APPENDIX

PART II

ACHONDRITES

CONTINUE TO
[PART I] Chondrites
[PART III] Irons
[PART IV] Stony-Irons
[PART V] Refractory Phases
[PART VI] Trends for Classification
[APPENDECTOMY]

Members of the achondrite classification can be divided into two distinct groups:

  1. Primitive—igneous-textured meteorites that are nearly chondritic in bulk composition and retain geochemical and isotopic characteristics of their precursors (lodranites, acapulcoites, winonaites, and IAB silicates, and according to Fe/Mg–Fe/Mn plots, the brachinites and ureilites (Goodrich and Delany, 2000). These meteorites are residues from low degrees of melting (< ~30%), and thus are actually somewhat evolved meteorites.
  2. Evolved—the primitive characteristics have become obliterated due to extensive igneous processing, i.e., melting (vestan [HEDs: howardites, eucrites, and diogenites], martian [SNCs: shergottites, nakhlites, chassignites, etc.], and lunar).

The primitive achondrite group has recently been further divided into two subgroups:

  1. Type A—members have some iron loss but preserve their main mafic chondritic silicate mineralogy (e.g. lodranites, acapulcoites, winonaites)
  2. Type B—members have lost their primary REE and siderophile abundance ratios as well as some light silicate components, but preserve some primitive components such as Fe/Mg ratios, C- and O-isotopic ratios, and volatile abundances (e.g. ureilites)
The Type A acapulcoite–lodranite clan has experienced a wide ranging thermal history. Of the most primitive acapulcoites, the least metamorphosed among them is GRA 98028, which exhibits relict chondrules, a very fine grain size, and contains no large FeS veins. The relict chondrules present in Monument Draw also probably reflect a relatively primitive nature. Further along the metamorphic sequence is Dhofar 125, which exhibits early stages of melting and some loss of sulfides. Another typical acapulcoite, Acapulco, has also experienced a high degree of thermal metamorphism and has a highly recrystallized texture. Certain Antarctic meteorites exhibit loss of plagioclase and sulfide phases, and are transitional to lodranites. Finally, Lodran and the other lodranites have experienced the highest temperatures. They were crystallized from residual melt material depleted in the low-melting point components such as plagioclase, troilite, and metal. One further stage in the metamorphic continuum is represented by an Antarctic meteorite that has trapped a partial melt component that was lost from the lodranite region. This meteorite has become an enriched acapulcoite.

Among the meteorite types that compose the evolved achondrite group, the eucrites have been petrologically divided into a metamorphic sequence comprising seven types (after Takeda and Graham, 1991; Yamaguchi et al., 1996):

  1. Type 1—most quickly cooled in the sequence; mesostasis-rich with glass phase and original chemistry preserved; exhibit pronounced Mg-Fe zoning in pyroxenes; represent the least altered basalt studied; e.g., clasts in Y-75011, Y-75015, and Y-74450
  2. Type 2—metastable Fe-rich pyroxenes are absent; mesostasis glass is no longer clear; e.g., Pasamonte
  3. Type 3—zoning from core to rim is less defined with an increase in Ca towards the rim; pyroxenes becoming cloudy; coarsening of pyroxenes resulting from augite exsolution lamellae; e.g., clast in Y-790266
  4. Type 4—only remnants of zoning still visible; cloudy pyroxenes present; mesostasis glass is recrystallized or absent; augite exsolution lamellae becoming resolvable in microprobe; e.g., Stannern, Nuevo Laredo
  5. Type 5—homogenous host composition with readily resolvable exsolved pigeonite lamellae; pigeonites extensively clouded by reheating; mesostasis glass recrystallized or absent; e.g., Juvinas, Sioux Co., Lakangaon
  6. Type 6—most slowly cooled eucrites in the sequence; the clinopyroxene pigeonite is partly inverted to orthopyroxene through slow cooling processes; pyroxenes contain Mg-rich cores and coarse augite exsolution lamellae; original mesostasis is absent; Ca is enriched in the rims; often have a brecciated texture; e.g., Millbillillie, Y-791186
  7. Type 7—recognized as the most metamorphosed in the sequence (Yamaguchi et al., 1996); e.g., Palo Blanco Creek, Jonzac, Haraiya, A-87272
Achondrites represent about 8% of all meteorite falls. They originated on chondritic bodies that were subjected to some degree of igneous melting and recrystallization. Their parent bodies were large enough to melt and segregate the denser metals from the lighter silicates, generally forming a metallic core, a magnesium-rich mantle, and a calcium-rich crust. Of the various achondrites, four recognized groups (howardites, eucrites, diogenites, and olivine diogenites) are believed to have originated on the asteroid 4 Vesta and constitute the largest achondrite group. These represent the brecciated surface materials (howardites), the extrusive/intrusive basalts (eucrites), the plutonic cumulates (diogenites), and a deeper cumulate layer (olivine diogenites). Recently, a dunite was identified as likely originating from even deeper in the mantle of 4 Vesta.

In addition, numerous meteorites comprising five main groups originated on Mars (shergottites, nakhlites, chassignites, etc.), and a growing number of meteorite finds are of lunar origin comprising lithologies and mixtures thereof from three distinct geochemical regions: the Feldspathic Highlands Terrane, the incompatible element-rich Procellarum KREEP Terrane, and the South Pole-Aitken Terrane (Jolliff et al., 2000).

The winonaites formed in the same region of the nebula, and possibly on the same parent body as silicate inclusions present in the IAB complex irons. There are still many theories proposed to explain the origins of the other achondrite groups, which include the second largest achondrite group, the ureilites, along with the less abundant acapulcoites, lodranites, brachinites, aubrites, and angrites. The angrites are being investigated as potential meteorites from the ancient disaggregated mantle of Mercury. Information on the possibilities of a meteorite from Mercury is explored on the mercurian meteorite page.



CONTINUE TO
[PART I] Chondrites
[PART III] Irons
[PART IV] Stony-Irons
[PART V] Refractory Phases
[PART VI] Trends for Classification
[APPENDECTOMY]


© 1997–2017 by David Weir