Paleoproterozoic amphibolites and gneisses - that are remarkably depleted in ^(18)O are found in the Belomorian Belt
in Karelia, Russia [1,2]. We mapped their extent to exceed 200x20km and affect metamorphosed mafic intrusions (est.
~2.4 Ga intrusion age) and host 2.6Ga gneisses found in this 1.9 Ga collisional belt. δ^(18)O values of –7 to –27.3‰
characterize minerals and rocks from several of these localities; some of these rocks are also remarkably depleted
with respect to δD (-212 to –235‰ amphiboles). All have typical terrestrial Δ^(17)O values of 0‰. Based on previous
paleogeographic reconstructions, we attribute the origin of these exotic O and H isotope compositions to the
hydrothermal alteration associated with subglacial rifting during the Paleoproterozoic panglobal ice ages, but discuss
additional possibilities: extremely low-δ^(18)O Paleo- proterozoic sea water, and excursion of Karelia to polar latitudes. Given
that at high-T hydrothermal exchange equilibrium Δ^(18)O(rockwater) is close to zero, but water-rock interaction is rarely
100% efficient, the lowest measured δ^(18)O value in silicates likely gives the upper δ^(18)O bound for the altering meteoric
fluid; we thus continues our quest to find the lowest δ^(18)O material such as a mineral assemblage or a tiny zircon
fragment that would provide record of δ^(18)Owater