Alberto E. Patiño Douce

What do experiments tell us about the relative contributions of crust and mantle to the origin of granitic magmas

Abstract The origin of different kinds of granitic rocks is examined within the framework of experimental studies of melting of metamorphic rocks, and of reaction between basaltic magmas and metamorphic rocks. Among the types of granitic rocks considered in this chapter, only peraluminous leucogranites represent pure crustal melts. They form by dehydration-melting of muscovite-rich metasediments, most likely during the fast adiabatic decompression that results from tectonic collapse of thickened intracontinental orogenic belts. All other granitic rocks discussed here represent hybrid magmas, formed by reaction of basaltic melts with metamorphic rocks of supracrustal origin. These hybrid rocks include Cordilleran granites, formed at or near convergent continental margins, strongly peraluminous ‘S-type’ granites, alumina-deficient ‘A-type’ granites, and rhyolites associated with continental flood basalts. The differences among these types of granites reflect differences both in their source materials and in the pressures at which mantle-crust interactions take place. In turn, these variables are correlated with the tectonic settings in which the magmas form. Hybrid mafic cumulates are also produced by mantle-crust interactions, simultaneously with the granitic melts. These cumulates range from orthopyroxene + plagioclase-rich assemblages at low pressure to clinopyroxene + garnet-rich assemblages at high pressure, and are known to be important constituents of the lower continental crust. With the exception of peraluminous leucogranites, generation of granitic magmas is almost always associated in space and time with growth, rather than just recycling, of the continental crust.

Melting of Crustal Rocks During Continental Collision and Subduction

The continental crust can partially melt to generate silicic magmas. Compositional diversity among these magmas is determined by the pressure of melting, by the availability of free aqueous fluids, and by the composition of the protolith. Because crustal rocks are subject to extreme pressure and temperature conditions during continental collisions, collisional orogens potentially are environments for magma generation. This chapter discusses the nature of the melts and solid residues likely to be formed in response to continental collision.

Anatexis and metamorphism in tectonically thickened continental crust exemplified by the Sevier hinterland, western North America

The generation of granitoid magmas by partial melting of crustal rocks during continental thickening events is well documented in many geological provinces throughout the world, including the late Mesozoic Sevier belt of western North America. We present a thermal and petrologic model of anatexis and metamorphism in regions of crustal thickening where the only mantle contribution is the normal conductive supply of heat through the base of the lithosphere (i.e. advection of mass and energy are excluded). We distinguish between formation of migmatites and generation of mobile granitoid magmas and examine the temporal and spatial relationships between these two distinct anatectic processes, between anatexis and regional deformation and between anatexis and metamorphism. A fundamental conclusion is that, if protoliths rich in hydrous minerals are present, regional anatexis is the end-product of classical Barrovian metamorphism in thickened continental crust, even in the absence of a free water-rich fluid phase. Barrovian metamorphic facies series are predicted with thickening ratios (maximum crustal thickness attained/initial crustal thickness) as low as 1.3, but mobile granitoid magmas are not formed if this ratio is less than approximately 1.5. Above these lower bounds, Barrovian metamorphism and anatectic granitoid magmatism occur independently of the magnitude of thickening and of the way in which thickening is accomplished. Both processes are sensitive to a diminished heat supply; lowering either Moho heat flow or crustal radioactive heat production results in blueschist-eclogite metamorphism and inhibits the formation of mobile granitoid magmas. We model anatexis under fluid-absent conditions and show that, with such a constraint, migmatization is always a syn-kinematic process (relative to the crustal thickening event), whereas generation of mobile granitoid magmas is in most cases post-kinematic (relative to crustal thickening) but can be syn-kinematic if thickening takes more than approximately 50 Myr. The typical time intervals for melting are consistent with geological observations; mobile granitoid magmas are predicted by most of our models within approximately 10 Myr of the end of the crustal thickening event. This “incubation period” results primarily from the temperature increase required for the dehydration-melting reactions capable of producing large melt fractions to occur. The energetic requirements of anatexis are relatively minor compared to conductive crustal thermal budgets, as shown by the fact that once the necessary P-T conditions are attained, melting reactions are completed within time intervals on the order of 1 Myr, i.e. 1–2 orders of magnitude smaller than the characteristic time scales of the tectonic processes involved in crustal thickening.

Experimental generation of hybrid silicic melts by reaction of high‐Al basalt with metamorphic rocks

Melting and crystallization experiments (T = 1000°C; P = 0.5, 0.7, 1.0, 1.2, and 1.5 GPa) were performed on two hybrid bulk compositions consisting of 50% anhydrous high-Al olivine tholeiite glass (HAOT) and 50% metamorphic rock (aluminosilicate-bearing metapelite and aluminosilicate-free biotite gneiss). The objective of the experiments was to determine phase equilibrium constraints attending bulk assimilation of crustal rocks by basaltic melts. Experiments generated 32–38 wt % of H2O-undersaturated granitic melt (SiO2 > 70 wt %). Thermal and phase equilibrium arguments show that production of this amount of granitic melt at the chosen experimental conditions is a plausible model for bulk assimilation in nature. The compositions of the melts and of the coexisting crystalline assemblages are affected to a comparable extent by pressure and by composition of the crustal source. Reaction of HAOT with biotite gneiss at P ≤ 1.0 GPa produces calc-alkaline melts in equilibrium with gabbronoritic cumulates (plag + opx + cpx). Reaction of HAOT with metapelite at P ≤ 0.7 GPa produces strongly peraluminous melts that resemble S-type granites from the Lachlan Fold Belt, in equilibrium with noritic cumulates (plag + opx ± spi ± gar). At P > 1.2 GPa, both source compositions produce strongly peraluminous leucocratic melts (<2 wt % FeO + MgO + TiO2) in equilibrium with garnet-rich and plagioclase-poor residues (opx + cpx + gar + plag from the biotite gneiss, gar + plag from the metapelite). The experiments show that a wide spectrum of high-SiO2 melts can be hybrids formed directly by reaction of basaltic melts with amphibolite-facies metamorphic rocks. Accumulation of the complementary mafic crystalline assemblages in the deep crust will generate granulites which are neither restitic nor the products of subsolidus dehydration. Mafic granulites and granitic melts formed by reaction of basaltic melts with metamorphic rocks will share isotopic and trace element signatures reflecting inheritance from both crustal and mantle sources.

Effects of pressure and H2O content on the compositions of primary crustal melts

Melting experiments with and without added H 2 O on a model metagreywacke and a natural metapelite demonstrate how pressure and H 2 O content control the compositions of melts and residual assemblages. Several effects are observed under isothermal conditions. Firstly, the stability field of biotite shrinks with decreasing pressure and with increasing H 2 O content, whereas that of plagioclase shrinks with increasing pressure and H 2 O content. Secondly, the ferromagnesian content of melts at the source (i.e. coexisting with their residual assemblages) decreases with decreasing H 2 O activity. Thirdly, with increasing pressure the Ca/Mg and Ca/Fe ratios of melts decrease relative to those of coexisting garnet. As a consequence, a wide spectrum of melts and crystalline residues can be generated from the same source material. For example, H 2 O-starved dehydration melting of metagreywacke at low pressure (≤10 kbar) generates K-rich (granitic) melts that coexist with pyroxene- and plagioclase-rich residues, whereas melting of the same material at high pressure (≍15 kbar) and with minor H 2 O infiltration can generate leucocratic Na-rich and Ca-poor (trondhjemitic) melts that coexist with biotite- and garnet-rich residues. An increased H 2 O content stabilises orthopyroxene at the expense of garnet + biotite + plagioclase, causing melts to shift towards granodioritic or perhaps tonalitic compositions.

Titanium substitution in biotite: an empirical model with applications to thermometry, O2 and H2O barometries, and consequences for biotite stability

Abstract An empirical model for solution of Ti in biotite is developed and calibrated on the basis of various equilibria among biotite and Ti-saturating assemblages. Sources of data used for the calibration include both experimental products and published analyses of natural assemblages. Ti solution in biotite can be satisfactorily modelled by means of the exchange component TiFe −2 , whose non-ideal behavior can be approximated empirically by means of a linear function of the relative concentrations of Ti and octahedral Al in biotite. Within assemblages which fix the chemical potential of the exchange component TiFe −2 , the concentration of this component in biotite is very sensitive to temperature and oxygen fugacity, making it possible to use biotite+oxide assemblages to obtain reliable T and f (O 2 ) estimates in igneous and metamorphic rocks. In assemblages which also contain alkali feldspar, it is possible to use breakdown equilibria of Ti additive components in biotite to estimate H 2 O fugacity. Such uses are likely to find important applications in rocks in which other assemblages sensitive to temperature, f (O 2 ) and/or f (H 2 O) are absent (in general, rocks containing a single oxide phase plus biotite as the only mafic silicate phase, ±alkali feldspar). The empirical model for solution of Ti in biotite also allows to calculate the loci of various terminal reactions for the assemblage titanian biotite+quartz, which, by comparison with equivalent equilibria in Ti-free systems, provide insight into the effect of Ti on the stability of biotite. Solubility of Ti in biotite increases with temperature in a strongly non-linear fashion. As a consequence, calculations predict that the effect of Ti on the stability of biotite is very small up to conditions typical of the beginning of the granulite facies of metamorphism, but beyond those conditions Ti concentration increases markedly and biotite is rendered very refractory. At mid to deep crustal pressures (≥ 10 kbar), the vapor-absent solidus of biotite+quartz in rocks containing Ti-saturating phases is likely to exceed 900°C, even in bulk compositions with subequal concentrations of Fe and Mg.

Fluid absent melting of a layered crustal protolith: implications for the generation of anatectic granites

We report the result of H2O-undersaturated melting experiments on charges consisting of a layer of powdered sillimanite-bearing metapelite (HQ36) and a layer of powdered tonalitic gneiss (AGC150). Experiments were conducted at 10 kbar at 900°, 925° and 950°C. When run alone, the pelite yielded ∼40 vol% strongly peraluminous granitic melt at 900°C while the tonalite produced only ∼5 vol% weakly peraluminous granitic melt. At 950°C, the pelite and the tonalite yielded ∼50 vol% and ∼7 vol% granitic melt, respectively. When run side by side, the abundance of melt in the tonalite was ∼10 times higher at all temperatures than when it was run alone. In the pelite, the melt abundance increased by ∼25 vol%. When run alone, biotite dehydration-melting in the tonalite yielded orthopyroxene and garnet in addition to granitic melt. When run side by side only garnet was produced in addition to granitic melt. Experiments of relatively short duration, however, also contained Al-rich orthopyroxene. We suggest that the large increase in melt fraction in the tonalite is mainly a result of increased activity of Al2O3 in the melt, which lowers the temperature of the biotite dehydration-melting reaction. In the pelite, the increase in the abundance of melt is caused by transport of plagioclase component in the melt from the tonalite-layer to the pelite-layer. This has the effect of changing the bulk composition of this layer in the direction of “minimum-temperature” granitic liquids. Our results show that rocks which are poor melt-producers on their own can become very fertile if they occur in contact with rocks that contain components that destabilize the hydrous phase(s) and facilitate dehydration-melting. Because of this effect, the continental crust may have an even greater potential for granitoid melt production than previously thought. Our results also suggest that many anatectic granites most likely contain contributions from two or more different source rocks, which will be reflected in their isotopic and geochemical compositions.

Fluid-absent melting of F-rich phlogopite+rutile+quartz

Abstract Fluid-absent melting is believed to be an important process in the generation of melts in the lower crust and upper mantle. Breakdown of phlogopite makes H2O available and thus controls the conditions at which fluid-absent melting occurs. Both F and Ti in biotite have been shown to affect strongly the thermal stability of biotite. To model better the fluid-absent melting of assemblages containing phlogopite, the reaction F-phlogopite + quartz + rutile = enstatite + melt has been studied experimentally. Experiments were performed at 7, 10, and 15 kbar using a natural F-rich phlogopite with a starting composition of F/(F + OH) = 0.43 and Mg/(Mg + Fe) (in molar proportions) = 0.94. Results indicate that the thermal stability of F-rich phlogopite + quartz + rutile is extended by as much as 450 °C relative to the KMASH system and by 300 °C relative to the Ti-free F-KMASH system. Approach to equilibrium in the experiments was assessed by convergence of results of melting and crystallization experiments. Phlogopite compositions from experimental products show that, although F-rich phlogopite incorporates relatively little Ti (2-3 wt% TiO2), the combination of F and Ti increases the stability of phlogopite to considerably higher temperatures (~300 °C) than that of either component alone. Melts formed by the fluid-absent melting of F-rich phlogopite + quartz + rutile at temperatures > 1000 °C are granitic and strongly peraluminous. The compositions of these melts suggest that the formation of metaluminous to peralkaline A-type granites by fluid-absent melting of halogen-enriched sources is unlikely.

Calculated relationships between activity of alumina and phase assemblages of silica-saturated igneous rocks: Petrogenetic implications of magmatic cordierite, garnet and aluminosilicate

Abstract Many peraluminous igneous rocks are characterized by the presence of Al-rich minerals such as cordierite, garnet and aluminosilicate polymorphs. Clearly, one of the important thermodynamic intensive variables controlling the stability of these minerals, relative to Al-poor phases such as ortho- and clinopyroxene, must be the activity of Al 2 O 3 ( a Al 2 O 3 ) in the melt. Calculated phase relationships of anhydrous mineral assemblages in equilibrium with quartz-saturated liquids show that, at crustal pressures (less than about 10 kbar), garnet, cordierite, and aluminosilicate are only likely to crystallize from melts in which a Al 2 O 3 is up to one order of magnitude higher than that required for orthopyroxene+clinopyroxene crystallization. In spite of this, peraluminous granitoid liquids saturated with orthopyroxene+garnet or orthopyroxene+cordierite do not ordinarily contain significantly larger amounts of excess Al (= Al in excess of that required to form feldspars) than peraluminous liquids saturated with orthopyroxene+clinopyroxene. These observations indicate that there must be profound compositional controls on the activity coefficient of Al in silicic melts, and the strongest of such controls appears to be the total alkali content of the melt. The activity coefficient of Al in silicic melts varies directly with total alkali oxide content, so that alkali-poor melts can become markedly peraluminous without ever becoming saturated in aluminous phases, whereas alkali-rich melts of similar normative corundum content can crystallize garnet, cordierite or even aluminosilicate. Because activity of alumina in a silicic melt is not a simple function of excess Al content, classification schemes for silicic igneous rocks based on model mineralogical assemblages, which reflect the value of the thermodynamic effective concentration of Al during crystallization, convey more petrologic information than those based on chemical parameters such as normative corundum content or alumina saturation index.

Metallic Mineral Resources in the Twenty-First Century. I. Historical Extraction Trends and Expected Demand

Industrial, technological, and economic developments depend on the availability of metallic raw materials. As a greater fraction of the Earth’s population has become part of developed economies and as developed societies have become more affluent, the demand on metallic mineral resources has increased. Yet metallic minerals are non-renewable natural resources, the supply of which, even if unknown and potentially large, is finite. An analysis of historical extraction trends for eighteen metals, going back to the year 1900, demonstrates that demand of metallic raw materials has increased as a result of both increase in world population and increase in per-capita consumption. These eighteen metals can be arranged into four distinct groups, for each of which it is possible to identify a consistent pattern of per-capita demand as a function of time. These patterns can, in turn, be explained in terms of the industrial and technological applications, and in some cases conventional uses as well, of the metals in each group. Under the assumption that these patterns will continue into the future, and that world population will grow by no more than about 50% by the year 2100, one can estimate the amount of metallic raw materials that will be required to sustain the world’s economy throughout the twenty-first century. From the present until the year 2100, the world can be expected to require about one order of magnitude more metal than the total amount of metal that fueled technological and economic growth between the age of steam and the present day. For most of the metals considered here, this corresponds to 5–10 times the amount of metal contained in proven ore reserves. The two chief driving factors of this expected demand are growth in per-capita consumption and present-day absolute population numbers. World population is already so large that additional population growth makes only a small contribution to the expected future demand of metallic raw materials. It is not known whether or not the amount of metal required to sustain the world’s economy throughout this century exists in exploitable mineral resources. In the accompanying paper, I show that it is nevertheless possible to make statistical inferences about the size distribution of the mineral deposits that will need to be discovered and developed in order to satisfy the expected demand. Those results neither prove nor disprove that the needed resources exist but can be used to improve our understanding of the challenges facing future supply of metallic raw materials.