John C. Schumacher

Nomenclature of amphiboles : additions and revisions to the International Mineralogical Association's amphibole nomenclature

The introduction of a fifth amphibole group, the Na-Ca-Mg-Fe-Mn-Li group, defined by 0.50 < B(Mg,Fe2+,Mn2+,Li) < 1.50 and 0.50 ≤ B(Ca,Na) ≤ 1.50 a.f.p.u. (atoms per formula unit), with members whittakerite and ottoliniite, has been required by recent discoveries of B(LiNa) amphiboles. This, and other new discoveries, such as sodicpedrizite (which, here, is changed slightly, but significantly, from the original idealized formula), necessitate amendments to the IMA 1997 definitions of the Mg-Fe-Mn-Li, calcic, sodic-calcic and sodic groups. The discovery of obertiite and the finding of an incompatibility in the IMA 1997 subdivision of the sodic group, requires further amendments within the sodic group. All these changes, which have IMA approval, are summarized.

Aragonite pseudomorphs in high-pressure marbles of Syros, Greece

Numerous rod-shaped calcite crystals occur in the blueschist to eclogite facies marbles of Syros, Greece. The rods show a shape-preferred orientation, and the long axes of the rods are oriented at a large angle to foliation. The crystals also have a crystallographic-preferred orientation: calcite c-axes are oriented parallel to the long axes of the rods. Based on their chemical composition, shape, and occurrence in high-pressure marbles, these calcite crystals are interpreted as topotactic pseudomorphs after aragonite that developed a crystallographic-preferred orientation during peak metamorphism. This interpretation is consistent with deformation of aragonite by dislocation creep, which has been observed in laboratory experiments but has not been previously reported on the basis of field evidence. Subsequent to the high-pressure deformation of the aragonite marbles, the aragonite recrystallized statically into coarse rod-shaped crystals, maintaining the crystallographic orientation developed during deformation. During later exhumation, aragonite reverted to calcite, and the marbles experienced little further deformation, at least in the pseudomorph-rich layers. Some shearing of pseudomorph-bearing marble layers did occur and is indicated by twinning of calcite and by a variable inclination of the pseudomorphs relative to foliation.

Intersector element partitioning in tourmaline: a potentially powerful single crystal thermometer

Hourglass sector zoning, and related polar overgrowths, are common features of metamorphic tourmaline, developing as a result of variations in element preference on the different growth surfaces. For sector-zoned crystals, three domains are present for each growth zone (c+, c− and a), with compositional differences most distinct for Ca and Ti, and among c+ and c− sectors. Intersector differences vary, commonly showing decreasing fractionation from core to rim attributed to increasing metamorphic grade. Here we show that intersector element partitioning is temperature dependent and derive empirical geothermometers based on c+–c−and c+–a partitioning of Ca and Ti. These thermometers are applicable over a range of temperatures and bulk-rock compositions. Intersector partitioning is not affected by re-equilibration and records and preserves complete T-histories of individual tourmaline grains from prograde to peak and on to retrograde growth. Information on element mobility is preserved by tourmaline composition, because intersector partitioning is independent of element concentration. These factors make intersector partitioning an ideal tool to elucidate the thermal history of tourmaline grains and thus their host environment and tourmaline’s refractory nature preserves these signatures even into the sedimentary record.

Hourglass sector zoning in metamorphic tourmaline and resultant major and trace-element fractionation

Abstract A new type of sector zoning, with an hourglass shape, has been identified in metamorphic tourmalines that formed under a wide variety of physical and chemical conditions. The two sectors in the c-direction are not equivalent due to asymmetry in the crystal structure of tourmaline along the c-axis. The c+ sector is characterized by low concentrations of Ti, Ca, Mg, and Na, although Al is high, and has a pale (commonly blue or pale-green) color. Conversely, the c. sector is low in Mg and Al, and high in Ca, Fe, and Ti (the latter two causing the dark-brown color of this sector). The a-sector has intermediate characteristics and probably approximates a sector-free tourmaline. Thin sectioning of these sector-zoned tourmalines perpendicular to the c-axis can produce three types of apparent radial zoning patterns: blue-green cores, dark-brown cores, or no distinct cores. These apparent cores will further vary in relative diameter depending on the sectioning level. Furthermore, .core. boundaries can be straight or ragged depending on whether the relative growth speeds for the different faces was constant or variable. These textures have been used to argue for a prograde or detrital origin of tourmaline cores. However, sector zoning is a more appealing explanation for most of these textures, and can further explain the textural resemblance among metamorphic tourmalines from highly variable bulk-rock composition, metamorphic history, and mineral paragenesis. The sector zoning that is described here develops by preferential uptake of elements on the r growth plane, resulting from a combined effect of differences in surface charge and morphology of this plane in the c+ and c. directions. This leads to the preferential incorporation of more positively charged elements in the c. direction, and a preference for a vacant X-site in the c+ direction. Because the compositional differences among the sectors are pronounced in both major and trace elements and in the same order of magnitude as growth zoning variability, the presence of sector zoning must be established and taken into account when making inferences from tourmaline chemistry.

The geothermobarometric potential of tourmaline, based on experimental and natural data

Abstract The geothermobarometric potential of tourmaline has been assessed by investigating element exchange among tourmaline and coexisting minerals in metamorphosed pelites and graywackes, and in experimental exchange between tourmaline and biotite. In the natural samples, a temperature dependence of tourmaline Mg-Fe exchange with biotite, staurolite, garnet, chlorite, and muscovite, and Ca-Na exchange with plagioclase is observed. Equilibrium calculations for the complete mineral assemblage show that tourmaline is in compositional equilibrium with all coexisting phases, which would allow for an internally consistent set of thermometers among all these phases to be defined. However, a prohibitively large spread is present in the KD vs. T relations. This is not caused by analytical effects, compositional zoning, or disequilibrium among the minerals. The experimental results show that it is the result of inter-site partitioning of elements over the Y and Z octahedral sites of tourmaline. Variations in the element distribution over these sites, their relative participation in the exchange and differences in the temperature dependence of exchange with each site, strongly affects the KD vs. T relation observed, with the slope actually changing sign depending on the elements residing at each site. Non-ideal interactions among the elements at each site will also affect this, and furthermore link every exchange to the bulk tourmaline composition, and hence the element mobility in the rock. The promising potential of tourmaline geothermobarometry can therefore not be fulfilled until effects of inter-site partitioning and non-ideal interactions are known.

Progressive reactions and melting in the Acadian metamorphic high of central Massachusetts and southwestern New Hampshire, USA

The purpose of this chapter is to describe briefly and summarize changes in the mineral assemblages and mineral compositions that are observed in various rock types along the metamorphic field gradient in the metamorphic high of central Massachusetts and southwestern New Hampshire, USA. The progressive metamorphism of rocks of basaltic composition is discussed here more extensively than for rocks of common pelitic compositions, because detailed descriptions of the progressive metamorphism of basaltic rocks at these metamorphic conditions are less common. This chapter is aimed, in part, at non-petrologists and postgraduate students. As a consequence, some basic subjects are discussed, including some general comments concerning P-T-t (pressure-temperature-time) paths and metamorphic field gradients. However, some aspects should interest petrologists looking for meatier topics.

Acadian metamorphism in central massachusetts and south-western New Hampshire: evidence for contrasting P-T trajectories

Early work by numerous investigators (Robinson 1963, Tracy et al. 1976, Robinson et al. 1982, 1986a, b) provides a structural and metamorphic framework for the Acadian metamorphic high in central Massachusetts within which changes in phase composition and assemblage in the rocks can be correlated. Six metamorphic zones have been distinguished (Tracy et al. 1976, Robinson et al. 1982), reflecting prograde (T) metamorphic conditions in graphite-bearing pelitic rocks (Fig. 1) that range from the amphibolite through to the lower granulite facies. The T and P estimates are based mainly on A. B. Thompson’s (1976) pioneering work in quantifying T and P using biotite-garnet Fe/Mg exchange and the garnet-cordierite-sillimanite-quartz assemblage, respectively. Background and setting Understanding of P-T trajectories in a region of complex deformation and metamorphism requires three types of information: (i) major and minor structural features that can be linked in a local kinematic sequence and regionally, to provide time correlation of the tectonic events; (ii) petrological features that give P-T information, and that can be correlated with the sequence of structural events; and (iii) significant geochronological information to provide an absolute time-frame for metamorphic events. The structural development and metamorphism of the region took place mainly during the Devonian Acadian orogeny and affected stratified and intrusive rocks of late Precambrian, Ordovician, Silurian, and Lower Devonian age. In termsof present tectonic inter-pretations, the area lay east (present geography) of the early Palaeozoic ocean, Iapetus, that closed during the Taconian orogeny, and east of the pertinent subduction zone. It also lay

Metamorphic style and development of the blueschist- to eclogite-facies rocks, Cyclades, Greece

The island of Syros, Greece is part of the Attic-Cycladic blueschist belt, formed during Mesozoic Eurasia-Africa subduction. The rocks of Syros can be broadly divided into three tectono-stratigraphic units: (I) metamorphosed sedimentary and volcanic rocks (marble-schist sequence), (II) remnants of oceanic crust with fault-bounded packages of blueschist/eclogite-facies mafic rocks and serpentinite (mafic-ultramafic rocks) and (III) the Vari gneiss, which is a tectonic klippe. Low-temperature, high-pressure assemblages are found on several islands in the Cyclades. The best preserved of these rocks are on Syros and Sifnos islands. Mineral compositions and peak metamorphic assemblages are similar on both islands. Both islands are considered to share similar P-T histories with highest-pressure mineral assemblages reflecting conditions of at least 15 kbar and about 500°C.