W. Compston

Mixture modeling of multi-component data sets with application to ion-probe zircon ages

A method is presented for detecting multiple components in a population of analytical observations for zircon and other ages. The procedure uses an approach known as mixture modeling, in order to estimate the most likely ages, proportions and number of distinct components in a given data set. Particular attention is paid to estimating errors in the estimated ages and proportions. At each stage of the procedure several alternative numerical approaches are suggested, each having their own advantages in terms of efficiency and accuracy. The methodology is tested on synthetic data sets simulating two or more mixed populations of zircon ages. In this case true ages and proportions of each population are known and compare well with the results of the new procedure. Two examples are presented of its use with sets of SHRIMP 238U206Pb zircon ages from Palaeozoic rocks. A published data set for altered zircons from bentonite at Meishucun, South China, previously treated as a single-component population after screening for gross alteration effects, can be resolved into two components by the new procedure and their ages, proportions and standard errors estimated. The older component, at 530 ± 5 Ma (2σ), is our best current estimate for the age of the bentonite. Mixture modeling of a data set for unaltered zircons from a tonalite elsewhere defines the magmatic 238U206Pb age at high precision (2σ ± 1.5 Ma), but one-quarter of the 41 analyses detect hidden and significantly older cores.

The Earth's oldest known crust: A geochronological and geochemical study of 3900–4200 Ma old detrital zircons from Mt. Narryer and Jack Hills, Western Australia

Detrital zircons with 207Pb/206Pb ages between 3908 and 4270 Ma from the Narryer Gneiss Complex, Western Australia, are the oldest terrestrial minerals found to date. They occur in small proportions (2–3%) together with 3.0–3.75 Ga zircons in quartzites and metaconglomerates of the 3.0 Ga Mt. Narryer (MN) and Jack Hills (JH) metasedimentary belts. We report the results of a geochronological and geochemical study of these zircons which, together with data on grain morphology and inclusion mineralogy, are used in an attempt to place constraints on their source rocks. Pre-3.9 Ga zircons from both localities show variable but typically low degrees of rounding and are optically homogenous to faintly growth-zoned. Among the few grains carrying visible inclusions, two stand out: one has a 40μm K-feldspar-granite inclusion; the other carries apatite, quartz, plagioclase (?), and monazite impurities. The zircons are concordant to near-concordant (some reverse discordancy is observed for MN grains) and have similar U abundances (100–650 ppm, MN; 60–413 ppm, JH) and Th/U ratios (0.3–1.1). Rare exceptions include low-Th/U (presumably metamorphic) overgrowths on two of the MN grains and high-Th/U discordant sites on some JH grains. At least three distinct age groups have been detected, ≥4.3, 4.2, and 4.15 Ga old, respectively, and low-Th/U rims on some of the grains were formed at ca. 3.93 Ga. Zr/Hf ratios (30–57) and low Sc contents (Sc measured for JH only) are consistent with granitic parent rocks. Rare-earth element (REE) patterns measured by ion probe show some variation (up to 15×) but are generally strongly fractionated (high Lu/La), with low La (<100 ppb) in a majority of cases, and pronounced negative Eu and positive Ce anomalies. The latter appear to be common in low-LREE (light rare-earth element) zircons (but have rarely been detected in previous studies) and most likely reflect minute amounts of Ce4+ in zircon parent melts. The REE patterns closely resemble those in dioritic-granitic zircons measured by ion probe, although the latter also contain grains with 103× higher LREE contents. Collectively, these morphological, mineralogical, and geochemical characteristics suggest a composite granitoid source for these zircons. This is supported by the striking similarities of zircons from both the pre-3.9 Ga and the post-3.75 Ga age groups in a given sample; based on whole-rock geochemical evidence, a continental provenance dominated by potassic granites has been inferred for the latter. The mixedsource lithologies implied by our data indicate the presence of at least three distinct pre-4 Ga age groups, and the occurrence of ca. 3.9 Ga metamorphic overgrowths indicates a differentiated continental source of substantial thickness rather than a provenance from felsic differentiates within dominantly mafic, oceanic-type crust.

Zircon ion microprobe studies bearing on the age and evolution of the Witwatersrand triad

Abstract The Ventersdorp and Witwatersrand Supergroups and the Dominion Group (collectively referred to as the Witwatersrand triad) represent some of the oldest and economically most important volcano-sedimentary basins within the Kaapvaal Craton of southern Africa. Precise age constraints which are vital to models aimed at understanding the formation, mineralisation and evolution of these basins have been hampered by analytical and geological problems. These problems have been overcome by this ion microprobe study on zircons from volcanic rocks within the three basins. Felsic and mafic volcanics from the Ventersdorp Supergroup yield ages of 2709 ± 4 Ma and 2714 ± 8 Ma, respectively constraining the timing of the closure of the Witwatersrand basin. A maximum age for the Witwatersrand basin is indicated by the age of 3074 ± 6 Ma for volcanics from the underlying Dominion Group. Thus a period of c. 350 Ma was available for the accumulation of the full Witwatersrand basin succession. Unfortunately, dating within the Witwatersrand basin itself is not complete and future work could show that extended time breaks may exist within the basin. These new data show that previous age determinations on the Ventersdorp, Witwatersrand and Dominion basins were inaccurate, and that the economically important Witwatersrand basin must now be recognised as Archaean.

87Sr/86Sr composition of seawater during the Phanerozoic

Abstract 87 Sr 86 Sr measurements of 108 sedimentary carbonate rocks have been used to trace variations in the strontium isotopic composition of seawater during the Phanerozoic. The lowest 87Sr/86Sr observed for any suite of carbonates is taken as the best approximation to the value in well-mixed contemporary seawater. Our data support the existence of low 87 Sr 86 Sr in the Cretaceous and Late Jurassic but they do not support further structure beyond a general trend through the Phanerozoic, which may correlate with the continental denudation rate.

DEFORMATIONAL MASS TRANSPORT AND INVASIVE PROCESSES IN SOIL EVOLUTION

Soils are differentiated vertically by coupled chemical, mechanical, and biological transport processes. Soil properties vary with depth, depending on the subsurface stresses, the extent of mixing, and the balance between mass removal in solution or suspension and mass accumulation near the surface. Channels left by decayed roots and burrowing animals allow organic and inorganic detritus and precipitates to move through the soil from above. Accumulation occurs at depths where small pores restrict further passage. Consecutive phases of translocation and root growth stir the soil; these processes constitute an invasive dilatational process that leads to positive cumulative strains. In contrast, below the depth of root penetration and mass additions, mineral dissolution by descending organic acids leads to internal collapse under overburden load. This softened and condensed precursor horizon is transformed into soil by biological activity, which stirs and expands the evolving residuum by invasion by roots and macropore networks that allows mixing of materials from above.

A search for ancient detrital zircons in Zimbabwean sediments

86 detrital zircon grains from Archaean quartzites from the Mweza and Shurugwi greenstone belts have been dated by the ion probe SHRIMP. Nine of these give ages of 3.75–3.80 Ga, older than any Zimbabwean rocks so far satisfactorily dated, and the Tokwe-Zvishavane gneisses are a possible source for these old grains. However, none are comparable in age with the 4.2 Ga zircons discovered in Western Australia. If such old grains exist in the sample it is unlikely (P>0.05) that their abundance could exceed 3%. Ages of the younger grains are broadly consistent with the presumed age of the quartzites sampled, though some high Th/U grains in the Shurugwi sample may be anomalously young.

Age and tectonic setting of Late Archean greenstone-gneiss terrain in Henan Province, China, as revealed by single-grain zircon dating

The authors report precise U-Pb zircon ages for single grains of a metarhyodacite from the Late Archean Dengfeng greenstone belt in Henan Province, China, near the southern margin of the North China craton. Most grains belong to an igneous population whose U-Pb isotopic systematics define a straight line intersecting concordia at 2512 +/- 12 Ma, and this is interpreted as the time of crystallization of the original greenstone volcanics. Several grains are distinctly older, between 2576 +/- 9 and 2945 +/- 44 Ma, and the authors interpret the older grains as xenocrysts of pre-greenstone continental crust that provide evidence for crustal derivation or crustal contamination of the original rhyodacitic lava. The xenocrysts suggest evolution of the Dengfeng greenstone belt in a continental environment that may be represented by the Taihua high-grade gneisses bordering the Dengfeng greenstones and for which the authors obtained ages of 2806 +/- 7 and 2841 +/- 6 Ma. The data add evidence to the now widely held concept that most Archean greenstones developed on or near older continental crust and were therefore prone to crustal contamination. In such cases, conventional zircon dating may not always record the precise age of rock formation.

3.96 Ga gneisses from the Slave province, Northwest Territories, Canada

Ion microprobe U-Pb analyses of zircons have identified the Acasta gneisses, from the westernmost Slave province, Canada, as the oldest known intact terrestrial rocks. Zircons from two samples indicate that the tonalitic to granitic protoliths of the gneisses crystallized at 3962 ±3 Ma, confirming earlier indications, from conventional zircon and Nd analyses, of the rock9s antiquity. The U-Pb analyses indicate that in addition to recent Pb loss, the zircons underwent an early episode of Pb loss and that new zircons crystallized ca. 3.6 Ga. The gneisses were derived from a source that had a long-lived enrichment in light REE, possibly from even older rocks that may be present in the Slave province.

Four zircon ages from one rock: the history of a 3930 Ma-old granulite from Mount Sones, Enderby Land, Antarctica

Ion microprobe U-Th-Pb analyses of zircons from a granulite-grade orthogneiss from Mount Sones, Enderby Land, Antarctica, record the ages of four principal events in the history of the gneiss, three of which already have been recognized through previous isotopic dating of other samples. The structure of the zircons indicates at least four different stages of growth. The several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed “stratigraphic succession” of growth and thereby defining separate U-Pb discordance patterns for each stage. The stratigraphically oldest zircon (rare discrete cores) is indistinguishable in age from the most common, euhedrally zoned zircon. Both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 3927±10 Ma ago, making the orthogneiss currently the oldest known terrestrial rock. The outer parts of most grains and some whole grains recrystallized at 2948±31/−17 Ma, during or immediately after possibly ∼100 Ma of high granulite grade metamorphism. The recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the Napier Complex at ∼1000 Ma. In the intervening time, at 2479±23 Ma, the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic Pb which left most of the crystals with radiogenic Pb deficiencies, but produced local radiogenic Pb excesses in others. A new generation of zircon, characterized by very high Th/U and low U, grew at that time. That event — deformation and possibly a minor rise in temperature — produced widespread perturbations of other isotopic systems throughout the Napier Complex.

The stratigraphy of the 3.5-3.2 Ga Barberton Greenstone Belt revisited: A single zircon ion microprobe study

Abstract Recent field and geochemical studies indicate a need to test the stratigraphy of the ca. 3.5 Ga Barberton Greenstone Belt as it is presently adopted [1,2]. This work uses the ion microprobe SHRIMP, to attempt such a test. Results show that: (1) Volcaniclastic sediments of the Theespruit Formation (