Experimental Constraints on the Origin of the 1991 Pinatubo Dacite

Abstract

Crystallization (dacite) and interaction (dacite-peridotite) experiments have been performed on the 1991 Pinatubo dacite (Luzon Island, Philippines) to constrain its petrogenesis. In the dacite-H 2 O system at 960 MPa, magnetite and either clinopyroxene (low H 2 O) or amphibole (high H 2 O) are the liquidus phases. No garnet is observed at this pressure. Dacite-peridotite interaction at 920 MPa produces massive orthopyroxene crystallization, in addition to amphibole ± phlogopite. Amphibole crystallizing in dacite at 960 MPa has the same composition as the aluminium-rich hornblende preserved in the cores of amphibole phenocrysts in the 1991 dacite, suggesting a high-pressure stage of dacite crystallization with high melt H 2 O contents (>10 wt %) at relatively low temperature (<950°C). The compositions of plagioclase, amphibole and melt inclusion suggest that the Pinatubo dacite was water-rich, oxidized and not much hotter than 900°C, when emplaced into the shallow magma reservoir in which most phenocrysts precipitated before the onset of the 1991 eruption. The LREEenriched REE pattern of the whole-rock dacite demands garnet somewhere during its petrogenesis, which in turn suggests high-pressure derivation. Partial melting of subducted oceanic crust yields melts unlike the Pinatubo dacite. Interaction of these slab melts with subarc peridotite is unable to produce a Pinatubo type of dacite, nor is a direct mantle origin conceivable on the basis of our peridotite-dacite interaction experimental results. Dehydration melting of underplated basalts requires unrealistically high temperatures and does not yield dacite with the low FeO/MgO, and high H 2 O, Ni and Cr contents typical of the Pinatubo dacite. The most plausible origin of the Pinatubo dacite is via high-pressure fractionation of a hydrous, oxidized, primitive basalt that crystallized amphibole and garnet upon cooling. Dacite melts produced in this way were directly expelled from the uppermost mantle or lower crust to shallow-level reservoirs from which they erupted occasionally. Magmas such as the Pinatubo dacite may provide evidence for the existence of particularly H 2 O-rich conditions in the sub-arc mantle wedge rather than the melting of the young, hot subducting oceanic plate.

Metasomatic interactions between slab-derived melts and depleted mantle: Insights from xenoliths within Monglo adakite (Luzon arc, Philippines)

“…An "intermediate" hypothesis is that the formation of amphibole-rich veins is related to interaction with the host lava (or a similar adakitic melt percolating the dunites before their uplift by the host lava), and that the orthopyroxene-rich veins are related to the circulation of a different type of slab-derived melt/fluid prior to the incorporation of the dunites within the host lava. The comparison of our data with the compositions of southern Kamchatka mantle peridotite xenoliths (Ishimaru et al, 2007) and with the experimental results of Prouteau and Scaillet (2003) gives strong support to this "intermediate" hypothesis. Indeed, Ishimaru et al (2007) describe southern Kamchatka peridotite xenoliths coated by hornblendites forming thin selvages as well as veinlets cutting the xenoliths.…”

“…Moreover, the amphiboles of our study display major element compositions similar to those of interstitial and discrete amphiboles from the amphibolites selvages coating the southern Kamchatka xenoliths. Finally the major element compositions of amphiboles from Monglo dunites are also similar to those of the amphiboles produced during the 1991 Pinatubo dacite-peridotite interaction experiments of Prouteau and Scaillet (2003).…”

“…6 and Table 2 of McInnes et al, 2001). On the other hand, their major element contents (lower Al and Ca and higher Si) are different from those of the orthopyroxenes produced during the Pinatubo dacite-peridotite interaction experiments of Prouteau and Scaillet (2003). Therefore, we propose that the formation of amphibole-rich veins in the studied dunites is related to their interaction with the host Monglo adakitic magma (or a similar adakitic melt) and that the orthopyroxene-rich veins are rather related to the circulation of a different type of slab-derived melt/fluid.…”

“…Indeed, experimental studies have shown that dacitic melts, and especially adakitic ones such as the Monglo host lava, may produce orthopyroxene, amphibole and phlogopite when they react with peridotitic paragenesis (e.g. [Rapp et al, 1999], [Prouteau et al, 2001] and [Prouteau and Scaillet, 2003]). An "intermediate" hypothesis is that the formation of amphibole-rich veins is related to interaction with the host lava (or a similar adakitic melt percolating the dunites before their uplift by the host lava), and that the orthopyroxene-rich veins are related to the circulation of a different type of slab-derived melt/fluid prior to the incorporation of the dunites within the host lava.…”

Is gold solubility subject to pressure variations in ascending arc magmas?

“…An elevated temperature (975-1000°C), close to or above the liquidus of dacitic compositions such as the two starting rocks (e.g., Prouteau and Scaillet, 2003), was adopted.…”