4.4 billion years of maturation of the continental crust?

Lead Research Organisation: University of Cambridge
Department Name: Earth Sciences

Abstract

The Earth is divided structurally into several different shells, an iron rich core, and silicate (rocky) mantle and crust. The continental crust is generally topographically high, and the oceanic crust is low lying, a consequence of their distinct density and composition. This bi-model crustal structure is unique within the solar system. Also unique is the free water at the Earth's surface, (the hydrosphere), and a vital part for the creation of life on Earth, constantly shaping and reacting with the rocks at the its surface. Geophysical methods, such as the transmission of seismic waves have mainly revealed the broad structure of the Earth, but geochemistry (the chemical study of geological samples) has revealed much about the processes by which the crust formed. The bulk Earths composition is similar to that of parent meteorite bodies, but different parts of Earth have very different compositions. The so called 'enriched' continental crust has a complementary composition to the so called 'depleted' mantle, consistent with the crust having been extracted from the mantle as a melt product early in Earths history. There is increasing evidence for the formation of the continental crust to be old, with a large portion of it having formed in the first half of Earth's History. However, the details of the processes by which the crust formed by partial melting of the mantle remain highly controversial. One difficulty with establishing and testing models for the extraction of the crust from the mantle, is that the crust is old, and its composition may have changed over Earth's history. Because much of the crust is old and continually reworked by sedimentary and plate tectonic processes, much of the crust has been altered or weathered. The soil that mantles much of the present surface of the Earth is the modern manifestation of such weathering. During continental weathering, many soluble elements (such as those found in a mineral water for example) get transferred to seawater. Some elements get cycled from seawater back into rocks. Calcium is precipitated as calcite shells by many marine organisms which eventually becomes limestone. Other elements, such as magnesium, are returned to the oceanic crust during water-rock interaction in the fluid convection cells that cools the hot oceanic crust after magma generation at mid-ocean-ridges. Over long time-scales, magnesium is transferred from the continents to the oceanic crust. Magnesium (a soluble element) is particularly depleted in the continental crust compared to mantle rocks, and it is important to constrain the extent to which this reflects weathering of the crust, or melt processes. This project proposes to evaluate the degree of alteration of the continental crust by exploiting very small differences (as low as 1 part per 10000 parts) in the isotope ratios of elements such as magnesium, (the fifth most abundant element in the continental crust). Such differences become imparted to the rock record during chemical reactions. In particular, low temperature reactions (such as weathering) create a distinct and larger signature compared to high temperature reactions (melting). Recent advances in mass spectrometry have made such minute differences in isotope ratios detectable. Detection of distinct Mg isotope ratios in the continental crust compared to the mantle will enable the quantification of the loss of Mg by weathering from the continental crust. This will enable refinement of models for the formation of the continental crust, because the initial composition will be better constrained.

Publications

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Related Projects

Project Reference Relationship Related To Start End Award Value
NE/G013764/1 01/02/2010 01/11/2011 £282,126
NE/G013764/2 Transfer NE/G013764/1 02/11/2011 01/05/2013 £134,819
 
Description Note that this is the same grant as NE/GE13764/2, and as such the findings are the same.

Key findings:The key finding from the project is that the continental crust is heterogenous in it's Mg isotope composition, and that the absolute composition of any given rock apparently depends on the age of the rock.This must relate to water-rock interaction (weathering) influencing the composition of the continental crust at the Earth's surface (Tipper et al in Prep).A second key finding is that the Mg isotope composition of the mantle is constant (Bourdon et al 2010).However, individual minerals from the mantle show a wide variation in Mg isotope composition, reflecting equilibration amongst individual phases (Stracke et al in prep). Another key finding is that Mg and Li isotopes co-vary in the Mackenzie River basin, and a model of clay formation has been developed to explain this (Tipper et al 2012).
Exploitation Route The findings help shape our understanding of how the Earth's crust (i.e., where we live) has evolved over time. This is of obvious importance to research and education, but is also of fundamental importance for our understanding of how our planet works.
Sectors Education,Environment,Other

 
Description This research is of fundamental scientific interest, and the results are beneficial to a wide range of Earth Scientists from many countries. Benefits to specific Earth-Science communities are specified below. 1. Researchers working on the geochemistry of crustal evolution. Better constraining the evolution of the continental crust will be beneficial to a wide range of researchers, from those working on crustal formation to the evolution of the crust and zircon geochemistry to oxygen isotope geochemistry. 2. Researchers studying the Earths weathering engine and chemical weathering:- better constraining the interaction into water-rock chemical reactions via the study of suspended sediment profiles in large rivers. 3. Modelers of crustal formation. 4. Modelers considering the impact of chemical weathering on climate. 5. Mantle geochemists. The determination of the Mg isotope composition of the mantle will interest mantle geochemists and cosmochemists who are currently debating whether the Earth has a chondritic composition or not. 6. Hydrothermal circulation and weathering of the oceanic crust. Mg plays a critical role in the weathering of the oceanic crust, because of the exchange for Ca during hydrothermal reactions, and quantitative removal of Mg from hydrothermal fluids. 7. Analytical development. The research requires the highest possible precision for Mg isotope ratios, requiring both development in chemical separation of Mg and plasma mass spectrometry techniques. 8. Sedimentologists and geomorphologists will be interested in the new geochemical data from the sampling of suspended sediment depth profiles. 9. Society. Much of the Earth Science taught at key stage 3 in secondary schools is based on work that is continually evolving. Topics such as the formation of the crust is of such fundamental importance that the knowledge will be rapidly passed on to society. The broad context of this work is global (bio)geochemical cycles, linking the geochemical cycles currently significant to climate change to solid earth geochemical cycles.
First Year Of Impact 2012
Sector Education,Environment,Other
Impact Types Societal