M. B. Baker

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

We present a new experimental technique for circumventing the quenching problems that have plagued high-pressure peridotite melting studies. A thin layer of ~50 μm diamonds is placed above a layer of peridotite powder. Partial melt extracted from the peridotite layer collects in the pore spaces between the diamonds and equilibrates diffusively with the residual peridotite mineralogy. Isolated from the crystalline residue, the melt quenches to a glass that records the composition of the liquid coexisting with the residual crystalline phases under the conditions of the experiment. We have used this technique to investigate partial melting of a fertile mantle composition at 10 kbar and a temperature range of 1270–1390°C. Oxide concentrations in the liquids from the longest duration runs (up to 151 hours) vary systematically with increasing temperature: TiO_2, Al_2O_3, and Na_2O decrease monotonically, while Cr_2O_3, FeO^∗, and MgO increase steadily. CaO shows more complicated behavior, first increasing and then decreasing, with the crest in the temperature-CaO trend approximately coincident with the disappearance of clinopyroxene from the residue between 1330 and 1350°C. Overall variation in silica content with temperature is small, and there appears to be a minimum at about 12% melting. The compositions of liquids produced in time series, temperature reversal, and two-stage experiments (conducted to test the technique) all indicate that our experimentally determined liquid compositions represent close approaches to equilibrium. Calculated melt fractions (F) also vary systematically with temperature. The slope of the T(°C)-F curve is not constant over the spinel lherzolite melting interval, but decreases as temperature increases from 1270 to 1330°C. Extrapolating the curve back to zero melt suggests that the anhydrous solidus temperature for our peridotite starting composition is ~ 1240°C. At temperatures below the cpxout curve, melt generation occurs via the reaction, 0.38 opx + 0.71 cpx + 0.13 sp → 0.22 oliv + 1.0 liq, and the proportions of minerals that enter the melt appear to be independent of temperature. At temperatures above cpx-out, the less well constrained melting reaction is: 1.06 opx + 0.04 sp = 0. 1 oliv + 1 liq. The fact that all of the 10 kbar melts have FeO^∗ contents that are substantially lower than those reported in any primitive MORB glasses further strengthens the conclusions that these glasses are not 10 kbar primary melts, that they involve a component of higher pressure partial melting, and that they have evolved by significant olivine fractionation from more primitive liquids. Our experimental data also provide an independent check of the results of recent peridotite partial melting calculations. Efforts to parameterize the experimental database on peridotite melting, and to calculate melt compositions as a function of P, T, and F are partially successful in reproducing the compositional trends determined in this study.

Alkalic magmas generated by partial melting of garnet pyroxenite

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.

Metasomatized Lithosphere and the Origin of Alkaline Lavas

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 1.5 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.

Coupled CaAl-NaSi diffusion in plagioclase feldspar: Experiments and applications to cooling rate speedometry

Abstract The rate of CaAl-NaSi interdiffusion in plagioclase feldspar was determined under 1 atm anhydrous conditions over the temperature range 1400° to 1000°C in calcic plagioclase (An80−81) by homogenizing coherent exsolution lamellae. The dependence of the average interdiffusion coefficient on temperature is given by the expression: D = 10.99 ( cm 2 /sec) exp (−123.4( kcal/mol )/RT), (T in °K). This value is for diffusion perpendicular to the (03 1) interface of the lamellae. CaAl-NaSi interdiffusion is 4 to 5 orders of magnitude slower than oxygen diffusion in the temperature range 1400° to 1200°C and possibly 10 orders of magnitude slower at subsolidus temperatures. The large differences in diffusion rates explain the apparent contradiction posed by the plagioclases of large layered intrusions (e.g., the Skaergaard), which retain delicate Ca, Na compositional zoning profiles on the micron scale, but have undergone complete oxygen isotopic exchange with heated meteoric groundwater from the surrounding wall rocks. CaAl-NaSi diffusion is slow, the closure temperature is high (within the solidus-liquidus interval), and Ca-Na zoning is preserved. Oxygen diffusion is faster, the closure temperature is lower (350°-400°C) and the feldspars exchange oxygen with the low-temperature hydrothermal fluids. The complex micron-scale oscillatory zones in plagioclase can also be used as cooling rate speedometers for volcanic and plutonic plagioclase. Cooling histories typical of large mafic intrusions (e.g. the Stillwater) are slow, begin at high initial temperatures (1200°C) and result in homogenization of oscillatory zones on the scale of 10 microns. The oscillatory zones found in the plagioclase of granodioritic plutons are preserved because cooling is initiated at a lower temperature (1000°C) limiting diffusion to submicron length scales despite the slow cooling rate of the intrusion.

Primitive basalts and andesites from the Mt. Shasta region, N. California: products of varying melt fraction and water content

Quaternary volcanism in the Mt. Shasta region has produced primitive magmas [Mg/(Mg+Fe*)>0.7, MgO>8 wt% and Ni>150 ppm] ranging in composition from high-alumina basalt to andesite and these record variable extents ofmelting in their mantle source. Trace and major element chemical variations, petrologic evidence and the results of phase equilibrium studies are consistent with variations in H2O content in the mantle source as the primary control on the differences in extent of melting. High-SiO2, high-MgO (SiO2=52% and MgO=11 wt%) basaltic andesites resemble hydrous melts (H2O=3 to 5 wt%) in equilibrium with a depleted harzburgite residue. These magmas represent depletion of the mantle source by 20 to 30 wt% melting. High-SiO2, high-MgO (SiO2=58% and MgO=9 wt%) andesites are produced by higher degrees of melting and contain evidence for higher H2O contents (H2O=6 wt%). High-alumina basalts (SiO2=48.5% and Al2O3=17 wt%) represent nearly anhydrous low degree partial melts (from 6 to 10% depletion) of a mantle source that has been only slightly enriched by a fluid component derived from the subducted slab. The temperatures and pressures of last equilibration with upper mantle are 1200°C and 1300°C for the basaltic andesite and basaltic magmas, respectively. A model is developed that satisfies the petrologic temperature constraints and involves magma generation whereby a heterogeneous distribution of H2O in the mantle results in the production of a spectrum of mantle melts ranging from wet (calc-alkaline) to dry (tholeiitic).

The Effect of Alkalis on the Silica Content of Mantle-Derived Melts

A large body of experimental evidence shows that at low and moderate pressure (<1.5 GPa), alkali-rich silicate liquids coexisting with Mg-rich olivine and orthopyroxene are richer in silica than typical basalts. This phenomenon is caused by the tendency of alkali ions to reduce the number of Si-O-Si linkages in the melt, which translates to negative deviations from ideality for mixing between alkalis and silica and which requires increases in alkalis to be accompanied by increases in silica for liquids in equilibrium with mantle peridotite. P_2O_5 and TiO_2 have an effect opposite to alkalis, and when these elements are also enriched in the liquid, the high silica contents caused by alkali-enrichment may be reduced or eliminated. The effect of alkalis on the silica content of melts equilibrated with magnesian olivine and orthopyroxene is reduced at higher pressure, such that silica enrichments in alkali-rich melts will be small if the equilibration pressure is greater than ∼1.5 GPa. This pressure effect is largely the result of decreases with pressure in the extent of polymerization for all olivine + orthopyroxene-saturated liquids. As pressure increases and liquids in equilibrium with olivine and orthopyroxene become less polymerized, proportionally fewer alkalis break up highly polymerized (Q^4) silica tetrahedra, and, therefore, alkalis have less effect on the activity coefficient of silica. Secondarily, the observed changes with pressure may also be related to changes in the energetics of alkali-silica interactions. Because equilibration of alkali-rich melts with mantle peridotite only produces high silica at low and moderate pressures, small degree partial melts of anhydrous peridotite formed during adiabatic upwelling will not typically be silica-rich. However, if liquids rich in alkalis, perhaps formed by selective leaching of Na_2O and K_2O from peridotite during upward percolation, equilibrate with the mantle at depths <1.5 GPa, they will become silica-rich. Such silica-rich liquids, now preserved as glass inclusions in spinel peridotite xenoliths, are probably restricted to the shallowest part of the mantle (<45 km).

The Petrochemistry of Jake_M: A Martian Mugearite

“Jake_M,” the first rock analyzed by the Alpha Particle X-ray Spectrometer instrument on the Curiosity rover, differs substantially in chemical composition from other known martian igneous rocks: It is alkaline (>15% normative nepheline) and relatively fractionated. Jake_M is compositionally similar to terrestrial mugearites, a rock type typically found at ocean islands and continental rifts. By analogy with these comparable terrestrial rocks, Jake_M could have been produced by extensive fractional crystallization of a primary alkaline or transitional magma at elevated pressure, with or without elevated water contents. The discovery of Jake_M suggests that alkaline magmas may be more abundant on Mars than on Earth and that Curiosity could encounter even more fractionated alkaline rocks (for example, phonolites and trachytes).

Assimilation of granite by basaltic magma at Burnt Lava flow, Medicine Lake volcano, northern California: Decoupling of heat and mass transfer

At Medicine Lake volcano, California, andesite of the Holocene Burnt Lava flow has been produced by fractional crystallization of parental high alumina basalt (HAB) accompanied by assimilation of granitic crustal material. Burnt Lava contains inclusions of quenched HAB liquid, a potential parent magma of the andesite, highly melted granitic crustal xenoliths, and xenocryst assemblages which provide a record of the fractional crystallization and crustal assimilation process. Samples of granitic crustal material occur as xenoliths in other Holocene and Pleistocene lavas, and these xenoliths are used to constrain geochemical models of the assimilation process.A large amount of assimilation accompanied fractional crystallization to produce the contaminated Burnt lava andesites. Models which assume that assimilation and fractionation occurred simultaneously estimate the ratio of assimilation to fractional crystallization (R) to be >1 and best fits to all geochemical data are at an R value of 1.35 at F=0.68. Petrologic evidence, however, indicates that the assimilation process did not involve continuous addition of granitic crust as fractionation occurred. Instead, heat and mass transfer were separated in space and time. During the assimilation process, HAB magma underwent large amounts of fractional crystallization which was not accompanied by significant amounts of assimilation. This fractionation process supplied heat to melt granitic crust. The models proposed to explain the contamination process involve fractionation, replenishment by parental HAB, and mixing of evolved and parental magmas with melted granitic crust.

Glass in the submarine section of the HSDP2 drill core, Hilo, Hawaii

The Hawaii Scientific Drilling Project recovered ~3 km of basalt by coring into the flank of Mauna Kea volcano at Hilo, Hawaii. Rocks recovered from deeper than ~1 km were deposited below sea level and contain considerable fresh glass. We report electron microprobe analyses of 531 glasses from the submarine section of the core, providing a high-resolution record of petrogenesis over ca. 200 Kyr of shield building of a Hawaiian volcano. Nearly all the submarine glasses are tholeiitic. SiO2 contents span a significant range but are bimodally distributed, leading to the identification of low-SiO2 and high-SiO2 magma series that encompass most samples. The two groups are also generally distinguishable using other major and minor elements and certain isotopic and incompatible trace element ratios. On the basis of distributions of high- and low-SiO2 glasses, the submarine section of the core is divided into four zones. In zone 1 (1079–~1950 mbsl), most samples are degassed high-SiO2 hyaloclastites and massive lavas, but there are narrow intervals of low-SiO2 hyaloclastites. Zone 2 (~1950–2233 mbsl), a zone of degassed pillows and hyaloclastites, displays a continuous decrease in silica content from bottom to top. In zone 3 (2233–2481 mbsl), nearly all samples are undegassed low-SiO2 pillows. In zone 4 (2481–3098 mbsl), samples are mostly high-SiO2 undegassed pillows and degassed hyaloclastites. This zone also contains most of the intrusive units in the core, all of which are undegassed and most of which are low-SiO2. Phase equilibrium data suggest that parental magmas of the low-SiO2 suite could be produced by partial melting of fertile peridotite at 30–40 kbar. Although the high-SiO2 parents could have equilibrated with harzburgite at 15–20 kbar, they could have been produced neither simply by higher degrees of melting of the sources of the low-SiO2 parents nor by mixing of known dacitic melts of pyroxenite/eclogite with the low-SiO2 parents. Our hypothesis for the relationship between these magma types is that as the low-SiO2 magmas ascended from their sources, they interacted chemically and thermally with overlying peridotites, resulting in dissolution of orthopyroxene and clinopyroxene and precipitation of olivine, thereby generating high-SiO2 magmas. There are glasses with CaO, Al2O3, and SiO2 contents slightly elevated relative to most low-SiO2 samples; we suggest that these differences reflect involvement of pyroxene-rich lithologies in the petrogenesis of the CaO-Al2O3-enriched glasses. There is also a small group of low-SiO2 glasses distinguished by elevated K2O and CaO contents; the sources of these samples may have been enriched in slab-derived fluid/melts. Low-SiO2 glasses from the top of zone 3 (2233–2280 mbsl) are more alkaline, more fractionated, and incompatible-element-enriched relative to other glasses from zone 3. This excursion at the top of zone 3, which is abruptly overlain by more silica-rich tholeiitic magmas, is reminiscent of the end of Mauna Kea shield building higher in the core.

The origin of abyssal peridotites: a reinterpretation of constraints based on primary bulk compositions

We calculated primary bulk compositions for a global suite of abyssal peridotites using primary mineral modes and either analyzed or calculated phase compositions. The latter were obtained through correlations between reported mineral compositions and modal olivine contents. Both the modal data and the mineral compositions were averaged by dredge site, drill hole, or fracture zone (FZ) depending on the amount of available data. Our calculated abyssal peridotite compositions yield major-element oxide-MgO trends that are generally in good agreement with those based on compilations of ultramafic nodules and peridotite massifs. In particular, we find no statistically significant correlation between FeO* (total Fe as FeO) and MgO and, therefore, no evidence for significant olivine accumulation. Previous reports of a positive correlation reflect an artifact of the regressions used to calculate missing phase compositions and result in a relationship between the Mg# of olivine and modal olivine abundance that is inconsistent with observed variations in abyssal peridotites. There is a slight positive correlation between bulk FeO* and MgO if individual thin sections are used to derive the mineral composition versus modal olivine regressions, but the large grain sizes and heterogeneous distributions of phases within abyssal peridotities make it unlikely that individual thin section modes accurately reflect phase proportions in meter-sized dredge-haul samples. The variability of Na and Ti contents in pyroxenes from plagioclase-free abyssal peridotites suggests to us, as it has to other workers, that a majority of these samples interacted to varying degrees with small amounts of melt. On the other hand, lower bounds on Na and Ti contents in the pyroxenes at a given dredge site as a function of modal olivine content are broadly consistent with calculated partial melting residues. Thus, abyssal peridotites may retain information both on the original partial melting process and on concurrent or later interactions with partial melts from other sources.