Claire Bendersky Mount Holyoke College Advisor Lysa Chizmadia Institute for Astronomy, University Hawaii at Manoa IFA Summer REU 2005 Using AOIs to Document Aqueous Alteration in the CO3 Astroidal Parent Bodies Introduction

Chondritic meteorites are some of the solar system’s earliest aggregates. It is thought that chondritic meteorites come from small (<500km) asteroids because they were never molten (FeNi metal is still dispersed throughout the meteorites) and there is a similarity in the reflectance spectra between the chondrites and C-type asteroids. Radioactive dating with long-lived nuclides (e.g. 204Pb, 206Pb, 235U, Sm-Nd, and Rb-Sr) of Ca-Al-rich inclusions (CAIs) within chondritic meteorites show they must have formed 4.55 Ga ago. This dating indicates that chondritic meteorites show CAIs must have formed just after the Sun and before the planets accreted. Furthermore, chondritic meteorites are thought to have formed in this solar system because their bulk compositions are nearly identical to that of the solar photosphere, indicating that they are probably very primitive representatives of the early solids to have formed in the solar nebula. Chondritic meteorites have experienced few changes, compositional or otherwise, since formation and are studied to understand how these first solids formed. The subtle changes that are seen are from aqueous and thermal sources and must have occurred early in the formation of the solar system. Short-lived radio-nuclides (e.g. 56Mn) indicate that the formation of carbonate assemblages occurred within 10 Ma of CAI formation. Studying these alteration effects can give clues about the role of water and heat in the early solar system. Background

Physical evidence from asteroids is gathered from meteorites that have fallen to earth. Asteroid is an umbrella term for bodies orbiting the sun that are smaller than planets (<2000km). A meteorite is any solid from interplanetary space that has fallen to the earth’s surface5. They are organized according to the chondrule content: achondrites and chondrites. Achondrites are meteorites without chondrules. In most cases they have experienced intense heat and have melted, in effect destroying evidence of their initial formation. Chondrites are meteorites containing chondrules, which are millimeter-sized spherules of rapidly cooled silicate melt1. The chondrules are embedded in the meteorite’s fine-grained matrix. Matrix is the <1-10µm-sized material occurring between the larger objects, such as chondrules and CAIs. Chondrites have not experienced a significant melting phase, preserving information from their initial formation.

Chondrites are further grouped on the basis of their Ca/Si and Al/Si ratios and matrix volume. Ca and Al have high condensation temperatures, higher than Si. They were two of the first elements to condense into solid material from the solar nebula. There are three groups of chondritic meteorites: enstatites chondrites, ordinary chondrites and carbonaceous chondrites. Enstatite chondrites (ECs) have a low Ca/Si (<4.0 at%), low Al/Si (<5.5 at%) and contain little matrix (<2 vol%)2,3. They are characterized by having experienced moderate to high metamorphic temperatures (> 600°C) and being breccias (impact events have broken up and reassembled the meteorites manifested by the presence of clasts with different degrees of aqueous alteration within an individual chondrite, pulverized portions, and broken chondrules and inclusions). Ordinary chondrites (OCs) are so named because they are the most common type of meteorites found on earth. They have intermediate Ca/Si (4 -5 at%), intermediate Al/Si (6-7 at%), and an intermediate matrix volume (5-15 vol%)2,3. Carbonaceous chondrites (CCs) have a high Ca/Si (>6.0 at%), high Al/Si (>8.0 at%) and contain a high matrix volume (>30 vol%)2,3. Carbonaceous chondrites contain considerable quantities of carbon, as carbonate or complex organic compounds, and water, in the form of hydrous minerals such as serpentine and clay minerals3. Therefore, these rocks may have been important in the delivery of organic matter and water to the early Earth, facilitating the origin of life.

Chondrites can also broken down into seven types based on the amount of aqueous and thermal alteration sustained. Those that have been the least aqueous and thermally altered are labeled as type 3. These chondrites are characterized by having opaque matrix, very sharply defined chondrules, clear isotropic primary chondrule glass, mean compositional deviations of pyroxene (≥ 5%), of olivine (≥ 3%), 2-10mg/g bulk carbon content, and 3-30 mg/g bulk water content. As the type decreases from 3 to 1, the chondrite has been exposed to more water: the matrix becomes increasingly fine grained, the mean deviation of compositional variation in pyroxene and olivine decreases to zero, the bulk carbon content increases to 26 mg/g in type 2 and to 50 mg/g in type 1 and the bulk water content increases to 160 mg/g and to 220 mg/g in types 2 and 1, respectively. As the type increases from 3 to 7 the chondrite has been exposed to increasing amounts of heat: the matrix becomes transparent, microcrystalline and starts to recrystallized by type 5, the chondrules become increasingly ill defined, the clear isotropic primary glass degrades until it starts to recrystallize by type 5, the mean deviation of compositional variation in olivine and pyroxene decreases to 0%, and the bulk carbon content and bulk water content steadily decrease to 0 mg/g 2.

Carbonaceous chondrites are further divided into eight groups (e.g. CI, CM, CB, CR, CO, CV, CK and CH) with distinct bulk compositional, oxygen isotopes4, chondrule abundances and refractory lithophile (e.g. Al, Ca, Mg etc.) element abundances. COs are described by having 48 vol% chondrule abundance, 34 vol% matrix abundance, 13 vol% refractory inclusion abundance, a range in δ17O%ο relative to standard mean ocean water (SMOW) from -4 to 0 and a range in δ18O %ο relative to SMOW4. COs are of particular interest because they are type 3 (least altered) and their group has been sub-divided into decimal subtypes, 3.0 – 3.9, of which 3.0 is the most primitive material that has suffered minimal hydrothermal processing4.

Previous studies have subtyped CO3s by analyzing different components. While all analyses show the same basic trend, most studies concluded that subtypes vary by ± 0.1. Both Scott and Jones 1990 and Kojima et al. 1995 analyzed Fe enrichment in olivine

in chondrules and secondary mineralization of CAIs, respectively. Chondrules and CAIs are good indicators of aqueous alteration. However, they have course sizes meaning they have a small surface area to volume ratio. Due to this fact, early stage alteration is not as apparent in chondrules as it is in finer grained objects. Chizmadia et al. 2002 analyzed amoeboid olivine inclusions (AOIs). AOIs are fine grained, irregularly shaped objects composed dominantly of olivine [(Mg, Fe)2SiO4] with minor amounts of anorthite [CaAlSi2O8], Al- Ti- rich diopside [CaMgSi2O6] and spinel [MgAl2O4]. The fine grain nature of AOIs and their abundance in CO3s make them an excellent candidate for analysis of the early stages aqueous alteration. The purpose of this study is to subtype seven previously un-subtyped CO3 chondrites to better understand the distribution of metamorphism on the parent asteroid.

Analytical Methods

Polished thin sections of seven CO3 chondrites (ALH82101, ALH85003, A-881632, DaG 055, EET92126, Y790992 and Y-791717) were mapped with back-scattered electrons (BSE) on the JEOL JSM5900 LV scanning electron microscope (SEM) in the Hawaii Institute for Geophysics and Planetology (HIGP) at the University of Hawaii (UH). 3-5 AOIs were identified in each thin section. Selected AOIs of two thin sections, EET92126 and ALH85003, which were hypothesized to be the least metamorphosed, were analyzed with back-scattered electrons (BSE) and secondary electrons on the Hitachi S-4800 Field Emission Gun SEM in Biological Electron Microscopy Facility (BEMF) at UH. The goal of this project was to analyze selected AOIs using the electron microprobe (EMP), which produces detailed elemental information. However, throughout the duration of this project the EMP required extensive repairs and was not available for analysis. Instead, the BSE images were compared to other images of previously sub-typed CO3 chondrites in Chizmadia et al. 2002 in order to estimate the petrologic subtype. Results The estimated petrologic subtypes are listed in Table 1. ALH85003 (Fig. 1b) and ALH82101 (Fig. 1c) are estimated to be CO3.5s, comparable to Lance (Fig. 1a). Their veins have widened into channels, 10-15 µm wide. In BSE images, the higher the average atomic number (Z) of a phase, the more white it appears. In meteorites, this usually correlates with Fe content, Fe being the most common high Z element. The relative brightness of the BSE images of the AOIs in ALH85003 and ALH82101 indicates that there is approximately an even volume of Mg-rich olivine and Fe-rich olivine. Most of the inclusions are Fe rich olivine6. Approximately 0.8 µm halos around relic low FeO olivine cores separate the low-FeO cores from the ferroan olivine veins, particularly in Lance (Fig 1a) and ALH770036 (Fig. 2a).

A-881632 (Fig. 2b) is estimated to be a C03.6, comparable to ALH77003 (Fig. 2a). Veins are no longer apparent; instead the Mg-rich olivine cores remain. These Mg-rich cores have 2-3 µm halos. The relative brightness of the AOIs in the BSE images of A-881632 and ALH77003 are very similar.

DaG 055 (Fig. 3b) is estimated to be a CO3.7, comparable to Warrenton (Fig. 3a). Only Fe-rich olivine remains in the AOIs. However the Fe contents of the AOI and

matrix have not equilibrated; they do not show similar levels of brightness in the BSE images.

Finally, Y-790992 (Fig. 4b), EET92126 (Fig. 4c) and Y-791717 (Fig. 4d) are estimated to be CO3.8s, comparable to Isna (Fig. 4a). There is no remnant Mg-rich olivine. The Fe content of the matrix and AOI has fully equilibrated; the brightness levels in the BSE images are similar. In fact, it is difficult to distinguish the AOIs from the surrounding matrix. Discussion Two main models, “onion skin” and “plum pudding,” describe the possible ways asteroids could experience aqueous alteration. The onion-skin model (Fig. 5a) describes a concentrically altered asteroid. The bulk of the heat is concentrated in the center of the asteroid and decreases toward the exterior. Water will generally flow along the heat gradient to the exterior of the asteroid, therefore, the petrologic subtypes are concentrically arranged. This model predicts a high abundance of low subtype CO3s. The plum pudding model (Fig. 5b) is similar to the onion skin model, but instead of all of the heat being concentrated in the center of the asteroid, it is more broadly distributed. Therefore, the subtypes are arranged concentrically around the heat centers. This model predicts a high abundance of high subtype CO3s. Conclusions

All seven CO3 chondrites in this study display moderate to heavy alteration. This is an important constraint on thermal and aqueous movement through the astroidal parent body and should be incorporated in astroidal models. These results favor the plum pudding model. Future Work These samples will be further studied. The constituent phases (olivine, diopside, anorthite, spinel, Fe-Ni metal sulphides and phosphates) will be analyzed on the Cameca SX50 electron microprobe (EMP) for 14 elements (e.g. Si, Mg, Fe, Mn, Al, Ca, Ti, Cr, S, Na, K, P, Ni and O) using 4 crystal spectrometers, natural mineral and synthetic standards, ~20 second counting times, PAP corrections (the Cameca version of ZAF corrections), beam current of ~11nA and an acceleration voltage of 15kV. References: [1] McSween, Jr. H. Y. (1987) Meteorites and Their Parent Planets Cambridge University Press, Cambridge, UK 237 pgs. [2] Wasson J. T. (1985) Meteorites: Their record of early solar-system history. W. H. Freeman and Co., USA. 274 pgs. [3] Sears D. W. G. and Dodd R. T. (1988) Overview and Classification of Meteorites. In Meteorites and the Early Solar System ed. by Kerridge and Matthews. University of Arizona Press, USA.

[4] McSween, Jr. H. Y., Sears D. W. G. and Dodd R. T. (1988) Thermal Metamorphism. In Meteorites and the Early Solar System ed. by Kerridge and Matthews. University of Arizona Press, USA. [5] Scott E. R. D., Barber D. J., Alexander C. M., Hutchison R. and Peck J. A. (1998) Primitive Material Surviving in Chondrites: Matrix. In Meteorites and the Early Solar System ed. by Kerridge and Matthews. University of Arizona Press, USA. [6] Brearley A. J. and Jones R. H. (1998) Chondritic Meteorites. In Reviews in Mineralogy: Planetary Materials vol. 36 J. J. Papike, ed.. Mineralogical Society of America, Washington, D. C., USA. [7] Bates L. R. and Jackson J. A. (1984) Dictionary of Geological Terms Third Edition American Geologic Institute, New York, USA 571 pgs. [8] Chizmadia L. J., Rubin A. E. and Wasson J. T. (2002) Mineralogy and Petrology of Amoeboid Olivine Inclusions in CO3 chondrites: Relationship of Parent-body Aqueous Alteration. Meteoritics and Planetary Science 37 1781-1796. [9] Scott E. R. D. and Jones R. H. (1990) Disentangling nebular and astroidal features of CO3 carbonaceous chondrite meteorite. Geochimica et Cosmochimica Acta 54 2485-2502. [10] Kojima T., Yada S. and Tomeoka K. (1995) Ca, Al-rich Inclusions in three Antarctic CO3 chondrites, Y-82050, and Y-790992: Record of low temperature alteration. Proceedings of the NIPR Symposium 8 79-96.

CO3 Carbonaceous

Chondrite Estimated Petrologic

Subtype ALH85003 3.5

ALH82101 3.5

A-881632 3.6

DaG 3.7

Y-791717 3.8

EET 92126 3.8

Y-790992 3.8

Table 1. The CO3 chondrites in this study and their estimated petrologic subtypes.

Figure 1a

Figure 1b Figure 1c Figure 1. Back-scattered electron (BSE) images of amoeboid olivine inclusions (AOIs) in a) Lancé (from Chizmadia et al. 2002), b) ALH85003 and c) ALH82101. These images show similar levels of Mg-rich and Fe-rich olivine, similar thicknesses of Fe-rich olivine veins between Mg-rich olivine cores with similar thicknesses of diffusive boundaries. Therefore, ALH85003 and ALH82101 have been estimated to have a similar petrologic subtype as Lancé, 3.5.

Figure 2a

Figure 2b Figure 2. Back-scattered electron (BSE) images of amoeboid olivine inclusions (AOIs) in a) ALHA77003 (from Chizmadia et al. 2002) and b) A-881632. These images show similar levels of Mg-rich and Fe-rich olivine, similar thicknesses of Fe-rich olivine veins between Mg-rich olivine cores with similar thicknesses of diffusive boundaries. Therefore, A-881632 has been estimated to have a similar petrologic subtype as ALHA77003, 3.6.

Figure 3a

Figure 3b Figure 3. Back-scattered electron (BSE) images of amoeboid olivine inclusions (AOIs) in a) Warrenton (from Chizmadia et al. 2002) and b) Dag055. These images show no remnant Mg-rich olivine, but the Fe content of the AOI is higher than that of the matrix. Therefore, Dag055 has been estimated to have a similar petrologic subtype as Warrenton, 3.7.

Figure 4a Figure 4b

Figure 4c Figure 4d Figure 4. Back-scattered electron (BSE) images of amoeboid olivine inclusions (AOIs) in a) Isna (from Chizmadia et al. 2002), b) Y-790992 and c) EET92126. These images show no remnant Mg-rich olivine and the Fe content of the AOI is similar to that of the matrix. Therefore, Y-790992 and EET92126 have been estimated to have a similar petrologic subtype as Isna, 3.8.

Figure 5a

Figure 5b Figure 5. Schematic illustrations of two models of asteroidal alteration, a) the onion skin model and b) the plum pudding model.