This proposal is for a combined geochronologic, petrographic, geochemical, and Sr-Nd-Pb-Os-O isotopic investigation of existing lava samples from the seamounts in the Adare Basin and from the adjacent continental Hallett Volcanic Province in Northern Victoria Land. A comparative study of contemporaneous intraplate oceanic and continental alkaline volcanism will be used to evaluate sources and processes responsible for magma production and differentiation. The seamount lavas will provide an opportunity to better characterize the composition of their underlying mantle source whereas the continental lavas will provide information on any effects of contamination by continental crust on their source signatures. The cause of Cenozoic magmatism in West Antarctica has been explained by a variety of models based primarily on geochemical evidence from continental volcanism. The proposed investigation will for the first time provide detailed geochronologic and geochemical data for both continental lavas and adjacent oceanic seamount lavas, thus offering a means by which to critically assess whether Cenozoic alkaline volcanism is due to plume or plume-related activities or magmatism unrelated to mantle plumes. The classic element of this controversy is whether the geochemistry of the magmas represents melting of mantle asthenosphere or subcontinental lithosphere and whether there is a degree of contamination as they rise through relatively thick and chemically heterogeneous continental crust.
Broader impacts: This project will support a PhD and MSc student. Undergraduate students will also be involved in sample preparation, analytical work and interpretation via honors/senior theses. A post-doctoral associate will be involved in the oxygen isotopic analysis of the samples. The students will be involved in a multi-institutional and international collaboration. The geochemical database generated will be made available to other researchers in the region and the results will be published in peer-reviewed journals.
According to the plate tectonics theory, the Earth’s outer layer consists of lithospheric plates that are constantly in motion and interacting with each another. A major manifestation of such phenomena is continental rifting. Two major geologic processes are complexly intertwined in continental rifting – tectonic and magmatic. Tectonic processes result from the buildup of stresses due to the interactions of lithospheric plates and/or release of pressure of magmas melted from the Earth’s mantle at depth. On the other hand, magmatic processes result from the escape of magmas from the mantle and/or in response to the tectonic processes on the surface. The composition of mantle melts or magmas such as that preserved in volcanic rocks can help scientists better constrain why continents break apart. Specifically, a popular working hypothesis is that continents break apart when they hover over hotspots (i.e. hot plumes rising from depth) in the mantle where copious melting occurs. This is referred to as "active" rifting. In general, geochemically "enriched" volcanic rocks from hotspots can be readily recognized when they are erupted through the normal, geochemically "depleted" oceanic plate; however, these are often hard to identify when erupted through the thicker, compositionally more heterogeneous and likewise geochemically enriched continental plate. Cenozoic magmatism associated with the west Antarctic rift system (WARS) has important implications for models of continental rifting. Although Cenozoic volcanic lavas in the WARS are believed to be localized along fault zones and, thus, WARS is considered magma-poor, some scientists propose that rifting here is active and mainly due to hotspot (plume) magmatism. In contrast, other scientists call upon continental magmatism caused by rifting ("passive"). Incidentally, previously available data for volcanic rocks from West Antarctica associated with rifting possess equivocal signals of either hotspot magmatism (i.e. active) contaminated by continental material or simply continental magmatism sampled through tectonism (i.e. passive). Through a pilot study funded by NSF-OPP, P.I. Pat Castillo from Scripps Institution of Oceanography, UCSD and co-P.I. Kurt Panter from Bowling Green State University were able to sample volcanic rocks through dredging of submarine volcanoes and outcrops of the seafloor in Adare Basin in the northwestern Ross Sea region (Fig. 1) for the first time during the NBP0701 cruise in the Ross Sea from Dec. 2006 to Jan. 2007. Now, oceanic volcanic rocks from Adare Basin are available for comparison with continental samples from West Antarctica. Together, these samples provide a compositional transect that allows an investigation of whether a continent-type compositional signal disappears from continental to oceanic environment or, conversely, a hotspot-type compositional signal disappears away from the ocean as it gets diluted/contaminated towards the continent. Major element, trace element and Sr-Nd-Pb-Os-O isotopic compositions of the volcanic rocks from Adare Basin seamounts (Fig. 1) resemble those from ocean islands (e.g., Hawaii) with a distinct, hotspot affinity. Furthermore, the compositions of the seamounts are remarkably similar to volcanic rocks from the continent adjacent to the Adare Basin. However, although the compositions are overall similar, key geochemical indicators of magma sources and processes vary systematically from continent to ocean. The orderly compositional variation toward the continent suggests that the volcanism was generated from a common mantle source at progressively greater melt fractions. Small degree partial melting preferentially taps a more easily fusible, hotspot-like component of the mantle whereas higher degree melting incorporates greater proportions of the adjacent ambient mantle. Significantly, the oxygen and osmium isotopic results for both continental and oceanic samples are consistent with such a partial melting scenario although they also suggest a small, but steady increase in crustal contamination toward the continent. Although the compositional signatures of the volcanism are hot-spot-like other evidence (e.g., mineral thermobarometry, melting models using trace element chemistry, age dating) indicate that the melting is originating within the relatively shallow mantle lithosphere and not from a plume, thus supporting a passive model for volcanism.
