During Leg 176, magnetic susceptibility was measured at a 4-cm spacing on all gabbroic rock recovered from Hole 735B using a Bartington sensor attached to the shipboard multisensor track. The instrument was particularly sensitive to the presence of magmatic magnetite, which is consistently intergrown with more abundant ilmenite. These oxide minerals together range from trace amounts to >30% of the rocks. The log of magnetic susceptibility thus produced a pattern of abrupt and pronounced spikes whenever oxide-bearing and oxide-rich gabbros were encountered. It provides a detailed, precise, high-resolution means of evaluating this aspect of gabbro lithology. The log reveals a far more intricate pattern of oxide gabbros cross-cutting and intruding primitive olivine gabbros and troctolites with low magnetic susceptibility than was possible to discern by eye during preparation of the shipboard core descriptions. Magnetic susceptibility confirms and reinforces the observation that oxide gabbros are typically more strongly deformed than olivine gabbros and troctolites, and it also shows that late-stage felsic veins are very strongly associated with oxide gabbros.
Because magmatic magnetite is always intergrown with ilmenite, there is a strong correlation between magnetic susceptibility and the bulk TiO2 contents of rocks with >0.7% TiO2, which represents ~1% by volume of oxide minerals in the rock. Among olivine gabbros and troctolites, which have <0.7% TiO2 contents, the correlation is weaker and is not significantly influenced by tiny variations in the proportions of magmatic oxides; instead, magnetic susceptibility among these rocks correlates more strongly with extent of differentiation, as marked by a decrease in bulk-rock MgNo. The correlation may indicate a higher proportion of exsolved magnetite in the somewhat more iron-rich ferromagnesian silicates and plagioclases of the olivine gabbros. The shipboard-based conclusion that the section cored consists of several sequential plutons, each more differentiated upward, breaks down in light of the magnetic susceptibility log. The upper parts of the three so-called plutons cored during Leg 176 instead are places where late-stage, intrusive disseminated oxide and oxide gabbros are particularly concentrated. The olivine gabbros and troctolites that they intrude have a different, unrelated pattern of compositional variability. In many cases, iron-rich melts penetrated the fine-scale porosity structure of crystallizing primitive gabbros, enriching them in oxide minerals but not modifying compositions of preexisting olivine. Several zones where particularly magnesian olivine gabbros and troctolites with very low magnetic susceptibilities are concentrated may represent places where the lower crust was significantly inflated by intrusion of primitive basalt at or near the ridge axis, before the oxide gabbros were emplaced and before significant crystal-plastic deformation occurred. The body of rock beneath a fault zone at 1100 meters below the seafloor is distinct, with more primitive olivine gabbros and troctolites than in the rocks above, far fewer seams of oxide gabbro, much less deformation, and far more widely separated felsic veins.
1Natland, J.H., 2002. Magnetic susceptibility as an index of the lithology and composition of gabbros, ODP Leg 176, Hole 735B, Southwest Indian Ridge. In Natland, J.H., Dick, H.J.B., Miller, D.J., and Von Herzen, R.P. (Eds.), Proc. ODP, Sci. Results, 176 [Online]. Available from World Wide Web: <http://www-odp.tamu.edu/publications/176_SR/chap_11/chap_11.htm>. [Cited YYYY-MM-DD]
2Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami FL 33149, USA. jnatland@rsmas.miami.edu
Initial receipt: 12 October 2000
Acceptance: 6 August 2002
Web publication:
24 October 2002
Ms 176SR-008