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Biotite from granite and gneiss in the southwestern part of the Yilgarn Craton near Perth has been dated by the Rb-Sr technique at about 430-500 Ma in a belt at the western edge of the craton. The belt widens southward from 30 km near Perth to 55 km east of Harvey. The eastern boundary of the belt passes along the east margin of the Saddleback greenstone belt. A transition zone 15-40 km wide separates the belt of young dates from an eastern chronological plateau where biotite dates, mainly 2300-2600 Ma, are marginally younger than regional Rb-Sr whole rock dates which average about 2550 Ma. In contrast to biotite dates, whole rock dates greater than 2500 Ma persist to the western edge of the craton. The 430-500 Ma dates probably represent resetting during uplift in the Early Palaeozoic.

In an endeavour to resolve reported discrepancies in the value of the branching ratio of176Lu at astrophysical energies, a new determineation of the175Lu (nγ)176mLu capture cross section has been measured as 958 ± 58 mb. This gives a value of the branching ratio of 0.21 ±0.05. This result indicates that some reequilibration of the ground and isomeric states of176Lu occurs in stellar environments undergoing s-process nucleosynthesis, and confirms that176Lu is not a reliable cosmochronometer. However the very existence of176Lu in the solar system implies that the ground state of176Lu was not completely depopulated, and provides the possibility of using this nuclide as a sensitive thermometer for stellar processes.

The Coppin Gap copper-molybdenum mineral deposit is situated in the NE of the Pilbara region, near the northern margin of the Mount Edgar Batholith. The prospect is associated with felsic intrusives occupying a position near the top of a suite of tholeiitic basalts and extends through an overlying mixed assemblage of lavas and tuffs. A Rb-Sr isochron age of 3234 ± 117 Ma (with an initial ratio of 0.7027 ± 0.0012), is reported for a suite of feldspar porphyry and granodiorite samples from the mineralized region. This age is consistent with reported Rb-Sr ages from the Mount Edgar Batholith, and suggests a close association between the mineralization at Coppin Gap and the batholith. The reported age represents a minimum age for the mineralization at Coppin Gap, and suggests that this deposit is one of the oldest 'porphyry style' copper-molybdenum deposits in the world.

New Rb-Sr, Sm-Nd and Pb-Pb data are presented from banded gneisses in the Mt Narryer region of the Western Gneiss Terrain, together with Rb-Sr and Sm-Nd data from cross-cutting granite sheets. The Meeberrie Gneiss, which previously gave a Rb-Sr isochron age of 3348 + 43 Ma and an initial ratio of 0.7037 + 0.0005, has given a Pb-Pb age of 3357 ± 70 Ma, whilst an additional suite of the same gneisses collected to the SW of the original samples gives a Rb-Sr age of c. 3100 Ma with an initial ratio of c. 0.705. Deformed sheets of recrystallized granite which intrude the Meeberrie Gneiss give a Rb-Sr age of 2579 + 122 Ma with an initial ratio of 0.7143 ± 0.0092. Sm-Nd analyses on samples of the Meeberrie Gneiss give TCHUR ages of 3710 ±30 Ma and 3620 + 40 Ma, which support the previously determined model age of 3630 ± 40 Ma. An additional TCHUR of 3540 ± 30 Ma for a sheet of Dugel Gneiss within the Meeberrie Gneiss coincides with a previously reported value of 3510 ± 50 Ma for another sample of the Dugel Gneiss. Two cross-cutting granite sheets give TCHUR ages of 3120 ± 30 Ma and 3070 ± 40 Ma. The geochronological data in this paper support and extend the isotopic evidence previously reported from the banded gneisses of the Mt Narryer region, and confirm that the Western Gneiss Terrain is the oldest known major segment of sialic crust in the Yilgarn Block, Western Australia.

Variably foliated, predominantly granodioritic plutonic rocks from the northern part of the Shaw Batholith in the east Pilbara Archaean craton are dated at 3,499±22 Ma (2s errors) by a whole-rock Pb-Pb isochron. These rocks intrude the surrounding greenstone sequence, and their age is indistinguishable from that sequence. High strain grey gneisses which occupy much of the western and southern Shaw Batholith are chemically and isotopically similar to the North Shaw suite and are inferred to have been derived from this suite by tectonic processes. Felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc-alkaline volcanic and plutonic province at ~3,500 Ma. This volcanic and plutonic suite is similar to modern calc-alkaline suites on the basis of major element, rare earh element and most other trace element contents. The Archaean suite contrasts with modern equivalents only in having lower concentrations of HREE and higher concentrations of Ni and Cr.

The anorthositic Windimurra Gabbroid is the largest gabbroid body in the central Yilgarn Block of Western Australia, and is approximately equivalent in size to the Stillwater Complex. It contrasts with most other Yilgarn layered gabbroid bodies in its generally fresh mineralogy, anorthositic affinities, apparent lack of regional or static metamorphic effects, and unusual greenstone setting, wherein mafic and ultramafic volcanics are absent. The contacts with surrounding granitoids and greenstones are tectonic, and it has not been possible to establish direct age relationships.The age of the gabbroids cannot be directly determined using the Rb/Sr whole-rock technique, because of the small range and low Rb/Sr ratios of the gabbroids, but a minimum age of 2669 ± 135 Ma is indicated from a Rb/Sr isochron for porphyry dykes - one of the three dyke sets herein dated that cut the complex; the other dykes give ages of about 2480 Ma. The Windimurra Gabbroid is thus Archaean in age.In contrast, a mineral isochron age of 1496 ±189 Ma, with an initial 87Sr/86Sr ratio of 0.7014 ± 0.0006, is obtained from gabbroid sample 66990. However, from the whole-rock Sr-isotopic data and constraints imposed by the ages of the dykes, an initial ratio range of 0.7007-0.7012 is indicated for the Windimurra Gabbroid. This is consistent with derivation of the gabbroid from an upper mantle source. The young mineral age probably results from isotopic re-equilibration of Rb and Sr in the minerals, caused by a relatively mild thermal event.The old age and low initial 07Sr/86Sr ratio of the Windimurra Gabbroid set it apart from other large stratiform bodies (e.g. Bushveld, Sudbury, Dore Lake, Great Dyke, Stillwater); yet structurally it is dissimilar to the pre-3000 Ma old 'stratigraphic' type anorthositic complexes (e.g. Fiskenaesset).The post-gabbroid crustal history of the Windimurra region is similar to that of the Agnew-Mt Keith region (about 190 km to the east), and a 2574 ± 74 Ma Rb-Sr isotopic age of the granitoids is in agreement with the well-documented period (2700-2550 Ma) of widespread emplacement and/or metamorphism of granitoid rocks elsewhere in the Yilgarn Block.

SummaryA metallic mass brought to the Western Australian Museum from the Wongan Hills district N.W. of Perth has been identified as an iron meteorite of unique type. It has graphite inclusions about I mm across distributed throughout the metal giving a ‘raisin bread’ appearance. Its nickel content (6·65 %) is comparable with that of coarse octahedrites but the kamacite grain structure is anomalous. Its gallium, germanium, and nickel contents place it close to, but outside, Wasson's chemical group IIb. Taenite is absent and troilite is rare. Neumann bands in the kamacite are distorted and the kamacite has flowed around large schreibersite inclusions. The latter have an exceptionally low nickel content (7·0 %) and probably formed at an unusually high temperature. The kamacite contains more phosphorus than normal iron meteorites, and small schreibersite grains in the kamacite are relatively nickel-poor (22 %). The unusual structure of this iron is thought to be due to one or more of the factors high carbon, high phosphorus, and relatively rapid cooling.

Within the Pilbara Block of Western Australia, a complex of migmatite, gneissic and foliated granite near Marble Bar is intruded by a stock of younger massive granite (the Moolyella Granite) with which swarms of tin-bearing pegmatites are associated. The age of the older granite has been determined by the Rb-Sr method as 3,125 ± 366 m.y., and that of the Moolyella Granite as 2,670 ± 95 m.y. Initial Sr87/Sr86 ratios suggest that the older granite is close to primary crustal material, but that the Moolyella Granite consists of reworked material. It probably formed by partial remelting of the older granite.

The Yilgarn Block is an ancient crustal block which extends over ∼650,000km2in the south-west of Western Australia and consists of high-grade gneiss and granite–greenstone terrains1. Being one of the largest segments of Archaean crust in the world, it is important to studies of early crustal evolution. We report here a geochronological study of banded gneisses near Mount Narryer, in the north-west of the Yilgarn Block, which has given a Rb–Sr whole-rock isochron age of 3,348±43 Myr with an initial 87Sr/86Sr ratio of 0.7037±0.0005. The data indicate that the rocks had a prior crustal residence time of ∼200 Myr. The validity of this early Archaean age is supported by model Sm–Nd ages of 3,510 Myr and 3,630 Myr for two of the samples. Taken together, these ages represent the oldest evolutionary sequence so far identified in the Yilgarn Block, and are comparable with old ages from other Archaean cratons elsewhere in the world.

THE discovery that environmental levels of mercury and lead present serious health hazards has led to intensified investigations into the effects of other heavy metals. Cadmium is a particularly toxic contaminant, and as with mercury, widespread environmental exposure to cadmium is related to its increased technological use1. We have developed an isotope dilution technique to measure the concentration of cadmium in aqueous solutions with a sensitivity of 0.003 µg 1−1. The concentration of cadmium in two river systems in Western Australia has been measured to establish an accurate set of low level reference determinations in a region which is relatively free of sources of industrial pollution.