Whole Rock Dissolution
For all of the isotopic studies done in this lab, the chemistry relies on separation of individual anions and cations based on elemental behavior. This is done in order to most purely analyze the isotopes of a single element, without any potential isobaric interferences. It is necessary to break down any bonds between elements in a rock, and produce a solution of anions and cations representative of a whole rock that can be used to purify for analysis of a single element.
There is a broad range in the concentration of desired elements in natural rock samples. Due to this, there is not a set value for the mass of rock needed for analysis. Instead, in order to ensure there is enough signal on the machine for good analytical statistics and replicate analyses if needed, we aim to have the following elemental mass for each of these systems for chemistry:
|Element||Goal Mass for Chemistry & Analysis|
If there are already elemental concentration data for the samples you are working with, you can use those values to calculate the amount of sample necessary. If not, use average values of similar rocks as a guide. Make sure to calculate the percentage of dissolved sample needed for each element of interest, these values will be used later when determining aliquot values.
Silicic rocks tend to have many refractory mineral phases that make total dissolution of a whole rock more difficult. It is important to have all of the rock dissolved prior to beginning column chemistry, as some mineral phases may concentrate elements of interest (e.g., zircon contains Nd, Sm & Pb, so a partial dissolution of a zircon bearing rock may not accurately represent the isotopic ratios of that rock). For dissolution of silicate rocks we use a jacketed Teflon dissolution vessel, which utilizes a HNO3-HF solution to dissolve rock powders at 180°C. This is higher than the vaporization temperature of HF, but due to the steel jacket enclosing the Teflon dissolution vessel, this raises the pressure inside the vessel and allows for an elevated ability to dissolve at lower temperatures.
It is important to use an appropriate amount of HF during dissolution. Too little HF will likely either cause a partial dissolution or a very slow dissolution. Too much HF will cause the formation of insoluble fluorides during dissolution and dry down, which have the ability to crystallize with elements of interest. We utilize a spread sheet that calculates the HF needed to break down the silicate bonds in a sample based on SiO2 content and sample mass.
Extreme caution should be used when doing whole rock dissolution. Concentrated reagents are used in this procedure. When opening the Teflon inserts following dissolution, always be under a fume hood. Do not open the Teflon inserts without the insert sitting firmly on the base of the hood. To open the inserts, rotate the cap of the insert while slowly prying the top open, pointing the opening direction away from your body. Not all inserts behave the exact same way, so exercise extreme caution throughout sample collection. DO NOT ATTEMPT THIS PROCEDURE WITHOUT THOROUGH TRAINING.
Once samples have been collected from the Teflon inserts, dissolution is not complete until the samples have been fluxed in concentrated HCl (6M). This helps break down any bonds that may have not been completely broken during initial dissolution, as well as break down any fluorides that may have formed. After samples have been fluxed in HCl, they can be aliquot’d and brought up in loading acids for column chemistry.