Open AccessInsights into the Galápagos plume from uranium‐series isotopes of recently erupted basalts
Author(s) -
Handley Heather K.,
Turner Simon,
Berlo Kim,
Beier Christoph,
Saal Alberto E.
Publication year - 2011
Publication title -
geochemistry, geophysics, geosystems
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.928
H-Index - 136
ISSN - 1525-2027
DOI - 10.1029/2011gc003676
Subject(s) - geology , basalt , plume , series (stratigraphy) , geochemistry , volcano , earth science , uranium , mantle plume , paleontology , tectonics , lithosphere , physics , materials science , metallurgy , thermodynamics
Uranium‐series isotopes ( 238 U‐ 230 Th‐ 226 Ra‐ 210 Pb), major element, trace element, and Sr‐Nd isotopic data are presented for recent (<60 years old) Galápagos archipelago basalts. Volcanic rocks from all centers studied (Fernandina, Cerro Azul, Sierra Negra, and Wolf Volcano) display 230 Th excesses (4%–15%) and steep rare earth element (REE) patterns indicative of residual garnet during partial melting of their mantle source. Rare earth element modeling suggests that only a few percent of garnet is involved. Correlations between ( 238 U/ 232 Th), radiogenic isotopes and Nb/Zr ratio suggest that the U/Th ratio of these Galápagos volcanic rocks is primarily controlled by geochemical source variations and not fractionation during partial melting. The lowest ( 230 Th/ 238 U) ratio is not observed at Fernandina (the supposed center of the plume) but at the more geochemically “depleted” Wolf Volcano, further to the north. Small radium excesses are observed for all samples with ( 226 Ra/ 230 Th) ranging from 1.107 to 1.614. The 226 Ra‐ 230 Th disequilibria do not correlate with other uranium‐series parent‐daughter nuclide pairs or geochemical data, suggesting modification at shallow levels on timescales relevant to the half‐life of 226 Ra (1600 years). The combination of 226 Ra and 210 Pb excesses is inconsistent with interaction of magma with cumulate material unless decoupling of 210 Pb (or an intermediate daughter, such as 222 Rn) occurs prior to modification of Ra‐Th disequilibria. An intriguing correlation of ( 210 Pb/ 226 Ra) 0 with Nb/Zr and radiogenic isotopes requires further investigation but suggests possible control via magmatic degassing and accumulation that may somehow be related to source heterogeneities.