We describe and model a potential re-equilibration process that can aect compositions of melt inclusions in magnesian olivine phenocrysts. This process, referred to as``Fe-loss'', can operate during natural pre-eruptive cooling of host magma and results in lower FeO t and higher MgO contents within the initially trapped volume of inclusion. The extent of Fe-loss is enhanced by large temperature intervals of magma cooling before eruption. The compositions of homogenised melt inclusions in olivine phenocrysts from several subduction-related suites demonstrate that (1) Fe-loss is a common process, (2) the maximum observed degree of re-equilibration varies between suites, and (3) within a single sample, variable degrees of re-equilibration can be recorded by melt inclusions trapped in olivine phenocrysts of identical composition. Our modelling also demonstrates that the re-equilibration process is fast going to completion, in the largest inclusions in the most magnesian phenocrysts it is completed within 2 years. The results we obtained indicate that the possibility of Fe-loss must be considered when estimating compositions of parental subduction-related magmas from naturally quenched glassy melt inclusions in magnesian olivine phenocrysts. Compositions calculated from glassy inclusions aected by Fe-loss will inherit not only erroneously low FeO t contents, but also low MgO due to the inherited higher Mg# of the residual melt in reequilibrated inclusions. We also demonstrate that due to the higher MgO contents of homogenised melt inclusions aected by Fe-loss, homogenisation temperatures achieved in heating experiments will be higher than original trapping temperatures. The extent of overheating will increase depending on the degree of re-equilibration, and can reach up to 50°C in cases where complete re-equilibration occurs over a cooling interval of 200°C.

Corundum xenocrysts from alkaline basalt fields differ in characteristics and hence lithospheric origins. Trace element, fluid/melt inclusion and oxygen isotope studies on two eastern Australian corundum deposits are compared to consider their origins. Sapphires from Weldborough, NE Tasmania, are magmatic (high-Ga, av. 200 ppm) and dominated by Fe (av. 3300 ppm) and variable Ti (av. 400 ppm) as chromophores. They contain Cl, Fe, Ga, Ti and CO 2 -rich fluid inclusions and give d 18 O values (5.1À6.2%) of mantle range. Geochronology on companion zircons suggests several sources (from 290 Ma to 47 Ma) were disrupted by basaltic melts (47 Ô 0.6 Ma). Gem corundums from Barrington, New South Wales, also include magmatic sapphires (Ga av. 170 ppm; d 18 O 4.6À5.8%), but with more Fe (av. 9000 ppm) and less Ti (av. 300 ppm) as chromophores. Zircon dating suggests that gem formation preceded and was overlapped by Cenozoic basaltic melt generation (59À4 Ma). In contrast, a metamorphic sapphire-ruby suite (low-Ga, av. 30 ppm) here incorporates greater Cr into the chromophores (up to 2250 ppm). Fluid inclusions are CO 2 -poor, but melt inclusions suggest some alkaline melt interaction. The d 18 O values (5.1À6.2%) overlap magmatic sapphire values. Interactions at contact zones (T = 780À940ºC) between earlier Permian ultramafic bodies and later alkaline fluid activity may explain the formation of rubies.

SummaryAl-rich spinels (100Cr/(Cr + A1) < 5, A1203 > 50 wt%) are common in alpine peridotites, both terrestrial and lunar marie and ultramafic cumulates, and in certain metamorphic rocks, but they are apparently rare in terrestrial volcanic rocks. Here we describe the occurrence of Al-rich spinel inclusions in olivine phenocrysts in island are volcanic rocks from five new localities: Bukit Mapas (Sumatra) and eastern Bali in the Sunda arc, and Epi, Merelava, and Ambrym islands in the Vanuatu arc. More commonly, relatively Cr-rich spinels also occur as inclusions in the same olivine phenocrysts, and it appears that the Cr-poor aluminous spinels must be in disequilibrium with the host basaltic melts. In the rocks studied, Al-rich spinels also coexist with trapped silicate glasses and highly aluminous clinopyroxene in melt inclusions in olivine. This paragenesis suggests an origin involving contamination by localised Al-rich melt pockets as opposed to a xenocrystic origin. Two mechanisms to produce this high-A1 melt in basaltic magma chambers are suggested: (1) localized high-A1 melt production by complete breakdown of assimilated lower crustal gabbroic rocks. In this model the high-A1 melt may crystallise Al-rich spinels which are subsequently trapped as solid inclusions by phenocryst phases of the host basaltic melt or may be trapped as melt inclusions in which Al-rich spinels and Al-rich clinopyroxene crystallise as daughter phases, and (2) incongruent breakdown of amphibole in amphibole-rich cumulates in sub-arc, or sub-OIB volcano magma chambers. The latter reaction produces a melt with 20-22% of A1203, aluminous clinopyroxene, Al-rich spinel and olivine. Mixing between these amphibole breakdown products and host basaltic melt may occur throughout the evolution of a magmatic system, but particularly during recharge with hot magnesian basalt batches. Aluminous spinels and aluminous clinopyroxene produced during amphibole breakdown, or perhaps crystallised from aluminous melt produced in the same reaction, are incorporated into the magma during recharge, and subsequently trapped, together with the coexisting Cr-spinels, by crystallising olivine and clinopyroxene. F. N. Della-Pasqua et al. Zusammenfassung AI-Spinelle in primitiven Inselbogen-VulkanitenAl-reiche Spinelle (100Cr/(Cr + A1) < 5, AlzO 3 > 50 Gew.~/o) sind in alpinen Peridotiten, in terrestrischen und lunaren mafischen und ultramafischen Kumulaten und in manchen metamorphen Gesteinen weit verbreitet, aber sie scheinen in terrestrischen, vulkanischen Gesteinen selten zu sein. Wir beschreiben bier das Vorkommen von A1-reichen Spinell-Inklusionen in Olivinkristallen von Inselbogen-Vulkaniten von 5 neuen Lokalit~iten: Bukit Mapas (Sumatra) und Ost-Bali im Sunda-Bogen und die Inseln Epi, Merelava und Ambrym im Vanuatu-Bogen. Relativ Cr-reiche Spindle kommen h~iufiger auch als Einschliisse in denselben Olivin-Kristallen vor, und es scheint, dab Chrom-arme Aluminiumspinelle im Ungleichgewicht mit ihren basaltischen Mutterschmelzen stehen. In den untersuchten ...

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