HED Group - Meteorites from Vesta
This group comprises three closely related classes of achondrites collectively known as the "HED" group, standing for howardites, eucrites, and diogenites. The members of the HED group represent more evolved achondrites that have experienced extensive igneous processing similar to the magmatic rocks found here on Earth. Consequently, they closely resemble terrestrial igneous rocks such as basalts, dolerites, gabbros, and other igneous rocks of volcanic or plutonic origin. We will discuss each of the three subgroups below.
Howardites, eucrites, and diogenites share a common oxygen isotope fractionation line as well as other chemical characteristics, indicating that their formation occurred on a common parent body. Their ancient crystallization ages of 4.43 to 4.55 billion years, determined from various radiogenic isotopes, suggest that the common parent has to be a large, differentiated asteroid. This asteroid became geologically inactive after a relatively short but intense igneous history. Early comparisons based on the reflectance spectra of certain meteorite classes and the main belt asteroids yielded the first match - the eucrites with the asteroid 4 Vesta, one of the largest asteroids in our solar system. Ongoing research has confirmed this match for the howardites and diogenites, and today it is widely accepted that Vesta is indeed the common parent body for the achondrites of the HED group. Together, they form the most abundant class of achondrites in our collections.
With a diameter of approximately 530 km, Vesta is the third-largest asteroid in our solar system. In contrast to the irregular, potato-shaped fragments that characterize most other asteroids, Vesta is a small, nearly spherical, differentiated body that shares many features with the terrestrial planets, e.g. Mercury, Venus, Mars, and Earth. However, Vesta isn't a perfect sphere. In fact, near its south pole we can resolve a 460 km-wide, 30 km-deep crater, the remnant of a huge impact that excavated Vesta's basaltic crust down to its mantle. Due to the immense force of this impact event, large chunks of matter were ejected from Vesta forming smaller asteroids of similar composition - the so-called Vestoids. Some of these "children of Vesta" subsequently entered a near-Earth orbit, and they are thought to be the actual sources of the howardites, eucrites, and diogenites that have made their passage to Earth in the form of meteorites.
The Eucrites are named for a Greek word meaning "easily distinguished". Representing the most common class of achondrites, more than 100 eucrites are known, excluding all probable pairings. Although they are easily distinguished from chondrites, they closely resemble terrestrial basalts. Actually, eucrites are extraterrestrial basalts, volcanic rocks of magmatic origin, representing the crust of their parent body, Vesta. They are primarily composed of the calcium-poor pyroxene, pigeonite, and the calcium-rich plagioclase, anorthite. Additionally, eucrites often contain accessory minerals such as silica, chromite, troilite, and nickel-iron metal. Based on mineralogical and chemical differences, the eucrites have been further divided into three distinct subgroups: the non-cumulate group, the cumulate group, and the polymict group.
Non-cumulate group: The eucrites of this group represent the upper crust of Vesta that solidified on a magma ocean after the core and the mantle of Vesta had already been formed by forces of igneous fractionation, gravitational segregation, and subsequent crystallization. Based on the enrichment of incompatible trace elements and the magnesium-iron ratio plotted against titanium oxide, these crustal rocks are further subdivided into three distinct trends: the eucrites of the main series, the Stannern trend, and the Nuevo Laredo trend.
- Main series eucrites are mostly monomict breccias, such as Juvinas, Béréba, NWA 769, and Aoufous. The latter eucrite is one of our most beautiful finds, resembling a lunar anorthositic breccia. However, there are some rare, unbrecciated members, such as Hammada al Hamra 262 - with its large, oriented pyroxene crystals, it resembles terrestrial basalt more than any other eucrite. Another example of an unbrecciated eucrite is our recent find Agoult, Morocco - a unique meteorite that exhibits a fine-grained, sugary texture. Continued research will show whether Agoult is indeed a member of the main series or if it represents a new trend of recrystallized eucrites, suggesting a different formation history.
- This trend is named for Stannern, a eucrite that fell in the Czech Republic in 1808. Other well-known members are Bouvante, a typical
Stannern trend eucrite that was found in France in 1978, and the most unusual Ibitira, a meteorite that fell in Brazil in 1957. Ibitira is a unique eucrite - it is unbrecciated, displays an unusual vesicular texture, and appears to be one of the oldest eucrites ever found. Its crystallization age dates back to the early history of the formation of Vesta's crust, but it must have been subsequently reheated by a major impact event, resulting in the formation of its unusual vesicular texture.
- This trend is named for Nuevo Laredo, an unusual eucrite that was found in Mexico in 1950. There are only a few eucrites belonging to this trend, and its latest member is one of our finds, Igdi from Morocco. The
Nuevo Laredo trend eucrites are thought to be the product of residual melts in the deeper layers of Vesta's crust, and they are closely related to the eucrites of the cumulate group.
Cumulate group: The rare eucrites of the cumulate group are the products of the gravitational settling of crystallized minerals, primarily pyroxene and plagioclase, within magma chambers trapped below Vesta's early crust. Consequently, they display abundant, oriented crystals enriched in magnesium, and depleted in incompatible elements and a minor basaltic component similar to the Nuevo Laredo trend eucrites. Rather typical members of the cumulate group are Talampaya, a meteorite that fell in Argentina in 1995, Dhofar 007, a more recent find from Oman, and a new find from Western Sahara, recently under publication, that has been recovered by our team in November 2000.
Polymict group: The eucrites of this group are polymict breccias that contain more than 90% eucritic material and less than 10% diogenitic clasts. This 9/1 ratio is an arbitrary dividing line to discriminate between polymict eucrites and the closely related howardites, the latter showing a more even distribution of eucritic and diogenitic clasts. However, polymict eucrites usually contain zoned pyroxenes within the basaltic clasts whereas the pyroxenes in howardites are mostly unzoned. Famous members of the polymict group are the falls of Pasamonte, USA, and Millbillillie, Australia. Other members were found in the hot deserts of Africa, e.g. Dar al Gani 480, Libya, and Smara, Western Sahara, a polymict fragmental breccia that was recovered by our team in April 2000. One of our more recent finds, NWA 049, has also been classified as a polymict eucrite - a unique meteorite that contains olivine and quartz. Continued research should reveal the secrets of its formation history.
The achondrites of this group are named for a Greek philosopher of the fifth century B.C., Diogenes of Apollonia. He was the first to suggest that meteorites actually have their origin in outer space, a fundamental realization that was subsequently forgotten for the next two thousand years. The diogenites are as rare as an ingenious spirit, and this group consists only of about 40 members if we exclude all probable pairings, especially those that have been found in the ice fields of Antarctica.
Mineralogically, the diogenites are composed primarily of magnesium-rich orthopyroxene, with only minor amounts of olivine and plagioclase. The pyroxenes are usually coarse-grained, suggesting a cumulate origin for the diogenites in magma chambers within the deeper regions of Vesta's crust. They are intrusive igneous rocks similar to plutonic rocks found on Earth, and they experienced much lower cooling rates than did the eucrites, which allowed the pyroxene to form sizeable crystals. This is especially true for Tatahouine, a unique diogenite that fell in Tunesia in 1931. This meteorite is renowned for its green, centimeter-sized pyroxene crystals. Other diogenites are monomict breccias, e.g. Bilanga, a recent fall from Burkina Faso, Johnstown, a famous fall from Colorado, USA, and Hamara, a most beautiful breccia from Morocco recovered by our own team in April 2000. More recently, our team has recovered another diogenite from Morocco, currently under publication, that is a polymict fragmental breccia containing less than 10% eucritic clasts. This analysis suggests that we have discovered the first polymict diogenite - a true scientific sensation.
Howardites are named for Edward Howard, a renowned British chemist of the 18th century and one of the pioneers of meteoritics - have a look at our history of meteoritics. They are nearly as rare as diogenites, and there are only about 50 members to this group if we exclude all probable pairings.
Consisting primarily of eucritic and diogentic clasts and fragments, howardites are polymict breccias. However, they also contain dark clasts of carbonaceous chondritic matter, other xenolithic inclusions, and impact melt clasts, indicating a regolith origin for the members of this group. The howardites represent the surface of Vesta, a regolith breccia, consisting of eucritic and diogenitic debris that was excavated by the large impact that created the enormous crater near Vesta's south pole. These fragments have been mixed with parts of the chondritic impactor, and this mixture has been subsequently pulverized and metamorphosed by smaller impacts and the solar wind to form an entirely new type of rock called regolith. Similar regoliths cover the surface of the Moon, and as with the howardites, these regolith breccias display high values for noble gases that have been implanted into the rock by the solar wind. Our own planet never formed any analogous rocks because it is protected by an atmosphere and magnetic field, both protecting us from a continuous meteorite bombardment and the destructive radiation of the solar wind.
Renowned members of the howardite group are the witnessed falls of Bialystok from Poland, Kapoeta from Sudan, and Pavlovka from Russia. Recently, several new howardite finds have been reported from the hot deserts of Africa and Asia, e.g. Dar al Gani 779, one of the most affordable howardites on the collectors market. We recently recovered another new howardite from the deserts of Northwest Africa that will be published as soon as a preliminary scientific analysis has been