Crystal origins and magmatic system beneath Ngauruhoe volcano (New Zealand) revealed by plagioclase textures and compositions.

The textural variation and compositional zoning of plagioclase in pre-historic and historic basaltic andesite lava flows from Ngauruhoe volcano reveals extensive crystal recycling from a multi-level magma system. Most phenocrysts have a calcic (~ An 80–90 ) resorbed core with diffuse or no zonation that is depleted in Fe and Mg. Some cores display patchy zonation from replacement by high An crystallization prior to resorption. The cores are mantled by oscillatory-zoned rims of lower An content (< An 60 ), and are enriched in Fe and Mg. Rim zones vary in relative thickness and textural complexity, and include sieve-textured bands, and/or cyclic calcic growth following dissolution events. A subordinate crystal population display similar features, but lack a resorbed core. These latter crystals display overall rimward enrichment in An, Fe and Mg. The resorbed cores crystallized from magmas more mafic than those erupted at Ngauruhoe, and slow cooling and prolonged storage resulted in loss of An zoning patterns and depletion of Fe and Mg by diffusion. These crystals are likely to have originated from deep cumulates or intrusions, and were subsequently entrained in ascending magmas. Patchy-textured cores were produced during decompression in a water under-saturated magma and staged ascent. The diversity in crystal cores reflect different conduits and ascent histories. The crystal rims grew in a more differentiated magma reservoir, and are in equilibrium with the erupted melt. Most of the zoning patterns in the rim zone require water pressure and/or temperature changes. These changes could have been caused by convective self-mixing in a closed system and/or the intrusion of hydrous melts of similar bulk composition. Other crystals display rimward elemental enrichments consistent with mafic recharge. Previously reported rimward enrichment in 87 Sr– 86 Sr compositions can be explained by the re-cycled origin of the crystal cores and progressive crustal assimilation at shallower depths in the magma system where subsequent crystal growth occurred. A steady-state balance of intrusion and crystallization in the deep crust, and remobilization and entrainment of co-magmatic crystals in newly formed and ascending magmas has produced relatively monotonous batches of basaltic andesite throughout the volcano's life. Trace element zonation (Fe and Mg) in the plagioclase mostly reflects variable diffusive loss through the crystal history, and the patterns in different crystals are highly variable. This, along with uncertainties in estimating trace element partitioning coefficients, highlights the difficulty of reconstructing melt compositions and subterranean residence time in such magmas. [ABSTRACT FROM AUTHOR]