ALBERT

Notes: We have developed a set of four synthetic standards for the rare earth and high field strength elements designed for use in the determination of those elements in silicates. The base material is a glass nominally at the eutectic of the MgO-Al2O3-SiO2 ternary system. The nominal doping level was 2 wt% of the elements as oxides. To avoid problems associated with peak interference, the elements were doped in four separate glasses.

Notes: Abstract Our current lack of understanding of the partitioning behavior of Sc, Y and the REE (rare-earth elements) can be attributed directly to the lack of a sufficiently large or chemically diverse experimental data set. To address this problem, we conducted a series of experiments using several different natural composition lavas, doped with the elements of interest, as starting compositions. Microprobe analyses of orthopyroxene, pigeonite, olivine, magnetite, ilmenite and co-existing glasses in the experimental charges were used to calculate expressions that describe REE partitioning as a function of a variety of system parameters. Using expressions that represent mineral-melt reactions (versus element ratio distribution coefficients) it is possible to calculate terms that express low-Ca pyroxene-melt partitioning behavior and are independent of both pyroxene and melt composition. Compositional variations suggest that Sc substitution in olivine involves either a paired substitution with Al or, more commonly, with vacancies. The partitioning of Sc is dependent both on melt composition and temperature. Our experimentally determined olivine-melt REE Ds (partition coefficients) are similar to, but slightly higher than those reported by McKay (1986) and support their conclusions that olivines are strongly LREE depleted. Y and REE mineral/melt partition coefficients for magnetite range from 0.003 for La to 0.02 for Lu. Ilmenite partition coefficients range from 0.007 for La to 0.029 for Lu. These experimental values are two orders of magnitude lower than many of the published values determined by phenocryst/matrix separation techniques.

Notes: Abstract An updated model for pyroxene-melt equilibria at 1 atm has been developed and calibrated using new and existing experimental data in order to refine calculations of liquid lines of descent, which simulate the effect of igneous differentiation processes. We combine the Davidson and Lindsley (1985) model for activities of components in clinopyroxene and orthopyroxene solid solutions, a i p , where i represents a quadrilateral endmember, with the Nielsen and Drake (1979) expressions for component activities in the melt, a i L (two-lattice melt model). The chemical potential differences for pyroxene-melt equilibria are expressed in the form: $$\Delta \mu _{\iota } = 0 = In \left( {{{a_i^p } \mathord{\left/{\vphantom {{a_i^p } {a_i^L }}} \right.\kern-\nulldelimiterspace} {a_i^L }}} \right) + A_i + {{B_i } \mathord{\left/{\vphantom {{B_i } T}} \right.\kern-\nulldelimiterspace} T}$$ Pyroxene compositions were projected to quadrilateral compositions with the method of Lindsley and Anderson (1983). The regression constants A i and B i were calculated from experimental data that consists of 282 pyroxene-melt pairs, including 83 orthopyroxene-melt pairs. These experiments were all performed at 1 atm and represent compositions ranging from basalts (alkali to lunar) to dacites (42–66 wt% SiO2). The model is calibrated for 1000〈T(°C) 〈1350 and pyroxene ‘others’ (Ti, Mn, Al, Cr, Na) less than 10 mole%. Using the input data as unknowns, the model expressions reproduce the experimental results within 10–20° C.

Notes: Abstract With few exceptions, geochemical models of igneous differentiation have assumed that crystals form homogeneously throughout the cooling magma chamber. This is in spite of field evidence and physical models suggesting for natural systems, that heat loss, and thus most crystallization, occurs primarily along the walls and roof of magma chambers. Such systems would produce aphyric, evolved composition lavas. Existing models of boundary layer crystal fractionation suggest that the products of such a process are significantly different from the products of homogeneous crystallization. To investigate this process further, we have developed a model using a numerical approach to boundary layer crystal fractionation based on new phase equilibria and trace element constraints. In addition to olivine, augite and plagioclase, this new model includes the ability to calculate the effects of oxide (magnetite and ilmenite), apatite and low-Ca pyroxene fractionation. This is critical if we are to effectively evaluate the geochemical signature of boundary layer fractionation. This is because the products of boundary layer fractionation are most different from homogeneous fractionation for high degrees of crystallization in the solidification zone. These are the same conditions where oxides, low-Ca pyroxene and apatite are saturated in most natural, differentiated mafic systems. The results of this model suggest that boundary layer fractionation can help to explain such phenomena as “phantom crystallization”, variation in incompatible element ratios, and the decoupling of major and trace element systematics. In addition, many of the geochemical patterns that we use to distinguish between the products of low pressure and high pressure differentiation are obscured by the fact that the liquid line of descent can be controlled by the removal of phases not in equilibrium with the entire magma chamber.

Notes: Abstract The development of petrogenetic models of igneous processes in the mantle is dependent on a detailed knowledge of the diversity of magmas produced in the melting regime. These primary magmas, however, undergo significant mixing and fractionation during transport to the surface, destroying much of the evidence of their primary diversity. To circumvent this problem and to determine the diversity of melts produced in the mantle, we used melt inclusions hosted in primitive plagioclase phenocrysts from eight mid-ocean ridge basalts from the axial and West Valleys of the Endeavour Segment, Juan de Fuca Ridge. This area was selected for study because of the demonstrated close association of enriched (E-MORB) lavas and incompatible element enriched depleted (N-MORB) lavas. Rehomogenized melt inclusions from E-MORB, T-MORB, and N-MORB lavas have been analyzed by electron and ion microprobe for major and trace elements. The depleted and enriched lavas, as well as their melt inclusions, have very similar compatible element concentrations (major elements, Sr, Ni and Cr). Inclusion compositions are more primitive than, yet collinear with, the host lava suites. In contrast, the minor and trace element characteristics of melt inclusions from depleted and enriched lavas are different both in range and absolute concentration. N-MORB lavas contain both depleted and enriched melt inclusions, and therefore exhibit the largest compositional range (K2O: 0.01 to 0.4 oxide wt%, P2O5: 〈0.01 to 0.2 oxide wt%, LaN: 7 to 35, YbN: 1 to 13, and Ti/Zr: 〈100 to 1300). E-MORB lavas contain only enriched inclusions, and are therefore relatively homogeneous (K2O: 0.32 to 0.9 oxide wt %, P2O5: 0.02 to 0.35 oxide wt%, LaN: 11 to 60, YbN: 4 to 21, and Ti/Zr: ∼100). In addition, the most primitive E-32 inclusions are similar in composition to the most enriched inclusions from the depleted hosts. Major element data for melt inclusions from both N-MORB and E-MORB lavas suggest that the magmas lie on a low pressure cotectic, consistent with a petrogenesis including fractional crystallization. However, the minor and trace element compositions in melt inclusions vary independently of the major element composition implying an alternative history. When fractionation-corrected, inclusion compositions correlate with their host glass composition. Hence, the degree of enrichment of the lavas is a function of the composition of aggregated melts, not of processing in the upper mantle or lower crust. Based on this fact, the lava suites are not produced from a single parent magma, but from a suite of primary magmas. The chemistry of the melt inclusions from the enriched lavas is consistent with a derivation from variable percentages of partial melting within the spinel stability field by a process of open system (continuous or critical) melting assuming a depleted lherzolite source veined with clinopyroxenite. The low percentage melts are dominantly enriched melts of the clinopyroxenite. In contrast, the depleted lavas were created by melting of a harzburgite source, possibly fluxed with a fluid enriched in K, Ba and the LREE. Such a source was likely melted up to or past the point at which all of its clinopyroxene was consumed. This set of characteristics is consistent with a scenario by which diverse melts produced at different depths travel through the melting regime to the base of the crust without homogenizing en route. The homogeneous major element characteristics are created in the lower crust by fractional crystallization and reaction with lower crustal gabbros. Therefore, the degree of decoupling between major and trace element characteristics of the melt inclusions (and lavas) is dictated by the reaction rate of the melts with the materials in the conduit walls, as well as the residence times and flux rate, in the upper mantle and lower crust.

Notes: Abstract Low pressure phase equilibria for natural mafic systems may be calculated utilizing empirically derived single component distribution coefficients and the constraints placed upon mineral-melt equilibria by stoichiometry and mass balance, without any knowledge of thermochemical properties of melts (Langmuir and Hanson 1981). Variations in distribution coefficients caused by differences in melt composition may be largely eliminated by the application of a two-lattice melt model based upon the models of Nielsen and Drake (1979), Drake (1976b), and Bottinga and Weill (1972). In the two-lattice melt model, the melt is assumed to be made up of two independent quasi-lattices, the network formers, composed of the components SiO2, NaAlO2, and KAlO2, and the network modifiers, composed of the free oxides of Ca, Mg, Fe, Al, Ti, and Cr. Compositionally independent mineral-melt distribution coefficients were calculated for all the major and some minor components in olivine, plagioclase, high-Ca pyroxene, low-Ca pyroxene, spinel, and ilmenite. Regression constants were calculated from data from 1 atmosphere, anhydrous, equilibrium experiments on natural and synthetic mafic compositions including data obtained from the literature and from new synthetic spinel-melt experiments in the Fo-Ab-An system doped with Ti, Mn, Ni, and Fe. The distribution coefficients are internally consistent within 3 mole % (1σ) for all components. The effects of variable oxygen fugacity on mafic mineral-melt equilibria were calculated utilizing the relations of Sack et al. (1980) for the determination of the Fe+3/Fe+2 ratio, and the results of Schreiber and Haskin (1976) for Cr+3/Cr+2. A computer program, EQUIL.FOR, incorporating the derived mineral-melt distribution coefficients, calculates the sequence of mineral and melt compositions, and liquidus temperatures for mafic melt compositions undergoing equilibrium or fractional crystallization. This program reproduces, within 15° C, 52 experimentally determined crystallization sequences collected from the literature. These published crystallization sequences are for mafic systems not represented in the data set used to calculate the mineral-melt distribution coefficients.