The compositional variation of the gabbros from Hole 1105A can be evaluated by the shipboard X-ray fluorescence (XRF) analyses (Shipboard Scientific Party, 1999). Selected elements as a function of depth in the hole (Fig. F21) confirm the petrographic observations of two distinct compositional groups. The volumetrically dominant and relatively primitive olivine gabbros have low TiO2 and Fe2O3 (total iron) contents, high Mg/(Mg + Fe) ratios, and anorthitic normative plagioclase compositions (weight percent CIPW norm). The Fe-Ti oxide-bearing gabbros have high TiO2 and Fe2O3 contents, low Mg/(Mg + Fe) ratios, and albitic normative plagioclase compositions. The apatite gabbros, in addition, have high P2O5 content (<4 wt%), reflecting modal apatite.
The interpretation of the compositional variation is complicated because the gabbros represent mixtures of the constituent primocryst minerals as well as interstitial crystallization and overgrowth. The problem of sampling the two closely intermixed gabbro components also attributes to the complications. The relative proportions and compositions of minerals control the bulk rock compositions. This means that volumetric combinations determined by the accumulative or segregative processes of up to eight minerals (olivine, plagioclase, augite, orthopyroxene, ilmenite, magnetite, apatite, and biotite) control the gabbro compositions. Thus, it cannot a priori be assumed that the two predominant gabbro groups identified by differences in the major oxide compositions also represent distinct liquid compositions. It is plausible, for example, that the compositional variation in TiO2 and Fe2O3 reflects variations in crystallization and accumulation processes and not the existence of two distinct liquid compositions. This is illustrated in Figure F22, which shows the Mg/(Mg + Fetotal) ratio as a function of the normative plagioclase composition (anorthite). The olivine gabbros show distinct positive correlation that may reflect the constituent minerals and, thus, the hidden cryptic variation in the dominant minerals. In contrast, the Fe-Ti oxide-bearing gabbros show limited variation in anorthite but large variation in Mg/(Mg + Fetotal) ratios. This latter likely reflects variable modal content of Fe-Ti oxide minerals and not necessarily the Mg/(Mg + Fetotal) ratio of the constituent minerals.
The compositions of the three main constituent minerals (olivine, plagioclase, and clinopyroxene) are illustrated as a function of depth in Figure F23, showing all analyses, and in Figure F24 as the averaged mineral compositions. The results confirm that the two main groups of gabbros observed from the bulk rock compositions are retained when mineral compositions are considered. At any depth interval in the core where both groups are represented, the gabbros show variation between primitive and evolved mineral compositions, often with an indication for a bimodal distribution. Only at some intervals can a near-complete compositional continuum be seen between the two groups. As already demonstrated, the olivine gabbros are relatively primitive, with Mg-rich mafic minerals and anorthitic plagioclases, and the Fe-Ti oxide gabbros are evolved, with Fe-rich mafic minerals and albitic plagioclases. The average compositions for each sample (Fig. F24) reveal the same compositional variation shown by the complete analytical data set but tend to enhance the compositional bimodality of the gabbros. The mineral compositions at any specific interval vary considerably between the two end-members because of crystallization and reequilibration. It is also apparent that some samples contain mixed populations of minerals grains (e.g., ~80 mbsf), probably resulting from mechanical redistribution during shear deformation. These latter are not further considered. It is the consistent variations in the most primitive and most evolved gabbro compositions that provides important first-order petrogenetic information and that is the principal object of the present discussion.
The olivine gabbros show large compositional variations, only tentatively defined based on the available sampling. If the most primitive compositions are considered, the observed variation appears to define two and possibly three large-scale, convexly zoned gabbro lenses. These lenses vary in thickness from 40 to 60 m, despite uncertainties such as limited sampling at certain intervals and lack of base and top of the core. There is a surprising lack of olivine gabbros between ~115 and 137 mbsf (Fig. F23); despite this, olivine gabbros were identified by Scientific Shipboard Party (1999) to dominate this interval. It is possible that the host gabbros of this interval were relatively evolved and saturated or near saturated in Fe-Ti oxides and, therefore, cannot petrographically easily be distinguished from the oxide gabbros. It is equally possible that this interval completely lacks olivine gabbros despite shipboard observations.
Each convexly zoned lens includes a lower segment that shows upward reversed zoning, a central maximum, and an upper segment that shows normal zoning upward, all for Mg/(Mg + Fetotal) ratios of augite and olivine and An mole percent content of plagioclase. There is a tendency for a symmetric zoning pattern in the olivine gabbro lenses. If this interpretation is correct, we can distinguish lens I from the bottom of the hole to 115 mbsf (42 m), lens II between 115 and 55 mbsf (60 m), and lens III from 55 mbsf to the top of the hole (39 m) (Fig. F24). Despite the large uncertainty in this interpretation (Figs. F23, F24), it is nevertheless clear that the cryptic variation preserved by the olivine gabbros is distinctly different from that preserved by the Fe-Ti oxide gabbros.
The Fe-Ti oxide gabbros show systematic upward normal variation toward decreasing Mg/(Mg + Fetotal) ratios of the mafic minerals and An mole percent of plagioclase irrespective of the variation in the host olivine gabbros. The upward variation in the most evolved plagioclase compositions amounts to ~10 mol% from An40 to An30. This is a normal differentiation trend typically observed from many layered intrusions and ophiolitic complexes.