Major and trace element composition of the depleted MORB mantle (DMM)

Knowledge of temperature and pressure, however qualitative, has been central to our views of geology since at least the early 19th century. In 1822, for example, Charles Daubeny presented what may be the very first “Geological Thermometer,” comparing temperatures of various geologic processes (Torrens 2006). Daubeny (1835) may even have been the first to measure the temperature of a lava flow, by laying a thermometer on the top of a flow at Vesuvius—albeit several months following the eruption, after intervening rain (his estimate was 390°F). In any case, pressure ( P ) and temperature ( T ) estimation lie at the heart of fundamental questions: How hot is Earth, and at what rate has the planet cooled. Are volcanoes the products of thermally driven mantle plumes? Where are magmas stored, and how are they transported to the surface—and how do storage and transport relate to plate tectonics? Well-calibrated thermometers and barometers are essential tools if we are to fully appreciate the driving forces and inner workings of volcanic systems.

This chapter presents methods to estimate the P-T conditions of volcanic and other igneous processes. The coverage includes a review of existing geothermometers and geobarometers, and a presentation of approximately 30 new models, including a new plagioclase-liquid hygrometer. Our emphasis is on experimentally calibrated “thermobarometers,” based on analytic expressions using P or T as dependent variables. For numerical reasons (touched on below) such expressions will always provide the most accurate means of P-T estimation, and are also most easily employed. Analytical expressions also allow error to be ascertained; in the absence of estimates of error, P-T estimates are nearly meaningless. This chapter is intended to complement the chapters by Anderson et al. (2008), who cover granitic systems, and by Blundy and Cashman (2008) and Hansteen and Klugel (2008), who consider additional methods …

Thermometers and Barometers for Volcanic Systems

A model for the generation of intermediate and silicic igneous rocks is presented, based on experimental data and numerical modelling. The model is directed at subduction-related magmatism, but has general applicability to magmas generated in other plate tectonic settings, including continental rift zones. In the model mantlederived hydrous basalts emplaced as a succession of sills into the lower crust generate a deep crustal hot zone. Numerical modelling of the hot zone shows that melts are generated from two distinct sources; partial crystallization of basalt sills to produce residual H2O-rich melts; and partial melting of pre-existing crustal rocks. Incubation times between the injection of the first sill and generation of residual melts from basalt crystallization are controlled by the initial geotherm, the magma input rate and the emplacement depth. After this incubation period, the melt fraction and composition of residual melts are controlled by the temperature of the crust into which the basalt is intruded. Heat and H2O transfer from the crystallizing basalt promote partial melting of the surrounding crust, which can include meta-sedimentary and meta-igneous basement rocks and earlier basalt intrusions. Mixing of residual and crustal partial melts leads to diversity in isotope and trace element chemistry. Hot zone melts are H2O-rich. Consequently, they have low viscosity and density, and can readily detach from their source and ascend rapidly. In the case of adiabatic ascent the magma attains a super-liquidus state, because of the relative slopes of the adiabat and the liquidus. This leads to resorption of any entrained crystals or country rock xenoliths. Crystallization begins only when the ascending magma intersects its H2O-saturated liquidus at shallow depths. Decompression and degassing are the driving forces behind crystallization, which takes place at shallow depth on timescales of decades or less. Degassing and crystallization at shallow depth lead to large increases in viscosity and stalling of the magma to form volcano-feeding magma chambers and shallow plutons. It is proposed that chemical diversity in arc magmas is largely acquired in the lower crust, whereas textural diversity is related to shallow-level crystallization.

/pdf/the-genesis-of-intermediate-and-silicic-magmas-in-deep-3g95p0zv3p.pdf

The Genesis of Intermediate and Silicic Magmas in Deep Crustal Hot Zones

In this manuscript we review experimental constraints for the viscosity of the upper mantle. We first analyze experimental data to provide a critical review of flow law parameters for olivine aggregates and single crystals deformed in the diffusion creep and dislocation creep regimes under both wet and dry conditions. Using reasonable values for the physical state of the upper mantle, the viscosities predicted by extrapolation of the experimental flow laws compare well with independent estimates for the viscosity of the oceanic mantle, which is approximately 10 19 Pa s at a depth of ∼100 km. The viscosity of the mantle wedge of subduction zones could be even lower if the flux of water through it can result in olivine water contents greater than those estimated for the oceanic asthenosphere and promote the onset of melting. Calculations of the partitioning of water between hydrous melt and mantle peridotite suggest that the water content of the residue of arc melting is similar to that estimated for the asthenosphere. Thus, transport of water from the slab into the mantle wedge can continually replenish the water content of the upper mantle and facilitate the existence of a low viscosity asthenosphere.

Rheology of the Upper Mantle and the Mantle Wedge: A View from the Experimentalists

Phase relations of natural aphyric high-alumina basalts and their intrusive equivalents were determined through rock-melting experiments at 2 kb, H2O-saturated with fO2 buffered at NNO. Experimental liquids are low-MgO high-alumina basalt or basaltic andesite, and most are saturated with olivine, calcic plagioclase, and either high-calcium pyroxene or hornblende (±magnetite). Cr-spinel or magnetite appear near the liquidus of wet high-alumina basalts because H2O lowers the appearance temperature of crystalline silicates but has a lesser effect on spinel. As a consequence, experimental liquids follow calcalkaline differentiation trends. Hornblende stability is sensitive to the Na2O content of the bulk composition as well as to H2O content, with the result that hornblende can form as a near liquidus mineral in wet sodic basalts, but does not appear until liquids reach andesitic compositions in moderate Na2O basalts. Therefore, the absence of hornblende in basalts with low-to-moderate Na2O contents is not evidence that those basalts are nearly dry. Very calcic plagioclase (>An90) forms from basaltic melts with high H2O contents but cannot form from dry melts with normal are Na2O and CaO abundances. The presence of anorthite-rich plagioclase in high-alumina basalts indicates high magmatic H2O contents. In sum, moderate pressure H2O-saturated phase relations show that magmatic H2O leads to the early crystallization of spinel, produces calcic plagioclase, and reduces the total proportion of plagioclase in the crystallizing assemblage, thereby promoting the development of the calc-alkaline differentiation trend.

Experimental investigations of the role of H2O in calc-alkaline differentiation and subduction zone magmatism

Determining the composition of high-pressure mantle melts using diamond aggregates

Recycled oceanic crust, with or without sediment, is often invoked as a source component of continental and oceanic alkaline magmas to account for their trace-element and isotopic characteristics Alternatively, these features have been attributed to sources containing veined, metasomatized lithosphere In melting experiments on natural amphibole-rich veins at 15 gigapascals, we found that partial melts of metasomatic veins can reproduce key major- and trace-element features of oceanic and continental alkaline magmas Moreover, experiments with hornblendite plus lherzolite showed that reaction of melts of amphibole-rich veins with surrounding lherzolite can explain observed compositional trends from nephelinites to alkali olivine basalts We conclude that melting of metasomatized lithosphere is a viable alternative to models of alkaline basalt formation by melting of recycled oceanic crust with or without sediment

http://science.sciencemag.org/content/sci/320/5878/916.full.pdf

Metasomatized lithosphere and the origin of alkaline lavas.

Many oceanic-island basalts (OIBs) with isotopic signatures of recycled crustal components are silica poor and strongly nepheline (ne) normative and therefore unlike the silicic liquids generated from partial melting of recycled mid-oceanic-ridge basalt (MORB). High-pressure partial-melting experiments on a garnet pyroxenite (MIX1G) at 2.0 and 2.5 GPa produce strongly ne-normative and silica-poor partial melts. The MIX1G solidus is located below 1350 and 1400 8C at 2 and 2.5 GPa, respectively, slightly cooler than the solidus of dry peridotite. Chemographic analysis suggests that natural garnet pyroxenite compositions straddle a thermal divide. Whereas partial melts of compositions on the silica-excess side of the divide (such as recycled MORB) are silica saturated, those from silica-deficient garnet pyroxenites can be alkalic and have similar- ities to low-silica OIB. Although the experimental partial melts are too rich in Al2O3 to be parental to highly undersaturated OIB suites, higher-pressure (4-5 GPa) partial melting of garnet pyrox- enite is expected to yield more appropriate parental liquids for OIB lavas. Silica-deficient garnet pyroxenite, which may originate by mixing of MORB with peridotite, or by recycling of other mafic lithologies, represents a plausible source of OIB that may resolve the apparent contradiction of strongly alkalic composition with iso- topic ratios characteristic of a recycled component.

http://www.mantleplumes.org/WebDocuments/Hirschmannetal2003Geology.pdf

Alkalic magmas generated by partial melting of garnet pyroxenite

This paper presents analyses of the trace element abundances and isotopic compositions in primitive lavas from the Mt. Shasta region, N California. These data are combined with estimates of pre-eruptive H2O contents and constraints from experimental petrology to develop a model of subduction zone magmatism. These lavas share geochemical characteristics of high-Mg andesites from the Setouchi volcanic belt in SW Japan and Adak-type high-Mg andesites of the western Aleutian arc. Estimates of the pre-eruptive water contents of the Shasta region lavas range from 8 wt % H2O. The pre-eruptive H2O content and an inferred melt of a harzburgitic residue are used to carry out a mass balance for the relative contributions from a mantle-derived melt and slab-derived fluid-rich component. We assume that elements are contributed either from melting of mantle peridotite or from a subduction-related fluid-rich component. Estimated fluid-rich component compositions are characterized by strong light rare earth element (LREE) enrichments ([La/Gd]N=3 to 7) and variable heavy rare earth element (HREE) depletions ([Dy/Yb]N=1 to 3). Sr and Ba abundances vary by approximately a factor of 2.5 in the fluid compositions calculated for the Mt. Shasta region lavas and large ion lithophile element (LILE) abundances are similar to those calculated by Stolper and Newman (1994) and Eiler et al. (2000). The major elements in the fluid-rich component are H2O (~55–68 wt%), Na2O (~25–33 wt%) and K2O (~5–13 wt%). This composition may be that of a supercritical fluid or a low-degree melt of the slab that has reacted with the overlying mantle wedge. Although the slab beneath Mt. Shasta is inferred to be hot (~600 – 650 °C), the calculated fluid-rich components do not resemble a pure slab melt. The calculated isotopic composition of the fluid-rich component is bimodal. One component has 87Sr/86Sr=0.7028 and eNd=+8, and is most similar to a MORB source. The second component has more radiogenic 87Sr/86Sr=0.7038 and eNd=+1 and is most similar to a sediment. These fluid-rich components probably represent a mixture of fluids and melts from the slab (serpentinized mantle, altered basalt, and sediment).

The role of an H2O-rich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region, N California

Mid-ocean-ridge basalts (MORBs) are probably a mixture of liquids formed by near-fractional melting of upwelling mantle over a range of pressures. Thus, an understanding of MORB genesis requires knowledge of the compositions of near-solidus melts of mantle peridotite, which have not been measured experimentally. Here we present the results of melting experiments on peridotite using a two-stage diamond-aggregate extraction technique, and the results of thermodynamic calculations of peridotite melting. Both the experiments and the calculations show that at 10 kbar, near-solidus melts (melt fraction F = 0.02-0.05) of fertile peridotite are enriched in SiO_2, A1_(2)O_3 and Na_(2)O and depleted in FeO* (all iron as FeO), MgO and CaO relative to higher-degree melts. At F≈0.02, the partial melt has ~57 wt% SiO_2 and is qualitatively similar to silica-rich melt inclusions found in spinel peridotites worldwide. At low melt fractions (F≤0.08), measured and calculated clinopv roxene/ liquid (cpx/liq) partition coefficients for TiO_2 are larger than those calculated from cpx-liq pairs in higher-melt-fraction experiments. This change in titanium partitioning just above the fertile peridotite solidus implies that other highly charged elements (such as Hf, Zr, Th and U) exhibit similarly complex behaviour during the initial stages of mantle melting.

Michael B. Baker

Papers

Determining the composition of high-pressure mantle melts using diamond aggregates

Metasomatized lithosphere and the origin of alkaline lavas.

Alkalic magmas generated by partial melting of garnet pyroxenite

The role of an H2O-rich fluid component in the generation of primitive basaltic andesites and andesites from the Mt. Shasta region, N California

Compositions of near-solidus peridotite melts from experiments and thermodynamic calculations