Joint Program students enrolled in the Geodynamics seminar are required to complete a project for the class. This includes research, an oral presentation during the last two or three seminar meetings, and a written paper due at the end of the semester. For first and second year students, the project must be on a topic related to the theme of the seminar and must be different from their main research interest. For more advanced students, the topic may be closely related to their dissertation research. The following paragraphs are a sampling of some of the student projects.
Solid advection in plumes: Implications on melting and melt transport
Several geochemical parameters have been defined which distinguish between lava derived from normal mid-ocean ridge sources and more enriched plume sources. The spatial and temporal distribution of these geochemical indicators have been used to kinematic models dynamic melting in the shallow mantle and the history of the position of the plume with respect to the surface. However, not much attention has been given to the physics of advection which place these two mantle sources in contact in regions where plumes and spreading centers interact. In addition, buoyant solid advection can induce pressure gradients which focus melt transport to the plume center. Using numerical models I explore the advective patterns of upwelling plumes and the effects entrainment has on the distribution of plume and MORB sources in the melting region. In addition, I examine the variation in the pressure field induced by solid flow to constrain the effects of plume-induced buoyancy-driven flow on melt migration.
Mark Behn, John Sinton, and Bob Detrick
Effect of the Galapagos Hotspot on Seamount Formation along the Galapagos Spreading Center
Studies along the Mid-Atlantic Ridge (MAR) and East Pacific Rise (EPR) have shown seamount formation to be a function of spreading rate. At the MAR, seamounts are a dominant morphologic feature of the inner valley floor, while at the EPR seamounts are rarely observed within the neovolcanic zone. The Galápagos Spreading Center (GSC) provides an excellent location to test the influence of a hotspot on the process of seamount generation at a relatively constant spreading rate. In this study we use multi-beam bathymetry data acquired during the G-PRIME cruise in April-May, 2000 to examine the distribution of axial seamounts along the GSC with distance from the hotspot. We use a numerical algorithm to identify isolated volcanic edifices, by searching bathymetry for closed, concentric contours protruding >50 meters above the surrounding seafloor. Seamount populations are fit with an exponential size-frequency model to estimate the total number of seamounts per unit area, no, and the characteristic seamount height, b-1. At distances >500 km from the hotspot (west of the 95.5ºW propagator) we find the total number of seamounts per unit area (no = 279 ± 16 per 103 km2) to be similar to the number observed along the MAR, but with a slightly lower characteristic height (b-1 = 36 ± 2 m). The parameter no is found to decrease systematically with decreasing distance to the hotspot. East of 92.6ºW, we find no to be 50 ± 9 per 103 km2, similar to the value observed at the EPR . In contrast, the characteristic height is not observed to vary dramatically along the GSC. We interpret these results to imply that the Galápagos hotspot plays an important role in controlling the style of volcanism along the GSC. Away from the hotspot the GSC behaves like a typical slow-to-intermediate spreading ridge, with seamounts generated by "point source" volcanic activity dominating the small-scale morphology of the inner valley floor. However, along regions of the GSC where gravity, seismic, and bathymetric data show the effect of the hotspot to be greatest, seamount formation becomes less significant, probably because low-relief lava flows emplaced by fissure eruptions along-axis are the dominant form of constructional volcanism.
Axial Volcano: Ridge-Hot Spot Interaction
Following a January 1998 diking/eruption event, a local network of seven ocean bottom hydrophones were deployed on Axial volcano, a hot spot rising 700 m above the nearby Juan de Fuca ridge. Sohn et al. (1999) estimated hypocenter locations for local events observed over a 166 day deployment by tracing rays from the instruments to a three dimensional grid superimposed on the study region and searching the grid for the minimum weighted rms residual between observed and calculated compression wave arrival times. A one dimensional velocity model for the nearby Cleft segment of the JdFR, corrected for Axial segment bathymetry, was used as the crustal velocity model in the study area. Since Sohn et al. (1999), a new compression velocity image of the volcano has emerged based on data collected during a 1999 active seismic experiment. Hydrophone data has already begun to be reanalyzed using this recent velocity profile. In this study, I will estimate hypocenter locations for the local events observed during the initial 166 day deployment, as well as new data retrieved from the hydrophones after the initial deployment using the new compressional velocity profile. By determining hypocenter locations for local, mircoscale earthquakes, it is hoped that we will be able to determine fault zones within Axial volcano. At shallow depths fault zones and hypocenter locations may be indicative of increased hydrothermal fluid flow, while at deeper depths, hypocenter locations may reveal melt migration pathways. Knowing the location of the fault zones also becomes important for modeling tectonic movement. Axial volcano lies along a rifting margin where the JdFR is deflected to meet the hot spot. Despite this deflection, there appears to be no steady-state connectivity between the magma chamber on Coaxial segment and the massive magma body supplying Axial Volcano. Information gleaned from seismic data and coupled with thermal and rheological models for the Axial Volcano/JdFR hot spot/ridge system, will allow us to better understand ridge-hot spot interactions.
Astri Kvassnes and Nobu Shimizu
A profile through the lower crust of an arc: Evidence for closed-system fractionation and local melt diversity
We have analyzed trace elements (Ti, V, Cr, Sr, Zr and Y) and REE (La, Ce, Nd, Sm, Dy, Eu, Er, Yb) a total of ten samples from three locations in the Lyngen Magmatic Complex, an ophiolitic arc-system dominated by gabbroic rocks in Northern Norway. The western suite of the complex, which stretches 100-km N-S and 10 km E-W, has previously been shown to have fractionation-trends that are similar to those found at mid-ocean ridges or back-arc basins. The eastern suite has been suggested to represent the cumulate section of a boninitic arc. The two suites are separated by an oceanic shear-zone associated with rocks with "normal" arc signatures. The three sections analyzed in this study are from the western suite, the transition zone and from the eastern suite. The sections all comprise layered mafic and ultramafic cumulates shown to have different fractionation trends (Kvassnes, 1997; Hetland, 1997). The samples were selected based on petrological and geochemical diversity. We show the trace-element fractionation of the cumulates, and how the cumulate-section of the eastern suite possibly originated from several different inter-fingering magmas. Additionally, we show negative Eu-anomalies in the most evolved cumulates, indicating closed system fractionation due to the crystallization of clinopyroxene together with plagioclase.