The rapid-turnaround shipboard analytical capability provided by the new ICP-AES instrument was fundamental to the success of Leg 187. At several points throughout the leg, data from one site were used in deciding which of two or even three alternate plans would be followed for the succeeding few sites. This reactive strategy enabled us to rapidly determine that the isotopic boundary is closely related to the eastern side of the regional depth anomaly. We were then able to focus with confidence on sites in the vicinity of the depth anomaly, to the exclusion of sites in eastern Zone A.
The Indian/Pacific mantle boundary was originally identified within the AAD on the basis of isotopic ratios in seafloor lavas, and these ratios, particularly 206Pb/204Pb, continue to be the only definitive discriminants between Pacific and Indian mantle provinces (Fig. F23). In planning for Leg 187, we worked exhaustively with our data from zero-age and young (<7 Ma) dredge samples throughout the AAD and Zone A to identify a reliable discriminant that could be analyzed on board the JOIDES Resolution. A single element, barium, appeared to be both reliable as a discriminant and relatively easy to determine with the necessary accuracy. For ease of use, we settled upon a single diagram, Zr/Ba vs. Ba, as providing a clear, visual discrimination between Indian- and Pacific-type lavas from our 0- to 7-Ma data set. A second element ratio, Na/Ti (expressed as Na2O/TiO2 throughout this report), also appeared to have potential as a discriminant, although both elements are susceptible to fractionation by a variety of magmatic processes. As it turned out, the 0- to 7-Ma dividing line on this diagram does not apply to most Leg 187 sites, so we relied almost exclusively on Ba-Zr systematics in our decision-making process. We recognize, however, that the Ba-Zr systematics may be an imperfect discriminant and that some sites could have been misidentified as Indian or Pacific type. In addition, some Leg 187 samples occupy a field (referred to as Transitional Pacific) not represented among our 0- to 7-Ma samples (see "Mantle Domain Recognition," below). Therefore shipboard determinations of mantle domain will remain tentative until isotope data become available.
The glasses from Leg 187 sites are all relatively primitive with MgO ranging from 9.4 to 7.2 wt%. With few exceptions, whole-rock samples have significantly lower MgO contents than the associated glasses. At the most extreme, whole-rock MgO values are lower by 3-4 wt%, but in most cases they are lower by only 0.5-1.0 wt%. These variations in MgO are seldom accompanied by coherent variations in other elements; therefore, they cannot be attributed to crystallization or other magmatic processes. A particularly clear example of this phenomenon is provided by Hole 1160B, which recovered seven lithologic units, three of which are massive flows interspersed with pillow flows. Each massive flow is overlain by a pillow flow of the same lithology. Whole-rock samples from all three massive flows and glasses from two of the overlying pillow flows are essentially identical in composition with ~8.9-9.1 wt% MgO (Fig. F24). Whole rocks from the pillow flows, however, have lower MgO contents, differing from the glasses by as much as 3-4 wt%. Most other elements in the pillow whole-rock samples are similar in abundance to the glass and massive flow samples. We conclude from these observations that MgO is selectively removed from pillow interiors as a result of the pervasive low-temperature alteration that has affected all our sites.
Element concentrations and variations other than Ba, Zr, Ti, Na, and MgO have received only cursory examination and discussion at most sites. However, significant differences are apparent in parental magma composition between Leg 187 lavas and present-day lavas from the same segment. For Segments B4 and B5, these variations are likely related to the westward migration of the depth anomaly relative to the segment boundaries. In Zone A, temporal variability appears to have been modulated by repeated rift propagation.
Throughout Leg 187, we used ICP-AES Ba and Zr data to determine mantle domain based on the 0- to 7-Ma data fields in the Zr/Ba vs. Ba diagram. Because of the potential for alteration of Ba content by seawater alteration, we used only hand-picked basalt glasses in making this determination. Figure F25 shows all available glass data from the AAD region. There is a clear division between glasses from the Indian mantle domain and those from the Pacific domain. A tie line connecting all the glasses from the transitional domain beneath the Segment B5 axis cuts across both fields (e.g., Segment B5 axis; Fig. F25). The only 0- to 7-Ma lavas that do not fall within the clearly defined Indian and Pacific fields are a small group of primitive lavas from propagating rift tips in Zone A.
Leg 187 glass data are plotted in relation to the 0- to 7-Ma Ba-Zr fields and the Segment B5 tie line in Figure F26A. Although many of the Leg 187 data plot within or very close to the 0- to 7-Ma fields and can readily be assigned to one of these fields, there is an important group of data that cannot. These data plot in the area below the Pacific field (i.e., toward lower Zr/Ba values and slightly higher Ba content than the majority of Pacific-type lavas). We refer to these lavas as Transitional-Pacific type because they appear to represent an extension of the Pacific field toward the compositions of the Zone A propagating rift tip lavas (Fig. F26A). This extension may reflect a temporal shift in any of several source parameters, including source composition and overall extent of melting or a shift in the mixing proportions of one or more mantle end-member components. It could also reflect addition of Ba to the samples from seawater, either directly by low-temperature alteration or indirectly by assimilation of altered crust into the magma. In terms of the Zr/Ba vs. Ba diagram, basaltic liquids plotting in the Transitional-Pacific field could also have been derived by crystal fractionation from Indian-type parents. We would expect this to produce trends that cut across the observed data array at a high angle. The data do not, however, support a fractionation origin for the Transitional-Pacific glasses, as there is no apparent progression to decreasing MgO across the Indian field or into the Transitional-Pacific field. Finally, lavas from the Transitional-Pacific field are distinct from transitional lavas along the Segment B5 tie line and are therefore unlikely to reflect mixing between Indian and Pacific domains as is presently defined by Segment B5 axial lavas.
For the 0- to 7-Ma glasses, a plot of Na2O/TiO2 values vs. MgO content effectively discriminates Indian-domain glasses from those of the Pacific domain, again with the Segment B5 tie line spanning the divide between domains (Fig. F25B). Unfortunately, this diagram does not apply in a straightforward way to the Leg 187 glasses. In Figure F26B, Leg 187 glass data are plotted on this diagram and coded as Indian, Pacific, or Transitional Pacific according to their position on the Zr/Ba vs. Ba diagram (Fig. F26A). According to this division, no Leg 187 Indian-type lava plots in the 0- to 7-Ma "Indian" field, and lavas of all three types are intermixed in the "Pacific" field.
Although it is offset to lower Na2O/TiO2 than the 0- to 7-Ma field, Leg 187 Indian lavas do define a discrete field, although this field also encompasses several Transitional-Pacific and Pacific lavas. The three Pacific lavas are, however, all from Site 1160; these also have the three lowest Ba contents of the Leg 187 Pacific lavas. In Figure F26A, they plot to the left of the Segment B5 tie line. The only other sample with comparable low Ba content is the borderline Pacific sample from Site 1164 that is plotted as a red triangle in the figure.
This offset of the Pacific/Indian field boundary appears to reflect the generally low Na2O contents of Leg 187 glasses relative to those of the AAD. Whether this apparently fundamental shift in primary magma chemistry reflects a temporal change in mantle temperature and/or extent of melting, a change in source composition or some other source parameter cannot be evaluated without further data.
Most of the Leg 187 sites are distributed along three north-south transects, one each in Segments B4 and B5 and one in western Zone A (Figs. F27, F28). There are two additional sites in eastern Zone A.
Indian-type mantle was present beneath all three Segment B5 sites and one Segment B4 site (1163) at their time of eruption. At two sites, basalts derived from two distinct mantle types were erupted in close proximity. Glasses from Hole 1155A are of Transitional-Pacific type, whereas those from Hole 1155B, 200 m away, are of Indian type. Glasses from Holes 1164A and 1164B are of Pacific and Indian types, respectively, although the Pacific-type glass plots very close to the boundary between the two fields. (This glass is represented by a red triangle in Fig. F26A.) Only Transitional-Pacific-type glass was recovered at Site 1152.
Six sites form a transect between crustal ages 28 and 14 Ma in western Zone A (Figs. F27, F28). Along this transect, Indian-type mantle was present at ~23 and 19 Ma, but in three out of four cases the transition between Indian and Pacific type is constrained to have taken place within no more than 2-3 m.y. The fourth transition, north of Site 1157, is not constrained as there is no site in this area.
Sites 1154 and 1160 in eastern Zone A are distinctly of Pacific type (Figs. F27, F28).
Taking the shipboard identifications of mantle domain at face value, three fundamental observations can be made:
From these observations we draw the following tentative conclusions, which will require careful testing as the isotopic data become available.
A discrete mantle boundary comparable to the present-day boundary in the AAD cannot be mapped through the entire 14- to 28-Ma time interval encompassed by Leg 187, although comparable boundaries may have existed for relatively short time intervals. For the longer term, it appears likely that the eastern limit of the Indian-mantle province corresponds closely to the eastern edge of the depth anomaly. The locus of this boundary is unconstrained, but it must lie close to the -500-m residual depth contour. West of this boundary and perhaps coinciding with the deepest region of the depth anomaly (-500 m), Indian mantle predominates, but occurrences of Transitional-Pacific- and even Pacific-type mantle are not uncommon. The western limit of Pacific- or Transitional-Pacific-type mantle is not well defined by our data, but it cannot be farther east than the -400-m residual depth contour on the west side of the depth anomaly.
The alternation of Indian-type sites with Pacific- and Transitional-Pacific-type sites along the western Zone A transect from 14 to 25 Ma suggests a rapid alternation of discrete mantle types on time scales of a few million years, comparable in time scale to the recent (0-4 Ma) migration in the AAD. These occurrences can be interpreted as discrete incursions, possibly of Indian mantle beneath Zone A, but more likely of Pacific-type mantle into the dominantly Indian region of the depth anomaly. If either interpretation is correct, then a discrete Indian/Pacific boundary likely existed for much of that time.
Transitional-Pacific-type mantle is not represented in our 0- to 7-Ma data set, and we have interpreted it as an extension of the Pacific-mantle field rather than a transition between the Pacific- and Indian-type domains as defined by present-day SEIR MORB glass. This type may manifest a mantle component that cannot be seen in the well-mixed, high-flux magma systems of Zone A. In the region of the depth anomaly, magma flux is greatly reduced and small magma batches with a variety of individual source signatures may be erupted with little modification.