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METEORITICS&
PLANETARY
SCIENCE
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Major element and primary sulfur concentrations in Apollo 12 mare
basalts: The view from melt inclusions
Major element and sulfur concentrations have been determined in
experimentally heated olivine-hosted melt inclusions from a suite of
Apollo 12 picritic basalts (samples 12009, 12075, 12020, 12018, 12040,
12035). These lunar basalts are likely to be genetically related by
olivine accumulation (Walker et al. 1976a, b). Our results show that major
element compositions of melt inclusions from samples 12009, 12075, and
12020 follow model crystallization trends from a parental liquid similar
in composition to whole rock sample 12009, thereby partially confirming
the olivine accumulation hypothesis. In contrast, the compositions of melt
inclusions from samples 12018, 12040 and 12035 fall away from model
crystallization trends, suggesting that these samples crystallized from
melts compositionally distinct from the 12009 parent liquid, and therefore
may not be strictly cogenetic with other members of the Apollo 12 picritic
basalt suite. Sulfur concentrations in melt inclusions hosted in early
crystallized olivine (Fo75) are consistent with a primary
magmatic composition of 1050 ppm S, or about a factor of 2 greater than
whole rock compositions with 400–600 ppm S. The Apollo 12 picritic basalt
parental magma apparently experienced outgassing and loss of S during
transport and eruption on the lunar surface. Even with the higher
estimates of primary magmatic sulfur concentrations provided by the melt
inclusions, the Apollo 12 picritic basalt magmas would have been
undersaturated in sulfide in their mantle source regions, and capable of
transporting chalcophile elements from the lunar mantle to the surface.
Therefore, the measured low concentration of chalcophile elements (e.g.,
Cu, Au, PGEs) in these lavas must be a primary feature of the lunar mantle
and not related to residual sulfide remaining in the mantle during
melting. We estimate the sulfur concentration of the Apollo 12 mare basalt
source regions to be ~75 ppm, significantly lower than that of the
terrestrial mantle.
Geochemistry and Ar-40-Ar-39 geochronology of impact-melt clasts in
feldspathic lunar meteorites: We studied 42 impact-melt clasts from lunar feldspathic regolith breccias MAC 88105, QUE 93069, DaG 262, and DaG 400 for texture, chemical composition, and/or chronology. Although the textures are similar to impact-melt clasts identified in mafic Apollo and Luna samples, the meteorite clasts are chemically distinct from those, with lower Fe, Ti, K, and P, thus representing previously unsampled impacts. 40Ar-39Ar ages on 31 of the impact-melts, the first ages on impact-melt samples from outside the region of the Apollo and Luna sampling sites, range from ~4 to ~2.5 Ga. We interpret these samples to have been created in at least six, and possibly nine or more, different impact events. One inferred impact event may be consistent with the Apollo impact-melt rock age cluster at 3.9 Ga, but the meteorite impact-melt clasts with this age are different in chemistry from the Apollo samples, suggesting that the mechanism responsible for the 3.9 Ga peak in lunar impact-melt clast ages is a lunar-wide phenomenon. No meteorite impact-melts have ages more than 1σ older than 4.0 Ga. This observation is consistent with, but does not require, a lunar cataclysm.
Ibitira: A basaltic achondrite from a distinct parent asteroid and
implications for the Dawn mission I have done a detailed petrologic study of Ibitira, a meteorite that has been classified as a basaltic eucrite since 1957. The mean Fe/Mn ratio of pyroxenes in Ibitira with <10 mole% wollastonite component is 36.4 ± 0.4; this value is well resolved from those of similar pyroxenes in five basaltic eucrites studied for comparison, which range from 31.2 to 32.2. Data for the latter five eucrites completely overlap. Ibitira pyroxenes have lower Fe/Mg than the basaltic eucrite pyroxenes; thus, the higher Fe/Mn ratio does not reflect a simple difference in oxidation state. Ibitira also has an oxygen isotopic composition, alkali element contents, and a Ti/Hf ratio that distinguishes it from basaltic eucrites. These differences support derivation from a distinct parent asteroid. Thus, Ibitira is the first recognized representative of the fifth known asteroidal basaltic crust, the others being the HED, mesosiderite, angrite, and NWA 011 parent asteroids. 4 Vesta is generally assumed to be the HED parent asteroid. The Dawn mission will orbit 4 Vesta and do detailed mapping and mineralogical, compositional, and geophysical studies of it. Ibitira is only subtly different from eucritic basalts. A challenge for the Dawn mission will be to distinguish different basalt types on the surface and to attempt to determine whether 4 Vesta is indeed the HED parent asteroid.
In situ survey of graphite in unequilibrated chondrites: Morphologies,
C, N, O, and H isotopic ratios We performed in-situ morphological and isotopic studies of graphite in the primitive chondrites Khohar (L3), Mezö-Madaras (L3), Inman (L3), Grady (H3), Acfer 182 (CH3), Acfer 207 (CH3), Acfer 214 (CH3), and St. Marks (EH5). Various graphite morphologies were identified, including book, veins, fibrous, fine-grained, spherulitic, and granular graphite, and cliftonite. SIMS measurements of H, C, N, and O isotopic compositions of the graphites revealed large variations in the isotopic ratios of these four elements. The δ15N and δ13C values show significant variations among the different graphite types without displaying any strict correlation between the isotopic composition and morphology. In the Khohar vein graphites, large 15N excesses are found, with δ15Nmax ~+955‰, confirming previous results. Excesses in 15N are also detected in fine-grained graphites in chondrites of the CH clan, Acfer 182, Acfer 207, and Acfer 214, with δ15N ranging up to +440‰. The 15N excesses are attributed to ion-molecule reactions at low temperatures in the interstellar molecular cloud (IMC) from which the solar system formed, though the largest excesses seem to be incompatible with the results of some recent calculation. Significant variations in the carbon isotopic ratios are detected between graphite from different chondrite groups, with a tendency for a systematic increase in δ13C from ordinary to enstatite to carbonaceous chondrites. These variations are interpreted as being due to small- and large-scale carbon isotopic variations in the solar nebula.
Noble gases in ten Nullarbor chondrites: Exposure ages, terrestrial
ages, and weathering effects We present concentration and isotopic composition of He, Ne, and Ar in ten chondrites from the Nullarbor region in Western Australia as well as the concentrations of 84Ke, 129Xe, and 132Xe. From the measured cosmogenic 14C concentrations (Jull et al. 1995), shielding-corrected production rates of 14C are deduced using cosmogenic 22Ne/21Ne ratios. For shielding conditions characterized by 22Ne/21Ne >1.10, this correction becomes significant and results in shorter terrestrial ages. The exposure ages of the ten Nullarbor chondrites are in the range of values usually observed in ordinary chondrites. Some of the meteorites have lost radiogenic gases as well as cosmogenic 3He. Most of the analyzed specimens show additional trapped Ar, Kr, and Xe of terrestrial origin. The incorporation of these gases into weathering products is common in chondrites from hot deserts.
Peak metamorphic temperatures in type 6 ordinary chondrites: An
evaluation of pyroxene and plagioclase geothermometry
Quantifying the peak temperatures achieved during metamorphism is critical
for understanding the thermal histories of ordinary chondrite parent
bodies. Various geothermometers have been used to estimate equilibration
temperatures for chondrites of the highest metamorphic grade (type 6), but
results are inconsistent and span hundreds of degrees. Because different
geothermometers and calibration models were used with different
meteorites, it is unclear whether variations in peak temperatures
represent actual ranges of metamorphic conditions within type 6 chondrites
or differences in model calibrations. We addressed this problem by
performing twopyroxene geothermometry, using QUILF95, on the same type 6
chondrites for which peak
Recipes for making synthetic CAIs, refractory residues, and minerals
for rim-forming experiments
Recipes are presented for synthesizing various type A and type B
Ca-Al-rich inclusions
Constraints on the depth and variability of the lunar regolith
Knowledge of regolith depth structure is important for a variety of
studies of the Moon and other bodies such as Mercury and asteroids. Lunar
regolith depths have been estimated using morphological techniques (i.e.,
Quaide and Oberbeck 1968; Shoemaker and Morris 1969), crater counting
techniques (Shoemaker et al. 1969), and seismic studies (i.e., Watkins and
Kovach 1973; Cooper et al. 1974). These diverse methods provide good first
order estimates of regolith depths across large distances (hundreds of
kilometers), but may not clearly elucidate the variability of regolith
depth locally (100 m to km scale). In order to better constrain the
regional average depth and local variability of the regolith, we
investigate several techniques. First, we find that the apparent
equilibrium diameter of a crater population increases with an increasing
solar incidence angle, and
Solid-state 13C NMR characterization of insoluble organic matter from
Antarctic CM2 chondrites: Evaluation of the meteoritic alteration level
Chemical structures of the insoluble organic matter (IOM) from the
Antarctic CM2 |
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This website is maintained by Agnieszka P. Baier. Website credits. Last updated: 10/27/06.
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