HR: 1340h
AN: V33A-1164    [Abstracts]
TI: Heterogeneity in the Oceanic Mantle From Nd Isotopic Composition of the Abyssal Peridotites
AU: * Mallick, S
EM: mallick@magnet.fsu.edu
AF: NHMFl and Dept. of Geological Sciences, 1800 E Paul Dirac Drive, Tallahassee, Fl 32304, United States
AU: Salters, V J
EM: salters@magnet.fsu.edu
AF: NHMFl and Dept. of Geological Sciences, 1800 E Paul Dirac Drive, Tallahassee, Fl 32304, United States
AB: Abyssal peridotites (APs) are believed to be the residue of the mid ocean ridge basalt (MORB) melting event, and thus provide information complementary to the basalts. We present the trace element concentrations, and Sr and Nd isotopic compositions of clinopyroxene (cpx) separates from the APs from three different slow to ultra-slow spreading ridges -Southwest Indian Ridge (SWIR), Mid Cayman Rise (MCR) and Mid Atlantic Ridge Kane Fracture Zone (MARK). These areas were chosen because they are thought to represent relatively low degrees of melting (based on ridge depth and Na₈), which thus would enhance the signature of an enriched (low solidus) components in the melt and thus provide maximum contrast between peridotites and basalts. At SWIR we have analyzed 15 peridotites from three dredges from 9⁰ to 16⁰ oblique segment. Dredge 85 peridotites, recovered from avolcanic segment, are harzburgitic (<5% modal cpx) and are LREE depleted (Ce/Yb)_N=0.001-0.035 and Ce_N=0.003-0.11. Dredge 78 peridotites have up to 5% of modal cpx, and are moderately depleted in REEs (Ce_N = 2-5). In contrast to other two dredges, dredge 96 peridotites are lherzolitic and two samples some show a pyroxenite vein. These APs have approximately 10% modal cpx indicating either a low degree of melting or refertilization. The light REE pattern of the cpx (Ce_N=8) is also less depleted and these are the only cpx whose REE pattern can be in equilibrium with MORB. Nd-isotopic compositions of the cpx from dredge 78 and 96, ¹⁴³Nd/¹⁴⁴Nd=0.513003-0.513046 and 0.512885- 0.512933 respectively, which falls within the Nd-isotopes ratios of the associated basalts. In contrast, peridotites from dredge 85 display a large range in Nd-isotopic composition(0.513086-0.513320), and their Nd isotopic compositions are more radiogenic than the associated ridge basalts. At MCR, peridotites from two dredges are dominated by lherzolite with relatively high modal abundance of cpx (up to 9 modal%). 13 peridotites analyzed from MCR are characterized by strong depletion in light REEs with (Ce/Sm)_N as low as 0.002 and by large range of variation in heavy REEs with Lu_N ~0.10-0.47. ¹⁴³Nd/¹⁴⁴Nd isotope ratios of the cpx range from 0.513111-0.513425 which partially overlaps with the associated basalts but extends to more radiogenic compositions. Peridotites from the MARK area are harzburgitic with depletions in light REEs with Ce_N as low as 0.004 and (Ce/Yb)_N=0.002-0.20. In comparison to SWIR and MCR, the cpx from MARK area are characterized by low abundance of middle to heavy REEs (Dy_N=0.8,Lu_N=1), indicating an overall larger degree of depletion. However, the Nd-isotopic compositions (0.513011-0.513190) of the cpxs are indistinguishable from the associated basalts. These results lead to several constraints on the ridge process: 1. Assuming that the peridotites traveled along similar pressure-temperature paths, the large range of variation in trace element ratios at a single dredge in a limited geographic area can not be explained by the degree of melting alone, requiring significant source variation. 2. The discrepancy between peridotites and basalts are too large to explain by radiogenic ingrowth during ascent (after melt extraction). 3. Although a low solidus component is required to explain the isotopic differences at the SWIR and MCR, the similarity of the Nd-isotopic composition of the basalts and peridotites at the MARK area indicates this low solidus component is not necessarily a ubiquitous feature of the depleted mantle.
DE: 1025 Composition of the mantle
DE: 1038 Mantle processes (3621)
DE: 1213 Earth's interior: dynamics (1507, 7207, 7208, 8115, 8120)
DE: 7208 Mantle (1212, 1213, 8124)
DE: 8124 Earth's interior: composition and state (1212, 7207, 7208, 8105)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2007 Fall Meeting