Open system crystallization and mixing in two-layer magma chambers

Abstract

A modeling technique is presented for describing trace element behavior in tholeiitic magmas evolving by either perfect or imperfect fractional crystallization and magma mixing in periodically replenished two-layer reservoirs, in which eruption events occur at various times. The models are based on density versus mafic index calibrations, and can be used to examine two replenishment situations: (1) dense (picritic) liquid ponds at the base of the magma chamber or, (2) light liquid rises buoyantly through the resident magma and collects at the top of the reservoir. Input parameters are bulk solid/liquid distribution coefficients ( D) and enrichment/depletion factors ( e) i.e. the ratios between concentrations of trace elements in the erupted and incoming liquids, respectively. The results show graphically the relations between the weight fractions of incoming liquid ( W), the crystallized magma ( X 1 and X 2) and the erupted magma ( Y t ). These models can be used to distinguish imperfect fractional crystallization from Rayleigh's law-dependent fractionation. They are applied to mid-ocean ridge basalts (MORB) dredged from Galapagos Spreading Center (GSC) at 85 °W and to rocks from the Newark Island Layered Intrusion (NILI). The most likely mechanism to explain the GSC MORB variations is imperfect fractional crystallization. By contrast, the NILI variations appear to have been chiefly controlled by perfect fractional crystallization. The Y t versus W relations for these two situations are very different. Indications are that at Galapagos, the erupted magma amounts to less than 5 wt.% of the reservoir, whereas at NILI, half of the mass of the initial replenishing magma was erupted during a cycle.