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Preparation of Authigenic Pyrite from Methane-bearing Sediments for In Situ Sulfur Isotope Analysis Using SIMS
Published on: August 31, 2017
Statistical geochemistry reveals disruption in secular lithospheric evolution about 2.5 Gyr ago
C Brenhin Keller1, Blair Schoene
1Department of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton, New Jersey 08544, USA. cbkeller@princeton.edu
Nature
|May 25, 2012
Summary
Earth
Area of Science:
- Geochemistry
- Geodynamics
- Earth History
Background:
- Earth has cooled over 4.5 billion years due to heat loss and declining radiogenic heat production.
- Igneous geochemistry records changing heat flux and Archaean geodynamics, but is complicated by data biases.
- Statistical sampling of 70,000 samples provides a comprehensive geochemical evolution record.
Purpose of the Study:
- To reconstruct secular geochemical evolution throughout Earth history.
- To investigate the influence of mantle cooling on geochemical trends.
- To identify and explain a geochemical discontinuity around 2.5 billion years ago.
Main Methods:
- Statistical sampling techniques applied to a large geochemical database (approx. 70,000 samples).
- Analysis of compatible and incompatible elements in basalts to track mantle melt fraction.
- Examination of Na/K, Eu/Eu*, and La/Yb ratios in felsic rocks for indicators of crustal melting.
Main Results:
- Basalts show gradually decreasing mantle melt fraction consistent with secular mantle cooling.
- A significant geochemical discontinuity around 2.5 billion years ago indicates reduced mantle melting and deep crustal processes.
- Increased crustal thickness and changes in felsic rock geochemistry coincide with this discontinuity.
Conclusions:
- Archaean mantle melting likely produced a thick, mafic lower crust, leading to delamination and tonalite-trondhjemite-granodiorite magmatism.
- The geochemical discontinuity marks a transition in Earth's crustal evolution.
- Changes in deep Earth geochemistry correlate with the rise of atmospheric oxygen at the end of the Archaean eon.
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