Laurite and ruarsite from podiform chromitites at Kraubath and Hochgrössen, Austria: New insights from osmium isotopes

The chemical and osmium-isotope composition of platinum-group minerals (PGM) [e.g., laurite-erlichmanite (RuS ₂-OsS ₂), ruarsite-osarsite (RuAsS-OsAsS) series and Os-Ir alloy (Os,Ir) from variably podiform chromitites of the Kraubath and Hochgrössen dunite-harzburgite massifs are reported for the first time. These massifs, the largest dismembered mantle relics in the Eastern Alps of Austria, were interpreted as a strongly metamorphosed ophiolite sequence, which forms part of the Speik Complex. Unaltered podiform chromitites from both localities display negatively sloped chondrite-normalized platinum-group element (PGE) patterns. The highly altered podiform chromitite at Kraubath is dominated by less refractory PGE (PPGE: Pd, Pt, Rh) over refractory PGE (IPGE: Os, Ir and Ru). The chemical composition of chromite varies from a Cr# [100 ^*Cr/(Cr + Al)] of 74 to 87 and a Mg# [100 ^*Mg/(Mg + Fe ²⁺)] of 44 to 61, values typical of podiform chromitites from the mantle section of an ophiolite. The PGM assemblage in the unaltered podiform ores is dominated by laurite (43% and 75% of all PGM at Kraubath and Hochgrössen, respectively). Sperrylite, PtAs ₂, is the most abundant PGM (61%) in the altered chromitite, whereas minerals of laurite-erlichnmanite series are subordinate (4%). At Kraubath, Os-bearing PGM (laurite, erlichmanite, ruarsite and Os-Ir alloy) occur as (a) single grains and (b) complex polyphase assemblages. At Hochgrössen, laurite and Os-Ir alloy are present as solitary grains only. In situ osmium-isotope measurements of 16 PGM grains from bedrock (e.g., laurite and ruarsite) by laser-ablation multiple-collector inductively coupled plasma - mass spectrometry (LA-MC-ICP-MS) revealed low ¹⁸⁷Os/ ¹⁸⁸Os and γOs _t=o values, indicative of a subchondritic source of the PGE in the mantle. Combined with less radiogenic Os isotopic values measured by negative thermal ionization mass spectrometry (N-TIMS), ¹⁸⁷Os ¹⁸⁸Os ranges from 0.11580 to 0.12437, and γOs _t=o values, from -9.97 to -3.31, respectively. The Os isotopic composition of PGM from altered chromitite at Kraubath is similar or even less radiogenic than that from typical podiform chromitite. The Re-Os system within PGM thus has not been disturbed during later metamorphic events. Furthermore, the wide range of ¹⁸⁷Os/ ¹⁸⁸Os values is similar to that in detrital Os-rich alloys derived from other dunite-harzburgite complexes worldwide (i.e., γOs _t=o ranges from -14.95 to -2.12). On the basis of the osmium isotopic composition of laurite and ruarsite, ultramafic protoliths in the Speik ophiolite complex yield model ages in the range of 1758 to 584 Ma. The mineral-isotopic data are consistent with a prolonged melting history of the parent ultramafic rocks from the mantle section of an ophiolite and provide further evidence for the closed-system behavior of the Re-Os isotopic system within PGM.