High Th/U ratios in subcontinental lithospheric mantle: mass spectrometric measurement of Th isotopes in Gaussberg lamproites
Abstract
230Th/ 232Th atom ratios as low as 2.75 × 10 -6 have been measured with an average precision of 0.5% (2σ mean) by thermal ionization mass spectrometry for leucite-bearing lamproites from the Gaussberg volcano, Antarctica. Most lavas have high and variable initial 230Th enrichments (45-60%), but constant Th/U ratios (7.55 ± 0.05). Th and Sr isotope ratios correlate negatively at constant ɛ Nd; higher 87Sr/ 86Sr ratios are associated with less excess 230Th and lower Sr concentrations. These differences are attributed to coupled variations in the degree of source metasomatism and percent melting. Low porosity melts are inferred to have been extracted fast enough that chemical equilibrium was not achieved. Source metasomatism has been multi-stage and increased the Th/U ratio, creating a Th-enriched reservoir in the subcontinental lithosphere that complements the Th-depleted asthenosphere.
- Publication:
-
Earth and Planetary Science Letters
- Pub Date:
- July 1992
- DOI:
- 10.1016/0012-821X(92)90183-V
- Bibcode:
- 1992E&PSL.111..257W