American Mineralogist, Volume 61, pages 603-615, 1976
Single-pyroxenegeothermometryand geobarometry
Jpnn-CleuoE C. MERCIER
Department of Earth and Space Sciences, State Uniuersity oJ New York
I 1794
Stonv Brook. New York
A bstract
T h e a v a i l a b l ee x p e r i m e n t adl a t a f o r t h e e q u i l i b r i u m r e a c t i o n st h a t c h a r a c t e r i z e i t h e r t h e
enstatite-diopsidejoin or the Al-concentration in pyroxenescoexistingwith an Al-rich phase
a r e r e d u c e dt o t h r e e o r f o u r t h e r m o d y n a m i cc o n s t a n t s ,e n t r o p y , e n t h a l p y ,a n d c h a n g ei n
v o l u m e ( t w o p a r a m e t e r s f o r t h e E n / Dj oi i n ) U s e d t o g e t h e r w i t h a s e t o f p a r t i t i o n c o e f f i c i e n t s
derived from natural assemblages,
these values allow independentestimatesof pressureand
t e m p e r a t u r et o b e m a d e f o r a n y s i n g l ep y r o x e n ep h a s ei n e q u i l i b r i u mw i t h a s e c o n dp y r o x e n e
a n d w i t h e i t h e r s p i n e lo r g a r n e t ;o t h e r w i s e m
, i n i m u m t e m p e r a t u r ea
snd maximum pressures
a r e o b t a i n e d .T h i s m e t h o d h a s b e e nc h e c k e dl o r c o n s i s t e n cby o t h i n t e r n a l l y( s i m i l a re s t i m a t e s
f o r c o e x i s t i n gp y r o x e n e si n e q u i l i b r i u m) a n d e x t e r n a l l y( a g r e e m e nwt i t h i n d e p e n d e not b s e r v a tions).
The geothermometersand geobarometersare applied to xenolithsfrom "kimberlites" of the
N a v a j o I n d i a n R e s e r v a t i o na n d t o o p h i o l i t e sf r o m W e s t e r nN e w f o u n d l a n d ,f o r b o t h o f w h i c h
partial recrystallizationinto hydrous phasesoften preventedthe use of any other technique.
This method is also applied to determine pressure-temperaturepaths for partially reequilibrated textures, to derive pyroxene geotherms from xenocrysts,to detect disequilibrium
b e t w e e nc o e x i s t i n gp y r o x e n e s ,a n d t o r e v e a l a c t u a l e q u i l i b r a t i o nf a c i e so f c o m p l e x p a r a g e n e s e sT. h e s e s t u d i e ss u p p o r t ( l ) a r e m a r k a b l es h o r t e n i n go f N e w f o u n d l a n d o p h i o l i t i c
s e q u e n c e so,r i g i n a l l y 1 0 0k m t h i c k , ( 2 ) a s h a l l o wo r i g i n f o r t h e N a v a j o R e s e r v a t i o n" k i m b e r l i t e s , " a n d ( 3 ) a n u n d e r t h r u s t i n go f o c e a n i c - t y p el i t h o s p h e r eb e n e a t ht h e w e s t e r nU n i t e d
S t a t e s 'c r u s t .
Introduction
P y r o x e n eg e o t h e r m o m e t r ya n d g e o b a r o m e t r yh a v e
b e c o m ef u n d a m e n t a li n d e t e r m i n i n g a
, s a f u n c t i o no f
d e p t h , t h e p e t r o l o g i c a cl h a r a c t e ra n d s t r u c t u r a ls t a t e
of the earth's upper mantle. There are, however,
m a n y i m p o r t a n t i n s t a n c e si n w h i c h o n l y o n e p y r o x e n e p h a s ei s c h e m i c a l l yk n o w n , p r e c l u d i n gd i r e c t a p p l i c a t i o n o f t h e c l a s s i c am
l e t h o d s .P r o v i d e dt h a t t h i s
phasewas originally in equilibrium with a second
p y r o x e n ea n d a n A l - p h a s e ,i t s t e m p e r a t u r ea n d p r e s s u r ea t e q u i l i b r a t i o nm a y b e c a l c u l a t e do n t h e b a s i so f
t h e r m o d y n a m i cp a r a m e t e r sd e t e r m i n e df r o m e x p e r i m e n t a l d a t a , a n d p a r t i t i o n i n gc o e f f i c i e n tds e t e r m i n e d
e m p i r i c a l l yl r o m n a t u r a l a s s e m b l a g e s .
E x p e r i m e n t a dl a t a a n d t h e o r e t i c a d
l e r i v a t i o n sp e r t i n e n t t o t h e e n s t a t i t e - d i o p s i djeo i n a n d t o A l - c o n c e n t r a t i o n si n p y r o x e n e si n e q u i l i b r i u m w i t h a n A l rich phase(spinel or garnet) have been reviewedrec e n t l y b y B o e t t c h e r( 1 9 7 4 ) a n d M e r c i e r a n d C a r t e r
( 1 9 7 5 ) ,a n d t h e s ed a t a h a v e b e e n u s e dt o c o m p u t e a
s e r i e so l p a r a m e t e r s( e n t h a l p y ,e n t r o p y , a n d c h a n g e
i n v o l u m e ) w h i c h c o m p l e t e l yd e s c r i b ea g i v e n s y s t e m
w i t h i n t h e p r e c i s i o no f t h e d a t a ( M e r c i e r a n d C a r t e r ,
1975)A
. s e to f e q u a t i o n si s d e r i v e dh e r ew h i c h a l l o w s
a c o m p u t a t i o n o f f u l l y c o r r e c t e dp r e s s u r ea n d t e m p e r a t u r e e s t i m a t e sf r o m t h e c o m p o s i t i o n o f e i t h e r
p y r o x e n ep h a s e ,p r o v i d e d t h a t t h i s p h a s eh a s b e e ni n
e q u i l i b r i u m w i t h a n o t h e r p y r o x e n ea n d a n A l - r i c h
p h a s e , o t h e r w i s e ,t h e e s t i m a t e so b t a i n e d a r e o n l y
m i n i m u m t e m p e r a t u r e sa n d m a x i m u m p r e s s u r e sI.n
a d d i t i o n t o t h e t h e r m o d y n a m i cp a r a m e t e r sr e f e r r e d
t o a b o v e , t h e s ee q u a t i o n sc o n t a i n a s e t o f p a r t i t i o n
coefficientsderived from hundreds of natural assemb l a g e s .W r i t t e n i n t h i s w a y , a n y r e f i n e m e n to f t h e
e x p e r i m e n t a ld a t a c a n b e e a s i l yt a k e n i n t o a c c o u n t
t h r o u g h r e p l a c e m e not f t h e c o n s t a n t ( s )a f f e c t e d .
T h e s i n g l e - p y r o x e n et e c h n i q u e i s f i r s t t e s t e d o n
t e x t u r a l l ye q u i l i b r a t e dm e t a m o r p h i cl h e r z o l i t e sa, l s e
603
JEAN-CLAUDE C MERCIER
assumedto be chemically equilibrated,to determine
the uncertainty related to the analysesused. This is
achievedby deriving independentlypressureand temperatureestimatesfor eachpyroxenephasein various
samples.Whereasthe experimentaldata usedto compute the thermodynamic parameters for the enstatiteldiopside solvus and for the Al solubility in pyroxenes(garnet facies) are generally accepted,data on
the Al solubility in pure enstatitefor the spinel facies
are still controversial.MacGregor's (1974) isopleths
are acceptedas a realisticmodel for the averagenatural pyroxene compositions (Mercier and Carter,
1975),and the validity of such a model is confirmed
by the agreementbetween maximum and minimum
depths of extension of the spinel facies,derived for
severallocalities from (l) the model basedon MacGregor's data, and (2) geophysical data for the
Moho. and/or the shallowestgarnet lherzolitesrecognized.
Inclusions in the diatremes in the Navajo Indian
Reservation (Utah-Arizona) and ophiolites from
Newfoundland are discussedhere as two applications
of the single-pyroxenetechnique, since low-temperature transformations (serpentinization,chloritizarion, etc . . .) and recrystallization had left a few
remnants of only one of the original pyroxenephases
in most of the samples.Pyroxenexenocrystsfrom the
Navajo Reservation diatremes have also been included in the study.
Deriuation of the equalions
The classicalpressureequation
P = (RTlnK'+ rAS - AH)/AV
has been used to fit K' composition-isoplethsin P-Z
space (Mercier and Carter, 1975) and to derive enthalpy, entropy, and change-in-volumevaluesfor the
equilibrium reactions related to the enstatiteldiopside join
(Mg, SirO6)"o*
- (Mg, Si2o6)"o*
En""
En""
(K-" LH., AS-, AV-)
a n d t h e A l - s o l u b i l i t y o f p y r o x e n e si n e q u i l i b r i u m
with an Al-phase
M g A l , S i O 6+ M g r S i O a
Ol
En""
MgAlrO' *
SP
3
MgrSirOa
En""
(sPinelfacies)
or
M g A l z S i O 6+ M g , S i O u
En""
En""
i
(garnetfacies)
MgaAlzSirOr,
(
K
"
'
,
AH", As"' A %)
Gt
The expressionsfor the various partition coefficients
K' are discussedin a later section. Within the preciThe generalmethod for fitting the availableexperi- sion of the data, enthalpy and entropy are constant
mental data and for extrapolation of the resultsfrom and the change in volume can be expressedas a
a given pyroxenephaseto the other has been recently function of the composition through:
discussedby Mercier and Carter (1975).The method
AV:u'*u"lnK'
describedhere emphasizesthe possibility of deriving
fully corrected temperature and pressure estimates where u' and D" are constant for a given reaction.
Consideringthe equilibrium reactionsgiven above,
from any single analysisof a pyroxene provided (or
assuming) an original equilibrium with a second one may write the system of equations
pyroxene and an Al-phase (spinel or garnet). Al- AH,,)/(u,,'I u"" ln K"')
used should also contain oli- P : (RT ln K.' * ZA.t,"
though the parageneses
The method
vine (reaction given below), the equations might apP : ( R T l n K , " t + r A S . , - A H . , ) / ( o - '* u , " " l n K - ' )
ply to some olivine-free assemblages,since no
significantdifferencesare found for estimatesderived Combining these two equations yields independent
from peridotites and coexisting reequilibrated py- temperatureand pressureequatlons:
roxenites (recrystallizedtectonites). In experiments,
the enstatite/diopsidejoin, at least,seemsinsensitive
to the degreeof Si saturation (Lindsley and Dixon,
- AH"(u-' * u*" 1n K-')
1976). However, many pyroxenites still have magmatic textures and therefore the estimates derived
: Z(R ln K,,' * A.S.)(u"' I o." ln K"')
from them cannot be representativeof regional con- AH*(ul -f ul' ln Kl)
ditions.
SI NGLE.P YROX E NE GEOTHERM OM ETRY A N D GEOBAROM ETRY
and
Tnst-r I . Enthal py, entropy, a nd change-in-volume data for
e n s t a t i t e - d i o p s i d ea n d A l - p y r o x e n e - A l - p h a s es y s t e m s *
P(R ln K"' *
AS")(u.' I
u-" ln K-')
+ AE.(R ln K"' *
:
P(R ln K-' *
-AH
AS,)
AS.)(u"' * u"" ln K"')
En/Di
which can be rewritten as:
lAH"(u-' I
u,," lnK,,') - AH",(o"'* u"" LnK"'))
/[(R ln n-"' + AS.Xu,,' * u-" ln K,,')
-
(R ln K,,' + AS,,)(u,,'t
-Art
v,
+ AH"(R ln K,,' + A.S.)
T :
-AC
callmo1
cal/rcL/oK
25I95
1 4. 1 5
cal,/kb
v',
cal/kb
A1-En/Sp
1 2 53 5
7.3I
168.0
A1-Di/sp
14977
9 .80
22Q.O
n
AI-Px/ct
6295
L 7T . 4
0.
* AV is
u"t' ln K,,'))
(v'
apptoxi@ted
+ v"
bg a
Jinear
function
LnKr) .
and
P :
[AH"(R \n K,"' I A^t,,)- AH",(R ln K"/ * AS.)]
/[(R ln i',' *
-
AS"Xu,,,'* u,,," ln K,,t)
P :
(R ln K,,' + AS.Xu"' * u"" ln K,,')l
[ ( u * "A H " ) \ n K - ' *
(u.'AH" - u"'AH-)]/ D
and
P :
I(RAH") ln K.' -
(RAH.) ln K"'
+ ( A r 1 " A . S .-
(n.ln K.' I
p".ln K"' + p,)/ D
where
For simplification,the dimensionlessdenominator comm o n t o b o t h e q u a t i o n sw i l l n o w b e d e s i g n a t e db y D .
The equations can be further simplified sinceDo": g
in all cases(Table 1), and they may be rewritten as
functions of the actual variableslnKo' and lnK*':
7 :
and
AH,AS")l/D
D:
ln K,,'.lnKi *
d,'ln K.'*
d.'ln K.'*
d,
The values for the various t, p, and d coefficientsare
calculatedas follows:
t, :
AH"f R
tz :
(u*'AH" - o,'AH-)fo-"R
P, :
AH"/u."
p, :
AH-/u-"
p,:
(AH"AS. -
:
706 kbar (from data in Table l)
AIl.49")fu-"R
dt -- AS"/R - u"'/u-"
where
: u-'/u*" : 2.26 (from data in Table 1)
( u , , 'A
, S, - D",R)ln K,,,, d"
: (u-t AS" - u"' AS-)/r,,," R
I @",'R)ln K,,' I (u",'AS" - u.,'A,S,,,) /"
By dividing all the coefficientsby o,"R, these ex- The numerical value for these coefficientsdepends
generally on the pyroxene and/or the faciesconsidpressionsbecome:
ered (Table 2), with the exceptionof pc and d", which
T : (t,.ln K.' * t")/D
are constant.
p : (u-"R)ln K_'.ln K"' I
T l s t - e 2 T h e r m o d y n a m i c p a r a m e t e r sf o r t h e t e m p e r a t u r ea n d p r e s s u r ee q u a t i o n s *
-t
A
'2
t- 1 ' l o K
- -\'
t2 (oK)
pr (kb)
p2 (kb)
At-En/Sp
- 8.387
+25.2I8
-6308.5
+45449.
+351.32
+ 95.99
Al-Di/sp
-I1.098
+32.765
- 7 5 3 7. 5
+61-152.
+4I9.76
+493.49
Al-Px/ct
- 6.208
+31.037
-3168.1
+53754.
+176.43
-264.90
* hrived
f rom data
in
Table
t.
JEAN.CLAUDEC MERCIER
M a c G r e g o r ' s e x p e r i m e n t a ld a t a , o n c e W o o d a n d
( 1 9 7 3 )c o r r e c t i o n sa r e a p p l i e dt o a c c o u n tf o r
W h e r e a sa s i n g l es e to f t h e r m o d y n a m i cp a r a m e t e r s B a n n o ' s
could
(AH, A^S,u' andu") is neededto definethe Ca solubil- s o l i d s o l u t i o n e f f e c t s .U n f o r t u n a t e l y ,i s o p l e t h s
d
i
o
p
s
i
d
e
s
a
s this
c
o
e
x
i
s
t
i
n
g
f
o
r
t
h
e
ity in either diopside or enstatite for either facies n o t b e o b t a i n e d
since
the
Indeed,
facies.
was
for
the
spinel
done
( s p i n e lo r g a r n e t ) a s a f u n c t i o n o f p r e s s u r ea n d t e m g e o b a r o m e t r yi s d e f i n e d( M e r i
n
A
l
u
s
e
d
e
q
u
i
v
a
l
e
n
t
perature (Mercier and Carter, 1975),a specificset of
1 9 7 5 )a s :
parametersmust be derived for each pyroxenephase c i e r a n d C a r t e r .
a n d f o r e a c h f a c i e st o c o m p l e t e l yd e f i n et h e A l s o l u - A : ( A l + C r - N a ) / 2
bility in pyroxenes.
(in atomicfractionsfor a 6-oxygenformula)
F o r t h e s p i n e lf a c i e s ,t h e t h e r m o d y n a m i cp a r a m e t e r s f o r A l ( E n ) d e r i v e d f r o m M a c c r e g o r ' s ( 1 9 7 4 ) .4 is negative for many diopsides due to the acmite
experimentaldata have been adopted as a model lor c o m p o n e n tw h i c h i s n e e d e df o r s t o i c h i o m e t r y .
t h e t r u e A l s o l u b i l i t y f o r t h e a v e r a g en a t u r a l p y r o x However, the unusual plot of (Al + Cr)6i/A"n
e n e c o m p o s i t i o n s c h a r a c t e r i s t i co f m e t a m o r p h i c a g a i n s tN a o , / 1 " " h a s p r o v e n t o b e a v a l u a b l et o o l f o r
l h e r z o l i t e s( o p . c i t . , C r l ( A l + C r - N a ) f r o m l 5 t o 2 5 o b t a i n i n ga g e o b a r o m e t ebr a s e do n d i o p s i d ec o m p o p e r c e n ti n d i o p s i d ea n d f r o m 7 t o I I p e r c e n ti n e n s t a - s i t i o n s f o r t h e g a r n e t f a c i e s .I f A l ( E n ) a n d A l ( D i )
t i t e ) . T h e v a l i d i t y o f t h i s m o d e l w i l l b e t e s t e di n a i s o p l e t h sh a d s i m i l a rs l o p e sw i t h l o ' : k . A " n ( w h e r ek
l a t e r s e c t i o n . A l ( D i ) i s o p l e t h sa n d e q u a t i o n s w e r e i s a c o n s t a n t )a n d i f t h e p y r o x e n e sc o n t a i n e dn o F e ' + ,
a l s o o b t a i n e db y p l o t t i n g t h e A l c o n t e n to f d i o p s i d e s t h e d a t a p l o t t e d w o u l d f i t a s t r a i g h t l i n e o f s l o p e
e q u i l i b r a t e d i n t h e s p i n e l f a c i e s ,a s a f u n c t i o n o f e q u a l t o l , s i n c e ,f r o m t h e d e f i n i t i o no f l :
t e m p e r a t u r e( d e r i v e df r o m t h e d i o p s i d ec o m p o s i t i o n )
A.^= (Al * Cr - Na)ar,/ft
a n d p r e s s u r e( f r o m t h e e n s t a t i t ec o m p o s i t i o n ) .
F o r t h e g a r n e t f a c i e s ,t h e A l ( E n ) e x p e r i m e n t aal n d w h i c h c a n b e r e w r i t t e na s :
t h e o r e t i c a li s o p l e t h sa v a i l a b l ea r e a l l i n a c c o r d w i t h
( A l t C r ) a r / A " n: k * N a 6 1 / 1 " , ( s l o p e: l )
Py roxene geobaro met ers
A d i : ( a l + c r- o 8 2 N o )/ 2
reflfn=o.teruo
( GARNET FACIES)
o
E
o
+
If the isopleth slopes were different for the two pyr o x e n e s ,f o r a g i v e n N a c o n t e n t a r a n g e o f ( A l +
C r ) a i / A " ^ v a l u e sw o u l d b e o b t a i n e da n d a l a r g es c a t t e r o b s e r v e df o r t h i s p l o t ( e . g .s p i n e lf a c i e s ,F i g . 2 ) . I f
t h e j a d e i t e / a c m i t er a t i o v a r i e d w i t h N a c o n t e n t , n o
linear fit could be obtained; a fortiori, if acmite was
s u d d e n l y a p p e a r i n g f o r N a - r i c h c o m p o s i t i o n s ,a
c h a n g ei n s l o p e s h o u l d b e o b s e r v e d .
Actual data plotted in Fig. l fit, with very limited
s c a t t e r ,a s t r a i g h tl i n e o f e q u a t i o n
( A l + C r ) 6 1 / A . , , :2 . * 0 . 8 2N a a r / , 4 " "
or
1 " " = ( A l * C r * 0 . 8 2N a ) a r , / 2 .
Nodi / Aen
F t c . I A l + C r u s N a f o r d i o p s i d e se q u i l i b r a t e d i n t h e g a r n e t
i a c i e s .T h e n o r m a l i z a t i o n t o , 4 " , l i m i t s t h e s c a t t e r i n gd u e t o p r e s s u r e o r t e m p e r a t u r ee l l e c t s a n d a l l o w s o n e t o d e r i v e t h e e m p i r i c a l
, 4 6 1e x p r e s s i o nt a k i n g i n t o a c c o u n t F e 3 + D a t a p o i n t s b e l o w t h e
d a s h e d l i n e c o r r e s p o n d t o p y r o x e n e sf o r w h i c h F e 3 * i s n e e d e df o r
s t o i c h i o m e t r yD a t a a r e f r o m N r x o n ( 1 9 7 3 ) ,B o y d ( 1 9 7 4 ) ,A h r e n s
et ol (1975) and include a few new analyses
t l v a l u ef o r d i o p s i d e s( a t
B y r e d e f i n i n gt h e e q u i v a l e nA
l e a s ti n t h e g a r n e t f a c i e s )a s ( A l - | C r - 0 . 8 2N a ) / 2 ,
o n e c o n c l u d e st h a t b o t h s l o p e sa n d o r i g i n s f o r t h e
A l ( E n ) a n d A l ( D i ) i s o p l e t h si n t h e g a r n e t f a c t e sa r e
p r a c t i c a l l yi d e n t i c a ls i n c e ,w i t h i n t h e p r e c i s i o no f t h e
data,A"nf Ao1becomcsequal to unity. Therefore,for
t h i s f a c i e s ,w e m a y t L s et h e e n t r o p y , e n t h a l p y , a n d
c h a n g e - i n - v o l u mdea t a d e t e r m i n e df o r t h e e n s t a t i t e ,
i f , 4 i s c o m p u t e d t o t a k e i n t o a c c o u n tt h e m i n i m u m
t r i v a l e n t F e p r e s e n ti n d i o p s i d e( F e * L : 0 . l 8 N a ) .
W h e r e a s t h e j a d e i t e / a c m i t er a t i o r e m a i n s p r a c -
SINGLE-PYROX ENE GEOTH ERM OM ETRY A N D GEOBAROM ETR Y
607
ticalfy constant(ld/ac: 4.56) and independentof the
Na content for diopsidesin the garnet facies,there is
no indication that this is true for the enstatiteor that
a similar relationshipstill holds in the spinelfacies(or
in the feldsparone) for which, on a similar plot (Fig.
2) more scatter is observed, in agreement with the
distinct slopes found for the isopleths(Mercier and
Carter, 1975).The steepfit obtained here is primarily
due to a progressiveenrichment in Na towards shallower levelsof the mantle (behavior opposite to that
of the garnet peridotites) until feldspar may form,
then strongly depleting the diopside of its jadeite
component.
Computation of the cofficients K,' and K*'
Mercier and Carter (1975) have derived,on a theoretical basis,the following expressionsfor the partition coefficientsKo' and Kw' for both facies,
-for the spinel facies,
K-' :
(t -
2vd)/(0.6s0 + 0.700w,r)
: 5 . 7 r 4 w " / (-r 2 t y " )
Ko' :
-for
K_' :
K""' A/(l -
c
o
E
()
+
g[,i: :
A)
the garnet facies,
(r -
2 W , ) / ( 0 . 8 6 2+ 0 . 2 7 6 W , )
.
A
:14.493W",/(t _ 2t1/")
Ku':
K""' A'(1 -
SPINEL FACIES
FELDSPAR FACIES
A)
where the subscripts d and e refer respectively to
diopside and enstatite, and where K",, and K",, are
correction factors to account for maior solid-solution
effects. In both facies,
Nodi /Aen
lY:
Ca/(Ca *
Mg *
Fe" *
Mn)
and
A:(At*Cr-n.Na)/2
where the elementsymbolsrepresentatomic fractions
for 6-oxygen formulae.
Although Na is generally entirely ascribed to a
jadeite component (op. cit.), the constant jadeite/acmiteratio (4.56) observedfor diopsidesin the
garnet facies might also characterizethe coexisting
enstatiteor both pyroxenesin the spinel facies.However,the Al/Na ratio for thesepyroxenesis usuallyso
large that the estimates are not really affected by the
model chosen(n : I or 0.82). The effect of Fe3+on
the constantsf : Di(Opx)/En(Cpx) (Fig. 3) is even
t o o s m a l l ( e . g .0 . 1 3 9 i n s t e a do f 0 . 1 3 8 f o r t h e g a r n e t
F t c . 2 . A l * C r u s . N a f o r d i o p s i d e se q u i l i b r a t e d i n s p i n e l a n d
feldspar facies. Even after normalization to ,4.", an increase in
p r e s s u r eo f e q u i l i b r a t i o n l o w e r s ( A l * C r ) f o r a c o n s t a n t N a , i n dicating a stronger pressure of dependence of Al for diopside.
Accordingly, Al(Di) isopleths (Mercier and Carter, 1975) are
steeper.The large slopes observed prevent any inference about the
e x i s t e n c ea n d / o r a m o u n t o f F e 3 + c o n t a i n e d i n t h o s e p y r o x e n e s .
Data are from Ross et al. (1954), white (1966), Kutolin and
F r o l o v a ( 1 9 7 0 ) ,G r i f f i n ( 1 9 7 3 ) a n d i n c l u d e n e w a n a l y s e s .
facies) to yield any appreciable change in the
coefficientsoI the K,' expressions.
On the other hand, as originally pointed out by
W o o d a n d B a n n o ( 1 9 7 3 ) , t h e F e z +c o n t e n t o f t h e
pyroxene may have an important effect on the Al
concentrationsobservedat a given pressure.For the
garnet facies,the correction factor,
JEAN.CLAUDE C. MERCIER
608
so
o
E
tc
9
o
En (CPX) mole o/c
F r c . 3 . D i ( O p x ) a s a f u n c t i o n o f E n ( C p x ) . T h e r e l a t i o n a p p e a r sl i n e a r a n d i n d e p e n d e n to f p r e s s u r eo r
temperature for both facies The precision is the same for both fits 1o = 0 007 or t lOoC on the
temperature estimate). Data from garnet pyroxenites equilibrated in the spinel-lherzolite facies(ariegites)
a r e o m i t t e d , a n d w o u l d p l o t i n i n t e r m e d i a t ep o s i t i o n s . M e r c i e r a n d C a r t e r ' s ( 1 9 7 5 ) o r i g i n a t s c a l e w a s
e r r o n e o u sa n d i s n o w c o r r e c t e d .D a t a a s i n F i g u r e s I a n d 2 .
K," -- (xf,b6f,b'/e - A)(x#)3
may be computed from the chemistry of enstatitel
and coexisting garnet. Mercier and Carter (1975)
showedthat the garnet composition could be inferred
with sufficient precision from the enstatite analysis
only, thus allowing a similar correction to be made
for xenocrysts,as the pe/Mg partitioning between
pyroxenesand garnet dependsmainly on temperature
and bulk composition. The correction has the effect
of reducing both temperature and pressure, and
therefore true corrected estimatesare obtained only
through an iteration readjustingalternativelytemperature, pressure,and correction factor. The isopleths
presentedhere (Fig. 4A, B) show the definitive K*"
valuesderived from enstatitein both cases,plotted as
functions of -lnK.' and total Mg (atomic fraction)
for either pyroxene:
K"" :
6.004-
3.025Mg
+ 0.702ln K,,.'
K " " : 3 . 2 9 8- 1 . 7 8 1M g
+ 0 . 1 2 8l n K , , '
Cr/(Cr + Al + Fe'*)"o: Crl(Cr * Al - Na)ar
(enstatite)
: 2.2 Cr/(Cr * Al - Na)""
the following empirical correction factors are pro( d i o p s i d e ) posed:
' T h e p y r o x e n es t r u c t u r a l f o r m u l a i s o b t a i n e d a s f o l l o w s :
Na and Mn in M2; Ti and Cr in M l.
i n M l ( e i t h e r 0 . 1 8 N a o r 0 ) ; F e ' ? +: F e - F d + .
'li,
: (Al
XL:
Al - XXi
Fd+
C r + N a ) , 2 2- Fe2+ (T - 400)/3000; XY3,*:
Fez+
Xofi,*.
is then distributed to obtain X't' : X-,".
T h i s f o r m u l a i s c o n s i s t e n tw i t h e i t h e r e x p r e s s i o nu s e d f o r , 4 . T h e
partition coefficient for Fe is approximated for the range of temp e r a t u r e sd e a l t w i t h f r o m V i r g o a n d H a f n e r ' s ( 1 9 6 9 ) e x p e r i m e n t a l
data and Wood and Banno's(1973)model.
-Ca,
-Fe'*
-XYi
-XY&-Mg
Mercier and Carter (1975) gavea similar theoretical expression for the correction factor K." (spinel
facies) and the corrections were negligible since K""
was generallycloseto I for natural peridotites.However, only divalent-cation effects were investigated
a n d , a s s h o w n b y M a c G r e g o r a n d B a s u ( 1 9 7 6 ) ,t h e
effectof Cr solid solutionsmay be very important. Cr
- Na)] isoplethswere con[or actually Cr/(Cr + Al
-lnK,'/A
space for pyroxenes from a
structed in
seriesof localities.The Cr isoplethswere found to be
practically parallel to the -lnK,' axis and K"" was
tentatively consideredas a function of Cr only, for
the common range of I values. A similar result is
o b t a i n e db y M a c G r e g o r a n d B a s u ( 1 9 7 6 )a s t h e i r C r
isopleths for spinel, in P-T space, are parallel to
MacGregor's (1974) Al isoplethsfor pure aluminous
enstatite.Considering the relationship illustrated in
F i g u r e4 C ,
K"" :
0 . 9 + 2 . 8 4( C r / A ) '
(enstatite)
K," :
0.9 + I .29 (Cr,/ tr|'t
(diopside)
Tests and applications
Before applying the above technique to solve geological problems for which fresh 4-phaseultramafites
, n e h a s t o d e t e r m i n et h a t ( 1 ) t h e A l
a r e n o t a v a i l a b l eo
609
SINGLE-PYROXENEGEOTHERMOMETRY AND GEOBAROMETRY
(E'|mi|t)
K; |SoFLETHS
6
Cr PARTITION
(Spinelfociasl
o rdnd - opr
. cpr - opr
t
^r/
o
o
./.. i.
o
,/
tl
a
t'l
a
o
a
_I '
C
c)
a
a/
-t
G
Y
c
o
x
o
o
o
2
I
a
()
o.l
o
o9
t.8
ilg ofom / loTmulo unit
B
r.o
Mg otom / tormulo unll
o
C
o.l
q llqr.Al-No)opr
Frc 4. K"" correction factors (garnet facies) and Cr partition (spinel facies). Data sourcesas in Figures I and 2. (A) K." for enstatite as
a f u n c t i o n o f - l n K . ' a n d M g , r e s p e c t i v e l ym a i n l y d e p e n d e n t o n t e m p e r a t u r ea n d b u l k c o m p o s i t i o n . K c " p l o t t e d i s t h e d e f i n i t i v ev a l u e
a f t e r i t e r a t i o n . ( B ) K " " f o r d i o p s i d e .K r " p l o t t e d h a s b e e n a c t u a l l y c o m p u t e d f r o m t h e c o e x i s t i n ge n s l a t i t ec o m p o s i t i o n .( C ) C r p a r t i t i o n
coefficients between enstatite and diopside or spinel for natural spinel peridotites.
solubility model for pyroxenes equilibrated in the
spinel facies is valid, and (2) the precision to which
the pressureand temperature estimatesare made is
not lowered when using the single-pyroxenetechnlques.
Verffication of the method
A sequenceof xenolith texturesmay be definedfor
any Vesuvian volcano, and through the comparison
of over 150 xenolith associationsthey are grouped
into four categories,each one assumedto representa
type of section through the upper mantle where basaltic magmas are generated.Among these various
types, one is highly metasomatizedand intruded by
abundant pyroxenitic veins,and another is characterized by dominant blastogranular to blastolaminar
textures,2and is systematicallyassociatedwith major
'zThese textures have been
described as "porphyroclastic" by
Mercier and Nicolas (1975) However, this early name appears
n o w s e m a n t i c a l l yi n c o r r e c t ( J . D . B l a c i c , p e r s , c o m m . , 1 9 7 5 ) a n d
its replacement by "blastogranular" (or by "blastolaminar" for the
most highly sheared facies) is suggestedto permit standardization
o f t h e t e x t u r a l n o m e n c l a t u r e .A l t e r n a t i v e l y , o n e m i g h t c o n s i d e r
these textures as porphyroclastic subtypes (Mercier, 1972).
lithospheric shear-zones(e.g. Baja California, Pyr6n6es).The last two types of sequences,referred to as
Bor6e(France) and Kilbourne (Kilbourne Hole, New
Mexico) types, are texturally equilibrated metamorphic lherzoliteswith a log-normal grain-sizedistribuiion (homogranular textures) for all the silicate
phases.Samples from these last two sequencesare
thereforeregardedas illustrating steady-stateflow in
the upper mantle.
The Bor6e and Kilbourne types of sequenceshave
no identical textural facies in common. However, a
similar grain-sizeis observed at any depth (Fig. 5),
and this grain size increasesregularly with depth, in
agreement with the conclusions previously drawn
from the study of xenolith associations(Mercier,
1972). The depth range obtained in this study for
Kilbourne Hole samples(25-60 km; Fig. 5) is in good
agreementwith that previouslyinferred from the petrology (equilibrium conditions betweenthe xenoliths
and host lava; MacGregor, 1970).The very high pressure estimates obtained from the high-temperature
peridotitesfrom the Boree-typesequenceare not incompatible with a spinel-faciesequilibrium; indeed,
the exceptionallyhigh bulk Crl(Cr * Al) ratio (over
610
JEAN.CLAUDE C. MERCIER
Inclusionsfrom the Nauajo Reseruation " kimberlites"
XENOLITHS IN BASALT
(conlinenlolvesuvionvolconism)
PYROXENE GEOTHERMS
I
U
F
lrj
=
lrJ
F
i4
SEOUEI
IKILBOURNE-TYPE
j e Protogronulor2(O3)
i !Equig.onulorT(O3)
a Msogronulor (lO)
y Prologronulor
| (2 5)
BOREE.TYPE SEQUENCE
o EquigronulorM (O 2)
v C o o r s G r o n u l o (r 2 5 )
O Poikiloblostic (40)
P R E S S T I R EK b
F I c . 5 . P y r o x e n eg e o t h e r m sf o r B o r 6 e - t y p ea n d K i l b o u r n e - t y p e
upper mantle sections, documented by xenoliths from Vesuvian
v o l c a n o e s ( n e w a n a l y s e s ) .T h e s e x e n o l i t h s a r e t e x t u r a l l y e q u i l i brated and illustrate steady-state flow. The mean grain sizes (for
olivine in the caseof the poikiloblastic texture) are indicated in
millimeters following the name of the textures The data are for
s a m p l e sf r o m B o r e e ( F r a n c e ) a n d B l a c k R o c k S u m m i t ( N e v a d a )
for the Bor6e-type sequence,and from Kilbourne Hole (New Mexico) and Cerzat (France) for the Kilbourne-type sequence. The
arrows indicate the maximum uncertainty due to the model
adopted, for (l) Kauai, Hawaii; (2) Dish Hill, California, and
B e a u n i t , F r a n c e( M o h o . ) ; ( 3 ) F r a n k S m i t h , S o u t h A f r i c a ( M o h o . ) ;
(4) Frank Smith, South Africa (sp,/gt facies boundary). Spinel
garnet facies boundary calculatedas for Figure ll. Data as in
Figures I and 2.
l/3, inferred from the mineral compositions) shifts
the spinel/garnet faciesboundary to relatively higher
pressures(MacGregor, 1970).
Plots of either temperatureor pressuredetermined
from one pyroxene, against that obtained from the
second one (Fig. 6), for apparently equilibrated assemblages,illustratethe degreeof confidenceattained
with this technique(Z a 30'C; P t 3 kbar) for either
spinel or garnet facies.Although exceptionallylarge
discrepanciesare consideredas an indication of disequilibrium, in which case the error made by using
the classicalmethods is several times the standard
deviation given above, systematicdivergencecould
also indicate analytical problems.
The "kimberlites" from the Navajo Indian Reservation (Utah-Arizona; near Four Corners) have been
studied by many (e.g. O'Hara and Mercy, 1966;
McGetchin and Silver, 1970) for their xenoliths
and/or xenocrysts.However, they do not have most
of the characteristicsof true South African kimberlites, such as penetrativeserpentinizationofboth lava
and inclusions, abundant metamorphic garnet-lherzolite xenoliths, pyroxene-ilmenite intergrowth nodules, etc. They are more comparable to
European basaltic diatremes (Mercier, 1972), although the Navajo Reservation diatremes contain
abundant pyrope xenocrysts,rarer crystalsof clinoand orthopyroxenes, amphiboles, serpentine, and
(limited to the southern diatremes;Fig. 7) abundant
olivines.Shoemaker(1956)recognizeda northeastern
minette province in which occur the "kimberlites,"
and a monchiquite province restricted to the Hopi
Buttes area. Whereas the monchiquite contains no
deep-crustal or mantle xenoliths, the minette has
abundant deep-crustalinclusionsand rare peridotites
(e.g. at"Ship Rock; Baldridge et al., 1975), and the
"kimberlites" offer abundant xenolithsof both types.
As is commonly observed,the "kimberlites" are associated with a dome structure of the basement,evidenced(Fig. 7) by the curvature of the restoredPliocene structural surface, which is interpreted as
illustrating post-Pliocenemagmatic activity, at least
at depth.
The lherzolitexenoliths (hundreds of samples)apparently fall into two distinct petrographicsuites,the
highly hydrated peridotiteswhich never contain fresh
brown picotite and are commonly transformed into
an aggregateof chlorite, amphibole, magnetite, and
chromite, and the fresh spinel lherzolites similar to
those found in many basalticvolcanoes(Mercier and
Nicolas, 1975). Smith (1974, 1975) noticed some
slight differences in composition, implying differences in equilibration conditions, for these two
groups; however, new data (Fig. 8) show a complete
overlap for the equilibration conditions of both
series. Pyroxene xenocryst compositions yield a
rather well-defined oceanic-typegeotherm (Fig. 8),
ignoring a few diopsides(dots in figure) on the basis
of their magmatic origin inferred from TilCr plots
(Fig. 9). Equilibration conditions derived from the
lherzolitesgenerallyyield much lower pressures,even
crustal conditions (Fig. 8), and temperaturesoften
well above the normal geotherm.
One hydrated peridotite still contains a large (0.4
SINGLE-PYROXENE
GEOTHERMOMETRY AN D CEOBAROMETRY
6ll
m m ) g a r n e t c r y s t a ls u r r o u n d e db y a t h i c k a n d c o m p l e x k e l y p h i t e c o r o n a i n c l u d i n g r e m n a n t so f a K l t y p e k e l y p h i t e( L a s n i e r , 1 9 7 2 :M K i n F i g s . 8 a n d 9 )
l a t e r r e e q u i l i b r a t e d( K 2 , K 3 ) , t h e l a s t e v e n t b e i n g a 3
o
general chloritization. The bulk composition of
t
c
g
p y r o x e n e i n c l u s i o n si n t h i s g a r n e t a g r e e sw i t h a n
o r i g i n a l s h a l l o w g a r n e t - f a c i e so r a r i e g i t e - s u b f a c i e sP
e q u i l i b r i u m . N o r e a l i s t i cp r e s s u r ea n d t e m p e r a t u r e t!e
estimates were derived for pyroxenes in this peri- F
d o t i t e o r i n m o s t o f t h e o t h e r s a m p l e sb y a s s u m i n ga E,
lrj
g a r n e t - f a c i eesq u i l i b r i u ma n d b y a p p l y i n gt h e c o r r e c - oI
EI
t i o n f o r e i t h e r t r u e g a r n e t c o m p o s i t i o no r t h a t d e - F
r i v e d l r o m t h e p y r o x e n ec h e m i s t r y .N e i t h e r o l t h e s e
peridotites can be regarded as metamorphic garnet
l h e r z o l i t e sc o m p a r a b l et o t h o s ef r o m S o u t h A f r i c a o r
f r o m o t h e r A m e r i c a n k i m b e r l i t e s .T h i s i s a l s o s u p p o r t e d b y g a r n e t f a c t o r a n a l y s i s( F i g . l 0 ) . W h e r e a s
TEMPERATURE'C(diopida)
the crystals from metamorphic lherzolites,depleted
or not, have a factor-8 value above 95 (Mercier,
1976a),in the presentcase the maximum value obtained (86) is still typical of magmatic ultramafic
parageneses.
Analysesof selectedgarnetsmost likely
o
o
to have come from lherzoliteson a color basis, tog e t h e r w i t h d o z e n so f a n a l y s e sf r o m t h e l i t e r a t u r e , oO
have been processedin a vain attempt to find evi- co
d e n c ef o r d e e p u p p e r - m a n t l ei n c l u s i o n s T
. h e T i a n d ItC r c o n c e n t r a t i o n so b s e r v e da r e i n a c c o r d w i t h t h e |lI
r e s u l t sf r o m f a c t o r a n a l y s i sw
, i t h t h e e x c e p t i o no f o n e G(n
f
x e n o c r y s tp r e s e n t l yn o t u n d e r s t o o d .T h e g a r n e tp e r i - an
lrj
d o t i t e d e s c r i b e da b o v e , t h u s , m a y b e e i t h e r a m e t a - Eos o m a t i z e dg a r n e t - l h e r z o l i t el a r g e l yr e e q u i l i b r a t e di n
the spinel facies(e.g. Eglazines;Mercier, 1972) or a
s p i n e l l h e r z o l i t ei n w h i c h a r i e g i t i cm a t e r i a lh a s b e e n
m e c h a n i c a l l ys c a t t e r e d( e . 9 . R o n d a , B e n i B o u c h e r a ;
L e B o i s d e s F e u i l l e s ;L a s n i e r ,1 9 7 2 ) .
T h e c o n d i t i o n sf o r r e e q u i l i b r a t i o ni n t o t h e p r e s e n t
PRESSURE Kb (diopsidcs)
hydrous phasesmust have been reachedfast enough
F t G . 6 . T / T a n d P / P p l o t s f o r e s t i m a t e sd e r i v e d i n d e p e n d e n t l y
t o q u e n c hm o s t o f t h e o r i g i n a l a s s e m b l a g e a
s ,s s o m e f r o m e n s t a t i t ea n d d i o p s i d e i n e q u i l i b r a t e d a s s e m b l a g e sT h e e s t i h y d r a t e d x e n o l i t h sm a y s t i l l h a v e p y r o x e n e se q u i l i - m a t e s a r e n o t c o r r e c t e df o r s o l i d s o l u t i o n e f f e c t si n t h i s p a r t i c u l a r
brated in the garnet facies,but also slow enough to p l o 1 . N o s y s t e m a t i cd e v i a t i o n h a s b e e n o b s e r v e da t a n y p r e s s u r e
a l l o w p a r t i a l r e e q u i l i b r a t i o ni n i n t e r m e d i a t ec o n d i - i n d i c a t i n gt h a t f ( F i g . 3 ) i s d e p e n d e not n t h e n a t u r eo f t h e A l - p h a s e
r a t h e r t h a n o n t h e p r e s s u r ei t s e l f . T h e p r e c i s i o n f o r t e m p e r a t u r e
t i o n s ( K l - g p e k e l y p h i t e ) .T h e r e f o r e ,t h e p e r i d o t i t e
a n d p r e s s u r ee s t i m a t e si s r e s p e c t i v e l y+ 3 0 " C a n d 1 : 3 k b a r , a s e m p l a c e m e nitn t o s h a l l o wl e v e l si s n o t r e l a t e dt o a n y s u m i n g i d e a l t h e r m o d y n a m i cp a r a m e t e r sa n d s t a n d a r d q u a l i t y
v o l c a n i ce v e n t b u t t o a s l o w e rt e c t o n i cp h e n o m e n o n , a n a fy s e s .D a t a a s i n F i g u r e s I a n d 2 .
a n d t h e t e m p e r a t u r e - p r e s s u rt ree n d o b s e r v e dw o u l d
c o r r e s p o n dt o a n e q u i l i b r i u m s i t u a t i o n ( g e o t h e r m )
a n d n o t t o a r e q u i l i b r a t i o np a t h . T h e a c t u a l p r e s e n t r e g i o n sw o u l d e x p l a i n t h i s s e c o n dg e o t h e r ma n d t h e
e q u i l i b r a t i o nc o n d i t i o n sa r e m u c h s h a l l o w e rt h a n i n - hydration and the quasi-absenceof sheared facies
ferred from geobarometry; thick serpentine verns only with difficulty, and would not be supported by
formed continuously before and after deformation a n y t e c t o n i c e v i d e n c e . H o w e v e r , H e l m s t a e d a n d
events in high-pressure-facies
rocks. An in situ dia- D o i g ' s ( 1 9 7 5 ) m o d e l , w h i c h c o n s i d e r st h e C - t y p e
p i r i c i n t r u s i o n o f u p p e r - m a n t l em a t e r i a l i n c r u s t a l e c l o g i t e ss o a b u n d a n ti n t h e N a v a j o R e s e r v a t i o nd i a -
JEAN-CLAUDE C. MERCIER
pentinization and/or recrystallizationof most of the
sequence,the technique described above has been
used, yielding a remarkably high thermal gradient,
even for oceaniclithosphere.The pyroxenegeotherm
derived (Fig. I I ) is defined for a pressurerange of
almost 40 kbar, much larger than expectedfrom the
reconstructedstructural section (8 kbar). Since the
possible errors due to the Al-solubility model
adopted (Fig. 5) are far smaller than the discrepanciesobservedbetweenthe pyroxene geotherm
and the structural section,the latter must have been
shortenedduring obduction.
From the gabbros (top) to the metamorphic aureole (base), the ultramafic sequence comprises
cumulate wehrlites and dunites, sheared(porphyroblastic) metamorphic dunites, protogranular harzburgites, and sheared (blastolaminar) lherzolites.
The protogranular harzburgitesexist within a depth
range equivalentto a few kilobars, in agreementwith
the section, but their equilibration at greater depths
indicates an intensive shortenins of the shallower
I N C L U S I O N SI N N A V A J O
F I c . 7 . T h e v o l c a n i c s o f t h e N a v a j o I n d i a n R e s e r v a t i o n ,n e a r
Four Corners: monchiquite (dots), minette (open circles) and
"kimberlites" (sotid circles). The contours are for the restored
Pliocene surface (modified after Shoemaker, 1956).
The Newfoundland ophiolites of the Western Platform (Fig. 1l, inset) have been recentlyinterpretedas
obducted slicesof lithospherefrom a Cambrian basin
(Mercier, 1976b) on the western side of the Eocambrian-Cambrian Iapetus (or Proto-Atlantic Ocean;
Dewey and Bird, l97l). Becauseof the intensiveser-
''
KIMBERLITES
PYROXENE GEOTHERMS
tremes as metamorphosedunderthrust oceaniccrust,
provides a satisfactory model for the hydrated per()
idotites as quenched"ophiolitic" slicescharacterized
by a typical oceanic paleogeotherm(also in agree- UJ
(r
ment with the textures).In any case,the actual geo- Fl
therm beneath the Navajo Reservationwould be of (r
lrJ
(L
oceanic type, and the diatremespresentno evidence
t!
whatsoever for a genesis at depths comparable to F
those for kimberlites.Thesegarnet-richdiatremesare
therefore regarded as minette-type magmas which
originated in the shallowerzones of the garnet facies
and were contaminatedas they passedthrough highly
hydrated ultramafites and eclogites. This interpretation also agreeswith the close field association
FIc.
of minettes and "kimberlites."
Western N ewfoundland ophiolites
''
ro
?o
30
40
50
PRESSURE Kb
8. Pyroxene geotherm derived from the pyroxene xenocrysts
i n t h e N a v a j o " k i m b e r l i t e s . " A s e c o n dt r e n d a t h i g h e r t e m p e r a t u r e
a n d l o w e r p r e s s u r ec o r r e s p o n d sm a i n l y t o t h e l h e r z o l i t ex e n o l i t h s ,
hydrated or not, plus some xenocrysts from Garnet Ridge. Abbrev i a t i o n s :S R = S h i p R o c k ( B a l d r i d g ee t a l . , 1 9 7 5 ) ; M l , M K , M L =
M o s e s R o c k g a r n e t l h e r z o l i t e M S S - l ; r e s p e c t i v e l y ,i n c l u s i o n s i n
g a r n e t , K l - p y r o x e n e d e p l e t e di n A l b y s u r r o u n d i n gK 2 - a m p h i b o l e ,
and pyroxenes in lherzolite. Some of the data are from 0'Hara and
Mercy (1966), Watson and Morton (1969) and McGetchin and
Silver (1970). The dotted lines correspond to the oceanic and
c o n t i n e n t a lg e o t h e r m so f M e r c i e r a n d C a r t e r ( 1 9 7 5 ) .S p i n e l / g a r n e t
facies boundary as in Figure I l. Dots and open circlesas in Figure 9
613
SI NGLE-PYROX ENE GEOTHERM OM ETRY A N D GEOBAROM ETRY
dunites in accordance with their texture. Thus,
rather than offering a continuous sectionthrough an
oceaniclithosphere,theseophiolites are formed of at
least two superposed sequences,as has also been
observed in Quebec (Laurent, 1975). The basal
shearedlherzolites,although presentlya few hundred
meters thick, correspond to a seriesoriginally more
than 30 km thick. In this case,the shorteningaffected
the entire formation, in agreementwith the field observationsof abundant fault zones subparallelto the
original shear surface. The maximum depths observed are consistent with the ariegite subfaciesin
which the lherzoliteswere equilibrated. Reequilibration paths are also obtained for the basal lherzolites
by comparing the estimates derived from the bulk
compositions of relict crystals and neoblasts. The
paths indicate a subhorizontal shear consistentwith
the early stage of obduction for the deepestlithospheric material. The drop in temperature (over
350'C) is explained by the original extraordinarily
high thermal gradient, and by the change from an
oceanic thermal regime to a continental one during
the obduction process.The effects of stresson the
enstatite-diopsidejoin are likely to be negligible,as
in some sheared peridotites (e.9. Lesotho, Mercier
and Carter, 1975;Beaunit, Mercier, 1972)the deformation (variable strains)occurswithout any simultaneous exsolution phenomenon in the pyroxenes,at
least to the resolution of optical microscopes.
Conclusions
D I O P S I D E S( NAVAJO,.KIMBERLITES,.}
Lherzolite
Eclogite
o
A
O\
o
? | oioo.io"
E
Xenolilhs
Xenolilhs
xsocrysrs
o 0 2 \rj\
\CRYS,ltLLIZATION
F
ool
.\*
'\"+.;
DEPLETION
Cr otom/ formulounif
Frc. 9. Ti,/Cr diagram for diopsides from the Navajo Country
diatremes. On the basis of this diagram, only part of the xenocrysts
(open circles) are likely to have been equilibrated in metamorphic
four-phase parageneses(Mercier, 1976a).
equilibrationsimpossibleto obtain by using any classical method. Further possibilitiesare the construcpaths for partially retion of pressure-temperature
crystallized and reequilibratedtextures, and also to
G
o
F
()
The method presented here for computation of f,
pressure and temperature estimates from pyroxene
compositions is more general than the classicalone
by which compositions are directly compared with
the availableexperimentalresults.Furthermore, with
the new evidenceof pressuresensitivityof the enstatite-diopside solvi (Warner and Luth, 1974;Lindsley
and Dixon, 1976), the determinationsshould be done
through iterations,and more calculationsare needed
for correcting the estimatesfor solid-solutioneffects.
FACTOR8
The method proposed allows the direct computation
FIc. 10. Garnet factor analysisfor the Navajo Country diaof fully correctedestimatesthrough the use of simple
equations,even when the composition of all the co- tremes. Many different paragenesesare representedby the variously
colored garnets, but none can be related to metamorphic upperexisting phasesare unknown. The equations are demantle material. Data sources as in Figure 8 Some other data are
rived in such a way that any further refinement of f r o m R e i d a n d H a n o r ( 1 9 7 0 ) .I n s e t :T i l C r d i a g r a m f o r s o m e o f t h e
either experimentaldata or empirical laws for natural garnet xenocrysts (Ti and Cr are given in atoms per formula unit).
assemblagescan be taken into account by simply The same conclusion is reached, with the exception of one crystal.
The open circles are for the Ship Rock and for the Moses Rock
changing the appropriate coefficient.
garnet-peridotites. Data sources as in Figure 8. The trends are for
Applications of this method are many and include
magmatic and metamorphic (depleted) mantle garnets (Mercier,
the use of partially reequilibrated assemblages,al- 1976a).The field for the undepleted garnets is shown by the heavy
tered facies,or xenocrystsfor deriving conditions of D a t t e r n .
614
JEAN-CLAUDEC MERCIER
DEPTH (Km)
50
roo
WESTERN NEWFOUNDLANDOPHTOLTTES
v
()
o
U
F
(r
U
=
l!
F
PRESSURE
Kb
F I c . I l . P y r o x e n eg e o t h e r m f o r t h e o b d u c t e d o p h i o l i t e s o f t h e
W e s t e r n P l a t f o r m , N e w f o u n d l a n d ( n e w a n a l y s e s ) .T h e g a r n e t l a c i e su p p e r b o u n d a r y i s c a l c u l a t e df r o m O ' H a r a e t a l ' s ( 1 9 7 1 \
experimentad
l a t a , c o r r e c t e df o r f r i c t i o n , a d j u s t e dt o R i c h a r d s o ne t
o / ' s ( 1 9 6 8 ) p r e s s u r es c a l ea n d m o d i f i e d f o r t h e a c t u a l C r l A l r a t i o
after MacCregor's (1970) data. The depth of the Moho for the
Baie Verte basin is inferred from the geology Asterisksare for
r e e q u i l i b r a t e dn e o b l a s t si n b l a s t o g r a n u l a r - t e x t u r e sda m p l e s .I n s e t :
Location of the ophiolites (black) obducted onto the Canadian
S h i e l d ( h e a v y p a t t e r n ) ; a l s o s h o w n , t h e r e m n a n t I a p e t u sc r u s t ( n o
patternl includes island-arc material) which has not been subd u c t e d( M e r c i e r , 1 9 7 6 b ) .
c h e c ke i t h e r t h e e q u i l i b r i u ms t a t eo f a p a r a g e n e s losr
t h e q u a l i t y o f t h e a n a l y s e sT
. he error on the estim a t e s ,a s s u m i n gi d e a l e x p e r i m e n t adl a t a , i s l e s st h a n
30"C and 3 kbar, and the maximum error may even
d e c r e a s eb y o n e - t h i r d f o r s u p e r i o r a n a l y s e s .I n a n y
case, using both pyroxene phases independently
yields maximum and minimum valuesfor the estim a t e si n s t e a do f a s i n g l es e to f a s s u m e de q u i l i b r a t i o n
conditions.
The geologicalexamplesjust discussedshow a few
characteristicsusef,ulfor a better understanding of
the earth's upper mantle and its related tectonics.
T h e g r a i n - s i z eo f p e r i d o t i t ei n c l u s i o n si n b a s a l t sa l w a y s i n c r e a s ew
s i t h d e p t h a s a p a r t i a l c o n s e q u e n coef
the elevation in temperature. However, the average
grain-sizeat depths over 200 km is about the sameas
t h a t o f s a m p l e se q u i l i b r a t e da t h a l f t h i s d e p t h .T h e s e
o b s e r v a t i o n sm a y g i v e s o m e c l u e sa b o u t t h e m o d e o f
f l o w o f t h e u p p e r m a n t l e , a s t h e g r a i n s i z ei s r e l a t e d
m a i n l y t o t h e a p p l i e d d e v i a t o r i cs t r e s s ,o r a l t e r n a tively to the temperatureand strain rate (Carter and
M e r c i e r , 1 9 7 6 ) .T h e s h e a r e dp e r i d o t i t e sm i g h t t h u s
i l l u s t r a t et r a n s i e n tc o n d i t i o n s r e s u l t i n gf r o m a c o n s i d e r a b l ei n c r e a s ei n s t r a i n r a t e o r s t r e s s ,a c c o m p a n i e d ( N e w f o u n d l a n d ,A ? " : 3 5 0 o C ,A o : 3 k b a r ;
M e r c i e r , 1 9 7 6 b )o r n o t ( L e s o t h o ;M e r c i e ra n d C a r t e r ,
1 9 7 5 ) b y a d e c r e a s ei n t e m p e r a t u r e ,a n d p o s s i b l y
c o m b i n e d w i t h m i n o r e f f e c t so f o t h e r p h y s i c a l o r
c h e m i c a lp a r a m e t e r sA. c c o r d i n g l y ,t h e s et e x t u r e sa r e
d o m i n a n t o n l y f o r i n c l u s i o n si n b a s a l t sf r o m a c t i v e
f a u l t z o n e s .O p h i o l i t e s ,g e n e r a l l yr e g a r d e da s s i n g l e
s l i c e s o f l i t h o s p h e r eo n t h e b a s i s o f c o h e r e n t a n d
c o n t i n u o u s s t r u c t u r a l s e c t i o n s ,m i g h t r a t h e r b e a
s e r i e so f s m a l l e rs l i c e sr e s u l t i n gf r o m i n h o m o g e n e o u s
strain, the deepestone having moved fastest,thereby
p r o d u c i n ga s h o r t e n i n go f t h e o r i g i n a l s e q u e n c eT. h e
d a t a o b t a i n e d f o r t h e i n c l u s i o n sf r o m t h e N a v a j o
I n d i a n R e s e r v a t i o np r o v i d e n e w e v i d e n c e( o t h e r e v i d e n c e b e i n g t h e a b u n d a n c eo f C - t y p e e c l o g i t e s )i n
f a v o r o f c o m p l e x u p p e r - m a n t l et e c t o n i c s ,w i t h p o s sible underthrusting of oceanic material along
e n o r m o u s d i s t a n c e s .I n a n y i n s t a n c e ,d e s p i t e t h e
a b u n d a n c e o f p y r o p e x e n o c r y s t s ,t h e s e v o l c a n i c s ,
c o m m o n l y r e f e r r e dt o a s " k i m b e r l i t e s , " a r e n o t r e a l
k i m b e r l i t eo f d e e p o r i g i n .
Acknowledgments
T h e w r i t e r i s i n d e b t e d t o D r S . N E h r e n b e r gf o r m a k i n g a v a i l a b l e u n p u b l i s h e da n a l y s e st,o D r . N . L . C a r t e r a n d D . A A n d e r s o n f o r c r i t i c a l c o m m e n t s a n d b e n e f i c i a l s u g g e s t i o n s a, n d t o C
M e r c i e rl b r t h e i l l u s t r a t i o n s .
T h i s w o r k w a s s u p p o r t e db y N a t i o n a l S c i e n c eF o u n d a t i o n g r a n t
A O - 3 I 5 6 9 - 0 0|
References
AsneNs, L. H., J B Drwsolr, A R. Durcnu nNo A. J. ERLANK,
E d i t o r s ( 1 9 7 5 ) P a p e r sp r e s e n t e da t t h e F i r s t I n t e r n a t i o n a l C o n f e r e n c eo n K i m b e r l i t e s . P h y s C h e m . E a r t h 9 , l - 9 4 0 .
Bnlonlocr, W. S., S. N. EHneNsenc eNo T. R. McGrrcum
( 1 9 7 5 ) U l t r a m a l i c x e n o l i t hs u i t e f r o m S h i p R o c k . N e w M e x r c o
(abstr.) EOS, 56, 464.
Bostrcurn, A. L. (1974) Review of symposium on deep-seated
r o c k a n d g e o t h e r m o m e t r yE. O S , 5 6 , 1 0 6 8 - 1 0 7 2 .
Bovo, F. R. (1974) Ultramaflc nodules from the Frank Smith
kimberlite pipe, South Africa Carnegie Inst Wash. Year Book,
73,285-294.
C l n r e n , N . L . n r n J - C . C . M r n c r e n ( 1 9 7 6 )S t r e s sd e p e n d e n coef
o f i v i n e n e o b l a s t g r a i n s i z e s .( a b s t r ) E O S , 5 7 , 3 2 2
D r w e v , J . F . n N o J M . B r n o ( 1 9 7 1 )O r i g i n a n d e m p l a c e m e not f
t h e o p h i o l i t e s u i t e :A p p a l a c h i a n o p h i o l i t e s i n N e w f o u n d l a n d J .
Geophys. Res 76, 3179-3206.
GntprtN, W. L. (1973) Lherzolite nodules from the Fen alkaline
complex, Norway. Contrib Mineral. Petol.38, 135-146.
SINGLE-PYROXENEGEOTHERMOM ETRY AN D GEOBAROMETRY
HeLustrrot, H. ,txn R. Dorc (1975)Eclogitenodulesfrom kimberlite pipes of the Colorado Plateau-samplesof subducted
Franciscan-typeoceanic lithosphere.Phys. Chem. Earth 9,
95-lll.
KuroLINV. A. eNoV. M. Fnor-ovr(1970)Petrology
of ultrabasic
inclusionsfrom basaltsof Minusa and Transbaikalianregions,
Siberia,USSR, Contrib.Mineral. Petrol.29, 163-179.
LesNten,B. (1972)Les p6ridotiteset pyroxOnolites
d grenat du
BoisdesFeuilles,Monts du Lyonnais,France.Contrib Mineral.
Petrol.34,29-42.
Leunerr, R. (1975)Occurrences
and origin of the ophiolitesof
southernQuebec,northernAppalachians.
Can.J. EarthScl. 12,
443-455.
Ltnosr-rv,D. H. ruo S. S. DrxoN(1976)Diopside-ensratite
equilibria at 850'-1400"C,5-35 kbar.Am J. Sci.in press.
MrcGnecon, L D. (1970)The effectof CaO, CrrOr, FerO, and
AlrOr on the stability of spinel and garnet peridotites.Pftys.
Earth Planet. Inter. 3. 372-377.
(19'74)The systemMgO-AlzO,-SiO,:Solubilityof Al,O, in
enstatitefor spinel and garnet peridotitecompositions.,42.
Mineral 59, I l0- I 19.
ANDA. R. Brsu (1976)Geologicalproblemsin estimating
mantlegeothermalgradients.Am. Mineral. 61,'l15-724.
McGercuttt,T. R. lNo L. T. Srlvpn(1970)Compositional
relationsin mineralsfrom kimberliteand relatedrocksin the Moses
Rock dike, San Juan County, Utah. Am. Mineral. 55,
r 7 3 8 - t ' t t7.
Mlnctnn, J-C. C. (1972) Structuresdes pdridotitesen enclaves
dansquelquesbasaltes
d'Europeet d'Hawaii.Regardssur la
constitutiondu manteausup6rieur.TheseDoct. 3" Cycle,Nantes,France.l-229.
(1967a\ Petrochemical
classification
of peridotites.Schweiz.
Mineral. Petrogr. Mitt., in press.
(1976b)Newfoundlandophiolitesand platetecronics.Tectonophysics,in press.
ANDN. L. Clnrrn (1975)Pyroxenegeotherms.
J. Geophys.
Res.80. 3349-3362.
ANDA. Nrcoles (1975)Texturesand fabricsof uppermantle peridotitesas illustratedby xenolithsfrom basalts../.
Petrol 16.454-487.
615
NtxoN, P. H., Editor (1973),LesothoKimberlites.LesothoNational DeuelopmentCorporation,Maseru,Lesotho,SouthAfrica.,
350p.
O'Heu, M. J. lxo E. L. P. Mrncv (1966)Eclogite,peridotite,
and pyropefrom the Navajo Country, Arizona and New Mexico.Am. Mineral.5l, 336-352.
S. W. Rrcs,rnosoNAND G. WrLsoN (1971) Garnet-peridotitestability and occurrencein crust and mantle.
Conrrib. M ineral Petrol. 32, 48-68.
lz.
Rrro, A. M. ANDJ. S. HnNon (1970)Pyropein kimberlite.
Mineral. 55, 1374-1379.
RrcHARDsoN,
S. w., P. M. Brn AND M. C. GILsenr (1968)
Kyanite-sillimaniteequilibrium between700"C and 1500'C.
Am J.Sci.266,513-541.
Ross,C. S., M. D. Fosren ANDA. T. Mvens (1954)Origin of
dunites and of olivine-richinclusionsin basalticrocks. Am.
Mineral. 39, 693-'137.
on
of uraniumin diatremes
Suoruerrn, E. M. (1956)Occurrence
Arizona, New Mexico and
the Navajo and Hopi reservations,
Utah. U. S. Geol.Suru Prof. Pap.30iJ,179-185.
Snrrn, D. (1974) Lherzolite inclusions from Green Knobs,
McKinfey County, New Mexico (abstr.).Geol.Soc.Am. Abstr.
6, 1066.
(1975)Relativeabundance,
and P-Thistories
compositions
of ultramaficxenoliths,Green Knobs, New Mexico,and implications for the mantle below the Colorado Plateau.(abstr.)
Geol.Soc. Am. Abstr. 1. 1275.
Vrnco, D. ANDS. S. HerNrn (1969)Fe'3+-Mg
order-disorder
in
heatedorthopyroxenes.
Mineral.Soc.Am. Spec.Pap.2, 67-81.
WrnNrn, R. D. rxp W. C. Lurs (1974)Thediopside-orthoenstatite two-phaseregionin the systemCaMgSirO.- MgrSi"O*Am.
Mineral.59,98-109.
in
WArsoN,K. D. eNo D. M. Morror (1969)Eclogiteinclusions
kimberfitepipesat Garnet Ridge, NortheasternArizona.Am.
Mineral. 54,267-285.
in basalticrocksfrom
Wulre, R. W. (1966)Ultramaficinclusions
Hawaii. Contr ib. M ineral. Pet rol. 12, 245-3| 4.
Wooo, B. J. lno S. BlNtto (1973)Garnet-orthopyroxene
and
relationshipsin simpleand comclinopyroxene-orthopyroxene
plex systems.Contrib. Mineral. Pelrol. 42, 109-124.