ANCIENT MANTLE HALOGEN (Cl, Br, I) COMPOSITION FROM ARCHAEAN KOMATIITES.
F. E. McDonald1, P. L. Clay1, K. H. Joy1, C. J. Ballentine2 and R. Burgess1, 1University of Manchester, SEAES, Oxford Road, Manchester, M13 9PL, UK ([email protected]), 2University of Oxford, Earth Sciences, South Parks Road, Oxford, OX1 3AN, UK.
Introduction: The halogens are volatile elements
that play a key role in terrestrial planetary processes.
For example, the halogens are strongly controlled by
fluid mobility and dominantly partition into melts influencing melt viscosity, solidus temperatures, and
rheology [1]. The halogens are not particularly affected
by fractional crystallisation or partial melting and can
be used as tracers for volatile concentrations and their
evolution on Earth [2].
Currently little is known about the origin of the
halogens, or about their abundance and behaviour within the mantle over time. Therefore, this study measures
heavy halogen (Cl, Br and I) concentrations and ratios
in Archaean komatiites, that have been selected as representative of the ancient terrestrial mantle.
Samples: Komatiite samples were selected on the
basis of age, geographical location, texture and alteration state. Samples are from four different greenstone
belt locations: Canada, (n=1, 2.7 Ga), South Africa
(n=3, 3.3 Ga), south east Baltic Shield (n=1, 2.4 Ga)
and Zimbabwe (n=4, 2.7 Ga). Examples of differing
mineral textures (olivine cumulative, pyroxene spinifex
and olivine spinifex) were chosen to reflect different
layers within a komatiite flow. All sample locations
show evidence of submarine emplacement of the komatiite flows (e.g. underlying pillow basalts) [3-6].
Each location has experienced a varying degree of serpentinisation, ranging from the Canadian sample which
is almost entirely serpentinised, to the remarkably fresh
Zimbabwe samples that have experienced little serpentinisation. The Zimbabwe and Baltic Shield samples
also contain abundant olivine-hosted melt inclusions.
Method: Samples of bulk rock and (where possible) olivine separates (~5 mg of each) were neutron
irradiated to convert the constituent halogens, Cl, Br
and I, into their respective noble gas isotopes, Ar, Kr
and Xe. The noble gases were released from samples
by both laser fusion and step heating for analysis by
noble gas mass spectrometry at the University of Manchester [7]. This technique permits few mg-sized samples to be analysed and as a by-product 40Ar-39Ar ages
may also be determined during the same analysis.
Results: The bulk halogen concentrations for the
samples from South Africa, Zimbabwe and Baltic
Shield are: Cl, ~84 to 315 ppm; Br, ~261 to 2319 ppb;
and I, ~3 to 30 ppb. The Canadian sample is considerably higher at ~470 ppm, ~4280 ppb and 180 ppb for
Cl, Br and I respectively. Except for the Canadian
sample which is greatly enriched in both Br and I, the
komatiite bulk halogen concentrations are within the
range determined for modern MORB [8-11].
Halogen ratios (Fig. 1) for both bulk and olivine
separates are between (~1.0-6.3)x10-5 for I/Cl, and
(1.2-3.3)x10-3 for Br/Cl, with the Canadian samples
again showing higher ratios at ~1.3x10-4 for I/Cl and
~4.8x10-3 for Br/Cl. Except for the Canadian sample,
the Archaean komatiites have ratios intermediate to
modern MORB [8-10] and the putative value of Archaean seawater [12].
During step heating, Br/Cl and I/Cl ratios do not
vary considerably with cumulative Cl-release. This
suggests that there is a single halogen-bearing component within the samples.
The 40Ar-39Ar ages are consistent with the literature
crystallisation ages [3-6], indicating that there has been
minor Ar loss or addition since closure in these particular samples. This limited post-emplacement alteration
indicates that these komatiites are capable of preseving
their primary Archaean (c3.3 – 2.4 Ga) mantle halogen
signature.
Figure 1: I/Cl and Br/Cl molar ratios for Archaean
komatiites (n=9) from four different locations. Bulk
rock (square) and olivine (triangle) halogen ratios are
depicted. Modern MORB [8-10], chondritic [13], Archaean seawater [12], and modern seawater halogen
ratios are indicated.
Discussion: The results suggest that mantle halogen
composition has not significantly changed during the
past three billion years. There is also no obvious relationship between halogen concentration with texture,
location, age or alteration.
The variation in halogen ratio between Archaean
seawater and modern MORB is consistent with their
submarine emplacement. Interaction with seawater
could have led to the incorporation of marine halogens
into the minerals.
Previously, a high 3He/4He ratio value of ~40 Ra
(where Ra is the atmospheric 3He/4He ratio of
1.386x10-6) has been determined for the Canadian
sample, inferred to be from an undegassed mantle
source [3]. The enrichment of halogens, and high Br/Cl
and I/Cl values in the Canadian sample (Fig. 1) may
therefore potentially be related to volatile-rich mantle,
distinct from the other komatiites analysed.
References: [1] Filiberto and Treiman (2009)
Chem. Geol., 263, (1-4) 60-68. [2] Pyle and Mather
(2009) Chem. Geol., 263, (1-4) 110-121. [3] Richard et
al. (1996) Science, 273, 93-95. [4] Nisbet et al. (1987)
Geology, 15, 1147-1150. [5] Lahaye et al. (1995)
Chem. Geol., 126, 43-64. [6] Puchtel et al. (1996) Contrib. Min. Pet., 124, 273-290. [7] Ruziè-Hamilton et
al., Chem. Geol. (submitted) [8] Jambon et al. (1995)
Chem. Geol. 126, 101-117. [9] Déruelle et al. (1992)
EPSL, 108, 217-227. [10] Kendrick et al. (2012a) Geology, 40, 1075-1078. [11] Schilling et al. (1980)
Trans.R. Soc. Math, Phy. Eng. Sci., 297, 147-178. [12]
Channer et al. (1997) EPSL, 150, 325-335. [13] Ballentine et al. (2014) AGU, Abs #P44A-04.
Acknowledgements: For their kind donation of
komatiite samples, thank you to Gary Byerly, Igor
Puchtel, Bernard Marty and Euan Nisbet. Funding
from the STFC and NERC are gratefully acknowledged.