Geology: The big melts
The Earth’s continental crust was not formed gradually but in stages, punctuated by large, potentially global, melting events. A report analyses the helium isotopic ratios from ocean island basalts, which reveal several distinct episodes of crustal growth.
Most Earth scientists agree that the continental crust formed by a process of partial melting from the mantle, although the timing and details of this process has remained controversial. Stephen Parman studied the chemical fingerprint of continental crust formation in the isotopic composition of helium isotopes recorded in ocean island basalts. The volatility of helium isotopes means that they are not reintroduced into the mantle during subduction and the record of mantle depletion is thus preserved for helium, whereas most other isotopes are continually recycled by mantle convection, largely obscuring such a record.
Parman found that peaks in the occurrence of helium isotope ratios are globally consistent and correspond with the ages of what have been proposed to be continental growth pulses. He concludes that the ultimate cause of these large melting events is still unclear, but could be related to large releases of heat from the mantle.
CONTACT
Stephen Parman (University of Durham, UK)
E-mail: stephen.parman@durham.ac.uk
Please note that the author is travelling in the US but should be contactable by email and mobile: Tel: +1 302 588 7369
Don Porcelli (University of Oxford, UK) N&V author
Tel: +44 1865 282 121; E-mail: don.porcelli@earth.ox.ac.uk