210Pb-226Ra and 228Ra-232Th systematics in young arc lavas: implications for magma degassing and ascent rates

Simon Turner, Stuart Black, Kim Berlo

Research output: Contribution to journalArticleResearchpeer-review

Abstract

New data show that island arc rocks have (210Pb/226Ra)o ratios which range from as low as 0.24 up to 2.88. In contrast, (228Ra/232Th) appears always within error of 1 suggesting that the large 226Ra-excesses observed in arc rocks were generated more than 30 years ago. This places a maximum estimate on melt ascent velocities of around 4000 m/year and provides further confidence that the 226Ra excesses reflect deep (source) processes rather than shallow level alteration or seawater contamination. Conversely, partial melting must have occurred more than 30 years prior to eruption. The 210Pb deficits are most readily explained by protracted magma degassing. Using published numerical models, the data suggest that degassing occurred continuously for periods up to several decades just prior to eruption but no link with eruption periodicity was found. Longer periods are required if degassing is discontinuous, less than 100% efficient or if magma is recharged or stored after degassing. The long durations suggest much of this degassing occurs at depth with implications for the formation of hydrothermal and copper-porphyry systems. A suite of lavas erupted in 1985-1986 from Sangeang Api volcano in the Sunda arc are characterised by deficits of 210Pb relative to 226Ra from which 6-8 years of continuous 222Rn degassing would be inferred from recent numerical models. These data also form a linear (210Pb)/Pb- (226Ra)/Pb array which might be interpreted as a 71-year isochron. However, the array passes through the origin suggesting displacement downwards from the equiline in response to degassing and so the slope of the array is inferred not to have any age significance. Simple modelling shows that the range of (226Ra)/Pb ratios requires thousands of years to develop consistent with differentiation occurring in response to cooling at the base of the crust. Thus, degassing post-dated, and was not responsible for magma differentiation. The formation, migration and extraction of gas bubbles must be extremely efficient in mafic magma whereas the higher viscosity of more siliceous magmas retards the process and can lead to 210Pb excesses. A possible negative correlation between (210Pb/226Ra)o and SO2 emission rate requires further testing but may have implications for future eruptions.

LanguageEnglish
Pages1-16
Number of pages16
JournalEarth and Planetary Science Letters
Volume227
Issue number1-2
DOIs
Publication statusPublished - 2004

Fingerprint

degassing
ascent
Degassing
magma
arcs
volcanic eruptions
volcanic eruption
Numerical models
Rocks
rocks
Volcanoes
island arcs
rate
young
Seawater
porphyry
volcanoes
rock
periodicity
partial melting

Cite this

@article{bb0cb71ec93441d4b4f1e7457bc9e19d,
title = "210Pb-226Ra and 228Ra-232Th systematics in young arc lavas: implications for magma degassing and ascent rates",
abstract = "New data show that island arc rocks have (210Pb/226Ra)o ratios which range from as low as 0.24 up to 2.88. In contrast, (228Ra/232Th) appears always within error of 1 suggesting that the large 226Ra-excesses observed in arc rocks were generated more than 30 years ago. This places a maximum estimate on melt ascent velocities of around 4000 m/year and provides further confidence that the 226Ra excesses reflect deep (source) processes rather than shallow level alteration or seawater contamination. Conversely, partial melting must have occurred more than 30 years prior to eruption. The 210Pb deficits are most readily explained by protracted magma degassing. Using published numerical models, the data suggest that degassing occurred continuously for periods up to several decades just prior to eruption but no link with eruption periodicity was found. Longer periods are required if degassing is discontinuous, less than 100{\%} efficient or if magma is recharged or stored after degassing. The long durations suggest much of this degassing occurs at depth with implications for the formation of hydrothermal and copper-porphyry systems. A suite of lavas erupted in 1985-1986 from Sangeang Api volcano in the Sunda arc are characterised by deficits of 210Pb relative to 226Ra from which 6-8 years of continuous 222Rn degassing would be inferred from recent numerical models. These data also form a linear (210Pb)/Pb- (226Ra)/Pb array which might be interpreted as a 71-year isochron. However, the array passes through the origin suggesting displacement downwards from the equiline in response to degassing and so the slope of the array is inferred not to have any age significance. Simple modelling shows that the range of (226Ra)/Pb ratios requires thousands of years to develop consistent with differentiation occurring in response to cooling at the base of the crust. Thus, degassing post-dated, and was not responsible for magma differentiation. The formation, migration and extraction of gas bubbles must be extremely efficient in mafic magma whereas the higher viscosity of more siliceous magmas retards the process and can lead to 210Pb excesses. A possible negative correlation between (210Pb/226Ra)o and SO2 emission rate requires further testing but may have implications for future eruptions.",
author = "Simon Turner and Stuart Black and Kim Berlo",
year = "2004",
doi = "10.1016/j.epsl.2004.08.017",
language = "English",
volume = "227",
pages = "1--16",
journal = "Earth and Planetary Science Letters",
issn = "0012-821X",
publisher = "Elsevier",
number = "1-2",

}

210Pb-226Ra and 228Ra-232Th systematics in young arc lavas : implications for magma degassing and ascent rates. / Turner, Simon; Black, Stuart; Berlo, Kim.

In: Earth and Planetary Science Letters, Vol. 227, No. 1-2, 2004, p. 1-16.

Research output: Contribution to journalArticleResearchpeer-review

TY - JOUR

T1 - 210Pb-226Ra and 228Ra-232Th systematics in young arc lavas

T2 - Earth and Planetary Science Letters

AU - Turner, Simon

AU - Black, Stuart

AU - Berlo, Kim

PY - 2004

Y1 - 2004

N2 - New data show that island arc rocks have (210Pb/226Ra)o ratios which range from as low as 0.24 up to 2.88. In contrast, (228Ra/232Th) appears always within error of 1 suggesting that the large 226Ra-excesses observed in arc rocks were generated more than 30 years ago. This places a maximum estimate on melt ascent velocities of around 4000 m/year and provides further confidence that the 226Ra excesses reflect deep (source) processes rather than shallow level alteration or seawater contamination. Conversely, partial melting must have occurred more than 30 years prior to eruption. The 210Pb deficits are most readily explained by protracted magma degassing. Using published numerical models, the data suggest that degassing occurred continuously for periods up to several decades just prior to eruption but no link with eruption periodicity was found. Longer periods are required if degassing is discontinuous, less than 100% efficient or if magma is recharged or stored after degassing. The long durations suggest much of this degassing occurs at depth with implications for the formation of hydrothermal and copper-porphyry systems. A suite of lavas erupted in 1985-1986 from Sangeang Api volcano in the Sunda arc are characterised by deficits of 210Pb relative to 226Ra from which 6-8 years of continuous 222Rn degassing would be inferred from recent numerical models. These data also form a linear (210Pb)/Pb- (226Ra)/Pb array which might be interpreted as a 71-year isochron. However, the array passes through the origin suggesting displacement downwards from the equiline in response to degassing and so the slope of the array is inferred not to have any age significance. Simple modelling shows that the range of (226Ra)/Pb ratios requires thousands of years to develop consistent with differentiation occurring in response to cooling at the base of the crust. Thus, degassing post-dated, and was not responsible for magma differentiation. The formation, migration and extraction of gas bubbles must be extremely efficient in mafic magma whereas the higher viscosity of more siliceous magmas retards the process and can lead to 210Pb excesses. A possible negative correlation between (210Pb/226Ra)o and SO2 emission rate requires further testing but may have implications for future eruptions.

AB - New data show that island arc rocks have (210Pb/226Ra)o ratios which range from as low as 0.24 up to 2.88. In contrast, (228Ra/232Th) appears always within error of 1 suggesting that the large 226Ra-excesses observed in arc rocks were generated more than 30 years ago. This places a maximum estimate on melt ascent velocities of around 4000 m/year and provides further confidence that the 226Ra excesses reflect deep (source) processes rather than shallow level alteration or seawater contamination. Conversely, partial melting must have occurred more than 30 years prior to eruption. The 210Pb deficits are most readily explained by protracted magma degassing. Using published numerical models, the data suggest that degassing occurred continuously for periods up to several decades just prior to eruption but no link with eruption periodicity was found. Longer periods are required if degassing is discontinuous, less than 100% efficient or if magma is recharged or stored after degassing. The long durations suggest much of this degassing occurs at depth with implications for the formation of hydrothermal and copper-porphyry systems. A suite of lavas erupted in 1985-1986 from Sangeang Api volcano in the Sunda arc are characterised by deficits of 210Pb relative to 226Ra from which 6-8 years of continuous 222Rn degassing would be inferred from recent numerical models. These data also form a linear (210Pb)/Pb- (226Ra)/Pb array which might be interpreted as a 71-year isochron. However, the array passes through the origin suggesting displacement downwards from the equiline in response to degassing and so the slope of the array is inferred not to have any age significance. Simple modelling shows that the range of (226Ra)/Pb ratios requires thousands of years to develop consistent with differentiation occurring in response to cooling at the base of the crust. Thus, degassing post-dated, and was not responsible for magma differentiation. The formation, migration and extraction of gas bubbles must be extremely efficient in mafic magma whereas the higher viscosity of more siliceous magmas retards the process and can lead to 210Pb excesses. A possible negative correlation between (210Pb/226Ra)o and SO2 emission rate requires further testing but may have implications for future eruptions.

UR - http://www.scopus.com/inward/record.url?scp=5044220882&partnerID=8YFLogxK

U2 - 10.1016/j.epsl.2004.08.017

DO - 10.1016/j.epsl.2004.08.017

M3 - Article

VL - 227

SP - 1

EP - 16

JO - Earth and Planetary Science Letters

JF - Earth and Planetary Science Letters

SN - 0012-821X

IS - 1-2

ER -