Experiments designed to measure the absolute palaeointensity of the geomagnetic field generally do so by comparing the ancient thermoremanent magnetization (TRM) retained by an igneous rock with a new TRM imparted in the laboratory. One problem with this procedure is that the relative magnitudes of the ancient and laboratory TRMs may be influenced, not only by the external field intensities at the time the two coolings took place, but also by the rate at which the coolings themselves occurred. Here, we present new measurements of this 'cooling rate effect' obtained from treatments in the laboratory differing in cooling rate by a factor of ∼200. Synthetic samples containing sized ferrimagnetic grains were used in the experiments. Theoretical considerations and previous experiments have indicated the cooling rate effect to be dependent on domain state. Increases in TRM magnitude of more than 7 per cent per order of magnitude decrease in cooling rate have been reported for assemblages of non-interacting single-domain (SD) grains. Here, we focus on magnetite grains in the less well-studied pseudo-single domain (PSD) and multidomain (MD) states using a range of applied field intensities to impart the TRMs. For the first time, we also measure the cooling rate effect in grains of titanomagnetite that have been oxyexsolved so that they contain strongly interacting SD lamellae. In all cases, the cooling rate effect measured was in the same sense as already observed in ideal magnetically non-interacting SD grains but was considerably weaker. On average, the effect did not exceed ∼3 per cent increase in TRM per order of magnitude decrease in cooling rate and did not show any systematic dependence on applied field intensity. In some samples containing coarser grains, the cooling rate effect was not distinguishable from zero. The sense and magnitude of the cooling rate effect remain uncertain in truly MD grains as different studies have produced discrepant results. For the more practically relevant case of PSD and interacting SD grains, which commonly dominate the TRM in igneous rocks, however, it appears that we can be more confident in our assertions. The cooling rate effect in such materials is in the same sense as in non-interacting SD grains but smaller: a consequence of long-range ordering. In lavas and small intrusions containing these, it is unlikely to exceed 10 per cent. Although a correction should always be attempted, the results of palaeointensity studies based upon such samples will generally not be severely biased.

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The offshore Gippsland Basin, underlies the continental shelf and slope between eastern Victoria and Tasmania.The basin is filled with up to 25,000' of sediment, varying in age from Lower Cretaceous to Recent. The Lower Cretaceous section is represented by at least 10,000' of nonmarine greywackes of the Strzelecki Group. The overlying sediments of Upper Cretaceous to Eocene age comprise the interbedded sandstones, siltstones, shales and coals of the Latrobe Group, with a cumulative thickness of about 15,000'. Offshore, the Latrobe Group is overlain unconformably by up to 1500' of calcareous mudstones of the Lakes Entrance Formation and up to 5000' of Gippsland Limestone carbonates. Pliocene to Recent carbonates, reaching a maximum thickness of about 1000', complete the sedimentary section of the basin.Australia's first commercial offshore field, the Barracouta oil and gas field, was discovered in the Gippsland Basin in February 1965. Further exploratory drilling over the following two and a half years led to the discovery of the Marlin gas field and the Kingfish and Halibut oil fields.The principal hydrocarbon accumulations are reservoired by sediments of the Latrobe Group within closed structural highs on the Latrobe unconformity surface. Seal is provided by the mudstones and marls of the Lakes Entrance Formation and Gippsland Limestone.A field development programme was initiated immediately after Barracouta had been confirmed as a commercial gas reservoir. By the end of 1967, the Barracouta 'A' platform had been erected. Construction and positioning of the Marlin, Halibut and the two Kingfish platforms followed.To date development drilling has been completed on the Barracouta and Halibut fields, while development of the Marlin field has been temporarily suspended following completion of four wells. Development of the Kingfish oil field which commenced in March 1970, is still in a relatively early stage.The Barracouta field has been producing gas and oil since March and October, 1969 respectively. The Marlin gas field was put on stream in November, 1969 and the Halibut oil field in March 1970. As yet no wells drilled in the Kingfish oil field have been completed for production.The four fields provide a major source of hydrocarbons for the Australian market. By the end of September, 1970 cumulative production of sales quality gas from the Barracouta and Marlin fields was almost 23 BCF. Cumulative production of stabilised oil from Barracouta was 2 million barrels and over 26 million barrels from Halibut.

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Did the Kyrenia Range of northern Cyprus rotate with the Troodos-Hatay microplate during the tectonic evolution of the eastern Mediterranean? Morris, A

Understanding Earth's geodynamo provides us a window into the evolution of the Earth's core, which requires accurate data about how its strength varies with time. Classic Thellier‐style paleointensity experiments assume that studied specimens contain only noninteracting single‐domain (SD) magnetic particles. Interacting grains commonly occur in volcanic rocks but are generally assumed to behave like equivalently sized SD grains. Multidomain (MD) grains can cause erroneous PI estimates or cause Thellier‐style experiments to fail entirely. Synthetic specimens containing naturally formed magnetite with MD grains and oxyexsolved titanomagnetite (closely packed SD grains) were subjected to various partial thermoremanent magnetization (pTRM) experiments, which tested nonideal behavior as a function of pTRM acquisition and loss inequality, thermal history, and repeated heating steps. For all grain sizes and domain states, pTRMc (heating and cooling in a nonzero field) gives larger values, compared to pTRMb (heating in a zero field and cooling in a nonzero field), by ∼5.5%. Oxyexsolved grains appear prone to the same concave‐up, nonideal Arai plots commonly observed in MD specimens, which also has potential implications for the multiple‐specimen, domain‐state corrected protocol. Repeated heatings cause additive deviations from ideality with relatively small impacts on Arai plot curvature for both grain types. Experiments with higher initial demagnetization temperatures had lower curvatures, with the most SD‐like behavior occurring in the uppermost 20 °C of the (un)blocking temperature range. Samples containing mixtures of magnetic domain sizes are likely to behave less ideally at lower temperatures but become more ideal with increasing temperature as the nonideal grains unblock.

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