Jacqueline E. Dixon

Recycled dehydrated lithosphere observed in plume-influenced mid-ocean-ridge basalt

A substantial uncertainty in the Earths global geochemical water cycle is the amount of water that enters the deep mantle through the subduction and recycling of hydrated oceanic lithosphere. Here we address the question of recycling of water into the deep mantle by characterizing the volatile contents of different mantle components as sampled by ocean island basalts and mid-ocean-ridge basalts. Although all mantle plume (ocean island) basalts seem to contain more water than mid-ocean-ridge basalts, we demonstrate that basalts associated with mantle plume components containing subducted lithosphere—‘enriched-mantle’ or ‘EM-type’ basalts—contain less water than those associated with a common mantle source. We interpret this depletion as indicating that water is extracted from the lithosphere during the subduction process, with greater than 92 per cent efficiency.

A hydrous melting and fractionation model for mid‐ocean ridge basalts: Application to the Mid‐Atlantic Ridge near the Azores

The major element, trace element, and isotopic composition of mid-ocean ridge basalt glasses affected by the Azores hotspot are strongly correlated with H2O content of the glass. Distinguishing the relative importance of source chemistry and potential temperature in ridge-hotspot interaction therefore requires a comprehensive model that accounts for the effect of H2O in the source on melting behavior and for the effect of H2O in primitive liquids on the fractionation path. We develop such a model by coupling the latest version of the MELTS algorithm to a model for partitioning of water among silicate melts and nominally anhydrous minerals. We find that much of the variation in all major oxides except TiO2 and a significant fraction of the crustal thickness anomaly at the Azores platform are explained by the combined effects on melting and fractionation of up to ~700 ppm H2O in the source with only a small thermal anomaly, particularly if there is a small component of buoyantly driven active flow associated with the more H2O-rich melting regimes. An on-axis thermal anomaly of ~35°C in potential temperature explains the full crustal thickness increase of ~4 km approaching the Azores platform, whereas a ≥75°C thermal anomaly would be required in the absence of water or active flow. The polybaric hydrous melting and fractionation model allows us to solve for the TiO2, trace element and isotopic composition of the H2O-rich component in a way that self-consistently accounts for the changes in the melting and fractionation regimes resulting from enrichment, although the presence and concentration in the enriched component of elements more compatible than Dy cannot be resolved.

SIMS analysis of volatiles in silicate glasses 1. Calibration, matrix effects and comparisons with FTIR

This paper describes microanalysis techniques using secondary ion mass spectrometry (SIMS) to measure the abundances and isotopic compositions of hydrogen, carbon, fluorine, sulfur and chlorine in volcanic glasses. SIMS measurement of total H_2O and total CO_2 abundances compare very well with measurements on the same glasses using vibrational spectroscopy techniques (FTIR). A typical 10-min SIMS measurement for volatile abundances is made on a singly polished specimen, sputtering a crater 15–30 μm in diameter and 2–3 μm deep, utilizing 1–5×10^(−9) g of sample material. Detection limits are routinely <30 ppm H_2O, <3 ppm CO_2, and <1 ppm F, S and Cl. Measurements of δD, δ^(13)C and δ^(34)S in volcanic glasses are currently reproducible and accurate to 2–5‰, depending on the concentration of the element. Because of their spatial selectivity, the SIMS methods allow resolution of magmatic volatile signatures from those carried by secondary phases, which can sometimes plague traditional vacuum extraction methods that require large amounts of sample (tens to hundreds of milligrams). Ease of sample preparation, rapid analysis and high sensitivity allow SIMS to be applied to volatile analysis of small samples such as melt inclusions, in which large numbers of individual analyses are often required in order to obtain a representative sample population. Combined abundance and isotopic composition data for volatile elements provide coupled constraints on processes relevant to magma genesis and evolution, including degassing, magma contamination, mixing, and source variability.

Degassing of alkalic basalts

Abstract In order to model quantitatively exsolution of volatiles over the range of basaltic melt compositions found on oceanic islands, I present compositional parameterizations of H2O and CO2 solubilities and use these parameterizations to develop vapor saturation and degassing models for alkalic basaltic liquids. Vapor-saturation diagrams generated as a function of melt composition are used to determine the pressure at which the melt was last in equilibrium with a vapor and the composition of the vapor phase based on measured H2O and CO2 contents in basaltic glasses. These models allow the calculation of the pressure at which a magma of known initial volatile content reaches vapor saturation and begins to exsolve a vapor phase. The higher solubility of CO2 in alkalic magmas causes vapor saturation in CO2-bearing alkalic magmas to be reached at lower pressures than in CO2- bearing tholeiitic magmas having identical volatile contents. However, if variations in major element and volatile concentrations were linked by variations in the extent of melting, then volatile-rich, strongly alkalic magmas would begin to exsolve a vapor at slightly higher pressures than volatile-poor alkali olivine basalts or tholeiites. Partitioning of H2O and CO2 into the vapor during volatile exsolution is controlled by the difference between H2O and CO2 solubilities. As melts become more alkalic, the relative difference between H2O and CO2 solubilities decreases, thus diminishing the preferential partitioning of CO2 into the vapor. Exsolution of volatiles from tholeiites is characterized by strong partitioning of CO2 into the vapor such that most or all CO2 is lost before any significant loss of H2O. In contrast, the combination of higher CO2 solubility and higher volatile contents (and perhaps higher CO2/H2O ratio) in alkalic melts results in less fractionation between CO2 and H2O during volatile exsolution.

Degassing history of water, sulfur, and carbon in submarine lavas from Kilauea volcano, Hawaii

Major, minor, and dissolved volatile element concentrations were measured in tholeiitic glasses from the submarine portion (Puna Ridge) of the east rift zone of Kilauea Volcano, Hawaii. Dissolved

DETERMINATION OF THE MOLAR ABSORPTIVITY OF DISSOLVED CARBONATE IN BASANITIC GLASS

H_{2}O

Submarine Strombolian Eruptions on the Gorda Mid‐Ocean Ridge

and S concentrations display a wide range relative to nonvolatile incompatible elements at all depths. This range cannot be readily explained by fractional crystallization, degassing of