Metamorphism of reduced granulites in low-CO2 vapour-free environment

Lamb, William M.; Valley, John W.

doi:10.1038/312056a0

Cited by 184 publications

(61 citation statements)

References 22 publications

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“…The anhydrous conditions required for granulite formation can be attained either by dry, vapourfree metamorphism (e.g. Lamb and Valley, 1984;Yardley and Valley, 1997) or through the input of internally- (Hansen et al, 1987) or externallyderived CO 2 -rich uids (Newton et al, 1980;Farquhar and Chacko, 1991;Wada and Santosh, 1995). External sources of CO 2 include magmatic, mantle-derived uids (Newton et al, 1980;Farquhar and Chacko, 1991;Satish-Kumar and Santosh, 1998), metamorphic decarbonation reactions (Hapuarachchi, 1977;Silva, 1987), and devolatilization reactions of organic matter (Dissanayake, 1994).…”

Section: Discussionmentioning

confidence: 99%

Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif)

et al. 2000

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Four distinct associations of graphite have been identified in the low-pressure, high-temperature belt of the Sierra de Aracena (SW Spain). Syngenetic occurrences include: (1) stratiform graphite mineralization within a calc-silicate series; (2) disseminated graphite within a terrigenous sequence; and (3) 'restitic' graphite within anatectic tonalites and their enclaves. Epigenetic graphite occurs as (4) veins cross-cutting mafic granulites.Graphite in all types of occurrences is highly crystalline, with the c parameter close to 6.70 A Ê . Such c values correspond to temperatures of formation of~8008C. The thermal properties of graphite are also typical of well-ordered graphite and provide DTA exothermic maxima ranging from 810 to 8588C depending on the mode of occurrence. The differences among the temperatures of formation estimated by graphite geothermometry, the position of the exothermic maximum in the DTA curves, and petrologic geothermometers are discussed in terms of the applicability of graphite geothermometry to granulite-facies rocks. Carbon isotope analysis yields d 13C values in the range from 31.6 to 21.4% for syngenetic graphite of types I, II and III attributable to biogenically-derived carbon. The heavier signatures for graphite in vein occurrences (d 13 C= 17.7 to 18.3%) with respect to syngenetic graphites suggest that isotopically heavy carbonic species were incorporated into the metamorphic fluids (probably as a consequence of decarbonation reactions of the calc-silicate rocks) from which graphite precipitated into the veins. These fluids were strongly channelled through structural pathways.KEY WORDS: graphite, calc-silicate rocks, low-P/high-T metamorphism, southern Spain.

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Section: Discussionmentioning

confidence: 99%

Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif)

et al. 2000

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“…Frost et al [1989] reported graphite on grain boundaries in plagioclase-rich rocks from the Laramie Anorthosite Complex using Auger electron spectroscopy. It has been considered that graphite, which is an excellent conductor, precipitates on grain surfaces from a CO 2 -rich fluid during cooling [Lamb and Valley, 1984]. However, Frost et al [1989] noted that the fine graphite film along the grain boundaries shows a disconnected nature, and they argued that decompression destroyed the connectivity of the graphite film when deep-seated rocks were tectonically exhumed.…”

Section: Application To Equilibrium Fluid Distributions Of Continentamentioning

confidence: 99%

Wetting properties of anorthite aggregates: Implications for fluid connectivity in continental lower crust

et al. 2002

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[1] The connectivity of C-H-O fluids in anorthite aggregate was assessed by measurements of the dihedral angle, q, at conditions of 800 -1000°C and 0.8 -1.2 GPa for duration of 1 -21 days. The longer-duration experiments show reduction of the effect of crystal anisotropy on pore geometry. Especially, fluid-filled small pores (<5 mm) in anorthite aggregate quickly approach to textural equilibrium controlled by minimization of surface energies in <3 days. Anorthite aggregates with CO 2 fluid show large dihedral angle over 120°. In the anorthite aggregates with aqueous fluid the dihedral angles are over 60°at pressure of 1.0 GPa and decrease across critical angle of 60°with increasing pressure. Over the range of experimental conditions, isopleths of dihedral angle show nearly zero to negative dP/dT slope with increasing temperature. The observed reduction in dihedral angle with increasing pressure and temperature may be attributed to increase in plagioclase solubility in aqueous fluid. Natural anorthite (with small amounts of albite component) shows smaller dihedral angle than pure anorthite. At higher temperature conditions the anorthite content of the run products deviates progressively from that of starting material. This suggests that preferential solubility of albite component relative to anorthite affects the equilibrium fluid distributions of plagioclase by aqueous fluid. Experimental results point to the likelihood that plagioclase-rich lower continental crust will not allow intergranular fluid flow of C-H-O fluids even in the presence of high geothermal gradient.INDEX TERMS: 3630 Mineralogy and Petrology: Experimental mineralogy and petrology; 3947 Mineral Physics: Surfaces and interfaces; 5112 Physical Properties of Rocks: Microstructure; 5114 Physical Properties of Rocks: Permeability and porosity;

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“…However, even when at concentrations as high as several weight percent, crystalline graphite typically forms isolated flakes which will not be as conductive as a continuous film at much lower concentration. Flakes of crystalline graphite have a high luster which makes them readily visible, even when present at trace levels, and while graphite is a common accessory mineral in some metasediments, it is rarely visible in the majority of granulite facies rocks [Lamb and Valley, 1984].…”

Section: Rate Of Drying Out Of the Lower Crustmentioning

confidence: 99%

The petrologic case for a dry lower crust

Yardley¹,

Valley²

1997

J. Geophys. Res.

325

166

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Abstract. Fluid pressure in the crest may be controlled by different mechanisms according to depth, temperature, and the mineralogy of the host rocks. Where rocks are fluid-saturated, fluid pressure may approach lithostatic or hydrostatic pressure depending on the ductility of the wall rocks and the connectivity of pores and fractures. However, if the host rocks contain minerals formed at temperatures higher than those currently prevailing, they will react with fluids to produce hydrated (or carbonated) retrograde minerals, and the fluid pressure will be limited by thermodynamic equilibrium between high-grade reactant minerals and retrograde products. The thermodynamically constrained parameter, water fugacity, may have a value of tens to hundreds of bars in the lower crest. In practice, this means that for typical igneous or high-grade metamorphic rocks now occurring in stable lower crest, notional fluid pressures are substantially (1 to 3 orders of magnitude) lower than lithostatic. No free, connected fluid phase can be present in deep stable crest, and alternative explanations must be sought for the relatively high electrical conductivity of such rocks. The proposal that high lower crustal conductivity is due to thin grain boundary films of graphite is also unlikely to be generally true because films of sufficient thickness would be readily visible on broken surfaces of hand specimens. An alternative explanation of the discrepancy between laboratory and field measurements of the conductivity of high-grade rocks is that laboratory measurements are not normally made under appropriate conditions of rockbuffered fluid pressure.

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Metamorphism of reduced granulites in low-CO2 vapour-free environment

Cited by 184 publications

References 22 publications

Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif)

Graphite occurrences in the low-pressure/high-temperature metamorphic belt of the Sierra de Aracena (southern Iberian Massif)

Wetting properties of anorthite aggregates: Implications for fluid connectivity in continental lower crust

The petrologic case for a dry lower crust

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