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Petrography, Petrology and Geochemistry of Xenoliths and Megacrysts from the Geronimo Volcanic Field, Southeastern Arizona
PETROGRAPHY, PETROLOGY AND GEOCHEMISTRY OF XENOLITHS AND MEGACRYSTS FROM THE GERÓNIMO VOLCANIC F I E L D , SOUTHEASTERN ARIZONA by P . D . KEMPTON, M.A. M E N Z I E S , and M.A.
DUNGAN
A b s t r a c t - The Gerónimo V o l c a n i c F i e l d i s n o t a b l e f o r i t s abundance o f p r i s t i n e T y p e I , T y p e I I and c o m p o s i t e x e n o l i t h s . T h e dominance o f u n f o l i a t e d , g r a n u l o b l a s t l c T y p e I h a r z b u r g i t e s and c l i n o p y r o x e n e - d e p l e t e d s p i n e l I h e r z o l i t e s s u p p o r t s the o c c u r r e n c e of a major d e p l e t i o n e v e n t in the mantle f o l l o w e d by a pe riod of reequi1ibration. LREE d e p l e t e d T y p e l a x e n o l i t h s a r e c o n s o n a n t w i t h s u c h a p a r t i a l m e l t i n g e v e n t (^1 b . y . ) . T y p e l b n o d u l e s a r e p e t r o g r a p h i c a l l y t h e same as T y p e l a i n c l u s i o n s , b u t h a v e LREE e n r i c h e d c l i n o p y r o x e n e s . T h e s e may r e c o r d a m a n t l e e n r i c h m e n t e v e n t ( < 0 . 5 b . y . ) c a u s e d by i n f i l t r a t i o n o f CO^+H^O fluids. " ^ C o m p o s i t e x e n o l i t h s a r e o f t h r e e t y p e s based on h o s t / v e i n m a j o r e l e m e n t com positions: a) Type I c r o s s c u t t i n g Type I, b) Type I I c r o s s c u t t i n g Type I, c) Type I I c r o s s c u t t i n g Type I I . T h e o r i g i n o f t h e s e c o m p o s i t e samples i s a t t r i b u t e d t o c r y s t a l l i z a t i o n o f b a s a n i t i c magma w i t h i n a p o p h y s e s o r a l o n g c o n d u i t w a l l s as t h e y a s c e n d t h r o u g h t h e m a n t l e . I n f i l t r a t i o n o f magmatic f l u i d s a l o n g g r a i n b o u n d a r i e s r e s u l t s i n p r o f o u n d m o d i f i c a t i o n o f c o n d u i t w a l l s on a t l e a s t a cm s c a l e a d j a c e n t t o t h e b o u n d a r y . T h e s e i n t e r a c t i o n s may i n v o l v e r e e q u i 1 i b r a t i o n o f i n c o m p a t i b l e t r a c e e l e m e n t s , REE and i s o t o p e s o r more c o m p l e t e m a j o r element readjustment. The spectrum of chemical e f f e c t s observed in composite x e n o l i t h s supports the e x i s t e n c e of a heterogeneous mantle beneath s o u t h e a s t e r n Arizona.
INTRODUCTION The Gerónimo V o l c a n i c F i e l d southwestern U . S . A . ,
number and v a r i e t y o f p r i s t i n e includes spinel
(GVF), located in southeastern A r i z o n a of
i s n o t e w o r t h y among x e n o l i t h l o c a l i t i e s due t o t h e ultramafic
nodules p r e s e n t .
amphibole p e r i d o t i t e
to 6 m.y.
basalt
r i n g s w h i c h r a n g e i n age
(Kempton, unpublished d a t a ) .
P r e v i o u s w o r k b y A r c u l u s and D e l a n o (1981) on t h e i n t r i n s i c in the t e r r e s t r i a l
between
The x e n o l i t h s are included in a l k a l i
t o b a s a n i t e c i n d e r c o n e s , f l o w s and p y r o c l a s t i c t u f f
suite
clinopyroxenite,
and t w o - p y r o x e n e g r a n u l i t e ; c o m p o s i t e r e l a t i o n s h i p s
t h e v a r i o u s r o c k t y p e s a r e common. f r o m 0.25
The x e n o l i t h
Iherzolite, harzburgite, websterite, wehrlite,
the great
oxygen
fugacity
m a n t l e f o u n d t h a t GVF a n h y d r o u s I h e r z o l i t e s a r e r e d u c e d
rel
a t i v e t o b a s a l t i c magmas and p r o b a b l y r e p r e s e n t f r a g m e n t s o f a n c i e n t m a n t l e 3 b . y . ) o n l y r e c e n t l y d i s t u r b e d and p e r h a p s m o d i f i e d
by b a s a n i t i c
O t h e r w o r k on t h e G e r ó n i m o V o l c a n i c F i e l d i n c l u d e s t h a t o f
(1-
volcanism.
L y n c h (1972)
and
E v a n s and Nash ( 1 9 7 9 ) . This
r e p o r t p r e s e n t s p e t r o g r a p h i c , m a j o r e l e m e n t , t r a c e e l e m e n t , REE and i s o -
t o p i c a n a l y s e s o f r e p r e s e n t a t i v e i n c l u s i o n s and l a v a s and d i s c u s s e s some o f petrogenetic implications
of the data.
T h e p e t r o g e n e t i c model p r o p o s e d
p r e t s t h e A l - a u g i t e c l i n o p y r o x e n i t e s as t h e f r a c t i o n a l
crystallization
the
inter product
72 o f F e - T i r i c h b a s a n i t e t h a t i n t r u d e d a p a r t o f t h e u p p e r m a n t l e i n i t i a l l y com posed o f v a r i a b l y d e p l e t e d s p i n e l
Iherzolite, similar
W i l s h i r e and S h e r v a i s
(1975) and I r v i n g
the southwestern U.S.
The i n t e r a c t i o n
ing from r e e q u i l i b r a t i o n e l e m e n t and m i n e r a l o g i c
p r o p o s e d by from
b e t w e e n t h e a s c e n d i n g magma and t h e
rock is recorded in composite x e n o l i t h s . h o s t / v e i n boundaries indicate
t o models
(1980) f o r o t h e r x e n o l i t h s u i t e s
that the extent of i n t e r a c t i o n
of incompatible
wall
Compositional v a r i a t i o n s r e l a t i v e is v a r i a b l e
to
rang
t r a c e e l e m e n t s t o more e x t e n s i v e m a j o r
readjustment.
PETROGRAPHY OF INCLUSIONS Xenoliths:
Type
I
Type I i n c l u s i o n s , a r e Mg-
following
the c l a s s i f i c a t i o n
from Gerónimo i n c l u d e s p i n e l
Type I i n c l u s i o n s e x h i b i t unfoliated
(1978), xenoliths
I h e r z o l i t e , h a r z b u r g i t e and w e b s t e r i t e ; b u t
n a n t among t h e s e a r e c l i n o p y r o x e n e - d e p l e t e d s p i n e l
textures
o f F r e y and P r i n z
and C r - r i c h r o c k s composed p r e d o m i n a t e l y o f o l i v i n e . T y p e I
limited textural
domi
I h e r z o l i t e s and h a r z b u r g i t e s .
variety.
In g e n e r a l ,
o r p o o r l y f o l i a t e d and h a v e g r a n u l o b l a s t i c - e q u a n t
they are
to coarse-equant
(nomenclature a f t e r H a r t e , 1977). T h e w e l l - e q u i l i b r a t e d
nature of
the
t e x t u r e s i s a p p a r e n t i n t h e common c o n f i g u r a t i o n o f g r a i n b o u n d a r y j u n c t i o n s 120^
angles.
Most s a m p l e s l a c k e x s o l u t i o n i n t h e m i n e r a l
phases,
but
in
clino
p y r o x e n e s f r o m some T y p e I w e b s t e r i t e x e n o l i t h s and v e i n s do show b r o a d e x s o l u tion lamellae.
These w i l l
relationships.
Type I x e n o l i t h s t y p i c a l l y
zoning in the mineral
be d e s c r i b e d more f u l l y
in the context of
composite
show o n l y m i n o r s e c o n d a r y m e l t i n g o r
phases.
R e c e n t w o r k has shown t h a t t h e p r e s e n c e o f m i c r o s c o p i c i n c l u s i o n s c a n h a v e a significant
e f f e c t on t h e t r a c e and r a r e e a r t h e l e m e n t a b u n d a n c e s i n o l i v i n e and
orthopyroxene in peridotites S t o s c h , 1982). ary melting,
( J a g o u t z , e t a l . , 1979;
Brief petrographic descriptions
alteration
Xenoliths:
Type
1980;
including the e x t e n t of second
and t h e p r e s e n c e o f f l u i d i n c l u s i o n s
I I and c o m p o s i t e x e n o l i t h s a r e g i v e n f o r i n d i v i d u a l
Type I I
S t o s c h and S e e k ,
i n 6VF T y p e I ,
Type
s a m p l e s i n Kempton ( i n p r e p ) .
II
i n c l u s i o n s a r e l e s s common t h a n T y p e I a t G V F , b u t may l o c a l l y
nate the x e n o l i t h population.
T y p e I I x e n o l i t h s a r e more F e , A l and T i
t h a n T y p e I x e n o l i t h s and i n c l u d e c l i n o p y r o x e n i t e , w e h r l i t e , k a e r s u t i t e
domi rich
perido
t i t e and r a r e w e b s t e r i t e . T e x t u r a l l y , x e n o l i t h s o f t h i s g r o u p h a v e an i g n e o u s a p p e a r a n c e i n w h i c h t h e modal p r o p o r t i o n s and t e x t u r a l phases a r e m a r k e d l y v a r i a b l e .
relationships
among t h e
C l i n o p y r o x e n e r a n g e s f r o m i r r e g u l a r l y shaped
g r a i n s o c c u r r i n g between equant o l i v i n e s t o l a r g e p o i k i l i t i c c r y s t a l s w h i c h t o t a l l y e n c l o s e o l i v i n e and s p i n e l . P e t r o g r a p h i c e v i d e n c e f o r m e t a m o r p h i c tallization
e x i s t s i n some s a m p l e s .
E x s o l u t i o n of aluminous spinel from
pyroxene is t y p i c a l ; e x s o l u t i o n of orthopyroxene occurs in Type I I
recrysclino
clinopyrox-
73 e n e s , b u t i s f a r l e s s common t h a n
spinel.
Amphibole r e p l a c e m e n t o f c l i n o p y r o x e n e i s most f r e a u e n t l y o b s e r v e d i n T y p e I I x e n o l i t h s . The replacement amphibole ranges from cm-sized c r y s t a l s
poikilit-
ically
to
e n c l o s i n g one o r more c l i n o p y r o x e n e s t o i r r e g u l a r
of microns in s i z e .
patches 10's
100's
P l a g i o c l a s e i s r a r e l y p r e s e n t and g a r n e t has n o t been o b
s e r v e d i n x e n o l i t h s o f t h i s t y p e . T h e F e - and A l - r i c h s p i n e l s a r e g r e e n t o i n t h i n s e c t i o n and h a v e s u b h e d r a l ent,
clinopyroxene is t y p i c a l l y
to euhedral shapes.
devoid of e x s o l u t i o n .
phases i n v a r i a b l y c o e x i s t i n amphibole p e r i d o t i t e s ,
When a m p h i b o l e In addition,
is
black pres
two o x i d e
a n e a r l y pure Mg-Al
spinel
and m a g n e t i t e . Xenoliths:
Composite
Structural categories: b) II
relationships a)
Type I I
Type I l i t h o l o g i e s
lithologies
lithologies
b e t w e e n T y p e I and T y p e I I x e n o l i t h s f a l l crosscutting
crosscutting
crosscutting
a Type I host
a host of Type I I a f f i n i t i e s
burgite hosts.
(Cr-diopside) veins crosscutting
Contacts between o l i v i n e - r i c h
assemblages a r e r e l a t i v e l y 2 cm.
three
(Type I/Type I ) ,
a T y p e I h o s t ( T y p e I I / T y p e I ) and c )
Type I / T y p e I composites have e i t h e r w e b s t e r i t e or c l i n o p y r o x e n i t e
into
s h a r p and p l a n a r ;
(Type I I / T y p e
Type
II).
(Cr-diopside + orthopyroxene) spinel
I h e r z o l i t e or harz-
h o s t r o c k s and p y r o x e n e - r i c h v e i n the veins are g e n e r a l l y t h i n ,
1 -
A d j a c e n t t o t h e v e i n , c l i n o p y r o x e n e may be d e p l e t e d and t h e h o s t p h a s e s
enlarged.
T h e p y r o x e n e s i n t h e C r - d i o p s i d e v e i n s and some w e b s t e r i t e s a r e
sig
nificantly
l a r g e r than the host phases.
ex
These large pyroxenes f r e q u e n t l y
h i b i t b r o a d e x s o l u t i o n l a m e l l a e w h i l e t h e h o s t p y r o x e n e s do n o t . T h i s t h a t the v e i n assemblage i s igneous in o r i g i n ,
indicates
having i n i t i a l l y c r y s t a l l i z e d
at
a h i g h e r temperature than the h o s t , above the p y r o x e n e s o l v u s . Type I I / T y p e I composites are e x t r e m e l y r a r e at GVF, although they appear be r e l a t i v e l y common i n o t h e r l o c a l i t i e s f r o m t h e s o u t h w e s t e r n U . S . 1980;
W i l s h i r e and S h e r v a i s , 1977;
P r e y and P r i n z ,
1979). C o n t a c t s between host
I h e r z o l i t e and c l i n o p y r o x e n i t e v e i n s a r e s h a r p and t h e o r i e n t a t i o n is t y p i c a l l y
oblique
to f o l i a t i o n in the host.
mm l a r g e r t h a n t h o s e i n t h e h o s t . The composite r e l a t i o n s h i p Type I I c h a r a c t e r i s t i c s l i t e hosts.
of thr
vein
The v e i n minerals average 1 to 2
V e i n w i d t h may be up t o 5 cm.
in which Type I I
lithologies
crosscut a host
i s t h e most common a t G e r ó n i m o . T h e s e c o n s i s t
o f d a r k A l - a u g i t e c l i n o p y r o x e n e and s p i n e l tacts
to
(Irving,
with
typically
+ o l i v i n e veins which crosscut wehr-
O l i v i n e i s f a r more a b u n d a n t i n t h e h o s t s t h a n i n t h e v e i n s .
b e t w e e n h o s t and v e i n r a n g e f r o m g r a d a t i o n a l
to sharp.
range from anastomosing, mm-size v e i n l e t s to c m - s i z e d i k e s . o f t h i s c o m p o s i t e t y p e w i t h o n l y one c o n t a c t
Con
Vein dimensions Frequent recovery
present suggests a l a r g e r average
w i d t h f o r t h e s e v e i n s t h a n f o r v e i n s f r o m T y p e I I / T y p e I o r T y p e I / T y p e I com posites.
A l - a u g i t e i n t h e h o s t commonly e n c l o s e s e q u a n t o l i v i n e c r y s t a l s
in a
74 poikill tic
to s u b p o i k i l i t i c r e l a t i o n s h i p .
The clinopyroxene increases in s i z e
and assumes a more e q u a n t h a b i t t o w a r d t h e v e i n . vary perceptibly
O l i v i n e s i n t h e h o s t do n o t
i n s i z e r e l a t i v e t o d i s t a n c e f r o m t h e h o s t / v e i n b o u n d a r y . Com
p o s i t i o n a l l y t h e h o s t p h a s e s a r e t y p i c a l l y more F e - r i c h t h a n t h o s e f r o m T y p e inclusions,
b u t f r e q u e n t l y n o t as F e - r i c h as o t h e r T y p e I I
that these Type I I
nodules.
hosts represent e x t e n s i v e l y modified Type I wall
chemical evidence w i l l
be p r e s e n t e d i n t h e f o l l o w i n g
rock.
Geo-
section.
A m p h i b o l e - b e a r i n g a s s e m b l a g e s o c c u r as s e l v a g e s on I h e r z o l i t e h o s t r o c k s GVF, but h o s t / v e i n r e l a t i o n s h i p s
h a v e n o t been f o u n d .
t a c t a p p e a r s s h a r p i n hand s p e c i m e n , b u t i n f i l t r a t i o n
at
The s e l v a g e assemblage
consists of k a e r s u t i t e + a p a t i t e + plagioclase + phlogopite.
T e x t u r a l l y the con
o f m e l t and
subsequent
c r y s t a l l i z a t i o n o f a m p h i b o l e w i t h i n t h e h o s t p r o d u c e s a more g r a d a t i o n a l on a t h i n s e c t i o n s c a l e .
I
We b e l i e v e
contact
Amphibole in the host I h e r z o l i t e p r e f e r e n t i a l l y
t a l l i z e s around the C r - s p i n e l s or replaces c l i n o p y r o x e n e .
crys
Adjacent to the
sel
v a g e s , a z o n e 1 - 2 mm w i d e o c c u r s i n w h i c h c l i n o p y r o x e n e has been c o m p l e t e l y r e p l a c e d by a m p h i b o l e ;
f u r t h e r from the s e l v a g e , incomplete replacement of
py
r o x e n e can be o b s e r v e d . Xenoliths:
Crustal
C r u s t a l x e n o l i t h s o c c u r o n l y rdre]y
at Gerónimo.
a r e t y p i c a l l y a l t e r e d by m e l t i n g and o x i d a t i o n ;
four r e l a t i v e l y pristine
p l e s , h o w e v e r , h a v e been a n a l y z e d . M i n e r a l o g i c a l l y , ulites.
T h o s e t h a t h a v e been f o u n d sam
these are two-pyroxene gran-
T h e y a r e composed p r e d o m i n a n t l y o f f e l d s p a r ,
f o l l o w e d i n a b u n d a n c e by
p y r o x e n e ( o r t h o p y r o x e n e and c l i n o p y r o x e n e o c c u r r i n g i n a p p r o x i m a t e l y e q u a l p o r t i o n s ) and l e s s t h a n a b o u t
5% a c c e s s o r y phases
( F e T i o x i d e s and
pro
apatite).
T h e y a r e f i n e g r a i n e d , c r y s t a l s r a n g i n g f r o m 0.5 t o 1 mm i n s i z e f o r t h e m a j o r phases. E x i s t i n g data i n d i c a t e t h a t the g r a n u l i t e s are c h e m i c a l l y d i v e r s e , par ticularly
w i t h r e s p e c t t o t r a c e e l e m e n t s : S r = 570 - 813 ppm; Rb = 0.9 - 22 ppm,
Y = 12.6 - 22.7
ppm;
La = 17.6 - 24.4 ppm; Ce = 35.8 - 47.2 ppm; Yb = 1.0
ppm; Ba = 550 - 1000 ppm; H f = 0.5 - 1.8
ppm.
i n c o r p o r a t e d i n o t h e r p h a s e s , most commonly p l a g i o c l a s e .
have t h i n e x s o l u t i o n
2.3
T h e a p a t i t e i n t h e s e s a m p l e s has
a v a r i a b l e h a b i t r a n g i n g f r o m t u r b i d c r y s t a l s 0.2 mm i n s i z e t o m i n u t e , inclusions
-
clear
Pyroxenes
lamellae.
Megacrysts M e g a c r y s t s o c c u r r i n g i n GVF l a v a s i n c l u d e : pyroxene, o l i v i n e ,
spinel, apatite,
amphibole
(kaersutite),
p l a g i o c l a s e and a n o r t h o c l a s e .
o f t h e s e phases c a n be f o u n d i n n e a r l y e v e r y f l o w i n t h e v o l c a n i c f i e l d . on t h e s i m i l a r i t i e s eral phases, i t
Al-clino-
One o r more Based
i n c o m p o s i t i o n b e t w e e n t h e m e g a c r y s t s and t h e x e n o l i t h m i n
is believed that the m a j o r i t y of these are d i s r u p t e d
A s i m i l a r c o n c l u s i o n was r e a c h e d by Wass
(1979).
xenoliths.
75 MINERAL CHEMISTRY Clinopyroxenes from Type I x e n o l i t h s concentrations
and i s o t o p i c
are Cr-diopsides.
characteristics
H o w e v e r , based on REE
T y p e I c l i n o p y r o x e n e s can be s e g r e
g a t e d i n t o two g r o u p s . T y p e l a c l i n o p y r o x e n e s h a v e LREE d e p l e t e d T y p e l b c l i n o p y r o x e n e s a r e LREE e n r i c h e d . F i g u r e 1. for
intermediate
types of patterns.
from other
localities
and P r i n z ,
1978).
Similar
REE s i g n a t u r e s
(0.70242 - 0.70470) and Nd (0.51328 - 0.51260) vs.
while
arbitrary reported
San C a r l o s ,
Frey
an e x t r e m e r a n g e i n S r
isotopic
composition.
On an Σ|^^
diagram, c l i n o p y r o x e n e separates o v e r l a p w i t h the "mantle a r r a y "
lie within 2.
is
h a v e been
( D r e i s e r W e i h e r , S t o s c h and S e e k , 1980;
T y p e s l a and l b n o d u l e s e x h i b i t
patterns
The d i v i s i o n
t h e q u a d r a n t c h a r a c t e r i z e d by m a n t l e d e p l e t e d
Thus, for
Concomitant w i t h t h i s enrichment
is a systematic v a r i a t i o n i n most a s p e c t s
in the C r / A l
i n LREE r e l a t i v e
variations
a r e not
I
I
La C e
I I Nd
I I I I I I I Sm E u
REE ATOMIC
Yb Lu
NUMBER
However,
clinopyroxene)
apparent. a r e LREE e n r i c h e d ,
I I I
Tb
ele
t o HREE
i n t h e c l i n o p y r o x e n e . F i g u r e 3.
( i . e . M g / ( M g + F e ) , t e x t u r e and modal abundance o f
I n common w i t h T y p e l b p y r o x e n e s . T y p e I I A l - a u g i t e s
10"
and Figure
t h e LREE e n r i c h e d T y p e l b c l i n o p y r o x e n e s , i s o t o p e s and t r a c e
ments a r e d e c o u p l e d .
systematic
in l i g h t REE,
I I I I La Ce
Nd
SmEu
REE ATOMIC
I
I
I
but
I Yb L u
Tb
NUMBER
F i g u r e 1. a ) REE c o n c e n t r a t i o n s i n T y p e l a c l i n o p y r o x e n e s n o r m a l i z e d t o c h o n d r i t i c abundances. Samples p l o t t e d i n c l u d e : • Z H l , X 2 0 - 1 0 H , # 2 0 - 9 , φ 2 3 B - 5 , A 23B-2. b) REE c o n c e n t r a t i o n s i n T y p e lb c l i n o p y r o x e n e s n o r m a l i z e d t o c h o n d r i t i c abundances. Samples p l o t t e d i n c l u d e : · 21-6, • ENOl, A 2 0 - 1 2 , φ Z H l . ZHl i s a t r a n s i t i o n a l sample based on t h e c r i t e r i a o u t l i n e d i n t h i s p a p e r , ( s e e F i g u r e 3) and has been p l o t t e d w i t h b o t h T y p e l a and T y p e l b . Dashed l i n e i n d i cates i n s u f f i c i e n t data to c o n f i d e n t l y determine t r e n d of REE.
76
.5133
GERÓNIMO VOLCANIC FIELD A R I Z O N A U.S.A
DEPLETED MANTLE
•ö Ζ •σ Ζ CO
M I N E R A L S IN SPINEL L H E R Z O L I T E INCLUSIONS
.5130 HOST ALKALINE MAGMAS " 1A · ' kDIOPSIDE IB Θ Δ AMPHIBOLE • MICA
.5127 Bulk E a r t h
_L 0.702
J0.703
ENRICHED MANTLE 0.704
0.705
0.706
^Sr/^^Sr
F i g u r e 2 . V a r i a t i o n i n Nd and S r i s o t o p i c c o m p o s i t i o n i n m i n e r a l s s e p a r a t e d f r o m s p i n e l I h e r z o l i t e ( T y p e l a and T y p e l b ) i n c l u s i o n s and a l k a l i b a s a l t s f r o m t h e Gerónimo V o l c a n i c F i e l d , A r i z o n a . Note t h a t s e v e r a l o f t h e u l t r a m a f i c x e n o l i t h s represent fragments o f subcontinental mantle v e r y d i f f e r e n t from t h e source r e gion o f t h e host b a s a l t s . Other mineral data from s e v e r a l sources (see Menzies, 1983, f o r r e f e r e n c e s ) . t h e i r c h o n d r i t e n o r m a l i z e d REE v a l u e s i n c r e a s e more s m o o t h l y f r o m h e a v y REE t o l i g h t REE and p o s s e s s a d i s t i n c t l y
c o n c a v e downward s h a p e . F i g u r e 4 .
Further
m o r e , t h e y a r e e n r i c h e d i n T Í O 2 ( 1 . 2 - 2 . 4 wt%) and d e p l e t e d i n C r ^ O ^ ( l e s s 0.1, w t % ) .
Analogous chemical d i s t i n c t i o n s
associated mineral
than
in mineral chemistry e x i s t f o r other
phases i n T y p e I and T y p e I I l i t h o l o g i e s .
n o s t i c m a j o r and t r a c e e l e m e n t s a r e s u m m a r i z e d i n T a b l e 1.
Ranges f o r d i a g
77 TABLE 1 C o n c e n t r a t i o n r a n g e s f o r s e l e c t e d m a j o r and t r a c e e l e m e n t s i n m i n e r a l s s e p a r a t e d f r o m T y p e l a . T y p e l b and T y p e I I x e n o l i t h s Type
T y p e I¿
lb
Type
II
Type la
ol i v i n e
Type
Type lb
II
clinopyroxene
Fo
88-90
87-90
74-79
Wo
46-48
45-47
44-50
Cr
62-160 (±7-27)
263 (±22)
6-577 (+2-14)
En
49-50
50-51
41-46
Fs
2-5
1-4
2485-2968 (+55-62)
2462 (+52)
232-850 (+23-51)
Ni
0.32-0.68
0.01-0.47
Cr^O^
0.45-1.04
0.84-1.34
—
—
Cr
orthopyroxene En
85-90
87-92
72
Cr
2941-1868 (+233-400)
4158 (+515)
n.d.
Ni
705-1272 (+16-27)
718 ( + 16)
n.d.
7.0-14.4
20.7-29.0 185-6230 (+16-473)
Cr Αΐ2θ3
52.8-59.4
36.1-46.5
56.3-62.7
Fe#
0.16-0.19
0.16-0.29
0.29-0.38
Ni
2775-3163 (+63-67)
1917 (+42)
336-445 (+11-33)
Ni
326-1093 (+27)
1.2-2.4
-
60-642 (+7-42) 35-163 (±3-27)
336-496 (+12-28)
Ce
2.91-5.71 6.33-19.62 10.76-17.88 (+0.48-1.38) (+0.51-1.68)(+0.77-2.72
Sm
1.41-1.62 0.60-2.93 3.47-4.97 (+0.03-0.04)(+0.01-0.05)(+0.06-0.23
Tb
0.38-0.64 0.33-0.49 0.61-0.84 (+0.03-0.10)(+0.03-0.10)(+0.04-0.07
Yb
1.68-1.85 0.56-1.79 1.64-2.20 (+0.15-0.27)(+Γ).06-0.15) (±0.17-0.57
spinel Cr203
5-9
Ti0,_,
Cr/Al
.019-.102
.097-.181
0-0.015
Note: C r , N i , C e , Sm, T b and Yb v a l u e s a r e g i v e n i n ppm: 2σ e r r o r g i v e n i n p a r e n t h e s e s . C o n c e n t r a t i o n s d e t e r m i n e d by I n s t r u m e n t a l N e u t r o n A c t i v a t i o n A n a l y s i s at NASA-JSC, Houston, T X . C r 2 0 3 , A l ^ O ^ and T Í O 2 v a l u e s a r e g i v e n i n w t / o : c o n c e n t r a t i o n s d e t e r m i n e d bv e l e c t r o n m i c r o p r o b e a t S o u t h e r n M e t h o d i s t U n i v e r sity, Dallas, TX. . F e ^ ^ / ( F e ^ ^ + Mg)V A R I A T I O N S IN MINERAL CHEMISTRY OF COMPOSITE Figure 5 i l l u s t r a t e s
XENOLITHS
the major element compositional
variation
in clinopyrox
ene w i t h d i s t a n c e f r o m t h e c o n t a c t b e t w e e n h o s t and v e i n f o r e a c h t y p e o f c o m posite relationship.
A l t h o u g h T y p e I / T y p e I c o m p o s i t e s show s l i g h t l y
Al^O^ in the vein r e l a t i v e to the host, h o s t and v e i n .
Type I I / T y p e
b u t show d i s t i n c t l y the host.
elevated
most m a j o r e l e m e n t s a r e i d e n t i c a l
in
I composites have u n i f o r m Mg/(Mg+Fe^^) and Ti02>
e l e v a t e d A l ^ O ^ and d e p l e t e d C r ^ O ^ i n t h e v e i n r e l a t i v e
Type I I / T y p e
to
I I c o m p o s i t e s e x h i b i t s i m i l a r AI2O3 and C r ^ O ^ t r e n d s ,
b u t a l s o show a m a r k e d d e c r e a s e i n M g / ( M g + F e ^ ^ ) and i n c r e a s e i n T i O - , i n t o t h e +2 vein.
Notice that the Type I I / T y p e
I I composite i s l o w e r i n Mg/(Mg+Fe
) and
C r ^ O ^ , b u t e l e v a t e d i n T Í O 2 f o r b o t h h o s t and v e i n r e l a t i v e t o t h e o t h e r c o m p o s -
78
ω c (D Χ
0-20
^
0-15
DEPLETED <
*ENRICHED
O i-
•
o
·
c
ΰ
0
•
10
c
<
•
1
• •
005
υ 1 10
.
_ ^
20
30
F i g u r e 3. R a t i o o f C r / A l c a t i o n s in recalculated pyroxene analyses ( r e c a l c u l a t i o n p r o c e d u r e o f Cam e r o n and P a p i k e , 1981) v s . Ce/Sm (chondrite normalized). A value o f Ce/Sm = 1 i n d i c a t e s no e n r i c h ment o f Ce r e l a t i v e t o Sm. T y p e l a c l i n o p y r o x e n e s h a v e Ce/Sm v a l ues l e s s t h a n 1; T y p e l b c l i n o p y r o x e n e s h a v e Ce/Sm g r e a t e r t h a n 1. N o t e t h a t as c l i n o p y r o x e n e s become p r o g r e s s i v e l y e n r i c h e d i n LREE t h e y a r e a l s o p r o g r e s s i v e l y enriched in C r / A l .
C e ^ / S m ^ in c l i n o p y r o x e n e
i t e t y p e s . C l i n o p y r o x e n e does n o t o c c u r i n t h e amphibole s e l v a g e s . H o w e v e r , t h e m i n e r a l o g i c c h a n g e s and c o m p o s i t i o n a l
variations
i n t h e host I h e r z o l i t e
pyroxenes w i t h d i s t a n c e from t h e s e l v a g e boundary a r e s i g n i f i c a n t .
clino
W i t h i n 2 mm
of t h e amphibole s e l v a g e c l i n o p y r o x e n e i s c o m p l e t e l y r e p l a c e d by amphibole and amphibole p a r t i a l l y
r e p l a c e s c l i n o p y r o x e n e o r c r y s t a l l i z e s as d i s c r e t e
t i a l c r y s t a l s f o r a d i s t a n c e o f a t l e a s t 2 cm away f r o m t h e s e l v a g e . tallization
intersti The c r y s
o f amphibole w i t h i n t h e I h e r z o l i t e g r e a t l y e n r i c h e s t h e w a l l
rock
i n REE and o t h e r i n c o m p a t i b l e e l e m e n t s ( M e n z i e s a n d M u r t h y , 1 9 8 0 ) . H o w e v e r , n o t i c e t h a t t h e h o s t c l i n o p y r o x e n e d e c r e a s e s i n A l ^ O ^ and T i O ^ a s w e l l a s C r ^ O ^ toward t h e s e l v a g e .
Furthermore, Cr/Al
Comparison o f these C r / A l
r a t i o d e c r e a s e s f r o m 0.056 t o 0 . 0 5 0 .
t r e n d s w i t h t h e v a l u e s f o r T y p e l a and T y p e l b c l i n o
pyroxenes i n F i g u r e 3 demonstrates t h a t t h e metasomatic e f f e c t
on t h e h o s t
l h e r z o l i t e associated w i t h amphibole c r y s t a l l i z a t i o n cannot account f o r t h e d i s -
F i g u r e 4 . REE c o n c e n t r a t i o n s i n Type I I clinopyroxenes normalized to c h o n d r i t i c abundances. Sam ples plotted include: ^ 21-M2, # 2 2 - 1 , A 22-11, • 23B-7, • 20-8. 23B-7 a n d 20-8 a r e a n h y d r o u s T y p e I I x e n o l i t h s ; 22-1 and 22-11 a r e k a e r s u t i t e p e r i d o t i t e xenoliths. 21-M2 i s a c l i n o p y r o x ene m e g a c r y s t .
10"
I I I I I I I I I I I I I I LaCe
Nd
SmEu
REE ATOMIC
Tb
NUMBER
Yb Lu
79
CPX
VEIN
CPX
HOST
M g / ( M g + Fe*2) 0-9
08
8-0
A I 2 0 3
70
Q
X O
6.0
10
C r p 3
05
-••-••^•-Λί'
0 2 0
LTiO, I I
10
-4-
JL 20
1 0
DISTANCE
FROM
0
HOST/VEIN
1-0
BOUNDARY
F i g u r e 5. M a j o r e l e m e n t c o m p o s i t i o n a l v a r i a t i o n i n c l i n o p y r o x e n e w i t h d i s t a n c e from h o s t / v e i n boundary in composite x e n o l i t h s . Samples p l o t t e d i n c l u d e : · 20-9 ( T y p e I / T y p e I ) ; A 20-10 ( T y p e I I / T y p e I ) ; • 21-8 ( T y p e I I / T y p e I I ) ; • 22-16 ( k a e r s u t i t e p e r i d o t i t e s e l v a g e on T y p e I s p i n e l I h e r z o l i t e ) . t i n c t major element m i n e r a l c o m p o s i t i o n o b s e r v e d i n T y p e lb
xenoliths.
I n c o n t r a s t t o t h e d i f f e r i n g m a j o r e l e m e n t v a r i a t i o n s b e t w e e n h o s t and v e i n , the incompatible
r a r e e a r t h elements are e s s e n t i a l l y i d e n t i c a l
f o r a g i v e n phase
i n b o t h h o s t and v e i n r e g a r d l e s s o f c o m p o s i t e x e n o l i t h t y p e . F i g u r e 6.
Compar
i s o n o f F i g u r e 6 w i t h F i g u r e s 1 and 4 d e m o n s t r a t e t h a t c l i n o p y r o x e n e s f r o m sam p l e 21-8 ( a T y p e I I / T y p e I I ) liths.
h a v e REE v a l u e s l i k e t h o s e o f normal T y p e I I
C l i n o p y r o x e n e s f r o m 20-9
s i g n a t u r e l i k e Type la I h e r z o l i t e c l i n o p y r o x e n e s . i t e 20-10
xeno
(a T y p e I / T y p e I ) h a v e a l i g h t REE d e p l e t e d I n t e r e s t i n g l y e n o u g h , compos
(a T y p e I I / T y p e I ) has REE c o n c e n t r a t i o n s i n t e r m e d i a t e b e t w e e n t h e s e
two t y p e s . Most i m p o r t a n t l y ,
however, w i t h i n a n a l y t i c a l
for a given host/vein pair are e s s e n t i a l l y
e r r o r , t h e REE p a t t e r n s
identical.
Comparison o f compatible t r a c e elements r e v e a l s s i m i l a r d i s t i n c t i o n s
between
h o s t and v e i n t o t h o s e s e e n i n t h e m a j o r e l e m e n t v a r i a t i o n s . C r i n p y r o x e n e and
80 s p i n e l and Ni i n o l i v i n e and s p i n e l a r e t h e most u s e f u l d i s c r i m i n a t o r s
because
t h e r a n g e s o f c o n c e n t r a t i o n s f o r t h e s e e l e m e n t s t e n d t o be d i s t i n c t f o r T y p e and T y p e I I x e n o l i t h s i n t h e s e p h a s e s . show t h a t f o r e a c h c o m p o s i t e t y p e , Type I / T y p e I composite 20-9, considered identical
C r and N i v a l u e s t a b u l a t e d
w i t h i n the a n a l y t i c a l
is small;
e r r o r on t h e d a t a .
is l a r g e r than expected f o r l i t h o l o g i e s
element mineral c o m p o s i t i o n s . A l t h o u g h i t trations
In
h o s t and v e i n c o u l d be In c o n t r a s t ,
m a g n i t u d e o f t h e v a r i a t i o n b e t w e e n h o s t and v e i n i n T y p e I I / T y p e 21-8
in T a b l e 2
v e i n m i n e r a l s h a v e l o w e r C r and N i .
the d i f f e r e n c e
II
w i t h s i m i l a r m i n e r a l o g i e s and m a j o r
m i g h t be a r g u e d t h a t t h e h i g h c o n c e n
o f C r i n c l i n o p y r o x e n e and Ni i n o l i v i n e r e s u l t f r o m
fractionation compatible
elements are r e l a t i v e l y high in the m e l t , the Cr c o n c e n t r a t i o n in s p i n e l o r d e r o f magnitude g r e a t e r than t h a t o b s e r v e d in any o t h e r Type I I is,
however, e a s i l y w i t h i n
l i e v e that Type I I / T y p e interaction
the range f o r Type I s p i n e l s .
i s an
xenoliths.
We, t h e r e f o r e , b e
I I c o m p o s i t e x e n o l i t h s f o r m as t h e r e s u l t o f e x t e n s i v e
b e t w e e n b a s a n i t i c m e l t and s p i n e l
I h e r z o l i t e wall
Such com
that interaction
to e x t e n s i v e m o d i f i c a t i o n
o f the upper mantle i n v o l v i n g enrichment in
N a , REE and p o s s i b l y Ca and A l .
between w a l l
rock.
posite xenoliths indicate
r o c k and magma may l e a d Fe, T i ,
I n c o m p e n s a t i o n , t h e magma ( a s r e p r e s e n t e d by
t h e v e i n a s s e m b l a g e s ) must a t l e a s t l o c a l l y become e n r i c h e d i n
Mg, C r and Ni
through chemical exchange w i t h the rock I h e r z o l i t e . • o • Δ • o
I
I I 21-8 21-8 20-10 20-10 20-9 20-9
the
composite
d u r i n g t h e e a r l y s t a g e s o f c r y s t a l l i z a t i o n when t h e c o n c e n t r a t i o n s o f
It
I
I I I I I I I HOST VEIN HOST VEIN HOST VEIN
I I I
p r e s e n t e d by I r v i n g
wall
Geochemical e v i d e n c e (1980) documents s i m
i l a r t r e n d s , a l t h o u g h f o r samples r e c o r d ing f a r l e s s i n t e r a c t i o n composites).
(Type I I / T y p e
The extensive
r e c o r d e d i n c o m p o s i t e x e n o l i t h 21-8 gests that greater interaction wall
I
modification sug
between
r o c k and a s c e n d i n g magma has
oc
c u r r e d i n t h e m a n t l e b e n e a t h GVF t h a n indicated
by s a m p l e s f r o m K i l b o u r n e H o l e .
F i g u r e 6. REE c o n c e n t r a t i o n s i n clinopyroxenes from composite xenoliths. 21 - 8 i s a T y p e I I / T y p e I I c o m p o s i t e ; 21 - 10 i s a Type I I / T y p e I composite; 20-9 is a Type I / T y p e I composite. REE ATOMIC
NUMBER
81 TABLE 2 C r and N i a b u n d a n c e s i n c l i n o p y r o x e n e , o l i v i n e and s p i n e l f r o m a s s o c i a t e d h o s t and v e i n m i n e r a l a s s e m b l a g e s o f composite x e n o l i t h s VEIN
HOST clinopyroxene
olivine
spinel Type I/Type
Cr
5881+587
Ni
386+19
89+11 2812+62
Cr
3747+190 378+10
141+13
3282+397
Ni
207+20
spinel
366+23
2844+62
n.d. n.d.
I , 20-10
52500+3990
642+42
577+47
5205+396
3163+66
357+10
643+14
3301+69
2639+55
90+10
62+7
4952+572
2857+63
Type I I / T y p e Cr
oli vine
I , 20-9
67360+3970
Type I I / T y p e
Ni
clinopyroxene
II ,
21-8
65465+6462
373+37
10+1
4374+421
1629+46
125+16
848+23
1093+27
1454+33
C o n c e n t r a t i o n s + 1σ a r e g i v e n i n ppm
DISSCUSSION AND SUMMARY P e t r o g r a p h i c o b s e r v a t i o n s combined w i t h m a j o r e l e m e n t , t r a c e e l e m e n t and i s o t o p i c data from x e n o l i t h s o f the Gerónimo V o l c a n i c F i e l d suggest a m u l t i s t a g e e v o l u t i o n f o r the mantle beneath the s o u t h w e s t e r n U . S . h a r z b u r g i t e s and c l i n o p y r o x e n e - d e p l e t e d s p i n e l
T h e dominance o f T y p e
I
I h e r z o l i t e s a t GVF s u p p o r t t h e
occurrence of a major widespread p a r t i a l m e l t i n g event t h a t produced a d e p l e t i o n i n the l i g h t REE, perhaps r e l a t e d t o c o n t i n e n t
formation.
E x t r a c t i o n of melt
(> 1 b . y . ) l e a v e s a I h e r z o l i t e r e s i d u e e v i d e n t as T y p e l a i n c l u s i o n s
(Menzies
and M u r t h y , 1980b; M e n z i e s , e t a l . , 1982; F r e y and G r e e n , 1 9 7 4 ) ; c l i n o p y r o x e n e s in T y p e la nodules have a r e f r a c t o r y major element c o m p o s i t i o n , pleted p r o f i l e s ,
l i g h t REE d e
r a d i o g e n i c Nd (0.5133 - 0 . 5 1 3 0 ) , and n o n r a d i o g e n i c S r ( 0 . 7 0 2 -
0.703) i s o t o p i c
c o m p o s i t i o n s . F i g u r e s l a and 2 .
time-integrated
r e s p o n s e t o t h e i n c r e a s e i n Sm/Nd.
Type lb x e n o l i t h s are enigmatic
in o r i g i n .
The isotopic
data r e p r e s e n t a
T h e i r LREE e n r i c h e d p r o f i l e s
can
n o t be a c c o u n t e d f o r as a r e s i d u e f r o m p a r t i a l m e l t i n g b a s e d on c u r r e n t k n o w l edge o f p a r t i t i o n c o e f f i c i e n t s . t r a t i o n o f a CO2 + H2O r i c h f l u i d
Mantle enrichment
(>0.5 b . y . ) caused by i n f i l
( a n a l o g o u s t o component Β p r o p o s e d b y F r e y and
G r e e n , 1974) i s one p o s s i b l e e x p l a n a t i o n f o r t h e s e i n c l u s i o n s . T h i s metasomatism r e s u l t s
infiltration
i n t h e l i g h t REE e n r i c h m e n t o f t h e c l i n o p y r o x e n e p h a s e , b u t
p r o d u c e s no p e t r o g r a p h i c o r m i n e r a l o g i c t r a n s f o r m a t i o n o f t h e p e r i d o t i t e s .
The
82 c o n s i d e r a b l e r a n g e i n Nd ( 0 . 5 1 2 8 - 0.5125) and S r ( 0 . 7 0 3 - 0 . 7 0 5 ) i s o t o p i c position visible
in Type lb nodules ( i . e . modified Type l a ) r e p r e s e n t s a p a r t i a l
r e s p o n s e t o t h e l o w Sm/Nd r a t i o .
However, a d i s t i n c t i v e major element
i s s u g g e s t e d by t h e c o r r e l a t i o n b e t w e e n t h e d e g r e e o f LREE e n r i c h m e n t and C r / A l
ratios
element mineral
com
in clinopyroxenes.
Furthermore, the modifications
composition which r e s u l t
control (Ce/Sm)
to major
i n a s s o c i a t i o n w i t h h y d r o u s and a n h y
drous v e i n i n g a r e not a p p r o p r i a t e to produce the d i s t i n c t i v e
m a j o r e l e m e n t com
position of Type lb x e n o l i t h s . Nonetheless, interaction
b e t w e e n T y p e l a and T y p e l b
s i l i c a t e m e l t e q u i v a l e n t i n c h e m i s t r y and i s o t o p i c n i t e produces v e i n i n g o f the I h e r z o l i t i c w a l l
(and T y p e I I ) and a
composition to the host basa-
rock.
Mineral assemblages
include
c l i n o p y r o x e n e + s p i n e l + o l i v i n e + k a e r s u t i t e + mica + a p a t i t e + f e l d s p a r . a s s o c i a t e d c o n t a c t metasomatism t r a n s f o r m s the p e r i d o t i t e
d u i t and i n some c a s e s i n t r o d u c e s a m p h i b o l e a n d / o r m i c a i n t o t h e w a l l
rock.
e v e n t i s b e l i e v e d t o be r e l a t i v e l y r e c e n t ( « 1 0 0 m . y . ) s i n c e m i c a i n t h e oy
I h e r z o l i t e has a l o w
The
adjacent to the con This
host
Of.
Sr/
S r = 0.70329 and a h i g h R b / S r r a t i o ;
similarly
the
v e i n a m p h i b o l e has a d e c o u p l e d r a r e e a r t h (Sm/Nd = 0 . 1 4 2 ) and a Nd i s o t o p i c position
com
(Σ|^|^ = + 8 ) .
The v e i n s are b e l i e v e d to r e p r e s e n t f r o z e n conduits or apophyses of melt (Menzies, et a l . ,
1982)
or composite r e l a t i o n s h i p s o f h o s t and v e i n :
a)
basanitic
t h a t s u r r o u n d a d e e p - s e a t e d magma p o c k e t .
a r e o f t h r e e t y p e s b a s e d on m a j o r e l e m e n t
Vein/host chemistries
Type I / T y p e I composites in which Type I spinel
Iherzo-
l i t e s or h a r z b u r g i t e s a r e c r o s s c u t by T y p e I w e b s t e r i t e s o r d i o p s i d e v e i n s ; b)
Type I I / T y p e I composites in which spinel
A l - a u g i t e c l i n o p y r o x e n i t e d i k e s ; and c )
I h e r z o l i t e i s c r o s s c u t by
Type
Type I I A l - a u g i t e c l i n o p y r o x e n i t e dikes crosscut w e h r l i t e hosts of Type I I element compositions.
These r e l a t i o n s h i p s
indicate
that the process of
v e i n i n g i s b o t h an a n c i e n t p r o c e s s ( T y p e I / T y p e I ) and t h a t i t s v a r i a b l e , ranging from r e e q u i 1 i b r a t i o n e x t e n s i v e major element a l t e r a t i o n the e x t e n t of m o d i f i c a t i o n
II
Type I I / T y p e I I composites in which
of incompatible
as w e l l
major
mantle
effects
are
t r a c e e l e m e n t s t o more
(Type I I / T y p e I I ) .
The c o n t r o l s
a r e u n k n o w n , b u t must depend on s u c h c o n d i t i o n s
t h e s i z e o f t h e c o n d u i t s , t h e a s c e n t r a t e o f t h e magma, t h e e x t e n t o f
on
as
modifica
t i o n e x p e r i e n c e d d u r i n g p r e v i o u s e p i s o d e s o f magma a s c e n t i n t h e same c o n d u i t and t h e t e m p e r a t u r e d i f f e r e n c e b e t w e e n t h e h o s t w a l l
r o c k and magma.
T h e magma
p o c k e t w i t h w h i c h t h e v e i n s a r e b e l i e v e d t o be a s s o c i a t e d i s p a r t o f t h e " g e n e r a t i o n o f magmas" t h a t u l t i m a t e l y
d i s r u p t s and e n t r a i n s t h e m a n t l e f r a g m e n t s .
T h e s e v e i n e d T y p e l a and T y p e l b n o d u l e s i l l u s t r a t e t h e f a c t t h a t m a n t l e
perido
t i t e s can be m o d i f i e d b y t h e v e r y p r o c e s s e s t h a t t r a n s p o r t them t o t h e s u r f a c e . T h e m a n t l e b e l o w t h e s o u t h w e s t e r n U . S . has an a n c i e n t i s o t o p i c unlike that of a residue (Type la) t e r been m o d i f i e d
l e f t a f t e r e x t r a c t i o n o f MORB.
by v a p o u r - r i c h f l u i d s and b a s a n i t i c m e l t s
signature
not
T h i s has
la
(composite
xenoliths
83 T y p e I i / T y p e I I , T y p e I I / T y p e I and T y p e I / T y p e I ) .
The a l k a l i basalts
from
GVF h a v e ^ ^ S r / ^ ^ S r = 0.70285 - 0.70327 and ^ ^ ^ N d / ^ ' ^ ^ N d = 0.51304 - 0 . 5 1 2 9 0 , F i g u r e 2.
All
the a l k a l i basalts
plot within
the f i e l d of mid-ocean r i d g e
h a v i n g been d e r i v e d f r o m a m a n t l e w i t h a t i m e - i n t e g r a t e d rare earth elements.
depletion
basalts,
of the
light
T h i s i s c o m p a t i b l e w i t h o b s e r v a t i o n s o f o t h e r B a s i n and
Range b a s a n i t e s and i m p l i e s
t h a t a MORB-type mantle e x i s t s below t h e
southwest
ern U.S. ACKNOWLEDGEMENTS T h i s r e s e a r c h has been s u p p o r t e d by t h e NASA G r a d u a t e T r a i n e e s h i p P r o g r a m and b y t h e I n s t i t u t e sity.
f o r t h e S t u d y o f E a r t h and Man, S o u t h e r n M e t h o d i s t
Univer
DUNGAN
A b s t r a c t - The Gerónimo V o l c a n i c F i e l d i s n o t a b l e f o r i t s abundance o f p r i s t i n e T y p e I , T y p e I I and c o m p o s i t e x e n o l i t h s . T h e dominance o f u n f o l i a t e d , g r a n u l o b l a s t l c T y p e I h a r z b u r g i t e s and c l i n o p y r o x e n e - d e p l e t e d s p i n e l I h e r z o l i t e s s u p p o r t s the o c c u r r e n c e of a major d e p l e t i o n e v e n t in the mantle f o l l o w e d by a pe riod of reequi1ibration. LREE d e p l e t e d T y p e l a x e n o l i t h s a r e c o n s o n a n t w i t h s u c h a p a r t i a l m e l t i n g e v e n t (^1 b . y . ) . T y p e l b n o d u l e s a r e p e t r o g r a p h i c a l l y t h e same as T y p e l a i n c l u s i o n s , b u t h a v e LREE e n r i c h e d c l i n o p y r o x e n e s . T h e s e may r e c o r d a m a n t l e e n r i c h m e n t e v e n t ( < 0 . 5 b . y . ) c a u s e d by i n f i l t r a t i o n o f CO^+H^O fluids. " ^ C o m p o s i t e x e n o l i t h s a r e o f t h r e e t y p e s based on h o s t / v e i n m a j o r e l e m e n t com positions: a) Type I c r o s s c u t t i n g Type I, b) Type I I c r o s s c u t t i n g Type I, c) Type I I c r o s s c u t t i n g Type I I . T h e o r i g i n o f t h e s e c o m p o s i t e samples i s a t t r i b u t e d t o c r y s t a l l i z a t i o n o f b a s a n i t i c magma w i t h i n a p o p h y s e s o r a l o n g c o n d u i t w a l l s as t h e y a s c e n d t h r o u g h t h e m a n t l e . I n f i l t r a t i o n o f magmatic f l u i d s a l o n g g r a i n b o u n d a r i e s r e s u l t s i n p r o f o u n d m o d i f i c a t i o n o f c o n d u i t w a l l s on a t l e a s t a cm s c a l e a d j a c e n t t o t h e b o u n d a r y . T h e s e i n t e r a c t i o n s may i n v o l v e r e e q u i 1 i b r a t i o n o f i n c o m p a t i b l e t r a c e e l e m e n t s , REE and i s o t o p e s o r more c o m p l e t e m a j o r element readjustment. The spectrum of chemical e f f e c t s observed in composite x e n o l i t h s supports the e x i s t e n c e of a heterogeneous mantle beneath s o u t h e a s t e r n Arizona.
INTRODUCTION The Gerónimo V o l c a n i c F i e l d southwestern U . S . A . ,
number and v a r i e t y o f p r i s t i n e includes spinel
(GVF), located in southeastern A r i z o n a of
i s n o t e w o r t h y among x e n o l i t h l o c a l i t i e s due t o t h e ultramafic
nodules p r e s e n t .
amphibole p e r i d o t i t e
to 6 m.y.
basalt
r i n g s w h i c h r a n g e i n age
(Kempton, unpublished d a t a ) .
P r e v i o u s w o r k b y A r c u l u s and D e l a n o (1981) on t h e i n t r i n s i c in the t e r r e s t r i a l
between
The x e n o l i t h s are included in a l k a l i
t o b a s a n i t e c i n d e r c o n e s , f l o w s and p y r o c l a s t i c t u f f
suite
clinopyroxenite,
and t w o - p y r o x e n e g r a n u l i t e ; c o m p o s i t e r e l a t i o n s h i p s
t h e v a r i o u s r o c k t y p e s a r e common. f r o m 0.25
The x e n o l i t h
Iherzolite, harzburgite, websterite, wehrlite,
the great
oxygen
fugacity
m a n t l e f o u n d t h a t GVF a n h y d r o u s I h e r z o l i t e s a r e r e d u c e d
rel
a t i v e t o b a s a l t i c magmas and p r o b a b l y r e p r e s e n t f r a g m e n t s o f a n c i e n t m a n t l e 3 b . y . ) o n l y r e c e n t l y d i s t u r b e d and p e r h a p s m o d i f i e d
by b a s a n i t i c
O t h e r w o r k on t h e G e r ó n i m o V o l c a n i c F i e l d i n c l u d e s t h a t o f
(1-
volcanism.
L y n c h (1972)
and
E v a n s and Nash ( 1 9 7 9 ) . This
r e p o r t p r e s e n t s p e t r o g r a p h i c , m a j o r e l e m e n t , t r a c e e l e m e n t , REE and i s o -
t o p i c a n a l y s e s o f r e p r e s e n t a t i v e i n c l u s i o n s and l a v a s and d i s c u s s e s some o f petrogenetic implications
of the data.
T h e p e t r o g e n e t i c model p r o p o s e d
p r e t s t h e A l - a u g i t e c l i n o p y r o x e n i t e s as t h e f r a c t i o n a l
crystallization
the
inter product
72 o f F e - T i r i c h b a s a n i t e t h a t i n t r u d e d a p a r t o f t h e u p p e r m a n t l e i n i t i a l l y com posed o f v a r i a b l y d e p l e t e d s p i n e l
Iherzolite, similar
W i l s h i r e and S h e r v a i s
(1975) and I r v i n g
the southwestern U.S.
The i n t e r a c t i o n
ing from r e e q u i l i b r a t i o n e l e m e n t and m i n e r a l o g i c
p r o p o s e d by from
b e t w e e n t h e a s c e n d i n g magma and t h e
rock is recorded in composite x e n o l i t h s . h o s t / v e i n boundaries indicate
t o models
(1980) f o r o t h e r x e n o l i t h s u i t e s
that the extent of i n t e r a c t i o n
of incompatible
wall
Compositional v a r i a t i o n s r e l a t i v e is v a r i a b l e
to
rang
t r a c e e l e m e n t s t o more e x t e n s i v e m a j o r
readjustment.
PETROGRAPHY OF INCLUSIONS Xenoliths:
Type
I
Type I i n c l u s i o n s , a r e Mg-
following
the c l a s s i f i c a t i o n
from Gerónimo i n c l u d e s p i n e l
Type I i n c l u s i o n s e x h i b i t unfoliated
(1978), xenoliths
I h e r z o l i t e , h a r z b u r g i t e and w e b s t e r i t e ; b u t
n a n t among t h e s e a r e c l i n o p y r o x e n e - d e p l e t e d s p i n e l
textures
o f F r e y and P r i n z
and C r - r i c h r o c k s composed p r e d o m i n a t e l y o f o l i v i n e . T y p e I
limited textural
domi
I h e r z o l i t e s and h a r z b u r g i t e s .
variety.
In g e n e r a l ,
o r p o o r l y f o l i a t e d and h a v e g r a n u l o b l a s t i c - e q u a n t
they are
to coarse-equant
(nomenclature a f t e r H a r t e , 1977). T h e w e l l - e q u i l i b r a t e d
nature of
the
t e x t u r e s i s a p p a r e n t i n t h e common c o n f i g u r a t i o n o f g r a i n b o u n d a r y j u n c t i o n s 120^
angles.
Most s a m p l e s l a c k e x s o l u t i o n i n t h e m i n e r a l
phases,
but
in
clino
p y r o x e n e s f r o m some T y p e I w e b s t e r i t e x e n o l i t h s and v e i n s do show b r o a d e x s o l u tion lamellae.
These w i l l
relationships.
Type I x e n o l i t h s t y p i c a l l y
zoning in the mineral
be d e s c r i b e d more f u l l y
in the context of
composite
show o n l y m i n o r s e c o n d a r y m e l t i n g o r
phases.
R e c e n t w o r k has shown t h a t t h e p r e s e n c e o f m i c r o s c o p i c i n c l u s i o n s c a n h a v e a significant
e f f e c t on t h e t r a c e and r a r e e a r t h e l e m e n t a b u n d a n c e s i n o l i v i n e and
orthopyroxene in peridotites S t o s c h , 1982). ary melting,
( J a g o u t z , e t a l . , 1979;
Brief petrographic descriptions
alteration
Xenoliths:
Type
1980;
including the e x t e n t of second
and t h e p r e s e n c e o f f l u i d i n c l u s i o n s
I I and c o m p o s i t e x e n o l i t h s a r e g i v e n f o r i n d i v i d u a l
Type I I
S t o s c h and S e e k ,
i n 6VF T y p e I ,
Type
s a m p l e s i n Kempton ( i n p r e p ) .
II
i n c l u s i o n s a r e l e s s common t h a n T y p e I a t G V F , b u t may l o c a l l y
nate the x e n o l i t h population.
T y p e I I x e n o l i t h s a r e more F e , A l and T i
t h a n T y p e I x e n o l i t h s and i n c l u d e c l i n o p y r o x e n i t e , w e h r l i t e , k a e r s u t i t e
domi rich
perido
t i t e and r a r e w e b s t e r i t e . T e x t u r a l l y , x e n o l i t h s o f t h i s g r o u p h a v e an i g n e o u s a p p e a r a n c e i n w h i c h t h e modal p r o p o r t i o n s and t e x t u r a l phases a r e m a r k e d l y v a r i a b l e .
relationships
among t h e
C l i n o p y r o x e n e r a n g e s f r o m i r r e g u l a r l y shaped
g r a i n s o c c u r r i n g between equant o l i v i n e s t o l a r g e p o i k i l i t i c c r y s t a l s w h i c h t o t a l l y e n c l o s e o l i v i n e and s p i n e l . P e t r o g r a p h i c e v i d e n c e f o r m e t a m o r p h i c tallization
e x i s t s i n some s a m p l e s .
E x s o l u t i o n of aluminous spinel from
pyroxene is t y p i c a l ; e x s o l u t i o n of orthopyroxene occurs in Type I I
recrysclino
clinopyrox-
73 e n e s , b u t i s f a r l e s s common t h a n
spinel.
Amphibole r e p l a c e m e n t o f c l i n o p y r o x e n e i s most f r e a u e n t l y o b s e r v e d i n T y p e I I x e n o l i t h s . The replacement amphibole ranges from cm-sized c r y s t a l s
poikilit-
ically
to
e n c l o s i n g one o r more c l i n o p y r o x e n e s t o i r r e g u l a r
of microns in s i z e .
patches 10's
100's
P l a g i o c l a s e i s r a r e l y p r e s e n t and g a r n e t has n o t been o b
s e r v e d i n x e n o l i t h s o f t h i s t y p e . T h e F e - and A l - r i c h s p i n e l s a r e g r e e n t o i n t h i n s e c t i o n and h a v e s u b h e d r a l ent,
clinopyroxene is t y p i c a l l y
to euhedral shapes.
devoid of e x s o l u t i o n .
phases i n v a r i a b l y c o e x i s t i n amphibole p e r i d o t i t e s ,
When a m p h i b o l e In addition,
is
black pres
two o x i d e
a n e a r l y pure Mg-Al
spinel
and m a g n e t i t e . Xenoliths:
Composite
Structural categories: b) II
relationships a)
Type I I
Type I l i t h o l o g i e s
lithologies
lithologies
b e t w e e n T y p e I and T y p e I I x e n o l i t h s f a l l crosscutting
crosscutting
crosscutting
a Type I host
a host of Type I I a f f i n i t i e s
burgite hosts.
(Cr-diopside) veins crosscutting
Contacts between o l i v i n e - r i c h
assemblages a r e r e l a t i v e l y 2 cm.
three
(Type I/Type I ) ,
a T y p e I h o s t ( T y p e I I / T y p e I ) and c )
Type I / T y p e I composites have e i t h e r w e b s t e r i t e or c l i n o p y r o x e n i t e
into
s h a r p and p l a n a r ;
(Type I I / T y p e
Type
II).
(Cr-diopside + orthopyroxene) spinel
I h e r z o l i t e or harz-
h o s t r o c k s and p y r o x e n e - r i c h v e i n the veins are g e n e r a l l y t h i n ,
1 -
A d j a c e n t t o t h e v e i n , c l i n o p y r o x e n e may be d e p l e t e d and t h e h o s t p h a s e s
enlarged.
T h e p y r o x e n e s i n t h e C r - d i o p s i d e v e i n s and some w e b s t e r i t e s a r e
sig
nificantly
l a r g e r than the host phases.
ex
These large pyroxenes f r e q u e n t l y
h i b i t b r o a d e x s o l u t i o n l a m e l l a e w h i l e t h e h o s t p y r o x e n e s do n o t . T h i s t h a t the v e i n assemblage i s igneous in o r i g i n ,
indicates
having i n i t i a l l y c r y s t a l l i z e d
at
a h i g h e r temperature than the h o s t , above the p y r o x e n e s o l v u s . Type I I / T y p e I composites are e x t r e m e l y r a r e at GVF, although they appear be r e l a t i v e l y common i n o t h e r l o c a l i t i e s f r o m t h e s o u t h w e s t e r n U . S . 1980;
W i l s h i r e and S h e r v a i s , 1977;
P r e y and P r i n z ,
1979). C o n t a c t s between host
I h e r z o l i t e and c l i n o p y r o x e n i t e v e i n s a r e s h a r p and t h e o r i e n t a t i o n is t y p i c a l l y
oblique
to f o l i a t i o n in the host.
mm l a r g e r t h a n t h o s e i n t h e h o s t . The composite r e l a t i o n s h i p Type I I c h a r a c t e r i s t i c s l i t e hosts.
of thr
vein
The v e i n minerals average 1 to 2
V e i n w i d t h may be up t o 5 cm.
in which Type I I
lithologies
crosscut a host
i s t h e most common a t G e r ó n i m o . T h e s e c o n s i s t
o f d a r k A l - a u g i t e c l i n o p y r o x e n e and s p i n e l tacts
to
(Irving,
with
typically
+ o l i v i n e veins which crosscut wehr-
O l i v i n e i s f a r more a b u n d a n t i n t h e h o s t s t h a n i n t h e v e i n s .
b e t w e e n h o s t and v e i n r a n g e f r o m g r a d a t i o n a l
to sharp.
range from anastomosing, mm-size v e i n l e t s to c m - s i z e d i k e s . o f t h i s c o m p o s i t e t y p e w i t h o n l y one c o n t a c t
Con
Vein dimensions Frequent recovery
present suggests a l a r g e r average
w i d t h f o r t h e s e v e i n s t h a n f o r v e i n s f r o m T y p e I I / T y p e I o r T y p e I / T y p e I com posites.
A l - a u g i t e i n t h e h o s t commonly e n c l o s e s e q u a n t o l i v i n e c r y s t a l s
in a
74 poikill tic
to s u b p o i k i l i t i c r e l a t i o n s h i p .
The clinopyroxene increases in s i z e
and assumes a more e q u a n t h a b i t t o w a r d t h e v e i n . vary perceptibly
O l i v i n e s i n t h e h o s t do n o t
i n s i z e r e l a t i v e t o d i s t a n c e f r o m t h e h o s t / v e i n b o u n d a r y . Com
p o s i t i o n a l l y t h e h o s t p h a s e s a r e t y p i c a l l y more F e - r i c h t h a n t h o s e f r o m T y p e inclusions,
b u t f r e q u e n t l y n o t as F e - r i c h as o t h e r T y p e I I
that these Type I I
nodules.
hosts represent e x t e n s i v e l y modified Type I wall
chemical evidence w i l l
be p r e s e n t e d i n t h e f o l l o w i n g
rock.
Geo-
section.
A m p h i b o l e - b e a r i n g a s s e m b l a g e s o c c u r as s e l v a g e s on I h e r z o l i t e h o s t r o c k s GVF, but h o s t / v e i n r e l a t i o n s h i p s
h a v e n o t been f o u n d .
t a c t a p p e a r s s h a r p i n hand s p e c i m e n , b u t i n f i l t r a t i o n
at
The s e l v a g e assemblage
consists of k a e r s u t i t e + a p a t i t e + plagioclase + phlogopite.
T e x t u r a l l y the con
o f m e l t and
subsequent
c r y s t a l l i z a t i o n o f a m p h i b o l e w i t h i n t h e h o s t p r o d u c e s a more g r a d a t i o n a l on a t h i n s e c t i o n s c a l e .
I
We b e l i e v e
contact
Amphibole in the host I h e r z o l i t e p r e f e r e n t i a l l y
t a l l i z e s around the C r - s p i n e l s or replaces c l i n o p y r o x e n e .
crys
Adjacent to the
sel
v a g e s , a z o n e 1 - 2 mm w i d e o c c u r s i n w h i c h c l i n o p y r o x e n e has been c o m p l e t e l y r e p l a c e d by a m p h i b o l e ;
f u r t h e r from the s e l v a g e , incomplete replacement of
py
r o x e n e can be o b s e r v e d . Xenoliths:
Crustal
C r u s t a l x e n o l i t h s o c c u r o n l y rdre]y
at Gerónimo.
a r e t y p i c a l l y a l t e r e d by m e l t i n g and o x i d a t i o n ;
four r e l a t i v e l y pristine
p l e s , h o w e v e r , h a v e been a n a l y z e d . M i n e r a l o g i c a l l y , ulites.
T h o s e t h a t h a v e been f o u n d sam
these are two-pyroxene gran-
T h e y a r e composed p r e d o m i n a n t l y o f f e l d s p a r ,
f o l l o w e d i n a b u n d a n c e by
p y r o x e n e ( o r t h o p y r o x e n e and c l i n o p y r o x e n e o c c u r r i n g i n a p p r o x i m a t e l y e q u a l p o r t i o n s ) and l e s s t h a n a b o u t
5% a c c e s s o r y phases
( F e T i o x i d e s and
pro
apatite).
T h e y a r e f i n e g r a i n e d , c r y s t a l s r a n g i n g f r o m 0.5 t o 1 mm i n s i z e f o r t h e m a j o r phases. E x i s t i n g data i n d i c a t e t h a t the g r a n u l i t e s are c h e m i c a l l y d i v e r s e , par ticularly
w i t h r e s p e c t t o t r a c e e l e m e n t s : S r = 570 - 813 ppm; Rb = 0.9 - 22 ppm,
Y = 12.6 - 22.7
ppm;
La = 17.6 - 24.4 ppm; Ce = 35.8 - 47.2 ppm; Yb = 1.0
ppm; Ba = 550 - 1000 ppm; H f = 0.5 - 1.8
ppm.
i n c o r p o r a t e d i n o t h e r p h a s e s , most commonly p l a g i o c l a s e .
have t h i n e x s o l u t i o n
2.3
T h e a p a t i t e i n t h e s e s a m p l e s has
a v a r i a b l e h a b i t r a n g i n g f r o m t u r b i d c r y s t a l s 0.2 mm i n s i z e t o m i n u t e , inclusions
-
clear
Pyroxenes
lamellae.
Megacrysts M e g a c r y s t s o c c u r r i n g i n GVF l a v a s i n c l u d e : pyroxene, o l i v i n e ,
spinel, apatite,
amphibole
(kaersutite),
p l a g i o c l a s e and a n o r t h o c l a s e .
o f t h e s e phases c a n be f o u n d i n n e a r l y e v e r y f l o w i n t h e v o l c a n i c f i e l d . on t h e s i m i l a r i t i e s eral phases, i t
Al-clino-
One o r more Based
i n c o m p o s i t i o n b e t w e e n t h e m e g a c r y s t s and t h e x e n o l i t h m i n
is believed that the m a j o r i t y of these are d i s r u p t e d
A s i m i l a r c o n c l u s i o n was r e a c h e d by Wass
(1979).
xenoliths.
75 MINERAL CHEMISTRY Clinopyroxenes from Type I x e n o l i t h s concentrations
and i s o t o p i c
are Cr-diopsides.
characteristics
H o w e v e r , based on REE
T y p e I c l i n o p y r o x e n e s can be s e g r e
g a t e d i n t o two g r o u p s . T y p e l a c l i n o p y r o x e n e s h a v e LREE d e p l e t e d T y p e l b c l i n o p y r o x e n e s a r e LREE e n r i c h e d . F i g u r e 1. for
intermediate
types of patterns.
from other
localities
and P r i n z ,
1978).
Similar
REE s i g n a t u r e s
(0.70242 - 0.70470) and Nd (0.51328 - 0.51260) vs.
while
arbitrary reported
San C a r l o s ,
Frey
an e x t r e m e r a n g e i n S r
isotopic
composition.
On an Σ|^^
diagram, c l i n o p y r o x e n e separates o v e r l a p w i t h the "mantle a r r a y "
lie within 2.
is
h a v e been
( D r e i s e r W e i h e r , S t o s c h and S e e k , 1980;
T y p e s l a and l b n o d u l e s e x h i b i t
patterns
The d i v i s i o n
t h e q u a d r a n t c h a r a c t e r i z e d by m a n t l e d e p l e t e d
Thus, for
Concomitant w i t h t h i s enrichment
is a systematic v a r i a t i o n i n most a s p e c t s
in the C r / A l
i n LREE r e l a t i v e
variations
a r e not
I
I
La C e
I I Nd
I I I I I I I Sm E u
REE ATOMIC
Yb Lu
NUMBER
However,
clinopyroxene)
apparent. a r e LREE e n r i c h e d ,
I I I
Tb
ele
t o HREE
i n t h e c l i n o p y r o x e n e . F i g u r e 3.
( i . e . M g / ( M g + F e ) , t e x t u r e and modal abundance o f
I n common w i t h T y p e l b p y r o x e n e s . T y p e I I A l - a u g i t e s
10"
and Figure
t h e LREE e n r i c h e d T y p e l b c l i n o p y r o x e n e s , i s o t o p e s and t r a c e
ments a r e d e c o u p l e d .
systematic
in l i g h t REE,
I I I I La Ce
Nd
SmEu
REE ATOMIC
I
I
I
but
I Yb L u
Tb
NUMBER
F i g u r e 1. a ) REE c o n c e n t r a t i o n s i n T y p e l a c l i n o p y r o x e n e s n o r m a l i z e d t o c h o n d r i t i c abundances. Samples p l o t t e d i n c l u d e : • Z H l , X 2 0 - 1 0 H , # 2 0 - 9 , φ 2 3 B - 5 , A 23B-2. b) REE c o n c e n t r a t i o n s i n T y p e lb c l i n o p y r o x e n e s n o r m a l i z e d t o c h o n d r i t i c abundances. Samples p l o t t e d i n c l u d e : · 21-6, • ENOl, A 2 0 - 1 2 , φ Z H l . ZHl i s a t r a n s i t i o n a l sample based on t h e c r i t e r i a o u t l i n e d i n t h i s p a p e r , ( s e e F i g u r e 3) and has been p l o t t e d w i t h b o t h T y p e l a and T y p e l b . Dashed l i n e i n d i cates i n s u f f i c i e n t data to c o n f i d e n t l y determine t r e n d of REE.
76
.5133
GERÓNIMO VOLCANIC FIELD A R I Z O N A U.S.A
DEPLETED MANTLE
•ö Ζ •σ Ζ CO
M I N E R A L S IN SPINEL L H E R Z O L I T E INCLUSIONS
.5130 HOST ALKALINE MAGMAS " 1A · ' kDIOPSIDE IB Θ Δ AMPHIBOLE • MICA
.5127 Bulk E a r t h
_L 0.702
J0.703
ENRICHED MANTLE 0.704
0.705
0.706
^Sr/^^Sr
F i g u r e 2 . V a r i a t i o n i n Nd and S r i s o t o p i c c o m p o s i t i o n i n m i n e r a l s s e p a r a t e d f r o m s p i n e l I h e r z o l i t e ( T y p e l a and T y p e l b ) i n c l u s i o n s and a l k a l i b a s a l t s f r o m t h e Gerónimo V o l c a n i c F i e l d , A r i z o n a . Note t h a t s e v e r a l o f t h e u l t r a m a f i c x e n o l i t h s represent fragments o f subcontinental mantle v e r y d i f f e r e n t from t h e source r e gion o f t h e host b a s a l t s . Other mineral data from s e v e r a l sources (see Menzies, 1983, f o r r e f e r e n c e s ) . t h e i r c h o n d r i t e n o r m a l i z e d REE v a l u e s i n c r e a s e more s m o o t h l y f r o m h e a v y REE t o l i g h t REE and p o s s e s s a d i s t i n c t l y
c o n c a v e downward s h a p e . F i g u r e 4 .
Further
m o r e , t h e y a r e e n r i c h e d i n T Í O 2 ( 1 . 2 - 2 . 4 wt%) and d e p l e t e d i n C r ^ O ^ ( l e s s 0.1, w t % ) .
Analogous chemical d i s t i n c t i o n s
associated mineral
than
in mineral chemistry e x i s t f o r other
phases i n T y p e I and T y p e I I l i t h o l o g i e s .
n o s t i c m a j o r and t r a c e e l e m e n t s a r e s u m m a r i z e d i n T a b l e 1.
Ranges f o r d i a g
77 TABLE 1 C o n c e n t r a t i o n r a n g e s f o r s e l e c t e d m a j o r and t r a c e e l e m e n t s i n m i n e r a l s s e p a r a t e d f r o m T y p e l a . T y p e l b and T y p e I I x e n o l i t h s Type
T y p e I¿
lb
Type
II
Type la
ol i v i n e
Type
Type lb
II
clinopyroxene
Fo
88-90
87-90
74-79
Wo
46-48
45-47
44-50
Cr
62-160 (±7-27)
263 (±22)
6-577 (+2-14)
En
49-50
50-51
41-46
Fs
2-5
1-4
2485-2968 (+55-62)
2462 (+52)
232-850 (+23-51)
Ni
0.32-0.68
0.01-0.47
Cr^O^
0.45-1.04
0.84-1.34
—
—
Cr
orthopyroxene En
85-90
87-92
72
Cr
2941-1868 (+233-400)
4158 (+515)
n.d.
Ni
705-1272 (+16-27)
718 ( + 16)
n.d.
7.0-14.4
20.7-29.0 185-6230 (+16-473)
Cr Αΐ2θ3
52.8-59.4
36.1-46.5
56.3-62.7
Fe#
0.16-0.19
0.16-0.29
0.29-0.38
Ni
2775-3163 (+63-67)
1917 (+42)
336-445 (+11-33)
Ni
326-1093 (+27)
1.2-2.4
-
60-642 (+7-42) 35-163 (±3-27)
336-496 (+12-28)
Ce
2.91-5.71 6.33-19.62 10.76-17.88 (+0.48-1.38) (+0.51-1.68)(+0.77-2.72
Sm
1.41-1.62 0.60-2.93 3.47-4.97 (+0.03-0.04)(+0.01-0.05)(+0.06-0.23
Tb
0.38-0.64 0.33-0.49 0.61-0.84 (+0.03-0.10)(+0.03-0.10)(+0.04-0.07
Yb
1.68-1.85 0.56-1.79 1.64-2.20 (+0.15-0.27)(+Γ).06-0.15) (±0.17-0.57
spinel Cr203
5-9
Ti0,_,
Cr/Al
.019-.102
.097-.181
0-0.015
Note: C r , N i , C e , Sm, T b and Yb v a l u e s a r e g i v e n i n ppm: 2σ e r r o r g i v e n i n p a r e n t h e s e s . C o n c e n t r a t i o n s d e t e r m i n e d by I n s t r u m e n t a l N e u t r o n A c t i v a t i o n A n a l y s i s at NASA-JSC, Houston, T X . C r 2 0 3 , A l ^ O ^ and T Í O 2 v a l u e s a r e g i v e n i n w t / o : c o n c e n t r a t i o n s d e t e r m i n e d bv e l e c t r o n m i c r o p r o b e a t S o u t h e r n M e t h o d i s t U n i v e r sity, Dallas, TX. . F e ^ ^ / ( F e ^ ^ + Mg)V A R I A T I O N S IN MINERAL CHEMISTRY OF COMPOSITE Figure 5 i l l u s t r a t e s
XENOLITHS
the major element compositional
variation
in clinopyrox
ene w i t h d i s t a n c e f r o m t h e c o n t a c t b e t w e e n h o s t and v e i n f o r e a c h t y p e o f c o m posite relationship.
A l t h o u g h T y p e I / T y p e I c o m p o s i t e s show s l i g h t l y
Al^O^ in the vein r e l a t i v e to the host, h o s t and v e i n .
Type I I / T y p e
b u t show d i s t i n c t l y the host.
elevated
most m a j o r e l e m e n t s a r e i d e n t i c a l
in
I composites have u n i f o r m Mg/(Mg+Fe^^) and Ti02>
e l e v a t e d A l ^ O ^ and d e p l e t e d C r ^ O ^ i n t h e v e i n r e l a t i v e
Type I I / T y p e
to
I I c o m p o s i t e s e x h i b i t s i m i l a r AI2O3 and C r ^ O ^ t r e n d s ,
b u t a l s o show a m a r k e d d e c r e a s e i n M g / ( M g + F e ^ ^ ) and i n c r e a s e i n T i O - , i n t o t h e +2 vein.
Notice that the Type I I / T y p e
I I composite i s l o w e r i n Mg/(Mg+Fe
) and
C r ^ O ^ , b u t e l e v a t e d i n T Í O 2 f o r b o t h h o s t and v e i n r e l a t i v e t o t h e o t h e r c o m p o s -
78
ω c (D Χ
0-20
^
0-15
DEPLETED <
*ENRICHED
O i-
•
o
·
c
ΰ
0
•
10
c
<
•
1
• •
005
υ 1 10
.
_ ^
20
30
F i g u r e 3. R a t i o o f C r / A l c a t i o n s in recalculated pyroxene analyses ( r e c a l c u l a t i o n p r o c e d u r e o f Cam e r o n and P a p i k e , 1981) v s . Ce/Sm (chondrite normalized). A value o f Ce/Sm = 1 i n d i c a t e s no e n r i c h ment o f Ce r e l a t i v e t o Sm. T y p e l a c l i n o p y r o x e n e s h a v e Ce/Sm v a l ues l e s s t h a n 1; T y p e l b c l i n o p y r o x e n e s h a v e Ce/Sm g r e a t e r t h a n 1. N o t e t h a t as c l i n o p y r o x e n e s become p r o g r e s s i v e l y e n r i c h e d i n LREE t h e y a r e a l s o p r o g r e s s i v e l y enriched in C r / A l .
C e ^ / S m ^ in c l i n o p y r o x e n e
i t e t y p e s . C l i n o p y r o x e n e does n o t o c c u r i n t h e amphibole s e l v a g e s . H o w e v e r , t h e m i n e r a l o g i c c h a n g e s and c o m p o s i t i o n a l
variations
i n t h e host I h e r z o l i t e
pyroxenes w i t h d i s t a n c e from t h e s e l v a g e boundary a r e s i g n i f i c a n t .
clino
W i t h i n 2 mm
of t h e amphibole s e l v a g e c l i n o p y r o x e n e i s c o m p l e t e l y r e p l a c e d by amphibole and amphibole p a r t i a l l y
r e p l a c e s c l i n o p y r o x e n e o r c r y s t a l l i z e s as d i s c r e t e
t i a l c r y s t a l s f o r a d i s t a n c e o f a t l e a s t 2 cm away f r o m t h e s e l v a g e . tallization
intersti The c r y s
o f amphibole w i t h i n t h e I h e r z o l i t e g r e a t l y e n r i c h e s t h e w a l l
rock
i n REE and o t h e r i n c o m p a t i b l e e l e m e n t s ( M e n z i e s a n d M u r t h y , 1 9 8 0 ) . H o w e v e r , n o t i c e t h a t t h e h o s t c l i n o p y r o x e n e d e c r e a s e s i n A l ^ O ^ and T i O ^ a s w e l l a s C r ^ O ^ toward t h e s e l v a g e .
Furthermore, Cr/Al
Comparison o f these C r / A l
r a t i o d e c r e a s e s f r o m 0.056 t o 0 . 0 5 0 .
t r e n d s w i t h t h e v a l u e s f o r T y p e l a and T y p e l b c l i n o
pyroxenes i n F i g u r e 3 demonstrates t h a t t h e metasomatic e f f e c t
on t h e h o s t
l h e r z o l i t e associated w i t h amphibole c r y s t a l l i z a t i o n cannot account f o r t h e d i s -
F i g u r e 4 . REE c o n c e n t r a t i o n s i n Type I I clinopyroxenes normalized to c h o n d r i t i c abundances. Sam ples plotted include: ^ 21-M2, # 2 2 - 1 , A 22-11, • 23B-7, • 20-8. 23B-7 a n d 20-8 a r e a n h y d r o u s T y p e I I x e n o l i t h s ; 22-1 and 22-11 a r e k a e r s u t i t e p e r i d o t i t e xenoliths. 21-M2 i s a c l i n o p y r o x ene m e g a c r y s t .
10"
I I I I I I I I I I I I I I LaCe
Nd
SmEu
REE ATOMIC
Tb
NUMBER
Yb Lu
79
CPX
VEIN
CPX
HOST
M g / ( M g + Fe*2) 0-9
08
8-0
A I 2 0 3
70
Q
X O
6.0
10
C r p 3
05
-••-••^•-Λί'
0 2 0
LTiO, I I
10
-4-
JL 20
1 0
DISTANCE
FROM
0
HOST/VEIN
1-0
BOUNDARY
F i g u r e 5. M a j o r e l e m e n t c o m p o s i t i o n a l v a r i a t i o n i n c l i n o p y r o x e n e w i t h d i s t a n c e from h o s t / v e i n boundary in composite x e n o l i t h s . Samples p l o t t e d i n c l u d e : · 20-9 ( T y p e I / T y p e I ) ; A 20-10 ( T y p e I I / T y p e I ) ; • 21-8 ( T y p e I I / T y p e I I ) ; • 22-16 ( k a e r s u t i t e p e r i d o t i t e s e l v a g e on T y p e I s p i n e l I h e r z o l i t e ) . t i n c t major element m i n e r a l c o m p o s i t i o n o b s e r v e d i n T y p e lb
xenoliths.
I n c o n t r a s t t o t h e d i f f e r i n g m a j o r e l e m e n t v a r i a t i o n s b e t w e e n h o s t and v e i n , the incompatible
r a r e e a r t h elements are e s s e n t i a l l y i d e n t i c a l
f o r a g i v e n phase
i n b o t h h o s t and v e i n r e g a r d l e s s o f c o m p o s i t e x e n o l i t h t y p e . F i g u r e 6.
Compar
i s o n o f F i g u r e 6 w i t h F i g u r e s 1 and 4 d e m o n s t r a t e t h a t c l i n o p y r o x e n e s f r o m sam p l e 21-8 ( a T y p e I I / T y p e I I ) liths.
h a v e REE v a l u e s l i k e t h o s e o f normal T y p e I I
C l i n o p y r o x e n e s f r o m 20-9
s i g n a t u r e l i k e Type la I h e r z o l i t e c l i n o p y r o x e n e s . i t e 20-10
xeno
(a T y p e I / T y p e I ) h a v e a l i g h t REE d e p l e t e d I n t e r e s t i n g l y e n o u g h , compos
(a T y p e I I / T y p e I ) has REE c o n c e n t r a t i o n s i n t e r m e d i a t e b e t w e e n t h e s e
two t y p e s . Most i m p o r t a n t l y ,
however, w i t h i n a n a l y t i c a l
for a given host/vein pair are e s s e n t i a l l y
e r r o r , t h e REE p a t t e r n s
identical.
Comparison o f compatible t r a c e elements r e v e a l s s i m i l a r d i s t i n c t i o n s
between
h o s t and v e i n t o t h o s e s e e n i n t h e m a j o r e l e m e n t v a r i a t i o n s . C r i n p y r o x e n e and
80 s p i n e l and Ni i n o l i v i n e and s p i n e l a r e t h e most u s e f u l d i s c r i m i n a t o r s
because
t h e r a n g e s o f c o n c e n t r a t i o n s f o r t h e s e e l e m e n t s t e n d t o be d i s t i n c t f o r T y p e and T y p e I I x e n o l i t h s i n t h e s e p h a s e s . show t h a t f o r e a c h c o m p o s i t e t y p e , Type I / T y p e I composite 20-9, considered identical
C r and N i v a l u e s t a b u l a t e d
w i t h i n the a n a l y t i c a l
is small;
e r r o r on t h e d a t a .
is l a r g e r than expected f o r l i t h o l o g i e s
element mineral c o m p o s i t i o n s . A l t h o u g h i t trations
In
h o s t and v e i n c o u l d be In c o n t r a s t ,
m a g n i t u d e o f t h e v a r i a t i o n b e t w e e n h o s t and v e i n i n T y p e I I / T y p e 21-8
in T a b l e 2
v e i n m i n e r a l s h a v e l o w e r C r and N i .
the d i f f e r e n c e
II
w i t h s i m i l a r m i n e r a l o g i e s and m a j o r
m i g h t be a r g u e d t h a t t h e h i g h c o n c e n
o f C r i n c l i n o p y r o x e n e and Ni i n o l i v i n e r e s u l t f r o m
fractionation compatible
elements are r e l a t i v e l y high in the m e l t , the Cr c o n c e n t r a t i o n in s p i n e l o r d e r o f magnitude g r e a t e r than t h a t o b s e r v e d in any o t h e r Type I I is,
however, e a s i l y w i t h i n
l i e v e that Type I I / T y p e interaction
the range f o r Type I s p i n e l s .
i s an
xenoliths.
We, t h e r e f o r e , b e
I I c o m p o s i t e x e n o l i t h s f o r m as t h e r e s u l t o f e x t e n s i v e
b e t w e e n b a s a n i t i c m e l t and s p i n e l
I h e r z o l i t e wall
Such com
that interaction
to e x t e n s i v e m o d i f i c a t i o n
o f the upper mantle i n v o l v i n g enrichment in
N a , REE and p o s s i b l y Ca and A l .
between w a l l
rock.
posite xenoliths indicate
r o c k and magma may l e a d Fe, T i ,
I n c o m p e n s a t i o n , t h e magma ( a s r e p r e s e n t e d by
t h e v e i n a s s e m b l a g e s ) must a t l e a s t l o c a l l y become e n r i c h e d i n
Mg, C r and Ni
through chemical exchange w i t h the rock I h e r z o l i t e . • o • Δ • o
I
I I 21-8 21-8 20-10 20-10 20-9 20-9
the
composite
d u r i n g t h e e a r l y s t a g e s o f c r y s t a l l i z a t i o n when t h e c o n c e n t r a t i o n s o f
It
I
I I I I I I I HOST VEIN HOST VEIN HOST VEIN
I I I
p r e s e n t e d by I r v i n g
wall
Geochemical e v i d e n c e (1980) documents s i m
i l a r t r e n d s , a l t h o u g h f o r samples r e c o r d ing f a r l e s s i n t e r a c t i o n composites).
(Type I I / T y p e
The extensive
r e c o r d e d i n c o m p o s i t e x e n o l i t h 21-8 gests that greater interaction wall
I
modification sug
between
r o c k and a s c e n d i n g magma has
oc
c u r r e d i n t h e m a n t l e b e n e a t h GVF t h a n indicated
by s a m p l e s f r o m K i l b o u r n e H o l e .
F i g u r e 6. REE c o n c e n t r a t i o n s i n clinopyroxenes from composite xenoliths. 21 - 8 i s a T y p e I I / T y p e I I c o m p o s i t e ; 21 - 10 i s a Type I I / T y p e I composite; 20-9 is a Type I / T y p e I composite. REE ATOMIC
NUMBER
81 TABLE 2 C r and N i a b u n d a n c e s i n c l i n o p y r o x e n e , o l i v i n e and s p i n e l f r o m a s s o c i a t e d h o s t and v e i n m i n e r a l a s s e m b l a g e s o f composite x e n o l i t h s VEIN
HOST clinopyroxene
olivine
spinel Type I/Type
Cr
5881+587
Ni
386+19
89+11 2812+62
Cr
3747+190 378+10
141+13
3282+397
Ni
207+20
spinel
366+23
2844+62
n.d. n.d.
I , 20-10
52500+3990
642+42
577+47
5205+396
3163+66
357+10
643+14
3301+69
2639+55
90+10
62+7
4952+572
2857+63
Type I I / T y p e Cr
oli vine
I , 20-9
67360+3970
Type I I / T y p e
Ni
clinopyroxene
II ,
21-8
65465+6462
373+37
10+1
4374+421
1629+46
125+16
848+23
1093+27
1454+33
C o n c e n t r a t i o n s + 1σ a r e g i v e n i n ppm
DISSCUSSION AND SUMMARY P e t r o g r a p h i c o b s e r v a t i o n s combined w i t h m a j o r e l e m e n t , t r a c e e l e m e n t and i s o t o p i c data from x e n o l i t h s o f the Gerónimo V o l c a n i c F i e l d suggest a m u l t i s t a g e e v o l u t i o n f o r the mantle beneath the s o u t h w e s t e r n U . S . h a r z b u r g i t e s and c l i n o p y r o x e n e - d e p l e t e d s p i n e l
T h e dominance o f T y p e
I
I h e r z o l i t e s a t GVF s u p p o r t t h e
occurrence of a major widespread p a r t i a l m e l t i n g event t h a t produced a d e p l e t i o n i n the l i g h t REE, perhaps r e l a t e d t o c o n t i n e n t
formation.
E x t r a c t i o n of melt
(> 1 b . y . ) l e a v e s a I h e r z o l i t e r e s i d u e e v i d e n t as T y p e l a i n c l u s i o n s
(Menzies
and M u r t h y , 1980b; M e n z i e s , e t a l . , 1982; F r e y and G r e e n , 1 9 7 4 ) ; c l i n o p y r o x e n e s in T y p e la nodules have a r e f r a c t o r y major element c o m p o s i t i o n , pleted p r o f i l e s ,
l i g h t REE d e
r a d i o g e n i c Nd (0.5133 - 0 . 5 1 3 0 ) , and n o n r a d i o g e n i c S r ( 0 . 7 0 2 -
0.703) i s o t o p i c
c o m p o s i t i o n s . F i g u r e s l a and 2 .
time-integrated
r e s p o n s e t o t h e i n c r e a s e i n Sm/Nd.
Type lb x e n o l i t h s are enigmatic
in o r i g i n .
The isotopic
data r e p r e s e n t a
T h e i r LREE e n r i c h e d p r o f i l e s
can
n o t be a c c o u n t e d f o r as a r e s i d u e f r o m p a r t i a l m e l t i n g b a s e d on c u r r e n t k n o w l edge o f p a r t i t i o n c o e f f i c i e n t s . t r a t i o n o f a CO2 + H2O r i c h f l u i d
Mantle enrichment
(>0.5 b . y . ) caused by i n f i l
( a n a l o g o u s t o component Β p r o p o s e d b y F r e y and
G r e e n , 1974) i s one p o s s i b l e e x p l a n a t i o n f o r t h e s e i n c l u s i o n s . T h i s metasomatism r e s u l t s
infiltration
i n t h e l i g h t REE e n r i c h m e n t o f t h e c l i n o p y r o x e n e p h a s e , b u t
p r o d u c e s no p e t r o g r a p h i c o r m i n e r a l o g i c t r a n s f o r m a t i o n o f t h e p e r i d o t i t e s .
The
82 c o n s i d e r a b l e r a n g e i n Nd ( 0 . 5 1 2 8 - 0.5125) and S r ( 0 . 7 0 3 - 0 . 7 0 5 ) i s o t o p i c position visible
in Type lb nodules ( i . e . modified Type l a ) r e p r e s e n t s a p a r t i a l
r e s p o n s e t o t h e l o w Sm/Nd r a t i o .
However, a d i s t i n c t i v e major element
i s s u g g e s t e d by t h e c o r r e l a t i o n b e t w e e n t h e d e g r e e o f LREE e n r i c h m e n t and C r / A l
ratios
element mineral
com
in clinopyroxenes.
Furthermore, the modifications
composition which r e s u l t
control (Ce/Sm)
to major
i n a s s o c i a t i o n w i t h h y d r o u s and a n h y
drous v e i n i n g a r e not a p p r o p r i a t e to produce the d i s t i n c t i v e
m a j o r e l e m e n t com
position of Type lb x e n o l i t h s . Nonetheless, interaction
b e t w e e n T y p e l a and T y p e l b
s i l i c a t e m e l t e q u i v a l e n t i n c h e m i s t r y and i s o t o p i c n i t e produces v e i n i n g o f the I h e r z o l i t i c w a l l
(and T y p e I I ) and a
composition to the host basa-
rock.
Mineral assemblages
include
c l i n o p y r o x e n e + s p i n e l + o l i v i n e + k a e r s u t i t e + mica + a p a t i t e + f e l d s p a r . a s s o c i a t e d c o n t a c t metasomatism t r a n s f o r m s the p e r i d o t i t e
d u i t and i n some c a s e s i n t r o d u c e s a m p h i b o l e a n d / o r m i c a i n t o t h e w a l l
rock.
e v e n t i s b e l i e v e d t o be r e l a t i v e l y r e c e n t ( « 1 0 0 m . y . ) s i n c e m i c a i n t h e oy
I h e r z o l i t e has a l o w
The
adjacent to the con This
host
Of.
Sr/
S r = 0.70329 and a h i g h R b / S r r a t i o ;
similarly
the
v e i n a m p h i b o l e has a d e c o u p l e d r a r e e a r t h (Sm/Nd = 0 . 1 4 2 ) and a Nd i s o t o p i c position
com
(Σ|^|^ = + 8 ) .
The v e i n s are b e l i e v e d to r e p r e s e n t f r o z e n conduits or apophyses of melt (Menzies, et a l . ,
1982)
or composite r e l a t i o n s h i p s o f h o s t and v e i n :
a)
basanitic
t h a t s u r r o u n d a d e e p - s e a t e d magma p o c k e t .
a r e o f t h r e e t y p e s b a s e d on m a j o r e l e m e n t
Vein/host chemistries
Type I / T y p e I composites in which Type I spinel
Iherzo-
l i t e s or h a r z b u r g i t e s a r e c r o s s c u t by T y p e I w e b s t e r i t e s o r d i o p s i d e v e i n s ; b)
Type I I / T y p e I composites in which spinel
A l - a u g i t e c l i n o p y r o x e n i t e d i k e s ; and c )
I h e r z o l i t e i s c r o s s c u t by
Type
Type I I A l - a u g i t e c l i n o p y r o x e n i t e dikes crosscut w e h r l i t e hosts of Type I I element compositions.
These r e l a t i o n s h i p s
indicate
that the process of
v e i n i n g i s b o t h an a n c i e n t p r o c e s s ( T y p e I / T y p e I ) and t h a t i t s v a r i a b l e , ranging from r e e q u i 1 i b r a t i o n e x t e n s i v e major element a l t e r a t i o n the e x t e n t of m o d i f i c a t i o n
II
Type I I / T y p e I I composites in which
of incompatible
as w e l l
major
mantle
effects
are
t r a c e e l e m e n t s t o more
(Type I I / T y p e I I ) .
The c o n t r o l s
a r e u n k n o w n , b u t must depend on s u c h c o n d i t i o n s
t h e s i z e o f t h e c o n d u i t s , t h e a s c e n t r a t e o f t h e magma, t h e e x t e n t o f
on
as
modifica
t i o n e x p e r i e n c e d d u r i n g p r e v i o u s e p i s o d e s o f magma a s c e n t i n t h e same c o n d u i t and t h e t e m p e r a t u r e d i f f e r e n c e b e t w e e n t h e h o s t w a l l
r o c k and magma.
T h e magma
p o c k e t w i t h w h i c h t h e v e i n s a r e b e l i e v e d t o be a s s o c i a t e d i s p a r t o f t h e " g e n e r a t i o n o f magmas" t h a t u l t i m a t e l y
d i s r u p t s and e n t r a i n s t h e m a n t l e f r a g m e n t s .
T h e s e v e i n e d T y p e l a and T y p e l b n o d u l e s i l l u s t r a t e t h e f a c t t h a t m a n t l e
perido
t i t e s can be m o d i f i e d b y t h e v e r y p r o c e s s e s t h a t t r a n s p o r t them t o t h e s u r f a c e . T h e m a n t l e b e l o w t h e s o u t h w e s t e r n U . S . has an a n c i e n t i s o t o p i c unlike that of a residue (Type la) t e r been m o d i f i e d
l e f t a f t e r e x t r a c t i o n o f MORB.
by v a p o u r - r i c h f l u i d s and b a s a n i t i c m e l t s
signature
not
T h i s has
la
(composite
xenoliths
83 T y p e I i / T y p e I I , T y p e I I / T y p e I and T y p e I / T y p e I ) .
The a l k a l i basalts
from
GVF h a v e ^ ^ S r / ^ ^ S r = 0.70285 - 0.70327 and ^ ^ ^ N d / ^ ' ^ ^ N d = 0.51304 - 0 . 5 1 2 9 0 , F i g u r e 2.
All
the a l k a l i basalts
plot within
the f i e l d of mid-ocean r i d g e
h a v i n g been d e r i v e d f r o m a m a n t l e w i t h a t i m e - i n t e g r a t e d rare earth elements.
depletion
basalts,
of the
light
T h i s i s c o m p a t i b l e w i t h o b s e r v a t i o n s o f o t h e r B a s i n and
Range b a s a n i t e s and i m p l i e s
t h a t a MORB-type mantle e x i s t s below t h e
southwest
ern U.S. ACKNOWLEDGEMENTS T h i s r e s e a r c h has been s u p p o r t e d by t h e NASA G r a d u a t e T r a i n e e s h i p P r o g r a m and b y t h e I n s t i t u t e sity.
f o r t h e S t u d y o f E a r t h and Man, S o u t h e r n M e t h o d i s t
Univer