Geochemistry and Resonance Ionization of Platinum-Group Elements

Geology, platinum, resonance ionization, meteorites

Blum, Joel David ; Wasserburg, Gerald J.

University:California Institute of Technology

Department:Geological and Planetary Sciences

关键词: Geology, platinum, resonance ionization, meteorites;   

Others  :  https://thesis.library.caltech.edu/8653/1/Blum_jd_1990.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

Experimental studies were conducted with the goals of 1) determining the origin of Pt-group element (PGE) alloys and associated mineral assemblages in refractory inclusions frommeteorites and 2) developing a new ultrasensitive method for the in situ chemical and isotopicanalysis of PGE. A general review of the geochemistry and cosmochemistry of the PGE isgiven, and specific research contributions are presented within the context of this broadframework.

An important step toward understanding the cosmochemistry of the PGE is thedetermination of the origin of POE-rich metallic phases (most commonly εRu-Fe) that arefound in Ca, AJ-rich refractory inclusions (CAI) in C3V meteorites. These metals occuralong with γNi-Fe metals, Ni-Fe sulfides and Fe oxides in multiphase opaque assemblages.Laboratory experiments were used to show that the mineral assemblages and textures observedin opaque assemblages could be produced by sulfidation and oxidation of once homogeneousNi-Fe-PGE metals. Phase equilibria, partitioning and diffusion kinetics were studied in theNi-Fe-Ru system in order to quantify the conditions of opaque assemblage formation. Phaseboundaries and tie lines in the Ni-Fe-Ru system were determined at 1273, 1073 and 873Kusing an experimental technique that allowed the investigation of a large portion of the Ni-Fe-Rusystem with a single experiment at each temperature by establishing a concentrationgradient within which local equilibrium between coexisting phases was maintained. A widemiscibility gap was found to be present at each temperature, separating a hexagonalclose-packed εRu-Fe phase from a face-centered cubic γNi-Fe phase. Phase equilibriadetermined here for the Ni-Fe-Ru system, and phase equilibria from the literature for theNi-Fe-S and Ni-Fe-O systems, were compared with analyses of minerals from opaqueassemblages to estimate the temperature and chemical conditions of opaque assemblageformation. It was determined that opaque assemblages equilibrated at a temperature of~770K, a sulfur fugacity 10 times higher than an equilibrium solar gas, and an oxygenfugacity 10⁶ times higher than an equilibrium solar gas.

Diffusion rates between -γNi-Fe and εRu-Fe metal play a critical role in determining thetime (with respect to CAI petrogenesis) and duration of the opaque assemblage equilibrationprocess. The diffusion coefficient for Ru in Ni (D^Ru_Ni) was determined as an analog for the Ni-Fe-Ru system by the thin-film diffusion method in the temperature range of 1073 to 1673Kand is given by the expression:

D^Ru_Ni (cm² sec^-1) = 5.0(±0.7) x 10^-3 exp(-2.3(±0.1) x 10¹² erg mole^-1/RT)where R is the gas constant and T is the temperature in K. Based on the rates of dissolutionand exsolution of metallic phases in the Ni-Fe-Ru system it is suggested that opaqueassemblages equilibrated after the melting and crystallization of host CAI during ametamorphic event of ≥ 10³ years duration. It is inferred that opaque assemblages originatedas immiscible metallic liquid droplets in the CAI silicate liquid. The bulk compositions ofPGE in these precursor alloys reflects an early stage of condensation from the solar nebulaand the partitioning of V between the precursor alloys and CAI silicate liquid reflects thereducing nebular conditions under which CAI were melted. The individual mineral phasesnow observed in opaque assemblages do not preserve an independent history prior to CAImelting and crystallization, but instead provide important information on the post-accretionaryhistory of C3V meteorites and allow the quantification of the temperature, sulfur fugacity andoxygen fugacity of cooling planetary environments. This contrasts with previous models thatcalled upon the formation of opaque assemblages by aggregation of phases that formedindependently under highly variable conditions in the solar nebula prior to the crystallizationof CAI.

Analytical studies were carried out on PGE-rich phases from meteorites and the productsof synthetic experiments using traditional electron microprobe x-ray analytical techniques.The concentrations of PGE in common minerals from meteorites and terrestrial rocks are farbelow the ~100 ppm detection limit of the electron microprobe. This has limited the scopeof analytical studies to the very few cases where PGE are unusually enriched. To study thedistribution of PGE in common minerals will require an in situ analytical technique with muchlower detection limits than any methods currently in use. To overcome this limitation,resonance ionization of sputtered atoms was investigated for use as an ultrasensitive in situanalytical technique for the analysis of PGE. The mass spectrometric analysis of Os and Rewas investigated using a pulsed primary Ar⁺ ion beam to provide sputtered atoms forresonance ionization mass spectrometry. An ionization scheme for Os that utilizes threeresonant energy levels (including an autoionizing energy level) was investigated and found tohave superior sensitivity and selectivity compared to nonresonant and one and two energylevel resonant ionization schemes. An elemental selectivity for Os over Re of ≥ 10³ wasdemonstrated. It was found that detuning the ionizing laser from the autoionizing energy levelto an arbitrary region in the ionization continuum resulted in a five-fold decrease in signalintensity and a ten-fold decrease in elemental selectivity. Osmium concentrations in syntheticmetals and iron meteorites were measured to demonstrate the analytical capabilities of thetechnique. A linear correlation between Os⁺ signal intensity and the known Os concentrationwas observed over a range of nearly 10⁴ in Os concentration with an accuracy of ~ ±10%, amillimum detection limit of 7 parts per billion atomic, and a useful yield of 1%. Resonanceionization of sputtered atoms samples the dominant neutral-fraction of sputtered atoms andutilizes multiphoton resonance ionization to achieve high sensitivity and to eliminate atomicand molecular interferences. Matrix effects should be small compared to secondary ion massspectrometry because ionization occurs in the gas phase and is largely independent of thephysical properties of the matrix material. Resonance ionization of sputtered atoms can beapplied to in situ chemical analysis of most high ionization potential elements (including all ofthe PGE) in a wide range of natural and synthetic materials. The high useful yield andelemental selectivity of this method should eventually allow the in situ measurement of Osisotope ratios in some natural samples and in sample extracts enriched in PGE by fire assayfusion.

Phase equilibria and diffusion experiments have provided the basis for a reinterpretation ofthe origin of opaque assemblages in CAI and have yielded quantitative information onconditions in the primitive solar nebula and cooling planetary environments. Development ofthe method of resonance ionization of sputtered atoms for the analysis of Os has shown thatthis technique has wide applications in geochemistry and will for the first time allow in situstudies of the distribution of PGE at the low concentration levels at which they occur incommon minerals.