and Lewis wollastonite deposits where samples containing wollas- ... Table 1. Mineralogy for samples containing magnetite +ilmenite, wollastonite, or hercynite.
Contributions to Mineralogy and Petrology
Contrib Mineral Petrol (1987) 96:485-495
9 Springer-Verlag 1987
Post-metamorphic C02-rich fluid inclusions in granulites William M. Lamb*, John W. Valley, and Philip E. Brown Department of Geology and Geophysics, University of Wisconsin, Madison, W1 53706, USA
Abstract. In granulite-facies samples from the Adirondack Mountains, NY, estimates of peak-metamorphic CO2 fugacities based on mineral equilibria are not consistent with estimates based on data for high-density, CO2-rich fluid inclusions. Of the 21 Adirondack samples investigated for this study, all contain CO2-rich inclusions. Inclusions occur in quartz, apatite, and garnet. They range in size from 3 to 50 vm and are without visible I-I20. In a few of the inclusions, freezing point determinations and preliminary Laser Raman spectroscopy show the presence of small amounts ( < 3%) of other fluids (N 2 and H2S). CO2 liquidvapor homogenization temperatures are between - 4 6 and +31~ corresponding to densities between 1.14 and 0.5 gm/cc. Some of these densities are consistent with peakmetamorphic entrapment (1.06 to 1.1 gm/cc). Peak metamorphic fluid compositions in these samples are inferred from fluid-buffering equilibria that restrict the fugacity of CO2 Q~CO2) directly (i.e., calcite + quartz + wollastonite) or buffer the fugacity of oxygen 0c 02). Assemblages that buffer f 0 2 are important because knowledge o f f O2 places an upper limit on f CO2. In 13 of the 21 samples, estimates of peak-metamorphic fluid compositions based on these equilibria show that the mole fraction of CO2 (XCO2) in equilibrium with the rock was low, in some cases less than 0.2. The contradiction of mineral equilibria and fluid inclusion data shows that the inclusions record post-metamorphic conditions. At present, there are no criteria to distinguish these "primary appearing" CO2-rich inclusions from those found in other granulite-facies terranes. Therefore, inferences of pressure-temperature conditions and peakmetamorphic fluid compositions based on fluid inclusions must be viewed with caution.
Introduction
Reduced water fugacities ( f H 2 0 ) are characteristic of many rock types found worldwide in granulite facies terranes. Low values o f f H 2 0 are inferred from equilibria involving orthopyroxene, amphibole, biotite, or silicate melts. Quantitative estimates of f H20 support this conclusion, with ~H20 = 0.01 to 0.6 (Bohlen et al. 1980b; Phillips 1980; Valley et al. 1983; Lamb 1983; Powers and Bohlen 1985; Bhat* Present address: Department of Geology, Texas A&M University, College Station, TX 77843, USA Offprint requests to: W.M. Lamb
tacharya and Sen 1986). The presence of high-density CO2rich fluid inclusions in rocks from granulite facies terranes is thought to provide insight into the meaning of these low water fugacities (Touter 1971; Hollister and Burruss 1976; Coolen 1982; Touret and Dietvorst 1983; Hansen etal. 1984; Rudnick et al. 1984; Santosh 1985; Schreurs 1985; Touret 1985). This study examines fluid inclusions in samples from the Adirondack Mountains, NY, and shows that the composition of the fluids found in many high-density CO2-rich fluid inclusions does not correspond to the composition of the peak metamorphic fluid. This suggests entrapment occurred after the peak metamorphic event. Various mechanisms have been proposed to explain low f H20 at the peak of metamorphism in granulites, including: (1) melting, where H20 is partitioned into a melt that may then be removed to shallower crustal levels (Fyfe 1973). In instances of partial melting without removal of magma, H20 dissolved in the silicate liquid will later be released on crystallization. Evidence for this fluid may be preserved as retrograde hydration or as post-metamorphic fluid inclusions. (2) Infiltration of CO2 to dilute the H20 evolved by dehydration reactions. Evidence from several granulite terranes has been interpreted to suggest that the deep crust is uniformly flooded by CO2 (Newton et al. 1980; Condie et al. 1982; Coolen 1982; Janardhan et al. 1982; Glassley 1983; Hansen etal. 1984; Friend 1985; Schreurs 1985), and this mechanism requires massive quantifies of fluid as estimates of the CO2/rock range from 0.01 to 0.25 (Newton et al. 1980; Lamb and Valley 1984; Newton 1986). The source of CO2 may be metacarbonates (Glassley 1983), the mantle (Newton et al. 1980), or carbonate-bearing magmas (Touret 1985). (3) Metamorphism of anhydrous rocks such as orthogneisses or rocks that have previously undergone high-grade metamorphism (Valley 1985). Only two of the above mechanisms are consistent with the formation of CO2-rich fluid inclusions at the peak of metamorphism: the infiltration of CO2 (mechanism 2) or partial melting (mechanism 1) in which case the H20 would be partitioned preferentially into the melt while CO2 remains as a residual fluid (Kadik and Lukanin 1973). If melting proceeds under fluid-absent conditions, or if rocks are already dry before metamorphism, then no free fluid phase existed at the peak of metamorphism. In this case, values of H20 and CO2 fugacities were buffered only by solid phases or silicate liquids, and fluid inclusions must have formed later. COz-rich fluid inclusions occur in rocks from several granulite facies terranes, including the Adirondack Moun-
486 tains, New York, Bamble, southern N o r w a y (Touret 1971 ; Touret and Dietvorst 1983; Touret 1985), southern India (Hansen et al. 1984; Santosh 1985), Kapuskasing, Ontario (Rudnick et al. 1984), British Columbia (Hollister and Burruss 1976), central Manitoba (Vry and Brown 1986), southwest Finland (Schreurs 1985), and southern Tanzania (Coolen 1982). If these fluid inclusions formed by either mechanism 1 or 2 then it m a y be possible that they were trapped at or near the peak of metamorphism. However, these fluid inclusions may also have been trapped after the peak o f metamorphism, possibly during a tectonically unrelated event. In this case, the compositions of such inclusions could not reflect peak-metamorphic fluids. A synmetamorphic origin o f CO2-rich fluid inclusions in granulites is sometimes inferred from textures that suggest the CO2-rich inclusions formed earlier than other inclusions. Early entrapment, however, does not preclude the possibility o f a post-metamorphic origin since all generations o f fluid inclusions could have formed after the peak of metamorphism. Also, fluid inclusions that were originally trapped during the peak o f granulite facies metamorphism may not preserve peak-metamorphic fluid compositions or densities. These properties may be altered through reequilibration by processes such as leakage, stretching, or diffusion of hydrogen out of the inclusion (Sterner and Bodnar 1985, 1986). The relatively high density ( > 0.9 gm/cc) o f some CO2rich fluid inclusions in granulites has often been cited as evidence of synmetamorphic entrapment. In some cases, geobarometry, based on the assumption that these inclusions contain the peak-metamorphic fluid, yields pressures that agree with independent, mineralogic barometry. Such results do not preclude the possibility that these fluid inclusions record post-metamorphic P - T conditions. In some terranes the transition from amphibolite-facies to granulite-facies corresponds to an increase in the abundance of CO2-rich inclusions relative to those that are H 2 0 rich (i.e., Bamble, southern Norway, Touret 1971; southwest Finland, Schreurs 1985; central Manitoba, Vry and Brown 1986), although the transition in inclusion compositions does not always exactly correspond to the location o f the orthopyroxene isograd ('Vry and Brown 1986). If these inclusions reflect the composition of the peak-metamorphic fluid, then either CO2-infiltration or anatexis may explain the presence of this transition. This study compares the composition of fluids trapped in inclusions with the composition of granulite facies fluids deduced from mineral equilibria. This approach allows the assessment of the time of trapping of COz-rich fluids (peakvs. post-metamorphic) without relying on possibly ambiguous evidence such as textural interpretations and fluid inclusion barometry. The granulite facies terrane of the Adirondack Mountains, N Y has been chosen for study because self-consistent and quantitative estimates o f peak metamorphic pressure, temperature, and fluid composition are available based on mineral equilibria.
Sample location and mineralogy Samples chosen for this study, from the Adirondack Mountains, NY (Fig. 1), contain mineral assemblages that constrain CO2 fugacities. They have been divided into three groups: (1) magnetite+ ilmenite-bearing samples, (2) wollastonite-bearing samples, and (3) hercynite-bearing samples.
~-AS-I ]
Fig. 1. Exposure of PreCambrian rock in the Adirondack Mountains, NY. Shown are the locations of samples containing coexisting magnetite + ilmenite as well as anorthosite (hatched) and selected charnockitic complexes (ruled). The Willsboro (W) and Lewis (L) wollastonite deposits are also located. The first appearance of orthopyroxene occurs in a zone shown by the dashed lines in the northwestern portion of the Adirondacks, with granulite facies to the south and east (see text). Sample LB-1, from within this zone, contains orthopyroxene and thus, all samples have experienced granulite facies conditions The Adirondacks are part of the Grenville Province and were metamorphosed during the Grenville orogeny (~ 1.1 b.y.). Metamorphic grade increases from upper amphibolite facies in the Lowlands (northwest) to granulite facies in the Highlands (east-central). Granulite facies pressures and temperatures range from 650_+50~ C at 7.0 kbar to 775+50~ C at 7.6 kbar; the highest pressures and temperatures are located in the central Highlands (Bohlen et al. 1985). Locations for the magnetite+ilmenite-bearing samples are in figure 1 (MM-5 is shown even though no fluid inclusion data exist for this sample). Also shown are the locations of the Willsboro and Lewis wollastonite deposits where samples containing wollastonite were collected (see Table 1). The hercynite-bearing samples are located adjacent to the western contact of the Diana charnockire complex (near sample LB-1 on Fig. l), and more detailed sample locations are given in Powers and Bohlen (1985). The zone of various orthopyroxene "isograds" is plotted on Fig. 1; for discussions on the placement of the orthopyroxene isograd see Wiener et al. 1984 and Bohlen etal. 1985. All the samples are located within or east of the zone of the orthopyroxene isograd (Fig. 1), and experienced granulite facies metamorphism. The peak-metamorphic mineral assemblages contained in all samples are listed in Table 1. Most samples also contain small amounts of retrograde minerals. Although the grain size of these minerals is small, making optical identification difficult, many samples appear to contain carbonate (usually calcite) 4-sericite___chlorite-I- secondary amphibole___secondary biotite. Of these minerals, calcite is the most readily identifiable as use of Alizarin stain and cathode luminescence revealed the presence of this mineral in almost every sample. In some samples the retrograde minerals occur in small veins. These veins are usually less than 2 mm in length and 0.2 mm in width, although in one case veins exceed t0 mm in length (SR-31). Carbonate (possibly calcite) also occurs as small (usually < 20 p.m) inclusions in quartz and feldspar. These mineral inclusions are often associated with CO2-rich fluid inclusions and both fluid and solid inclusions may be found in the same coplanar array. Other workers have described retrograde minerals in anorthosite samples from the Adirondacks and have shown that these
487
Table 1. Mineralogy for samples containing magnetite +ilmenite, wollastonite, or hercynite. Fluid inclusion analyses and mineralogy are from the same magnetite+ilmenite-bearing samples investigated by Bohlen and Essene (1977) and Bohlen et al. (1980a) and the same hercynite-bearing samples investigated by Powers and Bohlen (1985). New thin sections were prepared for this study Magnetite + Ilmenite-bearing samples
76-AS-1 SR-3 MM-13 N-5 TP-5 TP-1 t IN-10 LB-1 GVR-64 GVR-274 GVR-275 GVR-276
Mag+ Ilm Qtz
Kfs
PI
Opx
Cpx
X X X X X X X X X X X X
X X X X X
X X X X X X X X X X X X
X X
X X X X X X X X
X X X X X X X X X X X X
X X X X X X
X X
Grt
Amph
Bt
Ap
Zrn
Py
X X X X X X X
X X X X X X
X X X X
X
X X X X
X
X X X X X X
X X X X X X
Hercynite-bearing samples (from Powers and Bohlen 1985)
83-LB-5B 83-LB-21 83-LB-39B 83-LB-79 83-LB-81
Hc
Qtz
Kfs
P1
Grt
Bt
Sil
Ilm
X X X X X
X X X X X
X X X X X
X X X X X
X X X X X
X
X X X X X
X X X X
X X X
Crd
Crn
X X X X
X X
Wollastonite-bearing samples
84-AUS-1 a 84-AUS-3 ~ LEWIS A ~ W-50-388b
Wo
Qtz
Cal
Cpx
Grt
X X X X
X X X X
X X
X X X
X X X
" From the Lewis wollastonite deposit b From the Willsboro wollastonite deposit
Abbreviations: Amph Amphibole; Ap Apatite; Bt Biotite; Cal Calcite; Cpx Clinopyroxene; Crd Cordierite; Crn Corundum; Grt Garnet; Hc Hercynite; Ilm Ilmenite; Kfs K-feldspar; Mag Magnetite; Opx Orthopyroxene; Pl Plagioclase; Py Pyrite; Qtz Quartz; Sil Sillimanite; Wo Wollastonite; Zrn Zircon
minerals are commonly associated with CO2-rich fluid inclusions and document post-metamorphic hydrothermal activity (Morrison et al. 1986).
Fluid inclusion analysis Doubly polished thin sections, 100 to 200 gm in thickness, were prepared for fluid inclusion analysis. Measurements of CO2 homogenization and final melting temperatures were performed on a Fluid Inc. heating/freezing stage. These measurements are generally reproducible to _+0.1 o C and thermoeouple calibration using synthetic fluid inclusions indicates accuracy to within 0.3~ C for temperatures reported in this study.
C 0 2-rich inclusions All 21 samples contain CO2-rich fluid inclusions and m a n y contain more than one compositional type, including mixed H / O + CO2, a n d H20-rich with variable salinity. This investigation has focused primarily on the CO2-rich inclusions because of their possible significance for theories of granulite genesis. These inclusions range in size from 3 to
50 gm and comprise the vast majority of the inclusions in the m a g n e t i t e + i l m e n i t e - and hercynite-bearing samples. Other compositional types are present in m i n o r amounts, if at all. Conversely, the wollastonite-bearing samples generally contain a b u n d a n t H20-rich, and mixed H 2 0 + C O 2 fluid inclusions in addition to the CO2-rich fluid inclusions. Only those COz-rich inclusions that contain no visible evidence of fluids that are immiscible with CO2 at room temperature (e.g., I t 2 0 ) were considered in this study. COz-rich fluid inclusions in samples from the Adirondacks c o m m o n l y occur in planar arrays that often cut across grain boundaries. This " s e c o n d a r y " texture is illustrated in Fig. 2a. I n some cases, there are lone CO2-rich inclusions with no clear relation to other inclusions ("primary"). Sample LB-1 is particularly notable in this respect as it contains numerous " p r i m a r y " CO2-rich inclusions (Fig. 2b). The terms " p r i m a r y " and " s e c o n d a r y " are used here in accordance with Roedder (1984) and are m e a n t only as textural terms, no genetic meaning is intended. Most CO2-rich inclusions occur in quartz, although some inclusions are found in garnet and apatite. Every fluid
488 [ /
J
'
I
'
y
,
f/'
ADIRONDACK / GRANULITE / P-T
8t-
-
N 2
O.
200
~o6
400 600 TEMPERATURE =C
800
Fig. 3. CO2 isochores (labeled with density in gm/cc) calculated from the modified Redlich Kwong equation of Kerrick and Jacobs (1981). Also shown are the pressure-temperature conditions of granulite-facies metamorphism in the Adirondacks (Bohlen et al. 1985). The density of COz in fluid inclusions from the Adirondacks ranges from 0.5 to 1.14 gm/cc (see text). If entrapment at peakmetamorphic temperatures is assumed, these densities yield pressures from < 2 to 9 kbar
Fig. 2a, b. Photographs of CO2-rich fluid inclusions, a (top) shows a trail of "secondary" fluid incluisons from sample TP-5-1, while b shows "primary" fluid inclusions from sample LB-I. Scale bars represent 20 gm inclusion observed in apatite is CO2-rich. The fluid inclusions in apatite are generally
