Experimental and petrological constraints on lunar differentiation from the Apollo 15 green picritic glasses

Abstract

Petrogenetic modeling demonstrates that the Apollo 15 A-B-C glass trends could not have been formed by fractional crystallization or any continuous assimilation/fractional crystallization (AFC) process. The B and C glass compositional trends could not have been formed by batch or incremental melting of an olivine + orthopyroxene source or any other homogeneous source, though the A glasses may have been formed by congruent melting over a small pressure range at depth. The B compositional trend is well modeled by starting with an intermediate A composition and assimilating a shallower, melted cumulate, and the C compositional trend is well modeled by a second assimilation event. The assimilation process envisioned is one in which heat and mass transfer were separated in space and time. In an initial intrusive event, a picritic magma crystallized and provided heat to melt magma ocean cumulates. In a later replenishment event, the picritic magma incrementally mixed with the melted cumulate (creating the compositional trends in the green glass data set), ascended to the lunar surface, and erupted as a fire fountain. A barometer created from multiple saturation points provides a depth estimate of other glasses in the A-B-C trend and of the depths of assimilation. This barometer demonstrates that the Apollo 15 A-B-C trend originated over a depth range of ~460 km to ~260 km within the moon.

An experimental study of anorthosite dissolution in lunar picritic magmas: Implications for crustal assimilation processes

“…We have reproduced the plot of CaO vs. Al 2 O 3 including a greater number of picritic glass compositions in Fig. 7 (Delano, 1980(Delano, , 1986Hughes et al, 1988;Shearer and Papike, 1993;Elkins-Tanton et al, 2003). When extrapolated to Al 2 O 3 concentrations consistent with anorthite, the trend defined by Finnila et al (1994) does not intersect the measured lunar anorthite compositions.…”

“…The starting materials for the melts are a very low-Ti (VLT) Apollo 15 green glass (Elkins-Tanton et al, 2003), a low-Ti Apollo 15 yellow glass (Shearer and Papike, 1993), and a high-Ti Apollo 15 red glass (Delano, 1980), each synthesized from high purity oxides and carbonates (Table 1). In addition, an olivine-saturated low-Ti melt was made by adding 10wt% fine grained, optical clean olivine from Kilbourne Hole, NM (Mg# = 91, composition given in Table 1 of Morgan and Liang, 2005) to the lowTi glass.…”

Heat-producing elements in the lunar mantle: Insights from ion microprobe analyses of lunar pyroclastic glasses

“…Second, lunar pyroclastic glasses appear to represent melting of a diverse range of mantle mineral assemblages (Shearer and Papike, 1993Papike, , 1999. Third, high-pressure experiments indicate that the lunar pryoclastic glasses represent basaltic melts that were in equilibrium with olivine + orthopyroxene residua (Stolper et al, 1974;Green et al, 1975;Delano, 1980;Chen et al, 1982;Chen and Lindsley, 1983;Longhi, 1992;Hess, 2000;Elkins-Tanton et al, 2003). This residual mineral assemblage does not significantly fractionate Th from Sm during melting, which means that the Th/Sm ratio of the glasses approximates the Th/Sm ratio of the mantle sources from which the melts were derived (Shearer et al, 2002).…”

Compressibility of molten Apollo 17 orange glass and implications for density crossovers in the lunar mantle

“…ElkinsTanton et al, 2003a;Nicholis and Rutherford, 2009). Smith and Agee (1997) assessed the importance of crystal-liquid density inversions for the genesis of the low-Ti end-member ('Apollo 15 green C glass' containing 0.23 wt% TiO 2 ) and predicted a density crossover with orthopyroxene at 3.5 GPa, equivalent to a lunar depth of $800 km, which is significantly deeper compared to the multiple saturation depth at $1.3 GPa reported by Elkins-Tanton et al (2003b). This indicates that molten green glass would be buoyant with respect to its source region, facilitating eruption.…”