S. B. Simon

Shock metamorphism and petrography of the Shergotty achondrite

The Shergotty subsamples 1 and 12 consist of augite and pigeonite (67.5%), maskelynite (24%), ilmenite and titanomagnetite (2%), pyrrhotite (0.4%), whitlockite (1.8%), apatite (0.1%), quartz (0.5%), baddeleyite (trace), fayalite (0.4%), mesostasis (3%), and shock-induced local, polymineralic melt products (0.6%). The overall modal composition is similar to other Shergotty samples except for the rather high whitlockite content. The shock effects observed in the mineral constituents include mosaicism, deformation bands, planar fractures, and mechanical twin lamellae in clinopyroxene; isotropization of plagioclase with very rare remnants of birefringence; planar deformation structures, mosaicism, and strongly reduced birefringence in quartz; mechanical twinning of ilmenite; localized in situ melting of neighbouring minerals at the contact of low and high density phases. Based on the refractive index of maskelynite (average: 1.5467 with average An-content of 49%) and the degree of isotropization of the plagioclase an equilibrium shock pressure of 29 ± 1 GPa is derived. The inferred post-shock temperature is 200 ± 20°C. No heating event could have exceeded 400°C (DUKE, 1968). Local stress and temperature concentrations reach 60–80 GPa and 1600–2000°C. The observed shock effects can be explained by a single shock event. A second, weaker shock event as found by others appears to be highly improbable. Equilibrium shock pressures and post-shock temperatures for the other known shergottites are 31 ±2 GPa and 220 ± 50°C (Zagami), 43 ± 2 GPa and 400–800°C (ALHA 77005). The pressure estimate for EETA 79001 by Lambert (1985) is confirmed: 34 ± 1 GPa; the post-shock temperature is 250 ± 50°C. The abundance and textural setting of localized melt products in these meteorites confirm increasing shock pressures in the sequence Shergotty, Zagami, EETA 79001, ALHA 77005. Undoubtedly, the melts could have been formed by the same single shock which produced the equilibrium shock effects (e.g. maskelynitization) in these meteorites. The shock-induced particle velocities inferred from Hugoniot data of basalts are in the 1.5–2.0 km/s range for the parental rocks of the shergottites. Ejection velocities are therefore in the order of 3–4 km/s. Special ejection mechanisms are required in order to exceed the escape velocity of a planet like Mars without producing higher degrees of shock (e.g. melting) than those observed in the shergottites.

An ion microprobe study of the intra-crystalline behavior of REE and selected trace elements in pyroxene from mare basalts with different cooling and crystallization histories

Abstract To study the effects of crystallization sequence and rate on trace element zoning characteristics of pyroxenes, we used combined electron microprobe-ion microprobe techniques on four nearly isochemical Apollo 12 and 15 pigeonite basalts with different cooling rates and crystallization histories. Major and minor element zoning characteristics are nearly identical to those reported in the literature. All the pyroxenes have similar chondrite-normalized REE patterns: negative Eu anomalies, positive slopes as defined by Yb Ce , and slopes of REE patterns from Ce to Sm much steeper than from Gd to Yb. The characteristics of the REE patterns of pigeonite and augite can be rationalized in terms of REE size and charge and M2 site characteristics. Numerous REE and other trace element (V, Sr, Zr, Na, Sc, Cr) zoning characteristics can be correlated with quadrilateral composition. The total REE abundances increase with increasing Ca in the M2 site and indicate that the distribution coefficients of REE between pyroxene and melt vary systematically with the Wo component. The total REE abundances also increase with decreasing Mg (Mg + Fe) and increasing Al prior to plagioclase crystallization. The Eu anomaly in pyroxene is not substantially affected by plagioclase crystallization because Eu3+ is effectively partitioned into pyroxene relative to Eu2+ (( Eu 2+ Eu 3+ ) = .03 in pigeonite) and the plagioclase effect is diluted by the low Eu distribution coefficient for the more abundant pyroxene. The Cr and V components behave similarly in that Cr and V decrease with increasing Ti Al in basalt sample 15058 and decrease with crystallization and at similar Ti Al . Na and Sc increase in abundance from pigeonite to augite with similar Ti Al in both 15058 and 15499. In 15058, Na decreases coherently with increasing Ti Al , whereas Sc shows a limited variability. Sr and Zr in 15058 are somewhat more enriched in the augite with low Ti Al relative to pigeonite. The augite does not become enriched in Zr or Sr until the pyroxene has high Ti Al and low Mg (Mg + Fe) . In 15499, Sr and Zr show slight enrichment in augite relative to pigeonite. These trace element zoning characteristics in pyroxene and the partitioning of trace elements between pyroxene and the melt are intimately related to the interplay among the efficiency of the crystallization process, the kinetics at the crystal-melt interface, the kinetics of plagioclase nucleation and the characteristics of the crystal chemical substitutions within both the pyroxene and the associated crystallizing phases (i.e. plagioclase).

Ion microprobe studies of trace elements in Apollo 14 volcanic glass beads - Comparisons to Apollo 14 mare basalts and petrogenesis of picritic magmas

Abstract The major element geochemistry of picritic lunar glass beads indicates that they represent primary basaltic liquid compositions and, as such, provide unique information concerning the origin of mare basalts and characteristics of the lunar interior. This study used ion microprobe techniques for trace element analysis of individual glass beads representing seven compositionally distinct types of picritic glass beads from the Apollo 14 site [high-Ti glasses (17–11% TiO2): Red/Black, Orange; intermediate-Ti glasses (5–4% TiO2): Yellow; low-Ti glasses (2.8% TiO2): LAP (low alkali, picritic; Papike et al., 1989); very low-Ti glasses ( Ba Sr ) > 1 ] of these glasses are different from low-Ti mare basalts at other sites but are similar to crystalline basalts at the Apollo 14 site. The intermediate to high-Ti picritic glasses exhibit a flat to slightly positive LREE slope and a negative HREE slope. The Orange glass has a higher total REE abundance and a slightly larger negative Eu anomaly than the Red/Black and Yellow glasses. Relative to the Apollo 17 high-Ti glasses, Apollo 14 high-Ti glasses are enriched in REE, LREE/HREE, Y, V, Zr, Sr, Ba, and Ba Sr and are similar in alkali elements (Li, Rb), Co, and Sc. Trace element modeling, within the context of liquid lines of descent and major element characteristics, indicates that the picritic glass beads at the A-14 site are not related by low pressure fractional crystallization to each other or to crystalline basalts at the Apollo 14 or other landing sites. A possible exception is the relationship between LAP and basalts of the high-Al basalt suite. The wide range of primary magma compositions and the lack of petrogenetic linkage (via crystal fractionation) to crystalline basalts indicates that either a wide compositional range of evolved mare basalts has not yet been sampled or a unique mechanism is selectively tapping these picritic magmas directly from their mantle source region. The wide range of major and trace element characteristics of the volcanic glass beads is consistent with derivation from mineralogically distinct sources which consist of varying proportions of olivine + orthopyroxene ± clinopyroxene ± ilmenite ± plagioclase ± KREEP component. The evolved KREEP component may have been incorporated into these primary picritic magmas by either assimilation-fractional crystallization-type processes (AFC) or by hybridization of the mantle source. The former appears less likely due to the general systematic increase in incompatible elements relative to Ti concentration and the apparent lack of crystallization that is required in AFC-type models. The hybridization processes may be the result of “sinker” mechanisms as proposed by Ringwood and Kesson (1976), a manifestation of original mantle inhomogeneities, or a magma ocean stirred by large impacts.

Pegmatite/wallrock interactions, Black Hills, South Dakota: Progressive boron metasomatism adjacent to the Tip Top pegmatite

Abstract Interaction between country rock and fluids derived from the Tip Top pegmatite has resulted in a series of boron enriched assemblages. Between unaltered quartz-mica schist to the pegmatite contact is a succession of four mineral assemblages: 1. (1) Quartz-Biotite-Potassium Feldspar assemblage (Q-B-K), which consists essentially of the original metamorphic silicate assemblage plus anomalously high amounts of modal tourmaline 2. (2) Quartz-Biotite-Tourmaline assemblage (Q-B-T) 3. (3) Tourmaline-Quartz-Muscovite assemblage (T-Q-M) 4. (4) Tourmaline-Quartz assemblage (T-Q). Alkali elements (Cs, Rb, K, Li), SiO 2 , and Ba show a decrease from the Q-B-K assemblage to the T-Q assemblage. A1 2 O 3 , Ga, B, total Fe and Zn increase moderately from the Q-B-K assemblage to the T-Q assemblage. The mineral chemistries also change considerably. The Mg/(Mg + Fe 2+ ) ratios in biotites range from 0.54 to 0.50 in samples from the Q-B-K assemblage to 0.39 in the (Q-B-T) assemblage. The range in tourmaline end-member components from the Q-B-K assemblage to the T-Q assemblage is as follows: Q-B-K: Dravite .63 Schorl .23 Elbaite .05 Buergerite .09 T-Q: Dravite .23 Schorl .37 Elbaite .17 Buergerite .23 . Observed variations in mineral assemblage and whole rock chemistry within the alteration zone appear to a first approximation to be a function of μB 2 O 3 (boron metasomatism) and μK 2 O (alkali leaching). The breakdown of feldspar and biotite may be approximated by reactions: 2HCl + 2(K, Na)AlSi 3 O 8 /ai 2(K, Na)Cl + Al 2 SiO 5 + 5SiO 2 + H 2 O and 2 Annite + SiO 2 + 5Al 2 SiO 5 + 2NaCl + 6H 3 BO 3 /ai 2 Tourmaline + 2KCl + 7H 2 O. The alteration zone may represent either a single episode (B-, Cs-, Li-, Rb-enriched fluid) or multiple episodes (B, Zn, Mn fluid and Cs, Li, Rb fluid) of pegmatite fluid-schist interactions. In both situations, B in the aqueous fluid from the pegmatite reacts with the schist breaking down sheet silicate “traps” for Cs, Rb, Li, and K and forming tourmaline-rich assemblages.

Apollo 16 regolith breccias and soils: recorders of exotic component addition to the Descartes region of the moon

Abstract The subdivision of Apollo 16 regolith breccias into ancient ( ∼ 4 b.y.) and relatively younger samples on the basis of trapped 40 Ar/ 36 Ar ratios [1] makes possible, with the present-day soils as a third sample suite, a petrologic and chemical determination of regolith evolution and exotic component addition at the A-16 site. Petrologic data for the ancient breccias show that the early regolith was composed of fragments of plutonic rocks and impact melt rocks, and minerals and impact glasses derived from these lithologies. The presence of KREEPy glasses in the ancient breccias demonstrates that KREEPy lithologies and impact melts formed early in lunar history. Comparison of the ancient breccias with the young breccias and the soils shows that the mare components, mainly in the form of orange high-TiO 2 glass and green low-TiO 2 glass, were mostly added to the site after formation of the ancient breccias and prior to formation of the young breccias. The young breccias are petrologically similar to the soils. The major change in the regolith since the formation of the young breccias is an increase in maturity—the formation of fused soil particles with prolonged exposure to micrometeorite impacts. By contrast, the breccias formed at times when large impacts and soil mixing were dominant over small impacts and maturation.

The lunar regolith: Chemistry and petrology of Luna 24 grain size fractions

Chemical data are reported for the first time for lunar soil size fractions smaller then 2 μm. We report chemical data for 30 elements by INAA in eight size fractions (370−200, 200−94, 94−74, 74−40, 40−10, 10−5, 5−2 and 10 μm) are quite similar to each other but quite different from the fine fractions (<10 μm). The finer fractions (10–5, 5–2, <2 μm) become increasingly feldspathic and enriched in large-ion lithophile elements (LILE) with decreasing grain size. Chemical data for the finer fractions provide direct evidence in favor of efficient comminution of rock mesostasis and feldspar leading to their preferential incorporation into the finer fractions. High concentrations of meteoritic indicator elements (Ni, Au, Ir) in the finer fractions are consistent with the comminution process by micrometeorite impacts. The chemical data strongly support the F3 (fusion of the finest fraction) model for agglutinate formation. Based on grain size distribution, petrology, and LILE patterns of size fractions, the Luna 24 soils are less reworked than most lunar soils. The Luna 24 regolith appears to have formed as a result of mixing more mature and fine grained material with less mature coarse material in different proportions at different depth intervals.

Petrology, chemistry and origin of Apollo 15 regolith breccias

Abstract Variations in modal petrology, mineral compositions and bulk compositions were determined for ten Apollo 15 regolith breccias for comparison with local soils and assessment of the intrasite petrologic variability of the Apollo 15 regolith. Based on the above criteria the breccias are of local origin and mimic the soils from the corresponding sampling stations, with the exception of station 2 breccia 15205. This sample formed from an anomalous regolith and although not considered exotic to the site is not representative of the soil at the site. KREEP basalt and green glass components vary from trace amounts to dominant in the breccias, evidence that these materials entered the regolith prior to formation of the breccias. Breccias from the edge of Hadley Rille are modally richer in highland fragments than the soils, whereas at the base of Hadley Delta the reverse is true. This is explained by the loss of material into the Rille to be replaced by basalt-derived material, making the soils more basalt-rich. At the base of Hadley Delta highland material is accumulating and the soils are becoming more highland-rich. Over billions of years these processes have developed differences between the present day, evolving soils and “fossil” non-evolving soils represented by the regolith breccias. This shows that there has been little change in the geology and the morphology of the Apollo 15 site, probably since the eruption of mare basalts at the site (~3.3 b.y.).

Chemical migration by contact metamorphism between pegmatite and country rocks: natural analogs for radionuclide migration

Comparison of trace element signatures of country rocks as a function of distance from the contact with two pegmatites, Tin Mountain and Etta, in the Black Hills of South Dakota, suggests that some elements such as K, Li, Rb, Cs, As, Sb, Zn and Pb, have migrated to distances of 4 to 40 meters during contact metamorphism. The relative degree of migration varies depending on the element. On the other hand, there is virtually no migration of rare earth elements (REE), Al, Sc, Cr, Hf, U, and Th. Biotite and muscovite are effective trace element traps for Li, Rb, and Cs. Biotite has a greater affinity for Rb, Cs and Li than muscovite. 9 references, 5 figures, 1 table.