A single meteorite weighing 68.2 g was purchased by a team of American collectors in Safsaf, Morocco. This ordinary chondrite is a monomict breccia that belongs to the LL chondrite group. It has a low shock stage of S1 and a weathering grade of W1/2. A thorough analysis of NWA 1756 was conducted at Northern Arizona University (T. Bunch and J. Wittke). Based on texture and TL sensitivity data, though the latter method has shortcomings in distinguishing between types 3.0 and 3.1, it was determined that NWA 1756 is consistent with a LL3.0 subtype. Chondrules in NWA 1756 are closely similar to those in the highly unequilibrated LL chondrite Semarkona with respect to compositions, zoning profiles, and textures.
Recently, a new petrologic scheme was proposed by J. Grossman (2004), and J. Grossman and A. Brearley (2005). It is more discriminating at the lowest petrologic types, those associated with the highly unequilibrated chondrites (3.03.2). This new classification scheme, based on a sensitive analytical technique utilizing the variation in the distribution of Cr in ferroan olivine, is virtually unaffected by the processes of terrestrial weathering and aqueous alteration. The petrologic scale of the new decimal system has been extended as follows:
To discriminate among subtypes below type 3.2, it has been shown that the Cr content of ferroan olivine is an excellent indicator of metamorphism. Chromite exsolves from olivine in the incipient stages of metamorphism, initially producing heterogeneous Cr contents, and eventually homogeneously low Cr content of olivine. In a study by Chizmadia and Bendersky (2006), they determined that this sequence progresses from type 3.0, corresponding to high Cr2O3 contents of 0.30.4 wt%, to type 3.2, in which Cr2O3 constitutes less than 0.1 wt%. The gap between these subtypes represents type 3.1.
In addition, they have identified several other parameters, which, when used in combination, are instrumental in determining an accurate classification at the lowest petrologic grades:
At the onset of thermal metamorphism, 1) Cr is exsolved from ferroan olivine forming fine Cr-rich precipitates, which, with progressive metamorphism, become coarser within the olivine cores and form rims on the olivine surfaces; 2) very fine-grained FeS in chondrule rims and in fine-grained matrix become coarser, and secondary sulfides form within chondrules; 3) Fe and Mg in olivine are homogenized and metal grains are equilibrated; 4) abundances of presolar grains are diminished; 5) Na and other alkalis are initially lost from the matrix and enter type-I chondrules, causing zonation, only to reverse direction with progressive metamorphism; 6) albite crystallizes from type-II chondrules causing blue CL and increased TL sensitivity.
Based on this new scheme, NWA 1756 was found to be most consistent with a petrologic type 3.10 with thermal metamorphism occurring at low temperature conditions of 471°C (Kimura et al., 2008). Several pristine features are present in NWA 1756, including the following: 1) the chondrule mesostasis exhibits isotropism; 2) sub-millimeter-sized aggregates of Ni-rich metal (5060 wt%) are present in the matrix and occur in association with sulfide and carbide; 3) sub-µm-sized silica inclusions are present within the metal; 4) certain very rare minerals are present which do not exist in chondrites having even the lowest degree of metamorphism; and 5) presolar grains have been identified in cluster chondrite clasts that represent remnants of primary accretionary rocks (Metzler, 2012; Leitner et al., 2014).
Additional methods that can be utilized in establishing the lowest petrologic subtype have been suggested by Kimura et al. (2003). These are based on distinctions in the chemical compositions of spinel group minerals, Ti-oxides, and FeNi-metal between LL3.0 and higher types. Studies of the highly volatile element contents and of silicate heterogeneity are also useful.
Utilizing a different analytical procedure, Bonal et al. (2005) found that an accurate comparison could be made between the metamorphic grades of the CO and the ordinary chondrites using Raman spectrometry combined with petrographic analysis. Their method is based on the structural order of the chondritic organic matter, which was initially accreted in the same proportions in both CO and ordinary chondrites. This structural order is irreversibly transformed by thermal metamorphism to a commensurate degree across chemical classes. From their data, they concluded that the CO group would span a petrologic sequence from 3.1, as represented by Colony, to a type greater than or equal to 3.8, as represented by both Warrenton and Isna.
In a further expansion of this method, Quirico et al. (2006) determined that LL3.00 Semarkona has experienced thermal metamorphism beyond the onset stage, and they proposed a new petrologic scale to provide consistency in the range as follows: Semarkona would become petrologic type (PT) 1, with PT 0 being reserved for the stage of true onset of thermal metamorphism. In a study of FeNi-metal and sulfide composition and texture by Kimura et al. (2006), the ungrouped (probably CO-related; Simon and Grossman, 2015) carbonaceous chondrite Acfer 094 was determined to have experienced even less metamorphism than Semarkona and that it should be assigned a petrologic type 3.00, and perhaps a PT 0. All other meteorites analyzed to date would have a PT greater than 1, with Semarkona now considered to be petrologic type 3.01.
In subsequent studies of chromite zoning profiles along with the chromite content of individual ferroan olivine grains, Grossman (2008) was able to further resolve the petrologic type for chondrites at the lowest metamorphic stages. These two petrographic features provide a reference for a sequential history of increasing thermal metamorphism that is consistent among olivine grains within each meteorite. For metamorphic types 3.003.03, chromite zoning profiles are smooth and correlate with igneous FeO zoning profiles. In addition, at this lowest metamorphic stage chromite contents account for 0.30.5 wt% in the chondrite groups studied. While chromite contents in type 3.053.10 chondrites still reflect the lowest degrees of metamorphism, chromite now exhibits igneous zoning profiles which are no longer smooth. Upon reaching a degree of metamorphism equivalent to type 3.15, chromite zoning has diminished considerably, and chromite abundance is now only 0.10.2 wt%. With metamorphic types of at least 3.2, no zoning is observed and chromite abundance is mostly less than 0.1 wt%.
Image credit: J. N. Grossman & A. J. Brearley MAPS, vol. 40, #1, p. 87 (2005)
'The onset of metamorphism in ordinary and carbonaceous chondrites'
Following the scheme of J. Grossman and A. Brearley (2005), the LL chondrite Semarkona, the L chondrite NWA 7731, and the ungrouped (probably CO-related; Simon and Grossman, 2015) carbonaceous chondrite Acfer 094 (Kimura et al., 2006), have been assigned to the least equilibrated subtype 3.00; however, Semarkona has more recently been determined to represent a petrologic subtype of 3.01. This specific metamorphic type for Semarkona is also consistent with findings based on the FeNi-metal component, the features of which provide one of the most sensitive indicators for the onset of thermal metamorphism. The technique reveals that primary martensite decomposes to fine-grained plessite during very low degrees of thermal metamorphism in Semarkona, but which did not occurred in Acfer 094 (Kimura et al., 2008). Furthermore, they found that metal in and around Semarkona chondrules does not show a solar ratio of Co/Ni like that in Acfer 094, reflecting the greater degree of metamorphism that affected Semarkona. Moreover, low temperature aqueous alteration has occurred in Semarkona as attested by the presence of secondary alteration products such as smectite.
Kimura et al. (2008) also argue for the inclusion of the carbonaceous chondrites of groups CR, CH, CB, and CM as 3.00 type specimens, notwithstanding their general designation as type 2 due to aqueous alteration features. In light of this petrologic typing paradox, they propose that a separate scale be adopted to describe aqueous alteration distinct from that which describes thermal metamorphism.
Other work has shown that the CO-group meteorite ALHA77307 is consistent with a type 3.03, while three ordinary chondritesQUE 97008, MET 00526, and EET 90161have been assigned the next lowest petrologic subtype of 3.05; several meteorites share the less rigorously defined 3.1 subtype. Northwest Africa 1756 is a highly unequilibrated primitive chondrite which is among the very few to have escaped significant metamorphic processes on their parent bodies, and which preserve their primordial features. In a study of unequilibrated ordinary chondrites, Metzler (2012) found cm-sized clasts in NWA 1756 and other UOCs that exhibit low matrix abundances and have features of deformation consistent with hot, plastic collisions which occurred over a short interval during the earliest chondrule formation events in the protoplanetary disk. He named these primary accretionary clasts "cluster chondrites" and demonstrated that the chondrule apparent size is inversely correlated with the degree of deformation.
Utilizing the MIT Magellan telescope in Chile and NASA's Infrared Telescope
Facility in Hawaii, a compositional analysis of the ~270 m diameter near-Earth asteroid Apophis was obtained (Binzel et al., 2007). These results have been shown to be comparable to the composition measured for LL chondrites measured in the lab. The photo of NWA 1756 shown above is a 1.16 g specimen.