JOURNAL OF PETROLOGY
VOLUME 41
NUMBER 6
PAGES 739–757
2000
Mineralogical Evidence for Fluid–Rock
Interaction Accompanying Prograde Contact
Metamorphism of Siliceous Dolomites:
Alta Stock Aureole, Utah, USA
STEPHEN J. COOK∗ AND JOHN R. BOWMAN†
DEPARTMENT OF GEOLOGY AND GEOPHYSICS, THE UNIVERSITY OF UTAH, SALT LAKE CITY, UT 84112, USA
RECEIVED MAY 27, 1998; REVISED TYPESCRIPT ACCEPTED NOVEMBER 10, 1999
Contact metamorphism of siliceous dolomite in the southern part of
the metamorphic aureole of the Alta stock (Utah, USA) produced
the prograde isograd sequence: talc (Tc), tremolite (Tr), forsterite
(Fo), and periclase (Per). Calcite (Cc)–dolomite (Do) geothermometry and phase equilibria define a general prograde T–
X(CO2) path of decreasing X(CO2) with rising temperature for
the dolomite. High-variance assemblages typify the aureole. Per +
Cc and Fo + Cc + Do characterize the inner aureole (Per and
Fo zones), and Tr + Do + Cc and Tc + Do + Cc are
widespread in the outer aureole (Tr and Tc zones). Low-variance
assemblages are rare and the thickness of reaction zones (coexisting
reactant and product minerals) at the isogradic reaction fronts are
narrow (tens of metres or less). The mineral assemblages, calculated
progress of isograd reactions, and the prograde T–X(CO2) path all
indicate that massive dolomite was infiltrated by significant fluxes
of water-rich fluids during prograde metamorphism, and that the
fluid flow was down-temperature and laterally away from the
igneous contact. Fluid infiltration continued through at least the
initial retrograde cooling of the periclase zone. Down-T fluid flow
is also consistent with the results of Cc–Do geothermometry and
patterns of 18O depletion in this area. The close spatial association
of reacted and unreacted chert nodules in both the tremolite and talc
zones plus the formation of tremolite by two reactions indicate that
the outer aureole varied in X(CO2), and imply that fluid flow in
the outer aureole was heterogeneous. The occurrence of dolomite-rich
and periclase (brucite)-absent, high-18O marble layers within the
periclase zone indicates that fluid flow in the innermost aureole was
also heterogeneous. Estimates of the average time-integrated fluid
flux (qmTIFF) experienced by the periclase, forsterite, and tremolite
zones are 4·2 × 107, 6·65 × 105, and 2·0 × 105 mol fluid/
m2, respectively. The average value of qmTIFF for the periclase zone
∗Present address: Argonne National Laboratory, Environmental Research Division, 9700 South Cass Avenue, Argonne, IL 60439, USA.
†Corresponding author.
agrees well with the qmTIFF (3·4 × 107mol/m2) determined by
numerical simulation of the temperature and 18O depletion profiles
preserved in the southern aureole. The estimates of qmTIFF for the
forsterite and tremolite zones have much greater uncertainty, but
may indicate that fluid flux was considerably lower in these zones
than in the periclase zone. Given the outward (down-temperature),
subhorizontal flow geometry indicated by a variety of petrologic,
geochemical, and geothermometry evidence presented here and elsewhere, this decrease implies that fluid has leaked from the flow
system between the periclase and tremolite zones.
KEY WORDS:
Alta; fluid flow; infiltration; marble; mineralogical
INTRODUCTION
Both mineral assemblages and stable isotopes have been
interpreted with transport theory to evaluate the extent
of fluid–rock interaction and flow geometry in carbonate
rocks from contact metamorphic aureoles (Baumgartner
& Ferry, 1991; Ferry, 1991, 1994; Dipple & Ferry, 1992;
Jamtveit et al., 1992; Bowman et al., 1994; Roselle et al.,
1999). These applications have sometimes been controversial in establishing the direction of fluid flow with
respect to the temperature gradient (Ferry & Dipple,
1992; Nabelek & Labotka, 1993; Cartwright & Buick,
1996). Mineralogic records are a particularly valuable
 Oxford University Press 2000
JOURNAL OF PETROLOGY
VOLUME 41
record of fluid–rock interaction as these are an unambiguous link between the high-temperature conditions
of prograde metamorphism and fluid flow.
Geothermrometry, carbon and oxygen isotope data,
and petrologic evidence of fluid infiltration-driven metamorphism are preserved in siliceous dolomites of the
contact aureole surrounding the Alta stock in northern
Utah. Moore & Kerrick (1976) described a prograde
sequence of talc, tremolite, forsterite, clinohumite, and
periclase isograds in these rocks, and concluded that
isograd-forming reactions required interaction of rocks
with H2O-rich fluids. Calcite–dolomite geothermometry
and light stable isotope studies of the dolomites in the
southern part of the aureole (Bowman et al., 1994; Cook
& Bowman, 1994), and numerical models for fluid flow,
heat, and 18O/16O mass transport during the metamorphic event (Cook et al., 1997), confirmed the infiltration of H2O-rich fluid, and showed that fluid flow
was principally down-temperature (down-T) and laterally
away from the intrusive contact.
In this study we report petrologic evidence that corroborates the occurrence of down-T fluid flow during
prograde metamorphism. We present measurements of
reaction progress developed in the dolomites at each of
the isograds. We use a model for down-T fluid flow
and mass transport (Ferry, 1996) to estimate the timeintegrated fluid flux (qTIFF) responsible for prograde metamorphism based on measured reaction progress, locations
of isograds, and estimates of the T–X(CO2) conditions of
metamorphism. The petrologic estimate of qTIFF agrees
well with the value obtained by a numerical simulation
of fluid flow, heat transport, and transport and exchange
of 18O/16O that accompanied metamorphism of siliceous
dolomite in the southern Alta aureole (Cook et al.,
1997).
METAMORPHIC GEOLOGY
The Alta granodiorite (Wilson, 1961) is one of several
mid-Tertiary (38 Ma, Crittenden et al., 1973) stocks in
northern Utah’s central Wasatch Range (Fig. 1). The
stock intruded and contact metamorphosed a sequence
of Precambrian and Paleozoic sedimentary rocks that
consist primarily of quartzite (Precambrian Big Cottonwood and Cambrian Tintic Formations) overlain by
limestones and dolomites (Cambrian Maxfield and Mississippian Fitchville, Deseret, and Gardison Formations).
In the study area, preintrusive thrusting (Alta–Grizzly
thrust zone, Figs 1 and 2) has repeated a portion of
the carbonate section, placing the Cambrian Maxfield
Formation over the upper Mississippian Deseret–
Gardison Formations. Metamorphic effects associated
with the Alta stock are evident in carbonate rocks
NUMBER 6
JUNE 2000
Fig. 1. Geologic map of the Alta contact aureole. Geology after
Crittenden (1965) and Baker et al. (1966). Isograd locations based on
mineral assemblages in the siliceous dolomites after Moore & Kerrick
(1976). This study focuses on the metamorphism along the south–central
margin of the Alta stock. Isograds: Tc, talc; Tr, tremolite; Fo, forsterite;
Chm, clinohumite; Per, periclase.
(Maxfield, Fitchville, Deseret, and Gardison Formations,
Figs 1 and 2) up to >2 km south from the intrusive
contact.
Two other mid-Tertiary stocks are exposed in the
vicinity. To the east, the Alta stock cuts the 42 Ma
Clayton Peak granodiorite. Several kilometers to the west
is the 32 Ma Little Cottonwood quartz monzonite. Moore
& Kerrick (1976) reported that the western margin of
the Alta aureole has been overprinted by the contact
aureole of the Little Cottonwood stock (Fig. 1). The
south–central margin of the Alta aureole (Figs 1 and 2)
was selected for study for the following reasons: (1) it has
a well-developed prograde metamorphic sequence within
siliceous dolomite (Moore & Kerrick, 1976); (2) it has a
simple structure and excellent exposures of carbonate
units that allow reliable tracing of individual lithologies up
metamorphic grade; (3) it is unaffected by metamorphism
associated with the Clayton Peak stock to the east (Smith,
1972) and the Little Cottonwood stock to the west (Moore
& Kerrick, 1976).
Two lithologically distinct types of siliceous dolomite
are present in the study area (Figs 1 and 2): massive
dolomite strata that contain only sparsely disseminated
740
COOK AND BOWMAN
METAMORPHISM OF SILICEOUS DOLOMITES
MINERAL ZONES, ISOGRADIC
REACTIONS, AND MINERAL
ABUNDANCE
Talc zone
The outermost talc zone is distinguished by the mineral
assemblage Tc + Cc as reaction rims that surround
chert nodules in nodular dolomites. These were produced
by the reaction
3Do+4Qz+H2O=Tc+3Cc+3CO2.
Fig. 2. Geologic map of the south–central part of the Alta aureole
with locations of samples used in this study. Geology after Baker et al.
(1966) and Cook (1992). Stratigraphic units: Ct, Tintic; Co, Ophir;
Cm, Maxfield; Mf, Fitchville; Mdg, Deseret and Gardison; Tind,
intermediate dikes; Qal, alluvium. Alta stock shown by random bar
pattern; Fitchville (Mf ) by the horizontal line pattern. Isograds (bold
dashed lines) are labeled by index mineral: Tc, talc; Tr, tremolite; Fo,
forsterite; Per, periclase. The preintrusive Alta–Grizzly thrust (labeled
with teeth) has repeated part of the carbonate section.
(1)
The assemblage Tc + Cc + Dol is common in nodular
dolomites. Although quartz (chert) is found in cores of
nodules throughout the talc zone, it is invariably separated
from the dolomite matrix by a Tc + Cc reaction rim.
Few massive dolomite specimens in the south Alta aureole
exhibit this reaction even though they contain disseminated quartz; talc appears to be almost exclusively
restricted to the chert nodules (see Moore & Kerrick,
1976).
The extent of this reaction around nodules is variable.
Many nodules are unreacted, and some small nodules
have been completely reacted to Tc + Cc. Typically,
talc-bearing nodules exhibit partial reaction in which the
Tc + Cc assemblage (1–10 mm thick) mantles a chert
core. Talc-bearing nodules typically lie along individual
stratigraphic horizons. Within many outcrops, talc-bearing nodular strata alternate with talc-absent ones that
are separated by stratigraphic distances of only a few tens
of centimeters. The abundance of talc-bearing nodules
decreases with increasing distance from the stock. Because
the talc occurrences are almost exclusively restricted to
nodular dolomite horizons, and our focus is fluid infiltration in the massive dolomites, we have not measured
reaction progress in the talc zone.
Tremolite zone
quartz and nodular dolomite strata that consist of chert
nodules in a matrix of massive dolomite (Moore &
Kerrick, 1976). This study focuses on the massive dolomite strata, which, unmetamorphosed, consist of dolomite (Do) with typically <10 modal % each of quartz
(Qz) and calcite (Cc). In massive dolomite strata, prograde
metamorphism successively produced talc (Tc) (rare),
tremolite (Tr), forsterite (Fo), clinohumite (Chm), and
periclase (Per) (Moore & Kerrick, 1976). The periclase
was subsequently replaced by retrograde brucite (Br).
Except for the talc zone, these mineral zones lie approximately parallel to the intrusive contact. The periclase, forsterite, and tremolite isograds occur at average
distances of 200, 700, and 1200 m, respectively from the
contact with the stock.
The tremolite zone is defined by formation of Tr + Do
+ Cc, less commonly by Tr + Do and rarely by Tr +
Cc. Modal tremolite ranges from 1·6 to 17·1% in 12
representative massive dolomite samples (Fig. 2) from
the tremolite zone (Table 1). The absence of quartz
(Table 1) indicates that the tremolite-forming reaction
went to completion. Most samples from this zone contain
the assemblage Tr + Do + Cc, a result of either the
reaction
2Tc+3Cc=Tr+Do+H2O+CO2
(2)
5Do+8Qz+H2O=Tr+3Cc+7CO2.
(3)
or
741
Moore & Kerrick (1976) interpreted the greater abundance of Tr + Do relative to Tr + Cc to indicate that
JOURNAL OF PETROLOGY
VOLUME 41
NUMBER 6
JUNE 2000
Table 1: Tremolite zone, mineral modes
Sample:
88.76
88.77
Do
83·1
Cc
10·2
Tr
Phl
Opq
Total
89.10
89.11
89.14c
93·3
47·7
90·0
84·3
4·2
50·3
6·4
6·3
9·1
12·7
5·7
2·4
—
0·1
1·0
100·0
1·6
—
—
100·0
3·3
<0·1
89.16a
89.16b
89.18
89.20
89.22a
89.22e
59·1
70·4
91·3
90·4
93·2
94·9
79·0
28·0
12·5
6·4
8·3
3·4
3·5
14·6
—
17·1
0·2
0·4
0·3
0·3
100·0
100·0
100·0
—
—
—
100·0
100·0
2·1
0·8
0·1
0·4
3·2
—
1·6
—
89.25b
4·4
2·0
0·1
0·1
0·2
<0·1
0·2
100·0
100·0
100·0
100·0
100·0
Modes based on 2000 points.
reaction (2) formed most of the tremolite in massive
dolomite. However, reaction (1) followed by reaction (2)
is stoichiometrically equivalent to reaction (3). Any bulk
composition that underwent this reaction sequence could
not have produced Tr + Do without excess calcite.
The rare assemblage Tr + Do reflects either local
mineralogical (and hence compositional) zoning on a
scale larger than that of a thin section, or local metasomatism. The near absence of talc (and the presence of
Do + Qz) in massive dolomite in the talc zone and the
presence of Tr + Cc + Do in massive dolomite of the
tremolite zone is more consistent with the formation of
most tremolite by reaction (3). Also, samples that appear
to have originally contained small (<1 cm) chert nodules
now contain intergrown Tr + Cc.
Forsterite zone
The widespread assemblage Fo + Cc + Do defines the
forsterite zone. The modal abundance of forsterite in 12
representative samples (Fig. 2) of massive dolomite from
the forsterite zone ranges from 0·7 to 13·4% (Table 2).
The ubiquitous prograde zoning sequence in the massive
dolomite strata across the forsterite isograd is Tr + Do
+ Cc followed by Fo + Cc + Do, indicating that the
forsterite was produced by the reaction
Tr+11Do=8Fo+13Cc+9CO2+H2O.
(4)
The absence of tremolite (Table 2) indicates that reaction
(4) went to completion. The univariant assemblage corresponding to reaction (4), Tr + Do + Fo + Cc, is
rare in the massive dolomites of the forsterite zone.
Periclase zone
The assemblage Per + Cc in massive dolomite within
>200 m of the intrusive contact defines the periclase
zone. The periclase zone is not present throughout the
study area. Dolomites lying above the Alta–Grizzly thrust
zone along the southern margin of the stock (Fig. 2)
characteristically lack periclase, even within a few meters
of the intrusive contact (Cook, 1992; Cook et al., 1997).
The assemblage Per + Cc was produced by the reaction
Do=Per+Cc+CO2.
(5)
The periclase has been completely replaced with pseudomorphs of retrograde brucite, either by the reaction
Per+H2O=Br
(6)
or by retrograde reaction at the Per–Br–Cal–Do invariant
point (IV in Fig. 3):
Per+XIPCc+XIPCO2+(1−XIP)
H2O→(1−XIP)Br+XIPDo
(7)
where XIP is the X(CO2) value of the fluid at this invariant
point (see Ferry, 1996).
The volumetric (modal) abundance of brucite in 12
representative periclase zone samples (Fig. 2) ranges from
1·4 to 29·1%, but is bimodally distributed (Table 3). Five
of the 12 samples (samples 88·3, 88·12, 88·36, 88.A2,
and 88.A7) contain modest to abundant dolomite, but
no brucite, or in one case (88·12) a very small amount
of brucite (1·4%). Sample 88·12 is located above the
Alta–Grizzly thrust. Dolomite and calcite in these samples
exhibit textures indistinguishable from those typical of
dolomite-bearing rocks below the periclase isograd, and
hence dolomite is interpreted to be primary. Reaction
(5) has not been initiated in these five samples. All five
of these samples have 18O values >19·4‰, indicating
the absence of significant infiltration of water-rich fluids
necessary to drive reaction (5) (Bowman et al., 1994).
Sample 88·40 contains both brucite and dolomite in
significant quantities. This sample displays the same
textural relationships as observed in the first group,
suggesting the dolomite in sample 88·40 is primary as
well. This is a sample of the rare univariant assemblage
Per + Cc + Do. Sample 88·40 has an intermediate
18O value of +16·9‰, consistent with intermediate
fluxes of fluid infiltration sufficient to initiate, but not
complete reaction (5).
742
COOK AND BOWMAN
METAMORPHISM OF SILICEOUS DOLOMITES
Table 2: Forsterite zone, mineral modes
Sample:
88.7
88.8
88.16
88.18
88.20
88.33
88.53
88.55
88.59
88.60
88.C5
88.D2
Do
59·2
9·4
5·8
72·1
24·3
61·8
23·1
57·9
19·4
25·6
58·2
56·4
Cc
26·8
77·7
87·5
24·7
55·0
25·8
75·9
31·0
78·5
71·7
28·0
35·6
Fo
13·4
12·4
6·6
2·5
18·5
11·4
0·7
7·6
1·8
2·1
10·8
0·2
<0·1
0·1
1·9
Sp
—
0·2
—
—
—
—
—
7·8
—
Opq
0·3
0·3
0·2
0·4
1·2
0·9
0·3
1·1
<0·1
0·4
0·5
Phl
0·3
<0·1
0·3
0·8
0·1
<0·1
2·4
0·3
0·1
0·6
0·1
100·0
100·0
100·0
100·0
100·0
100·0
100·0
100·0
100·0
100·0
100·0
100·0
88.40
88.51
88.A2
88.A5
88.A7
88.B6
Total
0·2
—
Modes based on 2000 points.
Table 3: Periclase zone, mineral modes
Sample:
88.3
88.4
88.5
88.12
88.14
88.36
Do
13·7
10·6
0·5
89·9
12·4
98·2
19·4
8·4
6·9
4·3
15·9
4·4
Cc
75·2
68·9
62·5
7·5
57·5
1·1
59·0
59·0
81·1
73·1
71·6
72·3
Chm
—
Fo
1·3
4·3
—
27·2
—
16·8
29·1
—
—
—
0·1
—
0·3
0·6
Opq
0·6
—
—
—
0·1
Lud
—
—
Total
100·0
100·0
27·8
4·9
100·0
1·4
—
100·0
0·4
0·1
100·0
—
4·5
0·8
19·2
—
4·5
Sp
—
0·8
1·1
—
Br
—
2·4
9·6∗
0·3
—
—
1·8
11·2
—
—
—
7·5
9·6∗
20·6
—
—
15·4
0·2
0·2
2·5
0·6
<0·1
0·4
0·4
—
—
—
—
—
—
—
—
—
—
100·0
100·0
100·0
100·0
100·0
100·0
100·0
Modes based on 2000 points.
∗Includes minor clinohumite, quantity uncertain.
The remaining six samples contain significant amounts
of brucite (15–30 vol. %) and variable amounts of dolomite (0·5–12%). Dolomite in all of these samples is
preferentially associated with brucite pseudomorphs,
either as collections of isolated dolomite grains at brucite–
calcite grain contacts or as larger dolomite grains enclosing one or more brucite pseudomorphs. These
textures are consistent with the formation of this dolomite
by a series of retrograde reactions as described by Ferry
(1996) and Ferry & Rumble (1997). Reaction (5) has
gone to completion in these samples.
In a number of the periclase zone samples, original
forsterite reacted with dolomite to form clinohumite.
Close spatial association of calcite with clinohumite suggests the reaction
4Fo+Do+H2O+HF=Chm+Cc+CO2.
humite ranges from 0·8 to 7·5% in samples affected by
reaction (8) (Table 3). In most but not all cases (samples
88·36 and 88.A7 are exceptions), periclase zone samples
contain either forsterite or humite group minerals.
In summary, contact metamorphism of massive dolomite produced tremolite, forsterite, and periclase toward the contact with the Alta granodiorite. The
tremolite, forsterite, and periclase isograds approximately
parallel the intrusive contact on the south side of the
Alta stock, occurring at average distances of 1200, 700,
and 200 m, respectively. Few univariant assemblages were
observed, and no invariant assemblages were found. One
of the principal features of the metamorphic succession
in the Alta aureole is that all of the low-variance, isogradic
reaction zones are thin (less than tens of meters).
(8)
Petrographic differentiation between forsterite and clinohumite is difficult. Where identified and measured with
electron microprobe monitoring of F and Si, modal clino-
MINERAL CHEMISTRY
Compositions of the silicate minerals were determined
by electron microprobe analysis using the Cameca SX-
743
JOURNAL OF PETROLOGY
VOLUME 41
Table 4: Microprobe analyses of talc
Sample:
88.70a
89.37a
90.33a
90.38
NUMBER 6
JUNE 2000
Table 5: Microprobe analyses of chain silicates
90.40
Phase:
Tr
Tr
Tr
Tr
Tr
Sample:
88.76
88.77
89.11
89.14c
89.16b
wt %
SiO2
63·00
63·34
63·38
63·51
63·50
wt %
TiO2
0·02
0·02
0·03
0·02
0·03
SiO2
57·87
58·65
57·50
59·17
57·64
Al2O3
0·17
0·13
0·03
0·37
0·13
TiO2
0·08
0·04
0·17
0·04
0·07
FeO
0·05
0·03
<0·01
<0·01
0·07
Al2O3
1·45
0·60
2·24
0·28
1·85
MnO
<0·01
<0·01
<0·01
<0·01
0·03
FeO
0·09
0·03
0·17
0·12
0·12
MgO
31·32
31·44
31·29
31·03
31·13
MnO
0·02
<0·01
0·02
<0·01
0·05
CaO
0·15
0·15
0·24
0·06
0·22
MgO
24·42
24·54
23·85
24·64
24·08
Na2O
0·09
0·01
0·04
0·15
0·08
CaO
13·50
13·76
13·58
13·64
13·77
<0·01
<0·01
<0·01
0·01
0·02
Na2O
0·19
0·18
0·15
0·07
0·20
F
0·22
0·28
0·16
0·20
2·64
K2O
0·14
0·05
0·07
0·02
0·14
Cl
0·01
0·01
0·02
0·01
0·03
F
0·35
0·88
0·22
0·78
0·21
H2O
4·61
4·60
4·66
4·65
3·50
Cl
0·02
0·01
0·02
0·02
0·02
OoF+Cl
−0·09
−0·12
−0·07
−0·09
Sum
99·55
99·89
99·78
99·92
100·27
4·00
4·00
4·01
4·01
4·01
K2O
Si
AlIV
AlVI
—
0·01
—
0·01
—
—
—
−1·11
H2O
OoF+Cl
Sum
—
0·03
0·01
2·04
1·79
2·10
1·85
2·11
−0·15
−0·38
−0·10
−0·33
−0·09
100·02
7·85
7·94
7·79
7·99
7·81
0·06
0·21
0·01
0·19
0·04
0·15
—
—
—
—
AlVI
0·08
—
—
—
—
—
Ti
0·01
Mn
—
—
—
—
—
Ca
0·01
Na
0·01
0·01
—
2·95
0·02
0·01
2·92
—
0·02
K
—
—
—
—
O∗
10·00
10·00
10·00
10·00
F
Cl
OH
0·04
—
1·96
0·06
—
1·94
0·03
—
1·97
1·96
Fe
0·01
—
0·02
—
0·03
0·11
—
0·02
0·01
0·01
0·01
2·93
Mn
0·01
Mg
4·93
4·95
4·82
4·96
4·86
0·01
Ca
1·96
1·99
1·97
1·97
1·99
Na
0·05
0·05
0·04
0·02
K
0·02
0·01
0·01
O∗
22·00
22·00
22·00
22·00
22·00
F
0·15
0·38
0·10
0·33
0·09
—
10·00
0·04
—
100·17
0·15
—
2·96
100·30
AlIV
Fe
2·98
99·99
Si
Ti
Mg
100·15
0·53
—
1·47
Cl
OH
∗Number of oxygens per formula unit.
Tc, talc.
—
—
—
—
—
1·85
—
1·62
1·90
—
0·01
0·05
—
0·02
—
—
1·67
1·91
∗Oxygens per formula unit.
Tr, tremolite.
50 microprobe at the University of Utah. A 15 keV beam
current and 30 nA sample current were used.
Talc compositions approach those of the Mg end-member
except for substitution of fluorine in the hydroxyl site
(Table 4). Fluorine substitution ranges from 0·03 to 0·53
atoms per formula unit (p.f.u.) [Mg3Si4O10(OH)2]. The
observed variation of fluorine content within individual
samples was <0·15 atoms p.f.u.
from 0·09 up to 0·38 fluorine atoms p.f.u.
[Ca2Mg5Si8O22(OH)2]. Moore & Kerrick (1976) reported
that the fluorine contents of tremolite decrease with
metamorphic grade in the Alta aureole. This was not
confirmed by this study. Fluorine contents of individual
tremolite grains from each sample varied by <0·1 atoms
p.f.u. in all samples. In one sample (89·11), the amphibole
exhibits significant Al substitution in both the tetrahedral
and octahedral sites.
Amphibole
Olivine and clinohumite
Amphibole compositions are essentially end-member
tremolite (Table 5), with fluorine substitution ranging
Olivine compositions closely approach end-member forsterite (Table 6). Clinohumites also approach closely
Talc
744
COOK AND BOWMAN
METAMORPHISM OF SILICEOUS DOLOMITES
Table 6: Microprobe analyses of olivine and
clinohumite
Phase:
Chm
Chm
Fo
Fo
Fo
Sample:
88.51
88.B6
88.8
88.33
88.55
wt %
SiO2
38·59
38·03
42·06
42·66
42·56
TiO2
0·16
0·63
0·02
0·01
<0·01
Al2O3
0·02
<0·01
<0·01
0·02
<0·01
FeO
0·24
0·21
1·44
0·61
0·29
MnO
<0·01
0·04
0·08
0·11
<0·01
MgO
57·89
58·20
56·53
57·29
57·56
CaO
0·08
0·08
0·11
0·10
0·04
Na2O
<0·01
<0·01
<0·01
<0·01
<0·01
K2O
<0·01
<0·01
F
2·19
2·50
Cl
<0·01
<0·01
—
—
—
1·85
1·71
—
—
—
H2O
OoF+Cl
Sum
−0·92
100·10
Si
Al
−1·05
100·36
4·00
—
0·01
0·01
0·02
0·02
—
—
—
—
100·45
100·26
1·00
8·94
8·99
Ca
0·01
0·01
1·00
—
—
—
0·01
—
1·97
—
1·00
—
—
0·03
—
Mg
<0·01
—
—
—
Fe
<0·01
—
100·24
3·94
—
Ti
Mn
<0·01
—
0·01
—
1·99
—
1·99
—
Na
—
—
—
—
—
K
—
—
—
—
—
O∗
Cl
16·00
—
16·00
—
4·00
4·00
4·00
—
—
—
F
0·72
0·64
—
—
—
OH
1·28
1·36
—
—
—
∗Oxygens per formula unit.
Chm, clinohumite; Fo, forsterite.
end-member compositions. Substitution of fluorine for
hydroxyl is apparently required to stabilize the members
of this group (Duffy & Greenwood, 1979). Fluorine
contents range from 0·64 to 0·72 atoms p.f.u.
[Mg(OH)2·nMg2SiO4] in climohumite.
Whole-rock chemistry
Whole-rock analyses of 18 samples of massive dolomite
(Table 7) were performed by XRAL Activation Services
Inc. of Ann Arbor, MI, USA. All elements were analyzed
by X-ray fluorescence (XRF), except for fluorine and
boron, which were analyzed using wet chemical methods
and prompt gamma methods, respectively. Reported
detection limits are 0·01 wt % for XRF analyses, 20 ppm
for fluorine, and 0·5 ppm for boron.
Silica contents of individual samples range from 0·19
to 8·06 wt % SiO2, although most contain <3 wt %
SiO2. Aluminum and fluorine are the only components
in the marbles outside the CaO–MgO–SiO2–H2O–CO2
system that are abundant enough to significantly affect
phase equilibria. Al2O3 ranges from trace quantities to
2·1 wt %, although most samples contain <0·5 wt %.
Samples from all metamorphic grades contain measurable fluorine, with the highest values for samples from
the periclase (2200 ppm) and forsterite (1900 ppm) zones.
Six samples of the Alta stock contain from 520 to 740
ppm fluorine (Bowman & Cook, unpublished data, 1992).
Iron occurs at trace amounts in marble samples (typically <2800 ppm Fe2O3, Table 7) from all metamorphic
grades. Other components are typically <0·1 wt %, and
many approach the detection limits of XRF analysis (0·01
wt %). The only rocks that contain significant quantities
of iron are thin, bedding-concordant pyroxene (Hd40)–
garnet (Ad60) skarns within the periclase zone. The abundance and thickness of the Fe-bearing skarns generally
increase toward the contact with the stock or toward sills
and dikes emplaced—typically along bedding planes—
into the marbles. The skarn layers are depleted in 18O
compared with the original 18O values of the marbles,
and are essentially in oxygen isotopic exchange equilibrium with the stock (unpublished analyses, Kemp,
1985).
The unmetamorphosed siliceous dolomite and the talc
and tremolite zone marbles contain <0·5 ppm boron
(Table 7; Woodford, 1995). Samples from the Alta stock
display boron contents from 4·4 to 11 ppm ( J. R. Bowman
& S. J. Cook, unpublished data, 1992). Periclase zone
samples contain 20–41 ppm boron (Table 7). Reflecting
these high values, ludwigite (an Fe–Mg borate) is often
found in thin layers at bedding boundaries, in thin
layers that extend beyond layer-concordant skarns along
bedding, and as disseminated grains within layers that
contain abundant periclase (brucite). Periclase + ludwigite-bearing layers are invariably the most 18O-depleted
rocks within the periclase zone, with 18O values of
Ζ12‰ (Bowman et al., 1994; Cook et al., 1997). The
abundance of ludwigite within the periclase zone does
not correlate with distance from the intrusive contact.
Instead, like periclase, the abundance of ludwigite appears
to be bedding controlled. Within layers, ludwigite appears
to increase in abundance toward bedding-parallel skarn.
Ludwigite is not observed outside the periclase zone.
Despite the absence of ludwigite, several forsterite zone
samples also contain significant boron (in one sample,
up to 38 ppm; Table 7). The boron in these rocks appears
to reside in olivine, accessory phlogopite, and even calcite
(Woodford, 1995).
745
JOURNAL OF PETROLOGY
VOLUME 41
NUMBER 6
JUNE 2000
Table 7: Representative whole-rock chemical analyses
Zone:
Per
Per
Per
Per
Per
Per
Fo
Fo
Fo
Sample:
88.12
88.14
88.40
88.51
88.A7
88.B6
88.22
88.53
88.55
wt %
SiO2
1·10
1·27
1·79
1·17
3·84
5·40
0·19
0·44
TiO2
0·03
<0·01
<0·01
0·01
0·05
0·03
<0·01
<0·01
6·63
0·09
Al2O3
0·41
0·13
0·13
0·31
1·01
0·41
<0·01
0·04
2·10
Fe2O3
0·20
0·24
0·05
0·46
0·28
0·25
0·01
0·06
0·77
MgO
22·20
23·30
17·00
22·10
10·40
14·90
22·20
6·16
18·50
MnO
0·04
0·03
<0·01
0·01
<0·01
0·02
0·02
<0·01
<0·01
CaO
31·80
35·70
40·10
36·20
45·20
40·90
31·50
50·50
31·80
Na2O
0·03
0·02
0·02
0·02
0·03
0·05
0·03
0·03
0·07
K2O
0·01
<0·01
0·02
<0·01
0·01
0·01
0·01
0·01
0·46
P2O5
0·01
0·08
<0·01
0·01
0·02
0·02
<0·01
<0·05
0·02
LOI
44·10
39·20
40·70
39·40
39·30
38·70
46·40
41·70
38·60
Sum
99·94
99·99
99·84
99·72
100·20
100·70
100·40
99·03
99·06
F (ppm)
210
479
940
530
1900
2200
40
B (ppm)
n.a.
20
22
29
41
36
n.a.
Ca/Mg∗
1·03
1·10
1·70
1·18
3·12
1·97
260
0·5
1·02
900
38
5·98
1·24
Zone:
Fo
Fo
Fo
Tr
Tr
Tr
Tc
Tc
Tc
Sample:
88.59
88.60
88.C5
88.77
89.16a
89.25b
88.68
88.70a
89.39b
wt %
SiO2
1·17
1·82
6·41
1·57
8·06
5·59
0·78
1·03
1·48
TiO2
<0·01
0·01
0·09
<0·01
0·01
0·09
0·01
<0·01
0·02
Al2O3
0·06
0·37
1·82
0·05
0·08
1·74
0·08
<0·01
0·21
Fe2O3
<0·01
0·26
0·41
0·06
0·14
0·26
0·07
0·05
0·02
MgO
6·31
8·52
21·00
20·70
16·60
18·10
21·90
21·70
21·40
MnO
<0·01
<0·01
0·02
0·02
0·01
0·01
0·03
0·02
0·02
CaO
49·80
46·70
31·60
32·00
35·50
32·20
31·70
31·40
31·30
Na2O
0·04
0·02
0·05
0·02
0·03
0·06
0·02
0·02
0·02
K2O
0·03
0·05
0·27
0·02
0·02
0·30
0·02
<0·01
0·08
P2O5
0·01
0·01
0·02
0·04
0·01
0·02
0·02
0·02
0·02
LOI
43·00
41·30
37·50
45·70
39·10
41·40
44·90
45·00
45·00
100·40
99·11
99·20
100·20
99·57
99·78
99·54
99·24
99·57
Sum
F (ppm)
1900
560
800
B (ppm)
10
n.a.
14
Ca/Mg∗
5·67
3·94
1·08
160
300
<0·5
28
1·11
1·54
780
<0·5
1·28
130
<0·5
1·04
290
<0·5
1·04
500
<0·5
1·05
LOI, loss on ignition. n.a., not analyzed.
∗Molar ratio.
PHYSICAL CONDITIONS OF
METAMORPHISM
Pressure
Wilson (1961) used stratigraphic measurements to estimate lithostatic pressure (PL) for the aureole at
100–200 MPa (1–2 kbar). His reconstruction is possibly
complicated by stratigraphic thinning over an ancestral
Uinta Arch, or thickening from preintrusive thrusting.
Multiple preintrusive thrusts were mapped by Baker et
al. (1966) on both the north and south sides of the aureole
(Fig. 1). John (1991) based similar estimates for the depth
746
COOK AND BOWMAN
METAMORPHISM OF SILICEOUS DOLOMITES
of emplacement of the Alta stock on fluid inclusion
microthermometry in the stock. Mineral assemblages
from the Ophir Formation (biotite + andalusite +
potassium feldspar + quartz) in the innermost aureole
constrain pressures to <220 MPa (Kemp, 1985).
Temperature and fluid pressure
Calcite–dolomite geothermometry (Cook & Bowman,
1994) indicates temperature limits of 420 ± 50°C for
the tremolite isograd, 470 ± 50°C for the forsterite
isograd, and 580 ± 25°C for the periclase zone. The
minimum fluid pressure necessary to stabilize periclase
at 580°C and PL = 150 MPa in the inner aureole is
75 MPa (Figs 3 and 4). Cook & Bowman (1994) noted
that these temperatures underestimate peak metamorphic
temperature to some degree, particularly for the periclase
zone, because calcite in the Per zone has experienced at
least some retrograde dolomite exsolution (see above).
Hence Pf = 75 MPa should also be regarded as a
minimum value. Use of this minimum Pf in the calculation
of phase equilibria yields maximum values of X(CO2) at
isograds through application of geothermometry, and
hence produces conservative estimates of fluid fluxes.
PROGRADE T–X(CO 2 ) PATH
The sequence of reactions, coupled with phase equilibria
and geothermometry, define a prograde metamorphic
T–X(CO2) path. Definition of X(CO2) limits at the isograds
is needed to calculate fluxes of external fluids that drove
the decarbonation reactions in the marbles. Definition
of the prograde path is also useful because the trend in
X(CO2) with grade is diagnostic of the direction of fluid
flow relative to the metamorphic thermal gradient.
T–X(CO2) phase equilibria that governed the metamorphism of siliceous dolomite in the Alta aureole are
shown in Figs 3 and 4. The microprobe and whole-rock
chemical analyses indicate that phase equilibria in the
CaO–MgO–SiO2–H2O–CO2 system can, after adjusting
for effects of fluorine, be applied to the assemblages.
The phase equilibria were calculated assuming a mean
lithostatic pressure of 150 MPa and minimum fluid pressure of 75 MPa. Activities of hydrous silicates were corrected for the effects of fluorine substitution by assuming
typical fluorine contents for the affected phases (Tables
4–6). Also shown in Fig. 3 is the prograde path followed
by massive dolomite. This path is defined by combining
the sequence of prograde reactions, the results of calcite–
dolomite geothermometry, and the T–X(CO2) phase equilibria (Figs 3 and 4) as discussed below.
At the tremolite isograd, intergrown assemblages of Tr
+ Cc + Do, and the observed prograde sequence of
Do + Qz followed by Tr + Cc + Do in most samples
Fig. 3. T–X(CO2) phase equilibria applicable to prograde metamorphism of the massive siliceous dolomites at a constant lithostatic
pressure (PL) of 150 MPa and a fluid pressure (Pf ) of 75 MPa. Pf is the
minimum value required by geothermometry results for the periclase
isograd (Cook & Bowman, 1994). Geothermometry results used to limit
X(CO2) at the tremolite (Tr), forsterite (Fo), and periclase (Per) isograds
are shown by the brackets on either side of the diagram. Temperature
limits are considered minima because calcite, particularly in the Per
zone, has experienced some retrograde exsolution. Equilibria calculated
using thermodynamic data from Helgeson et al. (1978), except for
clinohumite, which was extracted from data of Duffy & Greenwood
(1979). Unit activities assumed for all nonhydrous solids. Activities of
the hydrous silicates corrected for the effects of fluorine substitution by
assuming typical fluorine contents for the affected phases (Moore &
Kerrick, 1976; Cook, 1992): clinohumite, aChm = 0·35; tremolite, aTr =
0·35; talc, aTc = 0·80. Fugacities of CO2 and H2O calculated using
the modified Redlich–Kwong equation of Bowers & Helgeson (1983).
The general path of prograde metamorphism is shown as the stepped
arrow with stipple pattern, and is based on the geothermometry results
and phase equilibria. Limits to fluid X(CO2) at the tremolite isograd
are poorly constrained. The lower limit corresponds to the output fluid
composition [X(CO2) = 0·38] from the higher-grade Fo isograd. Given
the considerable uncertainty in the pore fluid composition of invariant
point I, this value is consistent with formation of Tr by both reactions
(1) and (3). The upper limit is placed at X(CO2) = 0·95, defined by
T = 420°C for the Tr isograd from Cc–Do geothermometry. Between
isograds, pore fluid composition is presumably constant owing to the
lack of reaction capacity in the high-variance assemblages.
imply that most of the tremolite in massive dolomite
formed by reaction (3). Therefore, formation of tremolite
that contains a typical fluorine content by reaction (3)
(Fig. 3) requires conditions between invariant points I
and II [T = 375–450°C, X(CO2) = 0·54–0·99, at
Pf = 75 MPa]. A second-order polynomial fit to the
calcite–dolomite geothermometry results for the south
aureole (Cook & Bowman, 1994) yields a temperature
747
JOURNAL OF PETROLOGY
VOLUME 41
Fig. 4. T–X(CO2) phase equilibria governing the formation of clinohumite- and periclase-bearing mineral assemblages in the periclase
zone. Invariant point IV, Do + Cc + Per + Br + Fluid; invariant
point V, Do + Cc + Fo + Per + Chm + Fluid. Equilibria calculated
as in Fig. 3. Unit activities assumed for all solids except clinohumite
(aChm = 0·35).
of 420 ± 50°C for the tremolite isograd, but this limit
does not further constrain the pore fluid composition. The
narrow spatial interval (p50 m) in the aureole that contains
the univariant assemblage corresponding to reaction (3)
suggests that temperature changes—and resulting increases
in X(CO2)—across this reaction zone were modest.
At least some tremolite in massive dolomite formed
directly from talc by reaction (2) (Moore & Kerrick, 1976).
The tremolite isograd is thus a composite one at which
tremolite formed by two different reactions. The formation
of tremolite by two reactions indicates that pore fluid
compositions at the talc–tremolite boundary were variable,
and on both the low- and high-X(CO2) side of invariant
point I (Fig. 3). Rare univariant assemblages in nodular
dolomite recording reaction (2) (Moore & Kerrick, 1976)
suggest localized areas of elevated X(CO2) and effective
reaction buffering within nodular dolomite strata in the
outer aureole. However, the paucity of low-variance assemblages in both massive and nodular dolomite suggests
ineffective reaction buffering of pore fluid compositions in
both the tremolite and talc zones. Therefore the existence
of distinct prograde paths on both the low- and highX(CO2) side of invariant point I that is required to form
tremolite by two reactions indicates that fluid infiltration
was heterogeneous in character in the outer aureole.
The observed prograde sequence from Tr + Do +
Cc to Fo + Do + Cc across the forsterite isograd
indicates that most of the forsterite was produced by
reaction (4) (Fig. 3). A second-order polynomial fit to the
NUMBER 6
JUNE 2000
Cc–Do geothermometry results for the south part of the
aureole (Cook & Bowman, 1994) yields 470 ± 50°C at
700 m (the average distance of the forsterite isograd
from the igneous contact; Fig. 2). The total range of
temperature estimates for the isograd (420–520°C) would
limit X(CO2) values from 0·02 to 0·38 (Fig. 3). Cc–Do
temperatures are below 500°C in the outer forsterite
zone, which suggests maximum temperature of 490°C
or below and maximum X(CO2) = 0·25 at the isograd.
Phase equilibria and calcite–dolomite geothermometry
restrict the formation of periclase by reaction (5) (Figs 3
and 4) to H2O-rich conditions. A second-order polynomial fit to the Cc–Do geothermometry results for the
south part of the aureole (Cook & Bowman, 1994) yields
580 ± 25°C at 200 m (the average distance of the
periclase isograd from the igneous contact below the
Alta–Grizzly thrust; Fig. 2). The phase equilibria require
minimum temperatures of 578°C and minimum fluid
X(CO2) >0·021 at the periclase isograd (invariant point
IV). The assemblage Per + Chm + Cc is common in
the periclase zone. Clinohumite appears to result from
reaction (8). This assemblage requires maximum T–
X(CO2) conditions below invariant point V [Fig. 4; T =
602°C, X(CO2) = 0·038]. Reaction (8) is closely associated with reaction (5) in T–X(CO2) space (Fig. 4).
This close association requires thermal limits for this
reaction close to those for reaction (5) ( T = 578–602°C).
This temperature range would limit X(CO2) values from
0·028 to 0·038 at the periclase isograd.
The combined T–X(CO2) limits for the isograds define
a generalized prograde path for the massive dolomite in
the Alta aureole (shown by the arrow in Fig. 3). Limits
to X(CO2) decrease in steps from values greater than or
equal to invariant point I (approximately [0·4–0·5) at
the tremolite isograd, to between 0·04 and 0·38 at the
forsterite isograd, and to <0·04 at the periclase isograd.
A prograde path of H2O enrichment for the tremolite to
periclase zones is not a buffered one, because the latter
would evolve to progressively higher X(CO2) values with
increasing grade. Indeed, the paucity of low-variance
assemblages in all zones indicates that the buffering or
reaction capacity of these rocks has been exhausted, and
suggests ineffective buffering within the metamorphic
zones (with localized exceptions within the tremolite and
talc zones). Therefore the prograde path is illustrated as
a series of stepped increases in X(CO2) at each reaction
front with intervals of constant X(CO2) within each metamorphic zone.
FLUID–ROCK INTERACTION AND
FLUID FLUX IN THE ALTA AUREOLE
Geometry of fluid flow
A variety of mineralogical, geochemical and phase equilibria evidence demonstrate that siliceous dolomite in the
748
COOK AND BOWMAN
METAMORPHISM OF SILICEOUS DOLOMITES
south Alta Aureole experienced significant infiltration of
water-rich fluids during its prograde metamorphism and
that the fluid flow was in the direction of falling temperature.
Nature of prograde T–X(CO2) path
The prograde T–X(CO2) path defined by the mineralogy
and geothermometry data is a path of step-wise H2O
enrichment with increasing metamorphic grade for the
three inner zones of the south Alta aureole. This is the
trend predicted by simple models of down-temperature
fluid flow and mineral reaction (e.g. Ferry, 1991,1994).
For T–X(CO2) conditions where the [∂X(CO2)/∂T]P
slopes of mixed-volatile equilibria are positive [the case
for T–X(CO2) conditions in the Alta aureole], infiltrationdriven prograde metamorphism (e.g. decarbonation) will
increase X(CO2) values in the downstream direction of
flow, because of the production of CO2 at reaction fronts
and its subsequent transport downstream. This trend will
occur whether infiltration involves up-temperature flow
of equilibrium fluids or infiltration of disequilibrium
water-rich fluids either up- or down-temperature. As a
result, X(CO2) will increase with rising temperature in
response to up-temperature flow (Baumgartner & Ferry,
1991; Ferry, 1991), but will decrease with rising temperature in response to down-temperature flow of disequilibrium water-rich fluids (Ferry, 1991). These
opposing trends are also maintained for situations involving transient temperature gradients and when the
effects of reaction kinetics and dispersion are considered
(Lasaga & Rye, 1993; Huang, 1994). Thus the overall
trend of decreasing X(CO2) with increasing grade preserved in the dolomitic marbles of the south Alta aureole
is consistent with down-T flow during metamorphism,
in at least the periclase and forsterite zones and probably
the tremolite zone as well.
narrow (less than tens of meters) reaction zones that
characterize the periclase, forsterite, and tremolite isograds; (4) widespread formation of the high-variance
assemblages Tr + Cc + Do and Tc + Cc + Do.
Some other features of the Alta aureole are problematic. Moore & Kerrick (1976) observed the lowvariance assemblage Tr + Tc + Cc + Do + Qz as
reaction rims on chert nodules in dolomites from the
tremolite zone. In our experience, this assemblage is not
common. Furthermore, we have not observed quartz
and dolomite in contact within either the talc or tremolite
zone in samples that display talc or tremolite, nor have
we found evidence for coexistence of calcite and dolomite
within reaction rims around chert nodules. The most
common assemblages in the talc zone are either unreacted
Do + Qz or Tc + Cc + Do, in which reaction
rims of Tc + Cc separate quartz-bearing nodules from
dolomite rock matrix. However, such features suggest
that although down-T fluid flow is a general characteristic
of the aureole, the outer aureole shows evidence for
variable X(CO2). Fluid flow in the outer aureole is thus at
least somewhat heterogeneous and may be geometrically
complicated on several scales.
Boron and iron metasomatism
Marbles within the periclase and forsterite zones contain
more boron than do lower-grade equivalents. This boron
enrichment is bedding controlled in the periclase zone.
Regardless of metamorphic grade, the siliceous dolomite
samples contain trace element levels of iron. Iron-rich,
bedding-controlled skarn layers are found only within
the periclase and inner forsterite zones. Because the Alta
stock contains significant concentrations of both boron
and iron compared with the unmetamorphosed siliceous
dolomite, it is likely that these elements were introduced
into the periclase and forsterite zone marbles by beddingcontrolled fluid infiltration from the Alta stock.
Evidence from mineral assemblages
Forward models of fluid flow and decarbonation reactions
(Ferry, 1994; Dipple & Ferry, 1996) predict sequences
of mineral assemblages that would develop in siliceous
dolomite from up- or down-T fluid flow as a function of
the time-integrated fluid flux (qTIFF). For example, the
formation of periclase is likely to result from down-T flow,
whereas the widespread development of low-variance
assemblages such as Tc + Qz + Do + Cc or Tc
+ Tr + Do + Cc implies up-T flow. The mineral
assemblages of the Alta aureole suggest formation during
down-T fluid flow. The mineralogical evidence for this
include the: (1) presence of periclase up to 200 m from
the igneous contact in the inner aureole; (2) paucity of
low-variance assemblages in all metamorphic zones; (3)
Geothermometry and stable isotope results
Independent calcite–dolomite geothermometry and
stable isotope results also support down-T fluid flow. Cook
& Bowman (1994) noted that the maximum temperature
profile calculated for the south aureole from geologically
reasonable, two-dimensional conductive cooling models
of the Alta stock significantly underestimates the calcite–
dolomite geothermometry results at all grades (fig. 7,
Cook & Bowman, 1994). These calcite–dolomite temperatures are themselves likely minimum estimates of
peak metamorphic temperatures in the aureole (as discussed earlier). This discrepancy implies that significant
advective heat transfer occurred from the stock into
much of the aureole during contact metamorphism. The
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JOURNAL OF PETROLOGY
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geometry of the extensive 18O depletion pattern developed
up to 300 m into dolomitic marble in the south aureole
is consistent with down-temperature fluid flow laterally
away from the igneous contact (fig. 15, Bowman et al.,
1994). Significant bed-to-bed variations in 18O values
within the periclase zone correlate with the presence and
abundance of periclase (fig. 8, Cook et al., 1997). These
characteristics, together with the bedding-controlled development of ludwigite and iron-bearing skarn, indicate
that the down-T flow was also largely bedding controlled
and subhorizontal within the periclase and inner forsterite
zones.
Modeling approach
First-order estimates of time-integrated molar fluid fluxes
(qmTIFF) responsible for the infiltration-driven metamorphism in the south part of the Alta aureole can be
made with simple models of down-temperature fluid flow
linked with fluid–mineral reactions. The locations of
isograds, the prograde T–X(CO2) path, and the measured
abundances of minerals (mineral modes) provide the
quantitative basis for the calculation of these values of
qmTIFF.
Conceptual model
A conceptual one-dimensional model of coupled fluid
flow and mineral reaction based on Ferry (1996) is used
to compute values of qmTIFF and is illustrated in Fig. 5. Fluid
infiltration is down-temperature. Siliceous dolomites are
infiltrated with pure water (consistent with magmatic
fluid from the Alta stock), which is out of equilibrium
with the carbonate rocks. If the effects of diffusion and
dispersion are ignored and local equilibrium is attained
during decarbonation reaction, infiltration of water-rich
fluid produces decarbonation reaction front(s), which
migrate down the flow column as a function of the qmTIFF
and the reaction capacity of the rocks. A decarbonation
reaction buffers pore fluid composition only at the reaction front, which increases the X(CO2) value of the
fluid at, and flowing downstream from, this reaction
front. Because the decarbonation reaction will normally
proceed to completion, the rocks upstream from a reaction front have no remaining reaction capacity with
respect to this reaction. Hence the composition of the
infiltrating or input fluid within this metamorphic zone
will not be modified by these high-variance assemblages,
and will remain constant. For the same reason, the
composition of the output fluid leaving an upstream
reaction front is assumed to be the composition of the
infiltrating or input fluid for the next reaction front
downstream.
NUMBER 6
JUNE 2000
Ferry (1996) has shown that the time-integrated fluid
flux (qTIFF) responsible for migration of a decarbonation
reaction front during down-temperature flow is described
for one-dimensional flow without diffusion or dispersion
by
zf
qTIFF=(vCO2)(max)
1−X(CO2)
dz
XCO2−X(CO2)0
z0
(9)
+(vCO2)(max)(zf−z0)
where vCO2 is the number of moles of CO2 in the decarbonation reaction, max is the maximum reaction progress or reaction capacity of a rock (e.g. the value of
reaction progress at the completion of the reaction),
X(CO2)0 is the composition of the infiltrating or input
fluid responsible for driving the isogradic reaction front,
X(CO2) is the equilibrium fluid composition at the reaction front (and also the composition of the output fluid
downstream from this reaction front), and z0 and zf are
the initial and final distances, in meters, of the isogradic
reaction front (isograd) from the start of the flow path.
For these calculations, fluid flow is considered subhorizontal and away from the igneous contact; with
such geometry, z is equivalent to the observed average
distances of isograds from the igneous contact. This flow
geometry is relatively well established in the periclase
and inner forsterite zones by the bedding-controlled
nature of B and Fe metasomatism discussed above and
by stable isotope evidence presented elsewhere (Cook et
al., 1997). Subhorizontal flow is less well established for
the outer forsterite and tremolite zones, but the advective
heating implied for these zones by Cc–Do geothermometry is consistent with such flow geometry. The
assumption of horizontal flow provides a minimum estimate of z in the south Alta aureole, and therefore a
conservative estimate of qmTIFF.
Fluid composition limits
Estimates of qmTIFF values based on equation (9) will be
sensitive to limits placed on fluid compositions, just as
were earlier box model approaches to estimating water/
rock ratios (Wood & Graham, 1986). The estimated
values of input and output pore fluid compositions at each
isograd, average temperatures of reaction, temperature
ranges, and positions of isograds that are used in calculating qmTIFF are summarized in Table 8.
Initial rock compositions
750
Unmetamorphosed equivalents of Alta marbles contain
<10 modal % calcite, which corresponds to initial atomic
Ca/Mg ratios of 1·20 or less. The Ca/Mg ratios of
COOK AND BOWMAN
METAMORPHISM OF SILICEOUS DOLOMITES
Fig. 5. Idealized one-dimensional flow model used to estimate fluid infiltration in the southern Alta aureole (after Ferry, 1996). Periclase,
forsterite, and tremolite reaction fronts plotted at observed average distances of these isograds from the Alta stock. The values X(CO2)in and
X(CO2)out at each reaction front refer to the X(CO2) value of the infiltrating (input) and equilibrium (output fluid) pore fluid at the reaction front,
respectively.
ratios close to 1·2, and probably have not lost much Mg
during high-grade metamorphism. All of the calculations
presented in the following sections assume that the rock
retained its original Ca/Mg ratio throughout the metamorphic event. If samples with elevated Ca/Mg ratios
have experienced magnesium loss, reaction progress (and
qmTIFF) are probably underestimated to some degree in
these samples, particularly in the periclase zone. However, most of the high Ca/Mg rocks there (with the
exception of 88.B6) also have high 18O values (>17‰;
Cook et al., 1997), which suggest they have experienced
limited fluid flux.
Table 8: Prograde T–X(CO2) limits
Isograd
Reaction zf – z0
T (°C)
T range X(CO2)in X(CO2)out
Tr
3
1200
425
400–450 0·38
Fo
4
700
470
420–520 0·04
0·38
Chm
9
200
580
578–602 0·0
0·04
Per
5
200
580
578–602 0·0
0·04
0·95
Temperature at each isograd based on best polynomial fit to
calcite–dolomite geothermometry data (Cook & Bowman,
1994). Temperature range based on estimated errors in calcite–dolomite geothermometry and phase equilibria illustrated in Figs 3 and 4. X(CO2)out, maximum allowed by
geothermometry and phase equilibria.
Reaction progress
marble samples (Table 7) range from 1·03 to 5·98 but
are bimodally distributed. Almost half of the periclase
and forsterite zone samples exhibit Ca/Mg ratios in
excess of three. Lower-grade equivalents of these high
Ca/Mg rocks are rare, which suggests either that a
calcite-rich protolith is underrepresented in our lowgrade samples (Table 7), or that some higher-grade rocks
may have lost Mg (or gained Ca) during metamorphism.
However, half of the high-grade rocks exhibit Ca/Mg
Values of reaction progress () were measured on 12
representative samples from the tremolite, forsterite, and
periclase zones (Tables 9–11). Reaction progress is calculated using measured mineral modes (Tables 1–3)
together with knowledge of the mineralogical and chemical composition of the unreacted protolith, and following
the methods of Ferry (1986). End-member mineral compositions were assumed for all solid phases, which both
simplifies the calculation of reaction progress and is
reasonable given the mineral compositions in the Alta
aureole (Tables 4–6).
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JOURNAL OF PETROLOGY
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NUMBER 6
JUNE 2000
Table 9: Tremolite zone, reaction progress and fluid flux
Sample:
88.76
2·1 × 102 8·8 × 101 5·9 × 101 1·2 × 102 3·3 × 102 4·7 × 102 6·3 × 102 7·7 × 101 2·9 × 101 1·2 × 102 5·9 × 101 1·6 × 102
m
q
TIFF
v
q
TIFF
88.77
89.10
89.11
89.14c
89.16a
89.16b
89.18
89.20
89.22a
89.22e
89.25b
1·9 × 106 8·0 × 105 5·4 × 105 1·1 × 106 3·0 × 106 4·3 × 106 5·7 × 106 7·0 × 105 2·7 × 105 1·1 × 106 5·4 × 105 1·5 × 106
1·5 × 102 6·4 × 101 4·3 × 101 8·8 × 101 2·4 × 102 3·4 × 102 4·6 × 102 5·6 × 101 2·1 × 101 8·6 × 101 4·3 × 101 1·2 × 102
Calculations for sample modes given in Table 1 based on reaction (3) assuming X(CO2)in = 0·38, X(CO2)out = 0·95. in mol/
m3, qmTIFF in mol/m2, and qvTIFF (volume flux) in m3/m2. Volume flux calculations assume a mean molar volume of 8·0 × 10−5
m3/mol.
Table 10: Forsterite zone, reaction progress and fluid flux
Sample:
88.7
88.8
88.16
88.18
88.20
88.33
88.53
88.55
88.59
88.60
88.C5
88.D2
Fo3·8 × 102 3·5 × 102 1·9 × 102 7·1 × 101 5·3 × 102 3·3 × 102 2·0 × 101 2·2 × 102 5·1 × 101 6·0 × 101 3·1 × 102 2·2 × 102
Tr3·8 × 102 3·5 × 102 1·9 × 102 7·1 × 101 5·3 × 102 3·3 × 102 2·0 × 101 2·2 × 102 5·1 × 101 6·0 × 101 3·1 × 102 2·2 × 102
m
q
TIFF
Fo6·8 × 106 6·3 × 106 3·4 × 106 1·3 × 106 9·4 × 106 5·8 × 106 3·6 × 105 3·9 × 106 9·1 × 105 1·1 × 106 5·5 × 106 4·0 × 106
Tr2·0 × 106 1·9 × 106 1·0 × 106 3·8 × 105 2·8 × 106 1·7 × 106 1·1 × 105 1·2 × 106 2·7 × 105 3·2 × 105 1·6 × 106 1·2 × 106
qvTIFF
Fo4·4 × 102 4·1 × 102 2·2 × 102 8·2 × 101 6·1 × 102 3·8 × 102 2·3 × 101 2·5 × 102 5·9 × 101 6·9 × 101 3·6 × 102 2·6 × 102
Tr1·6 × 102 1·5 × 102 8·0 × 101 3·0 × 101 2·2 × 102 1·4 × 102 8·5 × 100 9·2 × 101 2·2 × 101 2·6 × 101 1·3 × 102 9·5 × 101
Calculations for sample modes given in Table 2 based on reaction (4) and assuming X(CO2)in = 0·04. Equilibrium or output
X(CO2)out = 0·38. in mol/m3 rock, qmTIFF in mol/m2, and qvTIFF in m3/m2. Volume flux calculations assume a mean molar fluid
volume of 6·5 × 10−5 m3/mol. Values for precursor Tr calculated as in Table 9.
Calculation of fluid fluxes
Tremolite zone
As discussed above, the bulk of the tremolite in massive
dolomite was produced by reaction (3). Measured reaction
progress in the 12 samples from the tremolite zone ranges
from 2·9 × 101 to 6·3 × 102 mol tremolite/m3 rock,
and averages 1·9 × 102 mol/m3 (Table 9). The absence
of quartz from tremolite zone samples indicates that
reaction (3) went to completion; hence the calculated
values of reaction progress are maxima, and correspond
to the reaction capacities (max) of these samples with
respect to reaction (3) and equation (9).
The input fluid X(CO2) of 0·38 corresponds to the
fluid composition of the output fluid from the highergrade forsterite zone. The output fluid X(CO2) of 0·95
at the tremolite reaction front is an upper limit based on
the best estimate of temperature (425°C) at the isograd
from the calcite–dolomite geothermometry and phase
equilibria (Table 8, Fig. 3). Given these conservative
estimates of fluid composition, the measured values of
reaction progress, and the average location of the tremolite isograd (1200 m from the igneous contact), calculated values of qmTIFF experienced by the tremolite zone
range from 2·7 × 105 to 5·7 × 106 mol/m2, and average
1·8 × 106 mol/m2 (Table 9). Because the widest possible
range of fluid X(CO2) was used, these fluxes are minimum
values of fluid flux. More restrictive limits for output
X(CO2) are suggested by the narrow tremolite reaction
zone, and would produce a corresponding increase in
the fluid flux required to produce the measured extents
of reaction and observed position of the tremolite isograd.
Forsterite zone
The forsterite isograd is the result of reaction (4). The
modal abundance of forsterite in the 12 samples in Table
2 ranges from 0·7 to 13·4 vol. %, corresponding to
752
COOK AND BOWMAN
METAMORPHISM OF SILICEOUS DOLOMITES
Table 11: Periclase zone, reaction progress and fluid flux
Sample:
88.3
88.4
88.5
88.12
88.14
88.36
88.40
88.51
88.A2
88.A5
88.A7
88.B6
Br—
7·8 × 103 1·1 × 104 5·3 × 102 1·1 × 104 —
6·8 × 103 1·2 × 104 —
8·4 × 103 —
6·2 × 103
Per—
7·8 × 10 1·1 × 10 5·3 × 10 1·1 × 10 —
6·8 × 10 1·2 × 10 —
8·4 × 10 —
6·2 × 103
3
4
2
6·6 × 10 2·2 × 10 —
1
Chm—∗
4
3
4
3
1·2 × 10 5·6 × 10 2·3 × 10 2·3 × 10 —
2
2
1
2
9·1 × 10 —∗
2
3·8 × 102
1
Fo2·7 × 10 3·4 × 10 1·1 × 10 2·0 × 10 6·0 × 10 1·1 × 10 1·1 × 10 1·1 × 10 3·2 × 10 4·6 × 10 2·7 × 10 1·9 × 102
2
1
2
1
1
1
2
2
2
1
2
Tr2·7 × 102 3·3 × 101 1·1 × 102 1·8 × 101 5·9 × 101 4·0 × 101 1·1 × 102 1·1 × 102 3·2 × 102 4·4 × 101 2·7 × 102 1·9 × 102
m
q
TIFF
Br—
1·6 × 106 2·3 × 106 1·1 × 105 2·2 × 106 —
1·4 × 106 2·4 × 106 —
1·7 × 106 —
1·2 × 106
Per—
4·1 × 107 5·9 × 107 2·8 × 106 5·8 × 107 —
3·6 × 107 6·2 × 107 —
4·4 × 107 —
3·3 × 107
6·4 × 105 2·9 × 105 1·2 × 106 1·2 × 106 —
4·8 × 105 —
2·0 × 106
3·5 × 105 1·1 × 106 —
Chm—
Fo1·4 × 106 1·7 × 105 5·5 × 105 1·0 × 105 3·0 × 105 5·8 × 104 5·8 × 105 5·8 × 105 1·6 × 106 2·3 × 105 1·4 × 106 9·7 × 105
Tr4·2 × 105 5·0 × 104 1·7 × 105 2·8 × 104 8·9 × 104 6·1 × 104 1·7 × 105 1·7 × 105 4·9 × 105 6·7 × 104 4·2 × 105 2·9 × 105
qvTIFF
Br—
1·0 × 102 1·4 × 102 7·2 × 100 1·4 × 102 —
8·7 × 101 1·5 × 102 —
1·1 × 102 —
8·0 × 101
Per—
2·6 × 10 3·8 × 10 1·8 × 10 3·7 × 10 —
2·3 × 10 4·0 × 10 —
2·8 × 10 —
2·1 × 103
4·2 × 10 1·9 × 10 7·8 × 10 7·8 × 10 —
3·1 × 10 —
3
3
2
2·3 × 10 7·5 × 10 —
1
Chm—
1
3
3
1
1
1
3
3
1
1·3 × 102
1
Fo9·0 × 10 1·1 × 10 3·6 × 10 6·6 × 10 2·0 × 10 3·6 × 10 3·6 × 10 3·6 × 10 1·0 × 10 1·5 × 10 9·0 × 10 6·3 × 101
1
1
1
0
1
0
1
1
2
1
1
Tr3·3 × 101 4·0 × 100 1·3 × 101 2·2 × 100 7·1 × 100 4·9 × 100 1·4 × 101 1·4 × 101 3·9 × 101 5·4 × 100 3·3 × 101 2·3 × 101
Calculations for sample modes given in Table 3 based on reactions (8) (Chm) and (5) (Per) assuming X(CO2)in = 0·0,
X(CO2)out = 0·038; reaction (6) (Br) assuming hydration of Per by pure water. in mol/m3 rock, qmTIFF in mol/m2, and qvTIFF in
m3/m2. Volume calculations for reactions (5) and (8) assume a mean molar fluid volume of 6·4 × 10−5 m3/mol; for reaction
(6), 5·5 × 10−5 m3/mol. Values for precursor Tr and Fo assuming conditions as in Tables 9 and 10.
∗Includes minor clinohumite, quantity uncertain.
reaction extents from 2·0 × 101 to 5·3 × 102 mol
forsterite/m3 of rock, and averaging 2·3 × 102 mol/m3
(Table 10). The absence of tremolite from the samples
indicates that reaction (4) has gone to completion, and
the calculated values of reaction progress are maxima.
The calculations use an input fluid X(CO2) of 0·04,
and an output fluid X(CO2) of 0·38 at the forsterite
reaction front. The latter is an upper X(CO2) limit consistent with phase equilibria for reaction (4) and with
calcite–dolomite geothermometry (Table 8). The former
corresponds to the fluid X(CO2) evolved by the highergrade periclase-producing reaction. Given these conservative limits to input and output X(CO2), the measured
values of reaction progress, and the average location of
the forsterite isograd (700 m), the calculated values of
qmTIFF experienced by the forsterite zone range from 3·55
× 105 to 9·4 × 106 mol/m2, and average 4·1 × 106
mol/m2 (Table 10). Because the maximum difference
between input and output fluid X(CO2) was used, these
fluid fluxes are minimum values.
Rocks from the forsterite zone also underwent subordinate extents of fluid–rock interaction associated with
formation of precursor tremolite. Table 10 also summarizes values of qmTIFF responsible for this earlier segment
of fluid–rock interaction experienced by the forsterite
zone rocks, calculated using the same conditions as the
tremolite zone samples. These calculations indicate that
values of qmTIFF required to produce tremolite before the
formation of forsterite range from 1·0 × 105 to 2·8 ×
106 mol/m2 (average 1·2 × 106 mol/m2).
Periclase zone
Five of the 12 samples measured from the periclase zone
contain no, or very little brucite, no clinohumite, and
still contain abundant dolomite (Table 3). These five
samples apparently did not have the permeability to
experience sufficient fluid fluxes to drive reactions (5)
and (8); their high 18O values (all >17·3‰; all but one
>19·4‰; Bowman et al., 1994) support this notion. The
other seven, brucite-bearing periclase zone samples
(Table 3) experienced formation of abundant periclase
by reaction (5), clinohumite by reaction (8), the hydration
of periclase to brucite via reaction (6) and/or (7), and
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JOURNAL OF PETROLOGY
VOLUME 41
the retrograde formation of dolomite. Table 11 summarizes the results of reaction progress and fluid flux
calculations for the prograde reactions and for hydration
of periclase to brucite during the initial cooling phase of
the innermost aureole.
Electron microprobe monitoring of F and Si shows
that most brucite-bearing samples contain only humite
minerals, whereas those lacking brucite contain only
forsterite. Two of the 12 samples contain both forsterite
and clinohumite. In accordance with this observation,
the fluid fluxes calculated for the samples in Table 11
assume either complete reaction of forsterite to clinohumite (in brucite-bearing samples) or completely unreacted forsterite (in samples lacking brucite). Modal
clinohumite ranges from 1·1 to 7·5 vol. % in those samples
affected by reaction (8) (Table 3). This corresponds to
a range of reaction extent from 5·6 × 101 to 3·8 × 102
mol clinohumite/m3 of rock, with an average of 1·7 ×
102 mol/m3 (Table 11).
The eight brucite-bearing samples in Table 3 (all but
sample 88.12 are located beneath the Alta–Grizzly thrust
fault) contain modal brucite from 1·4 to 29·1%. All but
sample 88.12 contain >15·4% brucite. Assuming moleper-mole replacement of periclase by brucite, modal
periclase would range from 0·6 to 13·3 vol. %. Compared
with constant volume replacement, mole-per-mole replacement yields less periclase, and thus a minimum
estimate of the time-integrated flux of infiltrating fluid.
This range of modal periclase corresponds to minimum
reaction extents from 5·3 × 102 to 1·2 × 104 mol
periclase/m3 of rock, with an average of 8·0 × 103 mol/
m3.
The calculations for reactions (5) and (8) use an input
fluid X(CO2) of 0·00, and an output fluid X(CO2) of 0·04
at the periclase reaction front. The latter value is the
maximum possible at the periclase reaction front from
calcite–dolomite geothermometry and phase equilibria
constraints (Figs 3 and 4). Calculations for reaction (6)
assume hydration of periclase in pure water to produce
brucite. Use of pure water for the input fluid in these
calculations is compatible with the notion of fluids infiltrating the periclase zone from the adjacent Alta stock
(see discussion below) and minimizes the estimated fluid
flux for these reactions. Given these limits to input and
output fluid composition, calculated values of qmTIFF from
2·9 × 105 to 2·0 × 106 mol fluid/m2 would be required
for the formation of the clinohumite in these samples.
Given these same limits to input and output fluid composition and the average location of the periclase isograd
(200 m from the igneous contact), calculated values of
qmTIFF required to form periclase by reaction (5) range
from 2·8 × 106 to 6·2 × 107 mol fluid/m2, with an
average of 4·2 × 107 mol/m2. Additionally, a molar
fluid flux ranging from 1·1 × 105 to 2·35 × 106 mol/
m2 is required to hydrate periclase to produce brucite by
NUMBER 6
JUNE 2000
reaction (6) during the initial retrograde cooling of the
periclase zone.
Rocks from the periclase zone also experienced subordinate extents of fluid–rock interaction associated with
earlier formation of forsterite and tremolite. Table 11
summarizes the values of qmTIFF responsible for these
previous stages of interaction using the same conditions as
the tremolite and forsterite zone samples. The calculations
indicate that fluid fluxes ranging from 5·8 × 104 to 1·6
× 106 mol fluid/m2 were required to produce forsterite,
and from 2·8 × 104 to 4·85 × 105 mol fluid/m2 for the
formation of tremolite before the formation of forsterite.
DISCUSSION
On average, the values of qmTIFF experienced by the
periclase, forsterite, and tremolite zones are 4·2 × 107,
6·65 × 105, and 2·0 × 105 mol fluid/m2, respectively.
The close spatial association of reacted and unreacted
chert nodules in both the tremolite and talc zones plus
the formation of tremolite by two reactions imply that
fluid flow in the outer aureole was heterogeneous. The
bimodal distribution of qmTIFF values for the periclase zone
samples indicates that fluid flow was also heterogeneous in
character in the innermost aureole. Some layers experienced significant values of qmTIFF; in response, dolomite was consumed by reaction (5) and these layers
became highly depleted in 18O. In contrast, other dolomite-bearing layers do not contain periclase (brucite)
and have high 18O values; these layers have not experienced significant fluid flux.
The average qmTIFF estimated for formation of the
periclase zone is about 200 times greater than the average
qmTIFF estimated for the tremolite zone and about 60
times greater than that estimated for the forsterite zone.
There are several possible explanations for part or all of
these large differences.
For this section of the Alta aureole, the idealized flow
geometry is radial within a horizontal plane (e.g. flow
concordant with subhorizontal sedimentary bedding) outward from the Alta stock, idealized as a vertical cylinder
(Cook et al., 1997). Because of the radial divergence of flow
lines in this version of one-dimensional flow geometry, the
qmTIFF will decrease as a function of the first power of
distance from the Alta stock. Given the relative locations
of the periclase (200 m), forsterite (700 m), and tremolite
isograds (1200 m) from the igneous contact, the consequence of radial flow would be that the qmTIFF at the
periclase isograd should be only 3·5 and six times that
at the forsterite and tremolite isograds, respectively. Even
accounting then for the effects of radial flow geometry,
the qmTIFF estimated for the periclase zone is still almost
20 times that for the forsterite zone, and 35 times
that estimated for the tremolite zone. Thus radial flow
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COOK AND BOWMAN
METAMORPHISM OF SILICEOUS DOLOMITES
geometry does not account for the differences in minimum qmTIFF estimated for the three isograds.
The large differences in qmTIFF may also reflect in part
the assumptions used to estimate qmTIFF. Our approach
of selecting the maximum possible differences between
the X(CO2) values of the input and output pore fluids
for the fluid flux calculations results in minimum estimates
of the quantities of qmTIFF for all three zones. However,
this difference can be limited far more closely at the
periclase isograd than at either the forsterite or tremolite
isograd (Fig. 3). This is because the greater uncertainty
in the calcite–dolomite geothermometry results below
500°C combined with the flatter [∂X(CO2)/∂T]P slopes
of the forsterite- and tremolite-forming reactions produce
far larger possible differences between the X(CO2) values
of the input and output fluids at the forsterite and
tremolite reaction fronts than at the periclase front.
Consequently, we potentially underestimate values of
qmTIFF at the former two isograds far more than at the
periclase isograd. To illustrate, calculated values of qmTIFF
can be increased in the forsterite and tremolite zones to
match that of the periclase zone (adjusted for radial flow)
by decreasing the equilibrium fluid composition at the
forsterite and tremolite reaction fronts to 0·16 and 0·52,
respectively. Both these reduced values are compatible
with the fluid composition limits defined by the existing
phase equilibria [given the large uncertainties in the
X(CO2) location of invariant point I; Fig. 3] and geothermometry data for these two reaction fronts. Indeed,
the observed narrow reaction zones (less than tens of
meters) that characterize both forsterite and tremolite
isograds suggest that the actual differences between input
and output fluid compositions at these reaction fronts may
be much less than the maxima allowed by uncertainties in
temperature.
Alternatively, the differences in values of qmTIFF calculated for the three zones may be real, and indicate
that the computed values of qmTIFF for the tremolite and
forsterite zones are much less than that for the periclase
zone. Because of the subhorizontal, bedding-concordant
fluid flow inferred for at least the inner Alta aureole (this
paper; Bowman et al., 1994; Cook et al., 1997), these
differences in qmTIFF would imply that fluid has leaked
out of the flow system between the periclase and tremolite
zones. Ferry (1994) has inferred similar behavior in
the Beinn an Dubhaich aureole. This is a potentially
significant result, and suggests that not only fluid fluxes,
but perhaps additionally the flow geometry, could be
distinct in the outer aureole from that documented by
this and previous studies for the inner aureole. These
results raise new questions about whether, how, and
where these fluids escape. Future work needs to focus on
locating these escape zones and documenting what they
look like geologically and geochemically. A number of
small high-angle faults located in the south aureole,
particularly one nearly coincident with the surface trace
of the forsterite isograd (Fig. 2), are possible candidates for
fluid escape. Geological relationships (e.g. cross-cutting
relationships) do not indicate whether these faults are
pre- or post-intrusion. Oxygen and carbon isotopes will
not be useful tracers to document any leakage along
these faults into the marbles above the Alta–Grizzly
thrust fault because most of these faults lie beyond the
carbon and oxygen isotope exchange fronts in the south
aureole (Bowman et al., 1994). Other petrologic and
geochemical tracers will need to be evaluated and applied
to document any such leakage.
Comparison with other estimates of timeintegrated fluid flux
The average qmTIFF value of 4·2 × 107 mol/m2 experienced by the periclase zone that is estimated from
reaction progress compares well with those determined
previously from oxygen isotope and geothermometry
evidence preserved in the Alta aureole. Bowman et al.
(1994) simulated the position of the oxygen isotope front
preserved in the dolomitic marbles beneath the Alta–
Grizzly thrust with a one-dimensional 18O/16O transport
model. The qmTIFF required to reproduce the location of
the oxygen isotope front with this one-dimensional model
is >1·2 × 107 mol/m2. This one-dimensional model
underestimates the actual qmTIFF experienced by the inner
Alta aureole because it does not account for the decrease
in qmTIFF produced by the radial flow geometry thought
to characterize this part of the Alta aureole. Nevertheless,
this estimate is on the order of the value of qmTIFF estimated
from petrologic reaction progress in this study.
Cook et al. (1997) applied a two-dimensional (axisymmetic) model of heat and mass transport to both the
oxygen isotope exchange front and to the temperature
profile recorded by calcite–dolomite geothermometry in
the south Alta aureole. Duplication of both profiles can
be accomplished simultaneously with a model permeability of 2·5 × 10−16 m2 and a qmTIFF of about 3·4
× 107 mol/m2. This model accommodates the effects of
diverging flow lines resulting from the radial flow geometry that actually occurs in the Alta aureole. This
estimate of qmTIFF is in excellent agreement with that
calculated here by petrologic methods.
Using one-dimensional models of fluid flow and mineral
reaction that predict the spatial distribution of mineral
assemblages developed in siliceous dolomites, Ferry (1994)
concluded that the sequence and spacing of the isograds in
the Alta aureole were compatible with down-temperature
flow and qmTIFF values of >1 × 107 mol/m2. This estimate
is equivalent to that estimated with one-dimensional
models of oxygen isotope transport (Bowman et al., 1994),
but is somewhat lower than estimated with petrologic
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JOURNAL OF PETROLOGY
VOLUME 41
reaction progress in this study and by Cook et al. (1997).
The Ferry (1994) model also predicts that primary brucite
would be expected at somewhat larger values of qmTIFF,
but primary brucite has not been observed in the Alta
aureole. However, given the approximations and assumptions incorporated into all of these models (e.g.
steady-state thermal profile in the reaction progress and
isotope models; equilibrium reaction and exchange; simplified flow geometry), the agreement in estimated qmTIFF
is probably reasonable.
SUMMARY
The prograde path followed by the massive dolomites in
the Alta aureole is one of decreasing X(CO2) with rising
temperature, and requires significant infiltration of a
H2O-rich fluid with increasing metamorphic grade. This
path, together with the widespread occurrence of highvariance assemblages such as Fo + Cc + Do, Tr +
Cc + Do, and Tc + Cc + Do suggest that the
infiltration-driven metamorphism involved down-T flow
of H2O-rich fluids away from the igneous contact. The
isograds apparently migrated outward from the intrusive
contact as a set of reaction zones driven by the infiltrating
fluids as the fluid flowed down the thermal gradient.
The widespread occurrence of high-variance assemblages
indicates that reactions were driven to completion at
reaction fronts, resulting in the general lack of buffering
within all three metamorphic zones. Boron and iron
metasomatism, the peak temperature profile based on
calcite–dolomite geothermometry, and whole-rock 18O
data also indicate that the infiltrating fluids entered the
marbles from the adjacent Alta stock, requiring fluid
flow down the metamorphic thermal gradient. This flow
geometry implies that the infiltrating fluids were initially
out of equilibrium with the adjacent siliceous dolomites,
and consequently the prograde metamorphic sequence
in Alta’s siliceous dolomites preserves a natural example of
metamorphism driven by disequilibrium fluid infiltration
down a metamorphic thermal gradient.
Estimates of time-integrated fluid flux can be obtained
from published models of fluid flow and mineral–fluid
reaction, given knowledge about the geometry of fluid
flow, the positions of the reaction fronts preserved as
mineral isograds, and measured reaction progress. Using
this approach, the average time-integrated fluid flux
required to produce the observed widths of the periclase,
forsterite, and tremolite zones is 4·2 × 107, 6·6 × 105,
and 2·0 × 105 mol/m2, respectively. The value for the
periclase zone is in good agreement with the value of 3·4
× 107 mol/m2 calculated by simulation of the observed
isotope exchange front and the temperature profile recorded by calcite–dolomite geothermometry in the south
Alta aureole (Cook et al., 1997). This value is also in
NUMBER 6
JUNE 2000
adequate agreement with fluid fluxes predicted by petrologically based fluid flow and reaction models.
The estimates of qmTIFF for the forsterite and tremolite
zones have much greater uncertainty, but may indicate
that fluid flux was considerably lower in these zones
compared with the periclase zone. Given the outward
(down-temperature), subhorizontal flow geometry indicated by a variety of petrologic, geochemical, and
geothermometry evidence presented here and elsewhere,
this decrease implies that fluid has leaked from the flow
system between the periclase and tremolite zones. Future
studies will attempt to verify the actual differences in
qmTIFF between the inner and outer aureole, and to identify
and characterize fluid escape zones.
ACKNOWLEDGEMENTS
This study represents a portion of the senior author’s
Ph.D. dissertation at The University of Utah. We thank
J. M. Ferry, G. M. Dipple, and G. T. Roselle for their
constructive and thoughtful reviews. Financial support
for this study was provided by NSF Grants EAR-8904948,
EAR-9205085, and EAR-9903274 to J. R. Bowman.
Additional support was provided by The University of
Utah Mining and Mineral Resources Research Institute
in the form of a research fellowship to the senior author.
756
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