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A series of new Sensitive High-Resolution Ion MicroProbe (SHRIMP) U-Pb ages is presented for Palaeozoic (mainly Devonian and Carboniferous) granites from Tasmania. In virtually all instances the new ages are significantly older than previously determined

Precise isotope dilution–thermal ionisation mass spectrometry (ID–TIMS) documentation is given for two new Palaeozoic zircon standards (TEMORA 2 and R33). These data, in combination with results for previously documented standards (AS3, SL13, QGNG and TEMORA 1), provide the basis for a detailed investigation of inconsistencies in 206Pb/238U ages measured by microprobe. Although these ages are normally consistent between any two standards, their relative age offsets are often different from those established by ID–TIMS. This is true for both sensitive high-resolution ion-microprobe (SHRIMP) and excimer laser ablation–inductively coupled plasma–mass spectrometry (ELA–ICP–MS) dating, although the age offsets are in the opposite sense for the two techniques. Various factors have been investigated for possible correlations with age bias, in an attempt to resolve why the accuracy of the method is worse than the indicated precision. Crystallographic orientation, position on the grain-mount and oxygen isotopic composition are unrelated to the bias. There are, however, striking correlations between the 206Pb/238U age offsets and P, Sm and, most particularly, Nd abundances in the zircons. Although these are not believed to be the primary cause of this apparent matrix effect, they indicate that ionisation of 206Pb/238U is influenced, at least in part, by a combination of trace elements. Nd is sufficiently representative of the controlling trace elements that it provides a quantitative means of correcting for the microprobe age bias. This approach has the potential to reduce age biases associated with different techniques, different instrumentation and different standards within and between laboratories.

Precise isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) documentation is given for two new Palaeozoic zircon standards (TEMORA 2 and R33). These data, in combination with results for previously documented standards (AS3, SL13, QGNG and TEMORA 1), provide the basis for a detailed investigation of inconsistencies in 206Pb/238U ages measured by microprobe. Although these ages are normally consistent between any two standards, their relative age offsets are often different from those established by ID-TIMS. This is true for both sensitive high-resolution ion-microprobe (SHRIMP) and excimer laser ablation-inductively coupled plasma-mass spectrometry (ELA-ICP-MS) dating, although the age offsets are in the opposite sense for the two techniques. Various factors have been investigated for possible correlations with age bias, in an attempt to resolve why the accuracy of the method is worse than the indicated precision. Crystallographic orientation, position on the grain-mount and oxygen isotopic composition are unrelated to the bias. There are, however, striking correlations between the 206Pb/238U age offsets and P, Sm and, most particularly, Nd abundances in the zircons. Although these are not believed to be the primary cause of this apparent matrix effect, they indicate that ionisation of 206Pb/238U is influenced, at least in part, by a combination of trace elements. Nd is sufficiently representative of the controlling trace elements that it provides a quantitative means of correcting for the microprobe age bias. This approach has the potential to reduce age biases associated with different techniques, different instrumentation and different standards within and between laboratories. Crown

We have carried out a magnetic fabric study on deformed, clay-rich rocks from the Carboniferous Texas beds and Coffs Harbour Association in order to test the hypothesis of oroclinal bending of the Texas and Coffs Harbour blocks of the southern New England Orogen, eastern Australia. The magnetic susceptibility is dominated by paramagnetic phyllosilicates, with site-dependent contributions from the ferromagnetic minerals pyrrhotite, magnetite and hematite. Pyrrhotite is ubiquitous in the Texas block and in the central part of the Coffs Harbour block, and may be important as an indicator of the grade of low-temperature metamorphism. The magnetic fabric results, in general, show good agreement with structural observations in the Texas and Coffs Harbour blocks. The magnetic foliation is related to the pervasive cleavage associated with accretionary deformation prior to oroclinal bending. The magnitudes of the anisotropy parameters and the plunge of the magnetic lineation indicate an increase in the intensity of deformation from east of the Coffs Harbour orocline towards the Texas orocline. An imprint from oroclinal deformation is suggested by a significant increase in the anisotropy parameter and by the development of steeply plunging magnetic lineations towards the hinges of the Texas and Coffs Harbour oroclines. The Terrica beds from the Early Permian (Allandale) Terrica inlier in the Texas orocline also show another, pre-tilt induced, tectonic imprint. Remanence data from volcanics in the Alum Rock inlier (293 Ma) and magnetic fabric data from this inlier tentatively constrain the onset of oroclinal bending as prior to extrusion of the Alum Rocks, and the completion of bending as post-Terrica beds deposition and pre-Illawarra Reversal (est. 265 Ma).

The role of the standard is critical to the derivation of reliable U–Pb zircon ages by micro-beam analysis. For maximum reliability, it is critically important that the utilised standard be homogeneous at all scales of analysis. It is equally important that the standard has been precisely and accurately dated by an independent technique. This study reports the emergence of a new zircon standard that meets those criteria, as demonstrated by Sensitive High Resolution Ion MicroProbe (SHRIMP), isotope dilution thermal ionisation mass-spectrometry (IDTIMS) and excimer laser ablation–inductively coupled plasma–mass-spectrometry (ELA–ICP–MS) documentation. The TEMORA 1 zircon standard derives from the Middledale Gabbroic Diorite, a high-level mafic stock within the Palaeozoic Lachlan Orogen of eastern Australia. Its 206Pb/238U IDTIMS age has been determined to be 416.75±0.24 Ma (95% confidence limits), based on measurement errors alone. Spike-calibration uncertainty limits the accuracy to 416.8±1.1 Ma for U–Pb intercomparisons between different laboratories that do not use a common spike.

Derivation of Phanerozoic zircon206Pb/238U ages by SHRIMP depends on calibration against an independently dated standard. The qualities of four different zircon standards (SL13, QGNG, AS3 and TEMORA 1) are assessed herein. Not all of these behave consistently on SHRIMP with respect to their ages as determined by IDTIMS. SL13, the most commonly used standard over the past decade and a half, is the most heterogeneous in Pb/U. In addition, when SL13 is used as the calibration standard, the varied ages resulting from that heterogeneity are generally younger than ages derived from the other three standards. AS3-calibrated ages are the oldest of the group. Only QGNG and TEMORA 1, when calibrated relative to each other, yield ages on SHRIMP that are consistent with their IDTIMS ages. Of these two, TEMORA 1 has the distinct advantage of producing consistent IDTIMS ages at high precision. Because of these factors and its availability, we recommend its use in geological studies where precise and accurate Pb/U zircon ages are imperative. Approximate conversion factors have been derived to improve quantitative inter-comparison between SHRIMP ages that have been calibrated against the different standards. These refinements significantly advance the role that SHRIMP can play in the numerical calibration of the Phanerozoic timescale.

The role of the standard is critical to the derivation of reliable U-Pb zircon ages by micro-beam analysis. For maximum reliability, it is critically important that the utilised standard be homogeneous at all scales of analysis. It is equally important that the standard has been precisely and accurately dated by an independent technique. This study reports the emergence of a new zircon standard that meets those criteria, as demonstrated by Sensitive High Resolution Ion MicroProbe (SHRIMP), isotope dilution thermal ionisation mass-spectrometry (IDTIMS) and excimer laser ablation- inductively coupled plasma-mass-spectrometry (ELA-ICP-MS) documentation. The TEMORA 1 zircon standard derives from the Middledale Gabbroic Diorite, a high-level mafic stock within the Palaeozoic Lachlan Orogen of eastern Australia. Its206Pb/238U IDTIMS age has been determined to be 416.75±0.24 Ma (95% confidence limits), based on measurement errors alone. Spike-calibration uncertainty limits the accuracy to 416.8±1.1 Ma for U-Pb intercomparisons between different laboratories that do not use a common spike.