August 2007 Issue - Volume 3, Number 4

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Frontiers in Textural and Microgeochemical Analysis

Dougal A. Jerram and Jon P. Davidson – Guest Editors

Overview

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2007 Topics

Overview

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2007 Topics

Thematic Articles

Frontiers in Textural and Microgeochemical Analysis

Igneous rocks display a large and varied range of textures and compositions, reflecting complex magma pathways, differentiation processes and cooling histories. Integrating geochemical analyses with petrographic and textural information allows us to gain valuable insights into the details of the magma system: the rock’s texture serves as a window into the crystallization history of the magma, while the compositions of the components (crystals and glass) document the conditions and pathways of evolution. In this issue of Elements we take a look at the frontiers in igneous petrology, focussing on the state of the art in textural and microgeochemical analysis and on how we can use some of the latest approaches to unravel the complexities of the magmatic system.

3D Analysis of Rock Textures: Quantifying Igneous Microstructures

By Dougal A. Jerram, Michael D. Higgins

An important goal of petrographers is to analyse rock textures (microstructures) and compositions in three dimensions, and thus fully quantify rock properties (porosity and permeability, geochemistry, crystal abundance, etc.). With the advent of serial sectioning techniques, X-ray tomography analysis and advanced image analysis, it is becoming increasingly easy to reconstruct rock textures in three dimensions. An exciting consequence is the potential to reconstruct crystal populations in three dimensions and relate their distribution to the chemical budget of a rock. Here we review the current state of the art in textural analysis techniques and consider the possibilities of virtual three-dimensional models of rock textures.

Igneous Textures: On the Kinetics behind the Words

By Taber G. Hersum, Bruce D. Marsh

That igneous textures can be collectively described, classified, and related to magma composition, style of emplacement, and spatial position speaks deeply to the existence of a specific set of fundamental kinetic processes controlling all magma crystallization. Textures record magma life history, telling the most recent, local conditions of cooling and also where the magma has been. Yet it is largely a mystery how silicate melts crystallize, how they become what they are, and, especially, how the final texture relates to the early transient textures more closely linked to the governing kinetics of nucleation and growth. These rich and intriguing processes can be understood by deciphering textures. This is done by first dismantling and quantifying them, then by rebuilding them and simulating magma crystallization and transport, and last by taking the results to the final court of appeal, the rocks themselves.

Isotopic Microsampling of Magmatic Rocks

By Jon P. Davidson, Daniel J. Morgan, Bruce L.A. Charlier

Radiogenic isotope ratios can be used as a kind of petrogenetic “DNA” to identify the source components of magmas. Technical advances allowing us to measure isotopic compositions at the sub-crystal scale have led to the realisation that many magmatic rocks are isotopically heterogeneous. Crystals traditionally regarded as phenocrysts grown from the host magma have now been shown to be wholly or partly out of isotopic equilibrium with the glass or groundmass in which they are contained. Many of these crystals are likely to be recycled from earlier cumulates. Combining these fingerprinting techniques with the other approaches described in this issue offers an unprecedented opportunity to understand the processes and timescales through which magmas are assembled, differentiated and delivered to sites of eruption or emplacement.

Crystal Zoning as an Archive for Magma Evolution

By Catherine Ginibre, Gerhard Wörner, Andreas Kronz

Spatial compositional variations in magmatic minerals record chemical and physical changes in the magma from which they grew. Electron-beam techniques allow high-resolution imaging and quantitative analysis of this compositional archive for major, minor and some trace elements. In this way, magmatic processes such as crystallization, recharge in a magma chamber, decompression during ascent, and convection in the magma chamber can be identified and the history of magmatic systems prior to eruption reconstructed.

Measuring Timescales of Magmatic Evolution

By Simon Turner, Fidel Costa

Advances in analytical methods have provided new insights into the timescales of magmatic processes. Data on the abundances of U-series isotopes in bulk rocks and crystal separates indicate magma differentiation over thousands of years. Residence and differentiation times of silicic magmas based on single-crystal, in situ age data vary from 10,000 to 100,000 years, with abundant evidence for crystal recycling from previous intrusive episodes. Chemical zoning patterns in single crystals indicate that processes such as mixing and mingling of magmas and crustal assimilation may occur over much shorter timescales of months to decades. Quantifying the rates of magma generation, emplacement and differentiation constrains the processes involved and may contribute to the evaluation of volcanic hazards.

August 2007 Issue - Volume 3, Number 4

Frontiers in Textural and Microgeochemical Analysis

Table of Contents

v3n5 Critical Zone: Where Rock Meets Life

Thematic Articles

Frontiers in Textural and Microgeochemical Analysis

3D Analysis of Rock Textures: Quantifying Igneous Microstructures

Igneous Textures: On the Kinetics behind the Words

Isotopic Microsampling of Magmatic Rocks

Crystal Zoning as an Archive for Magma Evolution

Measuring Timescales of Magmatic Evolution