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

Basalt, tomography, shock wave experiments, equation of state, mineral physics, volcanology, mantle dynamics

Rowan, Linda Rose ; Ahrens, Thomas J. (advisor)

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: Basalt, tomography, shock wave experiments, equation of state, mineral physics, volcanology, mantle dynamics;   

Others  :  https://thesis.library.caltech.edu/7397/1/Rowan_lr_1993.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

Basalts are the most ubiquitous rocks erupting at the earth's surface at the presenttime and they provide an important probe of the subsurface processes occurringwithin planetary interiors. Recent advances in both mineral physics and seismicanalysis have allowed me to undertake two independent studies related to the genesisand eruption of basaltic magmas. Chapters 1 and 2 are part of an experimental studyconducted in the shock wave laboratory on the equation of state of molten mid-oceanridge 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 atravel time tomography study of the three-dimensional structure of Kilauea Volcano,Hawaii in collaboration with Robert Clayton. Chapter 3 is currently in press in theJournal of Geophysical Research.

The EOS of molten MORB to 20 GPa was accomplished using the innovative silicateliquid shock wave measurement technique on the 40 mm propellant gun developedby Rigden [1986] and Miller [1990). This technique has been used to determinethe EOS for four synthetic melts and this thesis applies the technique to a naturalmelt, a MORB dredged from the Juan de Fuca ridge. The resulting EOS indicatesthat the MORB liquid is very compressible and therefore has a low bulk modulus of11.7 GPa. These results are consistent with low pressure static compression experimentson similar basalts, but are not consistent with the results of ultrasonic interferometry.The compressible nature of the MORB liquid is related to its compositionand this may be expressed best by comparing the MORB Hugoniot to the Hugoniotdetermined for An_(36)Di_(64) and komatiite. The MORB and An_(36)Di_(64) Hugoniots showsignificant increase in density at low pressure followed by a stiffening at high pressureswhere the liquid Hugoniot approaches its respective dense oxide high pressurecomposition. This may be related to gradual coordination changes from four-fold tosix-fold for the Si^(+4) and Al^(+3) which are essentially complete at the high pressurewhere the curve stiffens. The MORB is much more compressible than the komatiiteand overtakes the komatiite in density at a low pressure of 2.5 GPa. This is acompositional effect caused by the enrichment of the MORB in Al_2O_3 and SiO_2 anddepletion in MgO compared to komatiite. The compressible nature of the MORBallows it to become denser than the surrounding mantle near the base of the lowvelocity zone and therefore it is unlikely that MORB can be derived from very deepin the earth's upper mantle.

For most shock wave experiments, the sample is not recovered and nothing canbe determined about its structure or composition due to the passage of the shockwave. In a few of my EOS experiments on molten MORB, however, the shocked samplewas recovered and could be studied in detail. Observations of impact-inducedinteractions between the silicate liquid and its Mo container provide insight intoplanetary impact and differentiation processes involving metal-silicate partitioning.The shocked liquids showed extreme reduction and with increasing pressure the FeOcontent 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 particlesshow a similar texture as those found at impact sites on the earth and moonand 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 intricatemagmatic and tectonic environment. The technique proved to be a powerfulprobe of the volcano's intrusive plumbing because the presence of a dense seismicarray and many local earthquakes allowed for excellent coverage of complex subsurfacefeatures. Analysis and interpretation of the tomographic images leads to thefollowing inferrred model. The main shallow magma reservoir is delineated by aslow anomaly centered 2 km southeast of Halemaumau caldera. There is a distincthigh velocity region centered northwest of the summit from 0 to 2 km depth thatmay represent a dense wall and/or cap of intrusive rock that acts as a barrier orcontainment structure for the northern part of the reservoir. We suggest that theshallow reservoir is a narrow, compartmentalized region of sills and dikes because ofthe closely spaced high and low velocity anomalies near the summit. The rift zonesof Kilauea are imaged as major, high velocity entities, widening to the south withdepth until 6 km. These fast anomalies may be related to the sheeted dike complexesalong the rifts. On a finer scale, magma pockets centered at 0-2 km depth have beeninferred beneath Makaopuhi, Mauna Ulu and Puu Oo, along the east rift zone. TheHilina and Kaoiki fault zones, are imaged as slow features at shallow depths (less than 6km), related to their tensional structures that produce the open fractures and cracksin the basaltic edifice. The Koae fault system is imaged as a slightly fast shallowstructure (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 Hawaiianevents will allow the detailed development of a three-dimensional velocity modelfor Kilauea. Such a model will be extremely useful to seismologists and petrologistsalike for understanding the tectonic growth and magmatic evolution of this dynamicshield volcano.