Analysis of Partial Melting ^ of CR and R Chondritic Meteorites Experiments Analysis of Partial Melting ^ of CR and R Chondritic Meteorites Courtney King, Kathryn Gardner-Vandy, Dante Lauretta Lunar and Planetary Laboratory, University of Arizona

Objectives The study of meteoritics is important in order to learn about materials and environments of the early Solar System Understand the origin of oxidized (Fe2+) igneous meteorites Brachinites Other primitive achondrites Meteoritics is important in order understand what processes might have occurred in our early Solar System. In particular, I’m focusing on understanding the origin of oxidized Iron in igneous meteorites, when I say igneous I mean molten rock that has cooled. If we are able to understand the origin of oxidized Iron then perhaps we can understand the origin of a particular GROUP of meteorites, the Brachinites.

Background Types of meteorites: CR chondrite group R chondrite group Chondrites (undifferentiated) Primitive Achondrites Achondrites (differentiated) CR chondrite group Aqueous alteration R chondrite group Highly oxidized: Olivines and sulfides Brachinites Sulfides and oxides present Fe in Olivine There are three different types of meteorites: Chondrites, which are undifferentiated meteorites that contain spherical inclusions called chondrules. This is a picture of a chondrule. We also have achondrites which are differentiated, meaning they contain layers, similar to the Earth with a core, mantle and crust. And lastly we have primitive achondrites which are somewhere in between: these have been altered somewhat, but are not fully differentiated. I will be focusing on the CR group which have undergone some aqueous alteration. The graduate student I work with, Kathryn GV is working with R chondritic meteorites, which are highly oxidized. Meaning they contain olivine and sulfides rich in Iron. Both of us are essentially starting off with different materials hoping to produce the same results. We are interested in understanding how Brachinites are formed.

Steps for Analyzing a Meteorite Receive sample from Meteorite Working Group (MWG) Cut sample into pieces and weigh Mount a piece in epoxy and polish until smooth View/analyze the mounted sample using Electron Microprobe Perform a partial melting experiment Repeat mounting process to analyze partially melted sample using probe First let me take you through the process of analyzing a meteorite. We receive a sample, usually weighing only a few grams and cut it into 5-6 smaller pieces. After weighing each, we mount a piece in epoxy and polish the epoxy down until the surface is nice and smooth. In the picture you can see our sanding pads, the darker pads are coarser. We use ethanol as a lubrication so we don’t knick out large chunks of the sample. Next we place the sample into the electron microprobe which is a very powerful microscope that can analyze individual minerals for their specific element wt and atomic percents. (This is key to our data because we want to the abundance of olivine, sulfides, and metals). With another piece we will perform a partial melting experiment and then analyze it with the Microprobe.

Renazzo CR2 Rim of metals/sulfides Chondrule Type 2: Low grade aqueous alteration Rim of metals/sulfides Kamacite < 6% Ni Taenite > 6% Ni Chondrule Kamacite next to Taenite Kamacite next to sulfides  Pyrrhotite, Troilite Stoichiometric Equilibrium Presence of a Sulfur gas mixture: sulfurization of FeNi alloy (FeS) So far I have analyzed a thin section of a CR2 called Renazzo. The type 2 means that this meteorite has undergone some aqueous alteration. This picture is a backscattered electron image, meaning the whiter materials are heavier, usually metals or sulfides and the darker materials are silicates. As you can see here we have a rim of heavier materials. We have a few metal alloys-Kamacite [point] and Taenite. Kamacite is an FeNi metal alloy with less than 6 wt. % Ni, while Taenite has greater than 6 wt. % Ni. Here we have the chondrule from the earlier slide with a few analyzed points labeled. Now when I analyzed Renazzo, I expected to find the metal alloys grouped together and sulfides grouped together. Instead I found Kamacite with Taenite as well as Kamacite with sulfides Pyrrhotite and Troilite. After a bit of reading I found that Kamacite is in stoichiometric equilibrium with Troilite. Under the presence of a gas mixture containing sulfur, the Sulfur atoms will combine with iron, enriching the Ni. Pyrrhotite is has an iron sulfide ratio of a little less than 1, while Troilite’s ratio is 1. In essence the sulfur in the gas will steal the iron atoms away from the metal alloys when undergoing favorable processes such as heating.

Experimental Set-up Temperature Controller Pyrometer Induction Furnace Thermocouple Gas flow  Ar QMS Before I get too much further, let me explain the experimental setup. Now Kat and I have been working on this for most of the year, making sure all the machinery works correctly and there are no leaks in the system. In this picture you can see the temperature controller [point], which we program for a certain temperature and time interval. Also we have the pyrometer, and induction furnace. The pyrometer will read the temperature from the setup and tell the temperature controller. The temp controller will then tell the induction furnace to either amp up, down or stay the same, depending on the set point. Here we have a picture of the setup. I have drawn in a beam demonstrating how the pyrometer reads the temperature. You can also see the induction furnace, with its coils around the tube. The gas flow system allows gas to enter and then exit. We also have a QMS, which helps us double check to make sure the system is closed and no outside air is leaking into the system. Last we have the thermocouple, which is just another way to read temperature, it’s placed down into the tube, positioned just above the sample. Here is a close up of the tube: Inside is an Aluminia crucible, where the sample is placed. Around the crucible is a strip of platinum which glows when its heated to a high enough temperature. Close up: Al2O3 crucible Platinum

Experiments Partially melted R chondrite Temperature calibrations 1200º C, Ar gas Found: veins of troilite (FeS) surrounding unmelted olivine (FeMg)2SiO4 Texture: coagulated Temperature calibrations Halite, Troilite Ag, Cu So far we have done a couple partial melting experiments with an R chondritic sample. The parameters for the experiment were 1200 degrees C, for 5 days in an Ar rich environment. (We also did one at 1100 degrees C?). The temperature was high enough to melt the sulfides but not the silicates in the rock. We chose to make the run 5 days to ensure the sample was thoroughly heated. After analyzing the partially melting sample, we saw the texture of troilite change from inclusions to coagulated veins. The mineral formed in veins surrounding the iron rich olivine. Currently our next step is to calibrate the the temperature controller using minerals and elements of which we know their specific melting points. This will help us accurately measure the temperature for future experiments. A few minerals we’ve chosen include halite, troilite and then metals, silver and copper. Similar to what we see in primitive achondrites Perform better t calibrs Photo courtesy: Kathryn Gardner-Vandy

Implications of the Data Question: What happens when a meteorite containing reduced iron (Fe0) and aqueous alteration is heated? Possibility of producing oxidized iron (Fe2+); Olivine Metal/sulfide textures respond rapidly to melting What to look at: Kamacite < 6% Ni Taenite > 6% Ni Kamacite  Troilite (FeS) Sulfur gas, H2S The big question I’m asking of myself is “what happens if we heat a meteorite containing reduced iron and aqueous alteration?”. We see a fair amount of oxidized iron in Brachinites in the form of olivine so we want to know how that iron became oxidized. We believe that taking the starting materials of water and reduced iron and then heating it will give us the answer we’re looking for with a few steps in-between. As I’ve shown you, metal and sulfides respond rapidly to melting therefore by taking a meteorite with aqueous alteration, we want to look how the metal alloys will react with the water and surround elements as they are heated. We believe that this will produce the oxidized iron we are looking for. And perhaps in future experiments we will introduce sulfur gas or gas mixture and analyze the effects.

Dante Lauretta Kathryn Gardner-Vandy Katrina Jackson Thank you! Dante Lauretta Kathryn Gardner-Vandy Katrina Jackson