Shatsky Rise: Ar/Ar Geochronology and geochemistry of igneous rock samples from the Shatsky Rise Plateau (northwest Pacific Ocean)
Zusammenfassung der Projektergebnisse
The Shatsky Rise was drilled during IODP Exp. 324 (in 2009) at 5 individual locations on the three distinctive volcanic edifices of this oceanic plateau (Tamu, Shirshov and Ori). The recovered igneous rocks consist of massive basalts, pillow lavas and lava flows. Together with international collaboration partners, we analyzed samples from all three volcanic edifices to establish a comprehensive Sr-Nd-Pb-Hf isotope data set for the entire Shatsky Rise oceanic plateau and to constrain the magmatic source(s) and geochemical evolution of the plateau. The Nd, Hf, and Pb isotopic characteristics of Shatsky Rise lavas are largely consistent with the suggested genetic systematics based on trace element composition. The Normal and Low-Ti groups are isotopically identical supporting a common source. U1349 group lavas yield the most depleted isotope composition and overlap with Mesozoic and modern (agecorrected) Pacific MORB and may represent the local, upper mantle MORB source at the time of Shatsky Rise formation. The isotopic composition of the High-Nb group lavas suggests an involvement of isotopically enriched (EM-like) compositions possible representing recycled continental material (lower continental crust or subcontinental lithosphere). From the oldest Tamu Massif to younger Ori and Shirshov Massifs we observe that the source has become more heterogeneous with time. At least three isotopically distinct components can be identified in the magma source: 1) a volumetrically and spatially moderately depleted component of similar composition to modern East Pacific Ridge basalt, 2) an isotopically very depleted component which could represent local, early Cretaceous (entrained) depleted upper mantle, and 3) an isotopically enriched component, indicating the presence of (recycled) enriched (continental) material in the magma source. The majority of analyzed Shatsky lavas (i.e. U1349 type and the least radiogenic Normal and Low-Ti samples), however, possess Nd-Hf-Pb isotope compositions consistent with a derivation from a depleted (during the early history of the earth), nonchondritic, primitive reservoir (as a possible common source for all large oceanic igneous plateaus). Whereas lavas from the oldest edifice (Tamu) yield fairly uniform Nd-Pb-Hf isotopic compositions, a wider isotopic spread was found in lavas erupted on the younger massifs (Ori and Shirshov), suggesting that the Shatsky magma source become more heterogeneous through time. By comparing these results with petrological and other geochemical data, we conclude that Shatsky Rise was most like formed by interaction of a mantle plume (head) with a tripple junction. A second study on the temporal evolution of the Shatsky Rise is in preparation in collaboration with Anthony Koppers from OSU. Preliminary Ar-Ar age data indicate an age of 145 Ma ± 1 Ma (GEOMAR) for the stratigraphically oldest units drilled on Tamu massif i.e. unit XV, with distinctly younger ages of 140.2 ± 1.9 Ma (OSU) and 134 ± 2 Ma (GEOMAR) for the uppermost igneous units IX and IV respectively. This points to a decrease in volcanic activity during the waning stages of growth of the Tamu volcanic edifice, as indicated by intercalated layers of volcaniclastic sandstone. The integration of GEOMAR and OSU data will help to reconstruct the comprehensive temporal evolution for the whole Shatsky Rise. The main goals of project, the establishment of Sr-Nd-Pb-Hf and Ar-Ar isotope data sets for Sites U1347 and U1350, to evaluate the origin, temporal and geochemical evolution of the Shatsky Rise have been attained. With these new data, we have gained additional valuable insights into the evolution and the origin of the Shatsky Rise oceanic plateau.
Projektbezogene Publikationen (Auswahl)
- (2011) The origin of an oceanic plateau: Isotope geochemistry (Sr, Nd, Pb and Hf) of volcanic rocks from IODP Site U1347 and ODP Site 1213 (Hf data) on the Shatsky Rise (Northwest Pacific) In: AGU Fall Meeting 2011, 05.-09.12.2011, San Francisco, USA
Heydolph, K., Geldmacher, J. and Hoernle, K.
- (2012) New Insights into the Origin of the Shatsky Rise from Sr-Nd-Pb-Hf Isotopes of Volcanic Rocks from IODP Exp. 324 In: Goldschmidt Conference 2012, 24.-29.06.2012, Montreal, Canada
Heydolph, K., Geldmacher, J., Hoernle, K.
