BACKGROUND (continued)

Previous Investigations of Hole 735B
During Leg 118, a large intact 500-m section of gabbros was recovered from Site 735. These gabbros were unroofed and uplifted on the transverse ridge flanking the Atlantis II Fracture Zone. The complex internal structure and stratigraphy of the recovered section provided a first look at the processes of crustal accretion and ongoing tectonism, alteration, and ephemeral magmatism at a slow-spreading ocean ridge. Results from the leg showed that the section was not formed in a large steady-state magma chamber, but by continuous intrusion and reintrusion of numerous small, rapidly crystallized bodies of magma. There is little evidence of the process of magmatic sedimentation important in layered intrusions. Instead, new batches of magma were intruded into a lower ocean crust consisting of crystalline rock and semisolidified crystal mush. This led to undercooling and rapid initial crystallization of new magmas to form a highly viscous or rigid crystal mush, largely preventing the formation of magmatic sediments. Initial crystallization was followed by a longer, and petrologically more important, period of intercumulus melt evolution in a highly viscous crystal mush or rigid melt-crystal aggregate.

Thus, if the 437 m of gabbro recovered from Hole 735B is representative, long-lived magma chambers or melt lenses were virtually absent throughout most of the formation of the oceanic crust beneath the Southwest Indian Ridge (Bloomer et al., 1991; Dick et al., 1991a; Natland et al., 1991; Ozawa et al., 1991). Melts in the highly viscous or rigid intrusions were largely uneruptable throughout most of their crystallization. This explains the near absence of highly evolved magmas such as ferrobasalts along the Southwest Indian Ridge (Dick, 1989), as opposed to fast-spreading ridges where they are common, and a long-lived melt lens is believed to underlie the ridge axis (e.g., Sinton and Detrick, 1992).

Wall-rock assimilation occurring while small batches of melt work their way up through the partially solidified lower crust appears to have played a significant role in the chemical evolution of the section and, therefore, in the chemistry of the erupted basalt (Dick et al., 1992). This process has been largely unevaluated for basalt petrogenesis to date, but raises questions for simple models of the formation of mid-ocean-ridge basalt (MORB) drawn from experimental studies that assume equilibrium crystallization and melting processes throughout magma genesis.

An unanticipated major feature of the recovered core is the evidence of deformation and ductile faulting of the still partially molten gabbros (Bloomer et al., 1991; Dick, Meyer et al., 1991; Dick et al., 1992; Natland et al., 1991). This deformation apparently occurred over a narrow window, late in the cooling history of the gabbros (probably at 70%-90% crystallization) when they became sufficiently rigid to support a shear stress. This produced numerous small and large shear zones, creating zones of enhanced permeability into which the late intercumulus melt moved. This synkinematic igneous differentiation of intercumulus melts into the shear zones transformed the gabbro there into oxide-rich ferrogabbros. The net effect of these magmatic and tectonic processes was to produce a complex igneous stratigraphy with undeformed oxide-free olivine gabbros and microgabbros criss-crossed by bands of sheared ferrogabbro. Synkinematic differentiation is probably ubiquitous in lower ocean crust formed at slow-spreading ocean ridges, and should be recorded in ophiolite suites formed in similar tectonic regimes.

At Site 735, ductile deformation and shearing continued into the subsolidus regime, causing recrystallization of the primary igneous assemblage under granulite facies conditions and the formation of amphibole-rich shear zones (Cannat, et al., 1991; Cannat, 1991; Dick, Meyer et al., 1991; Dick et al., 1992; Mével and Cannat, 1991; Stakes et al., 1991; Vanko and Stakes, 1991). Here again, formation of ductile shear zones localized late fluid flow, with the most intense alteration occurring in the ductile faults (Dick, Meyer, et al., 1991). Undeformed sections of gabbro also underwent enhanced alteration at this time, principally by replacement of pyroxene and olivine by amphibole.

A consequence of simultaneous extension and alteration has been far more extensive alteration at high temperatures than found in layered intrusions that were intruded and cooled in a static environment (Dick, Meyer, et al., 1991; Stakes et al., 1991). An abrupt change in alteration conditions of the Hole 735B gabbros, however, occurred in the middle amphibolite facies with the cessation of shearing and ductile deformation (Dick, Meyer et al., 1991; Magde et al., 1995; Stakes et al., 1991; Stakes, 1991; Vanko and Stakes, 1991). Mineral vein assemblages changed from amphibole-rich to diopside-rich, reflecting different fluid chemistry. Continued alteration and cooling to low temperature occurred under static conditions similar to those found for large layered intrusions. These changes likely occurred due to an inward jump of the master faults defining the rift valley walls, thus transferring the section out of the zone of extension and lithospheric necking beneath the rift valley into a zone of simple block uplift in the adjoining rift mountains. Ongoing hydrothermal circulation, no longer enhanced by stresses related to extension, was greatly reduced, driven only by thermal-dilation cracking as the section cooled to ambient temperature.

The complex section of rock drilled at Site 735 formed beneath the very slow-spreading Southwest Indian Ridge (0.8 cm/yr half rate) and represents the slow end of the spectrum for crust formation at major ocean ridges far from hot spots. Such ridges have the lowest rates of ocean ridge magma supply, and crustal accretion is most heavily influenced by deformation and alteration. At the opposite end of the spreading rate spectrum (7-9 cm/yr), where the majority of the seafloor has formed, the crustal stratigraphy is likely different. Judging from the results of Hole 735B, the critical brittle-ductile transition has migrated up and down through the lower crust because of the waxing and waning of magmatism beneath the Southwest Indian Ridge. In contrast, this transition may be more stable near the sheeted dike gabbro transition at faster spreading ridges such as the East Pacific Rise, reflecting a near steady-state magma chamber or crystal mush zone. This should produce an internal stratigraphy for the lower crust quite different than that described here.

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