Paragenesis and Oxygen Isotopic Studies of Serpentine in Kimberlite

Abstract

Although serpentine is a ubiquitous mineral in kimberlites, its origin remains controversial. Some petrologists claim that all serpentine is secondary and produced by the introduction of externally derived fluids, whereas others suggest that serpentine is a primary mineral formed mainly from late-stage hydrothermal deuteric fluids. To date, the only investigations of the isotopic composition of serpentine in kimberlite have been by bulk analysis of samples that have undergone subaerial alteration, and hence undoubted interaction with meteoric water. Serpentine and chlorite in kimberlites occur as follows: (1) pseudomorphic retrograde lizardite and chlorite after primary olivine; (2) late-stage primary serpophitic serpentine; (3) prograde serpophitic serpentine replacing retrograde lizardite; and (4) serpentine–chlorite replacing magmaclast olivine and interclast phlogopite in diatreme zone rocks. Serpophitic or polygonal serpentine is a primary hydrothermal low temperature phase that common forms monomineralic segregations in the groundmass of hypabyssal kimberlites. In this study, the oxygen isotopic composition of all varieties of serpentine/chlorite was determined by ion microprobe in hypabyssal and diatreme zone kimberlites from Southern Africa, Canada and the USA. Data were acquired using the Edinburgh Cameca IMS 1270 ion microprobe and an antigorite (δ¹⁸O = 8.8 ‰) standard. In Iron Mountain hypabyssal kimberlites, the δ¹⁸O of pseudomorphic marginal and vein serpentine is similar and ranges between 4.3 and 6.3 ‰. For compositionally zoned pseudomorphic marginal serpentines, δ¹⁸O decreases from core-to-rim (cores 6.4 to 3.8 ‰ δ¹⁸O; rims 2.7 to 0.6 ‰ δ¹⁸O). Prograde pseudomorphic serpophite in the Ham West kimberlite ranges in δ¹⁸O from 4.1 to −0.5 ‰, with segregationary primary serpophite δ¹⁸O ranging from 1.6 to −1.8 ‰. In diatreme zone rocks at Letseng-la-terae, the earliest pseudomorphic serpentines range in δ¹⁸O from 5.0 to 3.5 ‰, later-forming rim and marginal serpentines from 4.3 to −1.8 ‰, with interclast serpophite from 3.4 to 1.5 ‰. At Kao, the earliest pseudomorphic serpentines are relatively uniform in composition and range in δ¹⁸O from 4.9 to 4.5 ‰, later rim and marginal serpentines from 4.9 to 4.8 ‰ and interclast serpophite from 6.3 to 3.8 ‰.These data show that the kimberlites investigated have not interacted with large volumes of light meteoric waters. Similar oxygen isotopic compositions in hypabyssal and diatreme zones rocks from different localities certainly preclude the introduction of meteoritic water as a cause of serpentinization. The trends from mantle oxygen δ¹⁸O to lighter oxygen are a result of either fluid compositional evolution or very minor introduction of light water. Assuming that parental olivine has a primary δ¹⁸O of about +6 ‰ and that this is decreased during serpentinization by ¹⁶O enrichment by reactions with isotopically lighter water (−10 to −20 ‰), at 200–300 °C, in closed or open systems, then “water/serpentine” ratios cannot exceed 0.8. These data are interpreted to suggest that there was not an influx of significant volumes of low-temperature meteoric water as a cause of serpentinization. This conclusion is supported by the absence of extremely light oxygen (i.e. δ¹⁸O < −5 ‰) in all of the serpentines investigated.