Carbon minerals in Frontier Mountain ureilites of the Museo nazionale dell'Antartide, Siena, Italy

Diamond formation in ureilites has been lively debated and is still controversial. The first hypothesis dates back to 1956, when Urey supposed that diamonds may form in static high-pressure conditions in the interior of the ureilites parent body (UPB), as their terrestrial analogues form in the Earth’s mantle [Urey 1956]. A few years later, Lipschutz proposed that diamonds formed by shock conversion of graphite during the catastrophic breakup of the ureilites parent body [Lipschutz et al. 1964]. A third hypothesis, supported by the different noble gases content of diamond and graphite in ureilites, states that diamond forms at low pressure in the solar nebula by chemical vapor deposition (CVD) [Fukunaga et al. 1987]. Nakamuta et al. 2000 attempted a first systematic study of diamond and graphite in monomict ureilites with different shock features. As a general trend, they observed that diamond and graphite occur together in low to high-shock ureilites, but not in very low-shock ureilites. Furthermore, in low-shock ureilites, diamond is associated with kamacite, inferring a catalytic role of the metal. The first hypothesis by Urey was revitalized by studies on diamonds from the Almahata Sitta polymict ureilite by Miyahara et al. [2015]. In the sample MS-170, [Miyahara et al. 2015] described large diamond grains (~40 μm) with similar crystallographic orientations which were considered as fragments of one large single crystal formed either in the deep interior of the parent body or by CVD. This crystal would have reached 100 μm, thus representing a unique case in ureilites. Later, Nabiei et al. (2018) reported in the same sample the presence of chromite, phosphate, and (Fe,Ni)-sulfide inclusions embedded in diamond. The authors suggested that the composition and morphology of the inclusions infer a formation pressure higher than 20 GPa, and thus a Mercury-size parent body. 
Investigating the relationships between the various carbonaceous phases could shed light on the precursor phase of diamonds and their formation process. Such information will give a contribution in reconstructing the evolutionary history of the asteroidal parent body in order to determine the asteroid-meteorite link. Furthermore, this will help us to understand the possible processes occurring in the asteroid belt of our Solar System, such as collision, migration, re-accretion and mixing of the asteroidal bodies.

References

Fukunaga, K. et al. (1987). Noble-gas enrichment in vapour-growth diamonds and the origin of diamonds in ureilites. Nature, 328(6126), 141.

Lipschutz, M. E. (1964). Origin of diamonds in the ureilites. Science, 143(3613), 1431-1434.

Miyahara, M. et al. (2015). Unique large diamonds in a ureilite from Almahata Sitta 2008 TC3 asteroid. Geochimica et Cosmochimica Acta,
163, 14-26.

Nabiei, F. et al. (2018). A large planetary body inferred from diamond inclusions in a ureilite meteorite. Nature communications, 9(1), 1327.

Urey, H. C. (1956). Diamonds, Meteorites, and the Origin of the Solar System. The Astrophysical Journal, 124, 623.

The main expected outcome of this project is to have further insights on the origin, formation and relationships among carbon phases in ureilites. Understanding the form and occurrence of carbon minerals in ureilites will contribute to constrain the petrogenesis of such meteorites and thus the evolution of their parent body. COMMANDER intends to characterize carbon phases in the ureilitic samples with different degrees of shock (S1 to S5/6) by single crystal X-ray micro-diffraction (SC- RD), micro-powder X-ray diffraction (mPXRD), micro-Raman spectroscopy, Scanning Electron Microscopy with variable pressure equipped with a Field Emission Gun (SEM-FEG) and Transmission Electron Microscopy (TEM), and to correlate the presence of the various polymorphs with the shock degree experienced by the meteorites.

The COMMANDER project demonstrates that the most reliable formation process for diamond and graphite in ureilites is the shock event(s). In detail, the carbon phases of the ureilitic fragments were investigated by Optical Microscopy (OP), Scanning Electron Microscopy (SEM), X-ray diffraction (XRD) and Micro Raman Spectroscopy (MRS). These analyses were carried out in order i) to investigate the carbon polymorphs as a function of the degree of shock and ii)  to assess at which shock level diamond occurs replacing graphite and iii) to evaluate if the degree of shock is related to the temperature recorded by graphite. The results of this project obtained on other ureilites allowed the COMMANDER research team to compare the results obtained on Frontier Mountain fragments with other ureilites investigated using the same analytical procedure.

The main results of this project can be summarized as follows:

1) X-Ray diffraction data indicate the coexistence of nano-diamonds together with micro-diamonds and nano-graphite in ureilites. In particular, this coexistence is present in FRO 01089, FRO 97013, FRO 01088 and FRO 01012 samples (with shock level from S3 to S6 by Stöffler et al. 2018);lwhile the X-Ray Diffraction analyses on the low-shock sample FRO 95028 (S2) show mostly nano-graphite with a very low abundance of diamond (only a low intensity peak at d-spacing 2.06 Å is present).  In addition, most of the X-ray diffraction patterns of the investigated medium and high shock ureilitic samples show a shoulder (d =2.18Å) on the highest peak of diamond (d = 2.06 Å) suggesting the presence of stacking disorder in diamond, which is a typical feature of shock (see Murri et al. 2019 and references therein);The powder diffraction patterns of FRO 01089 (S3), FRO 97013(S3/4) and FRO 01088(S5) show in the graphite highest peaks an asymmetry similar to that observed in the carbon bearing samples studied in Almahata Sitta 72, 209b and NWA 7983 samples. This asymmetry could be referred to “compressed graphite” (Nakamuta and Aoki 2000), which usually occurs at the initial stage of the direct transformation from graphite to diamond consequently to shock compression;

2) Micro-Raman Spectroscopy analyses on the investigated samples exhibit homogeneous values of G-band and D-band centers in their graphite Raman spectra;The temperature recorded by graphite, using the Cody et al. (2008) graphite geothermometer based on Raman spectroscopy, varies between 1291 and 1398 °C and, combining these results with all available data on ureilites with different degrees of shock, we can state that such temperature is unrelated to the degree of shock.

The results of COMMANDER project determine that diamond in ureilite is formed by shock event(s) that were simultaneously responsible for the grain-size reduction of graphite. The coexistence of different carbon phases, with different crystallite size, could be explained by: (i) the heterogeneous distribution of shock effects, that it is mainly ascribed to the heterogeneous propagation of the shock wave, (ii) the shock impedance contrast between contiguous phases (Ogilvie et al., 2011), (iii) the position of the fragments relative to the impact location and (iv) the abundance and distribution of Fe, Ni alloys acting as catalysts for diamond growth.

The results obtained were published in four high-impact scientific peer-reviewed journals (average I.F. 5.53), and two other papers are under review in MAPS and EPSL. In addition, these data were presented at 7 national and international conferences. 

Pubblication in peer-reviewed journals

1. F. Nestola, C. A. Goodrich, M. Morana, A. Barbaro, R. S. Jakubek, O. Christ, F. E. Brenker, M. C. Domeneghetti, M. C. Dalconi, M. Alvaro, A. M. Fioretti, K. Litasov, M. D. Fries, M. Leoni, N. P. M. Casati, P. Jenniskens, M. H. Shaddad. Impact shock origin of diamonds in ureilite meteorites. – PNAS, 2020, 117 (41), 25310-25318. (doi.org/10.1073/pnas.1919067117).

2. A. Barbaro, M.C. Domeneghetti, AC. Goodrich, M. Meneghetti, L. Litti, A.M. Fioretti, P. Jenniskens, M.H. Shaddad, F. Nestola. Graphite-based geothermometry on Almahata Sitta ureilitic meteorites - Minerals 2020, 10, 1005, 1-14. (doi:10.3390/min10111005 www).

3 A. Barbaro, M. C. Domeneghetti, K. D. Litasov, L. Ferrière, L. Pittarello, O. Christ, S. Lorenzon, M. Alvaro, F. Nestola, Origin of micrometer-sized impact diamonds in ureilites by catalytic growth involving Fe-Ni-silicide: The example of Kenna meteorite. – Geochimica et Cosmochimica Acta 2021, 309, 286-298 (doi.org/10.1016/j.gca.2021.06.022).

4 A. Barbaro, M. C. Domeneghetti, L. Pittarello, L. Ferrière, M. Murri, O. Christ, M. Alvaro, F. Nestola, Characterization of carbon phases of Yamato 74123 ureilite to constrain the meteorite shock history. –American Mineralogist 2022, 107, 377-384 (doi.org/10.2138/am-2021-7856).

5. O. Christ, A. Barbaro, F. E. Brenker, P. Nimis, D. Novella, M. C. Domeneghetti, F. Nestola.  Shock degree and graphite geothermometry in Ureilites NWA 6871 and NWA 3140 – Under review in Meteoritics and Planetary Sciences.

6. A. Barbaro, M.C. Domeneghetti, A. M. Fioretti, C. A. Goodrich, M. Alvaro, F. Nestola. Carbon polymorphs in Frontier Mountain ureilitic meteorites: a correlation with increasing the degree of shock. – Under review in Earth and Planetary Science Letters.

Meeting and conferences

1. Barbaro A.; Christ O.; Murri M.; Ferrière L.; Pittarello L.; Goodrich C. A., Domeneghetti M. C.; Fioretti A.M.; Alvaro M.; Brenker F.E.; Nestola F., Study of carbon phases in the Yamato 74123 and Kenna ureilites. XVI National Congress of Planetary Sciences, 3rd -7 th February 2020, Padova, (Italy).

2. Nestola F., Goodrich C.A, Morana M., Barbaro A., Christ O., Brenker F.E, Domeneghetti M.C., Dalconi M.C, Alvaro M., Fioretti A. M., Leoni M., Casati N. P. M., Jenniskens P.J., Shaddad M.H., Origin of diamond and graphite in ureilites: a timely topic in planetary geology. XVI National Congress of Planetary Sciences, 3rd -7 th February 2020, Padova, (Italy).

3. Barbaro A., M. Domeneghetti C., Meneghetti M., Litti L., Fioretti A. M., Goodrich C., Christ O., Brenker F. E., Shaddad M. H., Alvaro M., Nestola F., SHOCK TEMPERATURE RECORDED BY GRAPHITE IN UREILITES FROM ALMAHATA SITTA (#1480). 51st Lunar and Planetary Science Conference 2020 – 17th -21 st March 2020, Houston (TX, USA)

4. Barbaro A., Nestola F., Pittarello L., Ferrière L., Murri M., Christ O., Alvaro M., Domeneghetti M. C. Impact Shock Origin of Carbon Phases in Yamato 74123 Ureilite (#1123). 52nd Lunar and Planetary Science Conference 2021 – 15 th -19th March 2021, Houston (TX, USA) virtual Conference.

5. Barbaro A., Domeneghetti M. C., Litasov K. D., Ferrière L., Pittarello L., Christ O., Lorenzon S., Alvaro M., Nestola F. Carletonmooreite (Ni3Si) in shocked diamond-bearing Kenna ureilite (2609 id. 6066). 84th Annual meeting of the Meteoritical Society 2021 – 15th – 21st August 2021, Chicago (Illinois).

6. Barbaro A., Fabrizio N., Alvaro M., Domeneghetti M.C. Shock origin of carbon phases in ureilites. XVII National Congress of Planetary Sciences, 20^th -24^th June 2022, Napoli (Italy).

7. Barbaro A., F. Nestola and Domeneghetti M.C. CARBON POLYMORPHS IN FRONTIER MOUNTAIN UREILITIC METEORITES: A CORRELATION WITH THE INCREASING DEGREE OF SHOCK?. 85th Annual meeting of the Meteoritical Society 2022 – 14th – 21st August 2022, Glasgow.

Seminars

Fabrizio Nestola. C-phases in differentiated meteorites: from graphite to extraterrestrial diamonds, IAPS-INAF web seminar, 24th November 2021.