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.

A summary of the diagnostic mineralogic and chemical characteristics of differentiated achondrites is given in the table below.

AubriteHEDAngrite
EucriteDiogeniteHowardite
Texturecoarse-brecciatedbasalticcoarsebrecciated medium to coarse
Olivine/Pyroxene<<1<<1<<1<<11
OlivineFa0absentFa27-35Fa8-89 Fa11-66 (La0.1-48)
Olivine FeO/MnO--44-60-75-100
Low-Ca pyroxeneFs0.1-1.2traceFs23-33variable absent
Low-Ca pyroxene FeO/MnO-25-3827-35--
Ca-pyroxeneFs0-0.2Wo40-46variabletracevariable Fs12-50Wo50-55
Plagioclasetrace: An2-23An60-99An60-99 An60-98An86-99.7
Chromite, Cr/(Cr+Al)-62-8868-89--
Silicaabsenttracetracetraceabsent
Kamaciteminortrace-minortracetracetrace
Taenitetraceabsentabsenttraceabsent
Troilitetracetracetracetracetrace
Other mineralscubic sulfidestracetracetrace spinel, phosphates, oxides

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