Statistical geochemistry reveals disruption in secular lithospheric evolution about 2.5 Gyr ago

C Brenhin Keller ¹, Blair Schoene

⁰Department of Geosciences, Princeton University, Guyot Hall, Washington Road, Princeton, New Jersey 08544, USA. cbkeller@princeton.edu

Nature +

|

May 25, 2012

View abstract on PubMed

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.

Related Concept Videos