A determination of the intensity of the ancient lunar magnetic field

Thermal demagnetization of lunar breccia 15498,36 shows that the natural remanent magnetization is a simple thermoremanence carried by metallic iron. Using the classical Thellier-Thellier method the strength of the magnetizing field at the time of sample formation was found to be 2100 ±80 gammas.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973.     

Copper oxidation state in chalcopyrite: Mixed Cu d and d characteristics

The determination of the oxidation states of copper and iron in sulfides, and chalcopyrite (CuFeS₂) in particular, using 2p X-ray photoemission spectroscopy (XPS) and L_2,3-edge X-ray absorption spectroscopy (XAS) is revisited. Reassessment of the published spectra derived by these methods produces consistent results and reveals the ‘d count’ in the copper compounds to be intermediate between d⁹ and d¹⁰. Nevertheless, these covalent copper compounds can be divided into those nominally monovalent and those nominally divalent. The Fe L_2,3-edge XAS of chalcopyrite, along with Mössbauer data, confirm the presence of high-spin Fe³⁺. Chalcopyrite, despite recent published reports to the contrary, clearly belongs to the monovalent copper class.     

An expert system for the tectonic characterization of ancient volcanic rocks

The expert system approach enables geochemical evidence to be integrated with geological, petrological and mineralogical evidence in identifying the eruptive setting of ancient volcanic rocks. This paper explains the development of ESCORT, an Expert System for Characterization of Rock Types. ESCORT uses as its knowledge base a set of dispersion matrices derived from a geochemical data bank of some 8000 immobile element analyses, together with tables of magma-type membership probabilities based on a-priori knowledge, the dominant flow type, the facies of intercalated sediment, the basement to the volcanic sequence, the presence or absence of an iron-enrichment trend and the phenocryst phase assemblage. Inferencing is based on the Bayes Decision Rule, adapted to take into account possible uncertainties in the geological evidence. Upper and lower probability thresholds are used to decide whether an interpretation is likely or unlikely. Evaluation of the system shows that it is able to overcome many of the well-publicized ambiquities attached to geochemical discrimination diagrams, as well as providing a unified method of decision making and a measure of the correctness of the interpretation.     

Storm runoff processes and subcatchment characteristics in a new zealand hill country catchment

The quickflow responses of six subcatchment areas in a small hill country catchment in the Craigieburn Range, South Island, New Zealand, were compared for a range of storm sizes, rainfall intensities and antecedent wetness conditions. Topography and soil characteristics suggested that all subcatchments would receive subsurface stormflow input, but that some would receive larger saturation overland flow inputs than others. Quickflow yields and response ratios were positively correlated with storm size and antecedent wetness conditions in the subcatchment most suited to producing saturation overland flow. In subcatchments more likely to be dominated by subsurface flow, quickflow yields and response ratios were positively correlated with storm size, but were either not correlated, or negatively correlated, with antecedent wetness. Quickflow responses were either not significantly or negatively correlated with rainfall intensity variables. Quickflow from the subcatchment most suited to produce saturation overland flow providing an increasing proportion of total catchment quickflow in larger storms and as antecedent conditions became wetter. Subcatchment responses varied greatly in space and time and there was less pattern to the variation than had been expected. Where topographic and pedologic conditions permit substantial responses to storm rainfall by both saturation overland flow and subsurface stormflow, simple topographic and soil indicators may not be useful guides to the relative importance of runoff mechanisms, or to the identification of runoff-source areas.     

Grain boundaries as microreactors during reactive fluid flow: experimental dolomitization of a calcite marble

Limestone dolomitization is an example of a fluid-induced mineralogical transformation that commonly affects extensive rock volumes. To understand the mechanisms enabling these efficient replacement reactions, we investigated experimentally the dolomitization of a fractured calcite marble under flow-through conditions at mild hydrothermal conditions. Contrary to most earlier studies of coupled dissolution reprecipitation reactions that were conducted using small, individual grains, in this study, the integrity of the rock was preserved, so that the experiment explored the links between flow in a fracture and fluid–rock interaction. In these experiments, grain boundaries acted as microreactors, in which a Mg-poor ‘protodolomite’ formed initially, and then transformed into dolomite. The difficulty in nucleating dolomite played a key role in controlling the evolution of the porosity, by allowing for (1) initial dissolution along grain boundaries, and (2) formation of coarse porosity at the reaction interface. This porosity evolution not only enabled the reaction to progress efficiently, but also controlled the mineralogy of the system, as shown by brucite replacing calcite near the fracture once the fluid along calcite grain boundaries became sufficiently connected to the fluid flowing through the fracture. This study illustrates the role of grain boundaries, porosity evolution and nucleation in controlling reaction progress as well as the nature and textures of the products in pervasive mineralogical transformations.     

Shocked monazite chronometry: integrating microstructural and in situ isotopic age data for determining precise impact ages

Monazite is a robust geochronometer and occurs in a wide range of rock types. Monazite also records shock deformation from meteorite impact but the effects of impact-related microstructures on the U–Th–Pb systematics remain poorly constrained. We have, therefore, analyzed shock-deformed monazite grains from the central uplift of the Vredefort impact structure, South Africa, and impact melt from the Araguainha impact structure, Brazil, using electron backscatter diffraction, electron microprobe elemental mapping, and secondary ion mass spectrometry (SIMS). Crystallographic orientation mapping of monazite grains from both impact structures reveals a similar combination of crystal-plastic deformation features, including shock twins, planar deformation bands and neoblasts. Shock twins were documented in up to four different orientations within individual monazite grains, occurring as compound and/or type one twins in (001), (100), \(\left( 10\bar{1} \right)\), \(~\{110\}\), \(\left\{ 212 \right\},\) and type two (irrational) twin planes with rational shear directions in \([0\bar{1}\bar{1}]\) and \([\bar{1}\bar{1}0]\). SIMS U–Th–Pb analyses of the plastically deformed parent domains reveal discordant age arrays, where discordance scales with increasing plastic strain. The correlation between discordance and strain is likely a result of the formation of fast diffusion pathways during the shock event. Neoblasts in granular monazite domains are strain-free, having grown during the impact events via consumption of strained parent grains. Neoblastic monazite from the Inlandsee leucogranofels at Vredefort records a ²⁰⁷Pb/²⁰⁶Pb age of 2010?±?15 Ma (2σ, n?=?9), consistent with previous impact age estimates of 2020 Ma. Neoblastic monazite from Araguainha impact melt yield a Concordia age of 259?±?5 Ma (2σ, n?=?7), which is consistent with previous impact age estimates of 255?±?3 Ma. Our results demonstrate that targeting discrete microstructural domains in shocked monazite, as identified through orientation mapping, for in situ U–Th–Pb analysis can date impact-related deformation. Monazite is, therefore, one of the few high-temperature geochronometers that can be used for accurate and precise dating of meteorite impacts.