sign in sign up
<<
Home , Lonsdaleite

Lunar and Planetary Science XXX 1100.pdf

SHOCK-INDUCED ORIGIN OF IN . A. Bischoff1, C. A. Goodrich2, and T. Grund1; 1Institut für Planetologie, Wilhelm-Klemm-Str. 10, 48149 Münster, Germany ([email protected]). 2Max-Planck-Institut für Chemie, PO 3060, 55020 Mainz, Germany ([email protected])

Introduction: Most ureilites contain -rich rather than the less-precise scheme used so far for ureilites materials, which consist in various proportions of , [15]. , and and/or lonsdaleite. Analytical: We searched for diamonds by the Diamonds and/or lonsdaleite have been identified in cathodoluminescence (CL) technique, using a JEOL JSM ureilites mainly by X-ray diffraction. In this study we 6300F Field-Emission SEM. In order to eliminate the have identified diamonds in ureilites by possibility that observed diamonds were contaminants cathodoluminescence. from sample preparation, we also examined diamonds A shock origin for ureilitic diamonds has been widely from a commercial saw blade and found that their CL- accepted [e.g. 1], although the origin of the carbon itself spectra could easily be distinguished from those of has remained controversial. Some workers [e.g. 2-5] diamonds in ureilites. argued that the carbonaceous material was injected into Results: Under the assumption that all ureilitic the silicate assemblage by the shock event in diamonds show bright CL-properties, the following results which the diamonds were formed. Others [6,7] suggested can be summarized: a) Within the (Fig. 1) of that the carbon was part of the primary ureilite igneous weakly shocked ureilites (ALH78019, ALH83014), whose assemblage and that formation of diamonds by shock was olivines do not show planar fractures (shock stages S1 and secondary. The discovery of ALH78019, which was hailed S2) no diamonds could be identified. b) Diamonds were as a “very low-shock” ureilite, appeared to confirm both also not found in Novo 010, which we classify as S2/S3 the shock model for the origin of the diamond and the (few planar fractures). c) All other ureilites having well- primary nature of the carbon. ALH78019 contains mm- developed planar fractures (frequently filled with metal sized euhedral graphite crystals, sometimes completely and sometimes appearing to have been partially recovered enclosed in olivine, which were interpreted [7] to due to annealing) in olivine (S3) or showing fine-grained represent the primary, pre-shock form of carbon in recrystallized olivines (S6) contain diamonds of various ureilites. This interpretation was supported by the failure sizes (typically below 10 µm) and abundances (Figs. 2 and to detect diamond by X-ray diffraction in a C-rich residue 3). These diamonds occur as unoriented individual grains from ALH78019 [8]. (Figs. 2 and 3b) or as crystals arranged in chains (Fig. 3a). However, Matsuda and coworkers [e.g. 9-11] have d) Within the polymict ureilite Dar al Gani 164, carbon- argued that ureilitic diamonds were formed, not by shock, rich areas with a high abundance of diamonds coexist with but rather by chemical vapor deposition (CVD) from CH4 areas having a very low abundance of diamonds. gas in the solar nebula. Citing Mori and Takeda [12], they explained [11] the absence of diamonds in ALH78019 as due to their graphitization during high-temperature annealing in which the olivine underwent recovery of shock-produced dislocations. The CVD model is in agreement with [6,7] that the carbon (graphite and diamond) in ureilites is primary and that its origin is unrelated to shock. However, this model implies that ALH78019 and other diamond-free ureilites, rather than being those that are the least shocked, are actually those that have experienced the highest degree of shock, because it is only at the highest shock pressures that thermal metamorphism become the dominant effect and recovery of earlier formed shock-induced microstructures occurs [13]. This conclusion would have profound implications Fig. 1: Euhedral graphite crystal in olivine of the for petrologic studies, which so far have assumed that the ALH83014 ureilite. Within graphites of such weakly diamond-free ureilites show the most primitive igneous shocked ureilites, no diamonds could be identified by features. cathodoluminescence. SEI. We were concerned, therefore, to reevaluate the occurrence of diamonds in ureilites as correlated (or not) Conclusions: This preliminary study shows clearly with shock criteria independent of the state of the carbon - that the presence or absence of diamonds is related to the primarily, shock features in olivine. We examined the shock pressure recorded by olivine. Low-shock ureilites ureilites ALH78019, ALH83014, ALH82130, Hammadah do not contain diamonds. We rule out the idea that al Hamra 064, Havero, Y74123, Hajmah, Y790981, Novo diamond-free ureilites result from annealing and 010, Jalanash, and Dar al Gani 164, and identified their recrystallization of strongly shocked ureilites [11] because: shock levels according to the scheme of Stöffler et al. [14], a) formation of mm-sized euhedral graphites by recrystallization of mm-sized diamonds is difficult to Lunar and Planetary Science XXX 1100.pdf

SHOCK-INDUCED ORIGIN OF DIAMONDS: A. Bischoff et al.

understand; b) diamond-free ureilites contain shock effects rule out that some ureilites are very strongly shocked (S6), characteristic of low degrees of shock (e.g. undulatory it is possible that shock pressures experienced by ureilites extinction in olivine; shock lamellae in graphite); c) never exceeded ~35 GPa. texturally, olivines in diamond-free ureilites do not appear References: [1] Vdovykin G.P. (1970) Space Science to have seen recrystallization. Reviews 10, 483. [2] Göbel R. et al. (1978) JGR 83, 855. Considering ureilites shocked to levels above S2, we [3] Boynton W.V. et al. (1976) GCA 40, 1439. [4] have not found any correlation between shock level and Higuchi H. et al. (1976) GCA 40, 1563. [5] Wasson J. T. diamond abundance. This may be for the following et al. (1976) GCA 40, 1449. [6] Berkley J.L. et al. (1980) reason. During inspection of many ureilites it was found GCA 44, 1579. [7] Berkley J.L. and Jones J.H. (1982) that ureilites of the intermediate shock stages S4 and S5, Proc. LPSC 13, JGR, A353. [8] Wacker J. (1986) GCA characterized by mosaicism in olivine, are missing (though 50, 633. [9] Fukunaga K. et al. (1987) Nature 328, 141. we note that the recrystallized texture of shock stage S6 [10] Matsuda J. et al. (1995) GCA 59, 4939. [11] has been mistakenly called mosaicism in the ureilite Miyamota M. et al. (1988) LPS 19, 798. [12] Mori H. and literature [15]). If ureilitic target rocks were deeply buried Takeda H. (1983) LPS 14, 519. [13] Bauer J.F. (1979) and hot at the time they were shocked (consistent with the Proc. LPSC 10, 2573. [14] Stöffler D. et al. (1991) GCA generally accepted view that ureilites were excavated from 55, 3845. [15] Goodrich C.A. (1992) 27, 327. high temperatures - [15]) those shocked to levels S4 or S5 may have been annealed to a fine-grained recrystallized texture as a result of residual heat. Although we cannot

Fig. 2: Two diamond crystals within graphite of the Havero ureilite. (a) SEI. (b) corresponding CL-view .

Fig. 3: Within the polymict ureilite Dar al Gani 164, graphites with a high abundance of diamonds coexist with graphites having very low diamond abundance. In some diamond-rich fragments diamonds are arranged in chains (a), while in others they occur unoriented (b). CL-images. Diamonds are the bright spots.