CARBONACEOUS CHONDRITES AND AQUEOUS ALTERATION

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1 CARBONACEOUS CHONDRITES AND AQUEOUS ALTERATION Discussion Summarizer: Ariel Deutsch Hiroi et al., 1996 INTRODUCTION The authors present a thermal metamorphism study by comparing the 0.7 µm, 3 µm, and UV absorption strengths in the reflectance spectra of C, G, B and F asteroids to those of heated Murchinson samples. They suggest that these asteroids have experienced various degrees of thermal metamorphism and aqueous alteration, and that they may be the sources of CI, CM, and CR meteorites. REFLECTANCE SPECTRA Iron absorptions: We first discussed the observed absorption features associated with minerals on asteroids and caveats of interpretations that are often presented in the literature as fact rather than interpretation. Fig. 1 shows reflectance spectra of nine carbonaceous chondrites. The red box denotes a 0.7 µm feature that can arise due to Fe 2+ to Fe 3+ charge transfer, or due to crystal field splitting. If it is from crystal field splitting, given that the feature is relatively weak, it must be forbidden. A common element responsible for this feature is Fe 3+. By itself, this feature tells you nothing about the presence or absence of clay minerals. However, most clays do show this absorption feature. The 0.7 µm band is an iron feature. Therefore, if you want to argue that it is indicative of clays, you are making a few other assumptions. Furthermore, this feature has nothing to do with either OH or H2O, so it is not truly indicative of hydration. However, because we have substantially more data available in the VIS range than we do in the IR wavelengths, the proxy of this Fe absorption for hydration is often used. Beware that the literature is rife with improperly drawn conclusions about this Fe absorption indicating water or clay, but these are interpretations. The green box in Fig. 1 outlines a broader absorption feature centered around ~1 µm attributable to Fe 2+, due to the presence of olivine. Hydrated absorptions: Diagnostic hydration features are present in the UV wavelength range. For example, the blue box in Fig. 1 shows an example of a broad, U- shaped absorption feature centered near ~3 µm due to the presence of H2O. The yellow box in Fig. 1 outlines an example of a narrow, V-shaped absorption feature that is diagnostic of OH. Fig 1. Modified Fig. 1 from Hiroi et al., Reflectance spectra of carbonaceous chondrite powders (<100 or <125 µm), scaled to 1.0 at 1.95 µm and offset for clarity. 1

2 We posed the question, handling meteorite samples, how does one know that the water is not due to terrestrial contamination? One pattern that has emerged from reflectance studies is that carbonaceous chondrites typically have stronger water absorptions than other chondrite meteorites. Furthermore, Earthbased observations of C-type asteroids reveal consistent low albedo measurements and evidence for a 3-µm band. ATMOSPHERIC EFFECTS Fig. 2 illustrates the drastic effect that observing these bodies from within Earth s atmosphere can have on the resulting spectra. Beyond ~3.0 µm there are substantial error bars associated with some of the reflectance spectra due to the interference of Earth s atmosphere with Earth-based observations. This figure illustrates the need for increasing the number of space-based observations and surveys of the asteroid belt. Fig. 2. Fig. 2 from Hiroi et al., Telescopic reflectance spectra of the six selected C, G, B, and F asteroids, scaled to 1.0 at 1.95 µm and offset for clarity. THE 0.7-µm, 3-µm, AND UV ABSORPTION STRENGTHS Observations and experimental results: The authors examine thermal metamorphism by studying the 0.7 µm, 3 µm, and UV absorption strength. Fig. 3 plots the 3-µm band strength vs. the 0.7-µm band strength of the C, G, B, and F asteroids and carbonaceous chondrites. As the heated Murchinson samples (open squares) were exposed to increased heating temperatures ( C), the 0.7 µm-band strength began to disappear. Fig. 3. Fig. 4 (a) from Hiroi et al., Plots of the 3- µm band strength vs. 0.7-µm band strength. Asteroids points are plotted in their class name letters. Meteorites are plotted in symbols (open circle=ci; open square=cm; open diamond=cr; open inverse triangle=cv; open triangle=ck; filled circle=unusual CI/CM. Heated Murchinson samples are plotted in open squares with the heating temperatures ( C). The authors also plot the 3-µm band strength vs. the UV band strength and find a strong correlation with a linear correlation coefficient of when excluding the CV and CK meteorites (Fig. 4). This linear thermal relationship suggests that unaltered bodies clearly plot in the lower left while 2

3 those that have been thermally altered clearly plot in the upper right. Fig. 4. Fig. 4 (b) from Hiroi et al., Plots of the 3-µm band strength vs. UV band strength. We discussed that when you use one of these plots by itself, it is not that powerful, but the combination of these two is fairly diagnostic. Application to asteroids: Given that we see these trends of decreasing 0.7-µm band strength and UV absorption strength with increasing heating temperatures in the Murchison samples, the next step is to apply this to the asteroids to see where they plot. The C s, for example, are plotting as having some thermal metamorphism in both parameter spaces. Although some variability does exist, in combination these two plots used together are good diagnostic tools of thermal metamorphism. Heating techniques: Finally, we discussed how representative the lab heating techniques of the Murchinson samples are of the type of thermal metamorphism that is experienced by a parent body. Remaining questions are whether these processes operate in a vacuum or if they occur in the realm of hot fluids. Rivkin et al., 2002 INTRODUCTION The authors wrote a review paper that summarizes reflectance spectroscopy studies and astronomical evidence of hydrated (water- and hydroxyl-bearing) minerals on asteroids. DATABASE OF OBSERVATIONS Reflectance spectroscopy shows absorption features in the ~0.7 µm region and µm region, which are either used as a proxy for or diagnostic of hydrated minerals. Table 1 in Rivkin et al. (2002) numerates the number of asteroids observed at 3 µm and 0.7 µm at the time of this review paper. It is clear from this table that there are simply not many observations of asteroids, with sample sizes of each Tholen class between 1 and 32 at 3 µm, and between 1 and 45 at 0.7 µm. This reiterates our discussion previously that the database of observations is simply not that large. Thus supplementing the IR observations with the additional VIS observations, even though an Fe absorption is not directly indicative of hydration, may be used as a necessary proxy with the appropriately stated assumptions. HYDRATION ABSORPTIONS We also discussed the observed absorption features that are associated with hydrated minerals on asteroids (Table 1). Table 1. Table. 2 from Rivkin et al., Observed absorption features associated with hydrated minerals on asteroids. 3

4 This table is a good table to be able to go back and reference as necessary. As much of our earlier discussions noted, however, not all of these absorptions are truly indicative of hydrated minerals on asteroids. Also, the 0.7 µm absorption due to an intervalence charge transfer does not have to be in a phyllosilicate. Thus labeling the transition as in phyllosilicates is presumptuous and irrelevant. CHECKMARK VS. ROUNDED The hydrated mineral features on asteroids appear to have one of two general band shapes: checkmark or rounded (Fig. 5). The checkmark feature is associated with an OH absorption feature and is typically observed on low-albedo asteroids (Fig. 5, left panel). In contrast, the rounded feature is associated with a H2O absorption and is typically found on high-albedo asteroids (Fig. 5, right panel). Fig. 5. Fig. 3 from Rivkin et al., Checkmark vs. rounded spectral features associated with the 3 µm absorption. ROTATIONAL VARIATION Previous work predicts that there should be surface compositional diversity among smaller-diameter asteroids, and that largerdiameter low-albedo asteroids should exhibit homogeneity (Vilas and Sykes, 1996). Fig. 6 shows that the spectra for 105 Artemis changes in the absorption band at 0.7 µm as the asteroid rotates, indicating that 105 Artemis has an absorption feature on one hemisphere, but not the other. These variations indicate inhomogeneity in possibly the regolith, underlying material, or both. Fig. 6. Fig. 7 (upper panel) from Rivkin et al., Visible spectra of 105 Artemis obtained at different rotational phases using a 2.1-m telescope at McDonald Observatory. We discussed the different implications of the rubble pile model (which predicts variation in the location of thermal metamorphic grade on a given body) vs. the onion shell model (which predicts that the most thermally metamorphosed samples are found deepest in the body and grade outward in layers to the surface to more pristine samples). For example, if we are assuming an onion shell model, we would not expect the bodies to necessarily be heterogeneous if we were only excavating 50 m or so with an impact. THERMAL EVOLUTION Finally, we concluded our discussion of this review paper with conversations on thermal evolution. Models suggest that there should be more hydration features on larger asteroids than there are on smaller asteroids because the interiors of asteroids reach higher temperatures than the surfaces do. In these models, smaller asteroids are assumed to fragments of larger bodies, thus representative of the outer surfaces that experienced less heating. Table 2 presents a test of this size to band depth correlation by denoting the percentage of asteroids with a 0.7-µm feature for increasing diameter bins. Overall, there is somewhat of a trend 4

5 suggesting that a higher percentage of larger asteroids have a 0.7 µm feature, but it is not very clear (Table 2). However, updated research upholds and supports this trend. The authors do note, though, that this trend breaks down at the 3 µm band, for which the authors did not include a table. Table 2. Table. 3 from Rivkin et al., Percentage of Tholen CBFG asteroids testing positively for the 0.7-µm feature divided by diameter. We challenged whether it is a fair assumption to make that the smaller diameter asteroids are smaller, remnant pieces of bigger asteroids. An alternative explanation may be that these smaller asteroids are representative of their original size to which they accreted. We discussed, however, that the process of thermal alteration is a function of size. It is dependent on how much heat is radiated and how much the body can cool down. The larger diameter asteroids would undergo increasingly greater heating. If the body is too small, aqueous alteration may not occur at all. Therefore, the process depends on how much Al 26 is available, as well as the size of the object. Eiler and Kitchen, 2004 INTRODUCTION The authors present new hydrogen isotope data for separated matrix, hydrated chondrules, and hydrated coarse silicate fragments from CV, CO, CM, and CI chondrites. They find that as the CM chondrites increase in the degree of aqueous alteration, the less similar they are to the CI chondrites, in contrast to what is predicted by the oxygen isotope data alone. The authors conclude that the protoliths of CM chondrites are more volatile-rich and that CI chondrites are not simply more altered products of the same process that produced the CM chondrites. HYDROGEN ISOTOPE RESULTS The hydrogen isotope composition values (δd in per mil) are shown for the CV, CM, and CI chondrites in Fig. 7. It is unlikely that the CV and CO chondrites are protoliths of the CM chondrites. Instead, the protoliths of the CM chondrites were likely volatile-rich objects. The protoliths of CI chondrites must be volatile-rich as well, but these chondrites are likely not the products of more extensive aqueous alteration of the same type and of the same protoliths. Thus the CI chondrites likely evolved from different parent bodies and from processes that involved compositionally different infiltrating water. Fig. 7. δd values (vs. SMOW) of matrix (circles), altered chondrules (stars), and altered silicate fragments (squares) of CV, CM, and CI chondrites. All values are weighted averages of water released over the course of stepped heating experiments. BDL = below detection limit. VOLATILE RETENTION One question prompted from this model is, After aqueous alteration, why did CI chondrites retain their volatile component while CM chondrites did not? One possibility that we discussed is that CM chondrites have preferentially experienced 5

6 more impacts that vaporized the volatiles. This may be reasonable because impacts will not occur at a constant rate across the solar system, but the distribution of impacts depends on many factors including the target s location within the solar system and size of the body. This process would need to be able to leave behind an isotope ratio though and produce a significant enough difference that produces the observed trend. Alternative explanations discussed included the relative heating of the two groups, either from the CM chondrites traveling too close to the Sun or from preferential radiogenic heating of CM chondrites by Al 26. MY FAMILY IS COMPLICATED Overall, there is not a nice, clear projection of the CI and CM chondrites and the protoliths of these two groups. The hydrogen isotope composition data suggest that you cannot take a CM farther along the aqueous alteration path and produce a CI, but that these two meteorite groups stem from different protoliths. So how are these two groups related? What does it even mean to be chondritic? Ideally, we would want to know what these undifferentiated bodies started as so that we can determine the pathways of how they evolved from A to B, to the samples we have today. There is a whole sweep of carbonaceous materials that varies in isotopic space and in bulk chemistry, implying that there is massive heterogeneity in the disk. Just because Earth has one bulk composition, does not imply that Mars or Vesta have the same. And there is substantial variation within the chondrites themselves. The asteroid classifications are simply based on spectral properties, whereas meteorite classifications are made on the basis of in-depth geochemical analyses. We challenged how these classifications would change if we had more (and higher resolution) data. References Hiroi, T., Zolensky, M.E., Pieters, C.M., and Lipschutz, M.E. (1996) Thermal metamorphism of the C, G, B, and F asteroids seen from the 0.7-mm, 3-mm, and UV absorption strengths in comparison with carbonaceous chondrites. Meteoritics & Planetary Science 31, Rivkin, A.S., Howell, E.S., Vilas, F., and Lebofsky, L.A. (2002) Hydrated minerals on asteroids: The astronomical record. In Asteroids III (Eds. Bottke, Cellino, Paolicchi and Binzel) Univ. Arizona Press, Vilas, F., and Sykes, M.V. (1996) Are low-albedo asteroids thermally metamorphosed? Icarus 124, Eiler, J.M., and Kitchen, N. (2004) Hydrogen isotope evidence for the origin and evolution of the carbonaceous chondrites. Geochimica et Cosmochimica Acta 68,

Observing hydrated minerals on asteroids. Andy Rivkin (JHU/APL) JWST Workshop at DPS 10 October 2013 Denver CO

Observing hydrated minerals on asteroids Andy Rivkin (JHU/APL) JWST Workshop at DPS 10 October 2013 Denver CO Why asteroids? Asteroids represent leftover material not incorporated into the inner planets