DHOFAR 1988


CY2.2
Thermally metamorphosed/dehydrated to stage IV
(C2-ung in MetBull 104)

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Found December 2, 2011
19° 4.865' N., 54° 46.026' E.

A relatively fresh, partially crusted carbonaceous chondrite weighing 59 g was found in the Dhofar desert region of Oman by M. Cimala and L. Smula (see in situ photos below). A sample was sent for analysis to the Natural History Museum in Berlin (A. Greshake) and the University of New Mexico (oxygen isotopes; K. Ziegler), and Dhofar 1988 was classified as C2-ungrouped.

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Photos courtesy of Marcin Cimala

Dhofar 1988 and three other possibly paired brecciated meteorites were found within 3 km of each other (see Google Earth image below). These include the anomalous CM chondrite Dhofar 735 (oxygen isotope plot; photo courtesy of Thomas Witzke), the CM2 chondrite Dhofar 955, and the ungrouped carbonaceous chondrite Dhofar 2066 (oxygen isotope plot; photo courtesy of Lukasz Smula).

Google Earth Image of Dhofar (Zufar) Region, Oman

Dhof 735 (2002) 381 g: 19° 4.1' N, 54° 46.8' E

Dhof 955 (2003) 16 g: 19° 3.3' N, 54° 45.2' E

Dhof 2066 (2010) 13 g: 19° 3.728' N, 54° 44.665' E

Dhof 1988 (2011) 59 g: 19° 4.865' N, 54° 46.026' E

Coordinates from MetBull

A petrographic component analysis of Dhofar 1988 was conducted by Suttle et al. (2021) and presented in terms of area% as follows: ~60–70% matrix, ~20% coarse-grained chondrules (ave. 530 µm) and CAIs, and ~10–20% sulphides and carbonates. An XRD-based modal mineral analysis gave abundances of 73 vol% olivine (11 vol% primary and 62 vol% secondary), 14 vol% troilite, 7 vol% pyroxene, and 5 vol% calcite. Most of the chondrules in Dhofar 1988 have experienced at least partial aqueous alteration, whereas others have become pseudomorphs during a dehydration/recrystallization stage in which phyllosilicates were transformed into secondary olivine at temperatures of at least 500–750°C—heating stage IV of Nakamura (2005) and Kimura et al. (2009) (see table below). Suttle et al. (2021) ascertained that the primary olivine, which is present in chondrule cores and as large matrix grains, has a Mg-rich (Fo81.099.7; ave. Fo97.4) composition. They were able to infer that the former phyllosilicate matrix was predominantly an Al-poor serpentine with the chondrule cores having a higher Al content, and that an intermixed matrix component identified as micron-scale FeNi-sulfide grains reflects the former presence of abundant tochilinite–cronstedtite intergrowths (TCI, formerly PCP or "poorly characterized phases"). A comparison of aqueous alteration degree for CY chondrites based on the phyllosilicate fraction (PSF) scheme of Howard et al. (2015) is shown in the diagram below, where Dhofar 1988 is a subtype 1.3 within a range that encompasses 1.6–1.0; this is equivalent to subtype 2.2 in the scheme of Rubin et al. (2005).

Characteristic Features in CM and CY Chondrites Reflecting Secondary Heating; Nakamura (2005) and Kimura et al. (2009)
  Stage I Stage II Stage III Stage IV
Temperature <300°C 300–500°C 500–750°C >750°C
Major Features kamacite or martensite without plessite;
some pentlandite blebs in pyrrhotite
serpentine decomposes;
pentlandite blebs in pyrrhotite common
tochilinite and pentlandite decompose to pyrrhotite,
kamacite, and Ni-rich metal; secondary olivine forms
secondary low-Ca pyroxene crystallizes;
dehydration to anhydrous minerals
Examples Murchison, Murray, Nogoyo, Cold Bokkeveld A-881334, A-881655, Y-793321, Y-86695 A-881655, Y-82054, Y-82098 (or IV), Y-86029 B-7904, Dhofar 1988, Y-82162, Y-86720/789

Comparative Degree of Aqueous Alteration Among CY Chondrites
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Diagram credit: Suttle et al., GCA, vol. 295, p. 299 (2021)
'The alteration history of the CY chondrites, investigated through analysis of a new member: Dhofar 1988'
(https://doi.org/10.1016/j.gca.2020.11.008)

Dhofar 1988 has an O-isotopic composition that is enriched in heavy oxygen (18O, 17O), and has a plot very close to the C2-ungrouped Tagish Lake (oxygen isotope plot) and to the CM/CI-like, thermally metamorphosed/dehydrated Antarctic meteorite group termed 'CY' by Y. Ikeda (1992 [Consortium Summary]), which consists of Belgica 7904 (C2-ung), Y-82162 (C1/2-ung), Y-86029 (CI1), Y-86720 (C2-ung), Y-86789 (C2-ung, likely paired with Y-86720), and Y-980115 (CI1); see the MetBull oxygen isotope plot and diagrams below.

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Diagram credit: King and Russell, 50th LPSC, #1386 (2019)
See also open access article by King et al. in Chemie der Erde–Geochemistry, vol. 79 (2019)

standby for dhofar 225 oxygen isotope diagram
Diagram credit: Greenwood et al., GCA, vol. 277, p. 381 (2020, open access link)
'Linking asteroids and meteorites to the primordial planetesimal population'
(https://doi.org/10.1016/j.gca.2020.02.004)

Other possibly related dehydrated CM-like meteorites include WIS 91600 (CM2.2 [CI]), EET 96010 (CM2), and PCA 02012 (CM2). Furthermore, Nakamura (2006) identified two regolith breccias, Y-793321 (CM2) and A-881458 (CM2), that contain solar-wind-implanted noble gases and belong to this dehydrated group, while M.M.M. Meier (2014) found that the meteorite Diepenveen (CM2-an) also contains similar trapped solar gases. Both Y-86737 and Y-980134, which are classified as CI1, have similar oxygen isotopes (King and Russell, 2019). In addition, Goodrich et al. (2019) revealed that clast 28 in the polymict ureilite NWA 10657 has an O-isotope composition that plots near the CY group, and a Cr-isotope composition that is unique but plots near the CI group (see diagrams below). Cr isotopes should be obtained for other CY chondrites in the future for comparison. Notably, The CM-an Dhofar 225 has many features and an oxygen isotopic composition like Dhofar 735, which along with Belgica 7904, Y-86720, and PCA 02012 have experienced the highest temperatures (~900°C) over a brief time interval (PCA 02012 estimated at tens of hours; Nakato et al., 2013) compared to other members of the CY group.

standby for ureilite clasts oxygen isotope diagram
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Diagrams credit: Goodrich et al., 50th LPSC, #1312 (2019)

It was experimentally demonstrated by Lindgren et al. (2020) that the slope of 0.48, which represents the increase in heavy oxygen isotopes from heating of CM chondrite ALH 83100 to 800°C, does not intersect the CY compositional field (see diagram below). Therefore, they concluded that CY chondrites cannot be derived from CM2 chondrites through any thermal process.

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Diagram credit: Lindgren et al., GCA, vol. 289, p. 77 (2020, open access link)
'Signatures of the post-hydration heating of highly aqueously altered CM
carbonaceous chondrites and implications for interpreting asteroid sample returns'
(https://doi.org/10.1016/j.gca.2020.08.021)

Employing position-sensitive-detector X-ray diffraction (PSD-XRD), King et al. (2015) determined the modal mineralogy of a suite of CI and CY chondrites (see diagrams below). The total phyllosilicate abundances, ~83 vol% (81–84 vol%) for CI and ~79 vol% (including potentially back-transformed olivine) for CY, indicates that both parent bodies experienced similar extensive aqueous alteration to near completion. However, there is a significant difference in sulfide abundances between the two groups, ~6 vol% for CI and 19 vol% for CY, which reflects a difference in primary S content for the respective protoliths. King et al. (2015) also found a difference in the modal abundance of magnetite between the two groups, ~7–9 vol% for CI and 2 vol% for CY. Based on this data they inferred that the two groups likely originated on separate parent bodies.

X-Ray Diffraction Patterns for CI and CY Chondrites
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Diagrams credit: King et al., GCA, vol. 165, pp. 148–160 (2015, open access link
'Modal mineralogy of CI and CI-like chondrites by X-ray diffraction'
(https://doi.org/10.1016/j.gca.2015.05.038)

Employing micro-Computer Tomography along with high-precision triple oxygen isotope analysis, Suttle et al. (2020) investigated a number of the largest (>500 µm), least melted, and least weathered micrometeorites from the Transantarctic Mountains (TAM) collection. They found that two intensely hydrated 16O-poor micrometeorites, TAM50-25 and TAM19B-7, share a common oxygen isotope trend line with CO, CM, and CY chondrites, possibly reflecting differences in accumulated isotopically-heavy water ice (see diagram below). Although the two micrometeorites have significantly lower sulfide abundances as well as smaller chondrules compared to the CY chondrites, they maintain that the micrometeorites could have originated from the CY parent body.

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Diagram credit: Suttle et al., EPSL, vol. 546 (2020)
'Isotopic and textural analysis of giant unmelted micrometeorites—identification of new
material from intensely altered 16O-poor water-rich asteroids'
(https://doi.org/10.1016/j.epsl.2020.116444)

Comparative analyses of the known asteroid types and a suite of ungrouped and rare meteorites in multiple forms (bulk, powder, polished section), including NWA 2066 (paired with NWA 1988), were conducted by Krämer Ruggiu et al. (2021) utilizing petrographic, spectroscopic, and albedo data. They concluded that the best matches to NWA 2066, and by association NWA 1988, are the X-type asteroids Xc, Xe, and Xk; this is the first meteorite match to Xe-type asteroids (see diagram below).

Comparison of Meteorite and Asteroid Spectra
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Diagram credit: Krämer Ruggiu et al. Icarus, vol. 362, art. 114393 (2021, open access link)
'Visible-infrared spectroscopy of ungrouped and rare meteorites
brings further constraints on meteorite-asteroid connections'
(https://doi.org/10.1016/j.icarus.2021.114393)

Amsellem et al. (2020) utilized Rb–Sr systematics to date the thermal metamorphism and dehydration experienced by a number of CM chondrites, including MIL 07675, PCA 02010, PCA 02012, PCA 91008, and QUE 93005. For each of the meteorites they found that the heating was a late-stage event (<2 b.y. ago), possibly associated with a collisional disruption resulting in the formation of a C-type asteroid family (see diagram below).

Ages of Heated CM Meteorites and C-type Asteroid Families
squares: heated CM meteorites; diamonds: C-type asteroid families

standby for cm and c-type asteroid ages diagram
Diagram credit: Amsellem et al. Icarus, vol. 339, art. 113593 (2020, open access link)
'Timing of thermal metamorphism in CM chondrites: Implications for Ryugu and Bennu future sample return'
(https://doi.org/10.1016/j.icarus.2019.113593)

Further information about the thermally metamorphosed CM-like chondrites and the CY group can be found on the Dhofar 225 page. The specimen of Dhofar 1988 shown above and below is a 2.73 g slice that exhibits a brecciated texture with monomict clasts and chondrules surrounded by Fe-sulfide mantles.

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Photos courtesy of Marcin Cimala—PolandMET