Nature Geoscience 2, 640–643 (2009); published online: 2 August 2009; corrected after print: 22 November 2009.

It has been brought to our attention that the second sentence of the last paragraph on page 641 of this Letter and the sentence beginning “The microstructures are also present” in line 10 on page 642 of this Letter could be ambiguous. Both sentences have been amended in the HTML and PDF versions of the Letter.