Geochemistry of Magmatic and Xenocrystic Spinel in the No.30 Kimberlite Pipe (Liaoning Province, North China Craton): Constraints on Diamond Potential

Ren-Zhi Zhu,Guo-Guang Wang,Pei Ni,Jun-Ying Ding

doi:10.3390/min9060382

Ren-Zhi Zhu, Guo-Guang Wang + Show 2 more

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https://doi.org/10.3390/min9060382

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Journal: Minerals	Publication Date: Jun 24, 2019
Citations: 4	License type: CC BY 4.0

Affiliation: Nanjing University, University of Milano-Bicocca

Abstract

There are two genetic types of spinel (magmatic spinel crystallizing directly from kimberlite magma and xenocrystic spinel derived from mantle xenoliths) in the No.30 kimberlite pipe (Liaoning Province, North China Craton). Their geochemistry is investigated to reveal processes of diamond capture and resorption during kimberlite magmatism to constrain the diamond potential. Magmatic spinels are mostly euhedral to subhedral, 20 to 60 µm in size, and have compositional zones: the cores are classified as chromite with high Cr and Mg contents, and the rims are classified as magnetite with low Cr and high ferric Fe. The compositional trends suggest that magmatic spinel and olivine phenocrysts are crystallized contemporaneously during the early stages of kimberlite crystallization. During this period, temperature (T) and oxygen fugacity (fO2) values calculated at an assumed pressure of 1 GPa are in the range of 994–1274 °C and 1.6–2.4 log fO2 units below the nickel-nickel oxide (NNO) buffer, respectively. The high values of fO2 suggest heavy diamond resorption during kimberlite magmatism. Estimated temperatures of xenocrystic spinel range from 828 to 1394 °C, and their distributions indicate that only a small proportion of xenocrystic spinels are derived from the diamond stabilization field, which suggests a low potential of diamond capture. The low diamond capture and heavy diamond resorption during kimberlite magmatism contributed to the low diamond grade of the No.30 kimberlite.

Highlights

Diamond exploration relies on mantle-derived “indicator minerals” because they commonly occur as inclusions within diamonds, and play an important role in diamond formation
We report the trace element concentrations of xenocrystic spinels and discuss the differences between magnesiochromites derived from the diamond stability field and those from the barren levels of the mantle [4]
The xenocrystic and magmatic spinels were very resistant to alteration and survived minerals

Summary

Introduction

Diamond exploration relies on mantle-derived “indicator minerals” because they commonly occur as inclusions within diamonds, and play an important role in diamond formation. The use of chromite in diamond exploration has concentrated on the recognition of grains similar in the major-element composition to chromite inclusions within diamonds [14]. Combined major and trace element compositions for chromite are used to distinguish the source rock of chromite and to discriminate mantle-derived xenocrysts from magmatic spinels [4]. The Zn-in-chromite thermometer (proposed by Ryan et al [15] based on the strong temperature-dependence of the partitioning of Zn between chromite and olivine) further divides chromite xenocrysts from kimberlite into those derived from the diamond stability field and those from shallower, barren levels of Minerals 2019, 9, 382; doi:10.3390/min9060382 www.mdpi.com/journal/minerals. Single-grain temperature estimates are projected onto known geotherm to obtain a depth of origin for each grain [16]

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