Figure F4 is a plot of the edited measurements vs. depth, and it supplants figure F120 of the site report (Shipboard Scientific Party, 1999b). Scales for both MU and volume-corrected SI units are provided on the bottom and top of the figure, respectively. There are 476 individual peaks and peak regions with values of susceptibility >2000 x 10-6 MU, with the frequency of these spikes diminishing generally but sporadically downward. There are few such peaks below 1100 mbsf. Peak regions are collections of peaks spaced over several tens of centimeters that oscillate at high values of several thousand x 10-6 MU and that have no intermediate background values. Almost all of these were identified as intervals of oxide gabbro or oxide olivine gabbro in the core descriptions, although their internal fluctuations in magnetic susceptibility suggest that they have uneven internal concentrations of oxide minerals. In some cases, the internal oscillations are at average high values of magnetic susceptibility (Fig. F5), and in others, the fluctuations are more extreme (Fig. F6). In some places, individual peaks, some of them quite pronounced, are fairly widely spaced (Fig. F7). Some lithologic intervals have only small oscillations in magnetic susceptibility above background (e.g., gabbronorite interval 562 in Fig. F6). In all of these instances, there are at least some measurements, even within oxide gabbro intervals at background levels (red portions of the susceptibility variations shown in Figs. F5, F6, and F7). This attests to the presence of minor amounts of relatively primitive gabbro crosscut by, or perhaps occurring as xenoliths in, oxide gabbro at a very fine scale in these rocks.
Defining the width of the peak or peak region as the span of measurements having >2000 x 10-6 MU, and in this way clipping fall-offs representing the tail ends of the point-source response function, the average peak width is 13.4 cm, with a high standard deviation reflecting a range of widths between 176 and 4 cm. The true average is probably less than this because 4 cm is the lower limit of resolution of the MST and there are many narrow seams of oxide gabbro <1 cm in width. Some of these cross the core obliquely and are recorded in two or three consecutive measurements. Nevertheless, the lower 1000 m of core from Hole 735B is dominated by olivine gabbros and troctolites; these lithologies provide the background measurements of magnetic susceptibility. The olivine gabbros and troctolites are crosscut by a great many seams of oxide gabbro, the largest of which is <2 m thick in the portion of the hole cored during Leg 176.
Most of the wider peak regions between 500 mbsf and the bottom of the hole having widths of >50 cm are between 504 and 710 mbsf (Fig. F8). The percentage of core represented by peaks also diminishes downward in the section (Fig. F9). The region of core between 600 and 700 mbsf has >80% oxide-bearing and oxide-rich gabbro because many of the peak regions are fairly wide and closely spaced. The MST susceptibility measurements reveal that there is scarcely an interval of relatively primitive olivine gabbro or troctolite that is utterly without some proportion of rock enriched in oxide minerals, even if that material causes only small fluctuations in susceptibility.
Assuming that all peaks in magnetic susceptibility in some measure represent late-stage intrusive rocks, then there should be an underlying and possibly unrelated variability in the more primitive olivine gabbros and troctolites that magnetic susceptibility might reveal. Figure F10 shows that there are indeed variations beyond the general oscillation in background measurements. Fluctuations are indicated by two smoothing curves fit to the data. The black curve is a weighted fit, one of the standard ones provided by the plotting program, Kaleidagraph. This fit encompasses a data point and 10% of the surrounding data and intrinsically produces discontinuities at either end of the plot, where data are tied to the terminal data points. It reveals fluctuations in magnetic susceptibility, thus proportion of oxide minerals, at a fairly coarse scale of perhaps 100 m, assuming that there are approximately the same numbers of measurements in each 100 m of the core depicted. There are five oscillations, delimited at their minima by dashed horizontal lines, and numbered to the right.
The second curve with more spikes is an interpolated function that passes through sequential data points and matches the slopes at those points. There are 23 small peaks on this curve, defining fluctuations in slope presumably marking shifts in composition. This curve effectively indicates the preponderance of background measurements within intervals of ~20 m length. Finally, downward spikes on the overall susceptibility curve with values <250 x 10-6 MU indicate where the most primitive gabbroic rocks, olivine gabbros and troctolites, were detected.
The most important feature of Figure F10 is an abrupt offset in background magnetic susceptibility to lower values downcore at 1100 mbsf. This corresponds to a fault zone over ~30 m of the core, in which there are numerous small faults having both normal and reversed senses of displacement (Shipboard Scientific Party, 1999b). Background susceptibilities are, on the whole, higher above this fault than below it, and there are only two maxima at the 100-m scale of variability above the fault, whereas there are four lesser maxima, shifted to lower values, below it. There is also no strong correspondence between the coarse maxima in background susceptibility and those portions of the core having high proportions of oxide-rich seams, represented by peaks and peak regions in the downhole susceptibility trend.
Another way to consider these data is to assume that the material recovered in each core is fully representative of the interval cored, even if recovery was fairly low. Figure F11 thus shows the average background magnetic susceptibility by core plotted vs. depth, with another 10% weighted curve, this time smoothing many fewer data points. Nevertheless, the weighted curves in Figures F10 and F11 are very similar, and the core-by-core fluctuations in Figure F11 resemble the interpolated curve in Figure F10.
Of greatest interest is whether the shifts in background magnetic susceptibility reveal the pattern of cryptic variation in mineral compositions among the principal gabbroic facies cored in Hole 735B. The issue is complicated because the rocks are cumulates, and in such rocks the proportions of oxide minerals may vary not merely because one rock is more differentiated than another, but also because the percentage of intercumulus melt, from which the oxides crystallized, can vary because of differences in the amount of postcumulus overgrowth on surrounding silicates, expulsion of intercumulus melt by compaction and deformation, and even reintroduction of differentiated melt into the fine-scale porosity structure of the crystallizing rocks (Natland et al., 1991). A second possibility is that among these rocks, magmatic oxides may be present in such small abundance that they do not matter at all. If so, then another cause for variations in magnetic susceptibility must be sought. A correlation between magnetic susceptibility and the general extent of differentiation of rocks lacking magmatic oxides then becomes of unusual importance, no matter how it is caused. This would tie magnetic susceptibility to what petrologists have usually termed the cryptic variation (Wager and Deer, 1939) of intrusive bodies, namely the variation in composition of silicate minerals, which tracks differences of temperature of the magmas that produced the minerals (e.g., Usselman and Hodge, 1978). Then the fluctuations in magnetic susceptibility in Figures F10 and F11, whether construed as 2, 5, 23, or more in number, could reflect the volume and frequency of magma injection and the fluctuations in magma temperature that prevailed when this bit of ocean crust was accreting. This whole topic requires attention to the relationship between magnetic susceptibility and the bulk compositions and silicate mineralogy of the gabbros, which will be considered toward the end of this paper.