Petrology of basalts from the Central Indian Ridge (lat. 25°23′S, long. 70°04′E): estimates of frequencies and fractional volumes of magma injections in a two-layered reservoir

Abstract

Mineralogy, melt inclusions in crystals and reverse zoning in minerals of oceanic basalts can be used to get an insight into the functioning of a magma chamber beneath a spreading zone. Dredged basalts from the Central Indian Ridge (CIR), 21 km from the Rodriguez triple junction (RTJ) exhibit two groups of olivine and plagioclase of different composition. The first group is composed of forsteritic olivine (Fo 90) and very anorthitic plagioclase (An 89–90) resorbed megacrysts, containing numerous melt inclusions. Those observed in the core of the plagioclase megacrysts are the most primitive. Their compositions have been considered as the parental magma composition ( Mg-number= 0.77; low TiO 2: 0.31 wt.%; and low Na 2O: 1.2–1.4 wt.%). These magmas have a density about 2.70 g cm −3 and viscosity close to 100 poises at 1250°C (liquidus temperature). In the diagramFeO */MgO vs. (TiO 2MgO) × 100, the zigzag evolution of the melt inclusions from core to rim of these plagioclase megacrysts has been interpreted as the result of three cycles of injection of parental liquid, mixing and olivine crystallization. The second group of minerals is essentially composed of more evolved olivine (Fo 86–88) and plagioclase (An 67–72) euhedral mesocrysts. Some olivines are reverse-zoned (3 steps). They crystallized from hybrid liquids resulting from three successive mixings between batches rising from the base of the magma chamber and an evolved magma at the top of the reservoir. Time between the mixings can be calculated from the diffusion profiles of Ni in the reverse-zoned olivines. These three mixings have been achieved within 4 days. A two-layer model for the magmatic reservoir is proposed to account for the mineralogy, the melt inclusions and glassy matrix compositions of those basalts. An estimation of the total volume of the chamber has been made, knowing the volume of extruded magmas at zero age. The chamber would have a radius of 400–500 m. Relative volume of lower layer where the first group of minerals crystallized and the upper layer where the second group of minerals formed is around 1:3.