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Elastic properties of pyroxene polymorphs of MgSiO3 and implications for seismic models and discontinuities in the Earth’s upper mantle
Goldschmidt Conference Abstracts 2006
A357
Elastic properties of pyroxene polymorphs of MgSiO3 and implications for seismic models and discontinuities in the Earth’s upper mantle
The process of methane emanation at cold seeps and its correlation with sea-level changes throughout the last 210 thousand years
ROBERT C. LIEBERMANN1, JENNIFER KUNG1,3, BAOSHENG LI1, IAN JACKSON2
V. LIEBETRAU1, A. EISENHAUER1, J. FIETZKE1, K. HAMETNER2, D. GU¨NTHER2, P. LINKE1
1
Department of Geosciences and Mineral Physics University, Stony Brook, NY 11794, USA (Robert.Liebermann@ stonybrook.edu) 2 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia 3 Present address: Department of Earth Sciences, National Cheng Kung University, Tainan 70101, Taiwan Pyroxenes are important mineralogical constituents of petrological models of the Earth’s upper mantle. In the pressure and temperature range of the upper mantle, MgSiO3 pyroxenes exhibit various polymorphs, including orthoenstatite [OEN], protoenstatite [PEN], low-pressure clinoenstatite [LPCEN] and high-pressure clinoenstatit [HPCEN]e. To construct mineralogical models for Earth’s mantle, the pressure and temperature derivatives of the compressional and shear wave velocities of these different polymorphs are essential. In a series of ultrasonic interferometry experiments at high pressures and temperatures in solid-medium, multi-anvil apparatus, we have measured sound velocities in the OEN, HPCEN and LPCEN phases, as well as during the transitions between these phases (Kung et al., 2004, 2005a, 2006). In ultrasonic experiments in an internally heated, gas-medium apparatus, we have measured the wave velocities of an OEN specimen to temperatures of 1373 K at 300 MPa (Kung et al., 2005b). During the course of these ultrasonic experiments, we observe anomalous elastic behavior in these polymorphs, transitions between the polymorphs, and obtain robust data at high P & T for their elastic properties. These data are used to compare with seismic observations for the Earth’s upper mantle, in terms of various petrological models, following the approach suggested in Green and Liebermann (1976). doi:10.1016/j.gca.2006.06.722
1
Leibniz-Institut fu¨r Meeresforschung, IFM-GEOMAR, FB2 Marine Biogeochemie, Wischhofstr. 1-3, 24148-Kiel, Germany ([email protected]) 2 ETH-Ho¨nggerberg, HCI, G 113, 8093 Zu¨rich, Switzerland Cold seep ecosystems are often characterized by microbial mediated carbonate precipitation processes, due to interaction between methane-rich fluid and biological activity. Carbonates from these ecosystems provide unique archives of focussed marine methane emanation by their geochemical, geobiological, mineralogical and structural inventory. Beside geological setting and tectonic activity as general driving forces, oceanographic parameter like water temperature and sea-level changes are potential controls on episodic fluid flow. In this study carbonates, mainly from Hydrate Ridge (Cascadia Margin, Oregon) were investigated in order to decipher the geochemical archive of methane-related massive carbonate build-ups, supplied by a long lasting and focused venting system. Applied methods are MIC-ICP-MS (multi ion counting-inductively coupled-mass spectrometry) for U–Th age data and Laser Ablation (LA)-ICP-MS for high spatial resolution element ratios and concentration data. The age data reflect an episodic reactivation of a fluid pathway over a time interval of at least 210 ky. The coincidence of massive carbonate build-up and glacial climatic intervals point to the possibility that the formation of the chemoherm carbonates and, hence, the activity of the cold seep vent sites are directly related to the height of sea-level via the pressure difference between the height of the seawater column and the hydraulic head and buoyancy of the upward advecting fluids in the plumbing system of the sediments. Furthermore, the data set implies a dependency from the speed of sea-level fall and not only from the reached low stand. Sr and Ba LA-ICP-MS profiles identify high-frequency changes between high and low fluid flow precipitation phases. First results imply a breakthrough of deep seated fluids from below the bottom simulating reflector (BSR) between 55 and 42 ky after a phase of extensive gas hydrate destabilization. In order to reflect the observed correlation of paleoactivity of cold vents and sea-level changes, methane-related carbonates from different regions are topic of our actual research. The important relation to the chronology of changes in atmospheric greenhouse-gas concentration will be discussed. doi:10.1016/j.gca.2006.06.723
A357
Elastic properties of pyroxene polymorphs of MgSiO3 and implications for seismic models and discontinuities in the Earth’s upper mantle
The process of methane emanation at cold seeps and its correlation with sea-level changes throughout the last 210 thousand years
ROBERT C. LIEBERMANN1, JENNIFER KUNG1,3, BAOSHENG LI1, IAN JACKSON2
V. LIEBETRAU1, A. EISENHAUER1, J. FIETZKE1, K. HAMETNER2, D. GU¨NTHER2, P. LINKE1
1
Department of Geosciences and Mineral Physics University, Stony Brook, NY 11794, USA (Robert.Liebermann@ stonybrook.edu) 2 Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia 3 Present address: Department of Earth Sciences, National Cheng Kung University, Tainan 70101, Taiwan Pyroxenes are important mineralogical constituents of petrological models of the Earth’s upper mantle. In the pressure and temperature range of the upper mantle, MgSiO3 pyroxenes exhibit various polymorphs, including orthoenstatite [OEN], protoenstatite [PEN], low-pressure clinoenstatite [LPCEN] and high-pressure clinoenstatit [HPCEN]e. To construct mineralogical models for Earth’s mantle, the pressure and temperature derivatives of the compressional and shear wave velocities of these different polymorphs are essential. In a series of ultrasonic interferometry experiments at high pressures and temperatures in solid-medium, multi-anvil apparatus, we have measured sound velocities in the OEN, HPCEN and LPCEN phases, as well as during the transitions between these phases (Kung et al., 2004, 2005a, 2006). In ultrasonic experiments in an internally heated, gas-medium apparatus, we have measured the wave velocities of an OEN specimen to temperatures of 1373 K at 300 MPa (Kung et al., 2005b). During the course of these ultrasonic experiments, we observe anomalous elastic behavior in these polymorphs, transitions between the polymorphs, and obtain robust data at high P & T for their elastic properties. These data are used to compare with seismic observations for the Earth’s upper mantle, in terms of various petrological models, following the approach suggested in Green and Liebermann (1976). doi:10.1016/j.gca.2006.06.722
1
Leibniz-Institut fu¨r Meeresforschung, IFM-GEOMAR, FB2 Marine Biogeochemie, Wischhofstr. 1-3, 24148-Kiel, Germany ([email protected]) 2 ETH-Ho¨nggerberg, HCI, G 113, 8093 Zu¨rich, Switzerland Cold seep ecosystems are often characterized by microbial mediated carbonate precipitation processes, due to interaction between methane-rich fluid and biological activity. Carbonates from these ecosystems provide unique archives of focussed marine methane emanation by their geochemical, geobiological, mineralogical and structural inventory. Beside geological setting and tectonic activity as general driving forces, oceanographic parameter like water temperature and sea-level changes are potential controls on episodic fluid flow. In this study carbonates, mainly from Hydrate Ridge (Cascadia Margin, Oregon) were investigated in order to decipher the geochemical archive of methane-related massive carbonate build-ups, supplied by a long lasting and focused venting system. Applied methods are MIC-ICP-MS (multi ion counting-inductively coupled-mass spectrometry) for U–Th age data and Laser Ablation (LA)-ICP-MS for high spatial resolution element ratios and concentration data. The age data reflect an episodic reactivation of a fluid pathway over a time interval of at least 210 ky. The coincidence of massive carbonate build-up and glacial climatic intervals point to the possibility that the formation of the chemoherm carbonates and, hence, the activity of the cold seep vent sites are directly related to the height of sea-level via the pressure difference between the height of the seawater column and the hydraulic head and buoyancy of the upward advecting fluids in the plumbing system of the sediments. Furthermore, the data set implies a dependency from the speed of sea-level fall and not only from the reached low stand. Sr and Ba LA-ICP-MS profiles identify high-frequency changes between high and low fluid flow precipitation phases. First results imply a breakthrough of deep seated fluids from below the bottom simulating reflector (BSR) between 55 and 42 ky after a phase of extensive gas hydrate destabilization. In order to reflect the observed correlation of paleoactivity of cold vents and sea-level changes, methane-related carbonates from different regions are topic of our actual research. The important relation to the chronology of changes in atmospheric greenhouse-gas concentration will be discussed. doi:10.1016/j.gca.2006.06.723