High precision Pb isotopes in Indian Ocean MORB
Abstract
Several studies have shown that MORB from the Indian Ocean have different isotopic characteristics compared to those from the Pacific or Atlantic Oceans. A particular Pb isotope feature of Indian Ocean MORB is their high 208Pb/204Pb ratio compared to those from the Pacific or North Atlantic. These isotope differences have been interpreted as reflecting the presence of an old recycled component or lithospheric delamination in the Indian MORB reservoir. We present Pb triple spike data on MORB from three Indian spreading centers - South West Indian Ridge (SWIR), Central Indian Ridge (CIR) and South East Indian Ridge (SEIR) - and from the Rodriguez Triple Junction (RTJ). All samples have been previously analysed for major and trace elements and Sr, Nd and Pb isotopes. Our purpose is to use high precision Pb data 1) to investigate the Pb isotope relations between the three spreading centers and 2) to find whether differences in 207Pb/204Pb can also be resolved between Indian, Pacific and Atlantic MORB. So far, Pb isotope measurements have been obtained on CIR and SWIR, and those on SEIR and RTJ are on going. Samples from Atlantis FZ II (SWIR) have lower Pb isotope ratios (206Pb/204Pb= 17.47- 17.94, 207Pb/204Pb= 15.41-15.47, 208Pb/204Pb= 37.17-37.65) than CIR-MORB (206Pb/204Pb= 18.30-18.70, 207Pb/204Pb= 15.50-15.57, 208Pb/204Pb= 38.18-38.73). In both Pb isotope spaces, CIR and Atlantis FZ II MORB define two different linear arrays with distinct slopes. The 207Pb/204Pb-206Pb/204Pb slopes correspond to model ages of 1.97 (CIR) and 2.15 Ga (Atlantis FZ II), and values of κ =5.14 and 3.62, respectively. Our new Pb isotope data allow us to distinguish two Pb isotope domains, CIR and SWIR, while previous data show large scatter and overlap. Furthermore, the influence of the Réunion plume is reflected in the gradual increase of Pb isotope ratios along the CIR with proximity to the hotspot. Comparison of Indian MORB with those from East Pacific Rise (EPR) and South Atlantic [1] show that Pb isotope ratios increase in the order: EPR< South Atlantic ~ CIR < SWIR for 207Pb/204Pb and EPR < South Atlantic < SWIR < CIR for 208Pb/204Pb. 1. Galer, S.J.G., et al., Eos Trans. AGU, Fall Meet. Suppl., Abstract., 2001. 82: p. F1403.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.V61D..02N
- Keywords:
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- 1000 GEOCHEMISTRY (New field;
- replaces Rock Chemistry);
- 1040 Isotopic composition/chemistry