Neutral buoyancy and the structure of mid‐ocean ridge magma reservoirs
Authors: Ryan Michael P. 
Published: February 2008

The mechanical structure of the regions for shallow storage and lateral intrusion of mid‐ocean ridge basalt (MORB) is explored in part by examining the density range appropriate for picritic‐to‐tholeiitic liquids and liquid + crystal mixtures, and its relationship to the nonlinear density‐depth structure of the ridge. Magma density spans the range 2.6 to at least 2.82 Mg m−3 and incorporates the effects of fractional crystallization on the density evolution, the incorporation of olivine phenocrysts and the contributions of water in melt phase density reductions. The melt and melt + crystal density band crosses the in situ rock density curve over the ≈1 to ≈3 km depth interval beneath the volcanic surface. Within that averaged interval, picritic‐to‐tholeiitic melt and melt + crystal mixtures are in approximate mechanical equilibrium with their surroundings, and the interval is referred to as the MORB horizon of neutral buoyancy (HNB). The detailed locations of the transition regions between the negative and neutral buoyancy and between the neutral and positive buoyancy regimes are dependent on melt phase composition as well as the presence of suspended mineral phases and their identity. In the East Pacific Rise, the HNB has a 1:1 correspondence with the inferred location of the sheeted dike complex and the seismically detected minimum compressional wave velocity region. In ophiolite complexes the region corresponds to the sheeted dike complex and the uppermost isotropic gabbros. Above the HNB is the region of negative buoyancy (seafloor to an averaged ≈1 km depth) within which, negative buoyancy forces may induce the descent of nonvesiculated melt. This region corresponds to the pillow basalt ± sediment sequence. The region of positive buoyancy extends from ≈3 km to the depth of melt generation, and picritic melt ascends in dikes and veins of finite height ≈500 m to ≈11 km and relatively narrow width ≈20 mm to ≈100 cm. The finite strengths of high‐temperature fluid‐weakened rocks mandate finite height melt parcels along the ascent pathway. The magmatic source region is therefore never hydraulically connected with the surface nor with the near‐surface reservoir. Thus relatively locally integrated buoyancy has a natural role in the ascent of melt from the source region to the region of neutral buoyancy. During the final stages of magma ascent, hydrothermal fluid circulation enshrouds the HNB in a three‐dimensional envelope, as suggested by the depth extent of alteration mineralogy in ophiolite complexes. While hydrothermal cooling may play an important role in arresting the ascent of magma in very low spreading‐rate environments, gravitational equilibrium represents the effectively dominant theme in intermediate and high spreading‐rate regions such as the East Pacific Rise. Elastic crack penetration of rheological transition zones near the HNB identify modes of crack‐tip morphologic changes as a function of Young's modulus ratios (E2/E1) above and beneath the transition and affect the style of intrusion, while melt buoyancy determines the ultimate equilibrium resting place. Elastic crack stability considerations relate the vertical magma pressure gradients for tholeiitic (∇PT) and picritic (∇Pp) magmas to the vertically varying gradient in the horizontal stress component (∇σH) and suggest further subdivision into two horizons of neutral buoyancy: one for tholeiitic magma (HNBT), and one for the picritic liquid + olivine mixtures (HNBP). These intervals are very approximate but are estimated as ≈600 m to ≈1400 m and ≈1400 to ≈3000 m depth, respectively, beneath the East Pacific Rise axis. As oceanic lithosphere is created at the rise axis and spreads laterally, the horizons of negative buoyancy and neutral buoyancy ride with it and may continue to modulate the subsurface magma dynamics that influence off‐axis volcanism.