Three Reasons Why Petrologists Should Study Compaction

J. Connolly, ETH Zurich

What is compaction driven fluid flow?

expe lled flu id andm elt

D ep thro ckflow

flu idflow

reac tion -gene ra ted flu idand po rosity

Objectives

• Provide a conceptual understanding of porosity waves in a viscous rock matrix

• Insights from compaction on melt extraction at mid-ocean ridges

A simple model for regional metamorphism

e x p e l l e d f l u i d a n dm e l tD e p t h r o c kf l o wf l u i df l o wr e a c t i o n - g e n e r a t e d f l u i da n d p o r o s i t y

P o ro s ity

Dep

th

F lu id P re s su re

rea c tio n f ro n tp o re d ila tio n

p o re c o m p ac tio n

rea c tio n f ro n t

d ee p -se a te dh ea t so u rc e

What happens with time?

F lu id d ru c kP o ro s itä t

Tie

fe

R ea k tio n sfro n t

t= 1

t= 2

t= 3so litä re W elle n

Numerically computed porosity and pressure profiles above a metamorphic dehydration front

T im e (M y )

5 0 M P a

o v e rp re s su reu n d e rp re ssu re

P re s su re a n o m aly

2 0

1 6

2 4 6 8 1 0

1 2

1 6

1 2

8

4

0

4

0

8

2 0

5 %

P o ro s ity

p o ro s ityso lita ry w a v e

Dep

th (

km)

Dep

th (

km)

R ea c tio n f ro n t

Birth of the Blob Modelor

Fluid Flow through a 2D Rock Matrix with Constant Viscosity

Dep

th

P o rosity, = 010t

/ m a x 0

t= 3 .3 50

/m a x 0

t=10 0

3 .3 / m a x 0

2 km

10

2 km

10

In itia l cond itio n 1D M ode l 2D M ode l, B irth o f th e “B lob”

B lo b s a re b ig g e r, fa s te r a n d b e tte r lo o k in g .

· Length scale for fluid flow ~

Tod des Blobsoder

Fluidfluss durch eine sich aufwärts verstärkende Matrix

initial Störungen/ 0 = 10

Spherische StörungPorosität um = 22.5 0 (18 M j)/ 10 t t m a x 0 S inusform ige StörungPorosität um = 0 .5 0 (0 .4 M j)/ 50 t t m a x 0starke D eckschicht

in itia l S tö ru ngen/ 0 = 10

S ph erische S tö ru ngP orositä t um = 2 2 .5 0 (1 8 M j)

/ 10t t

m a x 0

S inusfo rm ig e S tö run gP orositä t um = 0 .5 0 (0 .4 M j)

/ 50t t

m a x 0

sta rk e D eck sch ich t

Has anyone ever seen a porosity wave?Porosity (%)250

3

Depth (km)

fluid compartment(oil und gas)

sedimentvelocity

$-drivenfluid flow

stationarycompaction-front

50

P orosity (% )25

0

3

Dep

th (

km)

f lu id co m partm en t(o il u nd gas)

sed im en tve locity

$ -d riv e nf lu id f lo w

sta tio narycom paction -fron t

50

Sedimentary Basin Compaction

Pannonian basin

P o ro s itä ts-P ro fil de s P ann o n isch en B eck ens

T

iefe

(km

)

P o ro sitä t (% )

4

2

0

5 1 5 2 5

A thy

G au ss (ko n s tan te G este in sv isk o sitä t)

P ann o n isch es B eck en

M o d e ll

D ieses R esu lta t häng t in e rs ter L in ie nu r v on de r

R h eo lo g ie un d d ie ab , d .h ., da s

E ndp ro fil is t un ab häng ig vo n S ed im en t-

P e rm eab ilitä t.

S ed im en ta tion rate

Inverse analysis of sedimentary compaction profiles for pressure solution creep parameters

S h a le sS an d s to n e s

1 5 0 0 2 0 0 0 2 5 0 0 (m ) z

1 0

2 0

3 0

4 0

2 0 .5

2 1 .5

2 2 .5

^^

Q̂ (

kJ/m

ol)

log(

[Pa-

s])

m

-2

0

2

Lateral flow during regional metamorphism?

A World Where Fluids Flow Upward => Mid-Ocean Ridges

How does melt produced during mantle upwelling get focused at mid-ocean ridges?

How can highly incompatible short-lived isotopes be fractionated and preserved in MORB?

A World Where Fluids Flow Upward => Mid-Ocean Ridges

How does melt produced during mantle upwelling get focused at mid-ocean ridges?

How can highly incompatible short-lived isotopes be fractionated and preserved in MORB?

How does melt get to the ridge?

C o rn e r F lo w S u c tio n E ffe c t

G o o d N e w s

F i ts m o s t g eo c h e m ic a l a n d g e o p h y s ic a l c o n s t ra in ts .

B a d N e w s

R e q u ire s h ig h m a n tle v is c o s ity ( > 1 0 P a -s ) . 21

L ith o s p h e r ic C h a n e llin g

G o o d N e w s

A s fo r c o rn e r f lo w .

B a d N e w s

E x p e c te d p a tte rn s n o t o b se rv e d in o p h io lite s .

M a n tle B u o y a n cy

G o o d N e w s

T h e o re t ic a lly ju s tif ie d .

B a d N e w s

P o o r m a tc h fo r g e o c h e m ic a l a n d g e o p h y s ic a ld a ta .

R e q u ire s lo w m a n tle v is c o s ity ( < 1 0 P a -s ) . 19

Steady State

Ridge Axis

Norm alized crustal th ickness

M O R Stead y Sta te ( = 1 8 % , = 9 % , = 1 cm /y, = 1 0 P a -s ) f f Wmax avg20

What was wrong?

P r e v i o u s m o d e l s

f t o t a lp p

d0

d t

p y S y

2

2 2

2a

US

L g

S t e a d y s t a t e p o r o s i t y e v o l u t i o n e q u a t i o n

fd0

dm p p

t

fmp y S y M

3

2 c c

sa

U LM

L g

M eltin g

100-

150

km10-10000 y

M id -O ce an R id g e S u b d u c tio n Z o n es

Fast Fluid Transport in Ductile Rocks

600-6000 y

40 0 transport0.1% 10 km y 1Myv

transport transport0.1km y 3%v

Initial State

Final State

Return of the Blob

Final State

Conclusion

The combination of models suggested here can reconcile the geochemical signature of MOR basalts, with the possible exception of near surface matrix-melt disequilibrium.

Reports of the death of the porosity wave model are premature and premised on a rheological model that is almost certainly false.

Viscoelastic porosity wave model for Pannonian Basin sediments

p seu d o e lastic tre n d

P a n n o n ia n tren d

b) P a n n o n ia n sa n d s to n es

(% )0 5 1 0 1 5 2 0 2 5

(% )1 0 1 5 2 0 2 50 5

z (k

m)

4

3

2

1 h y d ra u lic

v isc o u s in v erse m o d e l

v isc o u s in v erse m o d e lv isc o e las tic in v e rse m o d e l

v isc o e las tic in v e rse m o d e l

p seu d o e lastic tre n d

P a n n o n ia n tren d

v isc o e las tic f it

v isc o e las tic f it

a) P a n n o n ia n sh a les

Viscoplasticity

Viscous porosity waves are propagated by high fluid pressures. Under such conditions rocks even ductile rocks will deform plastically.

ov erp re ssu re

un de rp re ssu re

w eak

strong

pressu re in a v isco us b lob

schw ach e D eck sch ich t

2D P lastiz itä t

F lu id flu ssfok ussieru ng

sta rk e D eck sch ich t

2D Viskositä t

F lu id flu ssd ispe rs io n

Morb the Movie

What happens beneath a mid-ocean ridge?

P a r tia lly M o l te nA s th e n o s p h e re

6 0 k mL ith o s p h e re

1 -2 0 cm /y

O c e a n ic C ru s t (6 -7 k m ) Tem p e ra tu re

Dep

th

Meltin g C u rv e

What next?

c a rb o n a te s+ h y d ra te s

M e ltin g

S u b d u c tio n Z o n e s

H O + C O2 2

H O + C O2 2

In f il tra t io n -d riv e n d ec a rb o n a tio n

Composition and depth of devolatilization => global volatile budget, deep seismicity

Amount of pore fluid => subduction zone seismic structure

C o m p u te d P h a s e R e la tio n s , S e is m ic Ve lo c itie s

P l S a Bt C rd

P l Ms G rt B

t s il

P l Chl B

t Ms

P l Ms B t C

rd

C h l P l M s K fs

Ch l P l M s Ms Chl Ep ab

P l Ms Amph C

h l

C h l E p M s P g ab

Ms E

p Pg A

mph Chl Am

ph Pg C

hl Ms l

ws

M s Pg Ch l Amph

Am ph G rt M s Pg

Amph P

g Ms

Grt

P l

P l Bt M

s G rt

Pg A

mp

h Chl G

rt Ms

Ch l

Ms

Pg

lws

6

72

8

3

9

4 5

1 0

12

1 4

0 .5

1 .0

1 .5

4 00 5 00 T (° C )

P(G

pa)

0 .5

1 .0

1 .5

4 00 5 00 T (° C )

P(G

pa)

11

1 3

1

Model Formulation

I n c o m p r e s s i b l e c o r n e r f l o w s o l u t i o n f o r v e l o c i t y 1 ,v U f x y

M e l t i n g r a t e p r o p o r t i o n a l t o v e l o c i t y

, , 2aL

m e l tm e l t c c m e l t c c a

x ya

mv m v d x d y U L m L L

L

T o t a l p r e s s u r e i s l i t h o s t a t i c + c o r n e r f l o w e f f e c t

2g ,p y U f x y

V i s c o u s c o m p a c t i o n , i n c o m p r e s s i b l e s o l i d a n d m e l t

fp pv

M e l t f l o w b y D a r c y ’ s l a w , n e g l i g i b l e a d v e c t i o n

C o n s e r v a t i o n o f t o t a l m a s s a n d s o l i d m a s s

d

d 1

mfp p

t

1 1g fn m

f ff

p pkp

What next?

Experimental and microscopic models to characterize differential compaction rheology

The mantle wedge

• The models assumed constant porosity and lithostatic melt pressure.

• Lithostatic melt pressure is fundamentally inconsistent with expulsion.

• Variations in porosity, and therefore permeability, may cause significant focusing.

• To assess these effects it is essential to account both for the process that creates porosity (melting) and destroys it (compaction).

What was wrong with previous models of the corner flow effect?

What next?

Dynamic modelling of the matrix deformation, thermal controls of melting rates, and melt advection => details of the focusing

Ergo

The corner flow pressure effect is not dependent on the mantle viscosity and is capable of explaining extraction of asthenospheric melts at mid-ocean ridges

What next?

Evaluate the influence of the mantle compressibility on the strength of the pressure effect => future work?

Consider details necessary to explain geochemical peculiarites of MORB => next slide.

What is wrong with “conventional” porosity waves?

•Require high initial porosity to nucleate, but there is no Th/U fractionation at high porosity

•Unlikely to propagate at velocities > 3 v0

•Based on an inappropriate rheological model

So what point am I trying to make?

The first order control on the time and length scales of fluid flow in many petrologic systems is mechanical. To attempt to understand such processes solely through the study of petrological and geochemical tracers is like wagging a dog by its tail.

m e tam o rp h ic f lu id f lo w

Lateral flow during regional metamorphism?

What causes the pressure difference?

S t a t i c p r e s s u r e d e n s i t y .

V i s c o u s r h e o l o g y f l u i d t o t a l

r o c k

p pdv

d t

P ressu re

m echan ical equ ilib riu m

flu id p re ssu re

ro ck p re ssu reD

epth

f lu id -o verp re ssu re d ila tio n

flu id -u nde rp ressu re com pactio n