In situ chemical fractionation in thin basaltic lava flows: examples from the Auckland volcanic field, New Zealand, and a general physical model

Basaltic lava flows of the Auckland volcanic field, northern New Zealand, commonly contain near-vertical cylinders and sub-horizontal sheets of a rock more coarse grained and vesicular than the host. The cylinders and sheets, referred to collectively as pegmatoid autoliths, are 1–8 cm in diameter/thickness. The flows in which they occur are all less than 10 m thick and of pahoehoe type. Pegmatoid autoliths are enriched in most elements except Cr, Ni, Mg and Ca which are depleted, and Si, Al, and Sr which remain unchanged with respect to the host flow. Pegmatoid autoliths represent in situ chemical fractionation of a basaltic lava flow. Mass-balance calculations show that pegmatoid autoliths have the same composition as the interstitial liquid after 36–50% crystallisation of the host. The segregation process requires a rigid permeable crystal framework through which the interstitial liquid moves by gas filter-pressing. A physical model of gas filter-pressing in a cooling lava is developed and predicts that a combination of high permeability, low melt viscosity and thick lava should favour the segregation of interstitial melt. A review of geological observations shows that the occurrence of pegmatoid autoliths conforms with the modelʹs predictions.