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Experimental Studies of Melt-Rock Interaction

Reaction between basaltic magmas and harzburgite and lherzolite at moderate to low pressures has played an important role in melt migration in the upper mantle. In the presence of porous melt flow, such reaction often produces elongated, interconnected high porosity dunite channels in the melt migration region of the mantle.

We have examined the mechanisms and kinetics of harzburgite and lherzolite dissolution in basaltic magmas through laboratory experiments. At moderate pressures, reaction between a lherzolite and a basalt produces a reactive boundary layer that consists of a pyroxene-free dunite and a clinopyroxene-free harzburgite (Figure 1). The mineralogical boundaries between the various lithological units are sharp, while mineral compositions vary gradually across the reaction zone. The average grain size of olivine in the newly formed dunite and harzburgite increases as a function of experimental run time. These systematic variations in grain size, mineralogy, and mineral chemistry are consistent with a model that involves preferential dissolution of pyroxene and precipitation or re-precipitation of olivine. Similar variations in mineralogy and mineral chemistry have been reported in several ophiolites around the world, though the field data, in general, are more complicated (Figure 2). The overall objectives of our experimental studies are to understand the rate, consequences, and time scales of melt-rock reaction in the mantle. Concentration gradients across a sharp dissolution front, for example, can be used to infer the rate and time scales of melt extraction in the mantle, provided the kinetics and dynamics of reactive dissolution are well understood.

To better understand our experimental results and geological field observations, we have also examined the grain scale processes of melt-rock reaction. Through a combination of laboratory experiments and numerical simulations, we show that dissolution-reprecipitation has played an important role in affecting variations of the major and trace element abundances in reacted minerals and melt during melt percolation and melt-rock in the upper mantle.

Related References:
Liang, Y. (2003) Kinetics of crystal-melt reaction in partially molten silicates. 1. Grain scale processes. Geochemistry, Geophysics, Geosystems, 4(5), 1045, doi: 10.1029/2002GC000375.

Morgan, Z. T. and Liang, Y. (2003) An experimental and numerical study of the kinetics of harzburgite reactive dissolution with applications to dunite dike formation. Earth Planet. Sci. Lett. 214, 59-74.

Lo Cascio, M, Liang, Y., and Hess, P. C. (2004) Grain-scale processes during isothermal-isobaric melting of lherzolite. Geophys. Res. Lett. 31, L16605, doi: 10.1029/2004GL020602.

Morgan, Z. T. and Liang, Y. (2005) An experimental study of the kinetics of lherzolite reactive dissolution with applications to melt channel formation. Contrib. Mineral. Petrol. 150, 369-385, doi: 10.1007/s00410-005-0033-8.

Morgan, Z. T., Liang, Y., and Hess, P. C. (2006) An experimental study of the kinetics of anorthosite dissolution in lunar picritic magmas with applications to lunar crustal assimilation processes. Geochim. Cosmochim. Acta 70, 3477-3491.

Beck, A. R., Morgan, Z. T., and Liang, Y., and Hess, P. C. (2006) Dunite channels as viable pathways for mare basalt transport in the deep lunar mantle. Geophys. Res. Lett. 33, doi: 10.1029/2005GL024008.

Lo Cascio, M, Liang, Y., Shimizu, N., and Hess, P. C. (2008) An experimental study of the grain scale processes of peridotite melting: Implications for major and trace element distribution during equilibrium and disequilibrium melting. Contrib. Mineral. Petrol. 156, 87-102, doi: 10.007/s00410-007-0275-8.

Morgan, Z. T., Liang, Y., and Kelemen, P. B. (2008) Significance of the composition profiles associated with dunite bodies in the Josephine and Trinity ophiolites. Geochemistry, Geophysics, Geosystems, doi: 10.1029/2008GC001954.

Tursack, E. and Liang, Y. (2012) A comparative study of melt-rock reactions in the mantle: laboratory dissolution experiments and geological field observations. Contrib. Mineral. Petrol. 163, 861-876, doi: 10.1007/s00410-011-0703-7.

Brown faculty collaborators:

Paul Hess

Other project collaborators:

Emily Tursack, Nick Dygert, Lee Saper, Chunguang Wang

Figure 1. Lherzolite reactive dissolution in alkali basalt (1300°C, 1 GPa, 8 hrs). [a] Variations in Mg#s of olivine (red circles), orthopyroxene (open squares), and clinopyroxene (green diamonds) in the reaction zone as a function of distance in the charge. The scatter in the pyroxene data results from core-to-rim compositional variations. [b] BSE image of the charge with dash lines marking the sharp boundaries between the three lithologic units. The original lherzolite-basalt interface is marked by the capsule offset in graphite (black walls in this image). [c] False colored X-Ray concentration maps of Mg and Ca of the selected area in [b]. Here high to low concentrations are represented by red to blue color. The clinopyroxene, for example, is shown as red "dots" in the lower portion of the Ca map. This figure is taken from Morgan and Liang (2005).

Figure 2. Plot of measured Mg# in olivine and selected elements in olivine, clinopyroxene and spinel as a function of distance across a dunite-harzburgite-lherzolite-plagioclase lherzolite (DHL-PL) sequence from the Trinity ophiolites, Northern California. Based on our peridotite dissolution studies we believe that the DHL-PL sequence is formed by preferential dissolution of plagioclase, followed by clinopyroxene and the orthopyroxene during reaction between a plagioclase-bearing lherzolite and a basaltic magma in the shallow mantle. More complete mineral composition profiles were reported in graduate student Zach Morgan's Ph.D. thesis.

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