40Ar/39Ar age of the Rotoiti Breccia and Rotoehu Ash, Okataina Volcanic Complex, New Zealand, and identification of heterogeneously distributed excess 40Ar in supercooled crystals

Stephanie Flude; Michael Storey

doi:10.1016/j.quageo.2016.01.002

⁴⁰Ar/³⁹Ar age of the Rotoiti Breccia and Rotoehu Ash, Okataina Volcanic Complex, New Zealand, and identification of heterogeneously distributed excess ⁴⁰Ar in supercooled crystals

Stephanie Flude^*, Michael Storey

^*Corresponding author for this work

19 Citations (Scopus)

Abstract

Co-magmatic granitoid clasts erupted as part of the Rotoiti Ignimbrite (Rotoehu Tephra) contain euhedral K-feldspar and biotite crystals that protrude into miarolytic cavities and show textural evidence for growth in super-cooled conditions and are thus interpreted as growing during eruption. ⁴⁰Ar/³⁹Ar stepped heating experiments on single K-feldspar crystals reveal the presence of heterogeneously distributed excess ⁴⁰Ar, preferentially released at lower temperature steps (most likely from fluid/melt inclusions), which cannot reliably be characterised by, or corrected for using isotope correlation diagrams due to mixing between three reservoirs of ⁴⁰Ar (radiogenic, atmospheric and excess). This excess ⁴⁰Ar component is common, but not ubiquitous, and an age population unmixing algorithm applied to single-crystal fusion data identifies a younger group of K-feldspar and biotite crystals that appear to be largely unaffected by excess ⁴⁰Ar. This population gives a statistically robust weighted mean age of 47.4 ± 1.5 ka (1σ, n = 13) and an indistinguishable inverse isochron age of 50 ± 3 ka for this historically difficult to date eruption. The weighted mean age is significantly younger than previous age estimates of the Rotoiti eruption obtained by K/Ar and ⁴⁰Ar/³⁹Ar dating of bracketing lavas, but is indistinguishable from recent ¹⁴C and (U-Th)/He dates and estimates based on orbital tuning and sedimentation rates constrained by ¹⁴C ages.

Original language	English
Journal	Quaternary Geochronology
Volume	33
Pages (from-to)	13-23
Number of pages	11
ISSN	1871-1014
DOIs	https://doi.org/10.1016/j.quageo.2016.01.002
Publication status	Published - 2016
Externally published	Yes

Keywords

Ar/Ar
Excess-Ar
Rotoiti ignimbrite eruption
Taupo Volcanic Zone

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10.1016/j.quageo.2016.01.002

Cite this

@article{6d02a13e12274166b7f3738f524b796e,

title = "40Ar/39Ar age of the Rotoiti Breccia and Rotoehu Ash, Okataina Volcanic Complex, New Zealand, and identification of heterogeneously distributed excess 40Ar in supercooled crystals",

abstract = "Co-magmatic granitoid clasts erupted as part of the Rotoiti Ignimbrite (Rotoehu Tephra) contain euhedral K-feldspar and biotite crystals that protrude into miarolytic cavities and show textural evidence for growth in super-cooled conditions and are thus interpreted as growing during eruption. 40Ar/39Ar stepped heating experiments on single K-feldspar crystals reveal the presence of heterogeneously distributed excess 40Ar, preferentially released at lower temperature steps (most likely from fluid/melt inclusions), which cannot reliably be characterised by, or corrected for using isotope correlation diagrams due to mixing between three reservoirs of 40Ar (radiogenic, atmospheric and excess). This excess 40Ar component is common, but not ubiquitous, and an age population unmixing algorithm applied to single-crystal fusion data identifies a younger group of K-feldspar and biotite crystals that appear to be largely unaffected by excess 40Ar. This population gives a statistically robust weighted mean age of 47.4 ± 1.5 ka (1σ, n = 13) and an indistinguishable inverse isochron age of 50 ± 3 ka for this historically difficult to date eruption. The weighted mean age is significantly younger than previous age estimates of the Rotoiti eruption obtained by K/Ar and 40Ar/39Ar dating of bracketing lavas, but is indistinguishable from recent 14C and (U-Th)/He dates and estimates based on orbital tuning and sedimentation rates constrained by 14C ages.",

keywords = "Ar/Ar, Excess-Ar, Rotoiti ignimbrite eruption, Taupo Volcanic Zone",

author = "Stephanie Flude and Michael Storey",

year = "2016",

doi = "10.1016/j.quageo.2016.01.002",

language = "English",

volume = "33",

pages = "13--23",

journal = "Quaternary Geochronology",

issn = "1871-1014",

publisher = "Elsevier",

}

TY - JOUR

T1 - 40Ar/39Ar age of the Rotoiti Breccia and Rotoehu Ash, Okataina Volcanic Complex, New Zealand, and identification of heterogeneously distributed excess 40Ar in supercooled crystals

AU - Flude, Stephanie

AU - Storey, Michael

PY - 2016

Y1 - 2016

N2 - Co-magmatic granitoid clasts erupted as part of the Rotoiti Ignimbrite (Rotoehu Tephra) contain euhedral K-feldspar and biotite crystals that protrude into miarolytic cavities and show textural evidence for growth in super-cooled conditions and are thus interpreted as growing during eruption. 40Ar/39Ar stepped heating experiments on single K-feldspar crystals reveal the presence of heterogeneously distributed excess 40Ar, preferentially released at lower temperature steps (most likely from fluid/melt inclusions), which cannot reliably be characterised by, or corrected for using isotope correlation diagrams due to mixing between three reservoirs of 40Ar (radiogenic, atmospheric and excess). This excess 40Ar component is common, but not ubiquitous, and an age population unmixing algorithm applied to single-crystal fusion data identifies a younger group of K-feldspar and biotite crystals that appear to be largely unaffected by excess 40Ar. This population gives a statistically robust weighted mean age of 47.4 ± 1.5 ka (1σ, n = 13) and an indistinguishable inverse isochron age of 50 ± 3 ka for this historically difficult to date eruption. The weighted mean age is significantly younger than previous age estimates of the Rotoiti eruption obtained by K/Ar and 40Ar/39Ar dating of bracketing lavas, but is indistinguishable from recent 14C and (U-Th)/He dates and estimates based on orbital tuning and sedimentation rates constrained by 14C ages.

AB - Co-magmatic granitoid clasts erupted as part of the Rotoiti Ignimbrite (Rotoehu Tephra) contain euhedral K-feldspar and biotite crystals that protrude into miarolytic cavities and show textural evidence for growth in super-cooled conditions and are thus interpreted as growing during eruption. 40Ar/39Ar stepped heating experiments on single K-feldspar crystals reveal the presence of heterogeneously distributed excess 40Ar, preferentially released at lower temperature steps (most likely from fluid/melt inclusions), which cannot reliably be characterised by, or corrected for using isotope correlation diagrams due to mixing between three reservoirs of 40Ar (radiogenic, atmospheric and excess). This excess 40Ar component is common, but not ubiquitous, and an age population unmixing algorithm applied to single-crystal fusion data identifies a younger group of K-feldspar and biotite crystals that appear to be largely unaffected by excess 40Ar. This population gives a statistically robust weighted mean age of 47.4 ± 1.5 ka (1σ, n = 13) and an indistinguishable inverse isochron age of 50 ± 3 ka for this historically difficult to date eruption. The weighted mean age is significantly younger than previous age estimates of the Rotoiti eruption obtained by K/Ar and 40Ar/39Ar dating of bracketing lavas, but is indistinguishable from recent 14C and (U-Th)/He dates and estimates based on orbital tuning and sedimentation rates constrained by 14C ages.

KW - Ar/Ar

KW - Excess-Ar

KW - Rotoiti ignimbrite eruption

KW - Taupo Volcanic Zone

U2 - 10.1016/j.quageo.2016.01.002

DO - 10.1016/j.quageo.2016.01.002

M3 - Journal article

AN - SCOPUS:84955288964

SN - 1871-1014

VL - 33

SP - 13

EP - 23

JO - Quaternary Geochronology

JF - Quaternary Geochronology

ER -