PETROGENESIS OF NATIVE IRON-CARBON ALLOYS, DISKO ISLAND, GREENLAND

An investigation of the field relations and petrology of native iron-carbon alloys xenoliths found in basalts at Uivfaq, Disko Island, was undertaken in an attempt to determine the origin of the iron. Optical microscopy and electron microprobe techniques were used to study the iron masses, which are complex assemblages of iron, iron carbide, and minor iron phosphide, with inclusions of iron oxides, iron sulphides, and iron-rich silicates. The physical-chemical and geologic conditions of formation of the iron are inferred and the following model for the petrogenesis of the iron is proposed.; During early ((TURN)60 m.y.) rifting between Greenland and Baffin Island, basaltic magma erupted into a Cretaceous sedimentary basin on Disko and assimilated pieces of carbonaceous sediment. The carbon in these inclusions reduced iron oxide components of the magma to produce a molten high-carbon iron-carbon-(phosphorus) alloy at T = (TURN)1250 to 1150 C and logf(,O(,2)) = -12 to -13.5 at a depth of less than 3 km. This natural process was similar to that which is used to produce molten high-carbon iron (pig iron) in the blast furnace. Some of the molten "pig iron" was decarburized in a process similar to that used to manufacture low-carbon steels (an oxidation process) at logf(,O(,2)) = -12 to -9.5. The decarburization reaction is exothermic and could have raised temperatures near the iron at least 300(DEGREES)C. Consequently, molten low-carbon irons were produced. Both the low and high carbon irons were carried upward in basaltic magma as mm-sized grains and accumulated at a depth of (TURN)1 km.; FeO and P(,2)O(,5) were concentrated into basaltic magma by oxidation of the metal during the decarburization reaction. An Fe-S-O liquid separated from the silicate liquid. TiO(,2) was preferentially partitioned into the residual silicate liquid. The Fe-S-O liquid and (FeO + P(,2)O(,5) + TiO(,2))-rich silicate liquid were immiscible with respect to one another and to iron-carbon liquid. These liquids were trapped by the crystallizing and accumulating iron as intragranular and intergranular inclusions.; The intragranular silicate inclusions separted into an FeO-rich liquid and an SiO(,2)-rich liquid. These liquids were similar to immiscible liquids formed in lunar basalts. In some inclusions the onset of immiscibility was preceeded by crystallization of plagioclase (An(,50-60)) and chromian ulvospinel. . . . (Author's abstract exceeds stipulated maximum length. Discontinued here with permission of author.) UMI...