Notes for Supplementary Data Table

J values are based upon unencapsulated MMhb-1 hornblende standards with an assumed age of 520.4 Ma and ages are calculated with no correction for recoil loss. Volumes for Ar isotopes are in units of 1x10^-13 mlSTP. Error estimates are based on machine precision at the 1s level. The fraction of ^39Ar released (F39) is based solely on the release of ^39Ar within each individual analysis and is not corrected for the procedural blank. The total gas and retention ages are similarly not corrected for the procedural blank. ^36Ar volumes are corrected for interference from Ca and the decay of ^36Cl. ^37Ar and ^39Ar are corrected for decay from the effective midpoint of the neutron irradiation. ^38Ar volumes contain contributions from K, Cl and atmospheric Ar. ^39Ar is corrected for interference from Ca and ^40Ar is corrected for interference from K. All Ar isotope measurements were corrected for mass discrimination based upon daily measurements of atmospheric Ar.