Taking the Temperature of an Early Solar System Meteorite

Using textural observations and the chemistry of a chondritic meteorite, NWA-6104, that formed early in our solar system’s history, the temperatures that the meteorite experienced after formation were determined. Chondritic meteorites were created at the dawn of our solar system and, after ~4.5 billion years, still retain much of their original chemistry. Many planetary bodies that orbit our sun like Earth and Mars have undergone significant melting and differentiation (separation of core and mantle). Unlike these bodies, chondrites have not undergone significant melting or differentiation; the temperatures and pressures that chondritic meteorites have experienced simply haven’t been high enough to melt them. By studying their chemistry, conditions of the solar nebula within which they formed can be hypothesized. After their formation, chondrites were subject to differing degrees of metamorphism (solid-state alteration) by heat and fluids. Through evaluating the degree to which they were altered, conditions of planetesimals that did not undergo significant melting or differentiation can be constrained.

Artist's depiction of a solar nebula and subsequent evolution of our solar system (image taken from panoramicuniverse.com)

Chondrites contain distinct, millimeter-sized spheres called chondrules. These objects originated as free-floating molten droplets that were flash heated and later crystallized. Chondrules give chondrites the appearance of chocolate chip cookies, with chondrules representing the chocolate chips and a matrix representing the dough. For a research project, I used a petrographic microscope to characterize the textures of the chondrules within a meteorite that formed early in our solar system’s history to help determine the degree of thermal and aqueous alteration. I also used an electron microprobe, a machine that uses beams of charged particles to measure the chemistry of samples, to analyze its chemistry and classify it, as it had not been previously studied.

Photomicrograph of an olivine chondrule. Notice the round shape of a remnant chondrule in the center surrounded by matrix.

Textural observations and the chemistry of NWA-6104 classified it as an L5/6 chondrite. L stands for low iron bearing and 5/6 characterizes the degree of alteration, with 3 being the lowest possible degree. Meteorites with values above or below 3 experience either thermal or aqueous alteration, respectively. The degree of thermal alteration (5/6) was determined petrographically by certain textural criterion. Because the highest possible degree of thermal metamorphism is a rating of 6, this meteorite experienced significant thermal alteration. Chemical analyses and the use of a pyroxene-pyroxene geothermometer (a method of comparing the chemistry of two very closely related minerals, orthopyroxene and clinopyroxene) constrained the peak metamorphic temperatures that the parent body of NWA-6104 experienced to 800-900 ±50 °C. This range of temperatures is consistent with the degree of metamorphism that would be expected for a 5/6 ordinary chondrite.

To see the original paper from which this post was whittled, please go here.