Acapulcoites and lodranites

The acapulcoites (Fig. 6.7a) have recrystallized textures, but a few relict chondrules have been reported (McCoy et al., 1997a). The relative proportions of the minerals (olivine, orthopyroxene, diopside, plagioclase, metal) in acapulcoites are similar to those in ordinary chondrites, and the abundances of the major elements are nearly chondritic. However, 

---

Low-degree partial melts of undifferentiated chondritic material generate a range of different compositions. The first melt from a chondritic precursor appears at 950 °C and forms by melting of FeS, Fe-Ni metal, and phosphates. Melts of this composition are observed in primitive achondrites (acapulcoites and lodranites). As the temperature increases, silicates begin to melt. At 5 to 10% partial melting, the melt is basaltic in composition (McCoy et al., 1997). 

EET84302 is texturally and mineralogically intermediate between acapulcoites and lodranites (Takeda et al., 1994 Mittlefehldt et al, 1996 McCoy et al., 1997a,b). 

Figure 11 Oxygen isotopic compositions of whole-rock primitive achondrites. Data for a given class are much more scattered than data for differentiated achondrites shown in Figure 10, as a result of incomplete melting and homogenization. Several genetic associations are implied by the data (i) aubrites and enstatite chondrites (ii) acapulcoites and lodranites (iii) lAB irons and winonaites and (iv) ureilites and dark inclusions in carbonaceous chondrites. This and Figure 10 are drawn to the same scale for comparison (source Clayton and Mayeda, 1996).

Acapulcoites and lodranites. These meteorite classes, also called primitive... 

Garrison et al. (1995) and Weigel et al. (1999) conclude that SCR-Ne is also present or likely to be present in some acapulcoites and lodranites. For some reason, members of these meteorite classes often had a small preatmospheric size. It should be noted, however, that these conclusions are based on high Ne/ Ne ratios of samples without a documented relative position to each other (this is also true for most of the shergottites). A verification by means of depth profiles of Ne, Ne/ Ne as well as Al would be highly desirable. 

There are four main types of non-chondritic meteorites (Table 10.1). Primitive achondrites, such as the acapulcoites and lodranites, are thought to be from asteroids that experienced only incipient or limited melting (Table 10.1). In contrast, achondrites, iron meteorites, and stony-irons are considered to represent parent bodies that featured widespread melting processes, which ultimately led to planetary differentiation and the formation of a metallic core and a silicate-rich mantle and crust [14, 15]. 

Cosmic-ray exposure ages are between —5.5 Ma and 7 Ma for all acapulcoites and most of the lodranites, possibly indicating sampling from a single impact event on a common parent body (McCoy et al., 1996, 1997a,b). 

McCoy, T. J., Keil, K., Muenow, D. W. and Wilson, L. (1997a) Partial melting and melt migration in the acapulcoite-lodranite parent body. Geochimica et Cosmochimica Acta, 61, 639-650. 

McCoy T. J., Keil K., and Clayton R. N. (1992a) Petrogenesis of the lodranite—acapulcoite parent body. Meteoritics 27, 258-259. 

Figure 1 Na/Al versus FeO/MnO for achondrites. The FeO/MnO for acapulcoite-lodranite clan (ale), wino-naite-IAB-iron silicate inclusion clan (wic) and aubrites are for orthopyroxene. All other data are bulk rock values. Meteorite data are from sources listed in the text, while planet data are from McDonough and Sun (1995), Taylor (1982), and Wanke and Dreibus (1994).

The acapulcoite-lodranite clan is the prototypical primitive achondrite group. The acapulcoites have modal mineral abundances very much like those of ordinary chondrites, relict chondrules are reported from a few, and their bulk compositions are broadly chondritic (McCoy etal.,l 996 Mittlefehldt et al., 1996, 1998). The lodranites are distinctly nonchondritic. They are generally depleted in... 

Figure 3 Cl-normalized siderophile-chalcophile element ratios in acapulcoite-lodranite clan (ale), winonaite-IAB-iron silicate inclusion clan (wic) and some unique meteorites showing the effects of melting in the metal-sulhde system. Residual solid metal will have high Ir/Ni and low Se/Co, while metallic melt will have the opposite characteristics. Data from sources listed in the text, except for Divnoe (Petaev et al., 1994 Weigel et al., 1997) and Enon (Kallemeyn and Wasson, 1985).

Figure 5 Trapped Ar versus Na/Sc for acapulcoite-lodranite clan meteorites. For most acaplucoite-lodranite clan meteorites, there is a good correlation, indicating progressive gas loss with melting. Sodium and scandium data from sources listed in the text trapped Ar was taken from Weigel et al. (1999, table 8).

---