Titanian chondrodite- and titanian clinohumite-bearing metadunite from the 3800 Ma Isua supracrustal belt, West Greenland; chemistry, petrology and origin Available to Purchase

Titanian chondrodite (Ti-Ch) and clinohumite (Ti-Cl) have been discovered in a ser-pentinite from the 3.8 Ga Isua supracrustal belt, West Greenland, where they occur with olivine (≈ Fo₉₇), magnesite (X_Mg ≈ 0.99), magnetite, and Ni-Co-Pb sulfides in a matrix of antigorite. This serpentinite has the bulk composition of a dunite (Niggli value mg ≈ 0.89), with high Ni (∼2300 ppm) and Cr (∼950 ppm), and its REE pattern is broadly similar to those reported for cumulate dunites of igneous origin. The highly magnesian phase compositions indicate that the metadunite evolved from a serpentinite precursor, which experienced additional late serpentinization. Two different generations of olivine and antigorite, both with slightly different compositions, indicate an extremely complex history for the sample. However, the titanian humites locally occur as inclusions in olivine, indicating that they formed relatively early.

Ti-Ch (∼9 wt% TiO₂) occurs as separate grains and also forms lamellar intergrowths with Ti-Cl (∼6 wt% TiO₂). These phases differ significantly only in terms of Ti/Si, as X_Mg (∼0.98), X_Ni, X_Mn, X_Ti (∼0.45), and X_F (∼0.05) are similar. The high content of Ti and low content of F indicate nearly complete TiO₂Mg₋₁F₋₂ exchange, whereas the high X_Mg values make these the most magnesian Ti-rich humites yet reported.

The two-phase intergrowths may represent the reaction Ti-Ch + forsterite = Ti-Cl. The sample lacks geikielite or any magnesian ilmenite, indicating that the terminal breakdown of titanian humites by the reaction titanian humites = forsterite + geikielite + H₂O was not approached. Chemographic relations for the system MgTiO₃-Mg₂SiO₄-H₂O reveal that Ti-Ch should be the high-pressure humite phase, on account of the experimentally determined location of the Ti-Cl breakdown reaction (Engi and Lindsley, 1980).

The only other known occurrence of a Ti-Ch and Ti-Cl intergrowth is in kimberlite (Aoki et al., 1976), and the possibility that Ti-Ch is a mantle phase should be reconsidered. However, we do not regard the Isua sample to be of mantle origin, although the intergrowths may record decompression during the polymetamorphic history of the region.