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Magmatic evolution on the mid-atlantic ridge
EARTH AND PLANETARY SCIENCE LETTERS 2 (1967) 225-230. NORTH-HOLLAND PUBL. COMP., AMSTERDAM
MAGMATIC E V O L U T I O N ON THE M I D - A T L A N T I C RIDGE F. A U M E N T O
Geological Survey of Canada Received 21 April 1967
Recent field and e x p e r i m e n t a l evidence is used to produce a model which explains the generation and c h a r a c t e r i s t i c s of the b a s a l t s found on the Mid-Atlantic Ridge. Multiple cycles of partial melting of the upper mantle, related to tectonic p a t t e r n s beneath the axis of the Ridge, are thought to generate the different m a g m a types. The sudden s t r e s s r e l e a s e s due to faulting on the Median Nift Valley r e s u l t in considerable partial melting of a pyrolite mantle, which initiates a volcanic cycle with the subsequent ext r u s i o n of tholeiitic lava. As the original energy is consumed, the extent of p a r t i a l melting possible is gradually reduced, so that s m a l l e r quantities of magma, p r o g r e s s i v e l y enriched in alkalis, are generated. The l a s t extrusions of a volcanic cycle consist of s m a l l quantities of alkaline olivine basalt. Since the m a g m a s extrude onto an actively spreading ocean floor, t h e r e r e s u l t s a distribution of the different basalt types which can be c o r r e l a t e d to the topographic f e a t u r e s ot the Median Rift Valley.
1. I N T R O D U C T I O N R e c e n t s t u d i e s h a v e s h o w n t h a t a v a r i e t y of volcanic extrusives, including tholeiites, high alumina and alkali basalts and their differentiates, occur on the Mid-Atlantic Ridge. The orig i n s of t h e s e b a s a l t s u i t e s i n o t h e r p a r t s of t h e w o r l d h a v e l o n g b e e n t h e s u b j e c t of i n t e n s i v e discussion and controversy: opinions have been m o d i f i e d r e p e a t e d l y w i t h t h e d i s c o v e r y of new e v i d e n c e by e x p e r i m e n t a l o r f i e l d o b s e r v a t i o n s . A model for the Mid-Atlantic Ridge is therefore p r o p o s e d ; i t e x p l a i n s t h e p r o d u c t i o n of d i f f e r e n t basalt types, which it also correlates with recent o b s e r v a t i o n s on t h e R i d g e , m a d e b o t h b y t h e a u t h o r [1] a n d o t h e r r e s e a r c h e r s [ 2 - 9 ] . The more popular hypotheses for the product i o n of d i f f e r e n t b a s a l t t y p e s c a n b e s u b d i v i d e d t h u s : 1 ) f r a c t i o n a l m e l t i n g of t h e m a n t l e a t d i f f e r e n t d e p t h s u n d e r v a r y i n g c o n d i t i o n s of v e r y h i g h p r e s s u r e (e.g., K u n o [10], Y o d e r a n d T i l l e y [11], N i c h o l l s [7]), a n d 2) f r a c t i o n a l c r y s t a l l i z a tion under different pressure c o n d i t i o n s of a s i n g l e p a r e n t m a g m a ( E n g e l a n d E n g e l [2], M a c d o n a l d [12]). T h e new m o d e l i s b a s e d on a) new e x p e r i m e n t a l e v i d e n c e on t h e c r y s t a l l i z a t i o n of d i f f e r e n t b a s a l t t y p e s b e l o w 18 k b r e c e n t l y p r e s e n t e d b y G r e e n e t al. [13, 14], b) t h e r e c e n t o b s e r v a t i o n s o n t h e R i d g e , a n d c) t h e c o n c e p t of c o n v e c t i o n i n t h e m a n t l e a n d s p r e a d i n g of t h e o c e a n f l o o r b y i n s e r t i o n of new m a t e r i a l on t h e a x i s of t h e R i d g e ( H e s s [15]). In a n a t t e m p t to p u t l i m i t i n g
d a t e s o n t h e m o d e l , u s e i s a l s o m a d e of V i n e a n d M a t t h e w s ' [8, 9] h y p o t h e s i s on t h e n a t u r e of m a g netic anomalies over the Mid-Atlantic Ridge.
2. O B S E R V A T I O N S RIDGE
ON
THE
MID-ATLANTIC
T h e a u t h o r ' s o b s e r v a t i o n s a r e f o u n d e d on a r e c e n t l y c o m p l e t e d p e t r o l o g i c a l s t u d y of t h e f i r s t a r e a of t h e M i d - A t l a n t i c R i d g e , a t 45ON, to h a v e b e e n s a m p l e d a n d s u r v e y e d in d e t a i l ( L o n c a r e v i c e t al. [4], A u m e n t o [1]). R e p e a t e d s a m p l i n g a t 45ON on a t r a v e r s e f r o m t h e c e n t r e of t h e M e d i a n R i f t V a l l e y to t h e p e a k s on t h e c r e s t of t h e Mid-Atlantic Ridge yielded a complete and cont i n u o u s s e q u e n c e of b a s a l t t y p e s , f r o m o l i v i n e t h o l e i i t e s to a l k a l i b a s a l t s , t o g e t h e r w i t h h i g h alumina equivalents. These observations, and t h o s e p r e v i o u s l y m a d e e l s e w h e r e on t h e R i d g e ( E n g e l a n d E n g e l [2], M u i r a n d T i l l e y [ 5 , 6 ] , N i c h o l l s [7], a n d o t h e r s ) h a v e r e v e a l e d s e v e r a l important characteristics: 1) O l i v i n e t h o l e i i t e w i t h low to i n t e r m e d i a t e a l u m i n a c o n t e n t (14-16%) i s t h e m o s t c o m m o n r o c k t y p e f o u n d i n t h e d e e p e r p a r t s of t h e M i d - A t l a n tic Ridge. Associated rock types include common h i g h a l u m i n a o l i v i n e t h o l e i i t e s and, l e s s c o m m o n l y , q u a r t z t h o l e i i t e s ( E n g e I a n d E n g e l [2, 3], N i c h o l l s [7]). On t h e h i g h e r r e a c h e s of t h e R i d g e mountains there occur transitional olivine tholeiites with rare high alumina equivalents (Muir a n d T i l l e y [5, 6], A u m e n t o [1]). O c e a n i c i s l a n d s
226
F. AUMENTO
(such as the A z o r e s ) and the higher s e a m o u n t s [1] a r e capped with alkali olivine b a s a l t s and their differentiates. 2) B a s a l t s of d i f f er e n t c o m p o s i t i o n s o c c u r in c h a r a c t e r i s t i c a l l y d i f f e r e n t quantities. P l a c e d in descending o r d e r of abundance, the d i f f e r e n t types a r e as follows: Olivine t h o l e i i t e s High a l u m i n a olivine t h o l e i i t e s T r a n s i t i o n a l olivine t h o l e i i t e s Alkali b a s a l t s Quartz ~ o l e i i t e s High al u m i n a t r a n s i t i o n a l t h o l e i i t e s High al u m i n a b a s a l t s with s t r o n g e r alkali affinities The r e l a t i v e amounts quoted a r e b a s e d on y i e l d s f r o m r a n d o m d r ed g i n g s o v e r widely s e p a r a t e d a r e a s of the Ridge, c o r r e c t e d f r o m o b s e r v a t i o n s at 45ON. Although t h e r e is s t i l l an insufficient n u m b e r of dredge stations with which to p r o v i d e an absolute o r d e r of abundances, the quantities involved a r e r e m a r k a b l y c o m p a t i b l e with known amounts f r o m o t h er v o l c a n i c r e g i o n s in nono r o g e n i c settings. E v i d e n c e f r o m 45ON i n d i c a t e s that, c o n t r a r y to the c o n c l u s i o n s of Engel and Engel, high a l u m i n a b a s a l t may not be the m o s t c o m m o n r o c k type on the M i d - A t l a n t i c Ridge. T h e i r high al u m i n a b a s a l t c o n c e n t r a t i o n s a r e thought to be due both to insufficient s a m p l i n g and to the s t r a t i g r a p h i c s e q u e n c e of the e r u p t i r e s (as explained l a t e r ) , which, when r a n d o m ly dredged, would tend to y ie ld high a lu m in a b a salts. 3) A d i s t i n c t i v e and continuous e r u p t i v e c y c l e can be e s t a b l i s h e d . Olivine t h o l e i i t e s a r e the f i r s t to erupt, followed by h i g h - a l u m i n a e q u i v a lents, t r a n s i t i o n a l t h o l e i i t e s and alkali olivine b a s a l t s . V o l c a n o e s a r e often c o m p o s e d of a l a r g e t h o l e i i t i c shield capped by m i n o r amounts of a l kali basalt. 4) Olivine t h o l e i i t e s a r e the only b a s a l t types yet r e p o r t e d to have e x t r u d e d in the depths of the Median Rift V a l l e y (Aumento [1], Chase [16]). T h e s e b a s a l t s have an a l u m i n a content of 14-15% and about 5% n o r m a t i v e olivine. Bottom photographs, lack of s a m p l e w e a t h e r i n g o r m a n g a n e s e coating, and i m m e a s u r a b l y low c o n c e n t r a t i o n s of r a d i o g e n i c arg o n [1] indicate that t h e s e b a s a l t s a r e of v e r y r e c e n t origin. T h e s e may be the f i r s t p r o d u c t s of a new v o l c a n i c cycle. M a t e r i a l f r o m e a r l i e r c y c l e s of e r u p t i o n at d i f f e r e n t s t a g e s of evolution o c c u r s on s m a l l v o l c a n o e s along the i n n e r s l o p e s of the v a ll e y . On the adjacent v o l canic highlands which f o r m the c r e s t of the MidAtlantic Ridge a r e v o l c a n o e s which, t h e i r e r u p -
rive c y c l e s c o m p l e t e d , a r e now capped with a l kali b a s a l t s . D i f f er en t c y c l e s t h e r e f o r e show p r o g r e s s i o n to l a t e r s t a g e s with i n c r e a s i n g d i s tance f r o m the Median Rift Valley. 5) V o l c a n o e s on the flanks of the Median Rift V a l l e y eften o c c u r in p a i r s d i r e c t l y opposite each o t h er on e i t h e r side of the valley. 6) S t r o n t i u m isotopic r a t i o s [1] d e t e r m i n e d on s a m p l e s f r o m 45ON showed no v a r i a t i o n s in the value of 8 7 S r / 8 6 S r between olivine t h o l e i i t e s f r o m the Median Rift Valley to alkali b a s a l t s f r o m the adjacent c r e s t mountains (0.7033 + 0.0010). A c c o r d i n g to c u r r e n t t h e o r i e s of i s o topic abundances (Gast et al. [17], P o w e l l et al. [18]), t h ese constant v a l u e s may indicate a s i m i l a r p a r e n t m a g m a s o u r c e f o r all the r o c k types collected. 3. E X P E R I M E N T A L EVIDENCE No p r e v i o u s m a g m a t i c evolution t h e o r y can explain adequately the continuous and m u l t i p l e nature of the e r u p t i v e c y c l e s , the d i v e r s i t y and r e l a t i v e c o n c e n t r a t i o n s of the b a s a l t types c o l lected, and the isotopic c o m p o s i t i o n s . G r e e n et al. [13] have r e c e n t l y postulated, on the c o n c l u sion of En g el and En g el [3], a s e q u e n c e of f r a c tional m e l t i n g which should produce the c o r r e c t b a s a l t types. They su g g est that p a r t i a l m e l t i n g of p y r o l i t e ( a p p r o x i m a t e l y e q u i v a l e n t to a m i x t u r e of t h r e e p a r t s p e r i d o t i t e and one p a r t b a s a l t [19]) at a depth of 30 km p r o d u c e s high a l u m i n a tholeiites. Subsequent f r a c t i o n a l m e l t i n g at g r e a t e r depths would then be r e q u i r e d to p r o d u ce the alkali b a s a l t s . This i n t e r r u p t e d s e q u e n c e of m a g m a g en er at i o n , beginning with the i n c o r r e c t p r i m a r y m a g m a type, does not c o r r e s p o n d to the f i el d o b s e r v a t i o n s . The e x p e r i m e n t a l data of G r e e n et al. [13] can be used, h o w e v e r , to p r o v i d e an evolution t h e o r y which w i l l explain all the o b s e r v e d M i d - A t l a n t i c Ridge c h a r a c t e r i s t i c s . T h e i r d e m o n s t r a t i o n that changes in p r e s s u r e may be the main c o n t r o l l i n g f a c t o r s in the t r e n d of f r a c t i o n a l m e l t i n g and c r y s t a l l i z a t i o n of a p a r e n t p y r o l i t e m a n t l e m a t e r i a l is of p r i m e i m p o r t a n c e in the u n d e r s t a n d i n g of m a g m a g e n e r a t i o n under the M i d - A t l a n t i c Ridge. They showed that 30% p a r t i a l m e l t i n g of p y r o lite at p r e s s u r e s of 12-18 kb (35-60 km) w i l l y i el d an olivine tholeiite m a g m a with r e s i d u a l o l i v i n e and o r t h o p y r o x e n e s . A l e s s e r d e g r e e of p a r t i a l m e l t i n g (20%) will p r o d u c e an o l i v i n e r i c h alkali b a s a l t liquid with much l a r g e r r e s i d ual c o n c e n t r a t i o n s of al u m i n o u s e n s t a t i t e and a l u m i n o u s clinopyroxene.
MAGMATIC EVOLUTION F r a c t i o n a l c r y s t a l l i z a t i o n , with p r e c i p i t a t i o n of Mg-A1 p y r o x e n e , will take p l a c e if t h e s e l i q uids r e m a i n at s i m i l a r depths (and t h e r e f o r e p r e s s u r e s ) u n d er conditions of slow cooling. D e r i v a t i v e liquids will tend to have r a p id inc r e a s e s in alkali content, v e r y slight i n c r e a s e s in alumina, and slight d e c r e a s e s in s i l i c a content. T h e r e will be, t h e r e f o r e , as a r e s u l t of f r a c t i o n a l c r y s t a l l i z a t i o n , a continuous t r e n d f r o m olivine t h o l e i i t e through to alkali b a s a l t in the 35-60 km depth range. During t h e i r p a s s a g e through the upper z o n e s of the mantle, t h e s e liquids will be s u b j e c te d to d e c r e a s i n g p r e s s u r e s and t e m p e r a t u r e s , with a s s o c i a t e d changes in the t r e n d s of f r a c t i o n a l c r y s t a l l i z a t i o n . At p r e s s u r e s below 9 kb a t h e r m a l divide will a p p e a r which will not p e r m i t a l kali e n r i c h m e n t to take p la c e by p r e c i p i t a t i o n of p y r o x e n e s [11]. At t h e s e i n t e r m e d i a t e p r e s s u r e s (4.5-9 kb, 15-35 km) olivine and l o w - c a l c i u m pyr o x e n e s will p r e c i p i t a t e f r o m the r i s i n g liquids. D e r i v a t i v e liquids will m a i n t a i n t h e i r s i l i c a content, and i n c r e a s e t h e i r alkali content slightly, w h i l s t they will be c o n s i d e r a b l y e n r i c h e d in a l u mina. High a l u m i n a e q u i v a l e n t s of the t h o l e i i t e s and alkali b a s a l t s will t h e r e f o r e be anticipated. S ma l l amounts of f r a c t i o n a l m e l t i n g of p y r o lite at i n t e r m e d i a t e depths would a l s o p r o d u c e a h i g h - a l u m i n a t h o l e i i t e liquid; h o w e v e r , m o r e a l kaline v a r i e t i e s could not be produced, e i t h e r by d i f f e r e n t d e g r e e s of f r a c t i o n a l m e l t i n g o r by subsequent d i f f e r e n t i a t i o n of the high a l u m i n a tholeiites. At s h a l l o w e r depths (0-15 km, 0-4.5 kb) o l i vine p r e c i p i t a t i o n will be the m o s t a c t i v e p r o c e s s of differentiation. Its m a i n effect will be to shift o l i v i n e - n o r m a t i v e t h o l e i i t e s t o w a r d s s i l i c a s a t u r a t i o n and p r o d u c e q u a r t z t h o l e i i t e s . Conc e n t r a t i o n s of the l a t t e r could a l s o be a u g m e n t e d by f r a c t i o n a l m e l t i n g of p y r o l i t e at depths above 15 kin. H o w e v e r , w e r e q u a r t z t h o l e i i t e the p r i m a r y m a g m a type p r o d u c e d below the Ridge, m e c h a n i s m s would be lacking by which to d e r i v e the o t h e r m a g m a types. All the r o c k types found on the M i d - A t l a n t i c Ridge could t h e r e f o r e be p r o d u c e d by the s y s t e m of f r a c t i o n a l m e l t i n g and c r y s t a l l i z a t i o n of a p a r e n t p y r o l i t e m a t e r i a l outlined by G r e e n et al. A n u m b e r of deductions can be drawn f r o m t h e i r experimental evidence: a) A continuously v a r i a b l e , d e c r e a s i n g d e g r e e of p a r t i a l m e l t i n g at 35-60 km depth could p r o d u c e a continuous r a n g e in liquid c o m p o s i t i o n s f r o m ol i vine t h o l e i i t e s to t r a n s i t i o n a l t h o l e i i t e s , and e v e n t u a l l y alkali olivine b a s a l t s . The v o l u m e s of liquids p r o d u c e d p e r unit v o l u m e of p y r o l i t e will
227
a l s o v a r y c o n s i d e r a b l y . Olivine t h o l e i i t e will be the m o s t abundant due to l a r g e r am o u n t s of p a r tial melting; c o n v e r s e l y , alkali b a s a l t s will be the l e a s t abundant. b) T h e r e will be, as a r e s u l t of f r a c t i o n a l c r y s t a l l i z a t i o n at s i m i l a r depths, a continuous t r e n d f r o m olivine t h o l ei i t e through to alkali b a s a l t p a r a l l e l to the c o r r e s p o n d i n g t r e n d due to dec r e a s i n g f r a c t i o n a l melting. c) Since the o l i v i n e t h o l ei i t e m a g m a type will have a l a r g e r initial v o l u m e , it should p r o d u ce m o r e f r a c t i g n a t e d m a t e r i a l at i n t e r m e d i a t e depths (15-35 km) than the m o r e alkali m a g m a s . Indeed, the high a l u m i n a olivine t h o l e i i t e s a r e v e r y abundant on the Ridge, w h i l s t the high alumina t r a n s i t i o n a l t h o l e i i t e s a r e s c a r c e . In a c c o r d a n c e with the theory, a high alumina, high alkali b a s a l t should be s c a r c e , as is in fact the case. The r e l a t i v e p r o p o r t i o n s e x p e c t e d t h e r e f o r e c o r r e s p o n d to f i el d o b s e r v a t i o n s of abundances, both on the M i d - A t l a n t i c Ridge and in other volcanic provinces. d) A n u m b e r of p a r a l l e l s y s t e m s of m e l t i n g and c r y s t a l l i z a t i o n under d i f f e r e n t conditions of t e m p e r a t u r e and p r e s s u r e could c o n t r i b u t e to the production of the d i f f er en t b a s a l t types. T h i s s u g g e s t s that m a g m a t i c evolution on the M i d - A t lantic Ridge is not a s i m p l e p r o c e s s , but r a t h e r a co m b i n at i o n of s y s t e m s . It is p o s s i b l e , howe v e r , to e x t r a c t f r o m the e v i d e n c e p r e s e n t e d a single e v o l u t i o n a r y trend, to which o t h er p a r a l l el t r e n d s can be added without disrupting the seq u en ce; this s y s t e m will account f o r the MidAtlantic Ridge e x t r u s i v e s as f ar as they a r e known today.
4. THE MODEL RIDGE
FOR
THE
MID-ATLANTIC
N u m e r o u s v o l c a n i c c y c l e s of s h o r t duration a r e e n v i s a g e d , s u p e r i m p o s e d onto the l o n g e r c o n v e c t i v e c e l l c y c l e s in the m a n t l e as p r o p o s e d by H e s s [15]. Continuous c o n v e c t i o n c e l l s in the m a n t l e may p r o d u c e s t r e s s e s in the m o r e r i g i d upper m a n tle. It is s u g g e s t e d that a build-up of t h e s e s t r e s s e s in the u p p er mantle p r i o r to t h e i r r e l e a s e by r e n e w e d faulting on the Median Rift V a l l e y m ay p r o d u c e l a r g e am o u n t s of heat and t h e r e f o r e initiate an e r u p t i v e cycle. The r e l e a s e of t h e s e s t r e s s e s , by faulting on the a x i s and flanks of the Median Rift Valley, will c a u s e a sudden drop both in p r e s s u r e and in the m e l t i n g temperatures. On the e v i d e n c e of e a r t h q u a k e e p i c e n t r e s b e -
228
F. AUMENTO
CREST
W
MTNS.
MEDIAN
VALLEY
E 5 km.
o
I
RECENT
sea level
--
ALKALI
BA <.
TRANS.
THC
Ikm.
-- 2knz
- - ,,'7,k m . OLIVINE
ppt. of
THC
olivine --
15 km.~
ppt. of olivine & low Co pyroxene 35
ppt. of M g - A I pyroxene
.
% fractional melting
i,
20%
I
<::p= P'..OL,TE
25%
30%
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I
i
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P¥.OL,TE -------:--~".r..¥"OL'T E~------- P;'RO.,'rE convection
convection" ~ f
convection
Fig. 1.
neath the Ridge, s t r e s s r e l e a s e s o c c u r at the c o m p a r a t i v e l y shallow depth of 30 km, but s o m e may be as deep as 60 km (Heezen [20], M i s h a r i na [21]). Although the m e c h a n i c a l m o v e m e n t s have o c c u r r e d at t h e s e depths, the p r e s s u r e drop should o c c u r beneath the e a r t h q u a k e e p i c e n t r e s , for it is f r o m below that the convection c e l l s m a y be pushing a g a i n s t the m o r e r i g i d u p p e r mantle layer. Considerable quantities may t h e r e f o r e suddenly e x i s t in a highly s u p e r h e a t e d state at depths in the r a n g e of 40-70 km. A l a r g e amount of f r a c t i o n a l m e l t i n g should t h e r e f o r e o c c u r , producing an olivine t h o l e i i t e m a g m a in the d e e p e r l e v e l s , and g r a d a t i o n a l l y m o r e a l u minous m a g m a at the s i t e s of the s h a l l o w e r earthquake epicentres.
Some of the tholeiitic m a g m a will e x t r u d e r a p i d l y through the w e a k e s t spots in the c r u s t , e.g., onto the f l o o r of the Median Rift Valley. O t h e r f r a c t i o n s may r i s e m o r e slowly and f r a c tionate at l o w e r p r e s s u r e s to give high a l u m i n a or q u a r t z t h o l e i i t e s . The d i f f e r e n t i a t e s m a y be a u g m e n t e d with m a t e r i a l p r o d u c e d by f r a c t i o n a l m e l t i n g of p y r o l i t e at higher l e v e l s in the m a n tle, as a r e s u l t of s h a l l o w e r p r e s s u r e r e l e a s e s o r of m e l t i n g due to the heat t r a n s f e r of the hot, rising magmas. Throughout the e r u p t i v e cycle, continuous convection c y c l e s in the m a n t l e m a y b r i n g up f r e s h p y r o l i t e m a t e r i a l , so that s u c c e s s i v e d e r i v a t i v e liquids f o r m e d could r e m a i n at a constant c o m p o s i t i o n , given that o t h e r P - T f a c t o r s
MAGMATIC EVOLUTION remain constant. An unvarying composition could be maintained throughout a large part of a single convective cycle in the mantle. The superheating effects will diminish with time, however, and the energy source available for pyrolite melting will therefore gradually disappear. Fractional melting of pyrolite will only be possible to lesser extents, and will produce less voluminous derivative liquids with more alkaline affinities. These will erupt, or fractionate and then erupt, to give small amounts of the more alkaline basalts. These lavas will follow the previously established routes to the surface and extrude on top of great thicknesses of olivine tholeiites and differentiate layers. Furthermore, as a result of the continuous spreading of the ocean floor by insertion of new material (Hess [15]), the subsequent extrusive sites will no longer be in the Median Rift Valley, but rather in new positions on the flanks of the Valley. During the evolution of a volcanic cycle, with the associated transgression of its eruptive centres away from the Median Rift Valley centre, new stresses will build up below the Valley; new volcanic cycles will then begin, the products of which will erupt simultaneously to those of previous cycles in more advanced stages of evolution, but nearer to the axis of the Valley. The resultant oceanic crust expansion will be continuous, although not at a constant rate if observed over short periods of time only. However, the variations in expansion rate will occur with higher frequency than would affect Vine and Matthews' magnetic reversal dating hypothesis. The Median Rift Valley is today the site of a new cycle of volcanism in its early stages of evolution (outpourings of tholeiitic lava). P r e vious volcanic cycles at intermediate stages of development occur on the flanks of the Valley, whilst the final, alkali basalt stages occur oll the peaks of volcanoes some 5-10 km from the centre of the Valley. Assuming an average ocean floor spreading rate of 1 em/y (Vine [9]) the last complete volcanic cycle may be estimated to have taken about 1 m.y.
5. SUMMARY O b s e r v a t i o n s have shown that continuous, r e p e a t e d v o l c a n i c c y c l e s , c o m m e n c i n g with t h o l e i i t e s and ending with alkali b a s a l t s , a r e a c t i v e on the c r e s t of the M i d - A t l a n t i c Ridge. T h o l e i i t e s e x t r u d e on the f l o o r of the Median Rift Valley; subsequent e x t r u s i o n s capping the t h o l e i i t e s a r e p r o g r e s s i v e l y e n r i c h e d in a l k a l i s . The growth of
229
these volcanoes is accompanied by their gradual migration away from the Median Rift Valley as new material is inserted into feeder dykes in the Valley, causing the ocean floor to spread apart. By the time a volcano reaches the end of its cycle, extruding the last of the alkali lavas, it will have moved laterally a few km from its original axial position. Associated with this tectonic pattern are cycles of partial melting of pyrolite which occur below the earthquake epicentres. Initial stress releases will cause a considerable percentage of superheated pyrolite to melt differentially, producing olivine tholeiites; as the energy available for the melting is gradually depleted, less partial melting will take place, resulting in progressively more alkaline magmas to be generated. High alumina equivalents of these magmas will be produced by differentiation of the rising magmas at lower pressures, and/or by partial melting of more pyrolite at shallower depths. A continuous cycle from tholeiitic to alkali basalt magma generation will therefore take place, which can be correlated to the field evidence of abundances and spatial distributions. The evidence on which this model is based is primarily derived from observations at 45ON. The latter, detailed by oceanographic standards, are still far from being comparable to the detailed sampling usually undertaken on land. However, since the area 45ON seems to be a rather uncomplicated section of the Mid-Atlantic Ridge, the limited sampling may have provided an adequate representation of the geology. The model does not exclude variations. Amongst these, one could envisage new fissures opening along the lower reaches of volcanoes (as is often observed on land), extruding alkali lavas at greater depths than anticipated, possibly near the f l o o r of the Median Rift Valley. Again, w e a k e r t e n s i o n a l r e l e a s e s might only p r o v i d e sufficient e n e r g y for s m a l l am o u n t s of p a r t i a l m e l t ing to take place, r e s u l t i n g in t r u n c a t e d v o l c a n i c c y c l e s c o n s i s t i n g of single alkali p u l ses. O t h er s e c t i o n s of the M i d - A t l a n t i c Ridge could e a s i l y deviate in detail f r o m the s i m p l i f i e d m o d e l p r e s e n t e d h e r e , w h e r e a s the o v e r a l l m e c h a n i s m should s t i l l be o p e r a t i v e . M o r e c l o s e l y s p a c e d s a m p l i n g p r o g r a m s a c r o s s the axis of the Ridge, supported by d e t a i l e d photog r ap h i c and b a t h y m e t r i c c o v e r a g e , should be und e r t a k e n to t e s t the e f f e c t i v e n e s s of the m o d e l at d i f f e r e n t latitudes. We have now r e a c h e d a stage w h e r e r a n d o m s a m p l i n g will no l o n g e r i n c r e a s e our knowledge of the M i d - A t l a n t i c Ridge to any g r e a t extent.
230
F. A U M E N T O
ACKNOWLEDGEMENTS T h e a u t h o r w i s h e s to t h a n k D r . B. D. L o n c a r e v i c of t h e B e d f o r d I n s t i t u t e of O c e a n o g r a p h y f o r the opportunity of participating in t h e H u d s o n 1 9 - 6 6 C r u i s e , a n d D r . T . N. I r v i n e o f t h e G e o l o g i cal Survey of Canada for his helpful criticisms and discussions. T h i s p a p e r i s C a n a d i a n C o n t r i b u t i o n No. 157 to t h e I n t e r n a t i o n a l U p p e r M a n t l e P r o j e c t .
REFERENCES
[1] F. A u m e n t o , The M i d - A t l a n t i c Ridge n e a r 45ON. lI. B a s a l t s f r o m the a r e a of C o n f e d e r a t i o n P e a k (Subm i t t e d for p u b l i c a t i o n in Can. J. E a r t h Sci., 1967). [2] A . E . J . Engel and C . G . E n g e l , C o m p o s i t i o n of b a s a l t s f r o m the M i d - A t l a n t i c Ridge, S c i e n c e 144 (1964) 1330. [3] A. E. J. E n g e l . C . G . E n g e l and R. G. H a v e n s . C h e m ical c h a r a c t e r i s t i c s of o c e a n i c b a s a l t s and the Upp e r Mantle, Geol. Soc. A m e r . Bull. 76 (1965) 719. [4] B. D. L o n c a r e v i c . C . S . M a s o n and D.H. M a t t h e w s . The M i d - A t l a n t i c Ridge n e a r 45ON. I. The Median Valley, Can. J. E a r t h Sci. 3 {1966) 327. [5] I. D. M u i r and C. E. Tilley, B a s a l t s f r o m the n o r t h e r n p a r t of the Rift Zone of the M i d - A t l a n t i c Ridge, J. P e t r o l . 5 (1964) 409. [6] I. D. M u i r and C. E. T i l l e y , B a s a l t s f r o m the n o r t h e r n p a r t of the M i d - A t l a n t i c Ridge. II. T h e A t l a n t i s c o l l e c t i o n n e a r 30oN, J. P e t r o l . 7 (1966) 193. [7] G. D. N i c h o l l s , B a s a l t s f r o m the deep o c e a n floor, Min. Mag. T i l l e y 34 {1965) 373.
[8] F. J. Vine and D. H. M a t t h e w s , M a g n e t i c a n o m a l i e s o v e r o c e a n i c r i d g e s , N a t u r e 199 (1963) 947. [9] F. J. Vine, S p r e a d i n g of the o c e a n floor: new e v i d e n c e , S c i e n c e 154 (1966) 1405. [10] H . K u n o . O r i g i n of C e n o z o i c p e t r o g r a p h i c p r o v i n c e s of J a p a n and s u r r o u n d i n g a r e a s . Bull. Volc. 20 {1959) 1405. [11] H. S. Y o d e r and C. E. T i l l e y , O r i g i n of b a s a l t m a g m a s : a n e x p e r i m e n t a l s t u d y of n a t u r a l and s y n t h e t ic rock s y s t e m s . J. P e t r o l . 3 {1962) 342. [12] G. A. Macdonald, C u r r e n t p r o b l e m s of H a w a i i a n p e t r o l o g y , Geol. Soc. A m e r . M e e t i n g , San F r a n c i s c o , 1966. A b s t r . . p. 129. [13] D . H . G r e e n and A . E . R i n g w o o d , The g e n e s i s of b a s a l t m a g m a s . Dept. G e o p h y s . G e o c h e m . , A u s t . Nat. Univ. Publ. 444 {1966) 118. [14] T. H. G r e e n , D.H. G r e e n and A. E. Ringwood, The o r i g i n of h i g h - a l u m i n a b a s a l t s and t h e i r r e l a t i o n s h i p s to q u a r t z t h o l e i i t e s and alkali b a s a l t s , E a r t h P l a n e t . Sci. L e t t e r s 2 (1967) 41. [15] H.H. H e s s . H i s t o r y of o c e a n b a s i n s , Geol. Soc. A m e r . Buddington V o l u m e , e d s . E n g e l , J a m e s and L e o n a r d {1962) p. 599. [16] R. C. C h a s e , p e r s o n a l c o m m u n i c a t i o n (1966). [17] P. W. G a s t , G . R . Tilton and C. Hedge, Isotopic c o m p o s i t i o n of lead and s t r o n t i u m f r o m A s c e n s i o n and Gough I s l a n d s , S c i e n c e 145 (1964) 1181. [18] J. L. Powell and S. E. De L o n g , Isotopic c o m p o s i tion of lead and s t r o n t i u m in v o l c a n i c r o c k s f r o m Oahuo Science 145 (1966) 1181. [19] D. H. G r e e n and A . E . Ringwood, M i n e r a l a s s e m b l a g e s in a m o d e l m a n t l e c o m p o s i t i o n , J. Geophys. R e s . 68 {1963) 937. [20] B . C . H e e z e n , The r i f t in the o c e a n floor, Sci. A m e r . 203 (1960) 99. [21] L . A . M i s h a r i n a , On the s t r e s s e s in the s e i s m i c foci of the A t l a n t i c O c e a n , Akad. Nauk SSSR Izv. Geophys. Ser. 10 {1964) 1527.
MAGMATIC E V O L U T I O N ON THE M I D - A T L A N T I C RIDGE F. A U M E N T O
Geological Survey of Canada Received 21 April 1967
Recent field and e x p e r i m e n t a l evidence is used to produce a model which explains the generation and c h a r a c t e r i s t i c s of the b a s a l t s found on the Mid-Atlantic Ridge. Multiple cycles of partial melting of the upper mantle, related to tectonic p a t t e r n s beneath the axis of the Ridge, are thought to generate the different m a g m a types. The sudden s t r e s s r e l e a s e s due to faulting on the Median Nift Valley r e s u l t in considerable partial melting of a pyrolite mantle, which initiates a volcanic cycle with the subsequent ext r u s i o n of tholeiitic lava. As the original energy is consumed, the extent of p a r t i a l melting possible is gradually reduced, so that s m a l l e r quantities of magma, p r o g r e s s i v e l y enriched in alkalis, are generated. The l a s t extrusions of a volcanic cycle consist of s m a l l quantities of alkaline olivine basalt. Since the m a g m a s extrude onto an actively spreading ocean floor, t h e r e r e s u l t s a distribution of the different basalt types which can be c o r r e l a t e d to the topographic f e a t u r e s ot the Median Rift Valley.
1. I N T R O D U C T I O N R e c e n t s t u d i e s h a v e s h o w n t h a t a v a r i e t y of volcanic extrusives, including tholeiites, high alumina and alkali basalts and their differentiates, occur on the Mid-Atlantic Ridge. The orig i n s of t h e s e b a s a l t s u i t e s i n o t h e r p a r t s of t h e w o r l d h a v e l o n g b e e n t h e s u b j e c t of i n t e n s i v e discussion and controversy: opinions have been m o d i f i e d r e p e a t e d l y w i t h t h e d i s c o v e r y of new e v i d e n c e by e x p e r i m e n t a l o r f i e l d o b s e r v a t i o n s . A model for the Mid-Atlantic Ridge is therefore p r o p o s e d ; i t e x p l a i n s t h e p r o d u c t i o n of d i f f e r e n t basalt types, which it also correlates with recent o b s e r v a t i o n s on t h e R i d g e , m a d e b o t h b y t h e a u t h o r [1] a n d o t h e r r e s e a r c h e r s [ 2 - 9 ] . The more popular hypotheses for the product i o n of d i f f e r e n t b a s a l t t y p e s c a n b e s u b d i v i d e d t h u s : 1 ) f r a c t i o n a l m e l t i n g of t h e m a n t l e a t d i f f e r e n t d e p t h s u n d e r v a r y i n g c o n d i t i o n s of v e r y h i g h p r e s s u r e (e.g., K u n o [10], Y o d e r a n d T i l l e y [11], N i c h o l l s [7]), a n d 2) f r a c t i o n a l c r y s t a l l i z a tion under different pressure c o n d i t i o n s of a s i n g l e p a r e n t m a g m a ( E n g e l a n d E n g e l [2], M a c d o n a l d [12]). T h e new m o d e l i s b a s e d on a) new e x p e r i m e n t a l e v i d e n c e on t h e c r y s t a l l i z a t i o n of d i f f e r e n t b a s a l t t y p e s b e l o w 18 k b r e c e n t l y p r e s e n t e d b y G r e e n e t al. [13, 14], b) t h e r e c e n t o b s e r v a t i o n s o n t h e R i d g e , a n d c) t h e c o n c e p t of c o n v e c t i o n i n t h e m a n t l e a n d s p r e a d i n g of t h e o c e a n f l o o r b y i n s e r t i o n of new m a t e r i a l on t h e a x i s of t h e R i d g e ( H e s s [15]). In a n a t t e m p t to p u t l i m i t i n g
d a t e s o n t h e m o d e l , u s e i s a l s o m a d e of V i n e a n d M a t t h e w s ' [8, 9] h y p o t h e s i s on t h e n a t u r e of m a g netic anomalies over the Mid-Atlantic Ridge.
2. O B S E R V A T I O N S RIDGE
ON
THE
MID-ATLANTIC
T h e a u t h o r ' s o b s e r v a t i o n s a r e f o u n d e d on a r e c e n t l y c o m p l e t e d p e t r o l o g i c a l s t u d y of t h e f i r s t a r e a of t h e M i d - A t l a n t i c R i d g e , a t 45ON, to h a v e b e e n s a m p l e d a n d s u r v e y e d in d e t a i l ( L o n c a r e v i c e t al. [4], A u m e n t o [1]). R e p e a t e d s a m p l i n g a t 45ON on a t r a v e r s e f r o m t h e c e n t r e of t h e M e d i a n R i f t V a l l e y to t h e p e a k s on t h e c r e s t of t h e Mid-Atlantic Ridge yielded a complete and cont i n u o u s s e q u e n c e of b a s a l t t y p e s , f r o m o l i v i n e t h o l e i i t e s to a l k a l i b a s a l t s , t o g e t h e r w i t h h i g h alumina equivalents. These observations, and t h o s e p r e v i o u s l y m a d e e l s e w h e r e on t h e R i d g e ( E n g e l a n d E n g e l [2], M u i r a n d T i l l e y [ 5 , 6 ] , N i c h o l l s [7], a n d o t h e r s ) h a v e r e v e a l e d s e v e r a l important characteristics: 1) O l i v i n e t h o l e i i t e w i t h low to i n t e r m e d i a t e a l u m i n a c o n t e n t (14-16%) i s t h e m o s t c o m m o n r o c k t y p e f o u n d i n t h e d e e p e r p a r t s of t h e M i d - A t l a n tic Ridge. Associated rock types include common h i g h a l u m i n a o l i v i n e t h o l e i i t e s and, l e s s c o m m o n l y , q u a r t z t h o l e i i t e s ( E n g e I a n d E n g e l [2, 3], N i c h o l l s [7]). On t h e h i g h e r r e a c h e s of t h e R i d g e mountains there occur transitional olivine tholeiites with rare high alumina equivalents (Muir a n d T i l l e y [5, 6], A u m e n t o [1]). O c e a n i c i s l a n d s
226
F. AUMENTO
(such as the A z o r e s ) and the higher s e a m o u n t s [1] a r e capped with alkali olivine b a s a l t s and their differentiates. 2) B a s a l t s of d i f f er e n t c o m p o s i t i o n s o c c u r in c h a r a c t e r i s t i c a l l y d i f f e r e n t quantities. P l a c e d in descending o r d e r of abundance, the d i f f e r e n t types a r e as follows: Olivine t h o l e i i t e s High a l u m i n a olivine t h o l e i i t e s T r a n s i t i o n a l olivine t h o l e i i t e s Alkali b a s a l t s Quartz ~ o l e i i t e s High al u m i n a t r a n s i t i o n a l t h o l e i i t e s High al u m i n a b a s a l t s with s t r o n g e r alkali affinities The r e l a t i v e amounts quoted a r e b a s e d on y i e l d s f r o m r a n d o m d r ed g i n g s o v e r widely s e p a r a t e d a r e a s of the Ridge, c o r r e c t e d f r o m o b s e r v a t i o n s at 45ON. Although t h e r e is s t i l l an insufficient n u m b e r of dredge stations with which to p r o v i d e an absolute o r d e r of abundances, the quantities involved a r e r e m a r k a b l y c o m p a t i b l e with known amounts f r o m o t h er v o l c a n i c r e g i o n s in nono r o g e n i c settings. E v i d e n c e f r o m 45ON i n d i c a t e s that, c o n t r a r y to the c o n c l u s i o n s of Engel and Engel, high a l u m i n a b a s a l t may not be the m o s t c o m m o n r o c k type on the M i d - A t l a n t i c Ridge. T h e i r high al u m i n a b a s a l t c o n c e n t r a t i o n s a r e thought to be due both to insufficient s a m p l i n g and to the s t r a t i g r a p h i c s e q u e n c e of the e r u p t i r e s (as explained l a t e r ) , which, when r a n d o m ly dredged, would tend to y ie ld high a lu m in a b a salts. 3) A d i s t i n c t i v e and continuous e r u p t i v e c y c l e can be e s t a b l i s h e d . Olivine t h o l e i i t e s a r e the f i r s t to erupt, followed by h i g h - a l u m i n a e q u i v a lents, t r a n s i t i o n a l t h o l e i i t e s and alkali olivine b a s a l t s . V o l c a n o e s a r e often c o m p o s e d of a l a r g e t h o l e i i t i c shield capped by m i n o r amounts of a l kali basalt. 4) Olivine t h o l e i i t e s a r e the only b a s a l t types yet r e p o r t e d to have e x t r u d e d in the depths of the Median Rift V a l l e y (Aumento [1], Chase [16]). T h e s e b a s a l t s have an a l u m i n a content of 14-15% and about 5% n o r m a t i v e olivine. Bottom photographs, lack of s a m p l e w e a t h e r i n g o r m a n g a n e s e coating, and i m m e a s u r a b l y low c o n c e n t r a t i o n s of r a d i o g e n i c arg o n [1] indicate that t h e s e b a s a l t s a r e of v e r y r e c e n t origin. T h e s e may be the f i r s t p r o d u c t s of a new v o l c a n i c cycle. M a t e r i a l f r o m e a r l i e r c y c l e s of e r u p t i o n at d i f f e r e n t s t a g e s of evolution o c c u r s on s m a l l v o l c a n o e s along the i n n e r s l o p e s of the v a ll e y . On the adjacent v o l canic highlands which f o r m the c r e s t of the MidAtlantic Ridge a r e v o l c a n o e s which, t h e i r e r u p -
rive c y c l e s c o m p l e t e d , a r e now capped with a l kali b a s a l t s . D i f f er en t c y c l e s t h e r e f o r e show p r o g r e s s i o n to l a t e r s t a g e s with i n c r e a s i n g d i s tance f r o m the Median Rift Valley. 5) V o l c a n o e s on the flanks of the Median Rift V a l l e y eften o c c u r in p a i r s d i r e c t l y opposite each o t h er on e i t h e r side of the valley. 6) S t r o n t i u m isotopic r a t i o s [1] d e t e r m i n e d on s a m p l e s f r o m 45ON showed no v a r i a t i o n s in the value of 8 7 S r / 8 6 S r between olivine t h o l e i i t e s f r o m the Median Rift Valley to alkali b a s a l t s f r o m the adjacent c r e s t mountains (0.7033 + 0.0010). A c c o r d i n g to c u r r e n t t h e o r i e s of i s o topic abundances (Gast et al. [17], P o w e l l et al. [18]), t h ese constant v a l u e s may indicate a s i m i l a r p a r e n t m a g m a s o u r c e f o r all the r o c k types collected. 3. E X P E R I M E N T A L EVIDENCE No p r e v i o u s m a g m a t i c evolution t h e o r y can explain adequately the continuous and m u l t i p l e nature of the e r u p t i v e c y c l e s , the d i v e r s i t y and r e l a t i v e c o n c e n t r a t i o n s of the b a s a l t types c o l lected, and the isotopic c o m p o s i t i o n s . G r e e n et al. [13] have r e c e n t l y postulated, on the c o n c l u sion of En g el and En g el [3], a s e q u e n c e of f r a c tional m e l t i n g which should produce the c o r r e c t b a s a l t types. They su g g est that p a r t i a l m e l t i n g of p y r o l i t e ( a p p r o x i m a t e l y e q u i v a l e n t to a m i x t u r e of t h r e e p a r t s p e r i d o t i t e and one p a r t b a s a l t [19]) at a depth of 30 km p r o d u c e s high a l u m i n a tholeiites. Subsequent f r a c t i o n a l m e l t i n g at g r e a t e r depths would then be r e q u i r e d to p r o d u ce the alkali b a s a l t s . This i n t e r r u p t e d s e q u e n c e of m a g m a g en er at i o n , beginning with the i n c o r r e c t p r i m a r y m a g m a type, does not c o r r e s p o n d to the f i el d o b s e r v a t i o n s . The e x p e r i m e n t a l data of G r e e n et al. [13] can be used, h o w e v e r , to p r o v i d e an evolution t h e o r y which w i l l explain all the o b s e r v e d M i d - A t l a n t i c Ridge c h a r a c t e r i s t i c s . T h e i r d e m o n s t r a t i o n that changes in p r e s s u r e may be the main c o n t r o l l i n g f a c t o r s in the t r e n d of f r a c t i o n a l m e l t i n g and c r y s t a l l i z a t i o n of a p a r e n t p y r o l i t e m a n t l e m a t e r i a l is of p r i m e i m p o r t a n c e in the u n d e r s t a n d i n g of m a g m a g e n e r a t i o n under the M i d - A t l a n t i c Ridge. They showed that 30% p a r t i a l m e l t i n g of p y r o lite at p r e s s u r e s of 12-18 kb (35-60 km) w i l l y i el d an olivine tholeiite m a g m a with r e s i d u a l o l i v i n e and o r t h o p y r o x e n e s . A l e s s e r d e g r e e of p a r t i a l m e l t i n g (20%) will p r o d u c e an o l i v i n e r i c h alkali b a s a l t liquid with much l a r g e r r e s i d ual c o n c e n t r a t i o n s of al u m i n o u s e n s t a t i t e and a l u m i n o u s clinopyroxene.
MAGMATIC EVOLUTION F r a c t i o n a l c r y s t a l l i z a t i o n , with p r e c i p i t a t i o n of Mg-A1 p y r o x e n e , will take p l a c e if t h e s e l i q uids r e m a i n at s i m i l a r depths (and t h e r e f o r e p r e s s u r e s ) u n d er conditions of slow cooling. D e r i v a t i v e liquids will tend to have r a p id inc r e a s e s in alkali content, v e r y slight i n c r e a s e s in alumina, and slight d e c r e a s e s in s i l i c a content. T h e r e will be, t h e r e f o r e , as a r e s u l t of f r a c t i o n a l c r y s t a l l i z a t i o n , a continuous t r e n d f r o m olivine t h o l e i i t e through to alkali b a s a l t in the 35-60 km depth range. During t h e i r p a s s a g e through the upper z o n e s of the mantle, t h e s e liquids will be s u b j e c te d to d e c r e a s i n g p r e s s u r e s and t e m p e r a t u r e s , with a s s o c i a t e d changes in the t r e n d s of f r a c t i o n a l c r y s t a l l i z a t i o n . At p r e s s u r e s below 9 kb a t h e r m a l divide will a p p e a r which will not p e r m i t a l kali e n r i c h m e n t to take p la c e by p r e c i p i t a t i o n of p y r o x e n e s [11]. At t h e s e i n t e r m e d i a t e p r e s s u r e s (4.5-9 kb, 15-35 km) olivine and l o w - c a l c i u m pyr o x e n e s will p r e c i p i t a t e f r o m the r i s i n g liquids. D e r i v a t i v e liquids will m a i n t a i n t h e i r s i l i c a content, and i n c r e a s e t h e i r alkali content slightly, w h i l s t they will be c o n s i d e r a b l y e n r i c h e d in a l u mina. High a l u m i n a e q u i v a l e n t s of the t h o l e i i t e s and alkali b a s a l t s will t h e r e f o r e be anticipated. S ma l l amounts of f r a c t i o n a l m e l t i n g of p y r o lite at i n t e r m e d i a t e depths would a l s o p r o d u c e a h i g h - a l u m i n a t h o l e i i t e liquid; h o w e v e r , m o r e a l kaline v a r i e t i e s could not be produced, e i t h e r by d i f f e r e n t d e g r e e s of f r a c t i o n a l m e l t i n g o r by subsequent d i f f e r e n t i a t i o n of the high a l u m i n a tholeiites. At s h a l l o w e r depths (0-15 km, 0-4.5 kb) o l i vine p r e c i p i t a t i o n will be the m o s t a c t i v e p r o c e s s of differentiation. Its m a i n effect will be to shift o l i v i n e - n o r m a t i v e t h o l e i i t e s t o w a r d s s i l i c a s a t u r a t i o n and p r o d u c e q u a r t z t h o l e i i t e s . Conc e n t r a t i o n s of the l a t t e r could a l s o be a u g m e n t e d by f r a c t i o n a l m e l t i n g of p y r o l i t e at depths above 15 kin. H o w e v e r , w e r e q u a r t z t h o l e i i t e the p r i m a r y m a g m a type p r o d u c e d below the Ridge, m e c h a n i s m s would be lacking by which to d e r i v e the o t h e r m a g m a types. All the r o c k types found on the M i d - A t l a n t i c Ridge could t h e r e f o r e be p r o d u c e d by the s y s t e m of f r a c t i o n a l m e l t i n g and c r y s t a l l i z a t i o n of a p a r e n t p y r o l i t e m a t e r i a l outlined by G r e e n et al. A n u m b e r of deductions can be drawn f r o m t h e i r experimental evidence: a) A continuously v a r i a b l e , d e c r e a s i n g d e g r e e of p a r t i a l m e l t i n g at 35-60 km depth could p r o d u c e a continuous r a n g e in liquid c o m p o s i t i o n s f r o m ol i vine t h o l e i i t e s to t r a n s i t i o n a l t h o l e i i t e s , and e v e n t u a l l y alkali olivine b a s a l t s . The v o l u m e s of liquids p r o d u c e d p e r unit v o l u m e of p y r o l i t e will
227
a l s o v a r y c o n s i d e r a b l y . Olivine t h o l e i i t e will be the m o s t abundant due to l a r g e r am o u n t s of p a r tial melting; c o n v e r s e l y , alkali b a s a l t s will be the l e a s t abundant. b) T h e r e will be, as a r e s u l t of f r a c t i o n a l c r y s t a l l i z a t i o n at s i m i l a r depths, a continuous t r e n d f r o m olivine t h o l ei i t e through to alkali b a s a l t p a r a l l e l to the c o r r e s p o n d i n g t r e n d due to dec r e a s i n g f r a c t i o n a l melting. c) Since the o l i v i n e t h o l ei i t e m a g m a type will have a l a r g e r initial v o l u m e , it should p r o d u ce m o r e f r a c t i g n a t e d m a t e r i a l at i n t e r m e d i a t e depths (15-35 km) than the m o r e alkali m a g m a s . Indeed, the high a l u m i n a olivine t h o l e i i t e s a r e v e r y abundant on the Ridge, w h i l s t the high alumina t r a n s i t i o n a l t h o l e i i t e s a r e s c a r c e . In a c c o r d a n c e with the theory, a high alumina, high alkali b a s a l t should be s c a r c e , as is in fact the case. The r e l a t i v e p r o p o r t i o n s e x p e c t e d t h e r e f o r e c o r r e s p o n d to f i el d o b s e r v a t i o n s of abundances, both on the M i d - A t l a n t i c Ridge and in other volcanic provinces. d) A n u m b e r of p a r a l l e l s y s t e m s of m e l t i n g and c r y s t a l l i z a t i o n under d i f f e r e n t conditions of t e m p e r a t u r e and p r e s s u r e could c o n t r i b u t e to the production of the d i f f er en t b a s a l t types. T h i s s u g g e s t s that m a g m a t i c evolution on the M i d - A t lantic Ridge is not a s i m p l e p r o c e s s , but r a t h e r a co m b i n at i o n of s y s t e m s . It is p o s s i b l e , howe v e r , to e x t r a c t f r o m the e v i d e n c e p r e s e n t e d a single e v o l u t i o n a r y trend, to which o t h er p a r a l l el t r e n d s can be added without disrupting the seq u en ce; this s y s t e m will account f o r the MidAtlantic Ridge e x t r u s i v e s as f ar as they a r e known today.
4. THE MODEL RIDGE
FOR
THE
MID-ATLANTIC
N u m e r o u s v o l c a n i c c y c l e s of s h o r t duration a r e e n v i s a g e d , s u p e r i m p o s e d onto the l o n g e r c o n v e c t i v e c e l l c y c l e s in the m a n t l e as p r o p o s e d by H e s s [15]. Continuous c o n v e c t i o n c e l l s in the m a n t l e may p r o d u c e s t r e s s e s in the m o r e r i g i d upper m a n tle. It is s u g g e s t e d that a build-up of t h e s e s t r e s s e s in the u p p er mantle p r i o r to t h e i r r e l e a s e by r e n e w e d faulting on the Median Rift V a l l e y m ay p r o d u c e l a r g e am o u n t s of heat and t h e r e f o r e initiate an e r u p t i v e cycle. The r e l e a s e of t h e s e s t r e s s e s , by faulting on the a x i s and flanks of the Median Rift Valley, will c a u s e a sudden drop both in p r e s s u r e and in the m e l t i n g temperatures. On the e v i d e n c e of e a r t h q u a k e e p i c e n t r e s b e -
228
F. AUMENTO
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VALLEY
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o
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--
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.
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Fig. 1.
neath the Ridge, s t r e s s r e l e a s e s o c c u r at the c o m p a r a t i v e l y shallow depth of 30 km, but s o m e may be as deep as 60 km (Heezen [20], M i s h a r i na [21]). Although the m e c h a n i c a l m o v e m e n t s have o c c u r r e d at t h e s e depths, the p r e s s u r e drop should o c c u r beneath the e a r t h q u a k e e p i c e n t r e s , for it is f r o m below that the convection c e l l s m a y be pushing a g a i n s t the m o r e r i g i d u p p e r mantle layer. Considerable quantities may t h e r e f o r e suddenly e x i s t in a highly s u p e r h e a t e d state at depths in the r a n g e of 40-70 km. A l a r g e amount of f r a c t i o n a l m e l t i n g should t h e r e f o r e o c c u r , producing an olivine t h o l e i i t e m a g m a in the d e e p e r l e v e l s , and g r a d a t i o n a l l y m o r e a l u minous m a g m a at the s i t e s of the s h a l l o w e r earthquake epicentres.
Some of the tholeiitic m a g m a will e x t r u d e r a p i d l y through the w e a k e s t spots in the c r u s t , e.g., onto the f l o o r of the Median Rift Valley. O t h e r f r a c t i o n s may r i s e m o r e slowly and f r a c tionate at l o w e r p r e s s u r e s to give high a l u m i n a or q u a r t z t h o l e i i t e s . The d i f f e r e n t i a t e s m a y be a u g m e n t e d with m a t e r i a l p r o d u c e d by f r a c t i o n a l m e l t i n g of p y r o l i t e at higher l e v e l s in the m a n tle, as a r e s u l t of s h a l l o w e r p r e s s u r e r e l e a s e s o r of m e l t i n g due to the heat t r a n s f e r of the hot, rising magmas. Throughout the e r u p t i v e cycle, continuous convection c y c l e s in the m a n t l e m a y b r i n g up f r e s h p y r o l i t e m a t e r i a l , so that s u c c e s s i v e d e r i v a t i v e liquids f o r m e d could r e m a i n at a constant c o m p o s i t i o n , given that o t h e r P - T f a c t o r s
MAGMATIC EVOLUTION remain constant. An unvarying composition could be maintained throughout a large part of a single convective cycle in the mantle. The superheating effects will diminish with time, however, and the energy source available for pyrolite melting will therefore gradually disappear. Fractional melting of pyrolite will only be possible to lesser extents, and will produce less voluminous derivative liquids with more alkaline affinities. These will erupt, or fractionate and then erupt, to give small amounts of the more alkaline basalts. These lavas will follow the previously established routes to the surface and extrude on top of great thicknesses of olivine tholeiites and differentiate layers. Furthermore, as a result of the continuous spreading of the ocean floor by insertion of new material (Hess [15]), the subsequent extrusive sites will no longer be in the Median Rift Valley, but rather in new positions on the flanks of the Valley. During the evolution of a volcanic cycle, with the associated transgression of its eruptive centres away from the Median Rift Valley centre, new stresses will build up below the Valley; new volcanic cycles will then begin, the products of which will erupt simultaneously to those of previous cycles in more advanced stages of evolution, but nearer to the axis of the Valley. The resultant oceanic crust expansion will be continuous, although not at a constant rate if observed over short periods of time only. However, the variations in expansion rate will occur with higher frequency than would affect Vine and Matthews' magnetic reversal dating hypothesis. The Median Rift Valley is today the site of a new cycle of volcanism in its early stages of evolution (outpourings of tholeiitic lava). P r e vious volcanic cycles at intermediate stages of development occur on the flanks of the Valley, whilst the final, alkali basalt stages occur oll the peaks of volcanoes some 5-10 km from the centre of the Valley. Assuming an average ocean floor spreading rate of 1 em/y (Vine [9]) the last complete volcanic cycle may be estimated to have taken about 1 m.y.
5. SUMMARY O b s e r v a t i o n s have shown that continuous, r e p e a t e d v o l c a n i c c y c l e s , c o m m e n c i n g with t h o l e i i t e s and ending with alkali b a s a l t s , a r e a c t i v e on the c r e s t of the M i d - A t l a n t i c Ridge. T h o l e i i t e s e x t r u d e on the f l o o r of the Median Rift Valley; subsequent e x t r u s i o n s capping the t h o l e i i t e s a r e p r o g r e s s i v e l y e n r i c h e d in a l k a l i s . The growth of
229
these volcanoes is accompanied by their gradual migration away from the Median Rift Valley as new material is inserted into feeder dykes in the Valley, causing the ocean floor to spread apart. By the time a volcano reaches the end of its cycle, extruding the last of the alkali lavas, it will have moved laterally a few km from its original axial position. Associated with this tectonic pattern are cycles of partial melting of pyrolite which occur below the earthquake epicentres. Initial stress releases will cause a considerable percentage of superheated pyrolite to melt differentially, producing olivine tholeiites; as the energy available for the melting is gradually depleted, less partial melting will take place, resulting in progressively more alkaline magmas to be generated. High alumina equivalents of these magmas will be produced by differentiation of the rising magmas at lower pressures, and/or by partial melting of more pyrolite at shallower depths. A continuous cycle from tholeiitic to alkali basalt magma generation will therefore take place, which can be correlated to the field evidence of abundances and spatial distributions. The evidence on which this model is based is primarily derived from observations at 45ON. The latter, detailed by oceanographic standards, are still far from being comparable to the detailed sampling usually undertaken on land. However, since the area 45ON seems to be a rather uncomplicated section of the Mid-Atlantic Ridge, the limited sampling may have provided an adequate representation of the geology. The model does not exclude variations. Amongst these, one could envisage new fissures opening along the lower reaches of volcanoes (as is often observed on land), extruding alkali lavas at greater depths than anticipated, possibly near the f l o o r of the Median Rift Valley. Again, w e a k e r t e n s i o n a l r e l e a s e s might only p r o v i d e sufficient e n e r g y for s m a l l am o u n t s of p a r t i a l m e l t ing to take place, r e s u l t i n g in t r u n c a t e d v o l c a n i c c y c l e s c o n s i s t i n g of single alkali p u l ses. O t h er s e c t i o n s of the M i d - A t l a n t i c Ridge could e a s i l y deviate in detail f r o m the s i m p l i f i e d m o d e l p r e s e n t e d h e r e , w h e r e a s the o v e r a l l m e c h a n i s m should s t i l l be o p e r a t i v e . M o r e c l o s e l y s p a c e d s a m p l i n g p r o g r a m s a c r o s s the axis of the Ridge, supported by d e t a i l e d photog r ap h i c and b a t h y m e t r i c c o v e r a g e , should be und e r t a k e n to t e s t the e f f e c t i v e n e s s of the m o d e l at d i f f e r e n t latitudes. We have now r e a c h e d a stage w h e r e r a n d o m s a m p l i n g will no l o n g e r i n c r e a s e our knowledge of the M i d - A t l a n t i c Ridge to any g r e a t extent.
230
F. A U M E N T O
ACKNOWLEDGEMENTS T h e a u t h o r w i s h e s to t h a n k D r . B. D. L o n c a r e v i c of t h e B e d f o r d I n s t i t u t e of O c e a n o g r a p h y f o r the opportunity of participating in t h e H u d s o n 1 9 - 6 6 C r u i s e , a n d D r . T . N. I r v i n e o f t h e G e o l o g i cal Survey of Canada for his helpful criticisms and discussions. T h i s p a p e r i s C a n a d i a n C o n t r i b u t i o n No. 157 to t h e I n t e r n a t i o n a l U p p e r M a n t l e P r o j e c t .
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