Magnetic components contributing to the NRM of Middle Siwalik red beds

The components of the NRM in Middle Siwalik red beds are carried by two phases of hematite, a red pigment phase and a specular hematite phase. We present evidence in the form of a conglomerate test that the specularite phase carries a remanence acquired during or shortly after deposition. The red pigment, however, post-dates deposition, in many cases by at least one reversal boundary. This secondary component has a higher coercivity but a lower blocking temperature in these rocks and can therefore be selectively removed by thermal demagnetization to reveal a primary magnetization useful for magnetostratigraphic studies.     

A note on product differentiation and locational protection

This paper critically examines Steed's recent attempt to attribute dynamic product differentiation with a locationally protective function in the trade in high-cost clothing between the developed countries of Western Europe. An alternative explanation is offered which explains the distance restriction on trade in terms of the high market uncertainty in selling high-cost clothing, which restricts the size of firm and therefore its ability to export on a large scale. Product differentiation remains as a factor with trade expansionary rather than trade restricting characteristics.     

Mineralogy and stable isotope geochemistry of hydrothermally altered oceanic rocks

Data for the diffusion of cations in pyroxenes are relevant to a variety of sub-solidus processes including order-disorder and exsolution. Similar data must also be available if the reliability of geobarometers and geothermometers involving pyroxenes is to be assessed. Two types of diffusion experiment have been performed to determine cation diffusion rates in pyroxenes: (1) interdiffusion between single crystals of diopside and polycrystalline sinters enriched in Al and Fe, and (2) interdiffusion between single crystals of diopside and a glass of the same composition which was isotopically enriched in²⁶Mg and⁴³Ca. Following high-temperature annealing for periods up to several hundred hours, analysis of the diffusion couples, using an electron microprobe and an ion microprobe respectively, failed to show any measurable diffusion profiles. From these “null result” experiments the diffusion coefficients (D) for Al and Fe in diopside are estimated to be less than4×10^?14cm²s^?1 at 1200°C, and values ofD for Ca and Mg in diopside are estimated to be less than7 × 10^?14cm²s^?1 at 1250°C. These rates are significantly slower than published tracer-type diffusion data for Ca and Al.A review of studies of order-disorder, microstructural coarsening, and diffusion in pyroxenes suggest that activation energies for cation exchange are typically in excess of 60 kcal mol^?1. Transport rates will be assisted, and activation energies lowered by sample non-stoichiometry, inhomogeneities, high dislocation densities and the presence of water.The collective data for Al, Mg and Ca diffusion in diopside indicate diffusion coefficients? 10^?15cm²s^?1 at 1200°C. A comparison with data for diffusion in garnet, olivine and spinel suggests that pyroxenes may have the highest blocking temperatures.     

The distribution of 137Cs in surface intertidal sediments from the Solway Firth

The distribution of ¹³⁷Cs from the Sellafield (Windscale) nuclear fuel reprocessing plant has been examined in detail in the surface intertidal sediments of the inner Solway Firth by means of a hovercraft-borne radiometric survey. With the exception of a belt of relatively active sands to the south of Silloth, caesium distribution is generally consistent with that of fine-grained sediment such that the highest concentrations occur in mud flat and salt marsh sediments which are most extensive in sheltered coastal embayments. ¹³⁷Cs activities in July 1980 were typically 2–30 pCi g^?1 but locally exceeded 50 pCi g^?1. These levels are considerably lower than those recorded in locations, such as the outer Solway and Ravenglass estuary, which are closer to the Sellafield outfall.     

Triple junction magmatism: a geochemical study of Neogene volcanic rocks in western California

Inception of volcanism at late Oligocene to Recent centers in the eastern Coast Ranges of California (ECR suite) regularly decreases in age northward and is correlated with the northward migration of the transform-transform-trench Mendocino triple junction (MTJ). Miocene volcanism in the southern California basin (SCB suite) is spatially and temporally associated with the transform-ridge-trench Rivera triple junction (RTJ). The tholeiitic to calc-alkaline rocks in both suites were erupted through older trench melange while arc magmatism was occurring several hundred kilometers to the east. Therefore they are not related to subduction zone magmatism, but instead to interactions of the MTJ and RTJ with the continental margin.The ECR rocks, dominantly intermediate to silicic in composition, have relatively high δ¹⁸O values up to 11.3, ⁸⁷Sr/⁸⁶Sr ratios up to 0.7055, as well as relatively high Th contents, suggesting that crustal anatexis played a dominant role in their generation. Coupled crystal fractionation and crustal assimilation by an initially basaltic magma cannot explain the high δ¹⁸O values and ⁸⁷Sr/⁸⁶Sr ratios because greater than 95% of the basalt would need to crystallize. In contrast, the SCB rocks, dominantly mafic to intermediate in composition, have relatively low δ¹⁸O values down to 5.2 and ⁸⁷Sr/⁸⁶Sr ratios down to 0.7025 suggesting that these rocks were derived dominantly from a mantle source.Whether crustal anatexis occurs is determined largely by the type of stress a triple junction imposes upon the continental margin. Both the MTJ and RTJ are associated with high heat flow and magma fluxes from the mantle. The transform-transform-trench MTJ is associated with locally variable mild extension to compression and therefore allows pooling of basaltic magma in the crust to initiate crustal melting. The high rates of continental extension associated with the transform-ridge-trench RTJ prevents such pooling of magma.The space created by decoupling of the subducted slab at a transform-transform-trench triple junction might promote passive upwelling of mantle material to fill it and induce melting to generate basalts. Mafic volcanic rocks of this origin may provide a unique view of the subcontinental mantle at the continental margin. ⁸⁷Sr/⁸⁶Sr ratios as low as 0.70255 for mafic volcanic rocks in the Sonoma-Tolay center associated with the MTJ contrast with high La/Sm ratios of 1.1 to 1.3 and low Zr/Nb,Hf/Ta,La/Th, and La/Ta ratios of 5.0 to 6.7, 2.6 to 3.5, 4.7 to 8.8, and 10.2 to 12.5, respectively. These data suggest that the mantle beneath parts of western California may have originally been depleted but has been enriched relatively recently. Such enrichment might have occurred by metasomatic processes associated with crustal accretion and/or juxtaposition of differing lithospheric mantle in the Mesozoic and Cenozoic.     

Origin of low-18O metamorphic rocks from a Late Proterozoic shear zone in the Eastern Desert of Egypt

Anomalously low and variable ¹⁸O/¹⁶O ratios (whole rock ¹⁸O_SMOW=–1.9 to +6.0) have been measured in several Late Proterozoic metamorphic units associated with a large scale low-angle ductile shear zone at Meatiq Dome in the central Eastern Desert of Egypt. The low-¹⁸O units consist of granite gneiss, quartzofeldspathic mylonite, and quartz phyllonite. These are intruded by syntectonic diorite-tonalite-granodiorite and post-tectonic granite, both having normal ¹⁸O. No evidence for other low-¹⁸O country rock units has been found. Oxygen isotope fractionation between quartz and magnetite in the granite gneiss ( _QZ-MT=8.0 to 9.3) is consistent with equilibration at amphibolite grade temperatures, suggesting that ¹⁸O depletion occurred prior to metamorphism. Whole rock geochemical analyses indicate that the low-¹⁸O units were derived from two granitic protoliths, one of which apparently underwent local sericitic alteration prior to metamorphism; it is proposed that ¹⁸O depletion and sericitic alteration occurred simultaneously in a high-temperature meteorichydrothermal system associated with emplacement of these granitic protoliths. The relatively high ductility of the sericitically altered rock may have been an important factor in the subsequent localization of the shear zone.     

The origin of small-scale geochemical and mineralogic variations in a granite intrusion

A post-tectonic unzoned granite intrusion in the Meatiq Dome, a Late Proterozoic metamorphic complex in the Central Eastern Desert of Egypt, shows significant chemical and mineralogic heterogeneity on the scale of sampling (5 kg). Whole rock analyses of 21 samples indicate small variations in SiO₂, Al₂O₃, Na₂O and K₂O, and larger systematic variations in the less abundant major elements such as FeO, TiO₂, CaO, MnO and MgO, and trace elements such as Sc, Cr, Co, Rb, Sr, Zr, La, Ce, Nd, Sm, Tb, Yb, Lu, Ta, Th and U. These variations cannot be accounted for by processes such as marginal accretion, assimilation, alteration by late stage fluids, or multiple intrusion. Instead, we proposed a model involving 35 percent solidification of the granite magma followed by partial solid-liquid segregation during emplacement, resulting in rocks containing 7–71 volume percent early-formed solids. Such randomly distributed local segregation could have been caused by filter pressing, flow differentiation, and possibly gravity segregation, either singly or in combination. Thus, each sample is interpreted as a mixture of two end-members with nearly constant compositions: an early-formed solid assemblage of crystals and a complementary residual liquid. Early formed solids are enriched in TiO₂, FeO, CaO, P₂O₅, Sc, Co, Cr, Sr, La, Ce and depleted in SiO₂, Rb, Yb, Lu, Ta, Th, and U, while the residual liquid has complementary enrichments and depletions. This simple mixing model is consistent with field and petrographic observations, experimental studies pertaining to the crystallization sequence in the system Ab-Or-Qtz-H₂O at 1 Kb, and physical properties of silicic magmas. Furthermore, it is quantitatively supported by trace-element data for minerals, computed endmember compositions at 35% crystallization using mineral analyses and reasonable Kd values, and internal consistency in the percent solid for each sample computed from each of 17 elements in the inferred end-members. We suggest that this model might also apply to other small epizonal granite intrusions that show small-scale chemical heterogeneities.