Radioactive Geochronometry

A derivative of the Treatise on Geochemistry

Heinrich D Holland, Karl K. Turekian

• Publisher: Academic Press
• Year: 2010
• Print ISBN:
  9780080967080
• eBook ISBN:
  9780080967097
• Topic: Ciencias físicas
• Subtopic: Geología y ciencias de la Tierra

The history of Earth in the Solar System has been unraveled using natural radioactivity. The sources of this radioactivity are the original creation of the elements and the subsequent bombardment of objects, including Earth, in the Solar System by cosmic rays. Both radioactive and radiogenic nuclides are harnessed to arrive at ages of various events and processes on Earth. This collection of chapters from the Treatise on Geochemistry displays the range of radioactive geochronometric studies that have been addressed by researchers in various fields of Earth science. These range from the age of Earth and the Solar System to the dating of the history of Earth that assists us in defining the major events in Earth history. In addition, the use of radioactive geochronometry in describing rates of Earth surface processes, including the climate history recorded in ocean sediments and the patterns of circulation of the fluid Earth, has extended the range of utility of radioactive isotopes as chronometric and tracer tools. - Comprehensive, interdisciplinary and authoritative content selected by leading subject experts - Robust illustrations, figures and tables - Affordably priced sampling of content from the full Treatise on Geochemistry

1

Cosmic-Ray Exposure Ages of Meteorites

G.F. Herzog Rutgers University, Piscataway, NJ, USA

1.1 INTRODUCTION 2

1.2 CALCULATION OF EXPOSURE AGES 3

1.2.1 Basic Equations 3

1.2.2 Factors Influencing Production Rates 3

1.2.3 Measurement Units and Quantities 4

1.2.4 Calibration of Production Rates 5

1.2.4.1 ²⁶Al Versus ²¹Ne calibration 5

1.2.4.2 ⁸³Kr/⁸¹Kr Calibration 5

1.2.5 Equations for Calculating One-Stage CRE Ages 6

1.2.5.1 ²¹Ne–²² Nel²¹ Ne ages 6

1.2.5.2 ³⁸Ar–²²Ne/²¹Ne Ages 7

1.2.5.3 ³He Ages 7

1.2.5.4 ³⁶Cl/³⁶Ar Ages 8

1.2.5.5 ⁸¹Kr/Kr ages 8

1.2.5.6 ⁴⁰K/K ages 8

1.2.6 The Importance of Half-Lives 10

1.3 CARBONACEOUS CHONDRITES 10

1.3.1 CI, CM, CO, CV, and CK Chondrites 10

1.3.2 The CR Clan 11

1.4 H-CHONDRITES 12

1.5 L-CHONDRITES 12

1.6 LL CHONDRITES 13

1.7 E-CHONDRITES 13

1.8 R-CHONDRITES 14

1.9 LODRANITES AND ACAPULCOITES 14

1.10 LUNAR METEORITES 15

1.10.1 Overview 15

1.10.2 Construction of CRE Histories 15

1.10.3 Production Rate of Lunar Meteorites 18

1.11 HOWARDITE–EUCRITE–DIOGENITE METEORITES 18

1.11.1 Eucrites 18

1.11.2 Diogenites 19

1.11.3 Howardites 20

1.11.4 Kapoeta 20

1.12 ANGRITES 21

1.13 UREILITES 21

1.14 AUBRITES (ENSTATITE ACHONDRITES) 21

1.15 BRACHINITES 22

1.16 MARTIAN METEORITES 22

1.17 MESOSIDERITES 24

1.18 PALLASITES 25

1.19 IRONS 25

1.20 THE SMALLEST PARTICLES: MICROMETEORITES, INTERPLANETARY DUST PARTICLES, AND INTERSTELLAR GRAINS 26

1.20.1 Background 26

1.20.2 Micrometeorites and IDPs 26

1.20.3 Interstellar Grains 27

1.21 CONCLUSIONS 28

ACKNOWLEDGMENTS 29

REFERENCES 29

1.1 INTRODUCTION

The classic idea of a cosmic-ray exposure (CRE) age for a meteorite is based on a simple but useful picture of meteorite evolution, the one-stage irradiation model. The precursor rock starts out on a parent body, buried under a mantle of material many meters thick that screens out cosmic rays. At a time t_i, a collision excavates a precursor rock—a “meteoroid.” The newly liberated meteoroid, now fully exposed to cosmic rays, orbits the Sun until a time t_f, when it strikes the Earth, where the overlying blanket of air (and possibly of water or ice) again shuts out almost all cosmic rays (cf., Masarik and Reedy, 1995). The quantity t_f–t_i is called the CRE age, t. To obtain the CRE age of a meteorite, we measure the concentrations in it of one or more cosmogenic nuclides (Table 1), which are nuclides that cosmic rays produce by inducing nuclear reactions. Many shorter lived radionuclides excluded from Table 1 such as ²²Na (t_1/2 = 2.6 years) and ⁶⁰Co (t_1/2 = 5.27 years) can also furnish valuable information, but can be measured only in meteorites that fell within the last few half-lives of those nuclides (see, e.g., Leya et al., 2001 and references therein).

Table 1

Cosmogenic nuclides used for calculating exposure ages.

Nuclide	Half-life^a (Myr)
Radionuclides
^14C	0.005730
^59Ni	0.076
^41Ca	0.1034
^81Kr	0.229
^36Cl	0.301
^26Al	0.717
^10Be	1.51
^53Mn	3.74
^129I	15.7
Stable nuclides
^3He
^21Ne
^38Ar
^83Kr
^126Xe

^a http://atom.kaeri.re.kr

CRE ages have implications for several interrelated questions. From how many different parent bodies do meteorites come? How well do meteorites represent the population of the asteroid belt? How many distinct collisions on each parent body have created the known meteorites of each type? How often do asteroids collide? How big and how energetic were the collisions that produced meteoroids? What factors control the CRE age of a meteorite and how do meteoroid orbits evolve through time? We will touch on these questions below as we examine the data.

By 1975, the CRE ages of hundreds of meteorites had been estimated from noble gas measurements. Histograms of the CRE age distributions pointed to several important observations:

1. The CRE ages of meteorites increase in the order stones < stony irons < irons.

2. The CRE ages of stones rarely exceed 100 Myr; the average ages of stony irons are ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright page
• Introduction
• Contributors
• 1: Cosmic-Ray Exposure Ages of Meteorites
• 2: Early Solar System Chronology
• 3: The Origin and Earliest History of the Earth
• 4: Long-Lived Chronometers
• 5: The Geochemistry and Cosmochemistry of Impacts
• 6: Geochronology and Thermochronology in Orogenic Systems
• 7: Ages and Growth of the Continental Crust from Radiogenic Isotopes
• 8: Radiocarbon
• 9: Natural Radionuclides in the Atmosphere
• 10: Groundwater Dating and Residence-time Measurements
• 11: Cosmogenic Nuclides in Weathering and Erosion
• 12: Geochronometry of Marine Deposits
• 13: Chronometry of Sediments and Sedimentary Rocks
• 14: The Early History of Life
• 15: Heavy Metals in the Environment—Historical Trends
• APPENDIX 1: Periodic Table of the Elements
• APPENDIX 2: Table of Isotopesa
• APPENDIX 3: The Geologic Timescale
• APPENDIX 4: Useful Values
• Index