The interrelation between variations in magnetic anomaly amplitudes and basalt magnetization and chemistry along the Southeast Indian Ridge

Magnetic model studies, rock magnetization, and major element chemical analyses show that unusually large amplitude magnetic anomalies found along the easternmost segment of the Southeast Indian Ridge [zone A] are caused by the high intensities of magnetization of basalts intruded at the ridge axis. In turn, these high magnetizations are caused by a threefold increase in weight percent of titanomagnetites from normal midocean ridge basalts intruded in the west [zones B and C] to ferrobasalts intruded in the high magnetic anomaly zone to the east. The Fe-Ti increase in the opaques reflects a major elemental enrichment in Fe and Ti caused by extensive shallow fractionation of plagioclase, clinopyroxene, and olivine. This shallow fractionation appears related to unusually cold magma crystallization temperatures, perhaps caused by proximity to a postulated downwelling zone in the asthenosphere south of Australia. However, examination of these correlations in greater detail points to several ambiguities: although the entire region is deep, and therefore the lithosphere is integrally colder than normal plates, zone A is shallower than zones B and C, but geothermometrically, its axial magmas are colder. We do not know how cold magmas directly relate to ferrobasalt generation. Also, plumelike geochemical signatures are also seen in some of the analyses. It appears that plume mantle sources are mixing with MORB sources and then undergoing extensive shallow fractional crystallization in zone A.