Mineral Compositions of MORB from the Australian Antarctic Discordance (AAD): Implications for Mantle Source Characteristics

The chemical compositions of olivine, plagioclase, pyroxene, and spinel in lavas collected during Ocean Drilling Program Leg 187 in the Australian Antarctic Discordance, Southeast Indian Ridge (41°–46°S, 126°–135°E) were analyzed, and modeling of the theoretical equilibrium petrogenetic conditions between olivine and melt was conducted. The cores of larger olivine phenocrysts, particularly in the isotopic Indian-type mid-ocean-ridge basalt (MORB), are not equilibrated with melt compositions and are considered to be xenocrystic. Larger plagioclase phenocrysts with compositionally reversed zonation are also xenocrystic. The compositions of primary magma were calculated using a “maximum olivine fractionation” model for primitive MORB that should fractionate only olivine. Olivine compositions equilibrated with calculated primary magma and compositions of calculated primary magma suggest that (1) isotopic Pacific-type MORB is more fractionated than Indian-type MORB, (2) Pacific-type MORB was produced by higher degrees of partial melting than Indian-type MORB, and (3) primary magma for Indian-type MORB was segregated from mantle at 10 kbar (~30 km depth), whereas that for Pacific-type MORB was segregated at 15 kbar (~45 km depth). 1Sato, H. , 2004. Mineral compositions of MORB from the Australian Antarctic Discordance (AAD): implications for mantle source characteristics. In Pedersen, R.B. , Christie, D.M. , and Miller, D.J. (Eds.), Proc. ODP, Sci. Results, 187, 1–26 [Online]. Available from World Wide Web: . [Cited YYYYMM-DD] 2School of Business Administration/ Institute of Natural Science, Shenshu University, 2-1-1 Higashimata, Tama, Kawasaki, Kanagawa 214-8580, Japan. satohiro@isc.senshu-u.ac. jp Initial receipt: 17 May 2002 Acceptance: 17 July 2003 Web publication: 17 February 2004 Ms 187SR-202 H. SATO MINERAL COMPOSITIONS OF MORB 2 INTRODUCTION Mid-ocean-ridge basalt (MORB) erupted along the Indian Ocean spreading center are isotopically distinct from those erupted along the Pacific and Atlantic Ocean spreading centers (Dupré and Allègre, 1983; Hart, 1984). Indian Ocean MORB is characterized by high 87Sr/86Sr, 207Pb/204Pb, and 208Pb/204Pb for a given value of 206Pb/204Pb (e.g., Klein et al., 1988). Within the Australian Antarctic Discordance (AAD), located along the Southeast Indian Ridge (SEIR) south of Australia (Fig. F1A), a uniquely sharp isotopic boundary between the Indian Ocean and Pacific Ocean mantle provinces has been recognized (Klein et al., 1988). Based on isotopic analyses of MORB samples dredged from the present spreading axis in the AAD, only basalts younger than 3–4 Ma exhibit Pacific-type isotope signatures (Pyle et al., 1992; Christie et al., 1998). These data form the basis for a hypothesis that proposes rapid westward migration of Pacific mantle during the last 4 m.y. (Christie et al., 1998). During Ocean Drilling Program (ODP) Leg 187 (Shipboard Scientific Party, 2001), 23 holes were drilled at 13 sites across the hypothesized isotope boundary forming the eastern terminus of the AAD along a swath of 10to ~30-Ma seafloor interpreted from magnetic anomalies (Fig. F1B). Isotope mantle types, Indian, Pacific, and transitionalPacific, were assigned to basalts collected during Leg 187 based on shipboard geochemical analysis of Ba and Zr contents of basalt glasses of younger lavas from the AAD and east of the AAD (Shipboard Scientific Party, 2001). The following relationships are summarized from the shipboard identifications: (1) there is a plausible mantle province boundary east of the regional residual depth anomaly (difference between the depth of seafloor and that expected from depth-age relations found for other ocean basins: e.g., Marks et al., 1990), and (2) basalts derived from Indian and Pacific mantle types alternate on a timescale of a few million years between 28 and 14 Ma along a flowline in western Zone A (Shipboard Scientific Party, 2001). In this study, I will report chemical compositions of minerals in basalts collected from the AAD during Leg 187. Complemented with whole-rock major element compositions, I will discuss petrogenetic conditions, particularly pressure (depth) for magma segregation and estimate primary magma compositions for Indianand Pacific-type MORB. GEOLOGICAL SETTING AND LOCATION OF ODP SITES The AAD is a segment of anomalous “chaotic” bathymetry and gravity along the SEIR between 120° and 128°E. Weissel and Hayes (1971) subdivided the SEIR south of Australia into three zones, Zone A (east of the AAD), Zone B (the AAD itself), and Zone C (west of the AAD), based on striking contrasts in seafloor morphology and geophysical data. In spite of a regionally uniform spreading rate (~74 mm/yr full rate), axial morphology between Zone A and Zone B (AAD) is quite different; Zone A exhibits a bathymetry similar to the fast-spreading East Pacific Rise– type spreading axis, whereas Zone B (AAD) has a deeper axial ridge similar to the slow-spreading Mid-Atlantic Ridge (e.g., Palmer et al., 1993; Shipboard Scientific Party, 2001). 30° S