238U-230Th-226Ra-210Pb constraints on the genesis of high-Mg andesites at White Island, New Zealand

• Published in: Chemical Geology
• Main Authors: Heyworth, Zarah, Turner, Simon, Schaefer, Bruce, Wood, Bernard, George, Rhiannon, Berlo, Kim, Cunningham, Heather, Price, Richard, Cook, Craig, Gamble, John
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2007
• Subjects: New Zealand; Pacific; White Island
• Online Access: https://researchers.mq.edu.au/en/publications/59e96055-21d0-4ff3-897f-0fba5d1493b3; https://doi.org/10.1016/j.chemgeo.2007.05.012; http://www.scopus.com/inward/record.url?scp=34447567413&partnerID=8YFLogxK

On White Island, New Zealand, the intensified period of strombolian-volcanian and phreatomagmatic explosive activity that commenced in March 1977 led to eruption of unusually primitive, high-Mg andesites. These are Fo 80-93 olivine-saturated rocks that have MgO contents up to 10% (Mg# = 65-71) and SiO 2 of 56-58%. They have incompatible trace element characteristics that are typical of arc rocks. 87 Sr/ 86 Sr, 143 Nd/ 144 Nd and 176 Hf/ 177 Hf ratios (0.7049-0.7053, 0.51282-0.51266 and 0.28301-0.28298, respectively) are consistent with subducted sediment addition and/or crustal input but there is no clear correlation of either isotope ratio with MgO. The rocks have modest (3-10%) 238 U excesses at low ( 230 Th/ 232 Th) ratios (0.697 to 0.722). 226 Ra- 230 Th disequilibria are also restricted but, unusually, include both 226 Ra excesses and deficits with ( 226 Ra/ 230 Th) = 0.94-1.07. ( 210 Pb/ 226 Ra) o ranges from 0.98 to 1.52 requiring gas accumulation that may increase over time and with decreasing MgO. Sr/Y and Tb/Yb ratios are both low and relatively invariant at 8 and 0.3, respectively, and along with the 238 U excesses preclude an origin in which residual garnet was involved. The occurrence of some 226 Ra deficits suggests the presence of residual amphibole during partial melting for some samples. Rapid magma ascent (to preserve the 226 Ra disequilibria) limits the amount of possible melt-wall rock interaction that might reduce source-derived Tb/Yb ratios and in the mantle or raise 87 Sr/ 86 Sr in the crust. The White Island high-Mg andesites did not form by partial melting of eclogite in the subducting Pacific plate. Their primitive, olivine-saturated compositions suggest that their source was peridotitic and experimental data suggest that equilibration with peridotite at low temperatures at 0.5-1.5 GPa and in the presence of elevated alkalis can reconcile the high SiO 2 and MgO of the rocks. This may reflect stalling of the magmas beneath the continental lithosphere for a few 1000 years.