Geochemical Evidence for Mantle Origin and Crustal Processes in Volcanic Rocks from Popocatepetl and Surrounding Monogenetic Volcanoes, Central Mexico

Elemental, isotopic, and mineral compositions as well as rocktextures were examined in samples from Popocatépetl volcanoand immediately surrounding monogenetic scoria cones of theSierra Chichinautzin Volcanic Field, central Mexico. Magma generationis strongly linked to the active subduction regime to the south.Rocks range in composition from basalt to dacite, but Popocatépetlsamples are generally more evolved and have mineral compositionsand textures consistent with more complicated, multi-stage evolutionaryprocesses. High-Mg calc-alkaline and more alkaline primitivemagmas are present in the monogenetic cones. Systematic variationsin major and trace element compositions within the monogeneticsuite can mostly be explained by polybaric fractional crystallizationprocesses in small and short-lived magmatic systems. In contrast,Popocatépetl stratovolcano has produced homogeneous magmacompositions from a shallow, long-lived magma chamber that isperiodically replenished by primitive basaltic magmas. The currenteruption (1994–present) has produced silicic dome lavasand pumice clasts that display mingling of an evolved daciticcomponent with an olivine-bearing mafic component. The longevityof the magma chamber hosted in Cretaceous limestones has fosteredinteraction with these rocks as evidenced by the chemical andisotopic compositions of the different eruptive products, contact-metamorphosedxenoliths, and fumarolic gases. Popocatépetl volcanicproducts display a considerable range of ⁸⁷Sr/⁸⁶Sr (0·70397–0·70463)and _Nd (+6·2 to +3·0) whereas Pb isotope ratiosare relatively homogeneous (²⁰⁶Pb/²⁰⁴Pb 18·61–18·70;²⁰⁷Pb/²⁰⁴Pb 15·56–15·60). KEY WORDS: Popocatépetl; Sierra Chichinautzin Volcanic Field; arc petrogenesis; radiogenic isotopes     

The Fremont Butte,Washington Co., Colorado,Meteorite

Abstract The Fremont Butte meteorite was found near Fremont Butte, Colorado, in 1963. A single individual was found weighing 6.6 kg. It is an olivine-hypersthene or L group chondrite showing brecciation and a small number of well formed chondrules and olivine phenocrysts.     

Craters

Abstract Recent observational evidence emphasizes the importance of craters as a prominent surface characteristic of the smaller planetary bodies in the solar system. Craters of similar appearance may have very diverse origins. An initial assumption as to the type of origin should not influence an impartial and detailed physical study of any given crater.     

Carruthers and the theory of glacial undermelt: lessons from a pamphleteer?

In 1953, R. G. Carruthers, a respected geologist, published a private scientific pamphlet which challenged the established view of British Quaternary stratigraphy. Carruthers had no other route but private publication. Why was this, and what was the foundation for his ideas? We explore Carruthers' theory in the light of current knowledge and consider the lessons for scientific publication provided by it.