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Research

‡ indicates graduate student supervised by Dygert; † indicates undergraduate; § indicates postdoc

MELT MIGRATION AND MELT-ROCK REACTION IN THE MANTLE

Terrestrial and lunar basalts share a fascinating property: they are not in chemical equilibrium with mantle rock at low pressures. How are melts transported through dozens to hundreds of kilometers of mantle while remaining chemically insulated? To what extent do melts react with ambient mantle beneath mid-ocean ridges? How is melt-rock reaction expressed in basalts and mantle cumulates?

Multiple episodes of melt percolation and melt-rock reaction at Trinity ophiolite, northern California

A dunite-harzburgite-plagioclase lherzolite sequence at Trinity

Dygert, Liang, Kelemen, Journal of Petrology (2016)

Tabular dunite bodies (“dunite channels”) are thought to represent efficient pathways for melt extraction from the mantle. This study investigates trace element variations across a dunite-harzburgite-lherzolite-plagioclase lherzolite sequence at the Trinity ophiolite, demonstrating that tabular dunites can be sources of melt infiltration rather than melt extraction pathways, opposite the dunite channel paradigm.

 

Plate tectonic cycling modulates Earth’s 3He/22Ne ratio

diffusionprofile

Dygert, Jackson, Hesse, Tremblay, Shuster, †Gu, Earth and Planetary Science Letters (2018)

The MORB mantle has distinctly higher 3He/22Ne compared to the OIB source mantle. He diffusion in mantle rock may be orders of magnitude faster than Ne at mantle temperatures. Preferential 3He ingassing around dunite channels will increase the 3He/22Ne of the depleted mantle. Integrating this intuitive theoretical framework into a mixing model, we estimate mantle mixing timescales on the order of 400 million-billions of years. Our model suggests mantle convection is not and could not have been layered for most of geologic time.

 

Melt rock reaction: compositional controls on reaction products

Backscatter image showing an orthopyroxenite layer in a hydrous melt-rock reaction experiment

Wang, Liang, Dygert, Xu, Contributions to Mineralogy and Petrology (2016)

Wang, Liang, Xu, Dygert, Contributions to Mineralogy and Petrology (2013)

These publications present experimental results that can help field geologists interpret the conditions under which dunites, orthopyroxenites, and other lithologies form by melt-rock reaction in the mantle.

 


RHEOLOGY OF EARTH AND PLANETARY MATERIALS

In their earliest stages of evolution, terrestrial planets had deep magma oceans. Cooling magma oceans crystallize a stratified cumulate pile with abrupt lithological variations and more gradual compositional variations. What are the rheologies of mantle minerals and the multiphase aggregates they are composed of? What are the consequences for mantle convection? How does compositional layering shape the long term physical and chemical evolution of planets?

A flow law for ilmenite in dislocation creep: Implications for lunar cumulate mantle overturn

Dynamically recrystallized ilmenite deformed in a Griggs apparatus

Dygert, Hirth, Liang, Geophysical Research Letters (2016)

Zhang, Dygert, Liang, Parmentier, Geophysical Research Letters (2017)

Lunar basalts are Ti-rich, suggesting that ilmenite is a locally abundant component of the Moon’s mantle. Using rock deformation experiments conducted in a Griggs apparatus, we parameterized a dislocation creep flow law for synthetic ilmenite. Extrapolating the flow law to mantle conditions, we find that ilmenite is >3 orders of magnitude weaker than olivine. Small amounts of ilmenite may significantly weaken the lunar mantle, resulting in the development of dense downwelling diapirs after magma ocean crystallization.

 

Viscosity of magma ocean liquids: Implications for differentiation on Mercury and the Moon

A rhenium sphere in a falling sphere viscometry experiment

Mouser, Dygert, ‡Grambling, Anzures, Kono, Shen, Parman, LPSC (2019)

We measured the viscosity of S-bearing and S-free mercurian magma ocean analogue liquids. The viscosities of these liquids would promote fractional crystallization, suggesting Mercury’s cumulate mantle was dynamically mixed after magma ocean solidification.

Dygert, Lin, Marshall, Kono, Gardner, Geophysical Research Letters (2017)

The anorthitic lunar crust formed by flotation of positively buoyant plagioclase on the crystallizing lunar magma ocean. Experimental measurements of the magma ocean liquid viscosity and a crustal compaction model suggest formation of a pure lower lunar crust beneath a more impure older crust.

 

Great Basin mantle xenoliths record deformation associated with active lithospheric downwelling

Dynamically recrystallized olivine in a harzburgite

Dygert, Bernard, Behr, Geochemistry, Geophysics, Geosystems (2019)

Mylonitic mantle xenoliths from Lunar Crater volcanic field, central Nevada record chemical signatures and temperatures suggesting they originate from the base of the lithosphere. Recrystallized olivine grains demonstrate isochemical deformation and differential stress magnitudes of ~50MPa. Anomalies in shear-wave splitting orientations and magnitudes, seismic body wave velocities, heat flow, and topography suggest the presence of a downwelling Rayleigh-Taylor (R-T) instability beneath the volcanic field. We infer that the mylonites directly sample the detaching R-T instability.

 


TRACE ELEMENT PARTITIONING: CONSTRAINTS ON THE DYNAMIC EVOLUTION OF PLANETARY INTERIORS

Hedenbergite

Reflected light and backscatter micrographs of augite-melt partitioning experiments

Partition coefficients are fundamental tools used by petrologists and geochemists to understand igneous processes such as partial melting, melt migration, and element and isotope fractionation. High-pressure, high temperature experiments are conducted in the piston cylinder apparatus to measure partition coefficients at physically realistic conditions.

An experimental study of trace element partitioning between augite and Fe-rich basalts

Dygert, Liang, Sun, Hess, Geochimica et Cosmochimica Acta (2014)

The importance of melt TiO2 in affecting major and trace element partitioning between Fe–Ti oxides and lunar picritic glass melts

Dendritic quench growth and laser ablation pits in a high-Ti trace element partitioning experiment

Dendritic quench growth and laser ablation pits in a high-Ti trace element partitioning experiment

Dygert, Liang, Hess, Geochimica et Cosmochimica Acta (2013)

I conducted partitioning studies on lunar-relevant minerals in Fe- and Ti-rich systems relevant to magma oceans and evolved planetary interiors (Fe-Ti oxides and Fe-rich basalts, and clinopyroxene and Fe-rich basalts). Lunar magma ocean crystallization models that incorporate the experimental observations suggest REE fractionation during magma ocean crystallization was not efficient enough to produce a residual liquid with REE concentrations similar in slope to the lunar basalts. Dense, late lunar magma ocean cumulates were likely mixed back into the underlying cumulate mantle (by a process called cumulate overturn) and then partially melted to generate the observed trace element patterns.

 


MAJOR AND TRACE ELEMENT GEOSPEEDOMETRY: NEW INSIGHTS INTO THE THERMAL HISTORY OF PERIDOTITES AND ASTEROIDS

Geothermometers rely on temperature sensitive element partitioning among a mineral assemblage. Because trivalent rare earth elements (REEs) diffuse more slowly than divalent major elements, REE-based thermometers often record near-magmatic temperatures where major element thermometers record lower cooling temperatures. I use REE thermometry to decode the thermal histories of samples from the crust and mantle.

Temperatures and cooling rates recorded in REEs in ophiolitic and abyssal peridotites

affinityTemps

Dygert, Liang, Earth and Planetary Science Letters (2015)

Ophiolites are fragments of oceanic lithosphere obducted onto continental margins. I explored the global temperature systematics among ophiolitic peridotites, finding that those with a strong subduction influence were initially colder and cooled more slowly than peridotites dredged from abyssal basins. Subduction appears to profoundly influence the thermal regime beneath spreading centers.

 

Spatial variations in cooling rate in the mantle section of the Samail ophiolite in Oman: Implications for formation of lithosphere at mid-ocean ridges

coolingratefig

Wadi Tayin mantle cooled by conduction into overlying hydrothermally cooled crust

Dygert, Kelemen, Liang, Earth and Planetary Science Letters (2017)

In contrast to ophiolites with a strong subduction influence, the southern section of the Semail ophiolite in Oman cooled from high temperatures at rates similar to abyssal peridotites. The tectonic settings these samples originate from are very different: the Semail ophiolite has a thick crustal section and probably formed at a fast spreading center; abyssal peridotites are from fracture zones and amagmatic ridges that lack crust. These temperature systematics suggest that (surprisingly!) the rate of cooling beneath mid-ocean ridges is independent of spreading rate and the presence or absence of crust. Hydrothermal circulation probably extended to (but not far beneath) the crust-mantle transition zone at the spreading center that formed the Oman ophiolite.

 

Application of REE-in-two-pyroxene thermometry to ordinary chondrites: Evidence for multistage metamorphism

coolingratefig

A false color X-ray map of an LL chondrite (Alta’ameem)

§Lucas, Dygert, Patchen, Miller, McSween, LPSC (2019)

Dygert, Patchen, Miller, McSween, LPSC (2018)

Application of the REE-in-two-pyroxene thermometer to H and LL chondrites suggests their parent bodies were quenched by catastrophic fragmentation at high temperature. Fast cooling rates through high-temperature intervals stand in contrast to cooling rates orders of magnitude slower through lower temperature intervals. The observations can be reconciled if after fragmentation, the parent bodies reaccreted to form a slow-cooling rubble pile asteroid.

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