seafloor hydrothermal systems

7/27/2019 Seafloor Hydrothermal Systems

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Seafloor hydrothermal systems driven by the serpentinization

of peridotite

R . P. L o we l l1 a nd P. A . R on a2

R ec ei ve d 2 0 N ov em be r 2 00 1; r ev is ed 1 A pr il 2 00 2; a cc ep te d 1 5 A pr il 2 00 2; p ub li sh ed 12 J un e 2 0 02 .

[ 1 ] H ea t b al an ce m od el s p re di ct t ha t h ea t r el ea se d u po n

s er pe nt in iz at io n o f p er id ot it es c an r es ul t i n a w id e r an ge o f

h y d ro t h er m a l v e n ti n g t e m pe r a t ur e s . Te m p e ra t u re s r a n gi n g f r o m a

f e w t o a f e w t e n s o f C a r e l i k el y. Te m p e ra t u re s300C m a y b epossible in conjunction with significant heat input from below, low

f l ui d f l ow r a te (1 0 k g/ s ), a h ig h r a te o f s e rp e nt i ni z at i on (100k g /s ) , o r s om e c o mb in a ti on o f t h es e f a ct o rs . T he v o lu m e o f f r es h

l i t ho s p he r e a c c e ss i b le t o s e r p en t i ni z a ti o n m a y l i m it t h e d u r at i o n o f

a n e p i so d e o f h y dr o t he r m a l v e n t i n g w h e n t h e s e r pe n t in i z at i o n r a t e

e x ce e ds 1 0 k g/ s . H yd r ot he r ma l v e nt in g d r iv e n b y h i gh r a te s o f

s e r p en t i ni z a ti o n m a y o c c ur e p i so d ic a l l y a s n e w r e a c ti o n s u r f ac e sa r e m a d e a v a i l a b l e b y t e c t o n i c a c t i v i t y o r c r a c k p r o p a g a t i o n . G i v e n

t he u nc er ta in ty o f s ev er al p ar am et er s, t he c ur re nt e pi so de o f

v e nt in g a t t h e L o s t C i ty v e nt f i el d o n t h e M i d - At l an t ic R i dg e m a y

r a n g e f r o m102 t o 1 04 years. INDEX TERMS: 3 0 1 5 M a r i neG e o lo g y a n d G e o ph y s ic s : H e a t f l o w ( b e nt h i c) a n d h y dr o t he r m a l

processes ; 3210 Mathemati cal Geophysics: Modeling; 4832

Oceanography: Biological and Chemical: Hydrothermal systems

1. Introduction

[2] S e r p en t i ni z a t io n , w h i c h o c c u rs u p on r e a c t i o n o f u l t r a m a fi cr o c k s w i t h s e a wa t e r, i s c o m mo n l y o b s e r v ed i n p e r id o t it e s d r e d ge df r o m t h e s e a f l o o r a n d i n o p h i o l i t e s [Bonatti, 1 9 7 6 ;Nicholas, 1 9 8 9 ;

Cannat, 1 99 3 ] a s w e ll a s i n c o re s r e co ve r ed b y t h e o c ea n d r i l li n gprogram [Gillis et al., 1 9 9 3; Fruh-Green et al., 1 9 96 ;Karson andLawrence, 1 9 97 ] . T h is p r oc e ss y i el d s d i st i nc t iv e c h em i ca l s ol u -

t i o n s [Janecky and Seyfried, 1 9 8 6 ;Wetzel and Shock, 2 0 0 0 ;Kelleyet al., 2 0 01 ] c h a r a c t e ri z e d b y h i g h p H a n d h i g h r a t i os o f a b i o g e ni cCH4, H2, a n d C a t o M n a n d o t h e r m e t a l s . M o r e o v e r , t h e r e a c t i o n i sexothermic [Fyfe and Lonsdale, 1 9 8 1 ;Macdonald and Fyfe, 1 9 8 5 ] ,a n d t he h e at r e le a se d d ur i ng s e rp e nt i ni z at i on m a y d r iv e h yd r o-t h e r ma l f l o w [Rona et al., 1 9 8 7 ;Barriga et al., 1 9 9 8 ;Kelley et al.,2001].

[3] C o nd it i on s c o nd uc i ve t o s e rp e nt i ni z at i on i n cl u de a h ig hr a ti o o f t e ct o ni c e x te n si o n t o m a gm a ti sm , a l o w m a gm a b ud ge t ,a n d h i g h f r a c tu r e /f a u lt p e r me a b il i ty t h a t r e p ea t e dl y f a c il i t at e sa c ce ss o f s ea wa te r t o l ar ge v ol um es o f u pp er m an tl e. S uc hc o nd it i on s g e ne r al l y o c cu r a t s l ow s pr e ad i ng r i dg e s. M o un ti n g

e v i de n c e i n d ic a t e s t h a t f l u id s w i th s i g na t u re s o f s e r pe n t in i z a ti o nr ea c ti on s a re c om mo n a lo ng t he M id -A tl an ti c R id ge [Charlouet al., 1 99 1; Rona et al., 1 99 2; Charlou and Donval, 1 99 3;

Bougault et al., 1 9 98 ; Gracia et al., 2 00 0] . H ig h C H4 a nd H2c o nc en tr a ti on s i nd ic at e t he p re se nc e o f t he s e s ol ut io ns i n t heL o g at c h e v ( 1 445 0N, 44 58 0W ) a n d R a i n b ow ( 3 614 0N,33540W ) h i gh - te m pe r at u re ( 3 50 3 6 0C ) h y dr o th e rm a l f i el d sh o st e d i n g a bb r os a n d s e rp e nt i ni z ed p e ri do t it e s n e ar t h e a x is o f

t h e M i d -A t l an t i c R i d g e [Krasnov et al., 1 9 9 5 ;Donval et al., 1 9 97 ;Douville et al., 1 9 9 7 ] ; h o w e v e r , t h e h y d r o t h e r m a l f l u i d s f r o m t h e s e

t w o f i e ld s a l s o e x h ib i t c o n st i t ue n t s i n di c a t iv e o f b a s al t - se a w a te r reactions.

[4] T he r e ce n tl y d i s co v er e d L o s t C i ty f i el d i s a n e n d m e mb e r o f a h yd r ot h er m al s y st e m d r iv e n b y s e rp e nt i ni z at i on r e ac t io ns[Kelley et al., 2 00 1] . T hi s f ie ld i s l oc at ed a bo ut 1 5 k m w es t o f t he e as te rn i nn er c or ne r h ig h o f t he A tl an ti s t ra ns fo rm f au lt a t 30N, where a detachment fault has exposed upper mantle

peridotites that are undergoing serpentinization, presumably iso-l a te d f r om m a gm a ti c h e at s ou r ce s . T he v e nt i ng s i te i s s it u at e d

1 5 k m w es t o f t he s pr e ad in g a x is o n a b o ut 1 . 5 m i l li o n y e a rs o ldl i t ho s ph e r e [Cann et al., 1 99 7; Blackman et al., 1 9 9 8] . S e r p en t i -n i z a ti o n -d e r iv e d f l u id s a r e d i s ch a r gi n g a t t e m pe r a t ur e s u p t o 7 5Ca n d p r ec i pi ta t in g c a lc i um c a rb on a te a n d m a gn e si u m h y dr o xi d ec hi mn ey s, w hi ch h av e g ro wn u p t o 6 0 m h ig h [Kelley et al.,2001].

[5] To b e t t e r u n d e rs t a nd t h e r o l e o f s e r p e n ti n i za t i on r e a c t i o ns i nd ri vi ng h yd ro th er ma l f lo w i n t he o ce an ic c ru st , w e d ev el op as i m pl e h e a t b a l a n c e m o d e l t o e s t im a t e t e m p e r a tu r e a n d h e a t o u t pu t a n d t o c o mp a re t he r o le o f s e rp e nt i ni z at i on r e ac t io ns w i th o t he r h e a t s o ur c e s .

2. Heat Balance Model

[6] To q ua n ti f y t h e e f fe c t o f t h e s e r p en ti n iz a ti on r e ac t io ns o n

h y d ro t h er m a l o u t pu t w e c o n st r u c t s i m p l e h e a t b a l an c e e q u at i o ns .We f i rs t a s su me t ha t h y dr o th e rm a l f l ow m a y b e d r iv e n b y a b a sa lc o nd u ct i ve h e at f l ux H ( i n Wa t ts ) , c o rr e sp on di ng t o e it he r a

background or a magmatic heat source. If this heat flux is entirelyt r a n sp o r te d b y h y dr o t he r m a l f l o w t h e n

cfQT1 H 1

where cf i s t he s p ec i fi c h e at o f t h e f l u id ,Q i s t he t ot a l m a s s f l owr a te i n k g / s, a n d T1 i s t h e d i f f e r e nc e b e t we e n t h e t e m p e r a tu r e o f t h e h y dr o t he r m a l f l u i d a n d t h e b a c kg r o un d t e m pe r a t ur e . We t h e ns u pe r im p os e t h e t ot a l h e at f l ux r e su l ti n g f r om t h e l a te n t h e at o f s e r p en t i ni z a ti o n a n d r e c o gn i z e t h a t t h i s h e a t f l u x l o c a l l y h e a t s t h er oc k a nd r es ul ts i n a n a dd it io na l h yd ro th er ma l f lo w w it h

temperature

T2, w e w r i te

cfQT2 crMT2 LM 2

where cr i s t he s pe ci fi c h ea t o f t he r oc k, L i s t he l at en t h ea t o f r e a c ti o n , a n dM i s t h e r a t e o f s e r pe n t in i z a ti o n i n k g / s, r e s pe c t i ve l y.C om bi ni ng e qu at io ns ( 1) a nd ( 2) a nd s ol vi ng f or t he t ot altemperature change T = T1 + T2 yields

T LMcfQ crM

H

cfQ 3

[7] F ig ur e 1 sh ow s T a s a f u n c t i o n o f M f o r v a r i o u s v a l u es o f Q, f or v al ue s o f H r a ng in g f ro m 0 t o 1 00 M W. T he r an ge o f M

c o rr e sp o nd s t o a v ol u me r a te o f s e rp e nt i ni z at i on o f 1 k m3

/106

y att h e l o w e n d (M = 0 . 1 k g / s ) t o 1 k m3/102y a t t h e h i g h e n d (M = 103

GEOPHYSICAL RESEARCH LETTERS, VOL. 29, NO. 11, 1531, 10.1029/2001GL014411, 2002

1S c ho o l o f E a rt h a n d A t mo sp h er ic S c ie n ce s , G e or gi a I n st i tu t e o f Technology, Atlanta, USA.

2Institute of Marine and Coastal Sciences and Department of GeologicalS c i en c e s, R u t ge r s U n i ve r s it y, N e w B r u ns w i ck , U S A .

C o p yr i g ht 2 0 0 2 b y t h e A m e r ic a n G e o ph y s ic a l U n i on .0094-8276/02/2001GL014411

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k g /s ) . T he r a ng e o f Q v a l ue s c o r r es p on d s t o a h i g h- t e m pe r a t ur e(400C ) h e a t o u t p u t b e t w e e n 2 . 5 M W cfQT 2 5 0 0 M W a n d alow-temperature (10C ) h e a t o u t p u t b e t w e e n 0 . 0 4 M W cfQT4 0 M W . T h e p a r a m e t e r s u s e d t h u s e n c o m p a s s e s s e n t i a l l y t h e e n t i r er a ng e o f e x pe c te d h yd r ot he r ma l t e mp e ra t ur e s a n d h e at o ut p ut s .A l th o ug h t he l a te n t h e at m a y b e s om e wh a t d if f er e nt f o r d if f er e nt s e r p en t i ni z a ti o n r e a c ti o n s, w e u s e t h e v a l ue L = 2 .5 105 J/kg[Fyfe and Lonsdale, 1 9 81 ] .

[8] F ig ur e 1 a s ho ws t ha t f o r l o w v a lu es o f Q t h e t e m pe r a t ur ei n cr e as e r e su l ti n g f r om s e rp e nt i ni z at i on c a n a p pr o ac h 2 50C,

providedM = 103 k g / s. F o r h i g h er m a s s f l o w r a t e s a n d l o w e r r a t e s

of serpentinization (i.e., 10

2

kg/s Q 103

k g / s a n d 1 k g / s M100 kg/s, respectively), T i s a t m o s t a f e w t e n s o f d e g r e e s C e l s i u s .O n l y e x c e pt i o na l l y h i g h r a t e s o f s e r p en t i ni z a ti o n 103 k g / s w o ul dg i v e r i s e t o a c h a ng e i n f l u id t e m p e r a t ur e e x c ee d i ng 1 0 0C . A t t h es e r p en t i ni z a ti o n r a t e o f 0 . 1 k g/ s s u gg e st e d b y Macdonald and

Fyfe [ 1 98 5 ]; h ow e ve r, t he l a te n t h e at r e le a se w ou l d n ot r a is e t hef l u id t e m pe r a t ur e m e a su r a bl y a b o ve b a c kg r o un d .

[9] F i gu r es 1 b a n d 1 c , c o rr e sp on d t o p os s ib l e r e g io na l b a c k -g r o u n d h e a t f l u x e s ( a t 1 m . y . c r u s t a l a g e , h a l f - s p a c e c o o l i n g m o d e l sg iv e a h ea t f lu x d en si ty o f 5 0 0 m W/ m2 [e.g., Fowler, 1 9 90 ] ).Thus H = 0 . 5 M W c o rr e sp on ds t o b a ck g ro un d h e at f l ux b e in ge xt ra ct ed o ve r 1 k m2, w he r ea s H = 5 M W m ay r ep re se nt h ea t e x t r ac t i on f r o m a b r o ad e r a r e a o r e n h a n c e d ( m a g ma t i c ?) h e a t f l u xf r om b e lo w. F i gu r e 1 b a n d 1 c s ho w t h at i f t h e r a t e o f s e rp e nt i ni z a-t i o n i s l e s s t h a n 1 k g /s , t h e h y d r o t he r m a l t e m p e ra t u r e i s d o m in a t ed

by the heat flux from below. At higher rates of serpentinization, thee f fe c t o f l a te n t h e at r e le a se i s s u pe r im p os e d o n t he h yd r ot h er m alt e mp e ra t ur e r e su l ti n g f r om t h e b a sa l h ea t f l ux . F i gu r es 1 b a n d 1 ci n d i c a t e t h a t i n a n e a r a x i s s e t t i n g w i t h f l o w r a t e s o f 1 0 t o 1 0 0 k g / s ,t h e h y d r o t h e r m a l t e m p e r a t u r e w i l l r a n g e f r o m a f e w t e n s o f d e g r e e sC t o p er ha ps a s m uc h a s 1 50C . T h e h i g he r t e m pe r a t ur e s c o r r es p o ndt o t h e l ow e r f l ow r a te s . I n c a se s o f r a pi d s e rp e nt i ni z at io n ( e .g . ,102 k g /s ) , lo w f l ow r a te ( e .g . , 1 0 k g/ s ), a n d h ig h b a sa l h e at f l ux ( e .g . ,5 M W ) h y d r o t h e r m a l t e m p e r a t u r e s c o u l d r e a c h m o r e t h a n 2 5 0C.

[10] F i gu r e 1 d c o rr e sp on d s t o s e rp e nt i ni z at io n s up e ri m po s edu p on a h ig h- t em pe r at u re h yd r ot h er m al b a ck gr o un d t yp i ca l o f am a gm at i c s ys te m . F or s uc h a s ys t em , w hi c h w ou ld t yp i ca l lyc o rr e sp on d t o a r id ge a xi s e nv ir on m en t , s im pl e h e at b a la n cem o de l s y i el d f l ui d f l ow r a te s1 00 k g/ s o r g r ea t er [ e .g . Lowelland Germanovich, 2 00 0] . T he h yd r ot he r ma l t e mp e ra t ur e i s t he n

m a in l y d e t e rm i ne d b y t h e b a s a l h e at f l ux e v en a t t he h i gh e st r a te sof serpentinization.

[11] T he L os t C it y v en t f ie ld , s it ua te d o n 1 .5 m .y. o ld c ru st 1 5 k m a wa y f ro m t he M id -A tl an ti c R id ge a t 3 0N, is currentlyd i sc h ar gi n g f l ui d s b e tw e en 4 0 a n d 7 5C . B as ed o n t he r es ul tsd e p i c t e d i n F i g u r e 1 , t h i s s y s t e m m a y b e s t c o r r e s p o n d t o t h e c a s e i nw h i ch t h e r a t e s o f s e r pe n t in i z at i o n a n d f l u id f l o w b o t h l i e b e t we e n1 0 a n d 1 0 0 k g / s b a s a l h e a t f l u x i s l o w ( F i g ur e 1 b ) . A s e r p e n t in i z a-t i on r a te o f 1 0 k g/ s o c cu r ri n g s t e a di ly o ve r t he 1 . 5 m . y o ld h is t or yo f l i t ho s p he r e a t t h i s s i t e w o ul d r e s ul t i n c o m pl e t e s e r pe n t in i z at i o no f a r oc k v ol um e o f 1 50 k m3. I t s ee ms l ik el y t he re fo re t ha t c u r r en t v e n t i n g a c t i v it y i n t h e s y s t e m i s y o un g e r t h a n t h e 1 . 5 m . y .o l d l i th os p he r e o n w hi c h i t i s s i tu a te d . B e ca u se t e ct on i c a c ti vi t y

m ay h av e o pe ne d r eg io ns o f f re sh r oc k f or f lu id a cc es s a nds e r p en t i ni z a ti o n r e a c ti o n s t o o c c ur , t h e r e m a y h a v e b e e n m u l ti p l ee p is od e s o f a c ti v it y a t L o st C i ty v e nt f i el d .

3. Discussion

[ 12 ] T he t em pe ra tu re g iv en b y t he h ea t b al an ce m od el i ne q ua t io n ( 3 ) r e pr e se n ts t h e t e mp e ra t ur e a t w hi c h t he f l ui d l e av e st h e r e g i on o f s e r pe n t in i z at i o n. I n a p u r el y p e r id o t it e - ho s t ed s y st e ms u c h a s L o s t C i t y , t h e r e a c t i o n z o n e m a y e x t e n d t o t h e s e a f l o o r ; b u t i n a s ys te m i n w hi ch p er id ot it e i s o ve rl ai n b y b as al ts s uc h a sR a in b ow o r L o ga t c he v, t h e s e rp e nt i ni z a ti o n r e ac t i on s m a y b eo c cu r ri n g o n ly a t d e pt h . T h en e q ua t io n ( 3 ) y ie l ds t he d i sc h ar g et e mp e ra t ur e a t t he s e af l oo r o n ly i f h e at l o ss f r om t he d i sc h ar g e

z o n e i s n e g li g i bl e o n c e t h e f l u id l e a ve s t h e r e g i o n o f s e r pe n t in i z a-tion.

[ 13] A s t he f lu id p as se s t hr ou gh c ol de r c ou nt ry r oc k a ft er l e av i ng t h e r e gi o n o f s e rp e nt i ni z at i on , s om e h y dr o th e rm a l h e at c a n b e l o s t b y t h e rm a l c o n d u c ti o n t o t h e l o w e r p e r m e a b il i t y r o c k sa dj ac en t t o t he d is ch ar ge z on e. T he r at e o f l at er al h ea t l os sd e c r e a s e s a s t h e c o u n t r y r o c k i s h e a t e d , h o w e v e r ; a n d t h e d i s c h a r g ez o n e b e c om e s s e l f- i n su l a ti n g [ e . g ., Bodvarsson and Lowell, 1 9 72 ;

Lowell, 1 9 7 5] .[14 ] S up po se t he h yd ro th er m al f l ui d i n t he d is ch ar ge z on e

e n te r s t he c ol d er c o un tr y r o ck w i th f l ow r a te Q a n d t e m pe r a t ur eTi a n d e x i ts t o t he s e af l oo r w it h t h e s a m e f l ow r a te a t t e mp e ra t ur eTo. I f t he f a r f i el d t e mp e ra t ur e Tf i s a s su me d t o b e 0 , t he r at e a t w hi ch h ea t i s l os t t hr ou gh w al l a re a 2A c a n b e a p pr o xi m at e ds i m p l y b y 2 AlTm/d, w h e r el is the rock thermal conductivity, Tm =

(Ti +To) / 2 i s t h e m e a n t e m p e r a t u r e o f t h e f l u i d p a s s i n g t h r o u g h t h ed i s ch a r ge z o n e, a n dd i s t h e l a t e r a l d i s t a n c e o v e r w h i c h c o n d u c t i o n

Figure 1. H y dr o t he r m a l f l u i d t e m p e ra t u re u p on l e a v in g t h e r e a c ti o n z o n e v e r s u s t h e r a t e o f s e r p en t i ni z a ti o n i n k g / s f o r v a r i o u s v a l u es o f m a ss f l ow r a te Q . ( a) N o b a sa l h ea t f lu x. ( b) B as al h ea t f l u x i s 0 .5 M W. ( c) B as al h ea t f lu x i s 5 M W. ( d) B as al h ea t f lu x i s 1 00 M W.

26 - 2 LOWELL AND RONA: SERPENTINIZATION OF PERIDOTITE


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o c c ur s . A s su m i ng c o o li n g i n a h a l f- s p ac e , d ffiffiffiffiffiffiffipatp , w h er e a ist h e t h er m al d i ff us i vi t y o f t h e r o ck a n dt i s t h e t i m e. T hu s t h e h e at

balance can be written

cfQ Ti To lA To Ti ffiffiffiffiffiffiffipat

p 4

I f t h e f r a c ti o n al t e m pe r a t ur e r e t e nt i o nTo/Ti =f, t h e t i m e n e e d e d f o r f t o e x ce e d a c e rt a in v a lu e i s d e te r mi ne d f r om e q ua t io n ( 4 ) b y t he

inequality

t 2

c2fpa

1 f1 f

2A

Q

25

[15] F ig ur e 2 p lo ts t a s a f un ct io n o f Q f o r d i f f e r e nt v a l ue s o f A, a n d f o r f = 0 . 9 . T h e v a l u e s l = 2 . 0 W ( mC)1, cf = 4 103 J

(kgC)1, a = 106 m2 s1 w e re c h os e n f o r t h e m a te r ia l p r op e r-t ie s. F ig ur e 2 s ho ws t ha t i f A = 106 m2 and Q = 1 0 k g/ s, t heh y dr o th e rm a l d i sc h ar g e t e mp e ra t ur e w il l b e l e ss t h an 9 0% o f t het e mp e ra t ur e c a lc u la t ed u si n g e q ua t io n ( 3 ) f o r 8 0 00 y e ar s ,w h er e as i f A = 10

5

m

2

, t he t im e n ee de d i s8 0 y ea rs . A w al la re a o f 1 06 m2 i mp li es a d is ta nc e o f 1 k m a bo ve t he z on e o f s er pe nt in iz at io n a nd a l at er al d is ta nc e o f 1 k m. A s d is cu ss ed

previously, the most likely scenario for the Lost City vent field toh a ve a t e mp e ra t ur e4 0 7 5C is f or bot h M and Q t o r a ng e

between 10 and 100 kg/s, respectively. Thus, depending upon thea m ou nt o f c o nd uc t iv e c o ol i ng u nd e rg on e b y t he L o st C i ty v e nt f lu id s, t he a ge o f t he c ir cu la ti on m ay r an ge f ro m102 t o 1 04years.

[16] T he h e at b a la n ce m o de l o f e q ua t io n ( 3 ) a l s o a s s um e s t h a t a l l t h e h e a t r e l e as e d u p on s e r pe n t in i z at i o n i s r e a d il y a v a il a b le f o r t r a n sp o r t b y t h e h y dr o t he r m a l s y s t e m . T h i s a s s u m p ti o n r e q ui r e st h a t i n t er c o nn e c te d f l u id p a t hw a y s o c c ur t h r ou g h ou t t h e r e a c ti o nz o n e a n d r e m a in o p e n a s s e r pe n t in i z a ti o n o c c ur s . B e c a us e s e r p en -t in iz at io n i s a cc om pa ni ed b y a v ol um e i nc re as e o f u p t o 5 0%

[Coleman, 1 97 1; OHanley, 1 99 2] , t he r ea ct io n t en ds t o s ea lporosity.[ 17] S up po se , h ow ev er , t ha t t he m ai n f lu id f lo w o cc ur s i n

f r a c t ur e s d i s tr i b ut e d w i t hi n t h e h o s t r o c k a n d t h a t s e r p e n t in i z a ti o no cc ur s a s f lu id i nf il tr at es i nt o t he h os t r oc k t hr ou gh a s et o f m i c r of r a c t ur e s o r i e nt e d r o u gh l y p e r pe n d ic u l a r t o t h e m a i n c r a c k n e t wo r k . T h e n a s s e r p en t i ni z a ti o n o c c ur s , t h e v o lu m e e x p an s io nc o u ld e x e rt s t r e ss e s t h a t c r e a te n e w f r a c t u r e s u r f a c e s a l l o w in g t h er ea ct io n t o p ro gr es s i nt o t he r oc k [Macdonald and Fyfe, 1 98 5;OHanley, 1 99 2] . S uc h a p ro ce ss w ou ld r es ul t i n t he ke rn el

pattern observed in partially serpentinized rocks, in which a core ofu n re a ct e d p e ri d ot i te i s s u rr o un de d b y a z o ne o f s e rp e nt i ne w it hc r os s -f r ac t ur e s e x te n di n g f r om t h e s e rp e nt i ne t o t h e h o st r o ck

[ F ig ur e 3 ; OHanley, 1 99 2 ]. S e rp e nt i ni z at i on m a y t hu s b e c o n-t r ol l ed b y d i ff us i on o f w a te r f r om t h e m a i n c r ac k n e tw or k i n to t h eh o s t r o c k a s d e s c r i b e d b yMacdonald and Fyfe [ 1 9 8 5 ] , o r i t m a y b ec o n t r o l l e d b y t h e r a t e o f c r a c k p r o p a g a t i o n i n t o f r e s h r o c k . I n e i t h e r c a s e , p h y s i c a l p r o c e s s e s t h a t a r e n o t e x p l i c i t l y i n c o r p o r a t e d i n t o t h e

present heat balance model could control the rate of serpentiniza-t i o n a n d l a t e nt h e a t r e l e a s e . M o r e o v er , a s t h e s e r pe n t in i z at i o n f r o n t m o ve s a w ay f r om t he m a in c r ac k n e tw or k t he t r an s po r t o f l a te n t h e at t o t he h y dr o th e rm a l s ys t em b e co me s l e ss a n d l e ss e f fi c ie n t.T h e l i ke l ih oo d t ha t s e rp e nt i ni z at io n o f a v ol u me o f r o ck w o ul do c c ur e f f i ci e n tl y a t a c o n st a n t r a t e i s t h e re f o r e l o w . C o ns e q ue n t ly,t h e t e m pe r a t ur e c h a ng e s r e s u lt i n g f r o m s e r p en t i ni z a ti o n g i ve n b ye q u at i o n ( 3 ) a r e l i k el y t o b e o v e re s t im a t e s.

4. Conclusions

[18] O ur m o de l c a lc ul a ti o ns i n di c at e t h at e x ot h er m ic s e rp e nt i-n i za t io n r e ac t io ns e i th e r s e pa r at e ly o r i n c o nc e rt w it h h ea t f l uxf r o m b e l o w c a n a c c o u n t f o r t h e h e a t t o d r i v e s e a f l o o r h y d r o t h e r m a ls y s te m s w i t h a w i d e r a n g e o f v e n ti n g t e m pe r a t ur e s . S e r p en t i ni z a -t i on r e ac t io ns a r e l i ke l y t o r e su l t i n h yd r ot h er m al t e mp e ra t ur e sr a n gi n g f r o m a f e w t o a f e w t e n s o f d e g re e s C e l s i us . T e m p er a t ur e s300C i n p e r id o t it e - h os t e d s y s t e m s m a y b e p o ss i b le i n c o n ju n c -t i on w i th s i gn if i ca n t h e at i n pu t f r om b e lo w, l ow f l ui d f l ow r a te(1 0 k g /s ) , a h ig h r a te o f s e rp e nt in i za t io n (1 00 k g /s ) , o r s om ec o m bi n a ti o n o f t h e se f a c t or s . T h e v o l u m e o f l i t ho s ph e r e f r e s h t h a t i s a c c es s i bl e t o s e r pe n ti n iz a ti o n a n d d r iv e l o ca l h y dr o th e r ma lv e nt in g m a y l i mi t t h e d ur a ti on o f a h yd r ot he r ma l e p is od e w h ent h e s e rp e nt i ni z at i on r a te e x ce e ds 1 0 k g /s . H i gh r a te s o f s e rp e nt i-n i z a ti o n m a y o c c ur e p i so d i ca l l y a s n e w r e a c t i o n s u r f a c es a r e m a d ea v ai l ab l e b y t e ct o ni c a c ti v it y o r c r ac k p r op a ga t io n. T he t e ct o ni ca n d m a g m a t ic h i s to r y o f a r e g i on m a y e v e n t ua l l y b e d e r iv e d f r o mr a di o me t ri c d a ti ng o f h y dr o th e rm a l p r ec i pi t at e s e n er gi z ed b ys e r p en t i ni z a ti o n r e a c ti o n s ( c a r bo n a te s ) a n d m a g ma t i c i n t ru s i on s( s u lf i d e s) , r e s pe c t iv e l y. H y dr o t he r m a l s y s te m s d r i ve n b y e x o th e r -m a l s e r p e n t i n i z a t i o n r e a c t i o n s a n d a s s o c i a t e d s o l u t i o n s a r e l i k e l y t o

be a quantitatively significant component of hydrothermal outputo f s l o w - s p r e a d i n g o c e a n r i d g e s , w h i c h c o m p r i s e m o r e t h a n h a l f t h eglobal 6 0, 00 0 k m l en gt h o f t he o ce an r id ge s ys te m i n t heA t l an t i c, I n d ia n a n d A r c t ic o c e a ns .

[19] Acknowledgments. We t ha n k t h e r e vi e we r s a n d t h e a s s o ci a tee d i t o r f o r c o m m e n t s o n a n e a r l i e r v e r s i o n o f t h i s m a n u s c r i p t . T h i s w o r k w a ss u p po r t ed i n p a r t b y N S F G r a nt O C E 9 5 2 9 9 54 t o R . P .L .

Figure 2. T im e f o r h y d r o t he r m a l t e m pe r a t ur e t o r e a c h 9 0 % o f t h et e m pe r a t ur e o f t h e f l u i d l e a vi n g t h e r e a c ti o n z o n e a f t e r a c c o u n ti n g

f o r l a t er a l h e a t l o s se s p l ot t e d a g a i n st m a s s f l o w r a t e Q for differentv a lu e s o f h e at l o ss a r ea A .

Figure 3. D i a gr a m m at i c s k e tc h s h ow i n g p r o p a ga t i on o f c r o ss -f r a c t u r e s i n t o a v o l u m e o f p e r i d o t i t e a n d t h e r e s u l t i n g k e r n e l p a t t e r n[after OHanley, 1 9 92 ] .

LOWELL AND RONA: SERPENTINIZATION OF PERIDOTITE 26 - 3


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26 - 4 LOWELL AND RONA: SERPENTINIZATION OF PERIDOTITE

seafloor hydrothermal systems

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