Richard Hinton

The chemistry of zircon: Variations within and between large crystals from syenite and alkali basalt xenoliths

Single grains of zircon can contain zones indicating several generations of crystal growth, each of which should reflect the chemical and physical conditions occurring at the time of its formation. Trace element analyses have been made of large zircon crystals from rocks of alkaline affinities by ion microprobe. The chondrite-normalised rare earth element (REE) concentrations increase rapidly from La to Lu, as would be expected from the decrease in ionic radius and consequent easier substitution into the Zr site within the zircon lattice. Lanthanum, praseodymium, and neodymium are considerably lower than values observed in bulk analyses of zircon. The partition coefficients for the light rare earth elements (LREEs), between zircon and melt or whole rock, must therefore be significantly lower than those calculated using bulk analyses. Cerium is enriched relative to neighbouring REEs due to the presence of Ce4+. Estimates of partition coefficients of Ce3+ and Ce4+ between zircon and melt demonstrate that although the Ce anomalies are large the Ce4+Ce3+ ratio is very small (less than 3 × 10−3). The size of the Ce anomaly is variable and should be capable of providing information on oxygen fugacity changes.

Annual to sub-annual resolution of multiple trace-element trends in speleothems

This study aims to establish evidence for the widespread existence of preserved high-resolution trace element variations in speleothems that may have climatic significance. Ion microprobe analysis of speleothems reveals that annual to sub-annual variations in element chemistry exist at five, shallow western European cave sites (Crag Cave, County Kerry and Ballynamintra, County Waterford, Ireland; Uamh an Tartair, Sutherland, Scotland; Grotte Pere-Noël, Belgium; Grotta di Ernesto, NE Italy) with widely varying climatic, geomorphic and geological settings. The variations are not restricted to species (Mg, Sr and Ba) known to substitute directly for Ca in the calcite lattice, but include H, F, Na and P. Phosphorus (as phosphate) displays the greatest variability and may have the most significance as a proxy for the seasonal temperature cycle because of its role as a nutrient element. The technique allows estimation of growth rate of speleothems at any interval of interest, which is one of several possible uses in palaeoclimatology.

Do the trace element compositions of detrital zircons require Hadean continental crust

The trace element compositions of Hadean zircons have been used in two ways to argue for the existence of Hadean continental crust. One argument is based on low crystallization temperatures of Hadean zircons that have been determined using a novel geothermometer based on the Ti content of zircons in equilibrium with rutile. The second argument is based on using the trace element abundances in zircons to calculate their parental melt compositions, especially the rare earth elements. Here we demonstrate that zircons that grow from a melt formed by basalt differentiation at modern mid-ocean ridges cannot be unambiguously distinguished from Hadean zircons on either of these grounds. Thus, we conclude that the trace element compositions of Hadean zircons are permissive of models that do not include the generation of continental crust in the Hadean.

Cathodoluminescene at low Fe and Mn concentrations; a SIMS study of zones in natural calcites

ABSTRACT Point analyses of natural calcite cements by SIMS confirm the earlier suggestions that 20 ppm Mn is required to activate luminescence, whereas Fe concentrations in excess of 1400 ppm quench it partly, independently of the Mn content. Below this Fe threshold, luminescence is bright at Mn concentrations in excess of 225 ppm. In the range 20-225 Mn, all types of luminescence--dull, luminescent, and nonluminescent--can occur. This undifferentiated domain is evident also from a compilation of literature data. The type of luminescence is not a simple function of Fe and Mn concentrations; other parameters influence CL. Sinc most diagenetic calcite cements have Fe and Mn concentrations that place them within the field of mixed cathodoluminescence, these results force reassessment of models that equate the type of luminescence with the redox state of the diagenetic environment.

Ion microprobe analysis in geology

For many years it has been possible to measure accurately the small-scale variations in the major and minor element chemistry of minerals by the electron microprobe technique, in which the sample surface is bombarded with an electron beam and the X-ray signal is measured (Chapter 2). Abundances of isotopes, trace and ultra-light elements (e.g. H and Li) which cannot be determined by this method can, however, be measured by secondary ion mass spectrometry (SIMS; Shimizu, Semet and Allegre, 1978; Shimizu and Hart, 1982; Reed, 1989; Zinner, 1989). The ion microprobe is a SIMS instrument with a focused primary ion beam, which permits in situ microanalysis of minerals in samples prepared as standard polished sections. Nearly all elements from H to U can be detected and many can be analysed quantitatively down to part-per-million levels, or below.

Uranium enrichment in metalliferous sediments from the Mid-Atlantic Ridge

Abstract Previous studies of metal-rich sediments at mid-ocean ridge spreading centres have suggested that U/Fe ratios are relatively constant from site to site. A suite of cores from the Mid-Atlantic Ridge has been analysed for U and Fe content and most samples exhibit constant U/Fe ratios. Sulphide-bearing parts of one core, however, contain excess U compared with that observed in other metalliferous sediments but apparently only during oxidative alteration of pyrite phases. Fission track registration has demonstrated that this excess U is intimately associated with sulphide grains in the sediment. Scanning electron microscope examination indicates that this U enrichment is limited to very small areas within the oxidation layer and is associated with an enhanced P concentration. Ion probe analyses allow semi-quantitative analyses of various elements through a U-rich grain. The observed distribution of U coupled with its specific enrichment suggests that microbial mediation might be controlling this enrichment. U/Fe ratios in fully oxidised TAG sediments are constant and approximately an order of magnitude higher than in young plume particulates. This discrepancy is attributed to a second type of post-depositional U enrichment producing an homogeneous distribution of U throughout the sediment.

New SIMS U–Pb zircon ages from the Langavat Belt, South Harris, NW Scotland: implications for the Lewisian terrane model

Secondary ionization mass spectrometry (SIMS) U/Pb dating of zircon has been applied to rocks of the Lewisian Complex of South Harris (Outer Hebrides, NW Scotland), yielding insights into the complex geological evolution of this region and enhancing our understanding of terranes in the Lewisian Gneiss Complex. Results show that tonalite–trondhjemite–granodiorite magmatism occurred in the Tarbert Terrane between c. 2850 and 2830 Ma, equivalent to similar rocks in other parts of the Outer Hebrides. Magmatism was accompanied or closely followed by high-grade metamorphism at c. 2830 Ma, the first reported occurrence of such an event at this time. These rocks were affected by a subsequent high-grade metamorphic event at c. 2730 Ma, which has correlatives both in the Outer Hebrides and in the mainland Lewisian Gruinard Terrane. These data suggest that the ‘terranes’ forming the Outer Hebrides and parts of the mainland Lewisian were once contiguous and were fragmented by post-2730 Ma events. The c. 1700–1660 Ma ages from felsic sheets and pegmatites within the Langavat Shear Zone may record the reassembly of these crustal fragments.

Garnet pyroxenite xenoliths and pyropic megacrysts in Scottish alkali basalts

Synopsis Pyropic garnet occurs within garnet pyroxenite xenoliths and as discrete crystals in Carboniferous–Permian alkali basaltic tuffs and hypabyssal intrusions. Eight localities are now known from the Midland Valley together with another in the NW Highlands. The xenoliths are inferred to have been derived from minor pyroxenitic intercalations in spinel lherzolite within the lithospheric mantle. Although pyrope crystals at Elie Ness are regarded as high-pressure phenocrysts cognate with the basanitic host, it is indeterminable whether discrete garnets at the other localities are phenocrysts or xenocrystal fragments derived from garnet pyroxenites. All, however, whether phenocrystic, xenocrystic or integral components of pyroxenitic xenoliths, are regarded as cognate in the sense of being products of high-pressure crystallization of late Palaeozoic basanite magmas. Kaersutite, phlogopite, hercynitic spinel, ilmenite and pseudomorphs after orthopyroxene occur as minor components in some of the garnet pyroxenite xenoliths. Mineral compositional data (electron- and ion-microprobe) are presented, with comparisons to xenolith-megacryst suites elsewhere. The garnets have strong similarities with each other and with megacryst populations from alkali basalts outside the U.K. It is concluded that the pyroxenites originally crystallized from basaltic magmas as high-pressure cumulates from depths of >50 km and represent some of the deepest samples from the Scottish lithosphere.

Ionic diffusion in quartz studied by transport measurements, SIMS and atomistic simulations

Ionic diffusion in the quartz–β-eucryptite system is studied by DC transport measurements, SIMS and atomistic simulations. Transport data show a large transient increase in ionic current at the α–β phase transition of quartz (the Hedvall effect). The SIMS data indicate two diffusion processes, one involving rapid Li + motion and the other involving penetration of Al and Li atoms into quartz at the phase transition. Atomistic simulations explain why the fine microstructure of twin domain walls in quartz near the transition does not hinder Li + diffusion. (Some figures in this article are in colour only in the electronic version)

The anhydrous amphibole ungarettiite from the Woods mine, New South Wales, australia

We report ungarettiite from the Woods mine, New South Wales, Australia; this is the second known occurrence. Electron-microprobe and ion-probe analyses show it to be very close to end-member composition, NaNa 2 Mn 2+ 2 Mn 3+ 3 Si 8 O 22 O 2 . We record the first direct determination of H 2 O (