TY - JOUR
T1 - Magma evolution in the primitive, intra-oceanic tonga arc
T2 - Rapid petrogenesis of dacites at fonualeivolcano
AU - Turner, Simon
AU - Caulfield, John
AU - Rushmer, Tracy
AU - Turner, Michael
AU - Cronin, Shane
AU - Smith, Ian
AU - Handley, Heather
N1 - A corrigendum exists for this article and can be found in Journal of Petrology, 56(3), p.641–644, doi: 10.1093/petrology/egv009
PY - 2012/6
Y1 - 2012/6
N2 - Fonualei is unusual amongst subaerial volcanoes in theTonga arc because it has erupted dacitic vesicular lavas, tuffs and phreomagmatic deposits for the last 165 years. The total volume of dacite may approach 5 km 3 and overlies basal basaltic andesite and andesite lavas that are constrained to be less than a few millennia in age. All of the products are crystal-poor and formed from relatively low-viscosity magmas inferred to have had temperatures of 1100-1000°C, 2-4 wt % H 2O and oxygen fugacities 1-2 log units above the quartz-fayalite-magnetite buffer. Major and trace element data, along with Sr-Nd-Pb and U-Th-Ra isotope data, are used to assess competing models for the origin of the dacites. Positive correlations between Sc and Zr and Sr rule out evolution of the within-dacite compositional array by closed-system crystal fractionation of a single magma batch. An origin by partial melting of lower crustal amphibolites cannot reproduce these data trends or, arguably, any of the dacites either. Instead, we develop a model in which the dacites reflect mixing between two dacitic magmas, each the product of fractional crystallization of basaltic andesite magmas formed by different degrees of partial melting. Mixing was efficient because the two magmas had similar temperatures and viscosities. This is inferred to have occurred at shallow (2-6 km) depths beneath the volcano. U-Th-Ra disequilibria in the basaltic andesite and andesite indicate that the parental magmas had fluids added to their mantle source regions less than 8 kyr ago and that fractionation to the dacitic compositions took less than a few millennia. The 165 year eruption period for the dacites implies that mixing occurred on a similar timescale, possibly during ascent in conduits. The composition of the dacites renders them unsuitable candidates as contributors to average continental crust.
AB - Fonualei is unusual amongst subaerial volcanoes in theTonga arc because it has erupted dacitic vesicular lavas, tuffs and phreomagmatic deposits for the last 165 years. The total volume of dacite may approach 5 km 3 and overlies basal basaltic andesite and andesite lavas that are constrained to be less than a few millennia in age. All of the products are crystal-poor and formed from relatively low-viscosity magmas inferred to have had temperatures of 1100-1000°C, 2-4 wt % H 2O and oxygen fugacities 1-2 log units above the quartz-fayalite-magnetite buffer. Major and trace element data, along with Sr-Nd-Pb and U-Th-Ra isotope data, are used to assess competing models for the origin of the dacites. Positive correlations between Sc and Zr and Sr rule out evolution of the within-dacite compositional array by closed-system crystal fractionation of a single magma batch. An origin by partial melting of lower crustal amphibolites cannot reproduce these data trends or, arguably, any of the dacites either. Instead, we develop a model in which the dacites reflect mixing between two dacitic magmas, each the product of fractional crystallization of basaltic andesite magmas formed by different degrees of partial melting. Mixing was efficient because the two magmas had similar temperatures and viscosities. This is inferred to have occurred at shallow (2-6 km) depths beneath the volcano. U-Th-Ra disequilibria in the basaltic andesite and andesite indicate that the parental magmas had fluids added to their mantle source regions less than 8 kyr ago and that fractionation to the dacitic compositions took less than a few millennia. The 165 year eruption period for the dacites implies that mixing occurred on a similar timescale, possibly during ascent in conduits. The composition of the dacites renders them unsuitable candidates as contributors to average continental crust.
UR - http://www.scopus.com/inward/record.url?scp=84861809179&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/record.url?scp=84929753578&partnerID=8YFLogxK
UR - https://doi.org/10.1093/petrology/egv009
U2 - 10.1093/petrology/egs005
DO - 10.1093/petrology/egs005
M3 - Article
VL - 53
SP - 1231
EP - 1253
JO - Journal of Petrology
JF - Journal of Petrology
SN - 0022-3530
IS - 6
M1 - egs005
ER -