The thermal role of fluids in granulite metamorphism was presented. It was shown that for granulites to be formed in the middle crust, heat must be advected by either magma or by volatile fluids, such as water or CO2. Models of channelized fluid flow indicate that there is little thermal difference between channelized and pervasive fluid flow, for the same total fluid flux, unless the channel spacing is of the same order or greater than the thickness of the layer through which the fluids flow. The volumes of volatile fluids required are very large and are only likely to be found associated with dehydration of a subducting slab, if volatile fluids are the sole heat source for granulite metamorphism

Ashwal, Lewis D.

Morgan, Paul

English

NASA Technical Reports Server

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HEAT TRANSFER BY FLUIDS IN GRANULITE METAMORPHISM; 
Paul Morgan, Geology Department, Box 6030, Northern Arizona University, Flagstaff, AZ 
8601 1-6030, USA, and Lewis Ashwal, Lunar and Planetary Institute, 3303 NASA Road One, 
Houston, TX 77058-4399, USA. 
Granulite metamorphism represents the extremes of crustal conditions short of melting. 
These extreme conditions place important constraints upon models that can be used to explain 
the generation of granulites which we find exposed at the surface. In this short contribution 
we examine these constraints and discuss the role of fluids in granulite metamorphism, with 
special reference to the granulites of southern India. 
Requirements of Granulite MetamorI, his 
Granulite metamorphism requireszmperatures in excess of 700OC, and pressures are 
commonly recorded in exposed granulites indicating burial depths of 15 to 30 km. Nearly all 
examples of exposed granulite rocks contain at least some component of supracrustal rocks, 
including sediments, volcanics, or other rock units which formed at or near the Earth's 
surface (1). These granulites are now exposed on the surface of normal thickness crust (35 to 
40 km). Thus, three components are required in any mechanism proposed to explain granulite 
metamorphism: 
1) Transport of su racrustal rocks to 15-30 km; 
3) Re-exposure at the surface of normal thickness crust. 
2) Heating to 700 8 C or higher; and 
Models for Regional Metamom hism 
Models of regional metamorphism can be divided into two basic groups for the purpose of 
understanding granulite metamorphism: Monogenetic and Polygenetic models. In monogenetic 
models the transport of supracrustals to 15 to 30 km and the re-exposure at the surface are 
assumed to result from a single tectonic event. The commonly invoked mechanism invoked in 
this context is continental underthrusting and crustal thickening through which the 
supracrustals are transported to depth by underthrusting, and re-exposed by isostatic uplift 
and erosion of the thickened crust. The constraint that the exposed granulites are always 
underlain by 35 to 40 km of crust is inherent in this mechanism. In polygenetic models, 
transport to depth and re-exposure are assumed to result from different tectonic events. The 
supracrustals can be transported to depth either by underthrusting as in the monogenetic 
models, or by deep burial associated with multiple extensional crustal thinning events (isostatic 
considerations suggest that single events are unlikely to produce sedimentary basins much 
greater than 10 km, even under the most favorable assumptions). Re-exposure then results 
from an independent tectonic event, either a compressional event in which the granulites are 
overthrust onto a normal thickness crust, or through tectonic unroofing during extension, 
although the normal thickness crust constraint requires that this extension takes place in an 
overthickened crust. 
We have so far ignored the second necessary component of granulite formation, the 
heating to 70'C or more. This component places important constraints upon heat transfer 
associated with granulite metamorphism. In all monogenetic models of granulite formation, and 
the polygenetic models which invoke tectonic unroofing during extension, a minimum of 35 to 
40 km of crust beneath the granulites is required at the time of their heating and formation. 
As the granulite mineralogies commonly indicate burial depths of 15 to 30 km, they would 
have been formed roughly quarter to half-way down an overthickened crustal section. Taking 
a very simplistic view that the geothermal gradient is uniform throughout the crust, the Moho 
temperature would be expected to be two to four times the temperature at the depth of 
granulite formation: i.e., 70O0C at the granulite formation depth implies Moho temperatures of 
1400 to 2800OC. It is obviously over-simplistic to assume a uniform geothermal gradient 
throughout the crust, but as shown by Ashwal and others (l), if steady-state conductive 
conditions are assumed, it is impossible to devise a geotherm based upon realistic heat 
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HEAT TRANSFER BY FLUIDS 
Morgan, Paul and Ashwal, Lewis 
110 
production values and thermal conductivities that decreases sufficiently with depth to allow 
temperatures of 70O0C or more in the mid to upper crust without super-solidus temperatures in 
the lower crust. This constraint is removed if the granulites are formed in the lower crust 
and erosionally exposed following subsequent thrusting over Crust of normal thickness. 
Alternatively, the steady-state conductive geotherm may be modified by the effects of 
convection in the crust. 
Heat can be convected in the crust by the movement of fluids through the crust or by 
movement of the crust itself relative to the surface. We consider the latter form of 
convection fmt. General upward movement of the Crust in isostatic response to erosion or 
tectonic unroofing has the effect of making the geotherm convex upward and raising 
temperature in the upper crust. Numerous examples of this effect are given by England and 
Thompson (2) for a variety of initial assumptions for monogenetic regional metamorphism 
models. However, although the required geotherm can be modelled by this mechanism, 
examination of pressure-temperature-time (PTt) paths for rocks initially buried in the mid to 
upper crust, shows that these rocks cool as a result of uplift and only pass through the 
granulite formation temperature field for models in which massive melting (super-solidus 
temperatures) is predicted in the lower crust. Maximum temperatures are attained at any 
specified horizon when the erosion rate is a minimum after crustal thickening, Le., under 
steady-state conditions. Significantly increased temperatures at any specified horizon can only 
be generated by the upward convection of fluids through the crust, either magma or volatiles. 
Models of magmatic convection of heat into the crust and the resulting metamorphism 
have been presented by Wells (3). These models show that the sustained addition of magma to 
the crust profoundly influences the geotherm, primarily at levels in the crust below the depth 
of magmatic accretion to the crust. Thus, the most efficient mode in which to produce 
granulites by this mechanism would be through intrusions into the upper crust, above the level 
at which the granulites are formed. Unfortunately these intrusions would be eroded before 
the granulites could be exposed, but some evidence of the passage of magmas through the 
granulites may be expected to remain. If no evidence exists for magmatism associated with 
metamorphism, heat transfer by volatiles may be a viable mechanism. 
Advection of heat and matter by fluids during metamorphism has recently been studied by 
Bickle and McKenzie (4) for the case in which the rock is modelled as a porous medium, and 
heat transport is laterally homogeneous. At depth, however, it is likely that fluids flow 
through discrete fractures, and we have started to investigate metamorphism associated with 
fracture-controlled fluid-flow. Bodvarsson (5) has presented solutions for heating associated 
with water flowing through fractures for a limited set of flow conditions and single planar- 
fracture geometries. We use these solutions to estimate the effect of steady constant- 
temperature fluid-flow through a system of vertical planar-fractures in the crust. If vertical 
heat transfer in the rock medium is neglected, crustal temperatures are dominated by the 
temperature of the ascending fluid, which is of the fom: 
T = T erfc(A(d-z)) 
where To is the temperilture of the base o? the crust and the temperature at which the fluid 
enters the fracture, erfc( ) is the complementary error function, d is the crustal thickness, z is 
depth, and A is a constant, defined by the flow rate, the ratio of fluid to rock thermal 
properties, and the time since the flow started. Preliminary calculations suggest that, for 
water, very modest flow rates (of the order of 0.1 g/s per m of horizontal fracture length) 
can significantly modify the geothem, and that flows sustained over time periods of 1 ka to 1 
Ma, depending upon fracture spacing, can produce temperatures compatible with granulite 
metamorphism in the mid to upper crust without requiring melting in the lower crust. More 
complete numerical studies by Hoisch (6) support these results and conclusion. 
Granulites of Southern India 
Summaries of the geology of the Southern Indian Shield (7, 8) suggest that granulite 
metamorphism in the southern portion of this shield was associated with a late Archean and/or 
early Proterozoic mobile belt in which the crust was thickened by compressional deformation. 
HEAT TRANSFER BY FLUIDS 
Morgan, Paul and Ashwal, Lewis 
111 
There is abundant evidence for CO -K metasomatism throughout the shield, and we tentatively 
suggest that fluid-flow associatdwith this metasomatism was the primary agent of heat 
transport for the granulite metamorphism. Definition of a plate-tectonic regime associated 
with this deformation/metamorphism even is controversial, but it seems likely that compression 
and fluids for metasomatism/metamorphism were associated with early Proterozoic subduction. 
Acknowled eemen ts  
This study has benefitted greatly from discussions with Tom Hoisch, and we gratefully 
acknowledge access to the results of his numerical models studies prior to publication with 
which we have checked the approximations used in our analytical solutions. 
References 
(1) L.D. Ashwal, P. Morgan and W.W Leslie, Thermal constraints on high-pressure 
granulite metamorphism of supracrustal rocks, in Workshop on Cross Section of Archean Crust 
(L.D. Ashwal and K.D. Card, eds.), p. 13-19, LPI Tech. Rpt. 83-03, Lunar and Planetary 
Institute, Houston, 1983; (2) P.C. England and A.B. Thompson, Pressure-temperature-time paths 
of regional metamorphism I. Heat transfer during the evolution of regions of thickened 
continental crust, J. Petrology, 25, 894-928, 1984; (3) P.R.A. Wells, Thermal models for the 
magmatic accretion and subsequent metamorphism of continental crust, Earth Planet. Sei. Lett., 
46,253-265,1980; (4) M.J. Bickle and D. McKenzie, The transport of heat and matter by fluids 
during metamorphism, Contrib. Mineral Petrol, 95, 384-392, 1987; (5) G. Bodvarsson, On the 
temperature of water flowing through fractures, J. Geophys. Res., 74,1987-1992,1969; (6) T.D. 
Hoisch, Geol. Soc. Am., Heat transport by fluids during channelized flow and thermal 
consequences for regional metamorphism, Abs. w/ Prog., National Mtg., Phoenix, AZ, October, 
1987; (7) S.M. Naqvi and J.J.W. Rogers (eds.), Precambrian of South India, Geol. Soc. India, 
Memoir 4,575 pp., 1983; (8) R.C. Newton and A.T. Anderson (eds.), The Dharwar Craton of 
South India: An Archean Protocontinent, J. Geology, 94, pp. 127-299, 1986. 


Heat transfer by fluids in granulite metamorphism

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