Small-Scale Mantle Heterogeneities Beneath the Endeavour Segment, Juan de Fuca Ridge
Abstract
The Endeavour segment of the Juan de Fuca Ridge samples at least two different geochemical heterogeneities unrelated to adjacent hot spots. One is FOZO-like in isotopes and HFSE-rich. Another is K+LREE-rich. Both are sampled randomly in short spatial (few-km) and temporal succession through separate MORB melting events, but the FOZO-like MORB is restricted to axial magmas that rise along a major fault and by-pass the axial magma chamber. The large geochemical diversity and high density of submersible-collected samples creates opportunity for constraining melting processes and mantle dynamics beneath an intermediate spreading-rate ridge. Basalts from the Endeavour Axial Ridge Volcano (EARV) can be divided into N-type (normal) MORBs (K2O/TiO2 < 0.15), transitional, or T-MORBs (K2O/TiO2 = 0.15 - 0.25), and enriched, or E-MORBs (K2O/TiO2 > 0.25). N-MORBs have highest Zr/Nb and E-MORBs the lowest Zr/Nb. Recently we discovered the most trace element depleted N- MORB yet at Endeavour. This sample originates from the base of the western wall of the axial valley south of the Mothra vent field, located in the southern-most part of the EARV. However, none of the samples are as depleted as those from the sea floor beyond the ridge flanks or from the Heckle Seamounts to the north. Two subgroups of N-MORBs and three subgroups of T-MORBs are defined by consistent major and trace element characteristics. For example, T1 MORBs have the lowest SiO2; the T2 group has the lowest Na2O and higher Fe8 than T3. Trace element differences among the various groups can not be explained by crystal fractionation alone. Pb isotopes do not correlate with K2O/TiO2. T1 MORBs have the highest 206Pb/204Pb ratios and T3 MORBs generally have the lowest 206Pb/204Pb and lowest 143Nd/144Nd ratios. Pb isotopes of bulk rocks and sulfides define an array beneath the Northern Hemispheric Reference Line. The sulfide data (LaBonte et al., 2006; Cook, 1994; Tivey and Delaney, 1985) indicate deposition from basalt-sourced hydrothermal fluids that did not circulate through sediments. The lack of sedimentary influence and evidence for mixing in the data lead us to the interpretation that the EARV basalts represent sampling of a complex interaction of pristine, geochemically distinct mantle reservoirs. Cook, 1994, UC Santa Cruz M.S. Thesis LaBonte et al., 2006, Eos Trans., v. 87 Tivey and Delaney, 1985, Marine Mining, v. 5
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V51G..05H
- Keywords:
-
- 1032 Mid-oceanic ridge processes (3614;
- 8416);
- 1040 Radiogenic isotope geochemistry;
- 1065 Major and trace element geochemistry;
- 3618 Magma chamber processes (1036);
- 8416 Mid-oceanic ridge processes (1032;
- 3614)