R3.5–6, rumurutiiteMember of a rare group of chondrites, formerly named the Carlisle Lakes group, after a meteorite found in Australia in 1977. It is now named for the type specimen Rumuruti that fell in Kenya, Africa, in 1934. Rumuruti is the only witnessed fall of this group and only one small
27° 07.77′ N., 16° 20.61′ E. A single fragment weighing 205 g was found in the Libyan Sahara Desert. As with most other R chondritesChondrites are the most common meteorites accounting for ~84% of falls. Chondrites are comprised mostly of Fe- and Mg-bearing silicate minerals (found in both chondrules and fine grained matrix), reduced Fe/Ni metal (found in various states like large blebs, small grains and/or even chondrule rims), and various refractory inclusions (such Click on Term to Read More, Dar al Gani 013 is a chondritic genomict brecciaWork in Progress ... A rock that is a mechanical mixture of different minerals and/or rock fragments (clasts). A breccia may also be distinguished by the origin of its clasts: (monomict breccia: monogenetic or monolithologic, and polymict breccia: polygenetic or polylithologic). The proportions of these fragments within the unbrecciated material Click on Term to Read More enriched in solar-wind-implanted rare gases. This meteoriteWork in progress. A solid natural object reaching a planet’s surface from interplanetary space. Solid portion of a meteoroid that survives its fall to Earth, or some other body. Meteorites are classified as stony meteorites, iron meteorites, and stony-iron meteorites. These groups are further divided according to their mineralogy and Click on Term to Read More contains both equilibrated and highly unequilibrated lithic and mineralInorganic substance that is (1) naturally occurring (but does not have a biologic or man-made origin) and formed by physical (not biological) forces with a (2) defined chemical composition of limited variation, has a (3) distinctive set of of physical properties including being a solid, and has a (4) homogeneous Click on Term to Read More clasts together with impact-melt fragments, all incorporated within a fine-grained, olivine-rich, recrystallized matrixFine grained primary and silicate-rich material in chondrites that surrounds chondrules, refractory inclusions (like CAIs), breccia clasts and other constituents. Click on Term to Read More. Dispersed sulfide grains in the matrix are responsible for the darkening typically observed in R chondrites (Kallemeyn et al., 1996). The high noble metalElement that readily forms cations and has metallic bonds; sometimes said to be similar to a cation in a cloud of electrons. The metals are one of the three groups of elements as distinguished by their ionization and bonding properties, along with the metalloids and nonmetals. A diagonal line drawn Click on Term to Read More abundance (Pt, Os, Ir, AuThe astronomical unit for length is described as the "mean" distance (average of aphelion and perihelion distances) between the Earth and the Sun. Though most references state the value for 1 AU to be approximately 150 million kilometers, the currently accepted precise value for the AU is 149,597,870.66 km. The Click on Term to Read More, Ru) present in the pre-oxidized FeNi-metal component of the R chondrites was exsolved during the oxidationOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More phase, and was subsequently combined with sulfides, with the Pt residing in the unequilibrated phase, tetraferroplatinum, in some members.
of olivineGroup of silicate minerals, (Mg,Fe)2SiO4, with the compositional endpoints of forsterite (Mg2SiO4) and fayalite (Fe2SiO4). Olivine is commonly found in all chondrites within both the matrix and chondrules, achondrites including most primitive achondrites and some evolved achondrites, in pallasites as large yellow-green crystals (brown when terrestrialized), in the silicate portion Click on Term to Read More
|>5% mean deviation||homogenous||homogenous||homogenous|
|isolated intergrowths||networks forming||well-developed
|Sulfides||even distribution||even distribution||even distribution||mobilized|
Measurements of the coarser-grained feldspars present in higher petrologic types provide the most accurate ages; an Ar–Ar age of 4.53 (±0.01) b.y. has been determined for an equilibrated lithology of Rumuruti, which was age corrected for K decay bias to give an age of 4.56 b.y. (Trieloff et al., 2007; Buikin et al., 2006). A minimum range of ~170 m.y. in 39Ar–40Ar ages has been determined for various R chondrites (Dixon et al., 2003). They propose that the younger ages of some clasts could be the result of an early impact and subsequent deep burial with slow cooling in a thick regolithMixture of unconsolidated rocky fragments, soil, dust and other fine granular particles blanketing the surface of a body lacking an atmosphere. Regolith is the product of "gardening" by repeated meteorite impacts, and thermal processes (such as repeated heating and cooling cycles). Click on Term to Read More or rubble-pile. Alternatively, a late impact could have reset the 39Ar–40Ar chronometer of those lithologies exhibiting younger ages, with shallow burial and rapid cooling occurring afterwards. This latter scenario is consistent with the very early age of breccia formation, the measured CRE ages, and the abundant solar rare gases characteristic of this group. It is equally plausible that the extremely brief metamorphic history (crystallizationPhysical or chemical process or action that results in the formation of regularly-shaped, -sized, and -patterned solid forms known as crystals. Click on Term to Read More 4.56 b.y. ago), the pervasive brecciationThe formation of a breccia through a process by which rock fragments of of various types are recemented or fused together. Click on Term to Read More, and the high abundance of solar-wind-implanted rare gases could reflect the collisional disruption and gravitational reassembly of the Rumuruti parent bodyThe body from which a meteorite or meteoroid was derived prior to its ejection. Some parent bodies were destroyed early in the formation of our Solar System, while others like the asteroid 4-Vesta and Mars are still observable today. Click on Term to Read More soon after its formation (Buikin et al., 2007).The DaG 013 meteorite is almost unshocked (S1) and was initially given a weathering grade of W4 reflecting a high degree of terrestrial weathering based on the weathering scale of Wlotzka, (1993). More recently, a more useful weathering index (wi) was developed by Rubin and Huber (2005) for those oxidized meteorite groups lacking significant FeNi-metal phases, such as the CK and R chondrite groups. This index is based on the modal abundance of brown-stained silicates as visually determined on a thin sectionThin slice or rock, usually 30 µm thick. Thin sections are used to study rocks with a petrographic microscope. in transmitted light at ~100× magnification. It is thought that the brown staining in R chondrites (and CK chondrites) is caused by the terrestrial decomposition and mobilization of sulfides (mainly pyrrhotiteIron sulfide group of minerals whose composition ranges widely between its end members pyrrhotite (Fe7S8) whose crystal structure is monoclinic, and troilite (FeS) whose crystal structure is hexagonal. Its general formula is Fe1−xS (where x = 0 to 0.17). The troilite phase is found mainly in meteorites and in the Click on Term to Read More and pentlanditeFe-Ni sulfide, (Fe,Ni)9S8, that is often associated with troilite, and found in the matrix and chondrules of CO, CV, CK and CR chondrites. The color is yellow-bronze with light bronze-brown streak and metallic luster. It typically forms during cooling of magmatic sulfide melts during the evolution of parent silicate melt. The Click on Term to Read More) which are typically prevalent in these meteorites; e.g., Rumuruti, designated wi-0, contains 8.0 wt% sulfides. It was determined that DaG 013 has a weathering index of wi-5, signifying severe weathering. Dar al Gani 013 has a CRE age of 9.7 m.y. based on cosmogenic 21Ne, with an average age of 8.0 ±1.2 m.y. based on all cosmogenic nuclides studied (Schultz et al., 2005). Noble gasElement occurring in the right-most column of the periodic table; also called "inert" gases. In these gases, the outer electron shell is completely filled, making them very unreactive. Click on Term to Read More analyses of the known Northwest Africa R chondrites were conducted by Vogel et al. (2014). They have tentatively placed these numerous R chondrites into ~16 groupings representing possible common source craters and/or fallMeteorite seen to fall. Such meteorites are usually collected soon after falling and are not affected by terrestrial weathering (Weathering = 0). Beginning in 2014 (date needs confirmation), the NomComm adopted the use of the terms "probable fall" and "confirmed fall" to provide better insight into the meteorite's history. If Click on Term to Read More events. One of these groups comprises meteorites with a CRE age of ~10 m.y., and along with DaG 013 includes R chondrites with the NWA series designations 755, 845, 851, 978, 1471, 2198, and 5069. This is a unique group of highly oxidized chondrites with a higher volume of olivine (FeO-rich), a lower volume of pyroxene, and essentially no FeNi-metal as compared to all other chondrite groups. It has been suggested that the oxidation of the R chondrites occurred as a result of persistent equilibration with the nebular gas (Kallemeyn et al., 1996). The oxidant was probably water, and the oxidation of the various R chondrite components likely occurred near the snow line of the protoplanetary diskFlattened and rotating disk of dense gas and dust/solids orbiting a young star from which planets can eventually form. Click on Term to Read More where water-ice was abundant (Rout et al., 2009). A comparison of refractory lithophile abundances among R chondrites, E chondrites, O chondrites, and C chondrites indicated that the R-chondrite parent body formed at a heliocentricCentered around a sun. Our own Solar System is centered around the Sun so that all planets such as Earth orbit around the Sun. Note that 25% of Americans incorrectly believe the Sun revolves around the Earth. Click on Term to Read More distance greater than O chondrites and less than C chondrites (Khan et al., 2013). Notably, Rumuruti chondrites have the highest Δ17O values of any meteorite group, reflecting the isotopic composition of the accreted water. During metasomatic oxidation, FeNi-metal and pyroxene reacted with water to form olivine, while the remaining metal became enriched in Ni to form awaruiteNi-rich Fe metal, Ni3Fe, similar to taenite found in minor amounts in some meteorites. Awaruite is also known as josephinite, a mineral found as placer deposits in Josephine County, Oregon, and sometimes mistaken for a meteorite. Note: web.mineral.com incorrectly defines Awaruite as “Ni2Fe to Ni3Fe”, however the IMA Database of Click on Term to Read More (Isa et al., 2010). Pre-terrestrial hydrous phases identified in R chondrite genomict breccias include laihunite, goethite, anhydrite, and jarosite (Jamsja et al., 2011). Features related to oxidation conditions in R chondrites are not related to the metamorphic temperatures, i.e., petrologic type (Isa et al., 2011, 2013). The compositional makeup of known R chondrites reflects that of a regolith breccia to a greater degree (~48%) than any other asteroidal meteorite group. The R chondrites share certain similarities with H and L ordinary chondrites, such as refractory elementSubstance composed of atoms, each of which has the same atomic number (Z) and chemical properties. The chemical properties of an element are determined by the arrangement of the electrons in the various shells (specified by their quantum number) that surround the nucleus. In a neutral atom, the number of Click on Term to Read More depletions and siderophile elementLiterally, "iron-loving" element that tends to be concentrated in Fe-Ni metal rather than in silicate; these are Fe, Co, Ni, Mo, Re, Au, and PGE. These elements are relatively common in undifferentiated meteorites, and, in differentiated asteroids and planets, are found in the metal-rich cores and, consequently, extremely rare on abundances, and they have close similarities in chondrule size, textural type, and O-isotopic compositions. It is considered that these chondrite groups likely formed in the same O-isotopic reservoir during a similar timeframe within the nebular midplane (Greenwood et al., 2000). On the other hand, the R chondrites differ from the ordinary chondrites in having higher matrix/chondrule ratios and higher abundances of volatileSubstances which have a tendency to enter the gas phase relatively easily (by evaporation, addition of heat, etc.). and refractory lithophile elements, and they exhibit differences in other petrographic trends as well. Of particular interest is the higher bulk-rock Δ17O of R chondrites compared to ordinary chondrites, with a resulting higher FeO content and lower FeNi-metal content. The triple increase in Zn relative to ordinary chondrites is consistent with formation in a lower temperature, more oxidizingOxidation and reduction together are called redox (reduction and oxidation) and generally characterized by the transfer of electrons between chemical species, like molecules, atoms or ions, where one species undergoes oxidation, a loss of electrons, while another species undergoes reduction, a gain of electrons. This transfer of electrons between reactants Click on Term to Read More environment—between that of ordinary chondrites and carbonaceous chondrites. Rumuruti chondrites are enriched in high-temperature early nebular condensates still rich in refractory HREE, contrary to the later formed CV-group chondrites that contain a larger component of LREE (Khan et al., 2014). The difference in the O-isotopic abundances is greater between the R chondrites and ordinary chondrites than it is among the H, L, and LL ordinary chondriteWork in Progress Ordinary chondrites (OCs) are the largest meteorite clan, comprising approximately 87% of the global collection and 78% of all falls (Meteoritical Society database 2018)1. Meteorites & the Early Solar System: page 581 section 6.1 OC of type 5 or 6 with an apparent shock stage of S1, Click on Term to Read More groups, further resolving this group from the ordinary chondrite groups (Weber et al., 1997). Similar to carbonaceous chondrites, R chondrites have a high olivine content within a high proportion of matrix, reflecting their highly oxidized nature, indicating that they formed at a large heliocentric distance—perhaps even close to that of the least oxidized carbonaceous chondrites. Rumuruti chondrites have a higher Δ17O value than that of any other Solar SystemThe Sun and set of objects orbiting around it including planets and their moons and rings, asteroids, comets, and meteoroids. material, and therefore do not fall on the same O-isotope trend line as ordinary chondrites (Weisberg, et al., 1991). Oxygen-isotopic variations among equilibrated clasts suggest that some R chondrite material had higher Δ17O values than those determined from Rumuruti bulk rock (Δ17O = +2.9‰) (Greenwood et al., 2000). A study of unequilibrated R chondrite components was conducted by Isa et al. (2012). They revealed that olivines in R chondrite chondrules comprise two broad O-isotopic compositional groups: the first with low Δ17O values of ~–4‰ plotting on the CCAM line, and the second with high Δ17O values of up to ~3‰ plotting along the TFL. When compared to the highest values found in OC chondrules (up to ~1.6‰), the even higher values obtained for R chondrites suggest that the R chondrite precursor material had higher Δ17O values, attesting to differences in their respective formation environments. It has been argued by some that R chondrites and ordinary chondrites accreted from similar nebular components, but that R chondrites accumulated a higher abundance of the high-Δ17O oxidant water residing in the fine-grained matrix olivine component. The proportion of oxygenElement that makes up 20.95 vol. % of the Earth's atmosphere at ground level, 89 wt. % of seawater and 46.6 wt. % (94 vol. %) of Earth's crust. It appears to be the third most abundant element in the universe (after H and He), but has an abundance only Click on Term to Read More retained from this water was calculated to be ~23%. Modeling techniques and laboratory melting experiments (e.g., Gardner-Vandy and Lauretta, 2011; Gardner-Vandy et al., 2014; Sosa et al., 2017; Lunning et al., 2017) have demonstrated that an FeO-rich (oxidized) R chondrite-like precursor asteroid can undergo low-degree partial meltingAn igneous process whereby rocks melt and the resulting magma is comprised of the remaining partially melted rock (sometimes called restite) and a liquid whose composition differs from the original rock. Partial melting occurs because nearly all rocks are made up of different minerals, each of which has a different melting Click on Term to Read More (14–22%) at 1120–1140°C and an oxygen fugacityUsed to express the idealized partial pressure of a gas, in this case oxygen, in a nonideal mixture. Oxygen fugacity (ƒO2) is a measure of the partial pressure of gaseous oxygen that is available to react in a particular environment (e.g. protoplanetary disk, Earth's magma, an asteroid's regolith, etc.) and Click on Term to Read More of IW–IW+1 to produce a brachinite-like residue and a complementary evolved melt. Melt clasts with a basaltic andesite composition have been identified in DaG 013 (and PCA 91241) by Lunning et al. (2018) that are alkali-depleted analogs of lithologies which have been attributed to crustal components of the ureilite (ALM-A, MS-MU-011/035) and brachinite (GRA 06128/9) parent bodies. See further information about GRA 06128/9 on the brachinite Reid 013 page. A component of primitive, highly refractory forsterites which are Ca-rich and FeO-poor have been found in DaG 013. These forsterites have similar O-isotope ratios (16O-enriched) and chemical compositions to those found in some ordinary and carbonaceous chondrites. It was proposed by Pack et al. (2004) that these were formed as open-system condensates in a nebulaAn immense interstellar, diffuse cloud of gas and dust from which a central star and surrounding planets and planetesimals condense and accrete. The properties of nebulae vary enormously and depend on their composition as well as the environment in which they are situated. Emission nebula are powered by young, massive Click on Term to Read More setting, and that this entire suite of forsterites might have originated in a single reservoir where the dust/gas ratio was ~4–5 × the solar ratio. It is thought that they formed early in Solar SystemDefinable part of the universe that can be open, closed, or isolated. An open system exchanges both matter and energy with its surroundings. A closed system can only exchange energy with its surroundings; it has walls through which heat can pass. An isolated system cannot exchange energy or matter with history during a period intermediate between the formation of CAIs and the onset of chondrule formation. In DaG 013, a small number of rare FeNi-metal grains were identified in forsterites of type-3 clasts. A study of twenty R chondrites, including DaG 013, was undertaken to identify remnant CAIs and Al- and spinel-rich objects; 101 CAIs were eventually identified (Rout and Bischoff, 2007, 2008). The CAIs that were identified contain highly altered phases, some composed of secondary alteration products such as nepheline and sodalite thought to have formed in a nebular setting. High abundances of oxides are present. Some refractory inclusionsInclusions found predominantly in carbonaceous chondrites and are rich in refractory elements particularly calcium, aluminum and titanium that in various combinations form minerals such as spinel, melilite, perovskite and hibonite. There are two types of refractory inclusion: • Ca Al-rich inclusions (CAIs) • Amoeboid olivine aggregates (AOAs) Refractory inclusions were Click on Term to Read More are composed of concentric spinel with Al-rich diopside rims, while other concentric spinel-rich inclusions contain abundant fassaite, hiboniteRefractory mineral, Ca-aluminate (CaAl12O19) that occurs in terrestrial metamorphic rocks and in CAIs of many chondrites. Meteoritic hibonite tends to be blue as seen in the meteorite Isheyevo (Ch/CB). Hibonite is one of the most refractory minerals found in primitive meteorites. Click on Term to Read More, or alteration products rich in Na, K, Cl, and Al (probably nepheline and sodalite). The most abundant CAISub-millimeter to centimeter-sized amorphous objects found typically in carbonaceous chondrites and ranging in color from white to greyish white and even light pink. CAIs have occasionally been found in ordinary chondrites, such as the L3.00 chondrite, NWA 8276 (Sara Russell, 2016). CAIs are also known as refractory inclusions since they Click on Term to Read More type found in R chondrites is an irregular-shaped, complex spinel-rich inclusionFragment of foreign (xeno-) material enclosed within the primary matrix of a rock or meteorite. Click on Term to Read More, possibly associated with hibonite, plagioclaseAlso referred to as the plagioclase feldspar series. Plagioclase is a common rock-forming series of feldspar minerals containing a continuous solid solution of calcium and sodium: (Na1-x,Cax)(Alx+1,Si1-x)Si2O8 where x = 0 to 1. The Ca-rich end-member is called anorthite (pure anorthite has formula: CaAl2Si2O8) and the Na-rich end-member is albite Click on Term to Read More, and fassaite, along with secondary alteration products such as oligoclase that typically has diopside rims and a high Al content. Some of the least altered, type 3 lithologies contain concentric fassaite-rich CAIs that include rare perovskite along with its secondary transformation product ilmenite. Similarities were observed between these CAIs from R chondrites and those found in CO and CM chondrites, as well as to those in the ordinary and E chondrite groups. Based on Δ17O values, the CAIs in R chondrites were divided into 16O-rich (~ –23‰ to –26‰), 16O-depleted (~ –2‰), and heterogeneous (–25‰ to +5‰) (Rout et al., 2009). In a like manner to CAIs of other chondrite groups, R chondrite CAIs were likely formed in an 16O-rich nebular region, with some sustaining subsequent isotopic exchange with an 16O-depleted nebular gas or through metasomatism on the parent asteroid. In contrast to CAIs from other chondrite groups, no meliliteGroup of minerals found in the CAIs of meteorites such as CV chondrites. Melilite consists almost exclusively of the binary solid solution gehlenite (Ca2Al2SiO7) – åkermanite (Ca2MgSi2O7). The melilite in CAIs is closer to gehlenite in composition. The first-formed (highest-temperature) melilite crystallizing from a melt is relatively aluminum-rich and becomes progressively Click on Term to Read More or grossiteCalcium aluminate, CaAl4O7, first found in metamorphosed Israeli limestone and recently in CAIs in CV3 and CR-CH-CB carbonaceous chondrites. Click on Term to Read More was observed. This unique chondrite group was originally named for the Carlisle Lakes, Australia (49.5 g) specimen but was subsequently renamed for the only fall of the group—a 67 g stone from Rumuruti, Kenya (see photo below). There have recently been numerous new R chondrite finds in the hot and cold deserts of Africa, Australia, and Antarctica. The specimen of DaG 013 shown above is a 0.232 g cut fragment. Compare the effects of terrestrial weathering on DaG 013 (wi-5) to the fresh fall of the type specimen Rumuruti (wi-0).
Photos courtesy of Stefan Ralew—SR-Meteorite