Petrochemistry of Kimberlites, Lamproites and Diamonds in India

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HaggertyEAR-0106187Diamonds were first discovered in India some four millinnia ago. Many of the world's most famous diamonds (Great Mogul, Orloff, Shah, Darya-i-Noor, Regent, Nizam), as well as many infamous stones (e.g. Hope, Koh-i-Noor), were recovered from alluvial deposits on the banks of the Krishna River in the southern state of Andra Pradesh. The primary sources of these and many other extraordinary diamonds, in terms of size and clarity, have not been located. In common with the geological setting of diamond deposits worldwide, diamonds in India are also on cratons and in mobile belts. The two prominent diamond districts are Majhgawan (known since the 13th C), which is an operating mine in the Bundelkand Craton (Madhya Pradesh), and Wajrakarur in the Dharwar Craton of Andra Pradesh. The ages of intrusions in both districts are Proterozoic, and at about 1.1 Ga are similar in age to the Premier kimberlite (source of the Cullinan, the largest diamond diamond ever reported) in South Africa, and the Argyle lamproite (the world's largest producer, over 40 M cts/yr) in Australia. These two sites, along with Proterozoic alluvial deposits in central Africa and Brazil, lie along major lineaments in the reconstructed supercontinent of Rodinia. This is interpreted to be plume related. With over 50 new intrusives discovered in India over the past decade, these ancient cratons are an important new laboratory to study the state of the mantle, and its evolutionary relation the crust (TTG). The diamond host rocks in India are neither classical kimberlites nor conventional lamproites and are variously described as having both kimberlitic (mineralogically), and lamproitic (geochemically) affinities. This is a significant new challenge in mantle petrochemistry. Many intrusions have abundant mantle xenoliths, some are intensely eclogitic, and others are dominantly peridotitic, and both will provide new insights to compositions and thermal states of subcratonic lithospheric keels. India is well mapped geophysically and the mantle data base will provide an independent set of constraints to model the deep mantle. Mantle samples from 32 intrusions will be studied microscopically and analysed using electron microbeam and x-ray fluoresence techniques to obtain mineral and bulk rock compositions, respectively. Diamond morphologies will be examined by stereo- and scanning electron microscopy, and by Raman and infrared spectroscopy. These data will provide the first comprehensive study of Proterozoic mantle intrusives in India. Significant new insights are predicted and important new interpretations are anticipated, not only in India but in the broader context of diamond deposits in Rodinia.