- Webb Hall 1100
Jason Schmidt and Eric Schoettle. both UCSB graduate students, will each present a talk at Speakers Club on Thursday at 2:00 PM in Webb Hall 1100. Jason's talk is entitled Unraveling the petrogenesis of an isotopically enriched Samoan submarine lava dredge and Eric's talk is entitled Calculating uplift rates and timing in California’s Sierra Madre Mountains using Cosmogenic Radionuclide.
The Samoan islands and seamounts constitute an age progressive hot spot track located just north of the northern terminus of the Tonga trench. It is believed that an upwelling mantle plume, undergoing decompression partial melting, is responsible for generating the anomalous intra-plate volcanism in the region. Constraints on the extent of mantle heterogeneity are provided in part by studying the geochemistry and petrology of regional basalts that originate within the upper mantle. These geochemical signals are best expressed in ocean island basaltic (OIB) volcanoes. OIB lavas that exhibit the highest 87Sr/86Sr are thought to sample recycled terrigenous material derived from the upper continental crust, and therefore serve to constrain the Enriched Mantle 2 (EM2) endmember. Trachybasaltic to trachyandesitic submarine lavas dredged from the SW flanks of the westernmost Samoan island of Savai’i (ALIA-115 dredge lavas) exhibit the highest 87Sr/86Sr identified in an OIB to-date with whole rock 87Sr/86Sr up to 0.720469 (ALIA-115-21), and clinopyroxene antecrysts from the most geochemically-enriched lavas exhibit even higher 87Sr/86Sr ratios, up to 0.72163 (Jackson et al, 2007). Previously, combination of mixing and melting models suggested that the most enriched Samoan EM2 lavas could be magmas from a mantle source with 5 to 6% terrigenous sediment with 87Sr/86Sr in excess of 0.74 (Jackson et al, 2007). The observation of a strong correlation between 87Sr/86Sr and SiO2, as well as excellent correlation between 87Sr/86Sr and 143Nd/144Nd suggests that the ALIA-115 lavas could be products of magma (or mantle source) mixing. Here we present new major element data on clinopyroxene (cpx) hosted melt inclusions (MIs) from one of the most geochemically enriched lavas in the Samoan hot spot—Savai’i sample ALIA-115-18 —which has a whole rock 87Sr/86Sr of 0.718592 (Jackson et al., 2007). We utilize a thorough, canonical test for mixing to show that the petrogenesis of the ALIA-115 lavas are indeed best explained as products of magma (or mantle source) mixing between an isotopically enriched, high SiO2 endmember and a more primitive, isotopically depleted, lower SiO2 endmember. Major element analyses and modeling of diffusion and phase equilibria are used to constrain the initial composition of the MIs at time of entrapment and to predict their likely isotopic composition. Results suggest that the variability in MI composition, as well as their anomalously high SiO2 relative to their host lava, is an inherent property and not derived from post entrapment fractional crystallization (PEC). Analysis of host pyroxene compositional transects near MI rims suggests that post entrapment diffusion (PED) has modified major element ratios in the MIs. We present an innovative analysis strategy for maximizing the mixing signal (i.e. ‘seeing through’ PED), lending added weight to the mixing hypothesis. The predicted isotopic composition of the MIs is consistent with an extreme EM2 composition, and taken in conjunction with persuasive evidence for mixing, suggests the MIs could be representative of a relatively unmodified EM2 source partial melt.
Uplift rates and timing are essential to the study of tectonic geomorphology: uplift rates constrain the rates and types of surface processes in a landscape whereas timing connects tectonics to the broader geologic and tectonic story. However, they are difficult to obtain and as such are unknown or poorly constrained in many tectonically active regions. In this study we use cosmogenic radionuclide (CRN) techniques to calculate the rate and timing of uplift of California’s Sierra Madre Mountains, a minimally studied range within California’s coast range. To do this we use the stream profiling and the water gap formed by the Cuyama Gorge to argue that the landscape is in steady state, and allowing us to convert incision rate, calculated from exposure dating of bedrock strath terraces, to uplift rate. These rates are compared to catchment-averaged erosion rates obtained from detrital CRN analysis, which are in turn combined with bedrock lowering rates from a relict landscape to yield the age of onset of uplift. Uplift rates of the Sierra Madre are calculated to be approximately 1-2 mm/yr, beginning about 500,000 years ago. These rates are rapid compared to those found elsewhere in the Coast Ranges and are comparable to rates found in much more substantial mountain ranges.