Posted on Leave a comment

NWA 3329

Diogenite
Orthopyroxenite
(≥90 vol% orthopyroxene)

standby for northwest africa 3329 photo
Purchased Spring 2005
no coordinates recorded A fragmented stone weighing 252 g was collected in Algeria from a similar location as NWA 2968. The fragments were subsequently purchased in Er-Rachidia, Morocco by collector F. Kuntz. A sample of these fragments was analyzed at the University of Washington at Seattle (A. Irving and S. Kuehner) and NWA 3329 was determined to be a diogenite composed primarily of coarse-grained, dark brown orthopyroxene, together with interstitial plagioclase, silica, phosphate, FeNi-metal, and FeS.

The NWA 3329 fragments were subsequently studied by Barrat et al. (2010). Interestingly, some fragments from the batch were found to be identical to the dunitic diogenite NWA 2968, and petrographic evidence indicates that both lithologies were collected from the same location, with both showing similar degrees of weathering. Importantly, other recovered fragments consist of both lithologies together—diogenite and dunite—which are each identical to their respective type samples. Furthermore, trace element studies conducted on both lithologies were found to be consistent with pairing, and their Δ17O values are indistinguishable as well (Greenwood et al., 2015). These investigators, in accord with an earlier suggestion by Barrat et al. (2010), interpret the evidence to indicate that both the orthopyroxenite and dunite lithologies are fragments from a common fall, likely as components of a mesosiderite. It was also noted by Greenwood et al. (2015) that the REE pattern previously determined for the diogenite NWA 5613 (Barrat et al., 2010) is virtually identical to that for NWA 3329. Further evidence for a mesosiderite–HED genetic relationship is revealed by the identical Δ17O values among the diogenites, such as NWA 3329 and NWA 2968, and the olivine-rich (dunite) clasts that have been identified in most all mesosiderites (Greenwood et al., 2015, 2017). It is considered likely that the dunitic clasts in mesosiderites were initially formed as upper-crustal plutonic cumulates, which were subsequently disrupted through impacts and incorporated with other HED lithologies prior to the formation of the mesosiderites. standby for greenwood diagram
Diagram credit: Greenwood et al., 2015
For an explanation of the diagram components see the open access article in GCA, vol. 169, p. 130 (2015)
Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites:
Implications for the “Great Dunite Shortage ” and HED-mesosiderite connection’
(https://doi.org/10.1016/j.gca.2015.07.023)

standby for o-isotopic diagram
Diagram credit: Greenwood et al., 2017
For an explanation of the diagram components see the open access article in Chemie der Erde – Geochemistry, vol. 77, p. 25 (2017)
‘Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies’
(http://dx.doi.org/10.1016/j.chemer.2016.09.005)
A comparison of reflectance spectra of seven near-Earth asteroids to those of HED-group meteorites revealed that all of the pyroxene mineralogies were consistent with eucrites and howardites, but not to diogenites. Therefore, they suggest that there are no km-sized or larger objects composed strictly of diogenite material, but instead, diogenites might exist as a single component within a mixture of lithologies on the HED asteroid. Beck et al. (2012) identified the first olivine-rich melt material present in the howardites of the PCA 02009 pairing group. This olivine-rich material was likely derived from harzburgitic and dunitic lithologies exposed on the surface of Vesta. Further investigation employing the Antarctic DOM 10 howardite pairing group was conducted by Hahn et al. (2018). They sought to identify Mg-rich harzburgitic (distinguished from diogenitic) silicates (Mg# >80 and >85 for olivine and pyroxene, respectively) that represent HED mantle material. From results of a comprehensive geochemical analysis, they contend that these Mg-rich fragments are not related to cumulate diogenites, but instead are more consistent with a mantle residue that was affected by a late infiltration of metasomatic melt. In addition, they determined that QUE 93148 also likely represents a mantle residue from the HED parent body. The larger degree of partial melting (~35–55%) required to produce the observed Mg-rich lithologies, considered to be mantle residua, is attributed by Hahn et al. (2018) to a hybrid magma ocean model that combines aspects of the magma ocean model of Mandler and Elkins-Tanton (2013) and the shallow magma ocean model of Neumann et al. (2014) (see diagrams B and D below). standby for magma ocean diagrams
click on image for a magnified view

Diagram credit: Hahn et al., MAPS, vol. 53, #3, p. 541 (2018)
‘Mg-rich harzburgites from Vesta: Mantle residua or cumulates from planetary differentiation?’
(http://dx.doi.org/10.1111/maps.13036)
Further information regarding the origin of the dunitic clasts in our collections can be found on the Vaca Muerta page. To see an alternative classification system for the diogenites and dunites based on mineralogical and petrographical features, proposed by Beck and McSween (2010) and modified by Wittke et al. (2011), click here. The photo shown above is a 0.52g fragment of NWA 3329. The photo below is an excellent petrographic thin section micrograph of NWA 3329, shown courtesy of Peter Marmet. standby for nwa 3329 ts photo
click on image for a magnified view
Photo courtesy of Peter Marmet


Posted on Leave a comment

NWA 2968

Diogenite
Dunite
(Ungrouped achondrite in MetBull 91)

standby for northwest africa 2968 photo
Purchased November 2005
no coordinates recorded

Through the untiring efforts of nomads searching the vast Sahara Desert region of Algeria, a remarkable new meteorite type has been recovered. Only 268 g of small broken fragments of a coarse-grained, dark brown meteorite was found, and these fragments were subsequently sold in Erfoud, Morocco to meteorite dealer B. Reed. The small size range of these fragments (17 mm to at least 25 mm wide) is due to fracturing along compression and shear zones. Numerous pieces of this meteorite were submitted for analysis and classification (T. Bunch and J. Wittke, NAU; A. Irving, UWS; D. Rumble III, CIW).

Northwest Africa 2968 is a cumulate, olivine-rich (>95 vol%), dunitic rock, containing minor amounts of orthopyroxene and FeNi-metal, along with troilite and pyrrhotite which primarily fill fractures. Olivines exhibit shock features including domain offsets, mosaicism, and undulatory extinction. The FeO/MnO ratios and O-isotopic compositions of NWA 2968 are consistent with an origin from the howardite/eucrite/diogenite/ (HED) parent body, widely accepted to be the asteroid 4 Vesta. The averaged Δ17O value of –0.23 (±0.02) plots within the field of the HED group (Scott et al., 2009); see a linearized O-isotope plot (Miller, 2002). However, in contrast to the olivine and orthopyroxene in known howardites or diogenites, these minerals are considerably more highly magnesian in NWA 2968 (92.5 and 93, respectively), and likely crystallized from a less evolved parental melt. The composition and mineralogy of NWA 2968 is consistent with a mantle or lower crustal origin on a differentiated body, in a formation region analogous to that of the chassignites on Mars. standby for greenwood diagram
Diagram credit: Greenwood et al., 2015
For an explanation of the diagram components see the open access article in GCA, vol. 169, p. 130 (2015)
Geochemistry and oxygen isotope composition of main-group pallasites and olivine-rich clasts in mesosiderites:
Implications for the “Great Dunite Shortage ” and HED-mesosiderite connection’
(https://doi.org/10.1016/j.gca.2015.07.023)

standby for o-isotopic diagram
Diagram credit: Greenwood et al., 2017
For an explanation of the diagram components see the open access article in Chemie der Erde – Geochemistry, vol. 77, p. 25 (2017)
‘Melting and differentiation of early-formed asteroids: The perspective from high precision oxygen isotope studies’
(http://dx.doi.org/10.1016/j.chemer.2016.09.005)
It is still unresolved whether such dunitic lithologies represent higher-level cumulates or if they are instead mantle material (Greenwood et al., 2015). Mandler and Elkins-Tanton (2013) proposed a formation scenario for such dunites that involves a two-stage crystallization process: first, an equilibrium crystallization process from the late-stage liquid after 60–70% solidification of the global magma ocean; second, a fractional crystallization process within an ascended, high-level (crustal) pluton composed of the former extracted residual melt, ultimately resulting in the formation of a thin lower-crustal dunite layer along with more shallow olivine diogenite, diogenite, and cumulate eucrite lithologies. On the other hand, if the dunitic clasts are actually derived from mantle material, a scenario is required to explain how such material was incorporated into the regolith. However, it was argued by Barrat and Yamaguchi (2014) that magma chamber processes are unable to explain the chemical diversity of the diogenites (e.g., the range of heavy-REE ratios in diogenitic orthopyroxenes), and that neither assimilation of wallrock nor incorporation of a trapped melt component can account for this diversity. They contend that the diversity is more likely the result of variability in the respective initial parental melt compositions.

It is noteworthy that through continuing studies of MIL 03443, which is a cumulate, monomict, brecciated dunite previously classified as a mesosiderite clast, strong evidence has been developed for an origin on the HED parent body, and a relationship to diogenites specifically (Mittlefehldt, 2008; Beck et al., 2011). This evidence includes FeO/MnO and Δ17O values (see plot from Greenwood et al., 2015), the occurrence of olivine melt inclusions, and the abundances of pyrrhotite, Ni and Co. MIL 03443 has been shown to represent a fractional cumulate rather than a mantle restite (Beck et al., 2011). In a similar way, O-isotopic and trace element data for the unique 1.1 g olivine-rich (dunitic/harzburgitic?) achondrite QUE 93148 have led to the suggestion that it might be derived from the deep mantle of the HED parent body (Goodrich and Righter, 2000; C. Floss, 2003). However, due to its lower Co and Ni abundances than what would otherwise be expected for an olivine-rich mantle lithology or magma ocean cumulate, QUE 93148 could have actually originated on a distinct planetary body such as that of the main-group pallasites (Shearer et al., 2008; Shearer et al., 2010). Two other possible HED-related dunites, NWA 5784 and NWA 5968, will require further study to accurately assess their classification.

Notably, Beck et al. (2012) identified the first olivine-rich melt material present in the howardites of the PCA 02009 pairing group. This olivine-rich material was likely derived from harzburgitic and dunitic lithologies exposed on the surface of Vesta. Further investigation employing the Antarctic DOM 10 howardite pairing group was conducted by Hahn et al. (2018). They sought to identify Mg-rich harzburgitic (distinguished from diogenitic) silicates (Mg# >80 and >85 for olivine and pyroxene, respectively) that represent HED mantle material. From results of a comprehensive geochemical analysis, they contend that these Mg-rich fragments are not related to cumulate diogenites, but instead are more consistent with a mantle residue that was affected by a late infiltration of metasomatic melt. In addition, they determined that QUE 93148 also likely represents a mantle residue from the HED parent body. The larger degree of partial melting (~35–55%) required to produce the observed Mg-rich lithologies, considered to be mantle residua, is attributed by Hahn et al. (2018) to a hybrid magma ocean model that combines aspects of the magma ocean model of Mandler and Elkins-Tanton (2013) and the shallow magma ocean model of Neumann et al. (2014) (see diagrams B and D below). standby for magma ocean diagrams
click on diagrams for a magnified view

Diagram credit: Hahn et al., MAPS, vol. 53, #3, p. 541 (2018)
‘Mg-rich harzburgites from Vesta: Mantle residua or cumulates from planetary differentiation?’
(http://dx.doi.org/10.1111/maps.13036)
For further information about this dunitic meteorite and its potential pairing relationships see the NWA 3329 page. An alternative classification system for the diogenites based on mineralogical and petrographical features has been proposed by Beck and McSween (2010), and modified by Wittke et al. (2011). The photo shown above is a 5.8 g fragment of NWA 2968.


Posted on Leave a comment

Tatahouine

Diogenite
Orthopyroxenite
(≥90 vol% orthopyroxene)

standby for tatahouine photo
Fell June 27, 1931
32° 57′ N., 10° 25′ E. At 1:30 A.M., fragments of a rare diogenite fell over a radius of 500 meters, 4 km NE of a small village in Tunisia. Local Bedouins immediately collected over 12 kg of mostly minute fragments which were sent to the Muséum National d’Histoire Naturelle in Paris.

Because the larger mass broke up along mineral grain boundaries late in its entry, the unbrecciated orthopyroxene (hypersthene) crystals were not melted and fusion crust is rare (see photo below), rendering this meteorite difficult to recognize. Tatahouine is light olive-green in color, translucent-to-opaque, and is crisscrossed by small black veinlets up to 2 mm wide. Tatahouine has a cumulate texture with exceptionally large crystals, many reaching 2 cm long. Inclusions of silica, troilite, chromite, and minor metal occur throughout. Other diogenites with compositions similar to Tatahouine but with various grain textures have since been found, including NWA 1821 and NWA 3329.

In 1994, Dr. Alain Carion revisited the strewnfield and recovered many more specimens, raising the total recovered weight to ~13.5 kg. Interestingly, these recent specimens were found to be contaminated by secondary minerals including carbonates containing rod-shaped objects, these being similar in size and shape to those found in the martian meteorite ALH 84001. Other terrestrial weathering effects include increased concentrations of Ba and Sr, and formation of calcite and iron hydroxide.

Diogenites exhibit a wide range of incompatible element abundances. Tatahouine, NWA 5480, and MET 00424 represent a subgroup of diogenites exhibiting an HREE enrichment and Eu anomalies considered to be inherited from compositionally distinct parental melts, possibly a product of remelting of previously formed magma ocean orthopyroxene cumulates, some of which were contaminated by partial melts from the eucritic crust and a possible phase of fractional crystallization (Barrat et al., 2010). Consistent with this hypothesis is the conclusion by Yamaguchi et al. (2013) from their analyses of NWA 5480 that this olivine diogenite is an impact melt rock formed at the bottom of a large basin such as Rheasilvia. In a another study, Fukuoka et al. (1977) speculated that Tatahouine could be a restite of orthopyroxene cumulates. The REE patterns among the three recognized members of this subgroup are virtually indistinguishable, and they possibly formed from the same parental source magma (Barrat et al., 2010). While it is hypothesized that olivine diogenites like NWA 5480 are cumulates constituting distinct layered magmatic intrusions emplaced into the crust, the unusual features exhibited in NWA 5480 suggest a possible origin as a mantle residue; however, analyses by Yamaguchi et al. (2013) led them to conclude that this diogenite is an impact melt rock formed at the bottom of a large basin such as Rheasilvia.

Newly revised calculations of cosmic-ray exposure ages using the 10Be/21Ne method have shown that Tatahouine coincides with the major HED peak at 38 m.y. A more detailed scenario for the petrogenesis of the diogenites can be found on the Johnstown page. To see an alternative classification system for the diogenites based on mineralogical and petrographical features, proposed by Beck and McSween (2010) and modified by Wittke et al. (2011), click here. The specimen of Tatahouine shown above is a 2.9 g fragment containing multiple black shock veins. The photos below show a Tatahouine mass found in 2013 on which remnant fusion crust has been preserved, shown courtesy of Sergey Vasiliev.

standby for tatahouine crust photo
standby for tatahouine crust photo
standby for tatahouine crust photo
Photos courtesy of Sergey Vasiliev—SV-meteorites


Posted on Leave a comment

NWA 6928

Diogenite
Norite
standby for nwa 6928 photo
Purchased May 2011
no coordinates recorded A single stone weighing 223 g was purchased by G. Fujihara from a Moroccan dealer. A sample was submitted for analysis and classification to the University of Washington at Seattle (A. Irving). While NWA 6928 was initially considered to be an unusual basaltic eucrite, chemical analyses revealed that it was a rare noritic diogenite consisting of 80 vol% orthopyroxene (Fs33.4–34.1) and 19 vol% interstitial anorthitic plagioclase, along with accessory (<5 vol%) Ti–Al-bearing chromite, troilite, and merrillite (Irving et al., 2014). New terminology has been proposed in a revision to the diogenite classification scheme utilizing an IUGS-based system (Beck and McSween, 2010; Wittke et al., 2011).

Northwest Africa 6928 has a coarse-grained, cumulus texture and is considered to have crystallized from an ancient, relatively ferroan magma source, possibly a global magma ocean. It exhibits localized shock features including undulose extinction and cataclasis. A number of noritic and feldspathic diogenites have been recognized and described among the more recent meteorite recoveries from the dense concentration area of Northwest Africa, including NWA 8000, 8367, 8744, 10268, and 10388 (Irving et al., 2014, 2016).

The close compositional similarites that exist between the noritic diogenites and mesosiderites suggests that a genetic link might exist (Irving et al., 2016). If that is true, these diogenites probably derive from a parent asteroid distinct from 4 Vesta. The photo of NWA 6928 shown above is a 3.03 g slice expertly prepared by Montana Meteorite Laboratory. The top photo below is the reverse side of the 3.03 g slice, and the bottom photo shows the main mass before cutting. standby for nwa 6928 photo

standby for nwa 6928 photo
Photo courtesy of Big Kahuna Meteorites


Posted on Leave a comment

NWA 5312

Diogenite
Olivine–Orthopyroxenite
(orthopyroxene + 10–40 vol% olivine)

standby for nwa 5312 photo
Purchased 2007
no coordinates recorded A single stone weighing 354 g was purchased by F. Kuntz from in Morocco. Sample were submitted for analysis and classification to the Northern Arizona University (T. Bunch and J. Wittke). Previously, NWA 5312 was determined to be consistent with membership in the newly characterized olivine diogenite group, but in accordance with the new terminology proposed in a revision to the diogenite classification scheme, utilizing an IUGS-based system, NWA 5312 is now considered to represent the olivine–orthopyroxenite subgroup of diogenites (Beck and McSween, 2010; Wittke et al., 2011).

Northwest Africa 5312 is composed of 46 vol% orthopyroxene, 24 vol% olivine, and 18 vol% troilite, along with lesser amounts of merrillite, chromite, and taenite (Irving et al., 2009). Oxygen isotopic ratios determined for NWA 5312 overlap those of other HED members (D. Rumble III, Carnegie Institution). Other members of this rare diogenite group include GRA 98108, ALHA77256, LEW 88679, and NWA 5405.

For more details on the formation of diogenites visit the Johnstown page. The photo of NWA 5312 shown above and below is a 6.95 g partial slice. standby for nwa 5312 photo