See file, do part 2 only.
Metal-Silicate Differentiation & Magmatic Evolution Due: March 30, 20 21 Vesta Background: As the second largest object in the Asteroid belt (D = 525 km), Vesta is anomalous and special for a number of reasons. First , Vesta has been spectroscopically linked with HED meteorites based on oxygen isotope data. This was the first successful example of linking samples within meteorite collections to a body with the asteroid belt! The Howardites (H) are a mixt ure of lithologies. Eucrites ( E) are plagioclase and pyroxene bearing basalts. Diogenites (D) are pyroxenites with minor plagioclase and olivine. Eucrites are old, forming within the first few million years of our solar system. From earlier activities and homeworks, you know that this antiquity can result in the incorporation of large amounts of 26Al, which can contribute to heating, melting, and planetary differentiation. Did Vesta differentiate? It would seem so, both geophysical and geochemical evidence point to a core and a silicate reservoir that has differentiated further (e.g. crust and mantle). The problem set here is designed to explore the geochemical evidence for core and mantle differentiation. HED meteorites are silicate rocks, that are products of Vesta’s bulk silicate reservoir (i.e. mantle and crust) . Eucrites have been classified as basalts, and they are fine grained plagioclase and pyroxene bearing rocks. Some even exhibit flow structures and vesicles. With this small amount of information, you may be able to already guess the process and role these samples play in differentiation of Vesta’s silicate reservoir. The origin of Diogenites may not be as obvious. They are coarse grained, indicating crystallization at depth within Vesta and are made almost entirely of pyroxene. Wha t role do they play in silicate differentiation? In this activity and homework you will find out. P ART 1: Major element chemistry You have been provided with the major element compositional data for 3 HED meteorites - 2 eucrites (Juvinas & Serra de Magé) and a diogenite (Johnstown).Step 1: Compare the HED data to the composition of CHUR. This is done by normalizing, or dividing, the HED data by the CHUR data. Step 2: Plot each of these normalized datasets vs element (i.e. the x -axis will be the element name and the y -axis is the normalized concentration) . It is easier to see variations between the datasets if the y- axis is logarithmic. This is PLOT 1 , include a copy with your solutions .
1. Classify each element according to the Goldschmidt classification (i.e. lithophile, siderophile, chalcophile, or atmophile). The classification is provided in the Figure 1. 2.Identify which elements are depleted relative to chondrites. Do this for ALL three meteorites . [Hint: Remember that the data has been normalized to chondritic values (i.e.
CHUR), so a chondrite composition would equal 1 for all elements on this plot.] Figure 1: Goldschmidt ’s classification scheme for elements. Elements are classified as Lithophile (rock-loving), Siderophile (iron -loving), Chalcophile ( sulfur-loving) , and Atmo phile ( atmosphere -loving or gaseous). 3.Identify what Goldschmidt groups are depleted relative to chondrites. Do this for all three meteorites.
Step 4: In class we discussed how element compatibility within bulk silicate reservoirs ca n be quantified by a “distribution or partition coefficient” (D ). Recall that D = C s/C l.
C alculate distribution coefficients (D) for the lithophile major elements. Assume that Vesta started with a chondritic composition (or CHUR), and had to melt in order to differenti ate. In this melting scenario, CHUR would represents our solid phase and the meteo rites would represent the melted or liquid phase . Note that this calculation will only make sense applied to the Lithophile elements retained within the bulk silicate Vesta (BSV) .
4. Looking at PLOT 1, silicon is one of the two elements that exhibits an elevated value for ALL three samples relative to chondrites. What is the distribution coefficients for this element? Has this element behaved compatibly or incompatibly?
5. Explain why silicon is found to be in excess of (at higher concentrations than) CHUR.
The excess of this element relative to CHUR provid es an important clue as to the role that Diogenites and Eucrites play in silicate reservoir fractionation. [H int : Examine the differences betw een the Earth’s crust, mantle lithosphere, and primordial mantle for silicon (this data is also given in the spreadsheet) .] 6.Which element or elements exhibit elevated (relative to CHUR) values for just the Eucrite samples? Record their distribution coefficients and indicate if they are more or less compatible than silicon.
7.Examine the differences between the Earth’s crust, mantle lithos phere, and primordial mantle (i.e. mantle just following differentiation) for the element(s) you identified in question 6 . Which reservoir is the Eucrites most similar to chemically?
Based o n wh at we learned in class about the formation of Earth’s crust , how did the Eucrites form on Vesta ?
P ART 2 : Trace element chemistry You’ve been provided with the trace element , or rare earth element (REE) compositional data for the same 3 HED meteorites discussed in Part 1. Step 1: Plot the REE data for the Juvinas Eucrite …Ug, something is wrong, our data seems to reflect nucl eosynthetic processes. This is why we normalize data to CHUR as we did in P art 1.
Step 2: Fix the problem by normalizing to CHUR and replot the data for ALL 3 SAMPLES. Your newly created PLOT 2 should no longer exhibit a “saw tooth pattern”. Include a copy of PLOT 2 with your solutions . It may b e easiest to keep the x-axis in linear scale to answer the following questions, but remember the following figures have x- axes that are in logarithmic scale.
10. In Figure 2, y ou are been give n the distribution coefficients for all REE s within mafic minerals relative to a basalt liquid . Recall that D = C s/C l. Exclu ding the curve for garnet, a re the REEs all compatible or incompatible in mafic mi nerals?
1 1. Look at the R EE for the Eucrites in the PLOT 2 that you produced. Compare with the compositions for Earth ’s crust and depleted mantle in Figure 3. Which reservoir are Eucrites most similar to? Based on what you learned in class AND THE TRACE ELEMENT CHEMISTRY, how did Eu crites form? [I know you answered a similar question above, but for this question you s hould refer to the trace element or REE chemistry.] Figure 2 (left) : D values for REEs wi thin mafic minerals relative to a parent melt (or liquid) of basalt.
Figure 3 (right): REE compositions of Earth ’s oceanic crust, continental crust, and depleted mantle. 12. Looking at your PLOT 2 and the Figure 2, which mineral do you know to be in Serra de Mage and why? You may need to put have your x-axis in lin ear scale depending on your plotting program. [Obviously I have already given you the mineralogy in the in troduction , but I want to know why based on the chemistry] .
1 3. Is the formation of the Johnstown diogenite related to one of the E ucrites? [Hin t: For this question, it may be useful for your x -axis to be in logarithmic scale .] If so, which one and how?
