Abstract :
[en] The interpretation of the chemical evolution of layered intrusions and stratiform chromitite formation involves inversion of mineral composition trends to infer compositions of parent magmas. Three principal elemental re-equilibrated processes must be taken into account when interpreting mineral compositions in cumulate rocks: 1) changes in the composition of the parent magma due to different magma mixing proportions, fractional crystallization, contamination etc, proportions, consequently leading to chemical disequilibrium between the magmas and crystallizing minerals; 2) superimposed modification of mineral compositions as a result of crystallization of trapped interstitial or extraneous evolved melt/liquid; 3) redistribution of elements due to sub-solidus re-equilibration between silicate and oxide minerals. This thesis is an attempt to distinguish these effects, through a series of zoning information and stable isotopic systems.
To better understand and distinguish the role of inter-mineral diffusion during subsolidus processes and compositional shifts during melt/fluid infiltrating, we investigated the Fe and Mg isotopic compositions of olivine, orthopyroxene, and chromite separates from the Peridotite Zone of the Stillwater Complex. Olivine and orthopyroxene show limited Mg isotopic variations, with δ26Mg values ranging from -0.40 to -0.26‰ and from -0.29 to -0.22‰, respectively, similar to mantle peridotite values. In contrast, chromite displays extremely large Mg isotopic variations, with δ26Mg ranging from -0.05 to +0.84‰. The δ56Fe values in olivine and orthopyroxene range from 0.00 to +0.17‰ and -0.04 to +0.06‰, whereas chromites have δ56Fe values ranging from -0.09 to +0.13‰. Most olivine-orthopyroxene pairs in our samples show no clear Fe-Mg isotopic differences, whereas those between silicates and chromite display disequilibrium inter-mineral fractionation, as they fall off the theoretically predicted equilibrium fractionation lines. Higher δ56Fe values in silicates than chromite and higher δ26Mg values in chromite than silicates indicate that subsolidus re-equilibration between silicates and chromite is responsible for the observed disequilibrium Fe-Mg isotope fractionation.
The Cr isotopes are also analyzed in same samples, which olivine and orthopyroxene have δ53Cr values ranging from -0.09 to 0.25‰ and from -0.11 to 0.07‰, respectively, higher than the values of coexisting chromite with δ53Cr from -0.23 to -0.07‰. Evolving chromite Cr isotopic compositions may be a geochemical indicator of magmatic differentiation. Chromite-olivine pairs in silicate cumulates tend to have larger fractionation factors than those in chromitites, which is interpreted as the result of re-equilibration between interstitial melt/liquids.
Stable isotopes of Li-O in major minerals of chromitite, dunite, poikilitic harzburgite and bronzitite were also examined and measured in our thesis. The Li isotopes in olivine range from 4 to 26‰ in δ7Li with uniform Li contents of 1 to 3 ppm, whereas orthopyroxene and clinopyroxene have Li contents of 0.5 to 5 ppm and 4 to 8 ppm, and δ7Li ranges of -13 to 7‰ and -14 to -6‰, respectively. The δ18O values vary from 4.91 to 5.72‰ in olivine, from 5.11 to 5.87‰ in orthopyroxene, and from 4.64 to 5.86‰ in clinopyroxene. For a given sample, olivine displays more variable and higher δ7Li but lower δ18O values than orthopyroxene, indicating that olivine experienced more extensive compositional modification after crystallization relative to orthopyroxene. The general Li and O isotopic compositions are interpreted as the result of re-equilibration between interstitial liquids, from which pyroxenes crystallized, and cumulus minerals. The inter-mineral and inter-sample isotopic variations correlate with mineral assemblages, crystal sizes and major and trace element compositions, revealing that the interstitial liquids varied compositionally mainly due to mixing between fractionated magma and newly injected primitive magma. Abrupt mineralogical and geochemical changes from silicate rocks to chromitites imply that hydrous fluids, which collected on chromite surfaces and were later released from chromite seams, played an additional, critical medium of chemical exchange between minerals in the chromitites.
Our research results provided different results between Fe-Mg isotopes and Cr-Li isotopes. The inter-mineral element diffusion has been demonstrated by conjugated Fe-Mg isotopic variations in chromite and silicates. On the contrary, the Cr-Li isotopic elevations in silicates are interpreted as the result of re-equilibration between interstitial melt/liquids. For these distinctions, we hold the opinion that the elemental contents in minerals relative to the mineral/fluid should be the dominant factor. The Li and Cr content differences between in silicates and liquids are similar, nevertheless, the Fe-Mg elemental contents in cumulus minerals are obviously higher than those in interstitial melt/liquid. Therefore, we can also speculate that the major elemental and isotopic variations in cumulus minerals are controlled by inter-mineral diffusion, whereas the trace elements, fluid mobility elements and their isotopic variations are more likely to be changed by melt/liquid shifts.
Both the inter-mineral diffusion and diffusion between mineral and interstitial can be restricted by separates, which has significant in explain the disequilibrium isotopic composition in disseminated samples. However, the migration of fluids between different layers, especially for the chromitite and silicate cumulates, have still no limited. A FTIR study of H2O in silicates shows reverse H2O contents in olivine and poikilitic orthopyroxene in chromitite whereas co-increasing H2O contents in oikocrysts in silicate cumulates. Massive hydrated mineral inclusions in chromite and high fluid-mobile elements in included silicates support that the chromitite crystallized from a fluid-enriched parental magma, and the early crystallizated chromite microlites collecte fluids leading to their re-distribution between olivine and poikilitic orthopyroxene. On the other hand, chromite grains could be efficiently floated by these fluids, causing them to migrate away from the silicate minerals, leading eventually to formation of nearly monomineralic chromitite seams. This process demonstrates that the chromitite seams in layered intrusions could be the result of mechanical sorting.
