Volcanology and Petrology of the Taney Seamounts, Northeast Pacific Ocean

Transcription

1 MSc Research Proposal Volcanology and Petrology of the Taney Seamounts, Northeast Pacific Ocean Jason Coumans Introduction: Short chains of seamounts are observed near mid-ocean ridges and have been previously studied, particularly along the East Pacific Rise. These chains of seamounts are commonly oriented perpendicular to the ridge axis, indicating a relationship to plate motions. They appear to be most common at fast spreading ridge segments, but are also found at intermediate spreading segments [1]. The distribution of the seamounts is commonly asymmetric about the ridge axis with more seamounts appearing on one side of the ridge than the other. Morphologically, the volcanoes appear to be truncated cones, in some cases, with complex nested calderas stepping towards the ridge axis. Chronologically, it is assumed that the volcano closest to the ridge is younger than the volcano farthest from the ridge. The presence of large calderas suggests that there is some sort of shallow magma reservoir beneath the seamounts. A recent seismic study of Axial volcano [5] illustrates that there can be shallow magma reservoirs underlying seamounts. The Taney seamounts are an example of a short near ridge seamount chain in the northeast Pacific Ocean. The largest and oldest seamount in this chain (Taney-A, Fig 3) contains well exposed, nested calderas stepping towards the ridge axis. Using the principle of superposition, the nested caldera relationships define a relative chronology. The caldera walls were sampled to obtain stratigraphically controlled samples. This approach strongly contrasts previous seamount studies using dredge techniques for sampling, which eliminates the possibility of stratigraphic control. This study will use the nested calderas in order to study geochemical variation with time on a single seamount, as well as investigate mantle source processes. The Taney Seamounts: The Taney Seamounts (Fig 1) are a linear chain consisting of five aligned submarine volcanoes located on the Pacific Plate approximately 300 km west of San Francisco. Morphologically, the Taney Seamounts are characterized as truncated cones with nested calderas. Seamount Taney-A has been dated at ~26 Ma by Ar-Ar techniques [1] indicating that the seamount ages are close to that of the underlying seafloor. However, uncertainties in the geomagnetic timescale and the Ar-Ar dating [1] poorly constrain age differences between the seamounts and the underlying crust. The volcanoes are composed mainly of pillow lavas, but sheet flows have also been observed along with a volcaniclastic deposit on Taney-C.

2 Figure 1: Geographic relationship of the Taney Seamounts to other northeastern Pacific seamount chains. MBARI.org The largest and oldest seamount in the chain (Taney A, Fig 3) has been clearly modified by at least three successive caldera collapses which are breached towards the ridge axis. Due to the presence of the large calderas, it has been suggested that a shallow (<1km) magma reservoir resides underneath the caldera [1]. Presumably, most magma is transported into ridge-parallel faults as the calderas forms, while the rest fills the newly formed caldera [1].Therefore, the walls of each stepped caldera should systematically expose consecutive lava flows. In the youngest (most eastern) caldera there are pillow mounds representing the last stage of volcanism for Taney-A. Constructional features can also be observed in older, calderas possibly indicating an infilling of magma in the caldera basin.

3 Figure 2: High-resolution swath bathymetry and side-scan images of the Taney Seamounts (hull-mounted 30-kHz Simrad EM300) from [1] Figure 3: High-resolution swath bathymetry and side-scan images of Taney A (hull-mounted 30-kHz Simrad EM300). Available 1 m resolution AUV data overlay EM300. D represent separate dives, the ROV trajectories defined by the black lines. Sample locations are indicated by the red dots and numbered. (Eg. D175-R34 represents sample location 34 on dive 175.) From MBARI

4 Preliminary whole rock XRF data from Taney A has illustrated that there are at least two populations of HFSE. The lavas which encompass the older caldera walls all exhibit enrichment of incompatible Zr relative to the younger eastern pillow mounts which are depleted (Fig 4). A depletion in incompatible elements is counter-intuitive from that expected from fractionation in a shallow magma chamber, indicating that some other process or processes are acting. C1 > C2 > C3 > P Figure 4: Zr ppm versus MgO of Taney-A seamount basalts. There are two distinct populations of lavas with varying Zr concentrations at a given MgO wt.%, illustrating that there is geochemical variability with time at the single seamount scale. When comparing the geochemistry of all the seamounts a different pattern emerges. Both Zr and Nb for Taney-A show a rough positive correlation with TiO 2, which is an incompatible element (Fig 5 a,b). The major difference is that Nb is depleted relative to Zr with decreasing age of the seamounts (Fig 5b). Therefore the Zr/Nb ratio becomes larger with decreasing seamount age (Fig 6a,b). When plotting Zr against Nb, Taney-A and to a lesser degree Taney-C show rough correlations indicating that the two elements behave similarly in the system (Fig 6b). Relationships such as Nb depletion could indicate variations in magma source or melting conditions with time. A B Figure 5 A): Zr versus TiO2 wt%, showing a positive correlation suggesting that both are incompatible of the geochemical range. B): Nb versus TiO2 wt%, showing a positive correlation as well as a depletion of Nb with decreasing seamount age.

5 Zr/Nb A B Taney-A Taney-A Taney-C Taney-C Taney A Taney B Taney C TiO2 wt% Figure 6: A): Zr / Nb versus TiO 2 wt%, showing that there is a progressive decrease in relative Nb causing a Zr / Nb increase with decreasing seamount age. B): Zr versus Nb illustrating a rough correlation for Taney-A (and to a lesser extent Taney-C). Previous Studies of other Near Ridge Seamounts: Other near-ridge seamounts (President Jackson Seamounts) along the Pacific spreading centers exhibit similar physical characteristics to the Taney Seamounts. Like the Taney seamounts, the President Jackson seamounts are a short chain that parallel subaxial asthenospheric motion [2]. The President Jackson Seamount chain consists of eight volcanoes, either isolated cones or morphologically complex flat-topped structures [2]. Much like the Taney seamounts, the isolated cones have larger calderas forming nested sets breached towards the ridge axis. Samples from these seamounts and the related axis magmas were collected by dredging in Geochemically, the President Jackson and the majority of northeastern near-ridge seamounts are primitive in nature >8.0wt% MgO and lack evidence for magma mixing [2]. However, the associated ridge basalts illustrate chemical and mineralogical evidence for magma mixing involving fractionated melts [2]. This suggests that the President Jackson seamounts are fed by batches of melt which rapidly pass through crustal reservoirs underlying the calderas. The lava batch quickly solidifies or is drained into ridge parallel faults before the next batch arrives [2]. Interestingly, the Taney seamounts are more evolved (3-4 wt% MgO) contrasting with the majority of other near-ridge seamounts. This observation is supportive of more complex shallow magma chamber processes at Taney. Geochemical studies of other near-ridge seamounts [3],[4] have shown that the seamounts may have similar, more enriched, or more depleted compositions than the adjacent ridge segment with respect to incompatible elements [1]. Due to the assumption that near-ridge seamount magmas do not mix, they are thought to better preserve mantle sources then the associated ridges. Such a variety of compositions may be due to melting of a heterogeneous mantle. Geochemical variability can be observed on a single seamount and across the chain in all nearridge seamount chain examples.

6 Research Objectives: The main research objective of this project is to characterize the volcanology and petrology of Taney seamount A. Taney-A has been chosen because of well exposed cross-cutting calderas, which provides stratigraphic control absent at Taney-B,C,D. Particular emphasis will be placed on separating and characterizing geochemical signals produced by crustal processes and the mantle source by the use of relative chronology. To achieve this objective it will be necessary to elucidate i) petrographic microscopy of retrieved samples; ii) the major and trace element compositions of lavas with respect to the relative chronology outlined by the well exposed stepped calderas; iii) the compositions and zoning (or lack thereof) profiles for erupted phenocrysts; iv) the major and trace element compositions of glass inclusions. Methodology: Geochemical variations in the relative chronology during the formation of Taney A will be observed through the analysis of both whole rock and glassy margins. Whole rock geochemistry will be examined using XRF data from Washington State University and from McGill, a subset of the total suite. The XRF analyses will provide both major and trace element data. Two representative grains of glass from each Taney A sample with glassy margins will be first analyzed using the electron microprobe. The electron microprobe will provide major element data (SiO 2, TiO 2, Al 2 O 3, Fe 2 O 3, MgO, MnO, CaO, Na 2 O, K 2 O, P 2 O 5 ) as well as S -, Cl -. The glasses will then be examined using LA ICP-MS for trace element and REE concentrations. A bulk solution ICP-MS analyses of the glasses may be considered for comparison purposes, higher accuracy and precision. Picked plagioclase phenocrysts from the glassy margins were set into a grain mount. The grain mounts will be examined under a petrographic microscope and catholodoluminescence for zoning and glass inclusion characterization. The selected phenocrysts and glass inclusions will then be analyzed using the electron microprobe for major elements. Assuming there are glass inclusions of suitable size, a LA ICP-MS study will be useful to obtain the trace elements and REE concentrations. Timeline: (2011) March 8: April: - Electron microprobe analysis of Taney A glass samples. -Petrography and cathodoluminescence of plagioclase phenocrysts. - Electron microprobe analysis of plagioclase Phenocrysts. - Trace element analysis using LA ICP-MS of Taney A glasses. - Electron microprobe analysis of available glass inclusions. - Trace element analysis using LA ICP-MS of glass inclusions. - XRF of Taney A whole rock subset. - Petrography of thin sections, possible polished thin section analysis. May: June: July: August: September - December - Write Thesis

7 Scientific Significance: Previous studies of near-ridge seamounts involve dredge sampling which eliminated the possibility of relative chronology on the single seamount scale. Taney-A has at least three well exposed nested calderas stepping downwards towards the ridge axis. Precise sampling of lavas with an ROV will allow for a geochemical study encompassing relative chronology which would not be possible with dredging. This will provide new insights with regard to shallow level volcanological processes off-axis from ridge settings. Furthermore, a comparison will be made between melt source processes at Taney and at other northeast Pacific ocean seamounts (e.g President Jackson, Vance). A broad goal will be to use the different geochemical and petrological methods to characterize a possible poorly known link between mantle source and crustal processes. References: [1] Clague, David A., Jennifer R. Reynolds, and Alice S. Davis. "Near-ridge Seamount Chains in the Northeastern Pacific Ocean." Journal of Geophysical Research 105.B7 (2000): [2] Davis, Alice S., and David A. Clague. "President Jackson Seamounts, Northern Gorda Ridge: Tectonomagmatic Relationship between On-and Off-axis Volcanism." Journal of Geophysical Research 105.B12 (2000): [3] Fornari, D., M. Perfit, J. Allan, R. Batiza, R. Haymon, A. Barone, W. Ryan, T. Smith, T. Simkin, and M. Luckman. "Geochemical and Structural Studies of the Lamont Seamounts: Seamounts as Indicators of Mantle Processes." Earth and Planetary Science Letters 89.1 (1988): [4] Niu, Yaoling, Marcel Regelous, Immo J. Wendt, Rodey Batiza, and Mike J. O%u2019Hara. "Geochemistry of Near-EPR Seamounts: Importance of Source vs. Process and the Origin of Enriched Mantle Component." Earth and Planetary Science Letters (2002): [5] West, M., W. Menke, M. Tolstoy, S. Webb, and R. Sohn. "Magma Storage beneath Axial Volcano on the Juan De Fuca Mid-ocean Ridge." Nature 413 (2001).