Petrochemistry of opaque minerals in eclogites from the Western Gneiss Region, Norway

Abstract

Oxide minerals, especially those in the ilmenite group, occurring in several different eclogite pods in the Western Gneiss Region of southern Norway have been analysed with an electron microprobe.

The same oxide microassemblage often shows different ilmenite chemical compositions in different eclogite pods such that each pod, with its particular collection of oxide microassemblages, may be characterised by the trace-element chemistry of its ilmenites. Whilst Al, Cr, Fe³⁺, Mg, Mn and Ni display systematic variations in the magnetites and rutiles in these rocks, Mg and Mn are the only critical additional elements in the ilmenites and they can chemically distinguish the different petrographically-defined oxide microassemblages: clear IL, hemo-ilmenite, ilmeno-hematite, network texture IL and patchwork texture IL. Several reasons why the IL—HT solvus cannot be used for geothermometry are demonstrated.

In the eclogite pod at Liset, those ilmenites in the “patchwork texture” are simultaneously rich in Mg and Mn; some compositions are so rich in Mn (∼ 30 wt.% MnO) that they are Fe-pyrophanite or Mgpyrophanite rather than Mn-ilmenite or Mg-ilmenite. These compositions are recorded here for the first time as all other published analyses of the geikielite—ilmenite—pyrophanite (GIP) mineral group are rich in either Mg or Mn; their development is explained by a local Mn influx accompanied by Mg and Mn enrichment due to oxidation of ilmenite to produce hematite and “blitz” rutile during retrogressive amphibolitisation. Two chemical trends in the GIP compositions are related to the distinct petrography and Fe/Ti, Fe/Mg and Mg/Mn ratios in their host garnetite or pyroxenite environments.

Two non-eclogitised rock types at Liset can also be distinguished by the Mg and Mn contents of their ilmenites. The GIP triangular diagram thus holds promise as a petrogenetic discriminator for polymetamorphic eclogites and associated rocks as well as for carbonatites and kimberlites.