Paul D. Asimow

Research

My research combines thermodynamic and quantum modeling, experimental investigation by static and shockwave techniques, and analysis of actual rocks from the world’s oceans. The goal of all these studies is to characterize the mineralogy and melting of the top and bottom of earth's mantle, the formation of crust, and the nature of the core-mantle boundary, and to understand the role of these processes in the long-term differentiation and active heat engine of the earth.

Melting of the mantle by decompression at mid-ocean ridges and hotspots is a complex process. It takes place over a range of pressures and temperatures and may involve mutiple source compositions. The liquids are probably continuously extracted from the residue and mixed at some point during transit to the crust or in shallow magma chambers where the lavas differentiate before eruption. No single experiment can describe this entire process, so we use models of mantle melting, melt migration, and fractionation to try to understand what information is encoded in data like crustal thickness and composition of basalts and peridotites.

Our essential constraint on the progress of mantle melting is that it should be close to an adiabatic process. Under such a constraint, unfortunately, temperature is a dependent variable. Unfortunate because temperature is easy to understand and to control in the laboratory. Instead, mantle processes take place approximately at constant entropy (S) and controlled pressure (P). In an effort to understand the significance of this constraint, my work has combined analysis of simple systems that we can understand completely (such as the one-component system represented in pressure-entropy space in the figure) with a self-consistent thermodynamic model of silicate-liquid equilibria, MELTS. Marc Hirschmann, Ed Stolper, Mark Ghiorso, myself, and others have explored in a series of papers what MELTS tell us about mantle melting as currently calibrated, as well as where the shortcomings are that need to be corrected if the next generation of MELTS is going to tell us more. We have recently released our own front-end to the MELTS, pMELTS, and pHMELTS models, adiabat_1ph, which you can download for Windows, MacOS, or Linux.

Much of my recent work has focused on incorporating new ideas about the behavior of trace quantities of water in the mantle source region into melting models for normal and hotspot-affected mid-ocean ridges and back-arc basins. The discovery that water dissolves in nominally anhydrous mantle minerals and partitions into melts like a light rare-earth element allowed me to construct a hybrid model that considers the effects of water on melting equilibria in dynamic, near-fractional polybaric melting regimes.

Asimow PD (1997) A Thermodynamic Model of Adiabatic Melting of the Mantle, Ph.D. Thesis, California Institute of Technology. Advisor EM Stolper. Published on microfilm by UMI.

Asimow PD, Hirschmann MM, Ghiorso MS, O'Hara MJ, Stolper EM (1995) The effect of pressure-induced solid-solid phase transitions on decompression melting of the mantle. Geochimica et Cosmochimica Acta, 59:4489-4506.

Asimow PD, Hirschmann MM, Stolper EM (1997) An analysis of variations in isentropic melt productivity. Philosophical Transactions of the Royal Society of London, Series A, 355:255-281.

Hirschmann MM, Ghiorso MS, Wasylenki LE, Asimow PD, and Stolper EM (1998) Calculations of Peridotite Partial Melting from Thermodynamic Models of Minerals and Melts. I. Methods and comparison to experiments, Journal of Petrology, 39:1091-1115.

Asimow PD and Ghiorso MS (1998) Algorithmic Modifications Extending MELTS to Calculate Subsolidus Phase Relations, American Mineralogist, 83:1127-1132.

Asimow PD and Stolper EM (1999) Steady-state Mantle-Melt Interactions in One Dimension. 1: Equilibrium transport and Melt focusing, Journal of Petrology, 40:475-494.

Hirschmann MM, Asimow PD, Ghiorso MS, and Stolper EM (1999) Calculation of Peridotite Partial Melting from Thermodynamic Models of Minerals and Melts. III. Controls on Isobaric Melt Production and the Effect of Water on Melt Production, Journal of Petrology, 40:831-851.

Asimow PD (1999) Melting the Mantle, in H. Sigurdsson, editor, Encyclopedia of Volcanoes, Academic Press, Forthcoming.

Asimow PD (1999) A Model that Reconciles Major- and Trace-element Data from Abyssal Peridotites, Earth and Planetary Science Letters, 169:303-319.

Asimow PD, Hirschmann MM & Stolper EM (2001) Calculations of Peridotite Partial Melting from Thermodynamic Models of Minerals and Melts. IV. Adiabatic Decompression and the Composition and Mean Properties of Mid-ocean Ridge Basalts, Journal of Petrology, 42:963-998.

Asimow PD (2002) Steady-state Mantle-Melt Interactions in One Dimension. II: Thermal Interactions and Irreversible Terms, Journal of Petrology, 43:1707-1724.

Asimow PD and Langmuir CH (2003) The importance of water to oceanic mantle melting regimes, Nature, 421:815-820.

Asimow PD, Dixon JE & Langmuir CH (2004) A hydrous melting and fractionation model for mid-ocean ridge basalts: Application to the Mid-Atlantic Ridge near the Azores, Geochemistry Geophysics Geosystems 5(1):Q01E16, doi:10.1029/2003GC000568.

Asimow PD and Longhi J (2004) The significance of multiple saturation points in the context of polybaric near-fractional melting, Journal of Petrology 45:2349-2367, doi:10.1093/petrology/egh043.

Asimow PD (2004) Igneous Processes, in Selley R. C., Cocks R. & Plimer I.R. (Eds.), Encyclopedia of Geology, Academic Press.

Smith PM & Asimow PD (2005) Adiabat_1ph: a new public front-end to the MELTS, pMELTS, and pHMELTS models, Geochemistry Geophysics Geosystems, 6(2):Q02004, doi:10.1029/2004GC000816.

   We have a static high-pressure, high-temperature laboratory including a Rockland Research multianvil device, piston-cylinder device, TZM hydrothermal system, and 1-atmosphere gas-mixing furnaces. Together these technologies give us access to conditions up to at least 20 and perhaps 25 GPa and temperatures to about 2000 °C. We have developed large-volume cells for the multianvil experiments that allow us to make and recover intact samples several millimeters across from pressures above 14 GPa. We are pursuing studies of the solubility of water in mantle minerals, the partitioning of water among melts and minerals, the effect of water on peridotite melting, the thermodynamics of silicate liquids at high pressure, and synthesis of high-pressure mineral phases.

Direct studies of the melting of candidate mantle compositions provide the basic facts with which we interpret basalt data and around which we construct all our models of mantle melting processes. There is considerable evidence, seismic and geochemical, supporting the idea that melting may begin at depths under mid-ocean ridges beyond the pressure range of the piston-cylinder device. Advances in multianvil technology, particularly the large-volume cubic geometry, provide the low temperature gradients and working volumes necessary to study peridotite melting up to 6 GPa and to understand the small amount of hydrous, carbonated, and incompatible-element rich melts expected below the nominal solidus.

For thermodynamic approaches to mantle models, we need good data on the equations of state (that is, volume as a function of temperature and pressure) of all the phases involved. For minerals, which diffract X-rays, high precision molar volume data are being gathered at high pressure by placing multianvil and diamond anvil cells in the beamline of powerful synchrotron X-ray sources. Liquids do not diffract X-rays and are more compressible than solids so that extrapolation of one-atmosphere data fails more quickly than for solids, so we know much less. We can use experiments that achieve equilibrium between solids and liquids, however, to leverage our knowledge of solid equations of state into constraints on the partial molar volume of various components in the liquid phase. This work is proceeding, starting with SiO2 and TiO2.

The multianvil device extends Caltech's experimental capability into the pressure range of the mantle transition zone, 400 to 670 km (i.e., 14 to 24 GPa), where a number of phase transitions occur and denser minerals become stable. We have much to learn about these transition-zone and lower mantle phases, including majorite, wadsleyite, ringwoodite, and ilmenite and perovskite-structured silicates. Perovskite is presumed to be the dominant phase in the lower mantle, and therefore the most abundant mineral on earth. In collaboration with Tom Ahrens, we use the multianvil to prepare specimens of these dense minerals for study by shockwave techniques.

See the Caltech Shockwave Laboratory Website for more information on our minerals and melt physics studies at lower mantle pressures.

Gaetani GA, Asimow PD, and Stolper EM (1998) Determination of the Partial Molar Volume of SiO2 in Silicate Liquids at Elevated Pressures and Temperatures: a New Experimental Approach, Geochimica et Cosmochimica Acta, 62:2499-2508.

Luo S.-N., Mosenfelder J. L., Asimow P. D. & Ahrens T. J. (2002) Stishovite and its implications in geophysics: New results from shock-wave experiments and theoretical modeling, Physics-Uspekhi, 45:3-7 or, in Russian, Uspekhi Fizicheskikh Nauk 172:475-480.

Luo S.-N., Mosenfelder J. L., Asimow P. D. & Ahrens T. J. (2002) Direct Shock Wave Loading of Stishovite to 235 GPa: Implications for Perovskite Stability Relative to Oxide Assemblage at Lower Mantle Conditions, Geophysical Research Letters 29:10.1029/2002GL015627.

Luo S.-N., Ahrens T. J. & Asimow P. D. (2003) Polymorphism, superheating and amorphization of silica upon shock loading and release, Journal of Geophysical Research 108:10.1029/2002JB002317.

Luo S.-N., Tschauner O., Asimow P. D., & Ahrens T. J. (2004) A new dense silica phase: a possible link between tetrahedrally and octahedrally coordinated silica, American Mineralogist 89(2-3):455-461.

Staneff, G. D., Asimow P. D. & Caillat T. (2004) Synthesis and thermoelectric properties of Ce(Ru0.67Rh0.33)4Sb12, in Nolas, G.S., Yang J., Hogan T. P. & Johnson D. C. (Eds), Thermoelectric Materials 2003 – Research and Applications, Materials Research Society Symposium Proceedings 793, pp. 101-106. Warrendale, PA: Materials Research Society.

Luo S.-N., Akins J. A., Ahrens T. J. & Asimow P. D. (2004) Optical emission of shock compressed MgSiO3 glass, enstatite, olivine, and quartz, Journal of Geophysical Research, 109(B5):B05205, doi: 10.1029/2003JB002860.

Luo, S.-N., Swift D. C, Tierney T., Xia K., Tschauner O. and Asimow P. D. (2004), Time-resolved X-ray diffraction investigation of superheating-melting behavior of crystals under ultrafast heating, in Shock Compression of Condensed Matter--2003: Proceedings of the Conference of the American Physical Society Topical Group on Shock Compression of Condensed Matter, edited by M.D. Furnish, Y.M. Gupta and J.W. Forbes, American Institute of Physics, AIP Conference Proceedings 706:95-98.

Akins J. A., Luo S.-N., Asimow P. D. & Ahrens T. J. (2004) Shock-induced melting of MgSiO3 perovskite and implications for melts in Earth’s lowermost mantle, Geophysical Research Letters 39, L14612, doi:10.1029/2004GL020237.

Luo S.-N., Swift D. C, Tierney T.E. IV, Paisley D. L., Kyrala G.A., Johnson R. P., Hauer A. A., Tschauner O. and Asimow P. D. (2004) Laser-induced shock waves in condensed matter: Some techniques and applications. High Pressure Research, 24(4):409-422.

Tschauner O., Luo S.-N., Asimow P. D., Ahrens T. J., Swift D. C., Tierney T. E., Paisley D. L. and Chipera S. J. (2004) Shock-synthesized glassy and solid silica: intermediates between four and six-fold coordination. High Pressure Research, 24(4):471-479.

Asimow P. D., Stein L. C., Mosenfelder J. L. and Rossman G. R. (submitted) Quantitative polarized infrared analysis of trace OH in populations of randomly oriented mineral grains. American Mineralogist.

Mosenfelder J. L., Deligne N.I., Asimow P. D. and Rossman G. R. (submitted) Hydrogen incorporation in olivine from 2-12 GPa. American Mineralogist.

Luo S.-N., Tschauner O., Tierney T.E. IV, Swift D. C., Chipera S. J. and Asimow P. D. (in press) Novel crystalline carbon cage structure synthesized from laser-driven shock wave loading of graphite. Journal of Chemical Physics.

Tschauner O., Willis M. J., Asimow P. D. and Ahrens T. J. (in preparation) Efficient liquid metal-silicate mixing during shock: Implications for giant impacts. Science.

Tschauner O., Luo S.-N., Asimow P. D. and Ahrens T. J. (in preparation) Recovery of stishovite structure at ambient pressure out of shock-generated amorphous silica. American Mineralogist

  Theory and experiment are wonderful, but the truth lies in the field. I have working on samples from the Azores region of the Mid-Atlantic Ridge. The combination of a regional geochemical gradient associated with the Azores hotspot with the strongly segmented geometry typical of slow-spreading ridges makes this region a magnificent natural laboratory for separating overlapping influences of source composition, mantle upwelling, melt migration, and crustal processes. This work led to development of the water-bearing mid-ocean ridge melting model mentioned above, and to the curious conclusion that adding water to a mid-ocean ridge source increases the total amount of melting and hence crustal thickness while decreasing the mean extent of melting by drawing in lots of low-degree hydrous melts. We also used oxygen isotopes to fingerprint the recycled component of the Azores mantle source.


More recently I sailed on the R/V Kilo Moana to the Lau Basin back-arc spreading center in the Western Pacific. Forthcoming work on these samples will look at the effects of water on fractionation and the density structure of mid-ocean ridge crust and on tracing fluids through the mantle wedge to learn the differences between normal, hot-spot affected, and back-arc spreading center melting regimes.


Other work in collaboration with other Caltech professors and students has covered a number of subjects in igneous petrology.

Asimow P. D. and Langmuir, C. H. (2003) The importance of water to oceanic mantle melting regimes, Nature, 421:815-820.


Asimow P. D., Dixon J. E. & Langmuir, C. H. (2004) A hydrous melting and fractionation model for mid-ocean ridge basalts: Application to the Mid-Atlantic Ridge near the Azores, Geochemistry Geophysics Geosystems 5(1):Q01E16, doi:10.1029/2003GC000568.


Cooper K. M., Eiler J. M, Asimow P. D., & Langmuir C. H. (2004) Oxygen isotope evidence for the origin of enriched mantle beneath the mid-Atlantic Ridge, Earth and Planetary Science Letters 220:297-316.


Zeng L., Saleeby J. B. & Asimow P. D. (2005) Nd isotopic disequilibrium during crustal anatexis: a record from the Goat Ranch migmatite complex, southern Sierra Nevada, California. Geology 33:53-56, doi: 10.1130/G20831.1.


Zeng L., Asimow P. D. & Saleeby J. B. (in press) Coupling of anatectic reactions and dissolution of accessory phases and the Sr and Nd isotope systematics of anatectic melts from a metasedimentary source, Geochimica et Cosmochimica Acta.