Bytownite

Abstract: The Tongan Islands comprise a double island chain, and lie at the northern end of an active island arc extending NNE from northern New Zealand. The eastern chain are limestone covered, with an underlying pre-Upper Eocene gabbro-basalt-andesite series exposed on one island. The active volcanoes, the subject of this paper, form the western island chain, and lie some 100 km above a westward dipping seismic zone. Basaltic andesite dominates the Islands of Hunga Ha'apai and Late (and also Tofua and Kao, which are not described in this paper). Dacite is the main lava of Fonualei and the 1967-68 Metis Shoal eruption. Acid andesites occur only sporadically.Chemically, the lavas are moderately iron-enriched, with relatively low concentrations of K, Rb, Ba, Zr, REE, Pb, and U. K/Rb ratios are 500-700. The lavas are not typically 'calcalkaline'. Phenocryst minerals (normally poorly zoned) are dominated by bytownite with subordinate orthopyroxene, augite, and sometimes pigeonite in the basaltic andesites. Titanomagnetite occurs only in the more silicic andesites and dacites. The pyroxenes show a limited iron enrichment through the series. Coexisting pyroxenes exhibit equilibrium distributions of Mg, Mn, V, Sc, and Ni. Pyroxene partition coefficients for V, Sc. Mn, and Co increase with increasing iron enrichment. Groundmass minerals include intermediate plagioclase, pigeonite, hypersthene, magnetite, plus potash feldspar and quartz in the dacites. The Metis Shoal lava consists of rhyolitic glass with xenocrysts of forsteritic olivine and enstatite and phenocrysts of bytownite, hypersthene, augite, and titanomagnetite.The basaltic andesites are interpreted to be the product of direct peridotitic fusion within the mantle, presumably in the region overlying the seismic zone, and modified by olivine fractionation during ascent. Least squares numerical calculations, using major and trace element data, support the derivation of the more siliceous lava compositions by low pressure crystal fractionation of basaltic andesite Sr/Sr ratios exhibit a small range of variation (0.7036-0.7043), possibly indicative (except for the Metis Shoal lava) of minor limestone (?) contamination within the more silicic lavas during fractionation.

Abstract: Evolution of the magma chamber at Mount Mazama involved repeated recharge by two types of andesite (high-Sr and low-Sr), crystal fractionation, crystal accumulation, assimilation, and magma mixing (Bacon and Druitt 1988). This paper addresses the modal compositions, textures, mineral chemistry and magmatic temperatures of (i) products of the 6845±50 BP climactic eruption, (ii) blocks of partially fused granitoid wallrock found in the ejecta, and (iii) preclimactic rhyodacitic lavas leaked from the chamber in late Pleistocene and early Holocene time. Immediately prior to the climactic eruption the chamber contained ≳ 40 km3 of rhyodacite (10 vol% plag + opx + aug + hb + mt + ilm, ∼880° C) overlying high-Sr andesite and cumulus-crystal mush (28–51 vol% plag + hb ± opx ± aug + mt ± ilm, 880° to ≥950° C), which in turn overlay low-Sr crystal mush (50–66 vol% plag + opx + aug ± hb ± ol + mt + ilm, 890° to ≥950† C). Despite the well known compositional gap in the ejecta, no thermal discontinuity existed in the chamber. Pre-eruptive water contents of pore liquids in most high-Sr and low-Sr mushes were 4–6 wt%, but on average the high-Sr mushes were slightly richer in water. Although parental magmas of the crystal mushes were andesitic, xenocrysts of bytownite and Ni-rich magnesian olivine in some scoriae record the one-time injection of basalt into the chamber. Textures in ol-bearing scoriae preserve evidence for the reactions ol + liq = opx and ol + aug + liq(+ plag?) = hb, which occurred in andesitic liquids at Mount Mazama. Strontium abundances in plagioclase phenocrysts constrain the petrogenesis of preclimactic and climactic rhyodacites. Phenocryst cores derived from high-Sr and low-Sr magmas have different Sr contents which can be resolved by microprobe. Partition coefficients for plagioclase in andesitic to rhyolitic glasses range from 2 to 7, and increase as glass %SiO2 increases. Evolved Pleistocene rhyodacites (∼30–25,000 BP) and rhyodacites of the Holocene Llao Rock center (7015±45 BP) contain Sr-poor plagioclase and are derivatives from low-Sr magma. Rhyodacites of the Pleistocene Sharp Peak domes, Holocene Cleetwood flow (∼6850 BP), and climactic ejecta contain discrete Sr-rich and Sr-poor plagioclase phenocryst populations and are hybrids produced by mixing low-Sr rhyodacite (containing Sr-poor plag + opx + aug) with a more mafic high-Sr magma (with Sr-rich plag [ + hb?]). The data reinforce the conclusions of crystal-liquid mixing calculations (Bacon and Druitt 1988), and suggest some important refinements to the magma chamber model.

Abstract: The modal mineralogies of 20 mafic and ultramafic igneous rocks were determined from their thermal infrared emission spectra using a linear deconvolution approach, which uses a library of end-member mineral spectra to model a bulk rock spectrum. Over 90% of the modes obtained from thermal emission spectra agree with modes obtained by traditional optical analyses to within the stated error of the optical analyses (5–15 vol%). Library spectra of several compositions within a solid solution series (e.g., plagioclase feldspars labradorite and bytownite) were commonly used in each best fit model and are assumed to represent, in combination, a composition in the rock for which an identical mineral spectrum was unavailable in the library. The accuracy of this assumption was evaluated by calculating a weighted average solid solution composition for plagioclase and/or pyroxenes from the library minerals used in the model best fit of 14 rocks and comparing these compositions to the actual chemistries measured by electron microprobe or the optically estimated An#. The derived solid solution compositions are generally within 10–15 An# (or Mg#) of the measured composition. Modal data derived from the spectra via the deconvolution were summed and converted to weight percent (wt%) oxides for comparison to standard bulk chemistry data. SiO2, Al2O3, and Na2O + K2O were generally slightly overestimated, and FeO and MgO were typically slightly underestimated. Modes and wt% oxides were plotted on standard rock type classification diagrams and provide broadly accurate classifications, demonstrating that the linear deconvolution technique is successful at deriving useful mineralogical information from thermal infrared emission spectra of bulk rock samples. Furthermore, convolution of the laboratory data to spectral resolutions comparable to remote sensing instruments (specifically, the Mars Global Surveyor Thermal Emission Spectrometer) demonstrates only a ∼5 vol% increase in modal uncertainty for minerals present in abundances ≥10 vol%. This study thus provides a strong foundation for the application of this technique to mafic igneous sample spectra from both laboratory and remote sensing instruments.