112 research outputs found
Morphology and microstructure of chromite crystals in chromitites from the Merensky Reef (Bushveld Complex, South Africa)
The Merensky Reef of the Bushveld Complex consists of two chromitite layers separated by coarse-grained melanorite. Microstructural analysis of the chromitite layers using electron backscatter diffraction analysis (EBSD), high-resolution X-ray microtomography and crystal size distribution analyses distinguished two populations of chromite crystals: fine-grained idiomorphic and large silicate inclusion-bearing crystals. The lower chromitite layer contains both populations, whereas the upper contains only fine idiomorphic grains. Most of the inclusion-bearing chromites have characteristic amoeboidal shapes that have been previously explained as products of sintering of pre-existing smaller idiomorphic crystals. Two possible mechanisms have been proposed for sintering of chromite crystals: (1) amalgamation of a cluster of grains with the same original crystallographic orientation; and (2) sintering of randomly orientated crystals followed by annealing into a single grain. The EBSD data show no evidence for clusters of similarly oriented grains among the idiomorphic population, nor for earlier presence of idiomorphic subgrains spatially related to inclusions, and therefore are evidence against both of the proposed sintering mechanisms. Electron backscatter diffraction analysis maps show deformation-related misorientations and curved subgrain boundaries within the large, amoeboidal crystals, and absence of such features in the fine-grained population. Microstructures observed in the lower chromitite layer are interpreted as the result of deformation during compaction of the orthocumulate layers, and constitute evidence for the formation of the amoeboid morphologies at an early stage of consolidation.An alternative model is proposed whereby silicate inclusions are incorporated during maturation and recrystallisation of initially dendritic chromite crystals, formed as a result of supercooling during emplacement of the lower chromite layer against cooler anorthosite during the magma influx that formed the Merensky Reef. The upper chromite layer formed from a subsequent magma influx, and hence lacked a mechanism to form dendritic chromite. This accounts for the difference between the two layers
Mantle Pb paradoxes : the sulfide solution
Author Posting. © Springer, 2006. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 152 (2006): 295-308, doi:10.1007/s00410-006-0108-1.There is growing evidence that the budget of Pb in mantle peridotites is largely
contained in sulfide, and that Pb partitions strongly into sulfide relative to silicate melt. In
addition, there is evidence to suggest that diffusion rates of Pb in sulfide (solid or melt)
are very fast. Given the possibility that sulfide melt âwetsâ sub-solidus mantle silicates,
and has very low viscosity, the implications for Pb behavior during mantle melting are
profound. There is only sparse experimental data relating to Pb partitioning between
sulfide and silicate, and no data on Pb diffusion rates in sulfides. A full understanding of
Pb behavior in sulfide may hold the key to several long-standing and important Pb
paradoxes and enigmas. The classical Pb isotope paradox arises from the fact that all
known mantle reservoirs lie to the right of the Geochron, with no consensus as to the
identity of the âbalancingâ reservoir. We propose that long-term segregation of sulfide
(containing Pb) to the core may resolve this paradox. Another Pb paradox arises from the fact that the Ce/Pb ratio of both OIB and MORB
is greater than bulk earth, and constant at a value of 25. The constancy of this âcanonical
ratioâ implies similar partition coefficients for Ce and Pb during magmatic processes
(Hofmann et al. 1986), whereas most experimental studies show that Pb is more
incompatible in silicates than Ce. Retention of Pb in residual mantle sulfide during
melting has the potential to bring the bulk partitioning of Ce into equality with Pb if the
sulfide melt/silicate melt partition coefficient for Pb has a value of ~ 14. Modeling shows
that the Ce/Pb (or Nd/Pb) of such melts will still accurately reflect that of the source, thus
enforcing the paradox that OIB and MORB mantles have markedly higher Ce/Pb (and
Nd/Pb) than the bulk silicate earth. This implies large deficiencies of Pb in the mantle
sources for these basalts. Sulfide may play other important roles during magmagenesis:
1). advective/diffusive sulfide networks may form potent metasomatic agents (in both
introducing and obliterating Pb isotopic heterogeneities in the mantle); 2). silicate melt
networks may easily exchange Pb with ambient mantle sulfides (by diffusion or
assimilation), thus âsamplingâ Pb in isotopically heterogeneous mantle domains
differently from the silicate-controlled isotope tracer systems (Sr, Nd, Hf), with an
apparent âde-couplingâ of these systems.Our intemperance
should not be blamed on the support we gratefully acknowledge from NSF: EAR-
0125917 to SRH and OCE-0118198 to GAG
Pervasive melt percolation reactions in ultra-depleted refractory harzburgites at the Mid-Atlantic Ridge, 15° 20âČN : ODP Hole 1274A
Author Posting. © The Authors, 2006. This is the author's version of the work. It is posted here by permission of Springer for personal use, not for redistribution. The definitive version was published in Contributions to Mineralogy and Petrology 153 (2007): 303-319, doi:10.1007/s00410-006-0148-6.ODP Leg 209 Site 1274 mantle peridotites are highly refractory in terms of lack of residual
clinopyroxene, olivine Mg# (up to 0.92) and spinel Cr# (~0.5), suggesting high degree of partial
melting (>20%). Detailed studies of their microstructures show that they have extensively
reacted with a pervading intergranular melt prior to cooling in the lithosphere, leading to
crystallization of olivine, clinopyroxene and spinel at the expense of orthopyroxene. The least
reacted harzburgites are too rich in orthopyroxene to be simple residues of low-pressure (spinel
field) partial melting. Cu-rich sulfides that precipitated with the clinopyroxenes indicate that
the intergranular melt was generated by no more than 12% melting of a MORB mantle or by
more extensive melting of a clinopyroxene-rich lithology. Rare olivine-rich lherzolitic domains,
characterized by relics of coarse clinopyroxenes intergrown with magmatic sulfides, support
the second interpretation. Further, coarse and intergranular clinopyroxenes are highly depleted
in REE, Zr and Ti. A two-stage partial melting/melt-rock reaction history is proposed, in which
initial mantle underwent depletion and refertilization after an earlier high pressure (garnet field)
melting event before upwelling and remelting beneath the present-day ridge. The ultra-depleted
compositions were acquired through melt re-equilibration with residual harzburgites.Funding for this
research was provided by Centre National de la Recherche Scientifique-Institut National des
Sciences de lâUnivers (Programme Dynamique et Evolution de la Terre Interne)
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
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