Hydrogen isotope analysis of micro-scale apatite inclusions in Archaean zircon grains

A novel study of in situ hydrogen isotope composition (dD) measurements of micron-scale apatite inclusions hosted in zircon crystals was carried out by lateral high-resolution secondary ion mass spectrometry. The host rock of zircon is a tonalite belonging to the Nuvvuagittuq Supracrustal belt, Canada, which is considered to be one of the oldest supracrustal belts of Hadean-Archean age. Uranium-lead dating of host zircon crystals yielded an age of 3636 ± 20 Ma, which is consistent with previously reported ages of this unit. The dD values were measured for seven apatite inclusions, and range from –225 to +65 (vs. SMOW). A negative correlation was observed between dD values and the ¹⁸ O ^- / ¹ H ^- ratios of the examined apatite inclusions. This relationship can be explained by a mixing trend between the original hydrogen in the apatite inclusions and contamination from the analytical environment. The pristine dD value estimated for the apatite inclusion is +53 ± 60, which was clearly distinct from the dD value resulting from bulk rock analysis, –49 ± 2. The pristine dD value of the tonalitic magma source may provide new constraints on the formation of the primitive crust and/ or the ocean of the early Earth.