.8 0.4
24.2 6.5 2.4 4.0 0.4
H. Rayleigh calculations (Shaw, 1970) 15.0 280.0 7.5 14.25 2.5 0.9 0.25 1.0 1.2 20.0 30.0 15.0 350.0 27.0 500.0
Rb Ba La Ce Sm Eu Lu Th Hf Ni Co Cr V Se Sr
23.7 402.0 14.5 17.9 2.8 1.06 0.30 1.53 1.71 15.0 25.4 8.2 258.0 29.3 471.0
24.2 415.0 10.1 19.0 2.9 1.14 0.32 1.63 1.96 13.5 24.5 6.9 140.0 29.9 440.0
25.8 378.0 8.2 14.9 3.18 1.06 0.32 1.70 1.85 16.0 24.8 6.4 238.0 30.6 --
23.5 406.0 9.9 18.6 2.88 1.12 0.31 1.57 1.89 14.0 24.8 6.3 149.0 28.8 --
*Recalculated to 100% after FeO adjustment. **Determined by summing the squares of calculated residuals. * * * N o t determined by least-squares method (see text).
25.8 378.0 8.4 14.8 3.1 1.08 0.32 1.54 1.82 11.0 24.5 6.4 239.0 30.2 443.0
23.5 406.0 9.3 17.6 2.7 1.06 0.30 1.58 1.90 14.1 24.5 5.6 139.0 28.2 452.0
0 C.0
.o
0
0 0
*
234 TABLE 5 Model of crystal fractionation of Atitl~n parent producing the compositions observed in the stratigraphic sequence Parent*
AT-1
AT-2
obs.*
calc.
AT-3
obs.*
calc.
obs.*
calc.
I. Least-squares calculations (Wright and Doherty, 1970) 50.90 1.10 19.30 9.30 6.30 9.40 3.10 0.66
SiO 2 TiO 2 Al~O s FeO* MgO CaO Na~O K~O
54.28 1.00 18.96 8.50 4.24 8.10 3.83 1.09
54.27 1.00 18.96 8.50 4.25 8.11 3.83 1.09
55.75 1.04 19.49 7.15 3.83 7.34 4.09 1.31
0.0315"* Phases fractionated plagioclase clinopyroxene olivine orthopyroxene Fe oxides apatite***
55.71 1.04 19.49 7.17 3.85 7.36 4,08 1.30
57.68 1.02 17.97 7.34 3.38 7.26 4.01 1.34
0.0652
25.5 4.8 5.0 3.6 2.9 0.4
57.68 1.02 17.96 7.34 3.40 7.27 4.03 1.40 0.0296
30.9 7.5 4.3 5.2 4.3 0.5
35.7 6.2 9.7 -4.0 0.6
II. Ray leigh calculations (Shaw 1970) 17.5 300.0 7.0 14.2 2.4 0.85 0.20 1.1 2.0 40.0 30.0 30.0 315.0 20.0 550.0
Rb Ba La Ce Sm Eu Lu Th Hf Ni Co Cr V Sc Sr
27.3 475.0 10.7 22.8 3.34 1.26 0.30 1.63 3.29 17.5 24.9 13.3 212.0 24.4 504.0
29.5 467.0 10.4 19.7 3.01 1.19 0.32 1.88 3.42 21.0 24.6 14.1 184.0 23.7 511.0
36.8 543.0 13.3 25.6 4.30 1.27 0.31 2.38 3.80 16.0 19.4 9.8 165.0 20.4 517.0
35.8 550.0 11.8 19.7 3.2 1.31 0.35 2.29 4.17 18.4 22.6 7.9 115.0 23.0 509.0
*Recalculated to 100% after FeO* adjustment. **Determined by assuming the square of calculated residuals. ***Not determined by least-square method (see text).
33.3 607.0 11.9 19.5 3.35 1.19 0.27 2.86 3.82 21.0 20.4 10.5 168.0 24.0 472.0
38.4 578.0 12.3 22.9 3.3 1.37 0.39 2.46 4.48 13.3 20.5 10.3 140.0 26.8 471.0
O~
m
~
~MN~dN4M
~0 aO 0
t-t 0 0
O~ 0
c~
236 100 Covered Interval
e Reversal :i Proaabl
._~
?-_:_.
10
-
OI
I
Opx
0.1 ~ bottom
i
AT.6 AT'5
Fig. 1 i. Stratigraphic plot of fractionating phase percentages calculated by the least-squares computer program for the Atitl~n sequence (plag = plagioclase, ol = olivine, cpx = clinopyroxene, op = opaque minerals and opx = orthopyroxene).
(AT-3 to AT-1 and AT-6 to AT-5) are less fractionated towards the top of each group.
Other aspects of modeling The modal abundances of olivine in the Izalco and Atitl~n lavas contrast with the percentages of olivine calculated in the fractionation modeling. This is probably a result of the compositions chosen for the parents. The Izalco parent is a real composition that was very likely fractionated by plagioclase and olivine before eruption. Thus, the parent may have been relatively depleted in olivine, which would lower the fractionating phase percentage. For Atitl~n, the parent is an inferred composition that presumably reflects little of no differentiation. Calculations involving this parent could show the entire effect of the crystal fractionation process. The Atitl~n summit lavas may have been derived from a parent much like the Santa Marfa parent, b u t through a series of intermediate steps.
Apatite effect The least-squares and Rayleigh calculations are considered to be permissive of crystal fractionation as the cause of the chemical trends for both volcanoes.
237 However, one of the difficulties encountered in the modeling was the depletion of the rare earth elements relative to the predicted values. Fractionation at depth of pargasitic hornblende, the traces of which may be preserved as crystal clots (Stewart, 1975), would cause enrichment of the light relative to the heavy rare earth elements. This type of enrichment pattern may be present for the lavas of this study, b u t the lack of good analyses for the heavy rare earth elements makes it difficult to evaluate the trend. In any event, the few crystal clots observed in the Izalco and Atitl~n lavas are different from those described by Stewart (1975). In addition, it was found that the amounts of amphibole fractionating at depth necessary to account for the effect was over 25%, which the trace element modeling suggested was unreasonable. Another possible explanation for the depletion is fractionation of plagioclase phenocrysts that contain abundant apatite inclusions. Apatite has very high partition coefficients for all the rare earth elements and if only a small a m o u n t was involved in the fractionation, all the rare earth elements would be significantly affected. Since plagioclase, pyroxene, olivine and opaque minerals exclude P205, it is strongly partitioned into the residual glass as crystallization of a liquid proceeds, unless apatite is crystallizing. Anderson and Greenland (1969) have stated that apatite crystallization will only occur after a b o u t 70% total crystallization. However, their studies were of Hawaiian basalts which are relatively depleted in Cl and other volatiles (Anderson, 1974). Glass inclusion analyses from ashes erupted during the 1974 Fuego eruption show high concentrations of Cl and other volatiles (Rose et al., 1978). Increased volatile pressure, especially F and C1, might p r o m o t e early apatite crystallization. If a rock has an initial P2Os content of 0.2% and is 50% crystallized, the P2Os content is doubled; b u t 0.5% apatite crystallization will deplete the P:Os content by about the same amount, maintaining an almost constant P2Os content. It is thus generalized that for every 10% crystallization, 0.1% apatite crystallization will maintain a constant P2Os content. The Izalco lavas show no systematic variation in P2Os throughout the sequence despite the variation in K20. The Atitl~n lavas do display some changes, but it is noted that for the most part, the P2Os content does not follow the trend of K20, Cs, Th and Hf as would be expected if it were being partitioned solely into the residual glass (Fig. 12). Since the lavas of both volcanoes contain plagioclase phenocrysts with abundant apatite inclusions, it is assumed that apatite was an early crystallizing phase and apatite was therefore used in the Rayleigh calculations. The apatite proportions for the calculations were determined by the percent apatite crystallization percent total crystallization ratio explained above. The uncertainties in the partition coefficients allow a great deal of flexibility, but perhaps define a minimum and a maximum for the apatite effect.
238 0.3
Covered Interval
~ :
Probable Reversal
Atitl:in
71 / 0.2
bottom AT.6 0,2
AT,5
AT,4
AT,3
AT.2
]
-
P2O5 0.1
Izalco
bottom IZ.6
IZ.5
IZ.4
IZ.3
IZ'2
tnn i~"1
Fig. 12. Stratigraphic plots of P2Os for Izalco and Atitl~n. INTERPRETATIONS
The two volcanoes examined have different heights, volumes, eruptive frequencies and compositions. They also display different crystal fractionation trends. The general trend towards a less mafic composition with time that has been established for other volcanoes is well developed for Izalco. The stratigraphic sequence shows a trend that can be explained by successive eruptions from a continuously fractionating mafic parent, producing successively more silicic lavas towards the top of the sequence. At Atitl~n it is suggested that the observed trend may be explained by shallow crystal fractionation cycles. Successive batches of magma are fractionated by plagioclase, pyroxene and olivine, forming a series of small batches of zoned magma that are subsequently erupted and preserved as inverted sequences. The entire stratigraphic sequence may represent three separate zoned magmas, defined by AT-3 to AT-I; AT-4, from which several units appear to be missing; and AT-6 and AT-5. The different trends seen in the t w o stratigraphic sequences, one continuous and the other broken by reversals, may be explained by the physical differences between the t w o cones. At Izalco, the small structure has no strong inhibiting effect on eruptive frequency and allows nearly continuous eruptions of magma. The short repose times mean that crystal fractionation will take place for only a short time before eruption. The greater height of Atitl~n restricts eruptive activity because of the greater lithostatic versus hydrostatic pressure. Longer repose times allow extensive crystal fractionation to zoned magmas. These magmas are erupted in sequence, the top of the chamber first,
239
followed by deeper, more mafic lavas. Reversals in stratigraphic trends occur when repose allows advanced differentiation. Since periods of differentiation lengthen progressively as the cone grows, an oscillatory trend to gradually more silicic lava develops. CONCLUSIONS
The two volcanoes have different chemical trends for the stratigraphic sequences studied. The sequence at Izalco becomes continuously richer in SIO2, K20, Zr and La as CaO, MgO, Co and Cr are depleted from b o t t o m to top. In contrast, at Atithh~ the sequence is incomplete and shows reversals. In each of the sections separated by the reversals, SIO2, K20, Ba and Hf decrease and CaO, MgO, V and Co increase from the b o t t o m of the section to the top. The different groups may be distinguished from each other by the proportions of their mafic minerals. Both chemical trends can be explained by shallow crystal fractionation of the observed phenocryst phases. Because of the large partition coefficients of apatite for the rare earth elements, the effect of apatite crystallization is important in crystal fractionation trends, if apatite is included in the large fractionating phenocrysts. Early apatite crystallization can account for the depletion of all the rare earth elements relative to the other trace elements. ACKNOWLEDGEMENTS
The field work and the analytical work for this study was supported by NSF grant DES 74 19025. A.P. Ruotsala, N.K. Grant and D.C. Noble reviewed the original version of this work and provided many useful suggestions.
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