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Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox

Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox
Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox
To consider the 3He characteristics of plume-related lavas, we report a detailed survey of helium isotope (3He/4He) and concentration ([He]) variations along an 800-km transect of the Reykjanes Ridge (RR). 3He/4He ratios vary from 11.0 to 17.6 RA (where RA=air 3He/4He) whereas [He] ranges over three orders of magnitude from >5 cm3 STP/g – in the range of most mid-ocean ridge basalts (MORB) – to lows of 4 ncm3 STP/g. The lowest [He] and intermediate 3He/4He ratios occur along the northern RR (closest to Iceland) where eruption depths are shallow (<1000 m) and water contents of lavas are high (0.3–0.4 wt%). We suggest that low-pressure, pre-eruptive magmatic degassing is extensive in this region with degassed magmas susceptible to addition of radiogenic helium thereby lowering 3He/4He ratios. Along the southern RR, [He] reaches maximum values, and 3He/4He ratios display strong correlations with lead isotopes (206Pb/204Pb) consistent with binary mixing. These correlations indicate that the high-3He/4He plume component has higher absolute abundances of the primordial isotope 3He compared to the source of depleted MORB mantle. This finding implies that the so-called ‘helium paradox’ – the observation that plume-derived oceanic glasses apparently have lower 3He contents than MORB glasses – may be an artifact related to considering lavas (e.g. from Loihi seamount, Hawaii) which have not retained their source volatile inventory as well as those erupted along the southern RR.
REYKJANES RIDGE, HELIUM ISOTOPES, LEAD ISOTOPES, MANTLE, MIXING, DEGASSING
0012-821X
43-50
Hilton, D.R.
614511d2-7a07-4f72-ad16-b5a50ed62732
Thirlwall, M.F.
eef507cc-401b-4d16-bccf-953c37509e7e
Taylor, R.N.
094be7fd-ef61-4acd-a795-7daba2bc6183
Murton, B.J.
9076d07f-a3c1-4f90-a5d5-99b27fe2cb12
Nichols, A.
350b1f5d-4225-4e4d-95b4-5ae29cadb7f8
Hilton, D.R.
614511d2-7a07-4f72-ad16-b5a50ed62732
Thirlwall, M.F.
eef507cc-401b-4d16-bccf-953c37509e7e
Taylor, R.N.
094be7fd-ef61-4acd-a795-7daba2bc6183
Murton, B.J.
9076d07f-a3c1-4f90-a5d5-99b27fe2cb12
Nichols, A.
350b1f5d-4225-4e4d-95b4-5ae29cadb7f8

Hilton, D.R., Thirlwall, M.F., Taylor, R.N., Murton, B.J. and Nichols, A. (2000) Controls on magmatic degassing along the Reykjanes Ridge with implications for the helium paradox. Earth and Planetary Science Letters, 183 (1/2), 43-50. (doi:10.1016/S0012-821X(00)00253-3).

Record type: Article

Abstract

To consider the 3He characteristics of plume-related lavas, we report a detailed survey of helium isotope (3He/4He) and concentration ([He]) variations along an 800-km transect of the Reykjanes Ridge (RR). 3He/4He ratios vary from 11.0 to 17.6 RA (where RA=air 3He/4He) whereas [He] ranges over three orders of magnitude from >5 cm3 STP/g – in the range of most mid-ocean ridge basalts (MORB) – to lows of 4 ncm3 STP/g. The lowest [He] and intermediate 3He/4He ratios occur along the northern RR (closest to Iceland) where eruption depths are shallow (<1000 m) and water contents of lavas are high (0.3–0.4 wt%). We suggest that low-pressure, pre-eruptive magmatic degassing is extensive in this region with degassed magmas susceptible to addition of radiogenic helium thereby lowering 3He/4He ratios. Along the southern RR, [He] reaches maximum values, and 3He/4He ratios display strong correlations with lead isotopes (206Pb/204Pb) consistent with binary mixing. These correlations indicate that the high-3He/4He plume component has higher absolute abundances of the primordial isotope 3He compared to the source of depleted MORB mantle. This finding implies that the so-called ‘helium paradox’ – the observation that plume-derived oceanic glasses apparently have lower 3He contents than MORB glasses – may be an artifact related to considering lavas (e.g. from Loihi seamount, Hawaii) which have not retained their source volatile inventory as well as those erupted along the southern RR.

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Published date: 2000
Keywords: REYKJANES RIDGE, HELIUM ISOTOPES, LEAD ISOTOPES, MANTLE, MIXING, DEGASSING

Identifiers

Local EPrints ID: 8779
URI: https://eprints.soton.ac.uk/id/eprint/8779
ISSN: 0012-821X
PURE UUID: 40faaa12-800f-4fd4-a7ba-eb4eb5aeba31

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Date deposited: 13 Sep 2004
Last modified: 17 Jul 2017 17:12

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Contributors

Author: D.R. Hilton
Author: M.F. Thirlwall
Author: R.N. Taylor
Author: B.J. Murton
Author: A. Nichols

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