Paleoarchean variole-bearing metabasalts from the East Pilbara Terrane formed by hydrous fluid phase exsolution and implications for Archean greenstone belt magmatic processes

Archean greenstone belts contain low grade metamorphic volcanic sequences with localised examples of preserved original textures and relict magmatic minerals, which are invaluable for inferring processes by extension magma genesis. A feature observed in many but rare modern rocks is variolitic texture millimetre- to centimetre-sized leucocratic globular structures (varioles), mafic lavas. Models proposed explain this include rapid undercooling, mingling liquid immiscibility. Here, we investigate samples Paleoarchean variole-bearing pillow lavas from the ca. 3469 Ma Mount Ada Basalt Pilbara Supergroup, Western Australia. The clinopyroxene spinel preserve sharp contacts between varioles host. No plagioclase samples. mineral composition indistinguishable This despite variole material having lower MgO Fe2O3(total) higher SiO2 Al2O3 concentrations. However, host materials have TiO2, Cr2O3, NiO2 Sm-Nd isotope systematics. These observations consistent an immiscibility reaction a parental melt hydrous fluid phase conjugate liquid, where minerals that formed before become incorporated into both endmembers they continued grow diffusive exchange silicate melt. Two lines evidence indicate flux exsolution occurred due H2O saturation precursor magmas, including: (1) lack plagioclase, crystallisation supressed at high content; (2) Nb/Th < 7, indicates hydrated mantle source. part 10-Myr-duration Coongan Subgroup cycle occur interspersed tholeiite basalts minor komatiites felsic Duffer Formation. distinctly different rock types, εNdi −0.3 ± 0.4, ~1ε unit than contemporary tholeiites, there was variation genesis, included fluxed melting likely genetically related delamination, during 10 Myr formation cycle, as East Terrane formation.