33 research outputs found
Thermodynamic controls on element partitioning between titanomagnetite and andesiticâdacitic silicate melts
Titanomagnetiteâmelt partitioning of Mg, Mn, Al, Ti, Sc, V, Co, Ni, Cu, Zn, Ga, Zr, Nb, Mo, Hf and Ta was investigated experimentally as a function of oxygen fugacity (fO2) and temperature (T) in an andesiticâdacitic bulk-chemical compositional range. In these bulk systems, at constant T, there are strong increases in the titanomagnetiteâmelt partitioning of the divalent cations (Mg2+, Mn2+, Co2+, Ni2+, Zn2+) and Cu2+/Cu+ with increasing fO2 between 0.2 and 3.7 log units above the fayaliteâmagnetiteâquartz buffer. This is attributed to a coupling between magnetite crystallisation and melt composition. Although melt structure has been invoked to explain the patterns of mineralâmelt partitioning of divalent cations, a more rigorous justification of magnetiteâmelt partitioning can be derived from thermodynamic principles, which accounts for much of the supposed influence ascribed to melt structure. The presence of magnetite-rich spinel in equilibrium with melt over a range of fO2 implies a reciprocal relationship between a(Fe2+O) and a(Fe3+O1.5) in the melt. We show that this relationship accounts for the observed dependence of titanomagnetiteâmelt partitioning of divalent cations with fO2 in magnetite-rich spinel. As a result of this, titanomagnetiteâmelt partitioning of divalent cations is indirectly sensitive to changes in fO2 in silicic, but less so in mafic bulk systems.Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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Extreme enrichment of Se, Te, PGE and Au in Cu sulfide microdroplets: evidence from LA-ICP-MS analysis of sulfides in the Skaergaard Intrusion, east Greenland
The Platinova Reef, in the Skaergaard Intrusion, east Greenland, is an example of a magmatic CuâPGEâAu sulfide deposit formed in the latter stages of magmatic differentiation. As is characteristic with such deposits, it contains a low volume of sulfide, displays peak metal offsets and is Cu rich but Ni poor. However, even for such deposits, the Platinova Reef contains extremely low volumes of sulfide and the highest Pd and Au tenor sulfides of any magmatic ore deposit. Here, we present the first LA-ICP-MS analyses of sulfide microdroplets from the Platinova Reef, which show that they have the highest Se concentrations (up to 1200 ppm) and lowest S/Se ratios (190â700) of any known magmatic sulfide deposit and have significant Te enrichment. In addition, where sulfide volume increases, there is a change from high Pd-tenor microdroplets trapped in situ to larger, low tenor sulfides. The transition between these two sulfide regimes is marked by sharp peaks in Au, and then Te concentration, followed by a wider peak in Se, which gradually decreases with height. Mineralogical evidence implies that there is no significant post-magmatic hydrothermal S loss and that the metal profiles are essentially a function of magmatic processes. We propose that to generate these extreme precious and semimetal contents, the sulfides must have formed from an anomalously metal-rich package of magma, possibly formed via the dissolution of a previously PGE-enriched sulfide. Other processes such as kinetic diffusion may have also occurred alongside this to produce the ultra-high tenors. The characteristic metal offset pattern observed is largely controlled by partitioning effects, producing offset peaks in the order Pt+Pd>Au>Te>Se>Cu that are entirely consistent with published D values. This study confirms that extreme enrichment in sulfide droplets can occur in closed-system layered intrusions in situ, but this will characteristically form ore deposits that are so low in sulfide that they do not conform to conventional deposit models for CuâNiâPGE sulfides which require very high R factors, and settling of sulfide liquids
Precious and base metal geochemistry and mineralogy of the Grasvally NoriteâPyroxeniteâAnorthosite (GNPA) member, northern Bushveld Complex, South Africa: implications for a multistage emplacement
Architecture of the Oman-UAE ophiolite : evidence for a multi-phase magmatic history
The OmanâUnited Arab Emirates ophiolite is the
worldâs largest ophiolite. It is divided into 12 separate faultbounded
blocks, of which the northern three lie wholly or
partly in the United Arab Emirates. Extensive mapping has
shown that the United Arab Emirates blocks contain mantle
and crustal sections which correspond to the classic
âPenrose conferenceâ ophiolite definition but which are
cut by a voluminous later magmatic sequence including
ultramafic, mafic and felsic components. Samples from the
later magmatic sequence are dated at 96.4±0.3, 95.74±0.3
and 95.2±0.3 Ma; the early crustal section, which has not
been dated directly, is thus constrained to be older than c.
96.4 Ma. Petrological evidence shows that the early crustal
section formed at a spreading ridge, but the later magmatic
sequence was formed from hydrous magmas that produced
different mineral crystallisation sequences to normal midocean
ridge basalt (MORB). Mineral and whole-rock
geochemical analyses show that the early crustal rocks are
chemically similar to MORB, but the later magmatic
sequence has chemical features typically found in suprasubduction
zone (SSZ) settings. The ophiolite in the United
Arab Emirates thus preserves clear evidence for two stages
of magmatism, an early episode formed at a spreading
centre and a later episode associated with the onset of
subduction. Similar two-stage magmatism has been recognised
in the Oman sector, but the United Arab Emirates
contains the most voluminous SSZ magmatism yet described
from this ophiolite