Evolution of Crystallizing Interstitial Liquid in an Arc-Related Cumulate Determined by LA ICP-MS Mapping of a Large Amphibole Oikocryst

High-resolution chemical mapping of a large, single, oikocrysticamphibole grain from the Eriksberg gabbro, by laser ablationinductively coupled plasma mass spectrometry (LA ICP-MS), documentsthe progressive evolution of interstitial liquid in a hydrousbasaltic system. The amphibole has a nearly constant mg-numberand only minor variations in most major elements, in part becauseof the resorption of olivine buffering the liquid composition.However, non-buffered trace elements including Sc, V, Nb, Zr,Th, U, and the rare earth elements (REE) show at least an orderof magnitude variation. For example, V varies from <1 to>1000 ppm, Zr from 4·3 to >1000 ppm, Nb from 0·14to 12·3 ppm, and the REE patterns range from bowed upwith a negative Eu anomaly to bowed down with a positive Euanomaly—all within a single crystal. The distributionof amphibole compositions indicates that the interstitial liquidwas not uniformly distributed as crystallization proceeded.Rather, the compositional variations reflect progressively morechannelized flow of interstitial liquid during compaction. Whenthe interstitial liquid evolved so that the crystallizing amphibolehad     

Petrology and Mineral Compositions of the Middle Banded Series of the Stillwater Complex, Montana

The two olivine-bearing zones of the Middle Banded series ofthe Stillwater Complex are characterized by an increase in thenumber of cumulus minerals with height. In each, anorthositeand anorthositic troctolite dominate the lower part whereasolivine gabbro and gabbronorite form much of the upper portions.Electron microprobe analyses of cumulus minerals indicate littleor no variation of average mineral compositions with height.In addition, no significant lateral variations in cumulus mineralcompositions occur along 8 km of section. Plagioclase from throughoutthese zpnes shows the complex, reverse and oscillatory zonationpatterns also seen in plagioclase from the thick (>500 m)anorthosites that sandwich the zones. The data suggest thatthe entire Middle Banded series is genetically related and thereforerequires models for the origin of the thick anorthosites toalso explain the olivine-bearing rocks between them. However,textural features such as discordant troctolites, pegmatoids,slump structures and variably developed mineral laminations,and chemical features such as zonation in clinopyroxene producedby intergranular exchange with orthopyroxene and large within-samplevariations in the mg-number of olivine in low-olivine troctolites,indicate that significant modification of these rocks by postcumulusprocesses has taken place, thereby obscuring evidence of theirgenesis. KEY WORDS: Stillwater Complex; Middle Banded series; layered intrusions; mineral chemistry; postcumulus processes ^*Corresponding author. Telephone: (919) 681-8169. Fax: (919) 684-5833. e-mail: wpm{at}roguegeo.dukeedu     

Quantifying Three-Dimensional Silicate Fabrics in Cumulates Using Cumulative Distribution Functions

We present a new method for quantifying three-dimensional silicatefabrics and the associated uncertainties from grain orientationdata on three orthogonal sections. Our technique is appliedto the orientation of crystallographic features and, hence,yields a fabric related to the lattice-preferred orientation,although the method could be applied to shape-preferred orientationsor strain analysis based on passive linear markers. The orientationdata for each section are represented by their cumulative distributionfunction, and an iterative procedure is used to find the symmetricsecond-rank strain tensor that will simultaneously satisfy thecumulative distribution functions observed on each section.For samples with well-developed fabrics, this technique providesa much closer match to the sectional data than do previous techniquesbased on eigenparameter analysis of two-dimensional orientationdata. Robust uncertainty estimates are derived from a non-parametricbootstrap resampling scheme. The method is applied to two cumulates:one with a well-developed fabric and the other with a weak fabric,from the Stillwater complex, Montana. The silicate petrofabricorientations obtained for these samples compare favorably withindependent direct estimates of the volume fabric from electronbackscatter diffraction and magnetic techniques. KEY WORDS: cumulates; fabrics; quantitative; Stillwater complex; textural analysis