Planetary Science Research Discoveries

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Citation: Doyle, P. M. (June, 2015) Chondritic Asteroids–When Did Aqueous Alteration Happen? Planetary Science Research Discoveries. http://www.psrd.hawaii.edu/June15/Mn-Cr-fayalites.html (date accessed).

pdf version   PSRD-Mn-Cr-bonus-material.pdf

Bonus Material

Added December 23, 2015 for


Chondritic Asteroids–When Did Aqueous Alteration Happen? [ article ]


Making Synthetic Standards


Written by Patricia M. Doyle
University of Cape Town, South Africa, and previously at Hawai‘i Institute of Geophysics and Planetology, University of Hawai‘i

One advantage of making synthetic standards is that their composition can be matched to that of the unknown by mixing element oxides in specific (stoichiometric) proportions. In addition, the standards can be made under selected conditions. For example, our Hawai‘i team prepared synthetic olivines, doped with Mn, Cr and Ni, in a 1 atm vertical tube gas-mixing furnace (see photo below) in which mixtures of H2 (an explosive gas) and C)2 (~0.04% of Earth's atmosphere) controlled the amount of oxygen in the sample chamber. Iron, an important component in fayalite (Fe2SiO4), may occur as iron metal (Fe0), ferrous iron (Fe2+) or ferric iron (Fe3+) depending on the amount of oxygen (O) in the system. Buffer curves, defined by mineral equilibria, are a useful means of communicating the oxygen content (or more accurately, the oxygen fugacity) of the system as a function of temperature. For example, the iron-wüstite (IW) buffer defines conditions under which iron metal (Fe0) and wüstite (Fe2+O) coexist. The synthetic olivines made in Hawai‘i were prepared under an atmosphere slightly more oxygen-rich than the IW buffer curve, so Fe was present as Fe2+ and could easily enter the olivine structure.

Photo of the lab furnace and equipment.
(a) Vials hold oxides ready to be weighed and mixed using a pestle and mortar. (b) The main components of a 1 atm vertical tube gas-mixing furnace.


The furnace can reach red-hot temperatures of 1700 oC. Considering the range of temperatures available, our Hawai‘i team, who live on an island predominantly made of basalt (which melts ~1200 oC) used a phase diagram to guide our selection of the inserted oxide-mixture and experimental temperature (see diagram below). Those who live in icy-cold climates will probably be familiar with the importance of the ice-salt phase diagram for winter driving as water freezes at ~0 oC (32 oF), but water with dissolved salt remains liquid to much lower temperatures. Rock systems are much the same: the melting / freezing point will differ based on the bulk composition, and there are envelopes (immiscibility fields) where a solid will coexist with a liquid of a different composition. In phase diagrams, the solidus is the line defining the temperature below which everything in the system is solid, and the liquidus is the line above which (at high temperature) everything is liquid.

Olivine phase diagram with data points and images of products synthesized in the lab.
Phase diagram for the forsterite (Mg2SiO4) and fayalite (Fe2SiO4) solid solution showing the temperature-composition fields where liquid and solid components are stable. The solidus and liquidus lines were defined by Bowen and Schairer in 1935 using samples prepared under a nitrogen atmosphere. Also shown (in green dashed line) is the iron-wüstite (IW)+1 solidus, as calculated from the thermodynamic calculations of Uzi Nitsan in 1974. Three backscatter electron images are shown for representative samples that were quenched from completely solid, completely molten, and melt-crystal mixtures. Also shown is an image of one of the beads with liquidus phase olivine and quenched material.


Although the olivine phase diagram may appear simple, back in 1974, the results of Uzi Nitsan's thermodynamic calculations indicated that the composition of olivine (that is, the position of the solidus) depends on the amount of oxygen in the system, and our experimental results are in agreement with that prediction. Indeed, the line in green on the diagram defines the predicted olivine compositions for the oxygen abundance used in most of the experiments. When samples were prepared under conditions that were slightly more oxygen-rich or oxygen-poor, the olivine compositions were roughly as predicted by Uzi Nitsan's calculations. So, after more than 30 years, Uzi Nitsan's calculations are validated! (But we digress, let us get back to our olivine standards and their impact on dating minerals.)


We prepared synthetic olivine using three different approaches. Backscattered electron images of representative samples are shown in the diagram. Below the solidus, solid-state reactions turn the starting oxide mixture into olivine. The composition was just as we planned, but the grains were small and compositionally heterogeneous, with a few small iron-rich inclusions. At much higher temperatures, the oxide mixture melts completely. When cooled quickly, small needle-like olivine crystals formed. The brightly colored portions between the crystals are pockets of iron-rich melt (mesostasis) that was the last to crystalize. These grains were also too small to be useful as standards.


The leaf-shaped envelope in the phase diagram is a little like a Goldilocks zone: not hot enough for the run charges to be completely molten, and not cold enough for everything to be solid. Instead, olivine crystalizes out of the melt, so the quenched product is a mixture of crystals and melt. The olivines are as compositionally homogeneous as the widely used San Carlos forsterite standard, and are sufficiently large for SIMS analysis using 2–10 µm diameter primary beams. That makes them ideally suited for repeated use as a standard in isotope measurements.


References:

For full context of this bonus material and full reference list, return to the article: Chondritic Asteroids–When Did Aqueous Alteration Happen?

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