Petrogenesis and amphibole–melt trace element partitioning of the 156 ka Haramul Mic crystal-rich dacite, Ciomadul, Romania

Abstract

The 156 ka, crystal-rich (~ 40 vol%) Haramul Mic dacite represents eruption of a crystal mush. It marks the first eruptive product of the Ciomadul Volcanic Complex, East-Central Europe, following at least 100 kyr of dormancy. The mineral phases (plagioclase, amphibole, biotite, apatite, titanite and zircon) and the interstitial glass have relatively restricted major and trace element composition. Thermobarometric and hygrometric calculations indicate a low-temperature (~ 720 °C), low-pressure (~ 300 MPa), water-saturated (dissolved H₂O ~ 6.5 wt%) and oxidizing silicic magmatic system. A comprehensive set of amphibole-melt trace element partition coefficient data is provided, applicable to low-temperature, near-eutectic silicic volcanic and plutonic systems. The maximum partition coefficient (D₀ = 8.7 ± 0.1) and the optimal ionic radius (r₀ = 1.0494 ± 0.0008 Å) for trivalent elements, as determined from Onuma plots, are consistent with an evolved silicic magmatic system and a low-magnesian amphibole composition. Additionally, mineral-melt trace element partition coefficients are calculated for coexistent titanite, zircon, plagioclase and biotite. We note that in highly evolved silicic volcanic systems, partition coefficients remain consistently similar, even with slight variations in magma composition and conditions near the thermal minimum. In contrast to the subsequent eruptions, there is no evidence for mafic magma recharge and mixing as eruption initiation for the Haramul Mic. Instead, effective thermomechanical reactivation of a portion of the long-standing crystal mush, followed by rapid magma ascent is envisaged. Ciomadul provides an example where multiple processes can contribute to eruption initiation, as reflected in the textural and compositional characteristics of the crystal cargo, despite the relatively uniform composition of the erupted magma.