Differentiated Achondrites

A final philosophy of Earth [and planetary] history must be largely founded upon the unshakable facts known about igneous rocks. - R. A. Daly

Introduction

We have all seen erupting volcanoes on TV or in movies and some of us have been fortunate enough to view them first hand. Their fiery display is spectacular and memorable. Volcanic activity is tied to a parent body's growth and evolution. Basaltic eruptions emanate from deep within a parent body, where localized melting of pre-existing rocks is ongoing. Although temperatures in these localized hot spots are very high (~1100 to 1400° C), they are seldom sufficient to completely melt the surrounding rocks, and molten rock, called magma, forms by the partial melting of preexisting rocks. The unmelted portion is called residue and is left behind as the liquid magma ascends.

The magma may reach the surface with is composition unchanged or accumulate in deep chambers where cumulates may form, changing the composition of the remaining magma. Cumulates are built by the physical separation of newly formed crystals. The crystals are denser than the magma and settle to the floor of a magma chamber, analogous to the crystallization of snow flakes that fall through less dense air to the ground. Although this is one of several igneous mechanisms that produce global or regional differentiation, geologists like to have terms for everything and this cumulate process is called crystal fractionation.

The newly modified magma ascends further, stopping just below the surface to form shallow intrusions or erupting as extrusive lava flows (crystal fractionation can also occur in the shallow intrusive sills, further complicating matters). The main process that changes magma composition is called magmatic differentiation. As an analogy, envision a bowl of chocolate chip ice cream left out on a warm day. The milk portion, which melts at a lower temperature than that of the chips, melts and rises to the top, leaving a heavier, unmelted mass or residue of chocolate chips.

Volcanoes have been at work on all terrestrial planets (the rocky planets: Earth, Mercury, Venus, and Mars), the Moon, and some satellites of the outer gaseous planets (for example, Jupiter's moon Io). Similarly, the parent bodies of achondrite meteorites also underwent magmatic differentiation, producing basalt flows and plutonic (deep) cumulate rocks. From partially melted asteroids, we have received meteorite specimens that exemplify their geologic history to some degree.

The following sections present specimen illustrations and descriptions of differentiated achondrites, which includes the HED clan (howardites, eucrites, and diogenites), angrites, aubrites, and ureilites. A summary of the diagnostic mineralogic and chemical characteristics of differentiated achondrites is given in the table below.

Notes. minor, <5 vol. %; trace, <0.5 vol. %

The HED Clan

Howardites

Howardites are the H class in the HED achondrite clan (howard-eucrite-diogenite). Howardites are polymict breccias composed of angular clasts of eucrites and diogenites welded together by pulverized mineral dust. They do not have unique mineral characteristics like other meteorite classes.

Surface of the NWA 776 howardite. Consists of eucrite fragments or clasts (example of white plagioclase and dark pyroxene eucrite shown in upper left), diogenite pyroxene grains (green), and clasts (dark). Height is 2 cm. Image © 2005 T. E. Bunch.

Howardite ALH 76005. Image © 2005 T. E. Bunch

Although larger asteroids are accepted as the parents of meteorites, in all but one case, it is impossible to link a specific meteorite with a known asteroid. The exception is the HED suite of meteorites which most scientists believe originated from the asteroid Vesta. Spectral images from the Hubble Space Telescope show that Vesta (525 km in diameter, the third largest main-belt asteroid) has a surface composition identical to eucrite meteorites while some deeper areas exposed by impacts are very similar to diogenites. Thin ejecta blankets on the asteroid are mixtures of both and are equivalent to howardites. Deeper rocks exposed on crater floors of Vesta are olivine-rich, although asteroidal meteorites of this composition have not been found to date. In 1996, the Hubble Space Telescope detected a huge crater on Vesta, 430 kilometers across and perhaps a billion years old, that may be the source of the HED suite of meteorites. Recently scientists have discovered a series of 5-10 km size asteroids with the same spectral features as Vesta, which probably represent ejecta from Vesta.

Vesta. These two surface maps of the asteroid Vesta are derived from Hubble telescope images taken between November 28 and December 1, 1994 and show surface details as small as 35 miles across. The top panel indicates sharp contrasts in Vesta's surface color. The surface markings may represent ancient volcanic activity such as lava flows and, in addition, regions where major collisions have stripped away the surface. The bottom panel reveals that the Vestan surface is made up of igneous rock, indicating that either the entire surface was once melted or lava flowing from its interior once completely covered its surface. Source: http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/40/.

Vesta is a mini-planet where in its early history partial melting gave rise to layered plutonic rocks like diogenites after which melting in the near surface produced basaltic flows or eucrites. Impact mixing of these rocks made surface breccias or howardites. The age of the lava flows from which eucrites derived is 4.5 billion years old and less than 125 million years after the formation of the solar system. So, in its early history, Vesta's geological activity was intense, but very quickly became geologically inactive except for the effects of impacts that still occur today. Because of their much larger size, the geological evolution of true planets (Earth, Mars, etc.) has continued to the present. Dozens of Vesta-like asteroids probably existed at one time, but since then have been broken apart into smaller asteroids.

Spectral reflectance data for Vesta (above) allow construction of a geological map of the surface. Compare the bottom part of the figure with the Hubble map of Vesta (above). The upper part of this image shows the interpreted cross section of the asteroid. Credit: Harry Y. McSween, Jr., Department of Earth and Planetary Sciences, University of Tennessee. Source: http://geoweb.gg.utk.edu/Geology2003/Faculty/mcsween/displayimages/511.htm.

Eucrites

Eucrites are the E class of the HED achondrite clan (howardite-eucrite-diogenite). The HED clan of meteorites represents volcanic rocks (surface flows) and plutonic rocks (deeper crystallization) formed from basaltic magmas. Eucrites, from the Greek word eukritos, meaning "easily distinguished," are probably more similar to terrestrial rocks than any other asteroidal meteorite class. Like terrestrial basalts, eucrites formed by partial melting of parent rocks and are typically extrusive (surface flows). They are also referred to as "basaltic achondrites" because of their similarity to terrestrial basalts in texture and gross mineralogy, but differ in bulk composition and other characteristics. For example, they contain reduced, metallic nickel-iron (Ni-Fe) instead of oxidized iron minerals, e.g., magnetite (Fe₃O₄).

Fractured surface of the North African DaG 647 eucrite. Buff and dark grains are pyroxenes, cream is plagioclase. Height of the photograph is about 6 cm. Image © 2005 T. E. Bunch

Eucrites are composed of orthopyroxene, pigeonite, Ca-rich pyroxenes and plagioclase with minor amounts of chromite, ilmenite, troilite and Ni-Fe metal. A few eucrites, e.g., Moore County are similar to terrestrial gabbros (plutonic rocks) in having cumulate textures (preferred orientation of major minerals) that indicate crystal layering by settling-out in a magma chamber, analogous to the accumulation of fallen tree leaves into layered piles.

Textural similarity of the basaltic eucrite Stannern (left; pyroxenes white to gray; pyroxene, brown to buff) compared with a terrestrial basalt (right; same with highly colored olivine grains). Image © 2005 T. E. Bunch.

Example of a subophitic eucrite basalt with augite partially enclosed by plagioclase (white in both images; PPL, left; XPL, right). Height is 3 mm. Image © 2005 T. E. Bunch

Example of cumulate texture in the eucrite Moore Co.; plagioclase, white to gray. PPL on left, XPL on right. Height is 3 mm. Image © 2005 T. E. Bunch

Diogenites

Diogenites are the D class of the HED achondrite clan (howardite-eucrite-diogenite). They are composed essentially of orthopyroxene and hypersthene, with minor amounts of plagioclase, olivine, troilite, and chromite. Like eucrites, diogenites contain little NiFe metal. Although most diogenites have been crushed to various degrees, some remaining hypersthene crystal fragments are as large as 5 cm (~2 inches). Even then, component fragments show rotation and small pockets of pulverized material.

Sawn surface of the Johnstown diogenite that shows fragmented orthopyroxene clasts and crushed matrix. Base of specimen is 15.4 cm (6 in). Source: http://www.meteoritetimes.com/Back_Links/2002/april/Meteorite_of_Month.htm.

Unlike eucrites that formed from volcanic surface flows, diogenites probably formed by crystal separation from the parent magma. Eucrites and diogenites came from the same parent body, eucrites from the surface or near surface and diogenites from deeper within. Recently, several diogenites have been recovered that contain up to 30 vol. % of olivine and are classified as olivine diogenites. These meteorites may be samples of parent body rocks that are deeper than pyroxene diogenites. Little by little, we are receiving rock samples of Vesta that are from deeper and deeper depths. Eventually, we may receive Vesta meteorites that are mostly olivine and would represent the earliest differentiates.

Aubrites

Aubrites are enstatite achondrites that consist principally of nearly Fe-free enstatite with minor amounts of NiFe metal, troilite, plagioclase, olivine or diopside. Aubrites are named after the type example, Abres, that fell in France in 1836. They are igneous in origin and crystallized in an extremely reducing (oxygen-poor) magma that promoted the formation of small amounts of rare to unique minerals, for example, oldhamite (CaS), which is not found in terrestrial rocks.

Shallowater aubrite. Large crystals are enstatite with small olivine and augite grains. Height, 1.8 mm. Imagw © 2005 T. E. Bunch.

Nearly all of the aubrites are breccias, similar to ALH84007 above. Breccia clasts are remnants of huge enstatite crystals that may have been as large as 10-15 cm and are set in a matrix of crushed enstatite and the minor minerals. The Shallowater aubrite is unbrecciated which is unique among classic aubrites. There is a weak to moderate preferred orientation of enstatite crystals that suggests a cumulate origin or crystal settling in a magma chamber. This implies that the parent body was of sufficient size (>100 km in diameter) to have at least a weak gravitational field.

Angrites

Angrites, named after the type example, Angra dos Reis, that fell in 1869 in Brazil, are probably the most beautiful of all meteorites with the exception of the Martian meteorites, Shergotty, Nakhla, and Lafayette. The reason being, that angrites are unique in having large amounts of Ti-rich augite (fassaite). Fassaite is uncommon in terrestrial and areroidal rocks, but more common in lunar rocks. This pyroxene is reddish to purple, millimeters in size and ranges from 90 vol.% in Angra dos Reis (above) to 30 vol. % in SAH 99555. In addition to fassaite, angrites contain various amounts of olivine, a Ca-rich olivine (kirschsteinite), plagioclase, troilite, Ti-magnetite, NiFe metal, and apatite. One angrite, SAH99555, contains vesicles (gas bubbles) that are the largest observed in an asteroidal meteorite. Perhaps not asteroidal... recently, it has been proposed that angrites represent samples of Mercury.

Angrite d'Orbigny. (left, plane light) Crystals of purple-brown fassaite and white plagioclase. (right, XPL). Height, 3 mm. Image © 2005 T. E. Bunch

Angrites are one of the rarest of the meteorite classes with only 8 specimens known. They are basaltic in origin and are the oldest of all basaltic achondrites (4.57 billion years). Their textures, like that shown for the angrite D' Orbigny, the presence of vesicles in some examples, and elemental distributions suggest that the angrites crystallized rapidly, either within a meter of the surface or as surface flows.