Evidence for initial excess 231Pa in mid-ocean ridge zircons

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doi: 10.1016/j.chemgeo.2015.01.011
Author(s): Rioux, Matthew; Bowring, Samuel; Cheadle, Michael; John, Barbara
Author Affiliation(s): Primary:
Massachusetts Institute of Technology, Department of Earth, Atmospheric, and Planetary Sciences, Cambridge, MA, United States
University of Wyoming, United States
Volume Title: Chemical Geology
Source: Chemical Geology, Vol.397, p.143-156. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541 CODEN: CHGEAD
Note: In English. Includes appendices. 87 refs.; illus., incl. 2 tables, sketch map
Summary: A limiting factor in the accuracy and precision of U/Pb zircon dates is accurate correction for initial disequilibrium in the 238U and 235U decay chains. The longest-lived-and therefore most abundant-intermediate daughter product in the 235U isotopic decay chain is 231Pa (T1/2 = 32.71 ka), and the partitioning behavior of Pa in zircon is not well constrained. Here we report high-precision thermal ionization mass spectrometry (TIMS) U-Pb zircon data from two samples from Ocean Drilling Program (ODP) Hole 735B, which show evidence for incorporation of excess 231Pa during zircon crystallization. The most precise analyses from the two samples have consistent Th-corrected 206Pb/238U dates with weighted means of 11.9325 ± 0.0039 Ma (n = 9) and 11.920 ± 0.011 Ma (n = 4), but distinctly older 207Pb/235U dates that vary from 12.330 ± 0.048 Ma to 12.140 ± 0.044 Ma and 12.03 ± 0.24 to 12.40 ± 0.27 Ma, respectively. If the excess 207Pb is due to variable initial excess 231Pa, calculated initial (231Pa)/(235U) activity ratios for the two samples range from 5.6 ± 1.0 to 9.6 ± 1.1 and 3.5 ± 5.2 to 11.4 ± 5.8. The data from the more precisely dated sample yields estimated DPa zircon/DU zircon from 2.2-3.8 and 5.6-9.6, assuming (231Pa)/(235U) of the melt equal to the global average of recently erupted mid-ocean ridge basaltic glasses or secular equilibrium, respectively. High precision ID-TIMS analyses from nine additional samples from Hole 735B and nearby Hole 1105A suggest similar partitioning. The lower range of DPa zircon/DU zircon is consistent with ion microprobe measurements of 231Pa in zircons from Holocene and Pleistocene rhyolitic eruptions (Schmitt, 2007, 2011). The data suggest that 231Pa is preferentially incorporated during zircon crystallization over a range of magmatic compositions, and excess initial 231Pa may be more common in zircons than acknowledged. The degree of initial disequilibrium in the 235U decay chain suggested by the data from this study, and other recent high precision datasets, leads to resolvable discordance in high precision dates of Cenozoic to Mesozoic zircons. Minor discordance in zircons of this age may therefore reflect initial excess 231Pa and does not require either inheritance or Pb loss. Abstract Copyright (2015) Elsevier, B.V.
Year of Publication: 2015
Research Program: DSDP Deep Sea Drilling Project
IPOD International Phase of Ocean Drilling
ODP Ocean Drilling Program
Key Words: 02 Geochemistry; 03 Geochronology; Absolute age; Accuracy; Actinides; Atlantis II fracture zone; Basalts; Cenozoic; Chemical composition; Corrections; Crystal chemistry; DSDP Site 571; Dates; Deep Sea Drilling Project; East Pacific; Equatorial Pacific; Gabbros; Granophyre; IPOD; Igneous rocks; Indian Ocean; Ion probe data; Isotopes; Leg 138; Leg 179; Leg 85; Mass spectra; Metals; Mid-ocean ridge basalts; Mid-ocean ridges; Mineral assemblages; Miocene; Neogene; Nesosilicates; North Pacific; Northeast Pacific; ODP Site 1105; ODP Site 735; ODP Site 854; Ocean Drilling Program; Ocean floors; Orthosilicates; Pa-231; Pacific Ocean; Plutonic rocks; Precision; Protactinium; Radioactive decay; Radioactive isotopes; Rare earths; Silicates; Southwest Indian Ridge; Spectra; Tertiary; Textures; U-235; U/Pb; Uranium; Uranium disequilibrium; Volcanic rocks; Zircon; Zircon group
Coordinates: S324327 S324318 E0571618 E0571557
Record ID: 2015039569
Copyright Information: GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands