Abstract:
Leg 187 of the Ocean Drilling Program set out to evaluate the geological history of the unique Indian-Pacific geochemical mantle domain boundary as it is recorded on the seafloor between its present location within the Australian Antarctic Discordance (AAD) and the southern continental margin of Australia. We used shipboard trace element analyses of basalt glasses, later confirmed and refined by onshore isotopic data, to locate this boundary within + or -50 km through a grid of drill sites in seafloor of age 14-28 Ma. From these data, we infer that the Indian-Pacific mid-ocean-ridge basalt (MORB) mantle boundary is an intrinsic feature of the Australian-Antarctic Depth Anomaly (AADA), an anomalously deep region that spans the Southern Ocean Basin from the Australian to the Antarctic continental margins. For at least 25 m.y., the boundary has been located approximately 100 km east of the midline of the depth anomaly, between the -400 and -500 m depth anomaly contours. Pronounced westward deviations of the boundary along the present-day Southeast Indian Ridge (SEIR) and at approximately 20 Ma record transient westward propagation events of 3- to 4-m.y. duration. The origins of the distinct isotopic signatures that define the Indian and Pacific mantle provinces remain controversial, but it seems clear that a significant component of the Indian mantle source was derived, as a by-product of the breakup of Gondwana, from ancient continental material that includes ancient accreted island-arc derived material. The geochemical contrasts between lava populations from the Indian and Pacific domains have remained qualitatively constant through time. Basalts from the Indian domain are less evolved, with a smaller range of generally higher MgO contents than their Pacific counterparts. They are also more variable in other major and trace elements, consistent with derivation by generally smaller degrees of melting from a more variable melting regime. Quantitatively, however, the boundaries between the two groups have shifted, apparently reflecting a significant decrease in overall extents of melting. This shift, which occurred between approximately 14 Ma, the youngest age sampled during Leg 187, and approximately 7 Ma, the oldest age sampled by earlier dredging, most likely occurred when the northeastward absolute migration of the SEIR moved the spreading-axis segments that define the AAD into conjunction with the AADA and its underlying cooler mantle. In this cooler environment, overall extents of melting and magma supply rates were reduced, leading to the establishment of a predominantly extensional regime, characterized by extensive listric faulting within the AAD. Leg 187, in combination with earlier dredge sampling of 0- to 7-Ma seafloor, also provided an unusual opportunity to evaluate post-eruptive geochemical and biological modification of seafloor lavas. Culturable microbes were isolated from lavas at all Leg 187 sites, strongly suggesting that significant microbial activity persists in the shallow ocean crust for at least 28 m.y. However, microbial forms that are commonly visible in electron microscopic images of alteration fronts and zeolite mineral surfaces associated with fractures in young volcanic glasses are no longer detectable in Leg 187 glasses. This implies that the locus of microbial activity has shifted away from pillow-rim volcanic glass to some other location within the samples, most likely to fractures within the more crystalline pillow basalts. This shift in the dominant mode of microbial activity may reflect a change from open-system seawater circulation to closed-system circulation due to burial. This change is consistent with patterns of alteration. On the one hand, isotopic studies of altered lavas indicate that of isotopic exchange with circulating seawater had ceased prior to 14 Ma, whereas on the other hand, mineralogical studies document ongoing alteration throughout the 14- to 28-Ma time span.