Element zoning trends in olivine phenocrysts from a supposed primary high-magnesian andesite: an electron- and ion-microprobe study

Electron and ion-probe microanalysis have been used to obtain zoning profiles for major and trace elements in olivine phenocrysts from a high-magnesian andesite from Shodo-Shima island, southwest Japan. This rock was previously thought to represent undifferentiated, primary magma. Some crystals have unzoned cores, while others show cores which are reversely zoned with respect to Mg/ (Mg+Fe), Ni, Mn and Cr. In addition, some Ni profiles show a normally zoned hump at the most central portions of the reversely zoned crystals. All crystals show normally zoned rims. The Li concentrations are constant throughout the cores of all crystals studied, but rise sharply, by a factor of up to at least six, in the rims. The Ca and Co concentrations are essentially constant throughout all the crystals. Mechanisms for producing the observed zoning profiles are discussed, and it is concluded that the reverse zoning was produced by the introduction of crystals into a less differentiated magma than that in which they grew. The reversely zoned crystals could therefore represent xenocrysts which were introduced into an undifferentiated magma, or phenocrysts introduced into a more primitive magma by a magma mixing process. The Ni profiles are used to estimate the residence time of these crystals in the more primitive magma. The following trace element partition co-efficients have been estimated for the olivine-groundmass system in this rock: D ^Ni=16; D ^Mn=1.1; D ^Co=4.2; D ^Ca =0.02; D ^Ti=0.005; D ^V=0.05; D ^Sc=0.2; D ^Na=0.0002. Studies of trace element zoning will become increasingly important as the new generation of trace element microprobes become available but a larger database of experimentally determined values for trace element partition coefficients and diffusion coefficients in crystals and magmas, and a better understanding of other disequilibrium processes are required to fully exploit the new data.     

Mineralogy and Petrology of the Red Hill Alkaline Igneous Complex, New Hampshire, U.S.A.

The Red Hill intrusion, New Hampshire is one of the alkalineintrusions making up the White Mountain Magma Series. Earlierwork has shown that it consists of several units with ring-or plug- like form, in order of intrusion: Outer Coarse Syenite(OCS) plus Nepheline Sodalite Syenite (NSS Fire Tower Sycnite(FTS); Garland Peak Syenite (GPS) Watson Ledge Quartz Syenite(WLQS) Interior Fine Granite (IFG). New studies have revealedtrends of increasing alkalinity in both the OCS (OCS-AOCS-B)and NSS (NSS-A-B-C). Conventional K-Ar and Rb-Sr age datingon separated minerals and bulk rocks show that the OCS, NSS,FTS, and GPS have indistinguishable ages at 198.5?1.5 Ma whilethe IFG formed about 10 Ma later. These values are believedto represent intrusion ages. Amphiboles from the NSS vary from ferro-pargasites through taramitesto katophontes and pyroxenes from ferrosalites to aegirine-augitesthese trends follow the increasing degree of under- saturationin the NSS-A-C series. The NSS-A contain mafic syenite xenolithsas well as partially resorbcd salitic pyroxene cores withinamphiboles. Mineral and isotope data are consistent with theseinclusions being cognate suggesting derivation from more-basicparental magmas. Pyroxenes and amphiboles from saturated andoversaturated rocks vary from ferro-salites to ferro-hedenbergitesand hastingsites to ferro-edenites, respectively, but OCS-Bshow fractionation towards aegirine-augites and katophorites. Mineral assemblages crystallized close to the quartz-fayalite-magnetitebuffer at a pressure of about 1–1.5 kb under essentiallyfluid-saturated conditions. It seems likely that the complexformed by emplacement of fractionated magmas derived from alower-level magma chamber which initially contained a basalticparent magma. The first pulse of magma had an overall compositionsimilar to OCS plus NSS and was intruded as a sheet-like bodywhich differentiated in situ to give a central unit of highlyundersaturated magma (NSS-C). NSS-C samples with relativelyradiogenic Sr isotope compositions were modified by introductionof country-rock Sr via circulating fluids. Fractional crystallizationof alkali-rich amphiboles from critically undersaturated magma(NSS-A?) in the lower- level chamber led to the developmentof saturated and oversaturated magmas. These were intruded intothe OCS/NSS unit along ring fractures to form the FTS and GPSunits. Rock and mineral compositions, including isotope data,are consistent with the IFG forming by partial melting of countryrock and being intruded along pre-existing planes of weaknesssome 10 Ma after the main complex was formed.     

THE TROPICS: ENVIRONMENTS AND HUMAN IMPACTS UNDERSTOOD AND REINTERPRETED

In the 1950s, few people had travelled widely through different parts of the tropics and interpretations on tropical landforms, soils, vegetation and climate largely rested on impressions gained from residence or long sojourns in particular tropical countries. Most academic geography on the tropics was then written by expatriates and lacked the perspective gained from long experience of tropical regions. Particular problems of understanding arose as people attempted to extrapolate from one part of the tropics to another. Since then, understanding has advanced enormously, largely through the efforts of tropical scientists working in their own institutions and through the greater technological ability to study tropical lands both remotely and through easier field access. The establishment of a number of field stations, aimed primarily at the biological sciences, but also facilitating the work of geographers, has been a major stimulus to this effort. Concerns over practical issues, particularly local development and global climatic change have prompted new avenues of research, many of which have been assisted by the great increase in data collection in tropical countries. For the future, South-South dialogue between tropical physical geographers is needed to improve their ability to ask meaningful questions so that they can contribute to teach the rest of their discipline more about serious applied and applicable geography.