The Hf isotope composition of cratonic mantle: LAM-MC-ICPMS analysis of zircon megacrysts in kimberlites

There is a growing need for new zircon reference materials for in situ Hf-isotope analysis by laser ablation-multicollector inductively coupled plasma-mass spectrometry (LA-MC-ICP-MS). In this contribution we document the results of a preliminary investigation of seven natural zircons, conducted in order to test their suitability in this regard. Solution MC-ICP-MS data on separated Lu and Hf fractions provided reference compositional data while the results of ca. 750 in situ LA-MC-ICP-MS analyses allowed assessment of potential micrometre-scale heterogeneity. On the basis of these analyses and additional relevant considerations such as availability, size and (Lu)Yb/Hf ratio, we suggest that, of the currently available zircons, Temora-2 and Mud Tank are most likely to provide robust reference materials for Hf isotope determinations both at the present time and into the future. The former has the advantage of also being well-characterised for U-Th-Pb systematics and suitable for in situ age determination, while the latter is the most readily available and is of very large grain size. Additional materials such as BR266, and 91500, although limited in supply, show more consistent Lu/Hf ratios and are thus of use in monitoring elemental fractionation during ICP-MS analysis.

A Preliminary Appraisal of Seven Natural Zircon Reference Materials for In Situ Hf Isotope Determination

Fragments of the Earth’s mantle are frequently transported to the surface via volcanic rocks that are dominantly alkaline in nature. These fragments range up to sizes in excess of 1 m across. The term “mantle xenoliths” or “mantle nodules” is applied to all rock and mineral inclusions of presumed mantle derivation that are found within host rocks of volcanic origin. The purpose of this contribution is to review the geochemistry of mantle xenoliths.

Mantle Samples Included in Volcanic Rocks: Xenoliths and Diamonds

As a significant carrier of uranium, thorium and the rare earth elements (REEs) at ppm level, and hafnium at the percent level, zircon hosts a remarkable number of long-lived radioactive isotopes and their stable decay products. These include 238U, 235U and 232Th which decay via intermediate steps to 206Pb, 207Pb and 208Pb, respectively, 176Lu which decays to 176Hf, 147Sm which decays to 143Nd, and 138La which undergoes branched decay to 138Ce and 138Ba. This combination makes zircon one of the most versatile minerals available to the geochronologist. Whilst the majority of rock-dating studies involving zircon have utilized the U–Pb and Th–Pb decay schemes, as discussed in previous chapters, an increasing number of investigations are targeting the complementary age information available in the lesser known decay schemes. This chapter reviews the history of application of the Lu–Hf and Sm–Nd isotope systems in zircon and, in its conclusions, speculates on potential future use of the La–Ce system.

176Lu is an unstable radionuclide that undergoes spontaneous β− decay to stable 176Hf, with a half-life of approximately 35 billion years. Variations in the abundance of 176Hf are conventionally expressed with respect to 177Hf whose natural abundance is constant. Thus, the basic age equation for the Lu–Hf dating method, as applied to any closed system, is as follows:

\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \[(^{176}Hf/^{177}Hf)\_{t}\ =\ (^{176}Hf/^{177}Hf)\_{initial}\ +\ (^{176}Lu/^{177}Hf)_{t}\ {\cdot}\ (e^{{\lambda}t}\ {-}\ 1)\] \end{document} 

where t is the elapsed time, and λ is the 176Lu β− decay constant. The exact value of the decay constant λ is currently under review. Most geological studies have either used the value 1.94 × 10−11 y−1, originally calculated from the slope of a Lu–Hf isochron for the 4.56 Ga eucrite achondrite meteorites (Patchett and Tatsumoto 1980a, Tatsumoto et al. 1981), or …

Lu–Hf and Sm–Nd isotope systems in zircon

Bulk-rock geochemical compositions of hypabyssal kimberlites, emplaced through the Archaean Kaapvaal craton and Proterozoic Namaqua–Natal belt, are used to estimate close-to-primary magma compositions of Group I kimberlites (Mg-number 1⁄4 0 82–0 87; 22–28 wt % MgO; 21–30 wt % SiO2; 10–17 wt % CaO; 0 2–1 7 wt % K2O) and Group II kimberlites (Mgnumber 1⁄4 0 86–0 89; 23–29 wt % MgO; 28–36 wt % SiO2; 8–13 wt % CaO; 1 6–4 6 wt % K2O). Group I kimberlites are distinguished from Group II by their lower Ba/Nb ( 22) ratios. The distinct rare earth element patterns of the two types of kimberlites indicate a more highly metasomatized source for Group II kimberlites, with more residual clinopyroxene and less residual garnet. The similarity of Sr and Nd isotope ratios and diagnostic trace element ratios (Ce/Pb, Nb/U, La/Nb, Ba/Nb, Th/Nb) of Group I kimberlites to ocean island basalts (OIB), but more refractory Mg-numbers and Ni contents, are consistent with derivation of Group I kimberlites from subcontinental lithospheric mantle (SCLM) that has been enriched by OIB-like melts or fluids. Source enrichment ages and plate reconstructions support a direct association of these melts or fluids with Mesozoic upwelling beneath southern Africa of a mantle plume(s), at present located beneath the southern South Atlantic Ocean. In contrast, the geochemical characteristics of both onand off-craton Group II kimberlites show strong similarity to calc-alkaline magmas, particularly in their Nb and Ta depletion and Pb enrichment. It is suggested that Group II kimberlites are derived from both Archaean and Proterozoic lithospheric mantle source regions metasomatized by melts or fluids associated with ancient subduction events, unrelated to mantle plume upwelling. The upwelling of mantle plumes beneath southern Africa during the Mesozoic, at the time of Gondwana break-up, may have acted as a heat source for partial melting of the SCLM and the generation of both Group I and Group II kimberlite magmas.

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Geochemistry of South African On- and Off-craton, Group I and Group II Kimberlites: Petrogenesis and Source Region Evolution

Kimberlites from Southern Africa, along with their low-Cr megacrysts, have unusual Hf–Nd isotopic characteristics. Group I and Transitional kimberlites define arrays trending oblique to, and well below, the Nd–Hf isotope ‘mantle array’, defined by oceanic basalts, i.e. they have negative Hf values. Group I kimberlites have Hf values varying from –1.2 to –10.1. Low-Cr megacryst suites from individual Group I kimberlites have compositions that overlap those of their host kimberlites. The trend for all Group I kimberlite megacrysts (Hf values –1.0 to –9.0) shows a striking correspondence to that of the Group I kimberlite field. Group II kimberlites and their low-Cr megacrysts plot on or close to the mantle Nd–Hf array (Hf values 3.6 to –2.6). The data indicate a genetic link between kimberlites and the low-Cr megacryst suite. The negative Hf characteristics of Group I kimberlites and their megacrysts require a source component that is ancient (>1 Ga), and has evolved with low time-integrated Lu/Hf relative to Sm/Nd. Our preferred option is that this component originates beneath the lithosphere, from a reservoir of ancient, deeply subducted oceanic basalt that became incorporated into the convecting mantle source region for Group I and Transitional kimberlites.

/pdf/hf-isotope-systematics-of-kimberlites-and-their-megacrysts-wouercho7s.pdf

Hf Isotope Systematics of Kimberlites and their Megacrysts: New Constraints on their Source Regions

Origins of Kimberlites: A Mf isotope perspective

97 Improving outcomes for palliative lung cancer patients: experience of a multiprofessional parallel clinic model in a district general hospital

/pdf/the-source-regions-components-of-kimberlites-constraints-36e2qrsqns.pdf

C. B. Smith

Papers

Hf Isotope Systematics of Kimberlites and their Megacrysts: New Constraints on their Source Regions

Origins of Kimberlites: A Mf isotope perspective

97 Improving outcomes for palliative lung cancer patients: experience of a multiprofessional parallel clinic model in a district general hospital

The source regions/components of kimberlites: constraints from Hf-Nd isotope systematics