Journal of Conference Abstracts

Geochemical Evidence from Kimberlites and Lamproites for Proterozoic Mantle Heterogeneity beneath Southern India

N. V. Chalapathi Rao¹ (nvcr100@esc.cam.ac.uk), S. A. Gibson¹, D. M. Pyle¹ & A. P. Dickin²

¹ Dept. of Earth Sciences, University of Cambridge, Cambridge, CB2 3EQ, UK.

² Dept. of Geology, McMaster University, Hamilton, Ontario, Canada.

During the Mid-Proterozoic widespread small-volume mafic potassic-ultrapotassic igneous activity occurred across southern India. K-Ar dating of phlogopite separates suggests that this ranks among the Earth's earliest recorded mafic potassic magmatism (Chalapathi Rao et al 1996). Numerous and occasionally diamondiferous kimberlite pipes were emplaced through the Dharwar craton at Anantapur (1090 Ma) and Mahbubnagar (1360 Ma). Further east, lamproite pipes were emplaced into the intracratonic Cuddapah basin and at its NE margin (1380 Ma). All of these kimberlites and lamproites have high incompatible element concentrations (Th 6-22 ppm; Zr 96-627 ppm; Nb 86-240 ppm) and are strongly enriched in light REE relatively to heavy REE (La/Yb=74-177). Kimberlites have initial isotopic ratios (⁸⁷Sr/⁸⁶Sr=0.7027-0.7046; epsilon_Nd +0.22 to +4.12) similar to or greater than bulk-Earth. In contrast, lamproites have much higher initial ⁸⁷Sr/⁸⁶Sr (0.7052-0.7218) and lower epsilon_Nd (-6.4 to -7.7).

The incompatible trace element and combined isotopic ratios of the Anantapur and Mahbubnagar kimberlites are similar to Group I South African kimberlites. We suggest that the predominant contributing melt source for these magmas was the convecting mantle. The isotopic compositions of lamproites are similar to lamproites from West Kimberley, Western Australia, and suggest that their contributing melts were predominantly derived from ancient metasomatically-enriched sources in the subcontinental mantle (SCLM). Calculated T_DM Nd model ages suggest that the SCLM beneath the Cuddapah basin and at its NE margin was enriched at ~2 Ga.

The presence of contrasting melt sources contributing to the genesis of Proterozoic kimberlites and lamproites has important implications for the early evolution of SCLM beneath southern India.

Chalapathi Rao, N.V. et al., Precamb. Res. 79, 363-369, (1996).

Volume Behaviour of Lawsonite at Combined High Pressure and Temperature: Studies Using an On-Line Multi-Anvil Cell

Nicholas Chinnery¹ (Chinnery@fs2.man.ac.uk), Alison R. Pawley¹ (Alison.Pawley@man.ac.uk) & Simon Clark² (S.M.Clark@dl.ac.uk)

¹ Department of Earth Sciences, Manchester University, Oxford Road, Manchester M13 9PL.

² The Daresbury Laboratory.

Combined expansivity and compressibility data were determined for lawsonite, CaAl₂Si₂O₇(OH)₂.H₂O, up to 80 kbar and 900^oC using energy dispersive powder diffraction in a Walker type multi-anvil cell (Walker, 1990) on station 16.4 at the Daresbury Laboratory. This apparatus has recently been commissioned and so the work documented here not only provides new information on lawsonite but also shows the suitability of on-line multi-anvil apparatus for this type of measurement. Large-volume apparatus has the advantage, over devices such as the diamond-anvil type, of being capable of simultaneously sustaining both high pressures and temperatures.

The experimental results for the isothermal compressibility can be summarised by the bulk modulus K₂₉₈ (using the Murnaghan equation of state and assuming K' = 4): K₂₉₈ = 1271 ± 43 kbar. This result is in agreement with previous work by Comodi and Zanazzi (1996) carried out with a single crystal in a diamond-anvil cell but not that of Holland et al (1996) working with powdered synthetic sample, also in a diamond-anvil cell. It is not possible to obtain ambient pressure thermal expansivity data in the multi-anvil cell but the results at pressure are compatible with the expansivities previously measured (Comodi and Zanazzi, 1996, Pawley et al., 1996). The affect of temperature on the bulk modulus has been assessed by the measurement of lattice parameters at around seventy different combined pressures and temperatures.