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NWA R-Type (Unclassified)

R chondrite breccia, rumurutiite (tentative classification)
(possibly paired with NWA 4814) standby for northwest africa r chondrite photo
click on photo for a magnified view Found before 2007
no coordinates recorded Many possibly paired stones and fragments representing a unique rumurutiite are thought to be distributed in both Europe (300 g stone) and the United States (400 g fragments). The specimen shown above is reportedly from a lot of two large, fresh, partially fusion-crusted stones weighing together 2,635 g, acquired by American collectors/dealers. A type sample from one or more of these stones was sent to Northern Arizona University for analysis, assignment of a provisional NWA-series number, and classification; however, the exact provenance for the specimen shown above has been lost. As photographed, the specimen above exhibiting pronounced dark and lighter lithologies bears a strong resemblance to both the 300 g R3–6 NWA 2943 found in 2005, and to the 1,120 g R4 [R3–6] chondrite NWA 4814 found in 2006, either of which might or might not share a common source meteorite. A search for more information on these meteorites could help determine whether they derive from a common meteorite, or perhaps are launch paired.

This rumurutiite exhibits clasts representing a wide variety of petrologic types. Dark, highly unequiliberated clasts containing distinct chondrules represent type 3 material, while lighter clasts are more highly metamorphosed and represent petrologic types possibly as high as 6. A modified version of the Van Schmus–Wood classification scheme has been proposed by Berlin and Stöffler (2004) to accommodate the R chondrite metamorphic variation present in the pyroxene, feldspar, and sulfides, especially the lack of low-Ca pyroxene in types 5 and 6:

Modified Van Schmus–Wood Classification Scheme For R Chondrites
3 4 5 6
Homogeneity
of olivine
>5% mean deviation homogenous homogenous homogenous
Pyroxene predominantly
low-Ca pyroxene
low-Ca and
Ca-rich pyroxene
only Ca-rich
pyroxene
only Ca-rich
pyroxene
Feldspar small glassy
intergrowths
isolated intergrowths networks forming well-developed
networks
Sulfides even distribution even distribution even distribution mobilized

A more comprehensive look at the rumurutiite chondrite group can be found on the DaG 013 page. The photo shown above is a 1.89 g slice exhibiting a range of petrologic types in this R chondrite breccia.


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NWA 7514

R5, rumurutiite
standby for northwest africa 7514 photo
Found 2005
coordinates not recorded A large 23.62 kg stone covered by a thin, translucent, olive-gray crust, and not considered to be a meteorite, was gifted by a meteorite collector to T. Stout. In June 2012, a portion was sent to Cascadia Meteorite Laboratory in Oregon (A. Ruzicka and M. Hutson) for analysis, and it was determined that the stone was a meteorite after all—the largest R5 chondrite known, designated NWA 7514.

The interior of this fresh R chondrite appears homogeneous and somewhat featureless in hand section, with an overall gray color. In thin section, chondrules are evident but indistinct, embedded in a transparent matrix. Features are consistent with a shock stage in the range of S2–S4, and the meteorite is virtually unweathered, reflecting the weathering index (wi) value of 0 (Rubin and Huber, 2005). In contrast to some R chondrites which have experienced significant aqueous alteration of their sulfide minerals (mainly pyrrhotite and pentlandite) and contain abundant hydrous phases and brown staining (e.g., amphibole, phlogopite, and apatite in LAP 04840 and MIL 11207; Gross et al., 2013, 2017), the sulfides in NWA 7514 are virtually unaltered, reflecting a formation in dry conditions and a lack of terrestrial aqueous alteration (Ruzicka et al., 2013).

An oxygen three-isotope diagram was prepared from values determined at The Open University, United Kingdom (R. Greenwood), and the plot falls within the Rumuruti-chondrite field. The R-chondrite group is considered to be one of the most oxidized groups of meteorites, and NWA 7514 has the highest Δ17O value of any R chondrite measured to date. The specimen of NWA 7514 shown above is a 0.73 g partial slice. The photo below shows the unusual and awesome main mass. standby for northwest africa 7514 photo
Photo courtesy of Tim Stout and Rob Wesel—Nakhla Dog Meteorites


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NWA 1668

R5, rumurutiite
standby for northwest africa 1668 photo
Purchased October 2002
coordinates not recorded A very fresh 710 g stone with black fusion crust was found in Morocco or Algeria. It was subsequently purchased in Denver from a Moroccan dealer by D. Gregory. Analysis and classification was completed at the Department of Earth and Space Sciences, University of Washington (A. Irving and S. Kuehner), and this meteorite, named NWA 1668, was determined to be an R5 genomict breccia.

As is typical for R chondrites, chondrules are more sparsely distributed than in other chondrites. They have an average diameter of ~0.3 mm, which is larger than those in CO3 chondrites and smaller than those in ordinary chondrites (Imae and Zolensky, 2003), a size consistent with what is expected to occur at large heliocentric distances. With respect to mineralogy, NWA 1668 is primarily composed of olivine (Fa38.9), clinopyroxene, orthopyroxene, troilite, and Ti-chromite, along with minor sodic plagioclase, and rare, high-Ni metal (awaruite, composed of 72% Ni). The sparsity of metal, or lack thereof in some R chondrites, indicates that R chondrites experienced highly oxidizing conditions, probably both in the nebula and on the parent body.

Parent body metamorphism in an oxidizing, water-rich environment is attested by the hydroxyl-rich minerals amphibole, phlogopite, and apatite present in the R6 chondrites LAP 04840 and MIL 11207. It is considered that these chondrites experienced metamorphism of insoluble organic matter at high temperatures (~720°C) and at significant depth (tens of km) within a lithologic unit in which water with a high D/H ratio was pervasive. This deep burial is considered to be the result of reassembly following impact disruption on the R chondrite parent body (McCanta et al., 2006, 2008). In a study of these two hydroxyl-bearing R chondrites, Gross et al. (2017) reasoned that such high abundances of hydrous phases could only be established under conditions of high water vapor pressure (20–700 bar), and such pressures could only be maintained on this relatively small body through the emplacement of a solid ice shield.

Noble gas analyses of the known Northwest Africa R chondrites were conducted by Vogel et al. (2014). Based on the results, they have tentatively placed these numerous R chondrites into ~16 groupings representing possible common source craters and/or fall events. They have proposed a pairing exists between NWA 1668 and NWA 2897 which show a common CRE age of ~66 m.y.

Northwest Africa 1668 is one of the freshest R chondrites found to date, next to the 1934 fall in Rumuruti and the 23.6 kg NWA 7514. 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 section 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 pyrrhotite and pentlandite), which are typically prevalent in the R chondrite group as a result of formation in a high-sulfur fugacity; e.g., Rumuruti wi-0 contains 8.0 wt%.

In addition to NWA 1668 and Sah 99527 (both R5), several other unbrecciated R chondrites have been recovered so far, including the following: HaH 119, Ouzina, NWA 053, NWA 800, and Sah 98248 (all R4); NWA 753 (R3.9); Carlisle Lakes and NWA 978 (R3.8); NWA 755 (R3.7); and Acfer 217 (R3.8–5). Further details about the R chondrite group can be found on the DaG 013 page. The photo above shows a 4.0 g partial slice of this rumurutiite.


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NWA 978

R3.8, rumurutiite
standby for northwest africa 978 photo
Purchased August 1, 2001
coordinates not recorded Three fragments weighing together 722 g were purchased in Erfoud, Morocco. Classification was performed at UCLA, and Northwest Africa 978 was determined to be an R3.8 chondrite. Although it was initially assigned a relatively low weathering grade of W2, a more useful weathering index (wi) has since been 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 section 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 pyrrhotite and pentlandite), which are typically prevalent in this meteorite group; e.g., Rumuruti wi-0 contains 8.0 wt% sulfides. Northwest Africa 978 was determined to have a weathering index of wi-5, or severely weathered. This meteorite has been weakly shocked (S3).

In their noble gas studies, Schultz et al. (2005) suggested pairing several R chondrite finds of similar petrologic type with NWA 978 (21Ne-based CRE age of 10.8 (±0.4) m.y.), including NWA 755, NWA 845, NWA 851, and NWA 1471. Subsequent noble gas analyses of the known Northwest Africa R chondrites were conducted by Vogel et al. (2011, 2014). Their ~16 groupings vary slightly from the previous pairings suggested by other investigators, and they propose an ~10 m.y. CRE age group representing possible common source craters and/or fall events which includes R chondrites with the NWA series designations 755, 845, 851, 978, 1471, 2198, and 5069, and possibly DaG 013.

This is a unique group of chondrites having a higher volume of olivine (FeO-rich), a lower volume of pyroxene, and essentially no FeNi-metal as compared to all other chondrite groups. This is thought to be the result of metasomatic oxidation in which FeNi-metal and pyroxene reacted with water to form olivine (Isa et al., 2010). While R chondrites share certain similarities with ordinary chondrites including refractory element depletions and siderophile element abundances, they differ in volatile element abundances and petrologic trends. The triple increase in Zn relative to ordinary chondrites and the sparsity of metal, or lack thereof, observed in some R chondrites, indicates that this group experienced highly oxidizing conditions both in the nebula and on the parent asteroid.

Parent body metamorphism in an oxidizing, water-rich environment is attested by the hydroxyl-rich minerals amphibole, phlogopite, and apatite present in the R6 chondrites LAP 04840 and MIL 11207. It is considered that these chondrites experienced metamorphism of insoluble organic matter at high temperatures (~720°C) and at significant depth (tens of km) within a lithologic unit in which water with a high D/H ratio was pervasive. This deep burial is considered to be the result of reassembly following impact disruption on the R chondrite parent body (McCanta et al., 2006, 2008). In a study of these two hydroxyl-bearing R chondrites, Gross et al. (2017) reasoned that such high abundances of hydrous phases could only be established under conditions of high water vapor pressure (20–700 bar), and such pressures could only be maintained on this relatively small body through the emplacement of a solid ice shield.

The difference in O-isotopic abundances between the R chondrites and the ordinary chondrites is greater than it is among the H, L, and LL ordinary chondrite groups, further resolving the R 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. Because R chondrites have the highest 17O value of any other Solar System material, they plot on a different O-isotope trend line than the other chondrites. This plot is farther from the carbonaceous and enstatite chondrites than it is from the ordinary chondrites (Rubin and Kallemeyn, 1989). Rumuruti chondrites of type 3 contain noble metals such as platinum, osmium, and germanium that originated as nebular condensates. In higher metamorphic types, the volatile germanium phases are decomposed, while other noble metal phases are transformed into mostly arsenides and tellurides.

In addition to NWA 978 (R3.8), several other unbrecciated R chondrites have been recovered so far, including the following: Sah 99527 and NWA 1668 (both R5); HaH 119, Ouzina, NWA 053, NWA 800, and Sah 98248 (all R4); NWA 753 (R3.9); Carlisle Lakes (R3.8); NWA 755 (R3.7); and Acfer 217 (R3.8–5). The photo above shows a 3.5 g slice of NWA 978, while the top photo below shows the fresh fusion crust on the 361.6 g main mass. The bottom image is an excellent petrographic thin section micrograph of NWA 978, shown courtesy of Peter Marmet. standby for northwest africa 978 photo
Photo courtesy of Mike Farmer—Mike’s Meteorites and Tektites

standby for nwa 978 ts photo
click on image for a magnified view
Photo courtesy of Peter Marmet


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NWA 753

R3.9, rumurutiite
standby for northwest africa 753 photo
Found January 2000
coordinates not recorded Multiple fragments weighing together 12 kg were purchased in Rissani, Morocco, while the actual find location was probably the Kem Kem region of the Sahara. Classification was performed at the Institut für Planetologie in Münster, Germany. Northwest Africa 753 is a brecciated, very weakly shocked (S2) meteorite that is relatively fresh compared to most other R chondrite finds—designated W2 on the Wlotzka weathering scale (1993). Abundant sulfides, typically pyrrhotites, attest to a S-rich environment of formation (Jackson and Lauretta, 2010). It has been established through laboratory analyses that several pairings to NWA 753 exist, including NWA 1472, 1476, 1477, 1478, and 1566.

Results of Mn–Cr isotopic systematics have established an initial age for NWA 753 of 4.561 (±2) m.y., which is a few m.y. younger than the age established for carbonaceous chondrites (~4.566 m.y.) (Jogo et al., 2006). A possible correlation of Cr-isotopic compositions with O-isotopic variations for ordinary and R chondrites has been demonstrated.

In their search for CAIs and other Al-rich objects in R chondrites, Rout and Bischoff (2008) and Rout et al. (2009) discovered a high abundance of such objects in unequilibrated clasts from NWA 753. They determined that the abundance of CAIs in R chondrites is less than it is in carbonaceous chondrites, but greater than in ordinary and enstatite chondrites. These CAIs in R chondrites are significantly smaller than those in CM and CV chondrites, but similar in size to those in CH, O, and E chondrites. 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‰). As with 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. Taking into account the differences in mineralogy between the majority of R chondrite CAIs and those from other groups, R chondrites contain a unique subset of CAIs. That said, the investigators argue that certain types of CAIs present in R chondrites strongly resemble some of those found in the CO-chondrite group, especially those present in the more highly metamorphosed meteorites like Ornans, Moss, Isna, and Lancé. Other Al-rich objects in R chondrites share similarities to those from the O- and E-chondrite groups.

Kita et al. (2013) employed the same technique for NWA 753 that is applied to ordinary chondrite chondrules for determination of the petrologic subtype from 3.00 to 3.2, which is based on the alteration resistant Cr content in ferroan olivine (Grossman, 2004; Grossman and Brearley, 2005). They determined that the corresponding petrologic subtype for the least equilibrated material in NWA 753 is 3.15–3.20, the lowest subtype found among R chondrites. Crowther et al. (2015) utilized I–Xe chronometry in a preliminary study of four samples of NWA 753 having a petrologic type range of 3.15–3.9. They found that a correlation exists between the closure age and the petrologic type of a sample, with a younger age being correlated with a higher degree of metamorphism. This range of petrologic types reflects a difference in age of ~5 m.y. years and is consistent with cooling within an onion-shell structure on the R-chondrite parent body.

Consistent with the absence of solar gases, NWA 753 is not thought to have been part of a regolith for any significant length of time. Evidence gathered thus far indicates that NWA 753 experienced a relatively rapid transfer from the asteroid belt to the Earth, having a 21Ne-based CRE age of 12.8 m.y. (Schultz et al., 2005). Subsequent noble gas analyses of the known Northwest Africa R chondrites were conducted by Vogel et al. (2014). Their ~16 groupings vary slightly from the previous pairings suggested by other investigators, and they propose an ~14 m.y. CRE age group representing possible common source craters and/or fall events which includes R chondrites with the NWA series designations 753, 1472, 1476, 1477, 1478, 1566, 4360, 4419, and 5606.

This unique chondrite group was originally named for the Carlisle Lakes, Australia (49.5 g) specimen, but has since been renamed for the only fall of the group from Rumuruti, Kenya (67 g) in 1934. Two views of a 0.54 g partial slice of NWA 753 are shown above: the right photo shows the melted fusion-crusted surface, while the left photo shows the abundant tiny chondrules characteristic of this chondrite group.