Alan G. Whittington

Temperature-dependent thermal diffusivity of the Earth's crust and implications for magmatism

The thermal evolution of planetary crust and lithosphere is largely governed by the rate of heat transfer by conduction. The governing physical properties are thermal diffusivity (κ) and conductivity (k = κρCP), where ρ denotes density and CP denotes specific heat capacity at constant pressure. Although for crustal rocks both κ and k decrease above ambient temperature, most thermal models of the Earth’s lithosphere assume constant values for κ (∼1 mm2 s-1) and/or k (∼3 to 5 W m-1 K-1) owing to the large experimental uncertainties associated with conventional contact methods at high temperatures. Recent advances in laser-flash analysis permit accurate (±2 per cent) measurements on minerals and rocks to geologically relevant temperatures. Here we provide data from laser-flash analysis for three different crustal rock types, showing that κ strongly decreases from 1.5–2.5 mm2 s-1 at ambient conditions, approaching 0.5 mm2 s-1 at mid-crustal temperatures. The latter value is approximately half that commonly assumed, and hot middle to lower crust is therefore a much more effective thermal insulator than previously thought. Above the quartz α–β phase transition, crustal κ is nearly independent of temperature, and similar to that of mantle materials. Calculated values of k indicate that its negative dependence on temperature is smaller than that of κ, owing to the increase of CP with increasing temperature, but k also diminishes by 50 per cent from the surface to the quartz α–β transition. We present models of lithospheric thermal evolution during continental collision and demonstrate that the temperature dependence of κ and CP leads to positive feedback between strain heating in shear zones and more efficient thermal insulation, removing the requirement for unusually high radiogenic heat production to achieve crustal melting temperatures. Positive feedback between heating, increased thermal insulation and partial melting is predicted to occur in many tectonic settings, and in both the crust and the mantle, facilitating crustal reworking and planetary differentiation.

Water and the viscosity of depolymerized aluminosilicate melts

To determine the effect of water on the rheology of depolymerized melts, we have measured the viscosity of two series of hydrated synthetic aluminosilicate melts analogous in bulk polymerization to tephritic and basanitic liquids. The measurements have been made at 1 atm in the range 108.7 to 1013.7 Pa s, for water contents between 0 and 2.5 wt.% H2O. In all cases, water exerts a marked depressing effect on the viscosity but the reduction is much smaller than for the more polymerized compositions studied previously. With the addition of 2 wt.% H2O, for example, extrapolation of the results suggests that the viscosity decreases by ca. 4.5 and 1.5 orders of magnitude at 900 and 1200 K, respectively. An interesting consequence is that the viscosity and glass transition temperatures of the most polymerized melts become lower than those of depolymerized melts at water contents higher than ca. 1 wt.%. By analogy with natural magmatic compositions, hydrous rhyolites may become less viscous than hydrous basalts at high water contents and low temperatures. In nature, the viscosities of polymerized and depolymerized magmatic liquids should be similar because basalts are emplaced at higher temperatures but rhyolites typically have higher water contents.

Lithostratigraphic correlations in the western Himalaya—An isotopic approach

We present the results of a whole-rock Nd isotopic study of two contrasting regions of the western Himalaya, using the neodymium model age approach on the scale of a single orogen. High-grade metasedimentary rocks from Zanskar yield model ages ( T DM ) that are similar to those of the High Himalayan Crystalline Series ( T DM = 1.2–2.0 Ga; ϵ Nd = −6 to −16) and distinct from values from the Lesser Himalaya ( T DM = 2.3–3.4 Ga; ϵ Nd = −18 to −27). Hence these two lithological sequences can be recognized for 2000 km along the strike of the orogen. Data for the basement of the Nanga Parbat massif at the western extremity of the Himalaya ( T DM = 2.3–2.8 Ga; ϵ Nd = −18 to −30) suggest that these rocks are not equivalent to the High Himalaya, as previously supposed, but have affinities with the Lesser Himalaya. A thin metasedimentary cover sequence on the margins of the Nanga Parbat massif is isotopically indistinguishable from the High Himalaya ( T DM = 1.6–1.8 Ga; ϵ Nd = −10 to −14). The prior misidentification of the provenance of the massif stems from its high metamorphic grade, characteristic of the High Himalaya, but in this case related to the unique Neogene history of the Nanga Parbat massif, which has exhumed a higher-grade equivalent of the Lesser Himalaya that is not seen elsewhere.

Dynamics of Strombolian explosions: Inferences from field and laboratory studies of erupted bombs from Stromboli volcano

Strombolian activity is characterized by repeated, low energy, explosions and is named after the volcano where such activity has persisted for around 2000 years, i.e., Stromboli (Aeolian Islands, Italy). Stromboli represents an excellent laboratory where measurements of such explosions can be made from safe, but close, distances. During a field campaign in 2008, two 15 cm diameter bombs were quenched and collected shortly after a normal explosion. The bombs were characterized in terms of their textural, chemical, rheological, and geophysical signatures. The vesicle and crystal size distribution of the samples, coupled with the glass chemistry, enabled us to quantify variations in the degassing history and rheology of the magma resident in the shallow (i.e., in last 250 m of conduit length). The different textural facies observed in these bombs showed that fresh magma was mingled with batches of partially to completely degassed, oxidized, high-crystallinity, high-viscosity, evolved magma. This magma sat at the top of the conduit and played only a passive role in the explosive process. The fresh, microlite-poor, vesiculated batch, however, experienced a response to the explosive event, by undergoing rapid decompression. Integration of geophysical measurements with sample analyses indicates that popular bubble-bursting models may not fit this case. We suggest that the degassed, magma forms a plug, or rheological layer, at the top of the conduit, through which the fresh magma bursts. In this model we need to consider the paradox of a slug ascending too fast through a magma of varying viscosity and yield strength.

Experimental temperature-X(H2O)-viscosity relationship for leucogranites and comparison with synthetic silicic liquids

Viscosities of liquid albite (NaAlSi3O8) and a Himalayan leucogranite were measured near the glass transition at a pressure of one atmosphere for water contents of 0, 2·8 and 3·4 wt.%. Measured viscosities range from 1013·8 Pa. s at 935 K to 109·0 Pa. s at 1119 K for anhydrous granite, and from 1010·2 Pa. s at 760 K to 1012·9 Pa. s at 658 K for granite containing 3·4 wt.% H2O. The leucogranite is the first naturally occurring liquid composition to be investigated over the wide range of T-X(H2O) conditions which may be encountered in both plutonic and volcanic settings. At typical magmatic temperatures of 750°C, the viscosity of the leucogranite is 1011·0 Pa. s for the anhydrous liquid, dropping to 106·5 Pa. s for a water content of 3 wt.% H2O. For the same temperature, the viscosity of liquid NaAlSi3O8 is reduced from 1012·2 to 106·3 Pa. s by the addition of 1·9 wt.% H2O. Combined with published high-temperature viscosity data, these results confirm that water reduces the viscosity of NaAlSi3O8 liquids to a much greater degree than that of natural leucogranitic liquids. Furthermore, the viscosity of NaAlSi3O8 liquid becomes substantially nonArrhenian at water contents as low as 1 wt.% H2O, while that of the leucogranite appears to remain close to Arrhenian to at least 3 wt.% H2O, and viscosity–temperature relationships for hydrous leucogranites must be nearly Arrhenian over a wide range of temperature and viscosity. Therefore, the viscosity of hydrous NaAlSi3O8 liquid does not provide a good model for natural granitic or rhyolitic liquids, especially at lower temperatures and water contents. Qualitatively, the differences can be explained in terms of configurational entropy theory because the addition of water should lead to higher entropies of mixing in simple model compositions than in complex natural compositions. This hypothesis also explains why the water reduces magma viscosity to a larger degree at low temperatures, and is consistent with published viscosity data for hydrous liquid compositions ranging from NaAlSi3O8 and synthetic haplogranites to natural samples. Therefore, predictive models of magma viscosity need to account for compositional variations in more detail than via simple approximations of the degree of polymerisation of the melt structure.

Isotope studies reveal a complete Himalayan section in the Nanga Parbat syntaxis

Many models of orogenesis invoke simple anatomies for mountain belts, comprising a small number of major tectonic provinces separated by major faults. In the Himalayan arc, three main tectonic provinces (the Lesser Himalayan, High Himalayan and Tethyan Series) have been recognized over 2500 km along strike from Bhutan to Kashmir. However, their extension westward to the Nanga Parbat syntaxis remains equivocal. We have supplemented detailed field work in the area with isotopic analysis aimed at revealing distinct signatures for each of the three main provinces. Using Sr isotopic data to refine previous Nd-based discrimination, we demonstrate that the three main tectonic provinces of the central Himalaya also occur in the western syntaxis of northern Pakistan. These three units are thus continuous along the entire orogenic arc. However, their metamorphic grade is generally higher in the syntaxis than in the central Himalaya, challenging the validity of distinctions commonly drawn on this basis elsewhere in the mountain belt. The corollary is that these high-grade units probably continue beyond the syntaxis into northwestern Pakistan, which suggests that, although the precollisional materials may be identical, the western syntaxis marks a change in tectonic style from the main orogen. This conclusion in turn requires that the burial and exhumation history in the western Himalaya be radically different from that in the central Himalaya and thus necessitates a reexamination of models for India-Asia collision.

Gas-driven filter pressing in magmas: Insights into in-situ melt segregation from crystal mushes

Gas-driven filter pressing is the process of melt expulsion from a volatile-saturated crystal mush, induced by the buildup and subsequent release of gas pressure. Filter pressing is inferred to play a major role in magma fractionation at shallow depths (<10 km) by moving melt and gas relative to the solid, crystalline framework. However, the magmatic conditions at which this process operates remain poorly constrained. We present novel experimental data that illustrate how the crystal content of the mush affects the ability of gas-driven filter pressing to segregate melt. Hydrous haplogranite (2.1 wt% water in the melt) and dacite (4.2 wt% water in the melt) crystal mushes, with a wide range of crystallinities (34–80 vol% crystals), were investigated using in-situ, high-temperature (500–800 °C) synchrotron X-ray tomographic microscopy with high spatial (3 μm/pixel) and temporal resolution (∼8 s per three-dimensional data set). Our experimental results show that gas-driven filter pressing operates only below the maximum packing of bubbles and crystals (∼74 vol%). Above this threshold, the mush tends to fracture and gas escapes via fractures. Therefore, the efficiency of gas-driven filter pressing is promoted close to the percolation threshold and in situations where a mush inflates slowly relative to build-up of pressure and expulsion of melt. Such observations offer a likely explanation for the production of eruptible, crystal-poor magmas within Earth’s crust.

Interactions between deformation, magmatism and hydrothermal activity during active crustal thickening; a field example from Nanga Parbat, Pakistan Himalayas

Abstract The Nanga Parbat massif is a rapidly eroding, thrust-related antiform that is distinct from other regions of the Himalayan orogen in being both intruded by Late Miocene-Pliocene anatectic granites and permeated by a vigorous hydrothermal system. Exhumation is achieved by erosion during thrusting along the Liachar thrust in the apparent absence of extensional tectonics. At depths in excess of 20 km, small batches of leucogranitic melt have been generated by fluid-absent breakdown of muscovite from metapelitic lithologies. These melts ascend several kilometres prior to emplacement, aided by low geothermal gradients at depth and by interaction with meteoric water as they reach shallow levels. At intermediate depths ( ~ 15 km) limited fluid infiltration is restricted to shear zones resulting in localised anatexis. Within the upper 8 km of crust, magmatic and meteoric fluid fluxes are channelised by active structures providing a feedback mechanism for lbcusing deformation. Leucogranite sheets show a range of pre-full crystallization and high-temperature crystal-plastic textures indicative of strain localisation onto these sheets and away from the country rocks. At subsolidus temperatures meteoric fluids promote strain localisation and may trigger cataclastic deformation. Since nearsurface geothermal gradients are unusually steep, the macroscopic transition between distributed shearing and substantial, but localised, cataclastic deformation occurred at amphibolite-facies conditions (~600°C). Even with the greatest topographic relief in the world, the meteoric system of Nanga Parbat is effectively restricted to the upper 8 km of the crust, strongly controlled by active structures.

Exhumation overrated at Nanga Parbat, northern Pakistan

Abstract New 40 Ar 39 Ar laserprobe and thermobarometric data from the Nanga Parbat-Haramosh Massif (NPHM) in northern Pakistan provide important constraints on thermal models of the evolving geotherm of the massif. Simple thermal models indicate that the unusually young cooling ages from this area do not necessarily imply extreme exhumation rates of about 7 mm/y, because the advection of heat resulting from rapid exhumation leads to a steepened near-surface geotherm. The most important control on the present-day thermal structure of the NPHM appears to be the thermal structure resulting from obduction of the Kohistan Arc some 50 m.y. ago. Transient geotherms calculated for the past 10 Ma can only be reconciled with published geochronological and thermobarometric studies if exhumation rates lay in the range of 3 to 4 mm/y, still considerably greater than exhumation rates determined from the main Himalayan orogen.

Amorphous Materials: Properties, Structure, and Durability: The viscosity of hydrous NaAlSi3O8 and granitic melts: Configurational entropy models

Abstract We used configurational entropy theory to model the viscosity (η) of hydrous melts of NaAlSi3O8, haplogranite (SiO2-KAlSi3O8-NaAlSi3O8), and complex (natural) granite composition from available measurements and recently published configurational heat-capacity data. The equation log η = Ae + Be/TSconf(T), where Sconf is configurational entropy, reproduces viscosity data for individual samples as well as or better than the empirical three-parameter TVF equation (defined below), and has the advantage of being based on thermodynamic theory. The variables Ae, Be, and Sconf(Tg), where Tg is glass transition temperature, were parameterized as a function of water content for compilations of viscosity data for hydrous NaAlSi3O8, haplogranite, and peraluminous granite melts. With the simplest assumption of ideal mixing between silicate and water components, configurational entropy models with between 4 and 10 fitting parameters reproduce experimentally determined η-T-XH2O relationships significantly better than previous literature models based on empirical equations. Our preferred configurational entropy models have root-mean-square deviations of 0.26 log units for NaAlSi3O8 (n = 77), 0.16 log units for haplogranite (n = 55), and 0.28 log units for peraluminous granites (n = 79). The best statistical fits to the data sometimes require thermodynamically unlikely variations in Ae, Be, and Sconf(Tg) as a function of water content, however, such that further calorimetry data are needed to extract accurate thermodynamic information from viscosity data sets for hydrous melts.