Kinetic processes of mantle minerals

Abstract

This dissertation discusses the experimental results designed to constrain the processes of MORE generation. The main focus of this study is to investigate the location and the related processes of the transformation boundary from spinel to garnet peridotite facies at subsolidus conditions, because the presence of garnet in melting residues has significant influence to the conclusion drawn from geochemical/geophysical observations. Using an approach that monitors the rate of reaction progresses, the experimental results confirmed the presence of a region that garnet and spinel coexist in peridotite compositions. The trace element distribution among the product phases (opx and cpx) subsequent to the garnet breakdown reaction is in disequilibrium, due to the differences of diffusivity between major and trace elements. The presence of disequilibrium distribution in nature may be used to infer time scales of geodynamic processes. Diffusion coefficients of Al in diopside are experimentally determined, and used for modeling the equilibration of major elements in pyroxene during MORE genesis. In summary, this dissertation contributes two major inferences: the location of the transformation boundaries of the gamet-spinel peridotite; the presence of disequilibrium trace elements distribution with equilibrium major elements distribution in mantle pyroxenes.3

Early hydrous melting and degassing of the Martian interior

“…Garnet forms 50–800 μ m poikilitic crystals in experiments at pressures ≥2.0 GPa and temperatures ≥800°C. The garnet‐in curve on Figure 1 has been redrawn from experimental work on the spinel/garnet transition in terrestrial samples [ O'Hara et al , 1971; Koga , 2000]. However, it is in agreement with experimental constraints from this study and Bertka and Holloway [1994].…”

“…Experimental water‐saturated phase relations for a primitive mantle + crust Martian composition. Dry solidus is from Bertka and Holloway [1994], garnet‐in curve is from Koga [2000], and low‐pressure amphibole‐out curve is from Lykins and Jenkins [1992]. The shape of the chlorite‐out reaction has been modified from Jenkins and Chernosky [1986] and Pawley [2003].…”

“…Low‐pressure spinels are iron‐rich (Table 3), whereas high‐pressure spinels are enriched in chromium. In terrestrial compositions, spinel disappears at pressures just above the garnet‐in curve, leaving only a 0.2 GPa interval for garnet/spinel coexistence [ Koga , 2000]. However, the higher chromium content of the Martian mantle (Table 1) stabilizes spinel and allows coexistence of garnet and Cr‐rich spinel over at least 1.0 GPa.…”