HR: 0800h
AN: T41E-1352    [Abstracts]
TI: Abyssal Peridotites From Ultra-Slow Spreading Ridges: Mantle Heterogeneities Versus Melting Processes
AU: Sakaguchi, C
EM: csaka@misasa.okayama-u.ac.jp
AF: Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori, 682-0193 Japan
AU: * Warren, J M
EM: jmwarren@whoi.edu
AF: MIT/WHOI Joint Program, 360 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Shimizu, N
EM: nshimizu@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Dick, H J
EM: hdick@whoi.edu
AF: Woods Hole Oceanographic Institution, 360 Woods Hole Rd, Woods Hole, MA 02543 United States
AU: Nakamura, E
EM: eizonak@misasa.okayama-u.ac.jp
AF: Institute for Study of the Earth's Interior, Okayama University at Misasa, Tottori, 682-0193 Japan
AB: Abyssal peridotite geochemistry can provide important constraints on mantle convection, mid-ocean ridge processes, and earth composition. However, separating the chemical signatures of melt extraction processes at mid-ocean ridges from underlying source heterogeneity is not entirely straightforward. Ultra-slow spreading ridges are an ideal location to distinguish the effects of source versus process in abyssal peridotites, due to reduced crustal production resulting from low upwelling rates and on-axis conductive cooling. We present data on abyssal peridotites from ultra-slow spreading portions of the Southwest Indian Ridge (SWIR). At ultra-slow spreading rates, peridotites have a wide range in modal mineralogy and trace element composition, due to (i) variable degrees of melting, (ii) melt addition, and (iii) underlying mantle heterogeneity. In contrast, peridotites from fast spreading ridges have a narrow compositional range, reflecting high degrees of melting. SWIR peridotites from the Atlantis II Fracture Zone and the Oblique Segment are heterogeneous on all scales, from the >100 km ridge scale down to the hand specimen scale and in some cases, on the grain scale. Heterogeneity is observed in a variety of forms, including (i) veins, (ii) ranges in modal clinopyroxene content, (iii) variations in trace elements by up to three orders of magnitude, and (iv) isotopic variations. Some of this heterogeneity is clearly related to shallow level melt processes. For example, pervasive gabbroic veining and matrix plagioclase within one peridotite dredge indicates the freezing of melts in situ in the shallow mantle. However, the pyroxene veins in another dredge, which may be similar to compositional layering observed in orogenic lherzolites and ophiolites, are not as clearly related to melting processes. The isotopic composition of peridotite clinopyroxenes in this dredge extends to an enriched ¹⁴³Nd/¹⁴⁴Nd value of 0.512937. Pb isotopes in the clinopyroxenes are similarly enriched relative to depleted MORB mantle, with ²⁰⁶Pb/²⁰⁴Pb =19.6057, ²⁰⁷Pb/²⁰⁴Pb =15.5816 and ²⁰⁸Pb/²⁰⁴Pb =39.1233. Overall, the isotopic composition of the SWIR peridotites is variable, some of which may reflect underlying mantle compositional heterogeneity unrelated to the recent melt extraction events. Through detailed isotopic analysis on multiple samples from single dredges with well-defined characteristics, we attempt to distinguish between source and process in the SWIR abyssal peridotites.
DE: 1025 Composition of the mantle
DE: 1032 Mid-oceanic ridge processes (3614, 8416)
DE: 1040 Radiogenic isotope geochemistry
DE: 1065 Major and trace element geochemistry
DE: 8178 Tectonics and magmatism
SC: Tectonophysics [T]
MN: Fall Meeting 2005