Styles of Alteration of Ti Oxides of the Kimberlite Groundmass: Implications on the Petrogenesis and Classification of Kimberlites and Similar Rocks

Jingyao Xu,Joan Melgarejo,Montgarri Castillo-Oliver

doi:10.3390/min8020051

Abstract

The sequence of replacement in groundmass perovskite and spinel from SK-1 and SK-2 kimberlites (Eastern Dharwar craton, India) has been established. Two types of perovskite occur in the studied Indian kimberlites. Type 1 perovskite is found in the groundmass, crystallized directly from the kimberlite magma, it is light rare-earth elements (LREE)-rich and Fe-poor and its ΔNNO calculated value is from −3.82 to −0.73. The second generation of perovskite (type 2 perovskite) is found replacing groundmass atoll spinel, it was formed from hydrothermal fluids, it is LREE-free and Fe-rich and has very high ΔNNO value (from 1.03 to 10.52). Type 1 groundmass perovskite may be either replaced by anatase or kassite along with aeschynite-(Ce). These differences in the alteration are related to different f(CO2) and f(H2O) conditions. Furthermore, primary perovskite may be strongly altered to secondary minerals, resulting in redistribution of rare-earth elements (REE) and, potentially, U, Pb and Th. Therefore, accurate petrographic and chemical analyses are necessary in order to demonstrate that perovskite is magmatic before proceeding to sort geochronological data by using perovskite. Ti-rich hydrogarnets (12.9 wt %–26.3 wt % TiO2) were produced during spinel replacement by late hydrothermal processes. Therefore, attention must be paid to the position of Ca-Ti-garnets in the mineral sequence and their water content before using them to classify the rock based on their occurrence.

Highlights

Ti-rich minerals from kimberlite such as ilmenite, spinels, rutile and perovskite are important carriers of petrogenetic information
We describe the different alteration styles of groundmass Ti-rich oxide from SK-1 and SK-2 kimberlites (Eastern Dharwar craton, India), including the neoformation of pristine perovskite and Ti-rich hydrogarnets by subsolidus processes
Pristine primary perovskite grains in both kimberlites and carbonatites are often used for geochemical investigations and, in particular for U-Pb dating [5,6,10,12,13,16,17,18,19,20,21,41,42]

Summary

Introduction

Ti-rich minerals from kimberlite such as ilmenite, spinels, rutile and perovskite are important carriers of petrogenetic information. Xenocrystic Ti-rich oxides in kimberlitic rock, such as Cr-rich rutile, Ti-rich spinel and ilmenite, provide information about the metasomatic processes in the cratonic lithospheric mantle [1,2]. Perovskite [5,6] and rutile [7,8] could be used to determine the kimberlite emplacement age. In many cases these minerals undergo complex alteration processes during the hydrothermal or supergene late stages of the kimberlite crystallization sequence that could disturb the petrogenetic interpretations based on geochemical data. Perovskite is a principal host of LREE in SiO2 -undersaturated ultramafic and alkaline rocks [9]

Methods

Results

Discussion

Conclusion