Gabbroic rocks recovered by drilling and submersible at Hess Deep, eastern equatorial Pacific, record processes of melt migration and channelized magma flow that bear on the origin and persistence of magma lenses and the development of lower-crustal structure at the East Pacific Rise. Rocks from ODP Hole 894G near the summit of the intrarift ridge at Hess Deep, and sampled by submersible from an uplifted marginal horst on the nearby northern slope of the rift, are mainly gabbronorites and olivine gabbronorites that crystallized very near the base of sheeted dikes. North Slope gabbros were sampled without interruption right to the base of sheeted dikes, and differ from Site 894 principally in that the sequence contains oxide-rich ferrodiorites and is cut locally by narrow tonalite dikelets. The highly fractionated rocks plausibly represent the frozen residues of a narrow and thin magma lens which geophysical evidence suggests once lay beneath the axial neovolcanic zone of the East Pacific Rise and its feeder dikes. Site 894 rocks represent the now-frozen immediate substrate, upon which a magma lens once rested, and through which it was supplied with melt.
Bulk compositions and modal mineralogy (principally low abundances of oxide minerals) demonstrate that almost all the gabbroic rocks of Site 894, and North Slope gabbronorites deeper than about 200 m below the base of the dikes, are adcumulates with <7% (average 4.4%) of material crystallized from trapped intercumulus melt. Yet, in contrast to adcumulates in layered intrusions, none of these rocks crystallized in close proximity to a major magma body. Lithostratigraphic relationships at Site 894, and the presence of grain-size variations across sharp contacts in several of the North Slope dive samples, show that most crystallization and adcumulus crystal growth occurred in fracture networks or in narrow dike-like bodies averaging about 3 m, but ranging down to as little as 1 cm, in thickness. Expulsion of intercumulus melt was extremely efficient at all of these scales, and—based on evidence from drilling nearby at Site 895 at the crust-mantle transition—evidently was pervasive throughout the entire mass of gabbros down to the mantle. Melt expulsion was not only thorough, but occurred almost immediately, providing a nearly impermeable base into which dense, iron-rich magmas in the thin melt lens could not sink.
No gabbroic rock from either Site 894 or the North Slope is layered, although some of the rocks show preferred orientation of plagioclases. There are no monomineralic adcumulates; all are olivine-plagioclase-clinopyroxene or plagioclase-clinopyroxene adcumulates and mesocumulates which crystallized on cotectics. Strongly zoned plagioclases, some enclosed as broken crystals in clinopyroxene or orthopyroxene oikocrysts, and unusually high Cr-contents of clinopyroxenes, attest to initial stages of crystallization from primitive to moderately fractionated basaltic magmas, followed by post-cumulus migration of highly fractionated (ferrobasaltic to ferroandesitic) melts through an open crystal network, without complete reequilibration of the originally precipitated minerals. Some of the primitive magmas carried xenocrysts of highly calcic plagioclase (An80–95) which probably crystallized in the upper mantle or lower crust. Grain boundaries of all phases show substantial mutual interpenetration suggesting that pressure solution assisted in expulsion of intercumulus melts, thus aided in adcumulus growth. Reduction in melt porosity proceeded nearly to completion in most rocks before oxide minerals (mainly ilmenite with lesser magnetite) began to crystallize. The distribution of oxides therefore indicates the very late-stage porosity structure of the partially molten rocks, just before they froze completely.
Formation of adcumulates at fast-spreading ridges evidently can take place simultaneously in fracture networks throughout the entire 4-km section of crystallizing gabbros with only a small melt lens at the top. The melt lens does not precipitate a sequence of layered cumulates which then subsides into lower parts of the crust. Adcumulus growth occurs under conditions of stress which produce incessant rifting and fracturing alternating with intervals of compaction beneath a substantial overburden of dikes and extrusives. It is aided by a permanently positive, albeit fluctuating, temperature gradient downward into the upper mantle. This gradient guarantees the potential for some melt to exist anywhere in the crust beneath the physical location of the basalt solidus at the top of the magma lens. A magma lens thus may be sustained as a steady-state feature between inflation-eruption events by porous flow of melt expelled from the entire column of developing adcumulates. Persistent flow is required since the lens never completely freezes. The lens itself is a pool of highly fractionated, iron-rich and even siliceous melt that collects at the low-temperature top of the gabbro column, perhaps at the porous base of a cracking front which extends through the sheeted dikes. It is available to mix with more primitive magmas during major inflation-eruption cycles.
The extreme efficiency of adcumulate formation throughout the entire thickness of the gabbroic layer supports the operation of nearly ideal in situ fractional crystallization during the development of the basaltic liquid line of descent. Extrusive basalts and dikes at the East Pacific Rise, on average, will show two- to three-fold enrichments in incompatible elements to the extent that the gabbroic layer is depleted in those elements as a consequence of the formation of gabbroic adcumulates. These variable enrichments are acquired by unavoidable mixing with different proportions of highly fractionated melts in the magma lens, which directly underlies the narrow locus of eruption along the axial rift.
Date of initial receipt: 2 August 1994
Date of acceptance: 15 June 1995
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