Major element constraints on melting, differentiation and mixing of magmas from the Galapagos 95.5° W propagating rift system

Abstract

Projections in the pseudo-quaternary system diopside-plagioclase-olivine-quartz (di-pl-ol-Q) are used in this paper as a base on which to plot additional compositional, petrologic or tectonic variables. Used in this manner, the projections provide a sensitive means of displaying and evaluating the chemical variability of mid-ocean ridge basalt (MORB) suites, such as that of the Galapagos 95.5° W propagating rift system, and allow mantle controls on the chemical variability of MORB to be distinguished from the predominant effects of shallow-level crystal fractionation and mixing. Primitive lavas (mg # > 63) from the 95.5° W region form a broadly linear array within the di-pl-ol-Q tetrahedron, parallel to the di-ol join. In terms of two recent high pressure studies of MORB petrogenesis (Stolper 1980; Takahashi and Kushiro 1983), the primitive liquids in this array are not primary, but have evolved by less than 10% olivine fractionation from an array of primary liquids which separated from residual mantle over a range of pressures between 10–20 kb. With appropriate assumptions as to residual lithology and mg # of residual olivine, the nature and origin of 95.5° W primary magmas can be deduced: (1) Lavas of the normal rift are derived from a broad range of primary magmas that separated from compositionally uniform (in terms of major elements) mantle over a pressure range of 10–20 kb. (2) Beneath the newly forming propagating rift, melting is initiated, perhaps by pressure release, at shallow depth (9–11 kb). Degree of melting and source composition appear similar to those for the normal rift. Within a few km of the propagating rift tip, this shallow source is joined by a second, deeper (11–12 kb), more iron-rich source. Both sources persist for at least 100 km along the propagating rift, but the deeper source appears to become dominant farther behind the tip. Maximum pressure for melt separation anywhere beneath the propagating rift is less than 15 kb. In addition to these source effects, the projections allow a more complete description of the shallow-level fractionation and mixing effects along the propagating rift. Primitive lavas of restricted composition erupted near the rift tip give way, within 2–3 km, to an increasingly diverse suite of lavas characterized by variable extents of fractionation and by numerous, single mixing events consistent with magmatic evolution in small, only occasionally interconnected magma bodies. Beyond 15 km, the diversity gradually declines toward a restricted population of cpx-saturated lavas which characterizes the propagating rift beyond 75 km, and is consistent with the presence of a large, well-mixed magma chamber in this region.