Edward C. Hansen

Hypersaline fluids in Precambrian deep-crustal metamorphism

An outstanding debate about the evolution of Precambrian granulite facies terranes concerns the role of fluids in deep-crustal metamorphism. One line of thought ascribes dryness and large-ion element (LILE) depletion to dehydration melting of rocks containing biotite and amphibole, without important participation of a low-density fluid phase, and removal of the granitic melts to the upper crust during metamorphic episodes. This mechanism is supported for the highest-temperature granulite terranes by experimental rock-melting studies. An alternative approach emphasizes field evidence for alkali metasomatism and silica mobility on outcrop and thin-section scales, which seemingly demonstrate fluid-driven processes. The debate concerns whether or not fluids could have coexisted with partial melts in migmatitic terranes and have been important agencies in destroying silicate hydrates and transporting LILE, especially Rb and U, out of the lower crust. Nearly ubiquitous CO2-rich fluid inclusions in granulite facies rocks have been cited by many workers as evidence that important granulite facies fluids were carbonic, in contrast to the H2O-dominated fluids of lower-grade metamorphism. However, the low solubilities of silicate constituents in CO2-rich fluids and the low wetting ability (high dihedral angles) of such fluids relative to silicate mineral grain boundaries, inhibiting infiltration, have been revealed in experimental studies over the last two decades. These features seriously detract from the appeal of carbonic fluids as important agencies in deep-crustal metamorphism. Recent experimental work in the system NaClKclH2O at deep crustal temperatures and pressures demonstrates that concentrated brines have appropriate low H2O activity, high infiltration ability, and high alkali mobility (especially Rb affinity). Limited anatexis may be promoted by hyperfusible solutes such as F and B, but the low H2O activity restricts rock melting. Improved thermodynamic properties for biotite indicate that the H2O activities necessary for orthopyroxene stability in quartzofeldspathic rocks, in lieu of biotite, are considerably higher than estimates based on previously available data, and that brines which participate in granulite facies metamorphism at deep-crustal conditions could be only moderately concentrated, similar in character to those of fluid inclusions observed in many different kinds of rocks. Recent observations of brine inclusions in granulites support the concept that polyionic salt solutions, immiscible with CO2 in the high-grade metamorphic temperature range (700–850°C) are feasible granulite facies fluids. The importance of hypersaline fluids in high-grade metamorphic processes may have been greatly underestimated from previous fluid inclusion studies. Of the several conceptual sources of hypersaline fluids in the crust, volatile-rich alkaline basalts seem plausible because of their additional ability to deliver heat for metamorphism. The postulated magmatic emanations may split into concentrated brines and immescible CO2 during ascent. They transport alkalis and LILE upward in the crust. Alkalic basalts in modern continental settings are characteristic of oblique plate convergence or continential distension, and may represent remelting of subcontinental mantle previously enriched in volatiles and alkalis by subduction processes. A possible Phanerozoic example is the interior western U.S.A. This analogy leads to a ‘Basin and Range’ hypothesis (Hopgood and Bowes, 1990, Tectonophysics 174, 279–299), in which the ancient granulite facies terranes represent zones of cratonal deformation. Granulites of the uplifted and deeply eroded Precambrian mobile belts may record zones of extraordinary thermal and physicochemical activity effected by primitive plate tectonic processes involving continental collision and disruption.

Metamorphism, Partial Melting, and K-Metasomatism of Garnet-Scapolite-Kyanite Granulite Xenoliths from Lashaine, Tanzania

Xenoliths of garnet-plagioclase clinopyroxenite, garnet websterite, olivine websterite, and garnet anorthosite are relics of an igneous suite of olivine-normative alkali gabbros metamorphosed into granulites under Lashaine, Tanzania, at ∼1200 K and 14 kb. Most clinopyroxene megacrysts recrystallized into polygonal clusters of small grains, and plagioclase laths exsolved from the cores. Clinopyroxene (CATs 11 mole %, Jd 15) and plagioclase (An28–41, Or1–2) reacted into atolls of garnet. Meionitic scapolite with widely variable sulfate/carbonate developed from plagioclase, oxidized sulfides and CO2. Needles of bent, multiply-twinned kyanite pervade the plagioclase. Prolonged metamorphism homogenized most minerals, including plagioclase next to scapolite. Brown glasses (SiO2 38–56 wt %, Al2O3 15–23, K2O 0.5–8) and dark alteration products mainly occur as rims to garnet, clinopyroxene, and scapolite. One glass pocket with quench plagioclase and hollow clinopyroxene contains two glass populations attributed to melting of garnet, clinopyroxene, and plagioclase, followed by quench crystallization, and finally by K replacement of Na. One clinopyroxenite with 15% glass and quench spinel lacks garnet and scapolite. All properties are consistent with rapid decompression, quenching and K metasomatism. The estimated pre-emption temperature (1200 K) for the Lashaine granulites lies well above the temperatures at ∼14 kb for a standard shield geotherm (850 K) and even an oceanic geotherm (1060 K). Pressure-temperature estimates for granulite xenoliths from three sites (Delegate, Engeln, Lashaine) fall on a single trend tentatively called an alkaline-province geotherm.

Petrologic evidence for K-feldspar metasomatism in granulite facies rocks

Abstract We present evidence for K-feldspar metasomatism in charnockitic granulites from two well-known terranes: the Shevaroy Hills Massif, S. India (750°C, 8 kbars) and the Bamble Sector, S.E. Norway (790°C, 7.5 kbars) in the form of K-feldspar veins principally along plagioclase and quartz grain boundaries and in the form of highly variable antiperthitic patches of K-feldspar in an uneven scattering of plagioclase grains. With one exception, orthopyroxene or amphibole grains in contact with these K-feldspar veins show no alteration to secondary biotite, indicating that the H 2 O activity of the fluids responsible for these veins must have been relatively low. A high Ba concentration in these veins also suggests a metasomatic origin. Point counted, back-scattered electron photomicrographs, along with microprobe analyses, provide reintegrated K-feldspar and plagioclase compositions for 8 to 12 predetermined random areas per thin section for three samples from Bamble and three samples from the Shevaroy Hills. These reintegrated feldspar compositions plot over a range of temperatures on the feldspar ternary for each sample, and indicate saturation temperatures above the mean temperature for either region with a few reintegrated compositions above the 1000°C isotherm and with the lower cut-off temperatures at 700°C and 600°C for the Shevaroy and Bamble samples, respectively. These patterns suggest that exsolution alone could not have been responsible for the formation of the K-feldspar veins and patches in these rocks. We suggest that these veins are due to the influx of complex, supercritical, low H 2 O activity brines shortly after peak metamorphic conditions, that this influx continued during the initial phases of post-peak metamorphic uplift and that these fluids represent the first stage in a series of fluid influxes in which the H 2 O activity increased as uplift continued. Reaction of these brines with potassium-undersaturated plagioclase grains formed K-feldspar veins along grain boundaries and fractures, as well as diffusing into the plagioclase, which became supersaturated, and exsolved K-feldspar as antiperthite patches during uplift and cooling. Formation of secondary biotite halos around orthopyroxene in one sample associated with these K-feldspar veins can be explained by heightened H 2 O activity in the brines due to enrichment in H 2 O, emplacement of the veins at lower pressure during uplift or both compared to the other Shevaroy and Bamble samples.

Rb depletion in biotites and whole rocks across an amphibolite to granulite facies transition zone, Tamil Nadu, South India

Abstract Relatively low concentrations of Rb and high K/Rb ratios are characteristic of many granulite facies terranes. This depletion in Rb has been attributed to both the removal of a partial melt and exchange with a metamorphic fluid phase. These models have been tested using Rb concentrations in biotites and whole rocks from intermediate and felsic gneisses collected along a traverse from just north of Krishnagiri to just north of Salem in Tamil Nadu State, South India. Along this traverse, the northern amphibolite-facies zone gives way to a clinopyroxene zone in which clinopyroxene appears in intermediate and felsic gneisses. Further south is the lowland charnockite zone characterised by the presence of orthopyroxene and the scarcity of clinopyroxene in intermediate to felsic gneisses. The abundance of orthopyroxene increases southwards and it is the dominant ferromagnesium silicate in the highland charnockite zone. There is a good correlation between Rb in biotite and whole-rock Rb in samples collected throughout the traverse. Intermediate and felsic gneisses in the northern portion of this traverse have relatively high modal abundances of biotite, low Ti concentrations in the biotites, high whole-rock Rb concentrations, low K/Rb ratios and high Rb concentrations within the biotites. Ti concentrations in the biotites increase southward into the clinopyroxene zone and then remain relatively constant. High K/Rb ratios first appear at the southern boundary of the clinopyroxene zone. In the lowland and highland charnockite zones, the majority of the rocks have relatively low Rb concentrations and high K/Rb ratios. Low Rb concentrations in biotites (at or near the detection limit of 65 ppm) first appear in the lowland charnockite zone and persist into the highland charnockite zone. A smaller group of rocks in the highland charnockite zone contain biotites with moderate Rb concentrations. Most of these rocks also contain anomalously high biotite concentrations and low K/Rb ratios. It is not possible to produce Rb-depleted rocks containing low-Rb biotites by simple, equilibrium, partial-melting models beginning with a rock with the average composition of the amphibolite-facies gneisses (70 ppm) and using distribution coefficients from the literature. It is possible to produce such Rb-depleted rocks if the melt is removed in batches; however, it requires large amounts of partial melting (>40%). Low-Rb biotites can be produced by both equilibrium and batch melting if the original rocks have low whole-rock concentrations of Rb (10–20 ppm) to begin with. Attempts to model Rb depletion by exchange with a metamorphic fluid phase are hampered by a general lack of experimentally determined mineral/fluid distribution coefficients. Recent, experimentally determined exchange coefficients between phlogopite and a (K,Rb)Cl-rich supercritical brine at high pressure and temperature indicate that it would require a fluid/rock mass ratio of approximately 0.125 to produce Rb-depleted rocks with low-Rb biotites by exchange with a KCl-rich fluid. Alternatively, biotites low in Rb may have formed shortly after the peak of metamorphism in the rocks after they were depleted in Rb but while they were still under relatively high-grade conditions.

Elucidating aeolian dune history from lacustrine sand records in the Lake Michigan Coastal Zone, USA

Aeolian sand in two small lakes within a Lake Michigan coastal-dune complex southwest of Holland, Michigan, provides a more detailed higher-resolution history of dune activity, during the later half of the Holocene, than do combined palaeosol and OSL ages from the dunes themselves. The sand signal from four cores within these lakes consists of visible sand laminae and invisible sand peaks within mud and sapropel units. Early in the history of dune growth the sand may have come from backdunes immediately adjacent to the lakes, but in the upper portions of the core aeolian sand is interpreted as having been transported from 30—40 m high parabolic dunes ~200 m from the lakes. All but one episode of dune mobility suggested by radiocarbon-dated dune palaeosols or OSL-dated dune sand are represented by a peak in the radiocarbon-dated lacustrine sand signal. However, sand peaks occur in the lacustrine record that do not correspond to palaeosol or OSL ages, suggesting that the sand record from small lakes gives a more complete record of aeolian activity. Heterogeneity in sand deposition related to bathymetry suggests that the study of multiple cores from the same lake will yield the best results . No strong correlation between rising lake levels on the Lake Michigan relative lake level curve and aeolian activity was found, although such a correlation has been reported elsewhere.

The Origin of Dark Sand in Eolian Deposits along the Southeastern Shore of Lake Michigan

Dune deposits on Lake Michigan’s southeastern shore contain pin stripe laminations: thin (<3 mm) laterally continuous (10 m) layers of dark sand, with vertical separations of 0.5–100 cm between laminations. On modern dune surfaces concentrations of dark sand are observed in ripple troughs and form continuous sheets that pass under ripple crests. We explore the source of the dark grains and the processes that can concentrate them in layers, using direct field observations, textural analysis, and point counts of sand grain minerals using the energy-dispersive x-ray analysis function on an electron microprobe. Large and medium size fractions of dune sand are dominated by quartz while smaller size fractions contain high proportions of dark heavy minerals (Fe-Ti oxides, garnets, and Fe-Mg silicates). Pin stripe laminations and dark surface patches are enriched in this fine-grained component, suggesting that sorting by grain size is important in their origin. The fine-grained sand fractions of glacial tills have low concentrations of dark heavy minerals. Thus, the high proportions of heavy dark minerals in finer-grained fractions of dune sands developed during postglacial transport. The association of fine dark sands with ripples suggests that they form as translatent stratification when smaller grains preferentially collect in troughs during ripple migration. Dark laminations in dunes seldom display characteristic features of grainflow (upward coarsening in grain size, troughlike cross section), indicating that this mechanism does not account for the majority of pin stripe laminations. Grainfall has been observed creating a patch of dark sand and may account for the formation of some pin stripe laminations. Pin stripe laminations indicate the orientation of past dune surfaces and can help reconstruct past dune geometries and migration histories.

A Comparison between PIXE Studies and Electron Microprobe Studies of Rocks from Southern India.

The Hope College PIXE facility was used to perform an analysis of zircons, apatites and monazites prevalent in rocks collected in southern India. Thin sections of the samples were prepared for electron microprobe analysis at U. Chicago, and a number of rare earth elements were quantitatively measured. These samples were then fractured and individual crystals were analyzed with 2.3 MeV protons in an internal PIXE irradiation chamber. GUPIX II© software was used to identify and measure trace metals in the samples and the results have been correlated with the electron microprobe data. Whole rock analyses, prepared by making pressed pellets of the powdered rock components were also analyzed by PIXE and XRF. Each of these techniques can be used to address the geological distribution of large ion lithophiles in the mantle, and advantages and disadvantages of each technique will be presented.