Research

EDMOND A. MATHEZ

Curator

New York, NY 10014

Phone: 1 (212) 769 5379
Fax: 1 (212) 769 5339

mathez@amnh.org

Map of the UG2 chromitite and its footwall, Middelpunt Mine, Bushveld Complex.

phenomena
While mapping the UG2, we of course were wondering how it formed. Conventional wisdom holds that chromitites form when a chemically primitive magma intrudes into an extant chamber to mix with more evolved magma, forcing the mixture into the field of chromite-only stability. This hypothesis was developed by Neil Irvine nearly thirty years ago to explain the sequence of layering in the Muskox intrusion, and it became widely accepted in the layered intrusion community because it appeared to be firmly rooted in both phase equilibria and field observations.

According to this model, the rocks immediately above and below chromitite layers should be different. In the case of the UG2, they appeared to be similar. Therefore, Sisir Mondal, who worked with me as a post doc, and I conducted a detailed study of the section confirming the similarity (Mondal and Mathez, 2006). We investigated the various possibilities to account for the UG2 chromitite and concluded that rather than having been formed by magma mixing, it and other Bushveld chromitites formed by injection of new batches of magma with a composition similar to the resident magma but carrying a suspended load of chromite crystals, which then accumulated to form the massive chromitite layers.

E.A. Mathez, and J.L. Mey, 2005, Character of the UG2 chromitite and host rocks and petrogenesis of its pegmatoidal
footwall, northeastern Bushveld Complex. Economic Geology 100, 1617-1630.

Mondal, S.K., and E.A. Mathez, 2006, Origin of the UG2 chromitite layer, Bushveld Complex. Journal of Petrology 48, 495-510.

Compared to plagioclase, we found that the sulfides generally possess slightly higher 206Pb/204Pb ratios for equivalent 207Pb/204Pb ratios such that their compositions fall between the 2.06 and 1.86 Ga geochrons. The latter age is much younger than the cooling age. We interpreted that data to mean that the Bushveld Complex remained buried in the crust at temperatures of several hundred °C for about 200 Ma after solidification, and that any sulfides accessible to fluid continued to re-equilibrated during this time with more radiogenic Pb.

One question is where the sulfide Pb came from. Among the possibilities is that it was transported into the Bushveld Complex by fluids from an external reservoir when the rocks were still partially molten and thus permeable; another is that it originated mainly from radiogenic decay of U and Th present in minerals other than the sulfides in the immediately surrounding Bushveld rocks, followed by local redistribution of Pb by whatever fluid was present. We discovered some sulfides characterized by unusually high 208Pb/204Pb ratios, and for these at least an external source is unlikely. This observation and the fact that the sulfides display small-scale heterogeneity suggest that most, if not all, of the radiogenic sulfide Pb was locally derived.

E.A. Mathez and T.E. Waight, 2003, Lead isotopic disequilibrium between sulfide and plagioclase in the Bushveld Complex
and the chemical evolution of large layered intrusions. Geochimica et Cosmochimica Acta 67, 1875-1888.

E.A. Mathez and A.J.R. Kent, 2007, Variable initial Pb isotopic compositions of rocks associated with the UG2 chromitite,
eastern Bushveld Complex. Geochimica et Cosmochimica Acta 71, 5514-5527 .

Studies of apatite
Jim Webster and I conducted a series of experiments to understand the partitioning behavior of Cl among apatite, mafic silicate melt, and aqueous fluid and of F between apatite and melt (Mathez and Webster, 2005). We discovered that the value of DClapatite/melt remains constant at ≈ 0.8 for silicate melt containing less than about 3.8 wt. % Cl, but that at higher melt Cl contents, small increases in melt Cl concentration are accompanied by large increases in apatite Cl concentration, forcing DClapatite/melt to increase as well. Melt containing less than 3.8% Cl coexists with water-rich vapor; that containing more Cl coexists with saline fluid, the salinity of which increases rapidly with small increases in melt Cl content, analogous to the dependency of apatite composition on melt Cl content. We interpreted this behavior to be due to the fact that the solubility of Cl in silicate melt depends strongly on the composition of the melt, particularly its Mg, Ca, Fe, and Si contents. Once the melt becomes “saturated” in Cl, additional Cl must be accommodated by coexisting fluid, apatite, or other phases rather than the melt itself.

$The experimental data demonstrate that the Cl-rich apatites present in parts of the Stillwater and Bushveld Complexes equilibrated with highly saline fluids. We proposed a new mechanism to account for these fluids and the resulting apatite in which small amounts of initially hydrous, interstitial fluid dehydrated by reaction with initially anhydrous pyroxene. Although the amount of water that can dissolve in pyroxene is small, in the Bushveld and Stillwater environments the interstitial fluids “saw” and were able to reequilibrate with relatively large masses of crystals as the fluid percolated through the compacting crystal pile. E.A. Mathez and J.D. Webster, 2005, Partitioning behavior of chlorine and fluorine in the system apatite—silicate melt —fluid. Geochimica et Cosmochimica Acta 69, 1275-1286. Carbon in rocks, its influence on electrical conductivity, and possible bearing on earthquake precursory phenomena From a number of papers, colleagues and I have established that microcrack surfaces in otherwise carbon-free crystalline rocks are commonly coated with thin films of carbonaceous material (e.g., Mogk and Mathez, 2000). This naturally raises the question of how such films formed and, more generally, what happens when minerals fracture in the presence of C-O-H fluids. When a fracture opens, an adsorbed layer instantaneously forms on the new surface and apparently then grows to become microns thick. Despite the low abundance of carbon in the bulk rock, these layers have an important influence on the electrical conductivities of the rocks. In thinking about these questions, we had began to wonder if the formation of conductive layers in rocks could have something to do with electrical phenomena precursory to earthquakes. This led us to pose the hypothesis that as microfractures open in the time leading up to failure along a main fracture, carbon from fluids deposits as a continuous film on new, reactive mineral surfaces and causes electrical conductivity to increase. We set out to test this idea in a series of experiments. The test (Roberts et al., 1999) involved applying a uniaxial load to a sample at elevated temperature and pressure (typically 430°C and 150-180 MPa fluid pore pressure) in the presence of a carbonaceous atmosphere while simultaneously monitoring changes in electrical resistance across the sample. The vapor composition was purposely chosen to be metastable with respect to graphite at the run conditions. At the low strain rate of the experiments (10-6s-1) the samples experienced an extended period of dilatancy, during which time a microfracture network developed, before eventual catastrophic failure. In a typical experiment, the initial increase in load was accompanied by the decrease of resistance to a minimum value when maximum temperature was reached (see figure). At this point, as load steadily increased so did resistance, suggesting that the sample was drying out, oxidation state was changing, or deformation was progressively destroying the current-bearing pathways. In any case, two interesting phenomena were observed. First, resistance exhibited a precipitous drop from ≈150 to 100 M Ω immediately preceding catastrophic failure. Second, the end load displayed small decreases, thought to reflect the sudden opening of fractures, and these were accompanied by small decreases in resistance (arrows). Subsequent study of the sample revealed the presence of carbon in microfractures interpreted as members of conjugate sets formed during the dilatancy. These observations were interpreted to support our initial hypothesis. Because the growth of a conductive microcrack network represents a moving conductor, this in principle could account for EM disturbances associated with earthquakes, for example, those observed prior to the 1989 Loma Prieta event.$

Change in resistance and load during deformation of Nugget sandstone in the presence of an atmosphere of 95% CO2 + 5% CO at 435°C, 170 MPa confining pressure.

Since being introduced to the Stillwater Complex in southwest Montana by Stu McCallum in the 1970s, I have found layered intrusions to be endlessly fascinating. These fossil magma chambers display igneous layering on all scales, from centimeters to hundreds of meters; they include layers of nearly monomineralic lithologies, such as dunites, pyroxenites, chromitites, and anorthosites; and they host the major ore deposits of the platinum group elements, of chromium, and of vanadium, with some also containing ores of nickel, copper, and other base metals. These features reflect a variety of dynamic processes and transport mechanisms both within the magma chamber and in the partially molten crystal pile. Students of layered intrusions seek to understand these processes. My current research concerns mainly the Bushveld Complex. Here are summaries of several projects.

The UG2 is a layer of massive chromitite, typically slightly less than a meter thick, that can be traced with only minor interruption along a strike length of nearly 400 kilometers. It is the most important PGE resource in the Bushveld Complex (and thus in the world). One would think that such a remarkable layer would have been described in great detail. In fact, while there are may papers on the UG2 bearing on its existence as an ore deposit, no descriptions of the detailed field relations have been published in the open literature. Accordingly, former student Jacob Mey and I mapped the UG2 and its footwall on the scale of centimeters in a characteristic section exposed in a wall in the Middelpunt Mine, which is one of the Lebowa mines in the northeast part of the Bushveld Complex. Here’s our map (Mathez and Mey, 2005):

Variations in Pb isotopic compositions of coexisting plagioclase (circles) and sulfide (triangles) through an approximately 10 m stratigraphic section including the Merensky pyroxenite, Bushveld Complex.

phenomena
Adam Kent of Oregon State University and I just completed a follow-up study of the earlier one. Among other toys, Adam has a NuPlasma multicollector ICP-MS and NewWave 193 nm Excimer laser, the combination of which have proven to provide much improved sensitivity for Pb isotope analysis. In the new study we demonstrated that plagioclase compositions fall on the 207Pb/204Pb vs 206Pb/204Pb geochron of 2.06 Ga, which is the solidification age of the Bushveld Complex. The spread of plagioclase compositions on the geochron is significantly larger than that defined by analytical error, however, indicating that in terms of Pb at least two different magma compositions were present. The composition of the least radiogenic magma was approximated by that of the contemporaneous BSE with µ (238U/204Pb) and (232Th/204Pb) values of ≈ 9.0 and 35, respectively, suggesting a mantle derivation with little or no involvement of the continental crust, while the second magma possessed a Pb isotopic composition similar to the upper crust with µ ≈ 9.6.

Pb isotopic composition profiles across partially annealed cracks and grain boundaries of plagioclase, Bushveld Complex.

Above: Schematic illustration of the partitioning behavior of Cl between apatite and melt.

Left: Deduced topologies of the (a) silicate melt (m)—vapor (v) and (b) silicate melt—vapor—saline fluid (sf) systems.

Geochemical evolution of layered intrusions from study of Pb isotopes
With Tod Waight at the University of Copenhagen, I determined the Pb isotopic compositions of coexisting plagioclase and sulfide from the Bushveld Complex by laser ablation multi-collector ICPMS (Mathez and Waight, 2003). We studied samples associated with the Merensky Reef collected from one of the Lebowa mines and from drill core in the northeast corner of the Complex. We discovered large differences in the Pb isotopic compositions of the two minerals, even where they coexist in the same thin section. Partially annealed cracks in plagioclase also display compositions different than the bulk crystal composition. We interpreted the array of sulfide and plagioclase compositions to indicate the presence of Pb from multiple sources at the time of crystallization or soon thereafter.

RESEARCH PROJECTS

Array of plagioclase and sulfide compositions in rocks associated with the UG2 chromitite, Bushveld Complex.

$phenomena The measurement of miniscule changes in electrical resistance simultaneous with deformation demanded rather complex experiments, and ours were accompanied by a variety of problems. This motivated us to devise a simpler set of experiments to examine carbon growth on fracture surfaces during rock deformation. The new experiments are a collaborative effort with Andreas Kronenberg (Texas A & M), Stephen Karner (Exxon Production Research), Al Duba (AMNH), and Jeff Roberts (Lawrence Livermore National Laboratory) and myself. Our experiments involved deforming hollow cylinders of Sioux quartzite to failure in the presence of carbonaceous pore fluids, after which the samples were recovered for investigation of changes in electrical conductivity and carbon distribution. Samples were loaded at room temperature or 400oC by a hydrostatic pressure at their outer diameter, increasing pressure at a constant rate to ~290 MPa. Pore fluids consisted of pure CO, CO2, CH4 and a 1:1 mixture of CO2 and CH4, each with pore pressures of 2.0 to 4.1 MPa. We found that carbon occurs preferentially as quasi-continuous films on newly-formed fracture surfaces, but these films are absent from pre-existing surfaces in those same experiments. The observations support the hypothesis that electrical conductivity of rocks is enhanced by the deposition of carbon on fracture surfaces and imply that electrical properties may change in direct response to brittle deformation. They also suggest that the carbon films formed nearly instantaneously as the cracks formed. Carbon film deposition may accompany the development of microfracture arrays prior to and during fault rupture and thus may be capable of explaining precursory and coseismic geoelectric phenomena.$ shapeimage_25_link_0

$phenomena Mathez, E.A., J.J. Roberts, A.G. Duba, A.K. Kronenberg, and S.L. Karner, 2008, Carbon deposition during brittle rock   deformation: Changes in electrical properties of fault zones and potential geoelectric phenomena during earthquakes. Journal of Geophysical Research (in press). Mogk, D.W., and E.A. Mathez, 2000, Carbonaceous films in mid-crustal rocks from the KTB borehole, Germany, as characterized by time-of-flight secondary ion mass spectrometry. Geochemistry Geophysics and Geosystems 1, paper 2000GC000081. Roberts, J.J., A.G. Duba, E.A. Mathez, T.J. Shankland, and R. Kinzler, 1999, Carbon-enhanced electrical conductivity during fracture of rocks. Journal of Geophysical Research 104, 737-747.$

Polished surface perpendicular to axis of hollow cylindrical sample of Sioux quartzite deformed at 400°C with CO pore fluid (Q6) showing microcracks formed during the experiment, as observed by (a) secondary electron and (b) carbon x-ray intensity map. In the latter, spot density is proportional to concentration. It can be seen that the carbon films are well developed on the microcracks and fractures. The images are 150 µm on a side.