PHASE-EQUILIBRIUM CONSTRAINTS ON THE MAGMATIC ORIGIN OF LAURITE + Ru Os Ir ALLOY
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Bibliographic record
Abstract
To more completely assess the origin of associated laurite (RuS2) and Ru–Os–Ir (IPGE) alloy which are found as inclusions in near-liquidus phenocrysts such as chromian spinel, we conducted experiments to evaluate the effects of T and f (S2) on phase relations in the system Ru–Os–Ir–Cu–S. Cu–S melt (added as a flux) + IPGE metals were held in silica crucibles, and experiments were done in both vertical-tube gas-mixing furnace apparatus [low f (S2)] and evacuated silica tubes [higher f (S2) buffered by Pt–PtS] at 1200–1250°C for 1–3 days. At constant f (S2) of 10−1 atm, the two-phase field of laurite + alloy is restricted to only the most Ru-rich bulk compositions ( X Ru > 0.85) at 1250°C, and slightly expands to encompass more Ru-poor compositions ( X Ru > 0.6) at 1200°C. At this f (S2), laurite remains very close to pure RuS2. An increase in sulfur fugacity to 10−0.39 atm at 1200°C and 10−0.07 atm at 1250°C resulted in a considerable expansion of the two-phase field, with both laurite and alloy dissolving more Os + Ir. For example, at 1250°C and f (S2) of 10−0.07, the Os and Ir content of laurite increases to ~20 and ~12 at.%, respectively. Coexisting alloys in both sets of high- f (S2) experiments contain less than 15 at.% Ru. The compositions of laurite and IPGE alloy defined by high- f (S2) experiments show remarkable similarity to coexisting PGM compositions preserved in natural chromitite from several localities. If such phases are the product of entrapment at the magmatic stage, then high- f (S2) conditions are inferred. Similarly, the bulk compositions of laurite from suites in which IPGE alloy is absent also suggest similarly high f (S2), if high-temperature entrapment is assumed. Limits on the f (O2) of magmas that may precipitate alloy–laurite pairs stem from the requirement that such magmas remain sulfide-liquid-undersaturated, at least until PGM are trapped in their phenocryst host. Calculations suggest that for this to occur at high f (S2), laurite + IPGE alloy precipitation requires the involvement of relatively oxidized, low-FeO magmas.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.001 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.001 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.018 | 0.001 |
Machine scores (provisional)
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