Programs in Geosciences, The University of Texas at Dallas, Richardson, TX 75080 ... magmas in the active Volcano and Mariana island arcs, Western Pacific.
Journal of Volcanology and Geothermal Research, 18 (1983) 4 6 1 - - 4 8 2
461
Elsevier Science Publishers B.V., A m s t e r d a m -- Printed in The Netherlands
TRACE-ELEMENT A N D ISOTOPIC CONSTRAINTS ON THE SOURCE OF MAGMAS IN THE ACTIVE VOLCANO A N D M A R I A N A ISLAND ARCS, WESTERN PACIFIC
R O B E R T J A M E S S T E R N and EMI ITO
Programs in Geosciences, The University of Texas at Dallas, Richardson, TX 75080 (U.S.A.) Department of Geology and Geophysics, University o f Minnesota, Minneapolis, MN 55455 (U.S.A.) (Received O c t o b e r 20, 1982)
ABSTRACT Stern, R.J. and Ito, E., 1983. Trace-element and isotopic constraints on the source o f magmas in the active Volcano and Mariana island arcs, Western Pacific. In: S. Aramaki and I. Kushiro (Editors), Arc Volcanism. J. Volcanol. G e o t h e r m . Res., 18: 461--482. Analytical results o f the relative and absolute a b u n d a n c e o f L I L - i n c o m p a t i b l e trace elements (K, Rb, Cs, St, and Ba) and isotopic compositions (lsO/l~O, 8~Sr/86Sr, and 143Nd/144Nd) are s u m m a r i z e d for fresh samples f r o m active and d o r m a n t volcanoes o f the Volcano and Mariana island arcs. The presence of t h i c k e n e d oceanic crust (T ~ 1 5 - 20 km) b e n e a t h t h e arc indicates t h a t while h y b r i d i z a t i o n processes resulting in the modification o f primitive magmas b y anatectic mixing at shallow crustal levels c a n n o t be neglected, the e x t e n t and effects o f these processes on this arc's magmas are minimized. All c o m p o n e n t s o f the s u b d u c t e d plate disappear at the trench. This observation is used to reconstruct the c o m p o s i t i o n o f the crust in the Wadati-Benioff z o n e by estimating prop o r t i o n s o f various lithologies in the crust o f the s u b d u c t e d plate coupled with analyses f r o m DSDP sites. Over 90% o f the mass o f the s u b d u c t e d crust consists o f basaltic Layers II and III. S e d i m e n t s and seamounts, containing the bulk o f the incompatible elements, m a k e up the rest. Bulk Western Pacific seafloor has 87Sr/86Sr ~ 0.7032, 6~80 - +7.2, K / R b ~ 510, K]Ba ~ 46, and K/Cs ~ 13,500. Consideration o f trace-element data and c o m b i n e d ~ ~80 - 8~Sr/8~Sr systematics limits the participation o f sediments in magmagenesis to less t h a n 1%, in accord w i t h the earlier results o f Pb-isotopic studies. Combined 143Nd/144Nd ~ STSr]8~Sr data indicate little, if any, involvement o f altered basaltic seafloor in magmagenesis. Perhaps m o r e i m p o r t a n t t h a n m e a n isotopic and L I L - e l e m e n t ratios is t h e restricted range for lavas f r o m along over 1000 k m o f this arc. Mixtures o f m a n t l e w i t h either the s u b d u c t e d crust o r derivative fluids should result in strong heterogeneities in the sources o f individual volcanoes along the arc. Such heterogeneities would be due to: (1) gross variations o f crustal materials supplied to the s u b d u c t i o n z o n e ; and (2) lesser efficiency o f mixing processes a c c o m p a n y i n g induced c o n v e c t i o n b e t w e e n arc segments (parallel to the arc) as c o m p a r e d to t h a t perpendicular to the arc. The absence o f these heterogeneities indicates that either s o m e process exists for the efficient mixing o f m a n t l e and s u b d u c t e d material parallel to the arc or that s u b d u c t e d materials play a negligible role in the generation o f Mariana-Volcano arc melts. U.T.D. C o n t r i b u t i o n N u m b e r 421.
0377-0273/83/$03.00
© 1983 Elsevier Science Publishers B.V.
462 Consideration of plausible sources in the mantle indicates that (1) an unmodified MORB-like mantle cannot have generated the observed trace-element and isotopic composition o f this arc's magmas, while (2) a mantle similar to that which has produced alkaliolivine basalts (AOB) o f north Pacific " h o t s p o t " chains is indistinguishable in many respects from the source of these arc lavas. INTRODUCTION
The origin and evolution of magmas at convergent plate boundaries are among the most intriguing of all petrologic problems. Recycling of crustal materials back into the Earth's mantle is accepted as a critical corollary of plate tectonic axioms, and appears to be the only mechanism capable of impeding the irreversible differentiation of the earth (Hofmann and White, 1980). It follows that our understanding of the evolution of the Earth's crust and mantle depends on how well we know especially the following two aspects of the subduction process: (1) Over what portion of the earth's history has such activity been important? and (2) What are the relative proportion and absolute masses of material returned to the mantle compared with that distilled back into the crust as a result of arc magrnatism? The first of these questions is b e y o n d the scope of this paper. Results of our isotopic and geochemical studies in the western Pacific, however, bear on the second. In spite of two decades of field, petrologic, and geochemical investigations into problems of arc magmagenesis, none of the present generalized models have proven entirely acceptable. Considering the wide range in the composition of arc volcanic rocks, this is to be expected. Following the complicated interactions resulting from the differing thermal regimes, tectonic styles, and compositional variations present in subducted crusts and overlying mantle wedges of the Earth's convergent plate boundaries, this should result in a similar range in the processes and products of melting. Recognition of the potential independence of many dynamic processes among arcs thus suggests that the study of each arc should be based on the evidence from that arc alone instead of attempting to apply conclusions reached from studies of other arcs. The story that will emerge after integrating such studies will doubtless be complex, but should more accurately reflect reality. It is in this spirit that we outline trace element and isotopic constraints on magmagenesis in two related arc systems, the Mariana and Volcano arcs. We will argue in the next section that these arcs are built on primitive crust and that melts rising through this crust should not be contaminated. Concentration of the major elements and "compatible " trace elements (i.e. trace elements with Dxl/liq •/> 0.1)will vary in response to crystal-liquid equilibrium in the melt zone as well as re-equilibration during ascent and storage in magma chambers; unless the ascent rate approaches the adiabat, the abundance of these elements in eruptive rocks will reveal little about the composition of the source. Concentrations o f the large ion lithophile (LIL) incompatible elements (Dxl/liq • ~ .1; e.g. K, Rb, Cs, Ba, and to a lesser extent, Sr) will also vary as a function of the fractionation history of the magmas. However, inso-
463
far as mica and amphibole are not residual, the ratios of LIL incompatible elements K/Rb, K/Ba, K/Cs, etc., should change little during moderate degrees of fractionation. These phases should be entirely consumed during moderate degrees of melting and are very rare as phenocrysts in Volcano and Mariana arc lavas. Isotopic compositions of especially the heavier elements Sr, Nd, and Pb also should not change during fractionation, and the isotopic composition of O (expressed as 5180)1 should also be unaffected at magmatic temperatures. A recent paper by Kyser et al. (1981) suggests that oxygen may fractionate at magmatic temperatures, but we consider their evidence to be inconclusive. Thus we contend that LIL incompatible element ratios K/Rb, K/Cs, and K/Ba as well as isotopic ratios of O, Sr, Nd, and Pb in Mariana-Volcano arc lavas should be very similar to that of the source. In the succeeding section, we argue t h a t we can outline the incompatible elem e n t and isotopic characteristics of sources in the subducted crust and mantle wedge. By comparing these with the isotopic and trace-element compositions in fresh arc lavas, we will a t t e m p t to help resolve the source of melts in this system. GEOLOGIC SETTING: THE ROLE OF THE CRUST IN THE OVER-RIDING PLATE
The Mariana and Volcano arcs are southern parts of the volcanic chain that stretches over 2000 km from Tokyo to Guam (Fig. 1). The WadatiBenioff zone dips 45 ° in the north to depths of ~ 500 km. Beneath the Marianas, the seismic zone dips ~ 30 ° to 150 km, then plunges vertically to over 600 km. Beneath the Volcano arc seismicity is diffuse and a well-defined Wadati-Benioff zone does not exist. Seismicity beneath the Volcano arc is concentrated at depths shallower than 100 km (Katsumata and Sykes, 1969). This chain is built on oceanic crust as a result of the change in plate motion of the Pacific plate ca. 40 m.y. ago which converted a transform fault connecting segments of the Kula Pacific Ridge into the site of west-dipping subduction {Uyeda and Miyashiro, 1974). Subsequent back-arc rifting led to the development of the Parece Vela Basin in the Upper Oligocene and Lower Miocene and the Mariana Trough since the Pliocene (Crawford et al., 1981). The evolution of the Mariana-Volcano arc system thus reconstructed indicates it is built on Lower Tertiary oceanic crust. Geophysical surveys indicate this crust has since been thickened to 18--22 km (Murauchi et al., 1968; Sager, 1980}. This is an increase of 12--16 km over the thickness of normal oceanic crust (~ 5--6 km). Assuming thickening occurred continuously over the arc's history, this represents a thickening rate of 300--400 m/million years (~ 0.01 m m / y r ) . Since periods of little igneous activity in the arc are inferred for much of the Oligocene, Upper Miocene, and Pliocene (Crawford et al., 1981), this estimate of thickening rate is a minimum. Thickening m a y ~5 is 0 = ( R / R s M o w
--1) × 1000;R = lsO/160; SMOW = standard mean ocean water
464 130°
140 °
150 °
160 ° 1
E
TRENCH WATER DEPTH 4 KM I----I UATERNARY VOLCANO + SHATSKY R I ~
PACIFIC OCEAN
II98A
PMAKER
~5
o
MAGELLAN SEAMOUNTS o
0
~ o
• 199
-"-
I~
' ~'~"~
I-NEW GUINEA
CAROLINE
~
16
t~'i? SOLOMON;(I:'~
Fig. 1. Map of the western Pacific, modified after the chart of Chase et al. (1971). In addition to the location of major bathymetric features, volcanic islands studied in the course of this investigation are identified by numbers: 1 = O-Shima; 2 = Myojin Sho; 3 = Nishino-Shima; 4 = Iwo Jima; 5 = Fukujin (seamount); 6 = Uracas; 7 = Asuncion; 8 = Agrigan; 9 = Pagan; 1 0 = Guguan; 1 1 = Sarigan; 1 2 = Esmeralda Bank (seamount); 1 3 = Mariana Frontal Arc; 1 4 = Truk; 1 5 = Ponape; 1 6 = Kusaie (Kosrae). Also shown are the locations of Deep Sea Drilling Project sites 57.2,195B, 198A and 199. have been accomplished by the development of layered igneous bodies w i t h i n a n d b e l o w t h e c r u s t as w e l l as o u t p o u r i n g s o f l a v a ( I s s h i k i , 1 9 6 3 ; Stern, 1979}. T h e f a c t t h a t t h e M a r i a n a - V o l c a n o arc, o n e o f t h e y o u n g e s t a n d m o s t p r i m i t i v e o f all arcs, n e v e r t h e l e s s sits o n c r u s t w h i c h h a s t r i p l e d o r q u a -
465 drupled in thickness since the early Tertiary limits the confidence with which we can resolve whether the melts of this system were derived from subducted crust or mantle. Some lavas of arcs built on thickened crust elsewhere (Andean-type margins) are contaminated by anatectic crustal melts (e.g., Tilton and Barreiro, 1980}. We expect that while the possibility of similar processes occurring beneath the Mariana-Volcano arc cannot be precluded, a t t e n d a n t compositional changes in ascending melts should be minor. This conclusion results from the following considerations: ( 1 ) t h e thickened crust, composed of Lower Tertiary oceanic crust, anhydrous sub-volcanic gabbroic bodies, and arc lavas, will be relatively refractory; (2) the density of this crust is about ~ 2.9 g/cm 3 (Sager, 1980) compared to ~2.6 g/cm 3 for basaltic melts (Bottinga and Weill, 1970). Under these conditions, stagnation of basaltic melts in the crust should be limited to the development of shallow magma chambers beneath the larger edifices, leaving little opport u n i t y for chemical exchange between melt and crust; and (3) the chemical and isotopic similarity between basaltic melts rising through the crust and the crust itself should insure that what chemical exchange does occur will not severely affect derivative melts. In summary, while the Mariana-Volcano arc is one of the most promising of all arcs for "seeing t h r o u g h " the effects of crustal contamination suspected in other arcs, such contamination cannot be dismissed. We can only say that of all arc systems, the extent and effects of contamination should be minimized in this system. COMPOSITION AND ROLE OF SUBDUCTED CRUST AND SEDIMENTS The Volcano and Mariana arcs are a promising system in which to study problems of arc magmagenesis, n o t only because of their primitive crustal setting, but also because it appears that everything that arrives at the trench disappears down it. Samplings of the inner wall of the Mariana Trench have recovered peridotites, gabbros, and basalts as well as more exotic quartz diorites, dacites, and boninites (Dietrich et al., 1978; Anonymous, 1978; Bloomer and Hawkins, 1980). Offscraped pelagic sediments have not been recovered; we conclude that the western Pacific seafloor and pelagic sedim e n t cover are carried intact into the subduction zone. The crust arriving at the Mariana Trench is the oldest seafloor in the Pacific (Early Jurassic; Larson and Chase, 1972; Hilde et al., 1977). In contrast to arcs such as the Aleutians and Cascades where subducted sediments have an arc provenance and are, therefore, compositionally similar to the arc lavas, sediments subducted beneath the Mariana-Volcano arc are dominantly pelagic. The thickness of the sedimentary section is uncertain because DSDP drill holes have not reached basement. Sub-bottom penetration in DSDP Leg 20 holes approached 500 m, while reflection profiles in the region suggest basement lies 800 m or more beneath the seafloor (Davies, 1973; Zachariadis, 1973). We assume a conservative sediment thickness of 500 m in subsequent calculations. The lithologic proportions of this is estimated
466 f r o m D S D P Leg 6 a n d 20 s u m m a r i e s (Fischer and H e e z e n , 1971; H e e z e n et al., 1 9 7 3 ) ; this c o m p o s i t e section is c o m p o s e d o f 140 m T e r t i a r y v o l c a n o genic s e d i m e n t , 217 m M e s o z o i c a n d L o w e r T e r t i a r y chalk, 120 m M e s o z o i c abyssal clay, and 23 m Mesozoic chert. In a d d i t i o n to basaltic c r u s t and pelagic s e d i m e n t s , s e a m o u n t s are also b e i n g supplied to the t r e n c h . Since alkalic r o c k s have n o t b e e n r e c o v e r e d f r o m t h e inner wall o f t h e Mariana T r e n c h , it a p p e a r s these are also being s u b d u c t e d . T h e v o l u m e o f 95 s e a m o u n t s w i t h edifice heights o f at least 3 5 0 m w e r e c a l c u l a t e d for t h e region east o f the M a r i a n a - V o l c a n o arc, b e t w e e n 10 ° and 2 5 ° N a n d b e t w e e n 150 ° and 1 6 5 ° E , f r o m the b a t h y m e t r i c c h a r t o f Chase et al. (1971). T h e v o l u m e s , a s s u m e d densities, a n d masses o f t h e t h r e e c o m p o n e n t s o f w e s t e r n Pacific c r u s t are listed in T a b l e I. TABLE I Nature of the crust being subducted beneath the Mariana-Volcano arc
Basaltic seafloor 1 Sediments 2 Seamounts 4
Volume (× l0 s km 3)
Density (g/cm 3)
Mass (× 1018 kg)
% of total subdueted mass
82.5 7.5 3.64
3.0 1.52 3.0
24.8 1.143 1.09
91.8 4.2 4.0
1Thickness = 5.5 km. 2 Thickness = 500 m. 3Mass of sediments calculated using 140 m Tertiary volcanogenic sediment (p ~ 1.2 g/cm3), 217 m Mesozoic and Lower Tertiary chalk (p ~ 1.6 g/cm3), 120 m Mesozoic abyssal clay (p - 1.6 g/cm3), and 23 m Mesozoic chert (p - 2.3 g/cm3). 4 Volume based on point-counting bathymetric highsbetween 10--25° N and 150--160°E (Chase et al., 1971). T r a c e - e l e m e n t , STSr/S6Sr, and 5180 analyses for 10 r e p r e s e n t a t i v e s a m p l e s o f w e s t e r n Pacific s e d i m e n t s are r e p o r t e d in T a b l e II. Using the a s s u m e d lithologic p r o p o r t i o n s a l r e a d y p r e s e n t e d and densities listed in T a b l e I, t h e b u l k c o m p o s i t i o n o f s u b d u c t e d w e s t e r n Pacific s e d i m e n t s was d e t e r m i n e d {Table III). M o r e difficult t o assess is t h e b u l k c o m p o s i t i o n o f t h e s e a m o u n t s and basaltic crust. V e r y little is k n o w n a b o u t the s e a m o u n t s ; in T a b l e I I I these are a s s u m e d t o be c o m p o s e d o f t y p i c a l o c e a n island alkali basalt (Engel et al., 1965). Basaltic b a s e m e n t b e n e a t h the s e d i m e n t s has n o t b e e n recovered, b u t for m o d e l l i n g p u r p o s e s is a s s u m e d to consist o f 10% w e a t h e r e d basalt and 90% fresh M O R B ( H a r t et al., 1 9 7 4 ; H a r t , 1975). Using these values, t h e b u l k c o m p o s i t i o n o f t r a c e e l e m e n t s a n d Sr- and O - i s o t o p e s in t h e s u b d u c t e d w e s t e r n Pacific s e a f l o o r has b e e n r e c o n s t r u c t e d in T a b l e III. C o m p a r i s o n o f L I L i n c o m p a t i b l e e l e m e n t ratios b e t w e e n t h e s e s u b d u c t e d c o m p o n e n t s and t h e fields o c c u p i e d b y M a r i a n a - V o l c a n o arc lavas is s h o w n in Fig. 2. T h e f o l l o w i n g r e l a t i o n s h i p s b e t w e e n s u b d u c t e d s e a f l o o r a n d the arc lavas are a p p a r e n t :
588 17,884 15.1 0.018 0.697
8763 14.9 832 0.49 580 0.70841+5 30.1 --
380 7522 5.7 0. 01 0 0.655
3836 10.1 1035 0.51 678 0.70763~ 5 28.4 1.3
199 10-2 38-39
733 11,502 46.7 0.027 0.420
7476 10.2 381 0.65 160.2 0.70807,6 29.5 3.6
199 11-1 29-31
204 2787 82.2 1.070 2.66
31,218 152.7 1427 11.2 380.0 0.71499~3 -28.1
198A2-2 103-104
249 3056 84.7 0.645 1.89
30,867 124.2 192.7 10.1 364.3 0.71230,8 -27.5
198A3-3 77-78
308 4110 85.4 0.391 1.41
2589 8.4 21.5 0.63 30.3 0.71056~4 -3.3
198A CC
426 8517 176.2 2.26 5.47
511 1.2 0.53 0.06 2.90 -25.7 1.3 292 5355 2.04 0.016 2.30
2624 9.0 561.2 0.49 1288 0.70758, 8 27.4 0.9
195B 3-1 1 9 5 B 3 - 1 124-125 129-130
548 13,929 155.8 0.138 0.484
16,715 30.5 221.6 1.20 107.3 0.70554~8 -64.3
59.2 4-1 109-111
1847 3 9 ,9 5 5 213.4 0 .0 9 8 0 .8 4 7
39,156 21.2 2 1 6 .6 0.98 183.5 0.70526,9 -70.2
59.2 1-3 99-100
Lithologies: 199 1-2 92-93: L o w e r T e r t i a r y foram o o z e ; 199 10-2 38-39: P a l e oc e ne n a n n o c h a l k ; 199 11-1 29-31 : Paleocene l i m e s t o n e ; 1 9 8 A 2-2 103-104: Late C r e t a c e o u s c l a y ; 1 9 8 A 3 - 3 7 7 - 7 8 : Late C r e t a c e o u s c l a y ; 1 9 8 A C C : Late C r e t a c e o u s chert: 195B 3-1 124-125: L o w e r Cretaceous chert; 195B 3-1 129-130: L o w e r C r e t a c e o u s c h a l k ; 59.2 4-1 109-111: Early Miocene t uff; 59.2 1-3 9 9 1 0 0 ; Early Miocene tuff. Trace el em en ts analyzed by is ot ope d i l u t i o n . STSr/86Sr n o r m a l i z e d to ~Sr/88Sr = 0.1194 and 87Sr/86Sr = 0 . 7 0 8 0 0 . Ox y gen isotope data based on ~ ~sO for NBS-28 q u a r t z = 9.50°/,,,.
K/Rb K/Cs K/Ba Rb/Sr Ba/Sr
K (ppm) Rb Sr Cs Ba 87Sr/86Sr ~)xsO (°/oo) % loss (110°C)
199 1-2 92-93
Trace-element and i s o t o p i c c o m p o s i t i o n of western Pacific s e d i m e n t s
T A B L E II
468 TABLE III Composition of subducted western Pacific seafloor
K (ppm) Rb Sr Cs Ba 87Sr/86Sr 180 (°/oo) K/Rb K/Ba K/Cs Rb/Sr Ba/Sr
Sediments I
Seamounts 2
Basaltic crust 3
10205 32.3 361 2.3 382 0.7083 24.54
14027 33 815 0.35 498 0.7034 6.05
1265 1.7 125.6 0.06 11.5 0.7027 6.56
316 27 4440 0.09 1.06
425 28 40,000 0.04 0.61
744 110 21,000 0.01 0.09
Bulk western Pacific seafloor 2160 4.2 164 0.16 47 0.7032 7.2 514 46 13,500 0.03 0.29
1 Bulk western Pacific sediment composition based on weighted average of each of the 4 major lithologies in Table II. X i=
Z4
j=l
C~(1--Li)pjTi ~ 500
x i = mean concentration of element "i" in subducted sediment Cj = concentration of element " i " in lithology "j" L = fraction of weight lost during drying p = bulk density T = thickness of sediments in meters j = Chalk, chert, clay, volcanic ash 2 Elemental abundances for alkali basalt (Engel et al., 1965); STSr/8~Sr is for northern Pacific ocean islands (Hedge, 1978). Cs from K/Cs N 40,000 (Hofmann et al., 1978). 3 Hypothetical basaltic crust composed of 10% weathered basalt (AD5-18 intermediate; Hart et al., 1974) and 90% fresh "Average MORB" (Hart, 1975). 45180 for entire sedimentary column after Garlick and Dymond (1970), Savin and Epstein (1970), and Lawrence et al. (1975), as well as data from Table II. s Oceanic island alkalic lavas (Taylor, 1968) Muehlenbachs and Clayton, 1972. (1) S i n c e t h e fields o c c u p i e d b y s e d i m e n t s , b a s a l t i c c r u s t , b u l k w e s t e r n Pacific seafloor, or M O R B - t y p e m a n t l e do n o t c o r r e s p o n d to t h a t d e f i n e d by M a r i a n a - V o l c a n o arc lavas, n o n e o f t h e s e a l o n e c a n b e t h e s o u r c e o f t h e arc magmas. (2) M e l t i n g o f s u b d u c t e d s e a m o u n t s a l o n e , o r s e a m o u n t s p l u s a s m a l l a m o u n t o f w e s t e r n P a c i f i c b a s a l t i c c r u s t is a l l o w e d . (3) M e l t i n g o f 1 0 - - 2 0 % s e d i m e n t s a n d 8 0 - - 9 0 % b a s a l t i c c r u s t , w i t h o r w i t h o u t s e a m o u n t s , is a l l o w e d . (4) M i x i n g a n d m e l t i n g o f 2 - - 5 % s e d i m e n t a n d 9 5 - - 9 8 % M O R B - t y p e m a n t l e is a l l o w e d . (5) M e l t i n g o f a n o c e a n - i s l a n d o r " h o t - s p o t " t y p e m a n t l e is a l l o w e d .
469 120
W. PACIFIC BASALT
CRUST
IO0 8O K/Ba
e CHERT /
~
o~/ ~ A~o > ~ ~ ........
60
MARIANAVOLCANO
40
BULK ~
20
J
~. o' MORB-type o~:/ Mantle (213 ppm K, 0.2 ppm Rb 2.4 ppm Ba)
/ /1/
o '
~
CLAyO
IC
~,~m'BULK
W. PACIFIC
\'~______~_SEA _.,~FLOOR
SEOl MEN T c:f~d~~ 9 ~ Z ~ SEAMOUNTS J . . ' 4 " ~
:
~ Plagioclase-controlled fractionation
/
AOB- type Manlle 0
I
200
I
oCHALK I
400
I
1
600 K/Rb
I
I
800
I
I
iooo
I
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Fig. 2. I n c o m p a t i b l e e l e m e n t ratios o f M a r i a n a - V o l c a n o arc lavas and possible sources in t h e m a n t l e a n d s u b d u c t e d seafloor. F r e s h r o c k s f r o m t h i s arc have a l i m i t e d range in K / R b ( 3 5 0 - - 6 0 0 ) a n d K / B a ( 2 0 - - 5 0 ) . T h i s c o n t r a s t s w i t h m u c h greater ranges in m e a n r a t i o s f o r pelagic a n d w i n d - b l o w n s e d i m e n t s o f t h e w e s t e r n Pacific. Bulk s e d i m e n t (Table II) has a K / R b ~ 3 2 0 a n d K / B a ~ 27. M i x t u r e s o f b u l k s e d i m e n t w i t h w e s t e r n Pacific b a s a l t i c c r u s t i n t e r s e c t t h e field o f M a r i a n a - V o l c a n o arc lavas at 1 0 - - 2 0 % s e d i m e n t , a m u c h h i g h e r p r o p o r t i o n t h a n is p e r m i t t e d o n t h e basis o f Pb i s o t o p i c d a t a (Meijer, 1 9 7 6 ) . T h e c o m p o s i t i o n s o f sources e x p e c t e d in t h e o v e r l y i n g m a n t l e w e d g e are i n d i c a t e d b y stars. Values for t h e M O R B - t y p e m a n t l e were d e t e r m i n e d b y a s s u m i n g t h a t ( 1 ) M O R B = 20% partial m e l t o f t h e m a n t l e : a n d (2) K, R b , a n d Ba are t o t a l l y p a r t i t i o n e d i n t o t h e m e l t . T h u s , t h e M O R B - t y p e m a n t l e has 0.2× t h e c o n c e n t r a t i o n o f K, R b , and Ba o f t h e " A v e r a g e M O R B " r e p o r t e d b y Hart ( 1 9 7 5 ) . S u c h a m a n t l e clearly c a n n o t be t h e s o u r c e o f t h e s e arc m a g m a s , a l t h o u g h m i x t u r e s w i t h 2--5% s e d i m e n t d o i n t e r s e c t t h e field for arc lavas. F i n a l l y , a n A O B - t y p e m a n t l e similar t o t h a t r e s p o n s i b l e for t h e g e n e r a t i o n o f t h e average o c e a n i c alkali basalt o f Engel et al. ( 1 9 6 5 ) is very similar to t h e s o u r c e o f MarianaV o l c a n o arc lavas.
Combined use of oxygen and strontium isotopes is also useful for evaluating the role of the subducted crust. Subducted sediments have 6180 25%0 and 87Sr/S6Sr ~ 0.708 while the bulk subducted western Pacific seafloor should have 51so ~ 7.2%0 and 87Sr/S6Sr ~ 0.7033 (Table III). Both of these are different from Mariana-Volcano arc lavas (STSr/S6Sr = 0.7033-0.7034; 6 1 8 0 = 5.5--6.80/00; Ito and Stern, 1981; Douthitt and Dixon, 1981) While we cannot rule out mixing of bulk seafloor with the mantle to produce the arc values, the data limits the participation of sediments in the mixing or melting process to 1% or less (Fig. 3). The limited range of 6180 and STSr/S6Sr values exhibited by Mariana-Volcano arc lavas (1.3°/00 total variation in t~180, +0.04% variation in STSr/86Sr) contrasts strongly with the Banda arc (+3.6°/00 total variation in 5 ~sO, +0.35% variation in 87Sr/S6Sr), where compelling arguments favoring the mixing of sediments and mantle have been presented (Magaritz et al., 1978). On this basis, it is clear that suggestions 3 and 4 of the previous paragraph are wrong. Consideration of
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87Sr/86Sr Fig. 3. Plot o f the isotopic composition of O and Sr in the Volcano-Mariana arc (V-M; box with diagonal lines; data from Ito and Stern, 1981) and possible sources in the subducted crust and mantle. The composition of the bulk western Pacific pelagic sediments is from Table III. The range o f values expected from weathered and hydrothermally altered MORB is also shown. Also shown is the wide range of values reported for the Banda arc (Magaritz et al., 1978) where Whitford and coworkers have made a strong case that subducted components can be isotopically identified in analyzed lavas. Note that the range of isotopic compositions for the Banda arc, ~ 500 km long, is about an order of magnitude greater than that of the ~ 1000 km long Mariana-Volcano arc. For comparison, the range o f 6180 and 87Sr/86Sr reported for MORB-like basalts of the Mariana Trough (backarc basin) and Ayu Trough are also reported (solid black field) as well as the range of 8180 and 8VSr/86Sr for mantle-derived AOB or " h o t s p o t " basalts (field labelled " m a n t l e " , Kyser and O'Neil, 1978; Duncan and Compston, 1976). Also shown is a mixing line between sediments and MORB-like mantle• Note that no more than 1% sediment can be involved in magmagenesis beneath this arc.
t h e 5 1 8 0 - 87Sr/S6Sr d a t a c o u p l e d w i t h a r g u m e n t s b a s e d o n P b - i s o t o p i c a n a l yses of Mariana arc lavas and western Pacific sediments (Meijer, 1976) limits s e d i m e n t i n v o l v e m e n t in m a g m a g e n e s i s t o 1% o r less. T h e f a c t t h a t s e d i m e n t i n v o l v e m e n t in M a r i a n a - V o l c a n o a r c m a g m a g e n e s i s is in all c a s e s 1% o r less s u g g e s t s i t m a y n o t c o n t r i b u t e a t all. T h e c o m p o s i t i o n o f t h e b u l k s u b d u c t e d w e s t e r n P a c i f i c s e a f l o o r l i s t e d in T a b l e I I I is h i g h ly idealized. In reality, the relative proportions of the ratio of sediments to s e a m o u n t s , s e d i m e n t l i t h o l o g i e s , as w e l l as t h e t h i c k n e s s o f s e d i m e n t s d e livered to the subduction zone must vary enormously along the strike of the
471
arc. Note, for example, t h a t in Fig. 1 the Mapmaker and Magellan Seamounts represent probable " h o t - s p o t " chains now arriving at the trench in the northern parts of the Volcano and Mariana arcs, respectively. The proportion of clays to chalks within the pelagic sedimentary succession depends on the depth and thus on the age of the crust when it passed beneath the equatorial zone of high biogenic accumulation rates (Heezen et al., 1973). It is impossible at present to do more than point out that the bulk composition of subducted crust plus sediments plus seamounts varies greatly, both in time and in space. If mixing of this material with the mantle controls the composition of arc melts, these mixes should also be heterogeneous. While mixing processes in the overlying mantle wedge should be relatively efficient as a result of induced mantle convection (Toks6z and Bird, 1977), flow lines should be normal to the strike of the arc. Convective mixing of subducted crust, sediments, and derivative fluids with the mantle should thus be much less efficient along strike of the arc than for portions of the mantle perpendicular to the arc. Insofar as these processes are responsible for the generation of arc melts, the composition of lavas from volcanoes along the arc should also show considerable variation in the incompatible element trace element ratios and isotopic compositions. Such source region heterogeneity is not characteristic of the Mariana-Volcano arc. This is indicated in Fig. 4 which shows variations in STSr/a6Sr for the various subducted sources as well as data for the Mariana-Volcano arc. Clearly the source of these arc melts is remarkably homogeneous, especially considering these data come from islands as much as 1000 km apart. COMPOSITION AND ROLE OF THE MANTLE WEDGE
The recognition that the source of Mariana-Volcano arc melts is isotopically homogeneous and that sediment involvement must be less than 1% suggests an important role for the mantle wedge. Unfortunately no mantle xenoliths have been recovered from this arc, so we have no direct evidence bearing on the composition of the mantle wedge beneath the Mariana-Volcano arc. Instead, we will assume this mantle is similar to that of the suboceanic mantle elsewhere beneath the Pacific, and proceed from this to a discussion of whether or not such a mantle could be responsible for MarianaVolcano arc magmagenesis. The suboceanic mantle can be grossly subdivided into two reservoirs on the basis of isotopic and trace element composition. One is responsible for the generation of mid-ocean ridge basalts, the MORB-type mantle. This has been strongly depleted in the incompatible elements and light REE over much of the Earth's history; consequently it is characterized by low STSr/ 8~Sr (0.5131", e.g. White, 1979). Very *143Nd/144Nd v a l u e s d i s c u s s e d h e r e h a v e b e e n r e n o r m a l i z e d to BCR-1 = 0 . 5 1 2 7 1 a n d UCSD Nd = 0.51196.
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