A systematic assessment of the diamond trap method for measuring fluid compositions in high-pressure experiments

Abstract

A variety of experimental techniques have been proposed to measure the composition of aqueous fluids in high-pressure experiments. In particular, the “diamond trap method,” where the fluid is sampled in the pore space of diamond powder and analyzed by laser-ablation ICP-MS after the experiment, has become a popular tool. Here, we carried out several tests to assess the reliability of this method. (1) We prepared several capsules loaded with fluid of known composition and analyzed the fluid by laser-ablation ICP-MS, either (a) after drying the diamond trap at ambient condition; (b) after freezing and subsequent freeze-drying; and (c) after freezing and by analyzing a frozen state. Of these methods, the analysis in the frozen state (c) was most accurate, while the results from the other two methods were poorly reproducible, and the averages sometimes deviated from the expected composition by more than a factor of 2. (2) We tested the reliability of the diamond trap method by using it to measure mineral solubilities in some well-studied systems at high pressure and high temperature in piston-cylinder runs. In the systems quartz-H2O, forsterite-enstatite-H2O, and albite-H2O, the results from analyzing the diamond trap in a frozen state by laser-ablation ICP-MS generally agreed well with the expected compositions according to literature data. However, in the systems corundum-H2O and rutile-H2O, the data from the analysis of the diamond trap were poorly reproducible and appeared to indicate much higher solubilities than expected. We attribute this not to some unreliability of the analytical method, but instead to the fact that in these systems, minor temperature gradients along the capsule may induce the dissolution and re-precipitation of material during the run, which causes a contamination of the diamond trap by solid phases. (3) We carried out several tests on the reliability of the diamond trap to measure fluid compositions and trace element partition coefficients in the eclogite-fluid system at 4 GPa and 800 °C using piston-cylinder experiments. The good agreement between “forward” and “reversed” experiments—with trace elements initially either doped in the solid starting material or the fluid—as well as the independence of partition coefficients on bulk concentrations suggests that the data obtained are reliable in most cases. We also show that the rate of quenching/cooling has little effect on the analytical results, that temperature oscillations during the run can be used to enhance grain growth, and that well-equilibrated samples can be obtained in conventional piston-cylinder runs. Overall, our results suggest that the diamond trap method combined with laser-ablation ICP-MS in frozen state yields reliable results accurate within a factor of two in most cases; however, the precipitation of accessory minerals in the diamond trap during the run may severely affect the data in some systems and may lead to a gross overestimation of fluid concentrations.

The composition of subduction zone fluids and the origin of the trace element enrichment in arc magmas

“…Experiments starting with trace elements doped into the solids give results consistent with reversed runs, where the trace elements were doped into the solution (see also Rustioni et al 2019). The compositions of minerals were uniform throughout the entire capsule (see also Figure 7 in Rustioni et al 2021). In accordance with the results from the reversed experiments, this provides strong evidence that chemical equilibrium was reached.…”

“…As demonstrated by Rustioni et al (2021), solubilities in aqueous fluids measured with the diamond trap method yield results accurate within a factor of two. While this has a minor effect in the determination of trace element partition coefficients, which may vary by orders of magnitude, this factor may be significant for major element contents and it may explain the significant scatter in some of the data reported in Figs.…”

“…In each analysis Nist610 and a well-calibrated, natural Afghanite was used as external standards. For further details about the experimental and analytical procedures see Rustioni et al (2019Rustioni et al ( , 2021.…”

The single-crystal diamond trap (SCDT): a new method to determine the composition of high-P–T fluids

“…Minerals can also form within the diamond trap in response to temperature gradients if the temperature of the diamond trap is lower than that of the surrounding material. High field strength elements (HFSE; Nb, Ta, Zr, Hf, Ti) oxides such as rutile and zircon appear to be particularly sensitive to this effect (Tropper and Manning 2005;Bernini et al 2013;Rustioni et al 2021).…”

“…Another potential problem of the classical diamond trap method is elemental fractionation during LA-ICP-MS analysis. Rustioni et al (2021) analyzed a frozen diamond trap that contained small albite grains and H 2 O (and was pressurized, but was never heated) and obtained SiO 2 /Al 2 O 3 ratios that were about twice as high as the theoretical value of albite. In contrast, analyses on a larger piece of the same albite starting material that was analyzed on the same day with the same laser settings returned the correct SiO 2 /Al 2 O 3 ratio.…”