Partial melting of a lithologically heterogeneous mantle: inferences from crystallization histories of magnesian abyssal tholeiites from the Siqueiros Fracture Zone

Abstract

Summary Complex processes of magma mixing are revealed by the crystallization histories of several magnesian, porphyritic abyssal tholeiites dredged from the Siqueiros Fracture Zone at about 8°30′ N near its intersection with the East Pacific Rise. The compositions of chromian spinels, olivines, plagioclases and glass inclusions in these rocks indicate that polybaric crystallization and mixing occurred at high temperatures involving a range of refractory (low-TiO 2 , low-Na 2 O) melts. These later coalesced with cooler much less refractory components to produce the bulk rocks. The crystallization histories and comparisons with experimental data indicate that mantle sources of these basalts were lithologically heterogeneous, ranging from fertile (lherzolitic) to refractory (in the extreme, harzburgitic) with respect to a basaltic melt fraction. The separate magma strains going into a typical magnesian abyssal tholeiite were produced by simultaneous partial melting of the different lithofacies, after which they coalesced and mixed. Such mixing is more likely than batch melting to explain the ranges in CaO/Al 2 O 3 , and abundances of Na 2 O and TiO 2 , within the fairly small range and low (non-picritic) magnesium numbers (0.75–0.64) exhibited by all liquids including those inferred from mineral compositions. The early crystallization of minerals from refractory magma strains suggests that melting domains beneath spreading ridges consist of cores of refractory peridotite surrounded by cooler, more fertile peridotite. Sequential mixing of refractory and fertile magma strains is an unavoidable consequence of the ascent of magma batches from the interiors of complex melting regimes (diapirs) in the mantle. The diapirs may have been triggered into ascent by the buoyancy of low-density (low-iron) refractory peridotite. Several of the Siqueiros magnesian abyssal tholeiites are among the most depleted basalts ever obtained from the eastern Pacific, and yet alkalic basalt was also dredged from the fracture zone a few kilometres away. The tholeiites are even more depleted than typical, more fractionated, summit eruptives of the East Pacific Rise. The contrast may arise because of the homogenizing effects of axial magma chambers, within which truly depleted tholeiites are mixed with enriched alkalic magmas instead of erupting separately, as they do in the fracture zone. Mantle in this region thus appears to be segregated into strongly contrasting depleted and enriched peridotites, with depleted peridotite also ranging from refractory to fertile lithofacies. Primary lithological variability in the mantle thus was probably imposed by one or more ancient melting events that left distinctively enriched and depleted reservoirs in their wake.