CATION ORDERING OF ORTHOPYROXENES FROM THE SKAERGAARD INTRUSION - IMPLICATIONS FOR THE SUBSOLIDUS COOLING RATES AND PERMEABILITIES

We have determined the quenched cation ordering states of five orthopy roxene crystals collected from the marginal border group and the lower zone a and b of the Skaergaard intrusion, and modeled these data to r etrieve their closure temperatures (T-c) of Fe-Mg ordering and cooling rates. According to existing thermal models for the Skaergaard pluton , conductive cooling dominated the high and low temperature regimes, w hich were separated by an intermediate temperature regime in which the cooling was controlled primarily by convective fluid circulation. The cooling rates retrieved from the quenched cation ordering states of t he orthopyroxene crystals strictly apply to temperatures around the cl osure temperatures of the ordering states, similar to 340-400 degrees C, which fall at the transition from convective to the lower temperatu re conductive cooling. The cooling rates obtained from the cation orde ring states of orthopyroxene vary from similar to 1 to 270 K/ka. These results agree well with a thermal model calculated using an assumed a verage permeability of 10(-12) cm(2) for the pluton, but not completel y with a model calculated on the basis of an average permeability of 1 0(-13) cm(2), although both values produced shifts of delta(18)O that are comparable to those observed in the pluton.