Several different features of the core can be linked to peak regions and narrow spikes in magnetic susceptibility. These include oxide concentrates or seams—some of them deformed, felsic veins, and vein nets associated with oxide gabbros and other types of igneous breccias. I provide a sampling here, not an inventory.
Figure F14 depicts a peak region in Sections 176-735B-114R-1 and 11R-2; the several prominent spikes in magnetic susceptibility correspond to lithologic interval 561, an oxide gabbronorite. In the core photograph in the site report, oxide-rich material is systematically darker than rock composed mainly of silicate minerals. I have enhanced the contrast of a portion of the core photograph strongly to highlight the incidences of oxide-rich material. These appear as irregular patches in the true-scale image on the left, with a suggestion of a steeply dipping preferred orientation to some of them (arrows). Apart from this suggestion, the rocks are not described as being deformed in the core description, nor do they contain felsic veins (Shipboard Scientific Party, 1999b). A weaker orthogonal pattern of boundaries to the oxide-rich patches is probably shadowing along curving rock-saw marks. The left image is that of a single piece of rock that is in several subpieces (Section 176-735B-114R-1 [Pieces 6A-F]). These together comprise only a portion of the lithologic interval. I have compressed the image and widened it by a factor of three in the central image, including as well the rest of the core in the interval, to provide a direct correlation to the plot of magnetic susceptibility on the right. Each of the several shaded regions rich in oxide minerals corresponds to one of the spikes in magnetic susceptibility. The inclined boundaries to the oxide-rich patches are actually easier to see in the compressed image. In Section 176-735B-114R-1 (Pieces 2-5), the inclined pattern is not apparent, probably because the cores were split in a different orientation. This is because the rocks themselves are not foliated, and foliation was the principal criterion by which whole-round core was oriented in the core reference frame.
Figure F15 shows an unrolled image of a portion of Section 176-735B-154R-3. The entirety of lithologic interval 730, a disseminated oxide gabbro, was recovered within a single, unbroken piece of whole-round core. It corresponds to a peak region in magnetic susceptibility ~50 cm long, in which there are two spikes of 4000 x 10-6 MU. The unrolled image reveals that there are five fairly sharp planar boundaries with low dip paralleling the planar boundaries of the interval and three others that are more diffuse (solid and dashed lines in the sketch to the left). The upper and lower planar boundaries were identified as contacts in the site report, but not the intervening ones. Brackets therefore indicate six previously undetected subintervals, bounding darker or lighter rock that is coarser or finer grained or more leucocratic. The rock beneath is identified as a seventh subinterval on the scale of the scanned image. Susceptibility spikes correspond to subintervals 3 and 6. There are also two more steeply inclined fractures 95-118 cm from the top of the section, both clearly unrelated to the contacts above. Most of lithologic interval 730 is described as weakly foliated (degree of deformation = 1), but subinterval 6 and the top few centimeters of the olivine gabbro beneath it are more strongly deformed (degree of deformation = 3 and 2, respectively).
Figure F16 shows a peak region in Section 156-735B-156R-7 having similar but mostly more strongly deformed subintervals. The peak region corresponds to lithologic interval 742, an oxide gabbro that is variably deformed but again having rock that is darker and lighter or with variable grain size and extent of deformation. The maximum susceptibility is almost 10,000 x 10-6 MU, nearly the saturation limit of the Bartington detector. The extent of deformation is somewhat higher than in the core shown in Figure F15. There are 10 subintervals indicated in the sketch, with the lower interval boundary, described as a planar tectonic contact in the core descriptions, present at the base of the ninth. The upper boundary at the top of this piece of rock (and placed at the top of this section) was evidently not entirely recovered, although it is described as planar in the core descriptions.
The subinterval planar contacts are not parallel, as indicated by the shifting of their sinusoidal patterns in the unrolled image and sketch, although this is not so apparent in the core photograph. The upper two subinterval boundaries warp beneath a coarse-grained breccia. There are two igneous breccias and one interval with narrow felsic veinlets, with the most prominent peaks in magnetic susceptibility corresponding to one of the breccias (subinterval 3) and the veinlets (subinterval 5). Lithologic interval 742 is ~23 m below interval 730 shown in Figure F15. Interval and subinterval contacts have similar low dips in both.
The last three figures in sequence fairly characterize the range in lithologic variability represented by peak regions in magnetic susceptibility in the core recovered during Leg 176. The internal spikes and troughs of peak regions clearly represent variable lithologies and extents of deformation that have actually been imaged, although they were not described in detail during the leg. The planar subinterval boundaries of Figures F15 and F16 are almost certainly tectonic in origin because they parallel tectonic interval contacts, are distorted around breccias, and subdivide the rock into zones of different extents of deformation. Adjacent olivine gabbro in these two instances is undeformed except near the interval contacts, and this relationship is a common one at peak regions elsewhere.
A general impression is that peak regions with oxide gabbros having a diffuse or patchy distribution of oxides (e.g., Fig. F15) are more prevalent from 700 to 900 mbsf, whereas the narrower peak regions below that are more extensively deformed. The narrower bands of differentiated rock in the rocks beneath therefore seem to have served to concentrate deformation in narrow subintervals, whereas in portions of the section where oxide gabbros are wider and more abundant, it was more widely distributed or dispersed. Even at that, the oxide-rich zones in the rock shown in Figure F15 have the suggestion of planar boundaries about them, so that the rocks were probably not completely undeformed.
Whether oxide gabbros consistently served to localize high-temperature deformation, perhaps because they retained some melt at the time, was strongly debated during Leg 176. An association was noted between portions of core containing oxide gabbros and zones of highly deformed rock (Dick et al., 2000). This meant that although oxide gabbros might be close to deformed rock, and even themselves somewhat deformed, they were often not the most deformed rock in a zone of gradationally increasing or variable deformation fabric. This is borne out by the two examples of Figures F15 and F16. However, within these two lithologic intervals, described, respectively, as disseminated-oxide gabbro and oxide gabbro, the intervals themselves overall are more deformed than surrounding rock, and the most oxide-rich portions of them—revealed by individual peaks in magnetic susceptibility and contrast-enhanced images—are subintervals bounded by planar, tectonic, internal contacts. Deformation was clearly concentrated at each of the two more oxide-rich intervals, but it was not necessarily most concentrated where oxide minerals are most abundant in the rocks. Instead, in both cases, it is at the basal contacts, where differences in the rheological responses of the rocks to deformation presumably were greatest. The association between oxide gabbros and deformation therefore is not coincidental at all. It is intimate, if not precisely corresponding in degree.
A total of 203 felsic veins are sufficiently prominent to have been annotated in a vein-log spreadsheet (I-vein.xls) that is supplemental to the core descriptions (Shipboard Scientific Party, 1999b). These usually have sharp, planar contacts at high angles to full-round cored surfaces; however, some are irregular or form complicated vein networks. There are some inconsistencies between the core descriptions and the vein log. In particular, some veins are intimately bound with zones of deformation, and some of these are noted on the core barrel sheets but not included in the vein log. In some cases, the associated oxide gabbro seams are not noted, although they are evident in core photos, especially when the contrast is somewhat enhanced, and were detected by measurement of magnetic susceptibility.
The leading petrogenetic question concerning these veins is how they might be related to the next most differentiated rocks in the section, namely the oxide-bearing and oxide-rich gabbros. Magnetic susceptibility provides much information on this score and shows that the connection to oxide gabbros is extremely strong. Most of the veins are associated with oxide gabbros and, indeed, are present within oxide-rich peak regions that have, on the whole, the greatest magnetic susceptibilities in the section. However, there are many instances where the veins reached into primitive rock and modified them both physically and chemically.
The first large felsic vein encountered during Leg 176 is one of the latter (Fig. F17). The vein itself is almost 4 cm wide. It is embedded in olivine gabbro and contains aligned fragments of the host rock in its center. There is a bleached corona of reacted material on its margins. Above and below the vein, just out of the range of the photograph, the olivine gabbro has magnetic susceptibility at a background level given by the gray background on the plot next to the core photo. Within the piece of rock traversed by the vein, magnetic susceptibility is slightly higher than background, but it is just below background where the vein was most strongly detected by the Bartington sensor in whole-round core.
In Figure F18, a larger and more complicated vein network is shown using an unrolled image. A portion of the vein network is also shown in a close-up core photograph. The vein fills an array of vertical and inclined fractures. The associated magnetic susceptibility exceeds 2000 x 10-6 MU, with the peak corresponding to a slightly darker patch of oxides in the center of the unrolled image, between 33 and 38 cm from the top of the section.
Figure F19 depicts a 1-cm felsic vein in a deformation zone. The vein bisects a deformed seam of oxide gabbro that was detected as a sharp doublet in magnetic susceptibility, with a peak in excess of 6000 x 10-6 MU centered just above the vein and rising above background (see inset). The larger peak of the doublet lies just above the region of the inset, which shows only the lower of the two peaks. The deformation intensity is strongest right at the vein, although the vein itself is undeformed. The seam is not described as oxide gabbro, but was included within lithologic interval 693, olivine gabbro. Nevertheless the oxide minerals are plainly visible in the close-up core photograph. In Figure F18, to make this more apparent, I have expanded a portion of the photograph in the lower two images and both enhanced the contrast in the first enlargement and then selected only the oxides in the second. Adjusting the contrast brightens the specular reflection of the visible light spectrum from the alkalic feldspars in the vein more than from the calcic plagioclases in the surrounding olivine gabbro. The oxide minerals in the vein are part of the gabbroic host rock that the vein has engulfed.
Figure F20 depicts a similarly deformed zone in troctolite (lithologic interval 718) about 58 m farther down the core. Once again, a seam of foliated oxide gabbro with an extremely strong spike in magnetic susceptibility has the highest deformation intensity. Enhancing the contrast of the scan of a close-up photograph brings out its presence. Some brighter felsic material is distributed in the deformed rock above the seam, and there is an irregular lozenge—a deformed felsic vein—about 20 cm farther up. It is clearly caught up in the foliated fabric of the rock. Once again, neither the lithology of the felsic lozenge nor of the oxide-rich seam was noted in the core descriptions, although the deformation of both was depicted graphically. Thus, instead of being an example of strong deformation in rock other than oxide gabbro, this is a deformed oxide gabbro that incorporated a preexisting felsic vein.
The difficulty of identifying oxide-bearing and oxide-rich gabbro in deformed rock carried down to some very narrow zones of deformation. Figure F21 shows a strongly deformed band ~8 cm thick that was simply described as a shear zone in lithologic interval 550, an olivine gabbro. The extent of deformation is strongest at both contacts. Contrast enhancement of both the unrolled image and the scanned split surface of the core (slab) reveals streaks of felsic material in the deformed rock. The deformed zone also corresponds to a small but unmistakable peak in magnetic susceptibility, indicating a concentration of magmatic oxides at the basal, most deformed contact. Gabbro above and below the shear zone is very coarse grained, with that beneath almost appearing to be brecciated.
Felsic material in oxide-rich rock corresponding to some of the wider and undeformed peak regions is not always distributed in veins. Instead, it can be as patchy as the oxide minerals. Figure F22 shows four different images of the same portion of Section 176-735B-131R-1, taken from the core photograph and corresponding to lithologic interval 648, oxide gabbro. The swirly pattern of the felsic material is more distinctly contrasted against dark gray silicate minerals, mainly clinopyroxene, and black oxides in the adjacent image (Fig. F21). The oxides and felsic swirls are separately selected (black), respectively, in Figure F21. An analyzed sample from between 10 and 15 cm below the top of the section has very high TiO2 and enough Zr to show the influence of the felsic swirls (Snow et al., Chap. 12, this volume). This rock appears to exemplify imperfect segregation of felsic material from the oxide gabbro in which it differentiated.
A number of felsic vein networks (or net veins) and vein breccias are in the core recovered during Leg 176. These are rocks in which angular fragments of gabbro are bounded by fine, millimeter-scale felsic veins. In vein networks, the fragments are still in their original relative positions. In vein breccias, they are disrupted. The small size of the veins and their dispersed arrangement in the rocks meant that many of these were not noted in the igneous vein log. I have not investigated magmatic breccias with felsic veins systematically but provide examples showing a close connection between some of them and zones of high magnetic susceptibility in the core.
Figure F23A-F23F, is a gallery of net veins and vein breccias in slab samples, arranged in order of depth. Narrow and irregularly distributed felsic veinlets show up as light gray to nearly white, and the veins in each sample contain quartz, based on examination with a hand lens. The maximum of magnetic susceptibility is indicated below each image. The extent of plastic deformation in the darker matrix, ranging from slightly deformed (1) to mylonitic (4) is indicated to the right of each image. None are undeformed. Only one of these samples, shown in Figure F23C, has low magnetic susceptibility. The host rock is olivine gabbro. The breccias shown in Figure F23B and F23D were also identified as olivine gabbro, but this is inconsistent with the high magnetic susceptibility measured in the same intervals. The sample shown in Figure F23A was identified as a disseminated oxide olivine gabbro in the core descriptions, but again the same interval has the very high magnetic susceptibility of a much more oxide-rich gabbro. In the sample shown in Figure F23D, fragments of the host rock clearly constitute a disrupted breccia. Although the sample in Figure F23E appears almost equally brecciated, in fact most of the fragments are conformal to the direction of relatively weak foliation in the rock on either side. The sample in Figure F23F is olivine gabbro with gneissic fabric, coarsening downward to an abrupt contact with a fine-grained mylonitic oxide gabbro. Felsic veinlets are dispersed parallel to the gneissic fabric and into the mylonite.
In each case, formation of felsic veinlets followed plastic deformation. In each case, a drop in temperature accompanying magmatic differentiation was attended by a shift from plastic to brittle deformation. In each case, variably foliated rocks were then irregularly disrupted and infiltrated by the extreme late differentiates forming in the section. All of this occurred at high temperature, above the liquidus temperature for differentiates of basaltic parents (~1000°C for anhydrous liquids according to Dixon and Rutherford, 1979). Veining of deformed rock also occurred higher in the section, in the thick interval of oxide gabbros (lithologic Unit IV), where trondhjemite intrusion breccias crosscut foliated oxide gabbros (Dick et al., 1991a, plate 5.1). Near the top of the hole, trondhjemite, or fluids from which a felsic leucosome precipitated, intruded amphibole gneiss in the plane of foliation, possibly in the manner of lit-par-lit injection (Dick et al., 1991, plate 5.2). In every case, there is a strong association between felsic veins and oxide gabbros, which during Leg 176 were without exception indicated by high magnetic susceptibility.
The cores of Hole 735B contain dozens of magmatic breccias in oxide gabbros with few or no felsic veinlets. I show one example. Figure F24 depicts unrolled and slabbed images of a single piece of core entirely spanning lithologic interval 555, gabbro, and portions of the intervals above and below. Two fracture surfaces bound the breccia (sinusoidal patterns in the unrolled image), the upper of which appears to be magmatically annealed. The breccia fragments are extremely coarse grained and evidently were originally part of a pegmatitic gabbro. Now they are twisted and bent. Lithologic interval 555 also spans a strong spike in magnetic susceptibility, exceeding 7000 x 10-6 MU, although no corresponding concentration of oxide minerals was noted in the core descriptions. A second spike is in undeformed rock below the breccia. Some contrast enhancement applied to the unrolled image suggests that felsic material may have concentrated in porosity structure near the bounding fracture surfaces of the breccia.