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 orthopyroxene crystals collected from the marginal border group and the lower zone a and b of the Skaergaard intrusion, and modeled these data to retrieve 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, which 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 the orthopyroxene crystals strictly apply to temperatures around the closure temperatures of the ordering states, ∼ 340-400° C, which fall at the transition from convective to the lower temperature conductive cooling. The cooling rates obtained from the cation ordering states of orthopyroxene vary from ∼ 1 to 270 K/ka. These results agree well with a thermal model calculated using an assumed average permeability of 10^-12cm² for the pluton, but not completely with a model calculated on the basis of an average permeability of 10^-13 cm², although both values produced shifts of δ¹⁸O that are comparable to those observed in the pluton.