The crystalline lunar spherules: Their formation and implications for the origin of meteoritic chondrules

Abstract

Abstract-Crystalline lunar spherules (CLS) from three thin sections of Apollo 14 regolith breccias (143 18,6; 14318,48 and 14315,20) have been examined. The objects have been classified and their abundances, size distributions, bulk compositions, and (where possible) plagioclase compositions determined. By number, 64% consist predominantly of very tine-grained equant plagioclase grains but can also contain larger (-50 pm) feldspar crystals (type X), while 22% contain plagioclase lathes in a fine-grained mafic mesostasis (type Y ) . Plagioclase in both spherule types displays bright yellow cathodoluminescence that is conspicuous among the blue CL of the normal feldspar of the breccias. Type Z spherules (5%) contain feldspar with blue CL and minor amounts of olivine and pyroxene. Type Q spherules (4%) contain feldspar with yellow CL but in a luminescent mesostasis (of quartz or feldspar?). A few spherules are mixtures of type Y and type X textures. Most type X spherules, and a few type Y spherules, have fine-grained opaque rims. Compound objects were also found and consist of two or more CLS that appear to have collided while still plastic or molten. The CLS are thought to be impact spherules that crystallized in free flight, their coarse textures suggesting fairly slow cooling rates (-< 1 "Us). The abundance of the CLS resembles that of chondrules in the CM chondrite Murchison, and their cumulative size-frequency distributions are very similar to those of the chondrules in several meteorite classes. The bulk compositions of the CLS do not resemble regoliths at any of the Apollo sites, including Apollo 14, or any of the common impact glasses, but they do resemble the bulk compositions of several lunar meteorites and the impact glasses they contain. The Apollo 14 site is located on a region containing Imbrium ejecta, and we suggest that the CLS derive from the Imbrium impact. Ballistic calculations indicate that only impact events of this size on the Moon are capable of producing melt spherules with the required free flight times and slow cooling rates. Smaller impacts produce glassy spherules and agglutinates. As has been pointed out many times, the CLS have many properties in common with meteoritic chondrules. While much remains unclear, difficulties with a nebular origin and new developments in chondrule chronology, studies of asteroid surfaces and impact ejecta behavior, and the present observations indicate that meteoritic chondrules could have formed by impact.

Manganese‐chromium formation intervals for chondrules from the Bishunpur and Chainpur meteorites

“…It is reasonable to ask whether FeO/MnO variability among the chondrules results from a difference in volatility between Fe and Mn leading to differing fractional losses of each element from the chondrules as molten droplets. However, comparison of FeO/MnO for the chondrules to similar data for impact-melted lunar crystalline spherules (Symes et al, 1998;Ruzicka el al., 2000) suggests that volatility-driven fractionation of the Fe/Mn ratio during impact melting is unlikely. Symes et al (1998) reported the compositions of crystalline lunar spherules (CLSs) extracted from Apollo 14 breccias 143 15 and 14318.…”

“…However, comparison of FeO/MnO for the chondrules to similar data for impact-melted lunar crystalline spherules (Symes et al, 1998;Ruzicka el al., 2000) suggests that volatility-driven fractionation of the Fe/Mn ratio during impact melting is unlikely. Symes et al (1998) reported the compositions of crystalline lunar spherules (CLSs) extracted from Apollo 14 breccias 143 15 and 14318. The FeO/MnO ratios for two groups of seven spherules each were tightly grouped at 67 ?…”

Impactites of the Yaxcopoil‐1 drilling site, Chicxulub impact structure: Petrography, geochemistry, and depositional environment

“…In our context, it is noteworthy that D. Sears (University of Arkansas) envisions a chondrule forming process related to impact processes. He argues that chondrules formed on asteroids during an impact by a larger body (Symes et al 1998). Our impact "chondrules" were not formed in zero gravity but in an equally turbulent, fiery cloud of rock and melt particles.…”

Nebular or parent body alteration of chondritic material: Neither or both?

“…Crystalline lunar spherules, with physical properties similar to meteoritic chondrules but with highland compositions, are common at the Apollo 14 site that is thought to contain ejecta from the Imbrium impact. Thus, impact spherules take on the structural properties of chondrules only when the impact is sufficiently large in relation to the size of the target body and flight times are long enough for extensive crystallization (Symes et al, 1998). Only on asteroids that have experienced impacts large enough to produce craters with radii comparable to the radius of the asteroid would one expect the proportion of chondrules to melt spherules and agglutinates to be high.…”

“…However, we have previously suggested that gravitational perturbations by Jupiter might mean that impacts sufficient to create chondrules occur preferentially near resonances (Symes et al, 1998).…”