Extended Data Fig. 5: Olivine zoning.
Mg#, Ni, and Cr increase from rim to core in Baffin Island olivines. In spinel-rich olivine cores, the abundance of these elements extends to higher levels (Mg# > 87.5, Ni > 4.5 wt%, and Cr > 0.14 wt% in most grains) than in fluid inclusion-rich olivine rims. Mn profiles exhibit the opposite trend and zoning is less systematic with respect to Al and Ca. The two olivine populations—spinel-rich and fluid inclusion-rich—have overlapping Mg#, Ni, and Cr that are offset to lower values for fluid inclusion-rich grains by ~1, 0.5 wt%, and 0.2 wt%, respectively. Shown here are representative electron probe micro analyzer (EPMA) transects across a fluid inclusion-rich olivine (A and C) and a spinel-rich olivine (B and D). Both olivines are from sample PING18-H2. In A and B, backscatter electron images show the ___location of EPMA transects and the distribution of inclusions on the polished surface of the grain. In A, most inclusions are fluid inclusions (black). In B, the olivine has large spinel inclusions in its core (white) and some fluid inclusions (black). In both grains, the Mg# (100 × MgO / [MgO + FeO]) is greatest in the core and decreases towards the rim. Spinel-bearing olivines have higher Mg# in their cores than fluid inclusion-rich olivines without spinel. Some grains have spinel cores overgrown by lower Mg# rims that contain fluid inclusions. Complete EPMA data are available in SI Table 3.
