Hafnium isotopic variations in volcanic rocks from the Caribbean Large Igneous Province and Galapagos hot spot tracks
Hanan, B. B.
- Publisher: American Geophysical Union.
We report Hf isotope compositions of 79 lavas that record the early (�5–95 Ma) history of the\ud Gala´pagos plume volcanism. These include lavas from the Caribbean Large Igneous Province (CLIP;\ud �95–70 Ma), the accreted Gala´pagos paleo-hot spot track terranes (54–65 Ma) of Costa Rica (Quepos,\ud Osa and Burica igneous complexes), and the Gala´pagos hot spot tracks (<20 Ma) located on the Pacific\ud seafloor (Cocos, Carnegie, Malpelo, and Coiba Ridges and associated seamounts). These samples have\ud previously been well characterized in terms of major and trace elements, Sr-Nd-Pb isotopes and Ar/Ar\ud ages. As a result of the relative immobility of the high field strength and rare earth elements during synand\ud post-emplacement hydrothermal activity and low-temperature alteration, combined Lu-Hf and Sm-Nd\ud isotope systematics, when used in conjunction with Pb isotopes, provide a particular powerful tool, for\ud evaluating the source compositions of ancient and submarine lavas. The combined Nd-Hf isotope data\ud suggest that three of the isotopically distinct source components found today in the Gala´pagos Islands (the\ud Floreana-like southern component, the Fernandina-like central component, and the depleted Genovesa-like\ud eastern component) were present in the CLIP already by 95–70 Ma. The fourth Pinta-like northern\ud component is first recorded at about 83–85 Ma by volcanism taking place during the transition from the\ud plume head/CLIP to plume tail stage and has then been present in the hot spot track continuously\ud thereafter. The identification of the unique northern and southern Gala´pagos Plume Hf-Nd-Pb isotope source signatures within the CLIP and the oldest hot spot track lavas provides direct evidence that the\ud CLIP represents the plume head stage of the Gala´pagos hot spot. Hafnium isotopes are consistent with the\ud possibility that two types of sediment components may have contributed to the Hf, Nd and Pb isotope\ud compositions of the Gala´pagos plume lavas. One component, characterized by D207Pb/204Pb � 0 and high\ud positive DeHf has an isotope signature indicative of relatively recently recycled pelagic sediment, a\ud signature typical of the southern Gala´pagos island Floreana. The other component has an EM like isotopic\ud composition resembling modern seafloor sediments with positive D207Pb/204Pb and lower DeHf, a\ud signature typical of the northern Gala´pagos island Pinta.