Olivine gabbros, allied troctolitic gabbros, and troctolites, together making up the olivine gabbro suite, comprise some 76% of the section cored in Hole 735B on the eastern transverse ridge of the Atlantis II Fracture Zone, Southwest Indian Ridge. These rocks hold the keys to the processes of crustal accretion and magmatic differentiation beneath the floor of the rift valley. We develop the sequential stratigraphy of the olivine gabbro suite by itself, stripping out all later, crosscutting, usually more deformed, and more strongly differentiated oxide gabbros and felsic veins that comprise the remaining 24% of the section. The fundamental unit of crustal construction in the olivine gabbro suite appears as alternations in grain size that we term olivine gabbro sequences. Uniformly textured masses of coarse-grained olivine gabbro range from meters to tens of meters thick and alternate with finer-grained and usually more olivine-rich rocks that have higher Mg# and Ni, such as troctolite and microtroctolite. There are 97 such sequences in the core. However, only 35 are more than 10 m thick and contribute substantially to the section. Several sequences are >40 m thick, and one is 120 m thick. We consider that each sequence represents either a single pulse of magma, in which olivine tended to concentrate toward the base during injection, or several pulses of magma, mostly marked by sutured, interpenetrative igneous contacts and small changes in grain size, but all injected quickly and at one place in the section.
The sequences together make up three series of olivine gabbro and allied troctolite. The upper two occupy about the upper one-third of the cored section. They are more primitive than the lowest, being more calcic and magnesian in bulk composition and having more calcic plagioclase and forsteritic olivine. Each shows a general upward trend to more differentiated compositions, and the two are separated by the one portion of the core, some 50 m thick, of massive oxide gabbros, rich in the oxide minerals ilmenite and magnetite. The deepest and thickest series of olivine gabbro shows only subtle internal variation and no marked trend toward more differentiated compositions at any level, even though it is crosscut in hundreds of places by narrow seams of oxide gabbros. It crystallized from dozens of small injections of magma more differentiated than that which supplied the upper two series but not as differentiated as that which produced the oxide gabbros.
We interpret the upper two series of olivine gabbros as originally portions of cupolas or small magma chambers emplaced near the top of the gabbro section. These were repetitively injected by comparatively primitive magma and probably vented some lava to the seafloor. The third and deeper series consists of gabbro crystallized from more differentiated magma than ever erupted, but nevertheless which either leaked or was forcibly expelled laterally at or beneath the brittle-ductile transition in the crust from the tops or midsections of those or similar cupolas after their links to deeper sources in the lower crust or upper mantle closed off or were cleaved from their sources in the mantle. This explains the broad pattern of inverted or upside-down differentiation seen among olivine gabbros in Hole 735B and why evolved rather than primitive gabbro is so commonly recovered with peridotite in dredge hauls from transverse ridges worldwide.
The earliest stages of basaltic differentiation are not represented by any cumulates in Hole 735B, but those rocks may be restricted to narrow columns that froze from central feeders beneath each cupola, analogous to those recently imaged seismically along a portion of the Mid-Atlantic Ridge. Alternatively, the primitive, frozen relics of such conduits may have been asymmetrically displaced to the north beneath the rift valley while the gabbros of Hole 735B were being unroofed and uplifted along detachment faults to the summit of the transverse ridge to the south. Prior to this, beneath the axial rift, primitive magma ascending in the narrow conduits was free to incorporate ferrobasaltic melt dispersed in differentiated gabbro beneath and between active shallow cupolas through which the conduits had to pass. This explains the hybrid compositions of many abyssal tholeiites at slowly spreading ridges and the apparently inconsistent fact that unadulterated ferrobasalt does not itself erupt on such ridges.
The final stages of differentiation overlapped the first stages of high-temperature crystal-plastic deformation. Differentiation by deformation enhanced the efficiency of expulsion of buoyant intercumulus melt and its coalescence and ascent in an open network of cracks, gaps, fissures, and zones of concentrated shear. Seams of oxide gabbro and late-stage granitic veins lace the core at all levels but diminish in abundance with depth. The numerous small intrusions responsible for the massive oxide gabbro appear to have concentrated preferentially along a permeability pathway between the two upper plutons that caught and directed the flow of ascending late-stage, strongly differentiated melt.
1Natland, J.H., and Dick, H.J.B., 2002. Stratigraphy and composition of gabbros drilled in Ocean Drilling Program Hole 735B, Southwest Indian Ridge: a synthesis of geochemical data. In Natland, J.H., Dick, H.J.B., Miller, D.J., and Von Herzen, R.P. (Eds.), Proc. ODP, Sci. Results, 176 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/176_SR/synth/synth.htm>. [Cited YYYY-MM-DD]
2Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami FL 33149, USA. jnatland@rsmas.miami.edu
3Woods Hole Oceanographic Institution, Woods Hole MA 02543, USA.
Initial receipt: 26 March 2001
Acceptance: 6 June 2002
Web publication:
15 October 2002
Ms 176SR-002