The gabbroic rocks recovered from Hole 1105A are cumulates composed of a framework of touching minerals concentrated through fractional crystallization (Irvine, 1982). The relatively unzoned nature of the cumulus phases (±5% An) (Fig. F27) and their granular interlocking textures suggest that the amount of postcumulus material is small and most of the gabbroic rocks are classified as mesocumulates, adcumulates, and poikilitic adcumulates (or heteradcumulates) (Wager et al., 1960; Irvine, 1982). The large interstitial but homogeneous clinopyroxene grains found in many of the gabbros appear to have grown in equilibrium with the coexisting, largely unzoned plagioclase and pyroxene and do not represent intercumulus crystallization. Further evidence for intercumulus crystallization, such as the appearance of low-temperature mineral phases, is also commonly lacking in the gabbros.
Additional support for the cumulate nature of the gabbros is provided by the modal layering (including nearly monomineralic anorthositic layers) (Fig. F4) and the occasional preferred mineral orientations or lamination defined by plagioclase laths (Fig. F3A). Layering is an integral part of most gabbroic intrusions, mainly in the form of modally graded and rhythmically repeated layers (Irvine, 1982, 1987). Rhythmic layering is synonymous with crystal fractionation and is thought to reflect mechanical and density sorting, compaction and flow, fluctuations in nucleation and growth rates, or multiple injection of melt batches (Wager and Brown, 1967; Boudreau and McBirney, 1997; McBirney and Nicolas, 1997; Irvine et al., 1998). The layering observed in the Hole 1105A gabbros, in general, is caused by size variations and to a lesser extent by modal variations. As such, this layering differs from the typical modally graded rhythmic layering in the layered series of the Skaergaard intrusion and in many other intrusions. The grain-size layering in the Hole 1105A gabbros can be attributed to fluctuations in nucleation and growth rates in a boundary layer along the margins of the chamber (cf. Bloomer et al., 1991). In many respects, the Hole 1105A style of layering may be more akin to the layering observed in the marginal border groups of the Skaergaard intrusion (Naslund, 1984; Hoover, 1989; Irvine et al., 1998). The near-cotectic mineral proportions of the shipboard-analyzed olivine gabbros indicate that these are average gabbros without little redistribution into layers (Fig. F26A). The modes of Fe-Ti oxide minerals in the Fe-Ti oxide gabbros vary from a few percent to near the inferred cotectic proportions (Fig. F26A). The reasons for this apparent variation in the proportions of the Fe-Ti oxide minerals appear most likely to record mixed sampling of the intermixed olivine and Fe Ti oxide gabbros.
The strongest support for the cumulate nature of the gabbros is provided by the whole-rock concentrations of major oxides and excluded trace elements. The high Mg/(Mg + Fetotal) ratios and low incompatible element abundances (e.g., TiO2 and P2O5) in the olivine gabbros (Fig. F26B) clearly identified these as cumulates when compared to typical Atlantis II Fracture Zone basalts (Natland et al., 1991; Johnson and Dick, 1992). The low incompatible element content indicates low amounts of intercumulus material. This was also concluded by Natland et al. (1991) from a study of Hole 735B gabbros. The Fe-Ti oxide gabbros are compositionally highly variable in Fe2O3 and TiO2 and have low concentrations of excluded trace elements similar to the olivine gabbros. The relatively high P2O5 in some of the analyzed Fe-Ti oxide gabbros suggests that these contain apatite cumulus minerals.
The planar fabric and deformation textures, seen in most of the Site 1105 gabbros, point toward compaction as a factor in the development of the cumulate textures (Hunter, 1996). The effects of load on a crystal mush zone is to compact, reorganize, deform, and recrystallize the solid framework and to move the interstitial liquid upward toward lower pressure regimes (Irvine, 1980; Hunter, 1996). The amount of liquid in a crystal mush depends on many factors, including accumulation, solidification processes, and cooling rates (Sparks et al., 1985). The residual porosity of the cumulates may be estimated to be >20% from the porosity of the olivine gabbros (<5%) and the average amount of Fe-Ti oxide gabbros (~22%) in the core, assuming that the Fe-Ti oxide gabbro component represents redistributed and segregated residual liquid. The results from the lower part of Hole 735B (Dick et al., 2000) show that the amount of Fe-Ti oxides decreases significantly in the lower parts of the cores to a few percent. More than 20% trapped liquid near the top of a compacting crystal mush appears reasonable and is certainly much less than a plausible maximum attainable initial porosity (Wager and Brown, 1967; Irvine, 1980; Shirley, 1986).
It is significant that the olivine gabbros at Site 1105 suggest an extent of solidification approximately similar to the middle part of the lower zone (LZb) of the Skaergaard intrusion (35%-50%, depending on the trapped liquid). Despite this, the LZb of the Skaergaard reflects a much higher extent of compaction (5%-10% trapped liquid), although both cumulate packages predict the presence of comparable thick masses of cumulates and a magma reservoir amounting to a total of at least 50% by volume. The reasons for the preservation of a zone with high porosity and high trapped liquid in the upper parts of Sites 1105 and 735 gabbros may be a sudden uplift and removal of the residual liquid from the chamber, thus effectively terminating melt migration and trapping the residual liquid in the crystal mush (Thy and Dilek, 2000). Such removal may be caused by tectonic uplift and melt extraction, possibly by injection into a subaxial dike and sill system. It is possible that the trapped liquid distribution in the Site 1105 gabbros represents a "fossil" compaction profile in an interrupted or decapitated magma chamber.
Cooling and crystallization of the cumulate pile during liquid migration and final compaction will change the stable assemblage and eventually saturate the migrating liquid first in Fe-Ti oxide minerals (~1100°C) and subsequently in apatite (~1030°C). Continued flux through the same vertical or lateral channels may result in Fe-Ti oxide and silicate proportions that greatly differ from the cotectic proportions, leaving behind a trail of "second" cumulus zones either intermingled with the host or as narrow bands and networks. Although the gabbros from Hole 1105A do not allow cumulus zone divisions to be defined, crystallization orders of the mineral phases and the petrogenesis of the rocks can still be inferred from careful petrographic observations. Troctolitic gabbros were recovered from Hole 735B as bands in the most primitive gabbros (Dick et al., 1991a, 1992, 2000; Bloomer et al., 1991). This is an indication that the primitive liquids for Hole 735B were saturated, or nearly saturated, with respect to olivine and plagioclase. The dominant lithology of Hole 1105A indicates mutual equilibrium of olivine, plagioclase, and augite cumulus phases. Fe-Ti oxide minerals are present additionally in many gabbros and show that these minerals appear next in the crystallization order. Orthopyroxene (or perhaps inverted pigeonite) cumulus grains occur additionally in a few examined gabbros together with Fe-Ti oxides and suggest that this mineral appears after the Fe-Ti oxides in the crystallization order. The Fe-Ti oxide minerals are dominantly ilmenite, but magnetite also appears in significant amounts in textures, suggesting that both of these phases are primary in origin. Apatite is present in considerable amounts in some Fe-Ti oxide- and orthopyroxene-bearing gabbros. Finally, biotite is present in the most evolved gabbros together with apatite and Fe-Ti oxides. Thus, the petrographic information suggests the following crystallization order: olivine, plagioclase, clinopyroxene; ilmenite, magnetite; orthopyroxene; apatite; biotite. The results from the deepening of Hole 735B to ~1500 mbsf (Dick et al., 2000) do not suggest the presence of more primitive plutonic rocks below the deepest level of penetration of Hole 1105A (troctolites, dunites, and anorthosites). The petrographic observations further suggest that a gap may exist in olivine crystallization with the appearance of orthopyroxene as indicated by a decrease in abundance or the lack of olivine in gabbros that contain orthopyroxene. Regardless, this inferred crystallization order, in general, is similar to the crystallization order (or zone divisions) seen in the Skaergaard intrusion, as well as that predicted from low-pressure anhydrous experimental work on tholeiitic basalts. This was also pointed out by Meyer et al. (1989) in a study of dredge samples from the Southwest Indian Ridge (54°S-7°16´E). The complication, however, is that the rocks did not accumulate or accrete to the walls of the chamber in a systematic fashion that allows easy identification of cumulus zones, the volume relations, and the extent of fractionation. Therefore, we are unable to relate the magmatic evolution to any simple stratigraphic and volumetric scale as can be done to a certain extent for the Skaergaard intrusion (Wager, 1960).
The compositional variation in coexisting plagioclase, olivine, and augite can be relatively well duplicated by the modeling of perfect fractional crystallization (Figs. F26, F28). The small compositional differences between the gabbros from Holes 735B and 1105A can easily be accounted for by small differences in the parental melt compositions, such as Fe/(Mg + Fetotal) ratios and Na2O contents. The strong effect of the parental melt is particularly well illustrated by differences in cumulus plagioclase compositions seen between the Atlantis II gabbros (Holes 735B and 1105A) and the Skaergaard intrusion (Fig. F28). Despite the success in modeling the composition of the coexisting minerals, the actual income or zone boundaries for cumulus augite and Fe-Ti oxides are less perfectly predicted by the modeling (Fig. F26). This is probably understandable, considering the uncertainty in predicting the parental melt, as well as the actual crystallization temperatures, for major cumulus phases without direct experimental confirmation.
The cryptic variation in the Hole 1105A gabbros shows as a function of depth several features at odds with observations from Hole 735B (Ozawa et al., 1991; Bloomer et al., 1991; Natland et al., 1991; Hébert et al., 1991; Thy and Dilek, 2000). The olivine gabbros from Hole 735B record several small reversals to relatively more primitive compositions that may reflect relatively abrupt replenishment and mixing of the residual magma with more primitive magma similar in appearance to many layered continental and ophiolitic complexes (Jackson, 1961; Irvine, 1980; Wilson and Larsen, 1985; Thy et al., 1989). The present results from Hole 1105A suggest a somehow different pattern in the cryptic variation (Figs. F23, F24) and at a much smaller scale than that for Hole 735B. The olivine gabbros appear to define several symmetric and convexly zoned gabbro lenses in thicknesses from 40 to 60 m that may signify much slower chamber filling than for the Hole 735B gabbros. The pattern in the cryptic variation for Hole 1105A is relatively similar to that seen for the upper gabbros of the Troodos ophiolite (Thy et al., 1989). Such large-scale repeated variations in cumulus compositions can be used to impose limitations on chamber height (Browning, 1984). Indications from the Semail ophiolite suggest maximum chamber height of a few hundred meters (Browning, 1984; MacLeod and Yaouancq, 2000). Similar estimates for the Hole 1105A gabbros suggest chamber heights in the same range as those inferred for the Semail ophiolite (100-150 m). This is in contrast to estimates for the Hole 735B gabbros, which predict chamber heights easily approaching 1000 m or more. The cryptic variation in the Hole 1105A gabbros returns to approximately similar compositions after each cycle, showing little depth dependence. This may be seen as an indication that the gabbro cumulate sections were emplaced through a same size-limited magma lens at an active spreading center (Quick and Denlinger, 1993; Phipps Morgan et al., 1994). These observations are interesting because they imply that the gabbros associated with the ultraslow-spreading Southwest Indian Ridge show many features in common with gabbros from the fast-spreading Semail ophiolite.
The Fe-Ti oxide gabbros of Hole 1105A show cryptic variations as a function of depth that are independent of the host olivine gabbros (Figs. F23, F24). All silicate cumulus phases show systematic upward variations toward more evolved compositions and follow normal fractionation trends that lack the characteristic convex cryptic variation seen for the olivine gabbros. The cryptic variation in the Hole 735B Fe-Ti oxide gabbros shows irregular downward repeated trends toward evolved compositions (Ozawa et al., 1991; Thy and Dilek, 2000). These individual trends are the reverse of the normal fractionation trend seen in the host olivine gabbro cumulates. Ozawa et al. (1991) identified three cycles in the Fe-Ti oxide gabbros, each showing downward increasing fractionation. The Hole 1105A Fe-Ti gabbros are dominated by normal fractionation trends, whereas the Hole 735B Fe-Ti oxide gabbros are dominated by punctuated and repeated reversed fractionation trends. The results show that the cryptic variation in Hole 1105A systematically differs from that seen in Hole 735B. Thus, perhaps contrary to expectations, lithostratigraphic correlation between the two closely located holes is not possible based on the present information.
Despite the differences, the observations from Hole 1105A reinforce the conclusions from Hole 735B (Dick et al., 1991a) that the interstitial liquid, now represented by the Fe-Ti oxide gabbros, evolved independently of the host partially molten olivine gabbros. Flow distance and efficiency of fractionation of the interstitial trapped melt controlled differentiation of the migrating liquid, as measured in the mineralogy and compositions of the constituent minerals. Dick et al. (1991a, 1992), based on observations from the Hole 735B, suggested that partially molten gabbro bodies were affected by syntectonic compaction and ductile deformation during cooling, causing interstitial melt to migrate toward areas of pressure release. The irregular and punctuated pattern seen for the Hole 735B Fe-Ti oxide gabbros supports the suggestion of Dick et al. (1991a) that syntectonic processes in part controlled solidification. The interaction of ductile deformation bands with the partially consolidated crystal mush may be responsible for the localized reversed fractionation trends repeatedly seen in Hole 735B (Ozawa et al., 1991). The implications from Hole 1105A are that the trapped melt in the crystal mush experienced a significant component of upward migration, transgressing compositional boundaries in host olivine gabbro cells, and is reminiscent of compaction and upward migration prior to final solidification of the interstitial melt. There is no strong indication for ductile deformation and melt migration into deformation zones for the Hole 1105A gabbros.