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Equation of state of molten mid-ocean ridge basalt. Structure of Kilauea volcano.

Citation

Rowan, Linda Rose (1993) Equation of state of molten mid-ocean ridge basalt. Structure of Kilauea volcano. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/J6D0-GV56. https://resolver.caltech.edu/CaltechTHESIS:01142013-104747074

Abstract

Basalts are the most ubiquitous rocks erupting at the earth's surface at the present time and they provide an important probe of the subsurface processes occurring within planetary interiors. Recent advances in both mineral physics and seismic analysis have allowed me to undertake two independent studies related to the genesis and eruption of basaltic magmas. Chapters 1 and 2 are part of an experimental study conducted in the shock wave laboratory on the equation of state of molten mid-ocean ridge basalt and the chemical interactions of the shocked liquid with its Mo container. My advisors for this project were Thomas Ahrens and Edward Stolper. Chapter 3 is a travel time tomography study of the three-dimensional structure of Kilauea Volcano, Hawaii in collaboration with Robert Clayton. Chapter 3 is currently in press in the Journal of Geophysical Research.

The EOS of molten MORB to 20 GPa was accomplished using the innovative silicate liquid shock wave measurement technique on the 40 mm propellant gun developed by Rigden [1986] and Miller [1990). This technique has been used to determine the EOS for four synthetic melts and this thesis applies the technique to a natural melt, a MORB dredged from the Juan de Fuca ridge. The resulting EOS indicates that the MORB liquid is very compressible and therefore has a low bulk modulus of 11.7 GPa. These results are consistent with low pressure static compression experiments on similar basalts, but are not consistent with the results of ultrasonic interferometry. The compressible nature of the MORB liquid is related to its composition and this may be expressed best by comparing the MORB Hugoniot to the Hugoniot determined for An_(36)Di_(64) and komatiite. The MORB and An_(36)Di_(64) Hugoniots show significant increase in density at low pressure followed by a stiffening at high pressures where the liquid Hugoniot approaches its respective dense oxide high pressure composition. This may be related to gradual coordination changes from four-fold to six-fold for the Si^(+4) and Al^(+3) which are essentially complete at the high pressure where the curve stiffens. The MORB is much more compressible than the komatiite and overtakes the komatiite in density at a low pressure of 2.5 GPa. This is a compositional effect caused by the enrichment of the MORB in Al_2O_3 and SiO_2 and depletion in MgO compared to komatiite. The compressible nature of the MORB allows it to become denser than the surrounding mantle near the base of the low velocity zone and therefore it is unlikely that MORB can be derived from very deep in the earth's upper mantle.

For most shock wave experiments, the sample is not recovered and nothing can be determined about its structure or composition due to the passage of the shock wave. In a few of my EOS experiments on molten MORB, however, the shocked sample was recovered and could be studied in detail. Observations of impact-induced interactions between the silicate liquid and its Mo container provide insight into planetary impact and differentiation processes involving metal-silicate partitioning. The shocked liquids showed extreme reduction and with increasing pressure the FeO content of the initial melt was reduced to almost nothing by reaction with the Mo. These reactions produced metallic particles enriched in Mo, Fe and Si. These particles show a similar texture as those found at impact sites on the earth and moon and provide clues to the impact origin of metallic particles.

A travel time tomography study of local P wave data from Kilauea Volcano, Hawaii, was undertaken to determine the lateral heterogeneities produced by its intricate magmatic and tectonic environment. The technique proved to be a powerful probe of the volcano's intrusive plumbing because the presence of a dense seismic array and many local earthquakes allowed for excellent coverage of complex subsurface features. Analysis and interpretation of the tomographic images leads to the following inferrred model. The main shallow magma reservoir is delineated by a slow anomaly centered 2 km southeast of Halemaumau caldera. There is a distinct high velocity region centered northwest of the summit from 0 to 2 km depth that may represent a dense wall and/or cap of intrusive rock that acts as a barrier or containment structure for the northern part of the reservoir. We suggest that the shallow reservoir is a narrow, compartmentalized region of sills and dikes because of the closely spaced high and low velocity anomalies near the summit. The rift zones of Kilauea are imaged as major, high velocity entities, widening to the south with depth until 6 km. These fast anomalies may be related to the sheeted dike complexes along the rifts. On a finer scale, magma pockets centered at 0-2 km depth have been inferred beneath Makaopuhi, Mauna Ulu and Puu Oo, along the east rift zone. The Hilina and Kaoiki fault zones, are imaged as slow features at shallow depths (less than 6 km), related to their tensional structures that produce the open fractures and cracks in the basaltic edifice. The Koae fault system is imaged as a slightly fast shallow structure (less than 6 km) possibly related to intrusive diking from the adjacent rift zones. Continued inversions with the immense amount of seismic data collected for Hawaiian events will allow the detailed development of a three-dimensional velocity model for Kilauea. Such a model will be extremely useful to seismologists and petrologists alike for understanding the tectonic growth and magmatic evolution of this dynamic shield volcano.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Basalt, tomography, shock wave experiments, equation of state, mineral physics, volcanology, mantle dynamics
Degree Grantor:California Institute of Technology
Division:Geological and Planetary Sciences
Major Option:Geology
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Ahrens, Thomas J. (advisor)
  • Stolper, Edward M. (advisor)
  • Clayton, Robert W. (advisor)
Thesis Committee:
  • Ahrens, Thomas J.
  • Stolper, Edward M.
  • Clayton, Robert W.
  • Rossman, George Robert
  • Westphal, James A.
Defense Date:6 January 1993
Non-Caltech Author Email:rowan (AT) alumni.caltech.edu
Record Number:CaltechTHESIS:01142013-104747074
Persistent URL:https://resolver.caltech.edu/CaltechTHESIS:01142013-104747074
DOI:10.7907/J6D0-GV56
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:7397
Collection:CaltechTHESIS
Deposited By: Benjamin Perez
Deposited On:14 Jan 2013 19:18
Last Modified:21 Dec 2019 02:14

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