DOE/ID/12079-61
DOE/ID/12079--61
DE82 019951
OXYGEN-ISOTOPE GEOCHEMISTRY OF QUATERNARY RHYOLITE
UTAH, U. S. A.
FROM THE MINERAL MOUNTAINS,
_c
y
by
J. R. Bowman, S. H. Evans, J r , and W . P. Nash
March 1982
Department o f Geol ogy and Geophysics
U n i v e r s i t y o f Utah
S a l t Lake C i t y , Utah
84112
I
DISCLAIMER
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Reference t o a company name does not imply approval o r recommendation of
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exclusion of others t h a t may be suitable.
,
Department of Energy t o the
i
ACKNOWLEDGEMENTS
Financial assistance for this study was provided by the Department of
Energy, D i v i sion of Geothermal Energy Contract DE-AC07-79ET27002 and conti nued
under Contract DE-AC07-80ID12079.
I
I
We t h a n k Mr. Ray Lambert and Mr. John
Covert f o r performing the mass spectrometry a n a l y s i s of these samples.
ABSTRACT
Oxygen i sotope analyses were made 'of phenocryst and gl a s s s e p a r a t e s from
f o u r Quaternary r h y o l i t e flows and danes i n the Mineral Mountains, southwest
Utah.
With the exception of b i o t i t e i n a l l samples and a l k a l i f e l d s p a r i n the
r h y o l i t e danes, a l l minerals appear t o be i n c l o s e oxygen isotope exchange
equil ibrium.
The geothermometry equations proposed by Bottinga and Javoy
(1973) and Javoy (1977) for q u a r t z , a l k a l i f e l d s p a r and magnetite produce the
b e s t agreement w i t h temperature r e s u l t s from two-f e l d s p a r and i ron-ti t a n i urn
oxide geothermometry f o r these r h y o l i t e s .
If the r h y o l i t e s were generated by
p a r t i a l melting i n the crust, their whole-rock ( g l a s s ) 6I8O values (6.3 t o 6.9
permil ) a r e consistent w i t h generation from I-type (Chappell and White, 1974,
O'Neil and Chappell , 1977; O'Neil e t a1
., 1977) sources.
I NTRODUCTIO N
I n t h e Mineral Mountains o f southwest Utah, j u s t east o f t h e Roosevelt
.
Hot Springs thermal area, a s e r i e s o f Quaternary r h y o l i t e flows and domes
occur along t h e c r e s t o f t h e range.
The presence of young s i l i c i c volcanics
has been considered t o be an i n d i c a t o r o f t h e pyesence o f shal low magma
chambers and p o t e n t i a l heat sources f o r geothermal resources (Lipman e t a1
.,
1978; Evans and Nash, 1975, 1978), i n c l u d i n g t h e Roosevelt Hot Springs
system.
Because o f t h i s p o t e n t i a l l i n k w i t h t h e Roosevelt Hot Springs
geothermal system, chemical and p e t r o l o g i c s t u d i e s o f these v o l c a n i c s have
been made over t h e past several years t o d e f i n e petrogenesis, i n c l u d i n g P-T
c o n d i t i o n s o f c r y s t a l 1i z a t i o n and depth estimates f o r t h e magma chamber, and
t h e nature o f t h e magma source area.
These s t u d i e s (Evans and Nash, 1975,
1978; Nash and Evans, 1978) have applied several chemical geothermometers t o
phenocryst phases i n t h e r h y o l ites, incl udi ng t h e Fe-Ti oxide (Buddi ngton and
L i ndsl ey , 1964; Carmichael
, 1967;
Carmichael e t a1
., 1974) and two-fel dspar
(Stormer, 1975) geothermometers, t o d e f i n e temperature o f eruption.
The objectives of t h i s study are two-fold.
The f i r s t objective i s to
apply oxygen-i sotope geothermometers t o phenocryst phases i n t h e r h y o l it e s t o
t e s t t h e c a n p a t i b i l i t y o f i s o t o p e and chemical geothermometers, and t o t e s t
whether oxygen-isotope exchange e q u i l i b r i u m has been achieved i n r e l a t i v e l y
viscous r h y o ’ l i t i o volcanics.
The second o b j e c t i v e i s t o d e f i n e t h e S1*0 value
o f t h e r h y o l i t e magmas i n order t o place c o n s t r a i n t s on t h e source area f o r
these volcanics.
T h i s i n f o r m a t i o n w i l l h e l p evaluate whether o r n o t t h e
r h y o l i t e s were generated b y p a r t i a l m e l t i n g o f a source containing a
s i g n i f i c a n t canponent o f pel i t i c metasedimentary m a t e r i a l or one dominated by
p r i m i t i v e o r mantle-derived m a t e r i a l .
3
These two types o f source regions were
proposed by Chappell and White (1974) t o produce t h e two geochemically and
p e t r o l o g i c a l l y d i s t i n c t groups of Paleozoic g r a n i t o i d s i n southeast A u s t r a l i a
which they designated
' S-type'
(sedimentary) and ' I - t y p e '
(igneous)
.
O'Neil
and Chappel 1 (1977) subsequently demonstrated t h a t t h e S-type grani t o i d s had
h i g h e r 6l8O values than t h e I - t y p e g r a n i t o i d s .
I n t h e past several years,
several studies (Taylor and T u r i , 1976; Taylor and S i l v e r , 1978; Longstaffe,
e t a1
., 1980) have u t i l i z e d oxygen-isotope
data t o i n f e r t h e e x t e n t of
involvement o f p e l i t i c metasedimentary m a t e r i a l i n t h e f o r m a t i o n of
i n t e r m e d i a t e t o a c i d i c igneous rocks.
To these ends, oxygen-isotope analyses
were made on phenocryst and glass separates o f f o u r samples o f r h y o l i t e flows
and danes from t h e Mineral Mountains area.
GENERAL GEOLOGY
The geology o f t h e Mineral Mountains and o f t h e s i l i c i c volcanism has
been discussed i n Evans and Nash (1978) and w i l l not be repeated here.
The
Quaternary s i l i c i c volcanism was d i v i d e d i n t o t h r e e stages and types b y Evans
and Nash (1978):
1.
The e a r l i e s t stage i s recorded by o b s i d i a n flows o c c u r r i n g along
B a i l e y Ridge and Wild Horse Canyon.
c a l c u l a t i o n s o f PH20
2 3000
T h e i r macroscopic character and
bars (Evans and Nash, 1978; Nash and
Evans , 1978) suggest unusual f l u i d i t y f o r t h i s b u l k composition
.
Radiometric d a t i n g o f t h e B a i l e y Ridge f l o w i n d i c a t e s an age o f 0.77
f 0.08 m.y.
2.
(Lipman e t al.,
1978).
P y r o c l a s t i c rocks occupy an intermediate p o s i t i o n i n t h e
s t r a t i g r a p h i c sequence and a r e o f a s h - f a l l and ash-flow o r i g i n .
These rocks a r e mainly exposed i n Ranch Canyon.
K - A r d a t i n g on a
s i n g l e obsidian c l a s t from an ash-flow u n i t i n Ranch Canyon y i e l d e d
an age of 0.68 -I 0.04 m.y.,
p r o v i d i n g an o l d e r l i m i t f o r t h e age of
t h e p y r o c l a s t i c s (Lipman e t a1
., 1978) .
Recent K - A r d a t i n g of
anorthocl ase phenocrysts from these p y r o c l a s t i c rocks y i e l d s ages o f
0.61 f 0.04 and 0.66 f 0.03 m.y.
3.
(Evans and Brown, 1981).
The l a s t stage o f r h y o l i t e volcanism i s represented by a group o f a t
l e a s t 11 domal r h y o l i t e s d i s t r i b u t e d s p o r a d i c a l l y over 10 km near t h e
c r e s t o f t h e Mineral Mountains.
Age determinations on obsidian from
Bearskin Mountain and sanidine from L i t t l e Bearskin Mountain have
y i e l d e d K - A r ages of 0.58 f 0.12 m.y.
r e s p e c t i v e l y ( L i pman e t a1
and 0.53 f 0.05 m.y.,
.,-1978) .
Petrography, b u l k chemistry and t r a c e element chemistry data on these
r h y o l i t e s can be found i n Evans and Nash (1978) and Nash and Evans (1978).
For t h i s study, mineral and glass separates were made from samples o f t h e
r h y o l i t e f l o w s from B a i l e y Ridge and Wild Horse Canyon and from two o f t h e
danal r h y o l ites--Li ttl e Bearski n and North T w i n F1 a t Peak
.
ANAL YT ICAL TECHNIQUES
M i neral separates were obtained using standard heavy 1 i q u i d and magnetic
separation techniques.
Oxygen, fran s i 1 i c a t e minerals was extracted by
r e a c t i n g 5 t o 10 mg of sample w i t h 8rF5 a t 5 5 O O C i n n i c k e l r e a c t i o n vessels
f o r 1 4 hours (Clayton and Mayeda, 1963).
The evolved 02 gas was then
converted t o C02 f o r mass spectrometric a n a l y s i s by canbustion w i t h g r a p h i t e
( T a y l o r and Epstein, 1962)
.
I s o t o p i c measurements f o r C02 gas were made w i t h a Micromass 602 0 mass
spectrometer , a double c o l 1e c t o r , 90' sector magnetic def 1e c t i o n instrument o f
6 cm radius.
The i s o t o p i c data f o r oxygen are reported r e l a t i v e t o SMOW
A n a l y t i c a l e r r o r f o r oxygen isotope r a t i o s i s f 0.2 permil f o r
(Craig, 1961).
magnetite and f 0.1 permil f o r a l l o t h e r phases.
Notation
A l l i s o t o p i c data a r e reported i n t h e d e l t a notation, where
6
A = RA
- Rstd
n
x 1000.
'std
6A represents t h e 6l8O value o f sample A, and R i s t h e
sample o r standard.
180/160 r a t i o o f t h e
For c o e x i s t i n g phases A and B,
103 I n a A-B
= 6
A
- 6
B
-
-A
A-B
where a A-B i s t h e f r a c t i o n a t i o n f a c t o r , defined as
ANALYTICAL RESULTS
The oxygen-isotope canposi t i o n s measured f o r phenocryst and glass
separates fran t h e f o u r r h y o l i t e samples a r e c a n p i l e d i n Table 1.
Also
included f o r thermometry purposes i s t h e i s o t o p i c f r a c t i o n a t i o n f a c t o r , A, o r
d i f f e r e n c e i n 6l8O value, f o r t h e various p a i r s o f phases.
The minerals
e x h i b i t a c o n s i s t e n t order o f l8O enrichment, w i t h quartz (Qz)
f e l d s p a r (Pg)
>
b i o t i t e ( B i ) L m a g n e t i t e (Mt),
>
alkali
i n d i c a t i n g an approach t o
oxygen-isotope exchange e q u i l i b r i u m i n a1 1 r h y o l i t e samples.
The i s o t o p i c
f r a c t i o n a t i o n between minerals i s i l l u s t r a t e d i n t h e concordance p l o t s o f
Figures 1 and 2.
E q u i l i b r i u m isotope f r a c t i o n a t i o n f a c t o r s i n f e r r e d from
n a t u r a l data ( B o t t i n g a and Javoy, 1973, 1975; Javoy, 1977) a r e a l s o p l o t t e d i n
these f i g u r e s f o r comparison.
Concordance diagrams have been used i n previous s t u d i e s (Anderson e t a1
.,
1971; B o t t i n g a and Javoy, 1973, 1975; Clayton and Epstein, 1961; Javoy e t al.,
1970) t o ill u s t r a t e systematic c o r r e l a t i o n s o f oxygen-isotope data and t o
evaluate g r a p h i c a l l y t h e degree o f a t t a i r m e n t o f i s o t o p e exchange e q u i l i b r i u m
between mineral phases.
I n such p l o t s , a l l mineral p a i r s t h a t have
d i f f e r e n c e s i n dl8O values equal t o those defined b y t h e f r a c t i o n a t i o n
equations a r e i n t e r p r e t e d as having achieved i s o t o p e exchange e q u i l i b r i u m .
However, t h e r e i s disagreement between t h e experimentally and e m p i r i c a l l y
determined f r a c t i o n a t i o n f a c t o r s f o r many mineral pairs, i n c l u d i n g quartzp l agi o c l ase and quartz-magneti t e pai r s , which exceed a n a l y t i c a l e r r o r over
much o f t h e temperature range 300-10OO0C.
I n these cases, a conclusion
r e g a r d i ng attainment o f isotope exchange e q u i l i b r i u m depends on which
f r a c t i o n a t i o n equations a r e chosen.
Many o f t h e oxygen-isotope f r a c t i o n a t i o n s
based on experiment, i n c l u d i n g quartz and plagioclase, y i e l d temperatures
d i f f e r e n t from those based on empirical cal i b r a t i o n s .
The empirical
f r a c t i o n a t i o n s appear t o have achieved b e t t e r success a t p r o v i d i n g
g e o l o g i c a l l y reasonable temperatures ( B o t t i n g a and Javoy, 1973, 1975; Javoy,
1977) and greater i n t e r n a l consistency among temperatures from i n d i v i d u a l
mineral pairs, and a r e p r e f e r r e d here on t h i s basis.
However, i t i s n o t c l e a r
whether t h e observed c o r r e l a t i o n s o f oxygen-i sotope data from rock samples
r e s u l t s o l e l y fran f r o z e n - i n e q u i l i b r i u m thermal e f f e c t s .
B o t t i n g a and Javoy
(1975) noted t h a t t h e data from rock samples contained s c a t t e r which they
i n t e r p r e t e d as departures f r a n i s o t o p e exchange e q u i l ibrium.
Dienes (1977)
has cautioned t h a t n a t u r a l oxygen-i sotope c o r r e l a t i o n s f o r mineral p a i r s may
b e a d d i t i o n a l l y i n f l u e n c e d by random e r r o r s o r systematic r e t r o g r a d e e f f e c t s ,
p o s s i b l y re1 ated t o c o o l i n g o r i n c i p i e n t hydrothermal processes.
There a r e
thus u n c e r t a i n t i e s i n b o t h t h e experimental and e m p i r i c a l l y derived oxygeni s o t o p e f r a c t i o n a t i o n s f o r many mineral p a i r s which warrant some c a u t i o n i n
t h e i r application.
Despite t h e u n c e r t a i n t i e s i n t h e empirical c a l i b r a t i o n s ,
t h e r e i s good c o r r e l a t i o n o f 6180 values f o r natural quartz-pl agiocl asemagnetite t r i p l e t s ( B o t t i n g a and Javoy, 1973, 1975; Deines, 1977; Javoy,
1977). Further, t h e geothermometry equations based on these c o r r e l a t i o n s have
o f t e n produced g e o l o g i c a l l y reasonable temperatures ( B o t t i n g a and Javoy, 1973,
1975), which i s j u s t i f i c a t i o n f o r t h e i r continued use.
The Sl80 values of quartz, p l a g i o c l a s e and magnetite from samples 22 and
25 p l o t w i t h i n a n a l y t i c a l e r r o r o f t h e empirical concordance l i n e s i n Figures
1A and l B , suggesting t h a t these minerals are i n oxygen-isotope exchange
equilibrium.
Sample 24 p l o t s somewhat away from t h e concordance l i n e i n b o t h
Figures 1 A and 16.
Sample 24 p l o t s below t h e Qz-Mt/Qz-Pg concordance l i n e and
above t h e Qz-Mt/Pg-Mt
1 ine, i n d i c a t i n g t h a t t h e *(Qz-Pg) f r a c t i o n a t i o n f a c t o r
i s t o o l a r g e and t h e A(Pg-Mt) f a c t o r i s t o o small r e l a t i v e t o t h e A(Qz-Mt)
factor.
These discrepancies i n d i c a t e t h a t t h e Sl8O value o f p l a g i o c l a s e i s
t o o low f o r t h e phase t o be i n i s o t o p e exchange e q u i l i b r i u m w i t h quartz and
~
magnetite.
P i a g i o c l ase
n sample 24 has s u f f e r e d a t l e a s t minor sub-sol idus
oxygen-i sotope exchange , a f e a t u r e o f t e n observed i n igneous rocks ( B o t t i n g a
-
and Javoy, 1973, 1975; 0 N e i l and Taylor, 1967; Taylor, 1968, 1971, 1974).
Both samples 22 and 24 p l o t s i g n i f i c a n t l y o f f t h e concordance l i n e i n
Figure 2.
Because quartz and plagioclase i n sample 22 a r e i n approximate
oxygen-i sotope exchange e q u i l i b r i u m , then e i t h e r t h e b i o t i t e i n sample 22 has
not e q u i l i b r a t e d isotopical l y w i t h t h e quartz and p l a g i o c l a s e o r t h e
f r a c t i o n a t i o n f a c t o r used f o r t h e concordance l i n e i n Figure 2 does not
represent e q u i l i b r i u m f r a c t i o n a t i o n f o r b i o t i t e .
None o f t h e o t h e r proposed
b i o t i t e f a c t o r s (Table 2 ) produce concordance e i t h e r , so t h e b i o t i t e may
simply n o t be i n isotope exchange e q u i l i b r i u m .
Assuming t h a t t h e A(Qz-Si)
f a c t o r i s c o r r e c t , b i o t i t e would a l s o be o u t o f exchange e q u i l i b r i u m w i t h
quartz and plagioclase i n sample 24, because t h e p o i n t p l o t s above t h e
concordance 1ine, and i t has j u s t been demonstrated (Figures 1 A and 1B) t h a t
h(Qz-Pg) i s t o o l a r g e f o r sample 24.
These f i g u r e s demonstrate t h a t b i o t i t e
i n b o t h samples 22 and 24 and plagioclase i n sample 24 a r e depleted i n l 8 0
r e l a t i v e t o 6l80 values i n c o e x i s t i n g quartz and magnetite.
Apparently b o t h
b i o t i t e samples have e i t h e r not e q u i l i b r a t e d w i t h t h e magma, o r not maintained
e q u i l i b r i u m w i t h t h e magma, o r have undergone post-solidus i n t e r a c t i o n w i t h an
external , 180-depleted oxygen r e s e r v o i r , presumably meteoric water.
Hydrogen-
isotope analyses would be useful i n d e t e c t i n g 1 i m i t e d meteoric water-rock
i n t e r a c t i o n , b u t s u f f i c i e n t q u a n t i t i e s o f b i o t i t e f o r hydrogen-isotope
a n a l y s i s c o u l d not be separated.
I SOT0PE THERMOMETRY
The attainment of i n t e r n a l oxygen-isotope exchange e q u i l i b r i u m , as
i n d i c a t e d b y t h e concordance diagrams, does not n e c e s s a r i l y mean t h a t t h e
phenocryst phases have preserved t h e i r o r i g i n a l c r y s t a l l i z a t i o n
temperatures.
The concordance diagram allows an e v a l u a t i o n o f exchange
e q u i l i b r i u m and i d e n t i f i c a t i o n o f exchange w i t h external r e s e r v o i r s .
It does
n o t a1 1ow eval u a t i o n of t h e primary nature o f recorded temperatures, as shown
by numerous examples o f disagreement between i s o t o p e and chemical o r phase
e q u i l i b r i u m thermometers ( B o t t i n g a and Javoy, 1975; Javoy, 1977).
Experience
over t h e l a s t t e n t o f i f t e e n years w i t h oxygen-isotope geothermometry has
revealed t h a t i n slow-cool ing systems (metamorphic and p l u t o n i c rocks),
coexi s t i n g s i 1 i c a t e / o x i d e phases a r e vu1 nerabl e t o retrograde isotope
exchange.
Such exchange can occur under near-equil i b r i u m c g n d i t i o n s ,
produci ng concordant sl8O Val ues f o r mineral p a i r s b u t i s o t o p e temperatures
t h a t a r e lower than t r u e formation temperatures.
Fortunately, @O
values are
cmmonly "frozen" i n minerals formed i n r a p i d l y cooled basic volcanic rocks
(Anderson e t al.,
1971). L i t t l e i n f o r m a t i o n i s a v a i l a b l e f o r more a c i d i c
v 01 canic rocks , however.
A second problem w i t h oxygen-isotope geothermometry i s t h e l a c k o f
agreement as t o which geothermometer equation t o use.
Table 2 i s a
canpi1 a t i o n o f several geothermometer equations proposed f o r t h e mineral s
found i n t h e Mineral Mountains r h y o l i t e s o r computed f r a n a v a i l a b l e mineralwater f r a c t i o n a t i o n expressions.
The various equations are based on b o t h
experimental measurement [ f o r example Qz-Pg ( A ) ]
and empirical c a l i b r a t i o n o f
measured 6l80 values i n igneous and metamorphic rocks [ f o r example Qz-Mt (B)].The r e s u l t s o f a l l geothermometry equations are presented i n Table 3.
Unconstrained use of a1 1 geothermometry 'equations produces unacceptably 1 arge
ranges i n i s o t o p e temperatures.
For example t h e t h r e e a l t e r n a t e Q
-&
geothermometers g i v e a temperature range o f 344 t o 765OC f o r p r e c i p i t a t i o n o f
q u a r t z and p l a g i o c l a s e i n sample 22, with t h e lower ranges o f temperature
being f a r below t h e s o l i d u s f o r magma o f r h y o l i t e composition.
However some
o f t h e isotope geothermometers produce temperatures i n good agreement w i t h
those from independent chemical geothermaneters.
For sample 22, t h e
geotherometry equations proposed by B o t t i n g a and Javoy (1973, 1975) provide
temperatures o f 765OC, 724OC, and 725OC from quartz-pl agiocl ase, quartzmagnetite, and p l agioclase-magneti t e p a i r s respectively.
These temperatures
agree w i t h i n a n a l y t i c a l u n c e r t a i n t y w i t h t h e r e s u l t s o f Fe-Ti oxide ( 7 4 O O C )
and two-feldspar (77OOC) geothermometry (Evans and Nash, 1978) f o r t h i s sample
(Table 4).
The consistency o f these t h r e e i s o t o p e temperatures r e f l e c t s t h e
concordance of t h e mineral 6180 values w i t h t h e f r a c t i o n a t i o n f a c t o r s shown i n
Figures 1 A and l B , which were defined by t h e same geothermometry equations of
B o t t i n g a and Javoy (1973, 1975).
The agreement between t h e isotope and
chemical geothenometry r e s u l t s and t h e f a c t t h a t these temperatures a r e
geological l y q u i t e reasonable suggests these c l o s e l y represent f o r m a t i o n
( c r y s t a l l i z a t i o n ) temperatures f o r these phases i n sample 22.
These same
geothermometer equations produce concordant temperatures f o r sample 25 as
we1 1
.
However, t h e temperatures provided by t h e B o t t i n g a and Javoy 1973)
equations f o r quartz-pl a g i o c l ase, quartz-magnetite,
and p l agioclase-magneti t e
p a i r s fran sample 24 d i f f e r by an amount exceeding a n a l y t i c a l uncertainty.
The quartz-pl agiocl ase geothermometer ( B y Tab1 e 2) equation i n c o r p o r a t i n g
t h e experimental data of O'Neil and Taylor (1967) i s s i m i l a r t o t h a t proposed
by B o t t i n g a and Javoy (1973) and produces q u i t e s i m i l a r temperature r e s u l t s
(Table 3).
However, equation ( C ) f o r t h i s mineral p a i r produces temperatures
e i t h e r lower (samples 22 and 25) o r higher (sample 24) than t h e o t h e r two
versions.
The quartz-magnetite geothermmeter based on t h e experimental data
o f C1ayton e t a1
. (1972) f o r quartz produces temperatures systematical l y
higher than t h e o t h e r i s o t o p e geothermometers ( w i t h t h e exception o f those
i n c o r p o r a t i n g b i o t i t e ) and t h e Fe-Ti oxide and two-feldspar geothermometers.
A1 1 proposed i s o t o p e geothermometers in v o l v i ng b i o t i t e g i v e unacceptabl e o r
even i r r a t i o n a l values f o r temperature, i n c l u d i n g those based wholly o r i n
p a r t on t h e c a l i b r a t i o n s o f B o t t i n g a and Javoy (1973)
(B
and
E, Table 3).
T h i s disagreement i n d i c a t e s e i t h e r t h a t b i o t i t e i s out o f isotope exchange
e q u i l i b r i u m w i t h t h e o t h e r phenocrysts, o r t h a t none o f t h e proposed
geothenometer equations adequately accounts f o r t h e oxygen-isotope
>
I
.
c h a r a c t e r i s t c s o f magmatic b i o t i t e s .
One f a c t o r t h a t c e r t a i n l y needs t o be
evaluated i n t h i s regard i s t h e e f f e c t of Fe f o r Mg s u b s t i t u t i o n on t h e
oxygen-isotope composition o f b i o t i t e .
Temperature r e s u l t s f ran t h e geothermometers proposed by B o t t i nga and
Javoy (1973) a r e presented i n Table 4 f o r a l l f o u r samples, along w i t h r e s u l t s
from a p p l i c a t i o n o f chemical geothermometers (Evans and Nash, 1978) f o r
canparison.
-
and
There i s good agreement between t h e chemical and
geothermaneters f o r samples 22 and 25.
The
& - &,-&- Mt
&-
p a i r from
sample 23 provides a geological l y reasonabl e temperature (762OC), b u t no o t h e r
geothermometry data e x i s t f o r comparison.
The
& - & pair
from sample 24
provides a temperature (682) i n s a t i s f a c t o r y agreement w i t h t h e Fe-Ti oxide
and two-feldspar geothermmetry, b u t b o t h t h e Q
significantly different.
- & and & -
r e s u l t s are
I n sample 24, p l a g i o c l a s e has apparently experienced
some retrograde exchange and i s no longer i n i s o t o p i c exchange e q u i l i b r i u m
w i t h quartz and magnetite.
F o r t h e o t h e r samples, t h e agreement o f t h e
chemical geothermometers w i t h t h e r e s u l t s from t h e geothermometer equations
proposed by B o t t i n g a and Javoy (1973) and Javoy (1977) supports t h e c o n t e n t i o n
of t h e l a t t e r author t h a t these geothermmeters are i n t e r n a l l y more c o n s i s t e n t
and produce temperatures t h a t a r e o f t e n more reasonable geol ogical l y than
those based on experimental data alone.
MAGMA SOWCE
The 6l80 values of t h e g l a s s groundmass c l o s e l y approximate t h e 6l8O
values of t h e r h y o l i t e magmas, as t h i s phase c o n s t i t u t e s g r e a t e r than 95
volume % of these rocks.
6l80 values o f t h e glass range from 6.3 t o 6.9
1
I
permil
, which
a r e i n t h e low range of values f o r a c i d i c volcanics.
These
Val ues a r e consi s t e n t w i t h d e r i v a t i o n o f t h e Mineral Mountai ns r h y o l it e s from
an I - t y p e source (Chappell and White, 1974; O'Meil e t a1
., 1977).
The oxygen-
isotope data i n d i c a t e t h a t i f t h e r h y o l i t e s were generated b y p a r t i a l m e l t i n g
i n t h e c r u s t , t h e source r e g i o n cannot c o n t a i n a s i g n i f i c a n t component of
pel it i c metasedimentary m a t e r i a1
.
CONCLUSIONS
A p p l i c a t i o n o f oxygen- sotope thermometry t o phenocryst phases o f t h e
M i neral Mounta ins r h y o l it e s y i e l d s some temperature r e s u l t s t h a t are
geol o g i c a l l y reasonable and c o n s i s t e n t w i t h r e s u l t s from appl i c a t i o n o f
chemical geothermometers.
The geothermometry equations proposed by B o t t i n g a
and Javoy (1973, 1975) a r e most successful and y i e l d temperatures o f 716-765OC
f o r t h e r h y o l i t e flows and 626-686OC f o r t h e r h y o l i t e domes.
These r e s u l t s
are i n good agreement w i t h temperatures c a l c u l a t e d from Fe-Ti oxide and twof e l d s p a r geothermometers.
Some degree o f sub-sol idus r e t r o g r a d e re-
e q u i l i b r a t i o n may be i n d i c a t e d b y t h e isotope thermometry r e s u l t s f o r t h e
danal r h y o l ites.
However, 1 iqui dus temperatures as low as 650°C may be
poss ib l e i f f l u o r i n e and water were s u f f i c i e n t l y e f f e c t i v e i n l o w e r i n g t h e
r hyol it e l i q u i d u s .
The range o f Sl80 values f o r t h e r h y o l i t e - - + 6.3 t o + 6.9
permi 1-- s t y p i c a l o f I - t y p e magmas, in d i c a t i ng ,hat no s i g n i f i c a n t pel it i c
metasedimentary component i s invol ved i n t h e generation o f t h e Mineral
Mountains r h y o l i t e s
.
REF ERENC ES
Anderson, A. T., Jr., Clayton, R. N., and Mayeda, T. K.:
Oxygen i s o t o p e
thermometry o f mafic igneous rocks. J. Geol., v. 79, no. 6, p. 715-729,
1971
.
P
Becker, R. H.:
Carbon and oxygen isotope r a t i o s i n i r o n formation and
associated rocks from t h e Hammersley Range o f western A u s t r a l i a and t h e i r
imp1 i c a t i o n s . Ph.D. D i s s e r t a t i o n , Univ. o f Chicago, 138 p., 1971.
Bertenrath, R. and Friedrichsen, H.:
The oxygen i s o t o p e geochemistry of
b i o t i t e s and i t s a p p l i c a t i o n as a geologic thermometer. P a r t I
The
oxygen isotope f r a c t i o n a t i o n i n t h e system phl ogopite-annite ( i n prep .)
-
Bottinga, Y. and Javoy, M.:
Comments on oxygen i s o t o p e thermometry.
Planet. Sci. L e t t e r s , v. 20, p. 250-265, 1973.
Earth
: Oxygen isotope p a r t i t i o n i n g among t h e minerals i n igneous and
metamorphic rocks. Rev. Geophys. Space Phys., v. 13, no. 2, p. 401-418,
1975.
Buddington, A. F. and Lindsley, D. H.:
s y n t h e t i c equivalents. J. P e t r o l
i u m oxide minerals and
. v.I r o n5,- t ip.t a n310-357,
1964.
The i r o n - t i t a n i u m oxides o f s a l i c v o l c a n i c * r o c k s and
Carmichael , I.S. E.:
t h e i r associated ferromagnesian s i 1 icates. Contrib. Mineral . P e t r o l .,
14, p. 36-64, 1967.
V.
Carmichael, I. S. E., Turner, F. J., Verhoogen, J.:
McGraw H i l l , New York, 739 p., 1974.
.
Igneous Petrology.
Two c o n t r a s t i n g g r a n i t e types.
Chappell, B. W. and White, A. J. R.:
Geology, v. 8, p. 173-174, 1974.
Pacific
The use o f oxygen isotopes i n h i g h
Clayton, R, N. and Epstein, S.:
temperature geological thennometry. J. Geol , V . 69, p. 447-452, 1961.
I-
.
The use o f bromine p e n t a f l u o r i d e i n t h e
Clayton, R. N. and Mayeda, T. K.:
e x t r a c t i o n o f oxygen f r a n oxides and s i l i c a t e s f o r i s o t o p i c analysis.
Geochim. Cosmochim. Acta, v. 27, p. 43-52, 1963.
Clayton, R. N., O'Nei1, J, R., and Mayeda, T. K.:
between quartz and water. J. Geophys. Res.,
1972
.
Oxygen i s o t o p e exchange
v. 77, no. 17, p. 3057-3067
Standard f o r r e p o r t i n g concentrations o f deuterium and oxygen-18
Craig, H.:
i n n a t u r a l waters. Science, v. 133, p., 1833-1834, 1961.
,
I
On t h e oxygen isotope d i s t r i b u t i o n among mineral t r i p l e t s i n
Deines, P.:
igneous and metamorphic rocks. Geochim. Cosmochim. Acta, v. 41, p.
1709-1730, 1977.
Low-temperature r h y o l i t e s from t h e
Evans, S. H., Jr. and Nash, W. P.:
Roosevelt geothermal area, Utah. Geol. SOC. Amer. Abs. w i t h Programs, v.
7, no. 7, p. 1070, 1975.
Quaternary r h y o l i t e from t h e Mineral
Evans, S. H., J r . and Nash, W. P.:
Mountains, Utah, U. S. A. Univ. Utah Geo. Geophys. Dept., Topical Report
volume 77-10, DOE/DGE c o n t r a c t EY-76-S-07-1601, 1978.
--
Evans, S. H., Jr. and Brown, F. H,:
Summary o f K - A r d a t i n g
1981. Univ,
Utah Geol Geophys. Dept., Topical Report #12079-45, DOE/DGE c o n t r a c t
DE-AC07-80ID-12079, 1981.
.
Friedman, I. and O'Neil, J. R.:
Compilations o f stable-isotope f r a c t i o n a t i o n
f a c t o r s o f geochemical i n t e r e s t . U. S. Geol. Surv. Prof. Paper 440-KK,
1977.
I
Javoy, M.:
Stable isotopes and geothermometry. J. Geol. SOC. Lond.,
p. 609-636, 1977.
v. 133,
Javoyf6M.f8Fourcade,
S., Allegre, C. 3.:
Graphical method o f examination of
O/ 0 f r a c t i o n a t i o n s i n s i l i c a t e rocks. E a r t h Planet. Sci. L e t t e r s ,
V. 10, p. 12-16, 1970.
Lipman, P. W., Rowley, P. D., Mehnert, H., Evans, S. H., Nash, W. P., and
Brown, R. F.:
Pleistocene r h y o l i t e o f t h e Mineral Mountains, Utah:
geothermal and archaeological s i g n i f i c a n c e . Res. U. S. Geol. Survey,
V. 6, p. 133-147, 1978.
Longstaffe, F. J., Smith, T. E., and Muehlendachs, K., 1980, Oxygen Isotope
evidence f o r t h e genesis o f Upper Paleozoic g r a n i t o i d s from Southwest
Nova Scotia: Can. 3. E a r t h Sci., v. 17, p. 132-141.
F l u i d dynamic p r o p e r t i e s o f r h y o l i t e
Nash, W. P. and Evans, S. H., Jr.:
magmas, Mineral Mountains, Utah. Univ. Utah Geol. Geophys. Dept., F i n a l
Report, vol 77-13, DOE/DGE EY-76-S-07-1601, 1978.
.
Oxygen and hydrogen isotope r e l a t i o n s i n
O'Neil, J: R. and Chappell, B. W.:
t h e B e r r i d a l e B a t h o l i t h . J. Geol. SOC. Lond., v. 133, p. 559-571, 1977.
The oxygen isotope and c a t i o n exchange
O'Neil, 3. R. and Taylor, H. P., Jr.:
chemistry o f feldspars. Am. Mineral., v. 52, p. 1414-1437, 1967.
O'Neil , J. R., Shaw, S. E., and Flood, R. H.:
Oxygen and hydrogen isotope
canpositions as i n d i c a t o r s o f g r a n i t e genesis i n t h e New England
B a t h o l i t h , A u s t r a l i a . Contr. Pet. Mineral., v. 62, p. 313-328, 1977.
Stormer, J. C., Jr.:
A p r a c t i c a l two-feldspar geothermometer.
Mineral
v. 60, p. 667-674, 1975.
.,
Amer.
Taylor, H. P., Jr.:
The oxygen i s o t o p e geochemistry of igneous rocks.
Min. Petrol., v. 19, p. 1-71, 1968.
Contr.
: Oxygen i s o t o p e evidence f o r 1arge-scal e i n t e r a c t i o n between
meteoric ground waters and T e r t i a r y g r a n o d i o r i t e i n t r u s i o n s , western
Cascade Range, Oregon. J. Geophys. Res., v. 76, p. 7855-7874, 1971.
: The appl i c a t i o n o f oxygen and hydrogen isotope s t u d i e s t o
hydrothermal a l t e r a t i o n and o r e deposition. Econ. Geol., v. 69, p. 843883, 1974.
Taylor, H. P., Jr. and Epstein, S.:
The r e l a t i o n s h i p between l80/l6O r a t i o s
i n c o e x i s t i n g mineral o f igneous and metamorphic rocks. Geol. SOC. Am.
Bull
V. 73, p. 461-480, 1962.
0,
Taylor, H. P., Jr. and S i l v e r , L. T.:
Oxygen i s o t o p e r e l a t i o n s h i p s i n
p l u t o n i c igneous rocks o f t h e Peninsular Ranges B a t h o l i t h , Southern and
Baja C a l i f o r n i a . Fourth I n t ' l . Conf. Geochron., Cosmochron., and Isotope
Geology, Snomass, Colorado, U.S.A.
U.S. Geol Survey Open-File Report
78-101, 1978.
.
'
-
Taylor, H. P., Jr., and T u r i , 6.:
High l80 igneous r0ck.s from t h e Tuscan
magmatic province, I t a l y . Contrib. Mineral. Petrol
v. 55, p. 33-54,
1976.
.,
. .
,.
FIGURE CAPTIONS
I
Figure 1.
P l o t s o f t h e Mineral Range oxygen-isotope data on concordance
diagrams o f A(Qz-mt) versus A(Qz-Pg) i n F i g u r e 1 A and A(Qz-mt)
versus A(Pg-mt) i n Figure 18. The concordance l i n e s a r e based on
t h e data o f Becker (1971), B o t t i n g a and Javoy (1973) and Javoy
(1977). E r r o r b a r s f o r each mineral p a i r r e f l e c t t h e combined
a n a l y t i c a l e r r o r f o r i n d i v i d u a l mineral analysis.
Figure 2.
P l o t o f Mineral Range oxygen-isotope data on d concordance diagram
o f A(Qz-Bi) versus A(Qz-Pg). The concordance l i n e i s based on t h e
data of B o t t i n g a and Javoy (1973) and O'Neil and T a y l o r (1967).
E r r o r bars f o r each mineral p a i r r e f l e c t t h e combined a n a l y t i c a l
e r r o r f o r i n d i v i d u a l mineral analysis.
'.
.
\ -
I
I
\E
I
I
I
I
N
0
-m
I
N
a" .
4
2
-- .. - . - . ...
.
._ .- .-
..
2
Table 1. --Oxygen-isotope compositions and f r a c t i o n a t i o n f a c t o r s f o r mineral p a i r s from
r h y o l it e f 1ows and domes, Mineral Mountains, Roosevel t Hot Spri ngs Thermal Area
Rock U n i t
B a i l e y Ridge
Sample No.
6l80 (SMOW)
-
MR 7 6-22
quartz (Qz)
g l ass
f e l d s p a r (P )
biotite (Bi
magnetite (Mt)
7.2
6.3
6.3
7
W i l d Horse Canyon
L i t t l e Bearskin
N o r t h Twin F l a t Peak
MR-76-2 3
quartz
glass
magnetite
A(Qz-Pg)
0.9
A(Qz-Mt)
5.6
A(Qz-Bi)
A(Pg-Mt)
h(Pg-Bi)
b(Bi-Mt)
5 05
4.7
4.6
0.1
5.4
4.6
3.9
0.7
1.7
1.6
7 .1
-
5.2
6.3
1.9
MR -76 -24
quartz
glass
f e l dspar
biotite
magnet it e
7.5
6.9
6.0
2.1
1.4
1.5
6.1
MR-76-25
quartz
glass
feldspar
mag n e t it e
7.4
6.4
6.2
1.2
1.2
6.2
5.0
Table 2.
Mineral P a i r
Qz-Pg ( A )
Qz-Pg (B)
(C)
Qz-Mt ( A )
Q2-W
Qz-Mt (B)
Pg-Mt ( A )
Pg-Mt (B)
Pg-Mt (C)
Qz-Bi ( A )
Qz-Bi (B)
Qz-Bi (C)
Qz-Bi (D)
Qz-Bi ( E )
Pg-Bi ( A )
Pg-Bi (B)
Pg-Bi ( C )
B i - M t (A)
B i - M t (B)
Bi-Mt (C)
Compilation o f oxygen-isotope f r a c t i o n a t i o n f a c t o r s used i n geothermometry c a l c u l a t i o n s .
10%na
+
1-95 0 5 0 0 ° c )
+ 0.51 (<5OO0C)
'
Data Sources
Clayton e t a l . (1972); O'Neil and Taylor (1967)
1.19 (106T -2 ) ' - 0.29
0.97 (106T -2 )
+ 2.24
B o t t i n g a and Javoy (1973); O'Neil and Taylor (1967)
B o t t i n g a and Javoy (1973)
5.57 ( 1 0 F )
4.38 (106T-2) + 0.30
3.01 (106T -2 ) + 2.24
4.60 (106T -2
2.15 (106T-2)
6 -2 + 1.27 (>5OO0C)
3.02 (10 T ) - 0.17 (<5OO0C)
3.74 (106T- 2 ) 0.97
1OZT-i) - 0.17 (>5OO0C)
1 0 T' ) - 1.61 (<500°C)
4.92 (IO6T -2 ) - 2.41
3.69 (106T -2 ) - 0.60
2.55 (IO6T -2 ) - 0.68
3.73 (106T -2 ) - 2.12
2.72
0.60
1.83 (106T -2 ) + 0.97
0.65 (106T -2 ) + 2.41
6 -2
1.88 (10 T ) + 0.60
Becker (1971); Javoy (1977)
Becker (1971); B o t t i n g a and Javoy (1973); O'Neil and T a y l o r (1967)
-
-
Becker (1971); B o t t i n g a and Javoy (1973); Clayton e t a l . (1472)
B o t t i n g a and Javoy (1973); Clayton e t a l . (1972)
B o t t i nga and Javoy (1973) ;' Javoy (1977)
B o t t i n g a and Javoy (1975); Clayton e t a1 (1972); O'Neil and Taylor (1967)
B o t t i n g a and Javoy (1973, 1975); O'Neil and Taylor (1967)
Clayton e t a1 (1972)
Bertenrath and F r i e d r i c h s e n ( i n
.
B e r t e n r a t h and F r i e d r i c h s e n ( i n
B o t t i nga and Javoy (1973)
B o t t i n g a and Javoy (1973)
B o t t i n g a and Javoy (1975); O'Ne
Bertenrath and F r i e d r i c h s e n ( i n
a y l o r (1967)
O'Neil and Taylor (1967)
B o t t i n g a and Javoy (1973)
B o t t i n g a and Javoy (1973, 1975); O'Nei
and Tay o r (1967')
B e r t e n r a t h and F r i e d r i c h s e n ( i n prep); B o t t i n g a and Javoy (1973)
B o t t i n a"a and Javov
" 11973)
.
Compilation of --oxygen-isotope geothermometry
r e~s u l t s ("C) from the equations i n
~~-~
Table 3.
~
Table 2 f o r mineral p a i r s from r h y o l i t e flows and domes i n the Mineral Mountains.
Qz-Mt
Q2-N
Sample
A
B
A
C
~~
MR-76-2 2
344
727 765
MR-76-23
B
Qz-B i
Pg-Mt
A
Pg-Bi
B i-Mt
B
A
B
C
D
E
A
B
C
A
716
440
487
493
516
505
422
472
450
-1177
-257
-1666
727
448
493
500
521 511
473
514
504
-2330
-344
4063
B
C
725 833
C
~
815
724
886
762
MR- 76-2 4
670
542 531
742 682
736 856
MR-76-25
457
621 626
730 675
692
771 686
Table 4.
F1 ow Unit
Qz-Pg ( C )
Qz-Mt ( B )
Pg-Mt ( C )
Bailey Ridge
765
724
716
MR -76 -2 2
f 115
t 27
t 32
Wild Horse Canyon
762
MR-76-23
f 29
L i t t l e Bearski n
MR-76 -24
,
Comparison o f oxygen-isotope (Bottinga and Javoy, 1973; Javoy, 1977) and chemical
geothermmetry ("C) results for the Mi neral Mountains rhyol i t e s .
North Twin
MR-76-25
F1 a t Peak
531
f 60
62 6
f 75
.
682
Fe-Ti Oxide
Two-Feldspar
7 40
770
72 7
f 33
640
670
f 24
675
f 24
686
f 29
650
710