212 



not greatly increased by increasing water pressure. This and the extremely slow rate 

 of reactions below 300°C, are some of the more important data used in placing the 

 lower temperature limit on greenschist facies metamorphism. 



Even in 10,000 m of sea water (equivalent to about 1 kbar), the minimum 

 estimate of 3 kbar (Turner and Verhoogen, 1960) for greenschist-facies pressures 

 would not be obtained. Thus, if this minimum pressure estimate is correct, some 

 burial beneath the sea floor is required. If the metamorphism took place beneath a 

 5,000 m deep sea floor, an additional 9. 1 km of basah at a density of 2.8 g/ml would 

 be required to give a water pressure of 3 kbar assuming water pressure equals litho- 

 static pressure. 



An alternative approach to obtaining a rough estimate of lithostatic pressure 

 may be based on the minimum temperature of the greenschist facies (300°C, Turner 

 and Verhoogen, 1960). By assuming a steady state temperature gradient, a rock 

 conductivity of 0.004 cal./sec. deg, and using various heat-flow measurements in the 

 Mid-Atlantic Ridge, estimates of the depth to the 300°C isotherm may be obtained 

 (Table VIII). These depths for the high heat-flow value and the average heat-flow 

 value are less than that from the minimum, lithostatic pressure estimate. The temper- 

 ature gradient is inversely proportional to the rock conductivity in a steady-state 

 gradient, thus considerable error in the depth estimate may result from the uncer- 

 tainty in the assumed conductivity. However, based on the above assumptions, a 

 minimum depth of ca. 2 km of basalt would be required in even the areas of high 

 heat flow, such as those noted by Bullard and Day ( 1 96 1 ). Barth ( 1 962) suggests that 

 greenschist facies temperatures maybe as low as lOO^C. Thus, considerably less cover 

 may have been required than indicated by the temperature estimates of Turner and 

 Verhoogen (1960). On the other hand, Winkler (1965) estimates higher minimum 

 greenschist temperatures; 400''C at a pressure of 1 kbar. 



TABLE VIII 



DEPTH AT WHICH 300°C ISOTHERM WOULD BE REACHED BASED ON RECENT HEAT-FLOW MEASUREMENTS, 

 FROM THE MID-ATLANTIC RIDGE, AND ASSUMING STEADY STATE TEMPERATURE GRADIENT AND ROCK 



CONDUCTIVITY OF 0.004 caL/scc degree 



Heat-flow measurements^ 



^ Explanation: / = uncommonly high heat flow (Bullard and Day, 1961);2 = averageof 32 measure- 

 ments within 100 km of ridge (Lee and Uyeda, 1965); i = uncommonly low value (Lee and Uyeda, 

 1965>. 



^ Includes pressure increment due to an assumed 5,000 m of sea water (ca. 500 bars); rock density 

 assumed to be 2.8 g/cm^, a value typical for abyssal basalts. 



