A. E. Boudreau

Investigations of the Stillwater Complex: Part V. Apatites as indicators of evolving fluid composition

Variations in the F, Cl and OH contents of apatite are not constrained by crystal-chemical factors (in contrast to micas and amphiboles), and thus changes in the abundance of these components provide an indicator of halogen fugacity variations and insights into the degassing history of igneous rocks. Microprobe analysis of intercumulus apatites from the Stillwater Complex reveal that Cl-rich apatites, typically containing <0.4 wt % F and >6.0 wt % Cl, occur throughout the lower 1/3 of the complex excluding the Basal series. A change from Cl-rich to more F-rich apatite occurs within olivine-bearing zone I (OB I) of the Banded series, the host zone of the platiniferous J-M Reef. Although apatite compositions are somewhat variable above the J-M Reef, more F-rich apatites predominante and typically contain >1.2 wt % F and <3.0 wt % Cl. The most F-rich apatites occur in the uppermost exposed cumulates. Pristine apatites from coeval sills and dikes from below the complex and from the Basal series are similarly F-rich. In all apatites, the Cl and F contents are lower in rocks affected by later metamorphic fluids. Rare earth element (REE) concentrations in chlorapatites show a marked peak in the olivine-rich rocks of the J-M Reef, and contain up to 2 wt % Ce2O3 + La2O3. The trend of first increasing, then decreasing Cl/F ratios with stratigraphic height is modeled by a vapor-driven zone refining process occurring within the cumulate pile causing Cl-enrichment in the interstitial melt accompanied by degassing at the top of the magma chamber causing overall loss of Cl from the magma as crystallization proceeded. The abrupt change from Cl-rich to more F-rich apatites within OB I is interpreted as the result of a breakdown of the Cl-rich zone refining front and mixing with Cl-poor supernatant melt. Any high temperature fluids that exsolved and circulated through the lower 1/3 of the complex must have been enriched in Cl and could have transported REE and trace metals.

Compaction of Igneous Cumulates Part I: Geochemical Consequences For Cumulates and Liquid Fractionation Trends

The importance of crystallization during compaction of an 800 m section of cumulates in Olivine‐bearing zones III and IV of the Stillwater complex is assessed using whole‐rock and mineral compositions. Two sets of calculations provide estimates for the liquid fraction that crystallized in each sample. The first relies upon the effect that crystallized liquid has on the whole‐rock Mg#, and the second is based on the proportion of compatible and incompatible trace elements contributed by the crystallized liquid. Estimates based on trace‐elements vary with their bulk distribution coefficients (D) such that elements with higher Ds yield higher estimates of crystallized liquid. Comparison of estimates based on Y (average 9%) with those based on V (average = 45%) and on the whole‐rock Mg# (average 15%) show all three to be substantially different. The disparity in these estimates is interpreted to be a consequence of crystallization during compaction enriching the interstitial liquid in incompatible elements that were then expelled from the crystal pile prior to incorporation into late crystallizing minerals. Knowledge of the fate of liquid included in a growing crystal pile is of fundamental importance not only to interpreting the composition of cumulates but also to understanding how liquid compositions evolve by fractional crystallization. The return of evolved liquids from the crystal pile can decouple fractionation trends of major and trace elements producing apparently anomalous enrichment in the more incompatible trace elements.

Compaction of Igneous Cumulates Part Ii: Compaction and the Development of Igneous Foliations

Quantitative petrofabric analysis of 66 samples from olivine‐bearing zones III and IV of the Middle Banded series of the Stillwater complex, Montana, was conducted to allow direct comparison among textural and chemical variations. Systematic variations in foliations with stratigraphic height are observed in both zones. Statistical analysis of the orientations of inequant grains was used to calculate an “Alignment Factor” (AF), corresponding to the extent of the development of the foliation. Both petrofabric and petrographic evidence indicates that the foliation is best explained by compaction of the crystal pile. Comparison of AFs with estimates of the amount of compaction, based on whole‐rock compositions, shows approximate agreement. Variations in mole fraction Cl of interstitial apatite mimic the observed trends in the AF, indicating that local concentrations of fluids exsolved from interstitial liquid were important in the development of strong foliations. We conclude that: (1) differing amounts of compaction led to variations in the igneous foliations; (2) local volatile enrichment enhanced the development of the foliation; and (3) variations in plagioclase morphology (from tabular to blocky) were produced by growth and/or recrystallization under differing amounts of uniaxial stress.

A REEVALUATION OF CRYSTAL-SIZE DISTRIBUTIONS IN CHROMITE CUMULATES

Abstract Although studies have shown that igneous cumulates can form by in situ crystallization without requiring crystal settling, it has not been demonstrated that crystal-size distributions (CSDs) are consistent with such a process. Plots of crystal-size fractions per unit volume vs. crystal size for chromite grains from the Stillwater complex show a log-linear distribution with negative slope at larger sizes and a concave-down distribution at smaller sizes. The log-linear portion of these trends is similar to previously reported trends for other silicate and oxide phases in crystallizing magmas. However, the lack of smaller grain sizes indicates that the original population was altered during a postnucleation crystalaging period that resulted in the loss of the smaller size fractions, an interpretation consistent with textural and volume-balance evidence. The CSD trends imply that Cr was not concentrated by settling of chromite but was brought to the site of the growing chromite grains. Similarities between the Stillwater data and CSD trends for garnet in metapelites suggest that such trends are a characteristic feature of any geologic system undergoing crystal aging after an initial period of nucleation and crystal growth.

Halogens of Bushveld Complex, South Africa: δ37Cl and Cl/F evidence for hydration melting of the source region in a back-arc setting

Abstract The Bushveld Complex is one of only a few layered intrusions anomalously enriched in chlorine. This study presents the first chlorine stable isotope analyses of Bushveld and associated rocks designed to constrain the source of Bushveld Cl. Most samples overlap the range of values for pristine MORB and altered seafloor samples ( δ 37 Cl ∼+4‰ relative to sea water). This contrasts with distal metasedimentary floor rocks ( δ 37 Cl ∼+0‰), with little evidence of contamination away from the marginal rocks. The Cl isotope composition and high Cl/F ratio is therefore a fundamental characteristic of the Lower and Critical Zone magmas and not derived from infiltration from the country rock. These results, the high Cl/F ratio and the boninitic arc-like character of Bushveld magma, are consistent with hydration partial melting of a depleted source in a subduction-related setting with Cl supplied either by the dehydration of a subducted slab with a heavy Cl signature or an overridden “wet” mantle hotspot.

Infiltration metasomatism in layered intrusions — An example from the Stillwater Complex, Montana

Abstract The contact between Anorthosite zone II (AN II) and the overlying troctolite of Olivine-Bearing zone V (OB V) of the Stillwater Complex is characterized by a 5-m-thick, adcumulate anorthosite. This adcumulate anorthosite separates rocks of AN II, characterized by the apparent crystallization sequence plagioclase followed by orthopyroxene+clinopyroxene±quartz±Fe-Ti oxide assemblages, from those of OB V, where the apparent crystallization sequence is olivine+plagioclase followed by orthopyroxene. The grain size and compositional zoning of plagioclase in this transition zone is similar to that in the overlying troctolite and distinct from the coarser, complexly zoned plagioclase that makes up the underlying anorthosite. In contrast, the minor interstitial pyroxene in the transition zone compositionally overlaps that in the underlying anorthosite, and both have a lower molar Mg/(Mg+Fe) ratio than the pyroxene of the troctolite. These relationships suggest that the intercumulus liquids of AN II and OB V were compositionally distinct. Model phase diagram considerations suggest that infiltration of any plagioclase-saturated liquids into a troctolite assemblage will result in the assimilation of olivine and, depending on the actual composition of the anorthositic liquid, will also either assimilate or crystallize plagioclase as well. Hence, the monomineralic anorthosite zone at the top of AN II can be attributed to infiltration of interstitial liquids from AN II that were displaced upward into the troctolite during compaction of AN II. This reaction can be either exothermic or endothermic, depending on the actual composition of the infiltrating liquid and the bulk solids composition. Numerical models of the thermal effects of infiltration suggest that heat of reaction can significantly affect the liquid/solid ratio away from the infiltration boundary.

Volatile fluid overpressure in layered intrusions and the formation of potholes

Potholes that develop in layered intrusions show morphologic similarities with sedimentary features known as pockmarks. By analogy with pockmark formation theory, it is suggested that the cumulate section below the pothole region develops significant overpressures as solidifying intercumulus liquid separates a volatile‐rich fluid phase. A fluid overpressure develops as fluid separates from the intercumulus liquid faster than the fluid can escape from the crystal pile. The increase in pore pressure expresses itself as a dome‐shaped swelling on the chamber floor and in the uppermost part of the crystal pile, which eventually fractures and results in the violent escape of fluid. The combination of fluidization of the cumulates and incongruent melting, caused by the depression of liquidus temperature as a result of volatile elements added to less evolved liquid and crystal assemblages, leads to the formation of the pothole. Excess sulphide precipitation should accompany the fluid‐liquid mixing event as S‐bear...

STABLE CL ISOTOPES AND ORIGIN OF HIGH-CL MAGMAS OF THE STILLWATER COMPLEX,MONTANA

The first Cl isotopic determinations on biotite from the Ultramafic series and from the J-M reef of the Lower Banded series of the Stillwater Complex, Montana, have δ 37 Cl values (normalized to seawater, 95% confidence limit error is 0.2‰) ranging from 0.27‰ to −0.93‰; the average is −0.18‰. Assuming minimal fractionation on incorporation into biotite, these data imply an ultimate crustal source for the Cl. However, field, petrographic, and isotopic evidence suggests that high-Cl fluids did not infiltrate from the country rocks after emplacement of the Stillwater magma, nor was the Cl-isotopic signature the result of assimilation of Cl-rich crustal rocks. Stillwater parent magmas have boninitic affinities and can be modeled as second-stage melts from a metasomatically enriched, depleted mantle. We propose that the early melt-extraction event depleted the mantle in F and Cl. Subsequent metasomatism by a Cl-rich agent with a crustal Cl isotopic signature, probably a fluid, produced a source rock with a high Cl/F ratio. In modern magmas, high Cl/F weight ratios are associated with high overall volatile contents, suggesting that the Stillwater Complex and other high-Cl layered intrusions crystallized from magmas that may have contained in excess of 1 wt% water.

Crystal aging and the formation of fine-scale igneous layering

SummaryFine-scale layering in igneous intrusions shows parallels with experimentally produced banding observed in crystallizing salt solutions in which recent advances have demonstrated the importance of crystal aging on the development of banding. These experiments, as well as numerical models of fine-scale layering development during crystal aging, reproduce features observed in fine scale layering that are not readily explained by periodic nucleation and crystal growth models. These include the phenomenon of “lost segments”, in which the development of a layer may be reversed such that the layer disappears over time but does so without affecting the overall spacing of later-developing layers, and the development of “doublet” layers, which results when crystals at the margins of developing layers preferentially grow at the expense of those in the center. While these results suggest that crystal aging is the dominant process in the development of fine-scale layering, crystal aging may enhance the textural and modal features of igneous layering initiated by a variety of other crystal settling, nucleation or growth processes.ZusammenfassungFeine Schichtung in magmatischen Intrusionen zeigt Parallelen mit experimentell erzeugter Bänderung, wie sie in kristallisierenden Salzlösungen beobachtet werden kann. An solchen Lösungen haben jüngste Arbeiten gezeigt, daß Kristall-Alterung bei der Ausbildung einer Bänderung eine wichtige Rolle spielt. Diese Experimente reproduzieren ebenso wie numerische Modelle, die die Ausbildung einer feinen Schichtung während der Kristall-Alterung simulieren, Merkmale, die in feinen Schichten auftreten, über nicht leicht mit periodischer Keimbildung und Kristallwachstumsmodellen erklärt werden können. Diese Merkmale umfassen das Phänomen der “verlorenen Segmente”, bei dem die Ausbildung einer Lage in der Weise umgekehrt werden kann, daß die Lage mit der Zeit verschwindet, ohne dabei den Gesamtabstand der später gebildeten Schichten zu beeinflussen, oder die Ausbildung von „Lagenpaaren”. Diese erfolgt, wenn Kristalle an den Rändern der sich entwickelnden Lagen bevorzugt auf Kosten jener im Zentrum wachsen. Während diese Ergebnisse nahelegen, daß Kristall-Alterung bei der Ausbildung von feiner Schichtung der vorherrschende Prozeß ist, kann Kristall-Alterung auch die texturellen und modalen Eigenschaften einer magmatischen Schichtung verstärken, die durch eine Reihe anderer Prozesse wie Kristallsetzung, Keimbildung oder Wachstum eingeleitet wurden.

Pattern Formation During Crystallization and the Formation of Fine-Scale Layering

Fine-scale layering is characterized by the laminar segregation of mineral phases on a millimeter to centimeter scale. Where best developed, it is found in specific associations within the Stillwater Complex. In the Banded series, layer spacing is proportional to crystal grain size, with the best examples of fine-scale layering characterized by coarse rock textures and the development of secondary structures. These latter features include the division of primary banding into two or more layers and the development of “honeycomb” crystal arrangements within the plane of individual layers. Analogy with similar features formed in crystallizing salt solutions suggests that fine-scale layering can develop during textural coarsening of the rock. An assemblage of crystals is inherently unstable to minor perturbations in crystal size: larger grains will grow at the expense of smaller grains in order to diminish the total surface free energy of the system. Such a process can result in an orderly segregation of mineral phases and can produce layering when a gradient of grain size is present across the system. This condition would be expected in an advancing front of nucleation and crystal growth in a solidifying magma. Quantitative modeling can explain many of the features of fine-scale layering, and suggests that textural features of igneous layering in general can become better defined with time.