Meteorites as Forensic Evidence for the Early Solar System

We just saw how the fractionation of stable isotopes could be used as a forensic tool in planetary sciences. In this lecture we explore a few other sources of chemical data, with specific examples:

HED meteorites, differentiation, and trace element analysis:

We routinely assert that achondrites and iron meteorites came from bodies that started out chondritic but later differentiated, but what evidence do we actually have to support this? Is it possible that achondrites just have nothing to do with chondrites?


The Juvinas meteor
Consider the Juvinas meteorite (from France - right), an achondritic stony meteorite. Juvinas is a representative of a geochemical "family" of meteorites known as HED meteorites.

But what evidence do we have that its parent body was ever chondritic?


Trace element analysis: A comparison of the concentrations of REEs and other trace elements. Note: In Earth's highly evolved basalts, the end-products of a history of constant and repeated differentiation, fractional melting, and fractional crystallization, the pattern of REE concentrations is distinctly different from, that of CI chondrites. We would expect the rocks of any asteroid, even one that had undergone differentiation early in its history, to be less strongly differentiated than Earth basalts. Thus, if they started with chondritic parent material, their enrichment ought to be intermediate between chondrites and Earth basalts.

Reasons to reject:

In fact, the Juvinas profile is quite similar to that of a CI chondrite, except that all REEs that can be sampled are enriched by roughly the same factor. Strong evidence for the idea that the Juvinas magma had recently differentiated from chondritic material, with lithophile REEs segregating into molten silicates.


Vesta from Astro Bob
HED origins: The composition of HED meteorites broadly matches spectroscopic data for the large asteroid Vesta (right). Data from the Dawn spacecraft has confirmed that it is a differentiated body and scarred by impacts large enough to have lofted lots of material into orbit around the sun. Vesta is large enough plausibly to have differentiated early in the history of the Solar System when short-lived radioisotopes like 26Al were active.


Shergottite Alan Hills 84001 from Wikipedia

SNC meteorites, noble gasses, and the history of Mars:

If a meteorite came from a more strongly differentiated object than Vesta, would we know it? - Maybe. One interesting case involves the SNC meteorites.

SNC stands for the major petrologic types that make up this group:


Thus, the SNC meteorites are achondritic and, like HED meteorites, come from a differentiated body. Beyond that, isotopic, radiometric, and trace element analyses made it clear that these objects were not from a known asteroid belt source. But where were they from? An apparent answer was supplied by Viking lander data obtained in 1976.

Noble gas concentrations: The trace element and stable isotope analysis of the non-reactive noble gasses (He, Ne, Ar, Kr, Xe) is a growing area of planetary sciences. For poorly understood reasons, these gas' isotopes fractionate differently in chondrites, planetary atmospheres, and the Sun. A comparison of their concentrations in Mars' atmosphere, as measured by Viking, and in SNC meteorites revealed a nearly perfect match. This made Mars the hands-down favorite potential source.

Confirmation came from other lines of evidence:

The upshot are that these rocks formed on or in Mars, and were old when they were lofted into solar orbit by an impact. Accepting that they are Martian, what do they tell us about Mars:

Everything is consistent with other information we have covered, that Mars, early in its history, had a denser atmosphere and liquid water, but quickly became cold and dry.

Of course, ALH84001 is the celebrated meteorite once thought to contain fossil microbes. That is starting to look like a real stretch.

Summary:

We have seen how the following types of geochemical analyses of meteorites inform our view of the Solar System:



Key concepts and vocabulary:
Additional reading: