Rice geochemistry

Research in this group strives to wed observations in the laboratory and field to theory. We work at the scale of microns to the scale of the whole Earth, all in an attempt to better understand how our planet has differentiated into the diverse planet we experience today. Our observational tools are the rock hammer, the petrographic microscope, mass spectrometry for trace element and isotopic studies, and spectroscopy. We use our observations to drive simple geodynamic models, ranging from growth of thermal boundary layers to the development of soil profiles. Actually, we’re just trying to have fun.

:: The origin of continental crust

The tectonic and petrologic origin of continental crust is still an important, but not fully resolved, question. Central to this debate is the apparent paradox that continents are felsic in composition, but felsic melts cannot be generated by melting of the mantle. One of our ongoing studies is to investigate how a basaltic, mantle-derived magma differentiates into complementary felsic and mafic counterparts. Our project takes us to the Sierra Nevada and Peninsular Ranges batholiths in California, where we investigate the composition and nature of the deep crust and the overlying plutons.

:: Origin and dynamics of continental lithosphere

Beneath the Moho lies a part of the Earth’s mantle that translates more or less with the over-riding crust. This is the mantle lithosphere and, together with the crust, they make up part of the thermal boundary layer underlying continents. It turns out that much of the mantle part of the continental thermal boundary layers is chemically distinct from the ambient mantle in subtle ways. The mantle part of the chemical boundary layer appears to be melt-depleted and of slightly lower density than the surrounding mantle, a feature that has led to the suggestion that the longevity of continents is due to the intrinsic chemical buoyancy of the underlying chemical boundary layer. Our research is focused on 1) understanding the petrogenetic and tectonic origins of these chemical boundary layers, and 2) quantifying the physical properties of the rocks that make up the chemical boundary layers (1,2). We are particularly interested in how the nature of the continental mantle affects continental deformation. Is the strength of a continent sourced in the mantle part of the lithosphere or is it mostly in the crust? Towards these end, we are slowly mapping out the composition, thermal state and physical properties (seismic velocity, water content, density, rheology) of the mantle beneath western North America using xenoliths as a window into the mantle. This work will complement the USArray seismic study.

:: Serpentinization, the global water cycle, and element cycling

The upper part of the lithospheric mantle beneath oceans is believed to be serpentinized. The extent of serp entinization is not fully known, but a number of studies suggest that seawater can penetrate tens of kilometers into the lithosphere through faults and fractures. Understanding how much of the oceanic lithospheric mantle is serpentinized is key to understanding how much water is recycled into the Earth’s interior by subduction, one leg of the global water cycle. This in turn has important implications for the rheologic state of the mantle and even how this state has evolved with time because trace amounts of water play an important role in controlling viscosity. Graduate student Zhengxue Li and I have been investigating the trace and major element chemistry of serpentinites from the Feather River Ophiolite in California in order to place constraints on the mechanisms and depths of serpentinization. Together with post-doctoral fellow Arnaud Agranier, we are also investigating the Re-Os isotopic and platinum group element systematics of these serpentinites to help our interpretations. We are also quantifying the amount of fluid-mobile elements in these serpentinites (As, Pb, Li, B and soon halogens) in order to place bounds on the fluxes of these elements into subduction zones.

:: Mantle geochemistry (partitioning, redox, metasomatism)

We also do a lot of general mantle geochemistry. We are currently working on trace-element partitioning, redox evolution of the mantle (using the redox-sensitive behavior of V), and trace-element signatures of mantle metasomatism. Other projects include magma thermobarometry, core-mantle interaction and differentiation, platinum group element geochemistry, etc. See publication list below if you want to know more.

:: Continental weathering and the link to continent formation

Grad student Mark Little (that’s him on the right) and I (I’m taking the picture) have been investigating soil formation processes and associated element mobility. Some of this work has taken us to the volcanic highlands in East Africa. We are also modeling how chemical weathering, soil