Bulk Oxygen Isotopic Composition Research Articles

The fall, recovery, and classification of the Park Forest meteorite

Abstract— On the night of March 26, 2003, a large meteorite broke up and fell upon the south suburbs of Chicago. The name Park Forest, for the village that is at the center of the strewnfield, has been approved by the nomenclature committee of the Meteoritical Society. Satellite data indicate that the bolide traveled from the southwest toward the northeast. The strewnfield has a southeast‐northwest trend; however, this is probably due to the effects of strong westerly winds at high altitudes. Its very low 56Co and very high 60Co activities indicate that Park Forest had a preatmospheric mass that was at least ∼900 kg and could have been as large as ∼7 times 103 kg, of which only ∼30 kg have been recovered. The average compositions of olivine and low‐Ca pyroxene, Fa24.7 ± 1.1 and Fs20.8 ± 0.7, respectively, and its bulk oxygen isotopic composition, δ18O = +4.68%o, δ17O = +3.44%o, show that Park Forest is an L chondrite. The ferromagnesian minerals are well equilibrated, chondrules are easily recognized, and maskelynite is mostly ≤50 μm across. Based on these observations, we classify Park Forest as type 5. The meteorite has been strongly shocked, and based on the presence of maskelynite, mosaicism and planar deformation features in olivine, undulatory extinction in pyroxene, and glassy veins, the shock stage is S5. The meteorite is a monomict breccia, consisting of light‐colored, angular to rounded clasts in a very dark host. The light and dark lithologies have essentially identical mineral and oxygen isotopic compositions. Their striking difference in appearance is due to the presence of a fine, pervasive network of sulfide veins in the dark lithology, resulting in very short optical path lengths. The dark lithology probably formed from the light lithology in an impact that formed a sulfide‐rich melt and injected it into cracks.

Petrology and stable isotope geochemistry of the metamorphosed zinc-iron-manganese deposit at Sterling Hill, New Jersey

The Sterling Hill deposit is an isoclinically folded sequence of zinc-, iron-, and manganese-rich strata surrounded by the Franklin Marble. The most metal-rich layers are composed of combinations of willemite, franklinitc, zincitc, and calcite; other layers contain calc-silicate minerals and calcite. Structural, petrologic, and geochronologic evidence indicates that the deformation and mineral assemblages observed today were produced during granulite-grade Grenville metamorphism of the deposit at about 1.0 Ga.In general, the oxygen isotope and chemical compositions of individual minerals vary from lithologic layer to lithologic layer but are uniform within layers which suggests that mineral compositions were controlled by the bulk compositions of the rocks. Important exceptions to this rule occur near faults and fractures where the deposit was infiltrated by retrograde metamorphic fluids. In these areas, the rocks commonly contain hydrous minerals which have replaced anhydrous assemblages, and the minerals have been exchanged to larger delta 18 O values.Willemite and zincite are stable at higher f (sub O 2 ) /f (sub S 2 ) than pyrite and pyrrhotite which implies that the ores are not chemically equilibrated with the surrounding Franklin Marble but are higher in oxidation state and/or lower in sulfidation state. The deposit also contrasts with the surrounding marble in oxygen isotope composition. Bulk delta 18 O values for ore layers range from about 6 to 13 per mil, and thin marble layers interbedded with the ores are consistently about 15 per mil. The Franklin Marble is 20 to 25 per mil.Models for the protolith are presented in which the bulk oxygen isotope compositions and bulk chemical compositions of the rocks are assumed to reflect protolith compositions modified by metamorphic devolatilization. The mineralogy of the protolith is inferred from the bulk chemical compositions of the rocks and by analogy with the mineralogy of other metal-rich oxidized rocks, and the minerals are assumed to have equilibrated isotopically with the fluid which formed them. The application of the models which require the fewest assumptions is to the marbles interbedded with the ore layers. Their isotopic compositions appear to require either equilibration with 18 O-depleted meteoric water at the earth's surface temperatures, or equilibration with a more 18 O-enriehed water at temperatures characteristic of hydrothermal or metamorphic processes. Application of the models to the ore layers suggests that the last fluid-dominated event took place at 150 degrees + or - 50 degrees C rather than at the earth's surface temperatures. Structural features of the deposit also support the higher temperature formation in that they appear to be incompatible with an origin by weathering.Exposure to seawater is the most plausible alternative to subaerial weathering for producing the high oxidation state-low sulfidation state of the protolith. An extremely large volume of seawater would have been required to oxidize the entire metal inventory of the deposit. Thus, the formation of the oxidized protolith is best inferred to have taken place on the Proterozoie sea floor or in shallowly buried sediments which were well irrigated by seawater. The closest modern analogues for Sterling Hill, and by analogy the nearby Franklin Furnace deposit, are the sulfide-poor strata in the metalliferous sediments beneath the Red Sea brine pools. There are, however, significant differences between the two occurrences.

Local and exotic components of primitive meteorites, and their origin

Most of the material of chondrites was heavily reprocessed in the early Solar System, and hence retains only a dim memory of its interstellar origin even in the least altered meteorites. Opaque matrix, the most primitive material, seems to have taken up Fe2+ and changed its mineralogy and texture. Chondrules and Ca, Al-rich inclusions have been further altered by melting, oxidation or reduction, loss of volatiles, etc. None the less, small amounts of exotic components have survived, as indicated by isotopic anomalies. A dust component enriched in ieO is the most abundant and widespread. It survives in Ca, Al-rich inclusions as anomalous spinel grains, but is recognizable even in highly evolved meteorites and planets from variations in bulk oxygen isotopic composition. A few inclusions show small nucleosynthetic anomalies for many elements (Si, Ca, Ti, Cr, Sr, Ba, Nd, Sm), always coupled with mass fractionation of several of these elements, as well as 0 and Mg. Seven extinct radionuclides ( 26 AI, 41 Ca, 53 Mn, 107 Pd, 129 I, 146 Sm, and 244 Pu) have been recognized from their decay products, and provide clues to the chronology and nucleosynthetic sources of the early Solar System. Highly volatile elements, such as C, N and the noble gases, show especially large and numerous isotopic anomalies. The noble-gas components include Ne-E (monoisotopic 22 Ne from the (B+ decay of 2.6a 22 Na), Xe-HL (enriched 2-fold in the light and heavy isotopes), and Xe-S (enriched in the even-numbered, middle isotopes 128, 130 and 132). They are located in carriers that themselves are anomalous, e.g. carbon enriched up to 2.4-fold in 13 C or depleted by more than 30 % in 15 N, or spinel enriched in 16 O and 13 C. Other anomalies include nitrogen enriched 2-fold in 15 N and hydrogen enriched 5-fold in D ; probably relict interstellar molecules. A variety of astronomical sources seem to be required: novae, red giants, supernovae and molecular clouds.