Abstract— The Rumuruti meteorite shower fell in Rumuruti, Kenya, on 1934 January 28 at 10:43 p.m. Rumuruti is an olivine‐rich chondritic breccia with light‐dark structure. Based on the coexistence of highly recrystallized fragments and unequilibrated components, Rumuruti is classified as a type 3–6 chondrite breccia. The most abundant phase of Rumuruti is olivine (mostly Fa∼39) with about 70 vol%. Feldspar (∼14 vol%; mainly plagioclase), Ca‐pyroxene (5 vol%), pyrrhotite (4.4 vol%), and pentlandite (3.6 vol%) are major constituents. All other phases have abundances below 1 vol%, including low‐Ca pyroxene, chrome spinels, phosphates (chlorapatite and whitlockite), chalcopyrite, ilmenite, tridymite, Ni‐rich and Ge‐containing metals, kamacite, and various particles enriched in noble metals like Pt, Ir, arid Au. The chemical composition of Rumuruti is chondritic. The depletion in refractory elements (Sc, REE, etc.) and the comparatively high Mn, Na, and K contents are characteristic of ordinary chondrites and distinguish Rumuruti from carbonaceous chondrites. However, S, Se, and Zn contents in Rumuruti are significantly above the level expected for ordinary chondrites. The oxygen isotope composition of Rumuruti is high in δ17O (5.52 ‰) and δ18O (5.07 ‰). Previously, a small number of chondritic meteorites with strong similarities to Rumuruti were described. They were called Carlisle Lakes‐type chondrites and they comprise: Carlisle Lakes, ALH85151, Y‐75302, Y‐793575, Y‐82002, Acfer 217, PCA91002, and PCA91241, as well as clasts in the Weatherford chondrite. All these meteorites are finds from hot and cold deserts having experienced various degrees of weathering. With Rumuruti, the first meteorite fall has been recognized that preserves the primary mineralogical and chemical characteristics of a new group of meteorites. Comparing all chondrites, the characteristic features can be summarized as follows: (a) basically chondritic chemistry with ordinary chondrite element patterns of refractory and moderately volatile lithophiles but higher abundances of S, Se, and Zn; (b) high degree of oxidation (37–41 mol% Fa in olivine, only traces of Fe, Ni‐metals, occurrence of chalcopyrite); (c) exceptionally high Δ17O values of about 2.7 for bulk samples; (d) high modal abundance of olivine (∼70 vol%); (e) Ti‐Fe3+−rich chromite (∼5.5 wt% TiO2); (f) occurrence of various noble metal‐rich particles; (g) abundant chondritic breccias consisting of equilibrated clasts and unequilibrated lithologies. With Rumuruti, nine meteorite samples exist that are chemically and mineralogically very similar. These meteorites are attributed to at least eight different fall events. It is proposed in this paper to call this group R chondrites (rumurutiites) after the first and only fall among these meteorites. These meteorites have a close relationship to ordinary chondrites. However, they are more oxidized than any of the existing groups of ordinary chondrites. Small, but significant differences in chemical composition and in oxygen isotopes between R...

Rates of cation diffusion (magnesium, iron, and nickel) have been determined in olivine and its high-pressure polymorph, wadsleyite, at 9 to 15 gigapascals and 1100 degrees to 1400 degreesC for compositions that are relevant to Earth's mantle. Diffusion in olivine becomes strongly dependent on composition at high pressure. In wadsleyite, diffusion is one to two orders of magnitude faster than in olivine, depending on temperature. Homogenization of mantle heterogeneities (chemical mixing) and mineral transformations involving a magnesium-iron exchange will therefore occur considerably faster in the transition zone than at depths of less than 410 kilometers.