N1. Some undeformed gabbronorites may post-date the high temperature deformation, or - our preferred hypothesis - localization of the high temperature deformation preserved undeformed, pre-kinematic gabbronorites below the mylonitic shear zones
N2. It might seem simpler to envision 13° of clockwise rotation around a horizontal axis striking 270° to account for the shallow 15° inclination in the gabbros. However, to explain the steep 36° inclination in the peridotites via rotation around the same west-striking axis would require an antithetical, counter-clockwise rotation of 8°. It seems unlikely that rocks from the same hole would have undergone antithetical rotations.
N3. The discussion in the previous paragraphs considers only changes in the magnetic inclination due to rotation. However, tectonic rotations after acquisition of the magnetic remanence will also affect the magnetic declination and, hence, influence the reorientation of structural features. Figure F18 illustrates this effect, which can be as much as 50° for counterclockwise rotation around a horizontal or gently plunging axis striking 020°.
N4. In Figure F27, the approximate proportion of magnetite is quantified using measurements of magmatic susceptibility made on large individual core pieces. However, there are numerous uncertainties associated with this procedure. For example, the proportion of nonmagnetic ilmenite is not quantified, and the presence of Ti as ulvospinel component in the magnetite solid solution reduces its magnetic susceptibility, leading to an underestimate of magnetite proportion. Magnetite content inferred from susceptibility does not correlate well with estimates of oxide proportion in the visual core description, nor with estimates of oxide proportion based on chemical analyses and calculated mineral proportions, nor with estimates of oxide proportion based on thin section observations. However, these other estimates are not available on many samples and may be subject to a "nugget effect," leading to large variations in oxide proportion observed in thin sections on the centimeter scale, and inferred from geochemical analyses of small samples, so we have used the more continuous (and internally consistent?) data set derived from interpretation of magnetic susceptibility.
N5. If the constraint that tectonic rotation axes were parallel to the rift axis is dropped, smaller rotations around a horizontal axis striking 270° can account for the inclinations in the gabbroic rocks and the peridotites. However, this is problematic if Hole 1270B rocks are reversely magnetized, whereas Hole 1270C and 1270D samples are normally magnetized, because it requires opposite senses of rotation for the samples from the different holes. Alternatively, if all are normally magnetized, a minimum of 42° of clockwise rotation can account for the 14° inclination in the gabbroic rocks from Hole 1270B and a minimum rotation of 31° of clockwise rotation can account for the 3° inclination in the peridotites from Holes 1270C and 1270D. Given the scarcity of north- and south-facing slopes in the rift mountains flanking the Mid-Atlantic Ridge in this area, such large tectonic rotations around east-west axes seem unlikely.
N6. The discussion in the previous paragraphs considers only changes in the magnetic inclination due to rotation. However, tectonic rotations after acquisition of the magnetic remanence will also affect the magnetic declination and, hence, influence the reorientation of structural features. See "Site 1268" and Figure F18, and accompanying text for further discussion.
N7. About 7% of the recovered core from Hole 1274A consisted of poorly consolidated mud and matrix supported, cataclastic breccia, interpreted as fault gouge. This material had a dominantly ultramafic protolith, but observation of gabbroic clasts as well as peridotite clasts, and geochemical data indicate that it is a mixture including a substantial proportion of gabbroic material.
N8. Note that at Site 1271, just a few kilometers south of Site 1272, we recovered abundant gabbroic rocks that were clearly intrusive into harzburgite.
N9. If the normative proportions of orthopyroxene are primary, then some harzburgites have been incorrectly classified as orthopyroxene-bearing dunites
N10. Plagioclase contains more Sr and Ba than olivine, pyroxene, and spinel in residual peridotites, so that impregnated peridotites, together with carbonate-rich metaperidotites, have high Sr and Ba compared to other samples.
N11. The fact that H2O is conserved in this reaction suggests that it may be pressure sensitive, so perhaps there are conditions under which the assemblage brucite + talc is stable; however, this is doubtful for low-pressure hydrothermal alteration.
N12. Note that this analysis assumes that a melt was in equilibrium with olivine but does not assume that the olivine in the troctolites crystallized from that melt. It is not important for this discussion whether the olivine in the troctolites is entirely igneous in origin, or whether some of the olivine is relict, residual olivine from mantle peridotite.Table of Contents