Mineralogy and chemistry of Rumuruti: The first meteorite fall of the new R chondrite group

Abstract

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...

39Ar‐40Ar chronology of R chondrites

“…PCA 91002 and Acfer 217 both contain type R5-R6 clasts Rubin and Kallemeyn 1994); Rumuruti contains types R3, -5 and -6 clasts (Schulze et al 1994). In general, the matrices of the large light colored R5-R6 clasts are highly recrystallized.…”

“…There are two pyroxene phases: Ca-pyroxene, and low-Ca-pyroxene, with Ca-pyroxene being the more abundant phase, ~5.8 ± 2.0 wt%, similar to the abundance in ordinary chondrites (~4-5 wt%) (Kallemeyn, Rubin, and Wasson 1996). Calcic pyroxene is classified as subcalcic augite to diopside with compositions ranging from Fs 9 to Fs 16 (in ALH 85151 and PCA 91002, respectively Rubin and Kallemeyn 1989;Rubin and Kallemeyn 1994;Schulze et al 1994). Most of the radiogenic 40 Ar is expected to reside in plagioclase, but a minor amount may reside in the Ca-pyroxene.…”

“…Ten out of 18 previously studied R chondrites are regolith breccias (Schultz and Weber 2001). The brecciated R chondrites are comprised of clasts containing relict chondrules of olivine and pyroxene in a finer-grained matrix comprised of olivine, pyroxene, plagioclase, and minor sulfides (pyrrhotite and pentlandite) (Bischoff et al 1998;Kallemeyn, Rubin, and Wasson 1996;Rubin and Kallemeyn 1989;Rubin and Kallemeyn 1994;Schulze et al 1994). The olivine in R chondrites has a higher Fa content than any other chondrite group, Fa [37][38][39][40] , and a broad total range (e.g., Fa 0.4 to Fa 45.2 in ALH 85151) (Kallemeyn, Rubin, and Wasson 1996).…”

Expanding the application of the Eu‐oxybarometer to the lherzolitic shergottites and nakhlites: Implications for the oxidation state heterogeneity of the Martian interior

“…The fO 2 of lunar magmas is low and relatively constant at iron-wüstite (IW−0.5 ± 1.0) as indicated by the fairly ubiquitous presence of Fe-rich metal (Sato et al 1973;Usselman and Lofgren 1976;Haggerty 1978;Sato 1978;Nicholis and Rutherford 2005). With the notable exception of the Martian meteorites, the R chondrites (Nakamura et al 1993;Bischoff et al 1994;Rubin and Kallemeyn 1994;Schulze et al 1994;Kallemeyn et al 1996), and some carbonaceous chondrite groups (e.g., Larimer and Wasson 1988;Brearley and Jones 1998), most meteorite parent bodies also have compositional characteristics (Femetal phases) requiring they crystallized at fO 2 values of IW or below (e.g., eucrites : Stolper 1977, acapulcoites and lodranites: Benedix andLauretta 2006, ureilites: Walker andGrove 1993).…”

Smithsonian Institution

“…CM, CO, and CV groups, researchers now recognize the CK group of thermally metamorphosed carbonaceous chondrites and the metal-rich CH and CR groups. Also recently recognized are the highly oxidized R chondrites (e.g., Schulze et al, 1994). Chondritic groups are subdivided according to petrologic type (1-6), with 1 being the most aqueously altered, 3.0 being the least altered, and 6 being the most thermally altered.…”