February 2013 Issue - Volume 9, Number 1

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One Hundred Years of Geochronology

Daniel J. Condon and Mark D. Schmitz – Guest Editors

Overview

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2013 Topics

Overview

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2013 Topics

Thematic Articles

One Hundred Years of Isotope Geochronology, and Counting

In 1913, Frederick Soddy’s research on the fundamentals of radioactivity led to the discovery of “isotopes.” Later that same year, Arthur Holmes published his now famous book The Age of the Earth, in which he applied this new science of radioactivity to the quantification of geologic time. Combined, these two landmark events did much to establish the field of “isotope geochronology” – the science that underpins our knowledge of the absolute age of most Earth (and extraterrestrial) materials. In celebrating the centenary, this issue brings together modern perspectives on the continually evolving fi eld of isotope geochronology – a discipline that reflects and responds to the demands of studies ranging from the early evolution of the Solar System to our understanding of Quaternary climate change, and the 4.5 billion years in between.

Precision and Accuracy in Geochronology

By Blair Schoene, Daniel J. Condon, Leah Morgan, Noah McLean

Geochronology in Earth and Solar System science is increasingly in demand, and this demand is not only for more results, but for more precise, more accurate, and more easily interpreted temporal constraints. Because modern research often requires multiple dating methods, scrupulous inter- and intramethod calibration in absolute time is required. However, improved precision has highlighted systematic analytical biases and uncovered geologic complexity that affects mineral dates. At the same time, both enhanced spatial resolution through microbeam geochronology and creative uses of disparate data sets to inform age interpretations have helped explain complexities in age data. Quantifying random and systematic sources of instrumental and geological uncertainty is vital, and requires transparency in methodology, data reduction, and reporting. Community efforts toward inter- and intracalibration of chronometers will continue to help achieve the highest possible resolving power for integrative geochronology.

High-Precision Geochronology

By Mark D. Schmitz, Klaudia F. Kuiper

High-precision geochronology is integral to testing hypotheses regarding the correlation, causes, and rates of events and processes in Earth history. Recent studies have sought to reconcile very precise, but apparently conflicting, ages for the same geological samples and events using different chronometers. Both systematic (decay constants, ages of standard materials) and geological (daughter-nuclide loss, inheritance) complexities contribute to the challenges of rock-clock calibration. Community-wide efforts to improve radioisotope geochronology have successfully mitigated many of these factors, and have brought high-precision geochronology to a threshold of unprecedented integration with stratigraphic and geochemical proxies of Earth systems dynamics.

High-Spatial-Resolution Geochronology

By Alexander A. Nemchin, Matthew S. A. Horstwood, Martin J. Whitehouse

High-spatial-resolution isotope analyses have revolutionised U–(Th–)Pb geochronology. These analyses can be done at scales of a few tens of microns or less using secondary ion mass spectrometry or laser ablation inductively coupled plasma mass spectrometry. They allow determination of the internal age variation of uranium- and thorium-bearing minerals and as a consequence much greater understanding of Earth system processes. The determination of variation on the micron scale necessitates the sampling of small volumes, which restricts the achievable precision but allows discrimination of discrete change, linkage to textural information, and determination of multiple isotopic and elemental data sets on effectively the same material. High-spatial-resolution analysis is being used in an increasing number of applications. Some of these applications have become fundamental to their scientific fields, while others have opened new opportunities for research.

Dating the Oldest Rocks and Minerals in the Solar System

By Yuri Amelin, Trevor R. Ireland

Meteorites originating from asteroids are the oldest-known rocks in the Solar System, and many predate formation of the planets. Refractory inclusions in primitive chondrites are the oldest-known materials, and chondrules are generally a few million years younger. Igneous achondrites and iron meteorites also formed in the first five million years of the protoplanetary disk and escaped accretion into planets. Isotopic dates from these meteorites serve as time markers for the Solar System’s earliest history. Because of the unique environments in the protoplanetary disk, dating the earliest meteorites has its own opportunities and challenges, different from those of terrestrial geochronology.

Time Constraints and Tie-Points in the Quaternary Period

By David A. Richards, Morten B. Andersen

The Quaternary Period, by virtue of the near-surface preservation and widespread accessibility of its environmental archives, provides fundamental data to test models of climate change, sea level variation, geomagnetic field variation, human and faunal migration, cultural evolution and more. Spatially disparate records of past environmental change with subannual to multimillennial temporal resolution are compared to examine the relative timing of events and consider causal mechanisms, and this analysis puts great demands on the chronological tools available. Highly precise and accurate age estimates are required, in concert with correlative tools or chronostratigraphic markers. We focus on radioisotope chronometers (e.g. U-series, 40Ar/39Ar and 14C) and illustrate their application in three vignettes for which different strategies are required: (1) the dramatic decades of the last deglaciation (~14.7 ka), (2) before and after one of the last geomagnetic excursions (~41 ka) and (3) the glacial–interglacial cycles of the Middle Pleistocene (125–780 ka).

Revolution and Evolution: 100 Years of U–Pb Geochronology

By James M. Mattinson

U–Pb geochronology has its roots in a spectacular decade of discovery. Within about ten years of the discovery of radioactivity in 1896, old ideas about the nature of matter were overthrown and the seeds of geochronology were planted. After giants of physics like Ernest Rutherford moved on to other research, Arthur Holmes nurtured the new field of geochronology, combining physics, chemistry, and geology to produce the earliest quantitative geologic timescale. Over the following decades, geochronology experienced a series of revolutionary and evolutionary advances, and became a vital part of almost all fields of geology.