The University of Southampton
University of Southampton Institutional Repository

Petrogenesis and geochemistry of Archean Komatiites

Petrogenesis and geochemistry of Archean Komatiites
Petrogenesis and geochemistry of Archean Komatiites
Komatiites are products of decompression melting of mantle so hot that they are almost exclusively restricted to the Archean. The high degree of partial melting (F) and pressure (P) required for their generation facilitates comparison between the magma composition and its mantle source. To investigate compositional variations in Archean komatiites, a global selection of 38 Archean komatiites spanning five cratons (Kaapvaal, Zimbabwe, Yilgarn, Pilbara, Superior) were analysed for their major and trace element contents. Included are the Aluminium-Depleted (ADK, Barberton-Type) and Aluminium-Undepleted (AUK, Munro-Type) petrogenetic types that have been equated with high P/moderate F and moderate P/high F, respectively, on the basis of their Al/Ti and Gd/Yb ratios. Following calculation of the primary magma composition of each suite, we show that the absolute Al content at a specified MgO proves a more sensitive indicator of P than either of the above two ratios and hence we introduce a new classification using Al. The Mg# is a reliable proxy for F, independent of the two endmember melting styles, fractional and batch. We demonstrate that most komatiites form by batch melting, ceding to fractional melting with decreasing pressure as the density contrast between the liquid and solid grows. The Munro AUKs are the only suite to show evidence of fractional melting, with melt extraction occurring at the lowest F and P, ?25% melting at 5?GPa (mantle potential temperature, TP?=?1750°C) whereas the ADKs of Barberton segregated at the highest F and P (?40%, 9?GPa, TP?=?1950°C). The petrogenetic type is a combination of P and F, where, at a given pressure, higher F will produce AUKs over ADKs as majorite is consumed in the source. Through numerical simulations, it is shown that both types can occur within the same mantle plume, with ADKs forming in its cooler, distal fringes whereas AUKs occur along its axis. Furthermore, and contrary to previous views, there is no temporal distinction between the two komatiite types, with both AUKs and ADKs occurring throughout the Archean. By contrast, younger, 2·7 Ga komatiites tend to have sources that are more depleted than those of older, 3·5 Ga komatiites. Komatiites are invaluable records of the mantle’s chemical and physical evolution during the Archean.
komatiite, partial melting, geochemistry, thermodynamics, temperature, mantle plume
0022-3530
147-184
Sossi, Paolo A.
21430383-4895-473d-ad38-20646846550c
Eggins, Stephen M.
dbd78280-ca98-4654-999d-53bfada31e43
Nesbitt, Robert W.
6a124ad1-4e6d-4407-b92f-592f7fd682e4
Nebel, Oliver
5bc20a52-7ade-44d8-a205-debee7bf65a7
Hergt, Janet M.
fe6c1cc5-b5b5-482d-ba96-f88d1eeb50ee
Campbell, Ian H.
8e224757-9adf-4465-bdf3-4046669532e4
O’Neill, Hugh St.C.
84c063ec-2bca-4bac-bcad-9bdb060029a8
Van Kranendonk, Martin
0ffe4f01-01ab-49af-b01b-cc27675d740b
Davies, D. Rhodri
45b7db26-cd29-4fcc-8dce-35af070bf9cc
Sossi, Paolo A.
21430383-4895-473d-ad38-20646846550c
Eggins, Stephen M.
dbd78280-ca98-4654-999d-53bfada31e43
Nesbitt, Robert W.
6a124ad1-4e6d-4407-b92f-592f7fd682e4
Nebel, Oliver
5bc20a52-7ade-44d8-a205-debee7bf65a7
Hergt, Janet M.
fe6c1cc5-b5b5-482d-ba96-f88d1eeb50ee
Campbell, Ian H.
8e224757-9adf-4465-bdf3-4046669532e4
O’Neill, Hugh St.C.
84c063ec-2bca-4bac-bcad-9bdb060029a8
Van Kranendonk, Martin
0ffe4f01-01ab-49af-b01b-cc27675d740b
Davies, D. Rhodri
45b7db26-cd29-4fcc-8dce-35af070bf9cc

Sossi, Paolo A., Eggins, Stephen M., Nesbitt, Robert W., Nebel, Oliver, Hergt, Janet M., Campbell, Ian H., O’Neill, Hugh St.C., Van Kranendonk, Martin and Davies, D. Rhodri (2016) Petrogenesis and geochemistry of Archean Komatiites. Journal of Petrology, 57 (1), 147-184. (doi:10.1093/petrology/egw004).

Record type: Article

Abstract

Komatiites are products of decompression melting of mantle so hot that they are almost exclusively restricted to the Archean. The high degree of partial melting (F) and pressure (P) required for their generation facilitates comparison between the magma composition and its mantle source. To investigate compositional variations in Archean komatiites, a global selection of 38 Archean komatiites spanning five cratons (Kaapvaal, Zimbabwe, Yilgarn, Pilbara, Superior) were analysed for their major and trace element contents. Included are the Aluminium-Depleted (ADK, Barberton-Type) and Aluminium-Undepleted (AUK, Munro-Type) petrogenetic types that have been equated with high P/moderate F and moderate P/high F, respectively, on the basis of their Al/Ti and Gd/Yb ratios. Following calculation of the primary magma composition of each suite, we show that the absolute Al content at a specified MgO proves a more sensitive indicator of P than either of the above two ratios and hence we introduce a new classification using Al. The Mg# is a reliable proxy for F, independent of the two endmember melting styles, fractional and batch. We demonstrate that most komatiites form by batch melting, ceding to fractional melting with decreasing pressure as the density contrast between the liquid and solid grows. The Munro AUKs are the only suite to show evidence of fractional melting, with melt extraction occurring at the lowest F and P, ?25% melting at 5?GPa (mantle potential temperature, TP?=?1750°C) whereas the ADKs of Barberton segregated at the highest F and P (?40%, 9?GPa, TP?=?1950°C). The petrogenetic type is a combination of P and F, where, at a given pressure, higher F will produce AUKs over ADKs as majorite is consumed in the source. Through numerical simulations, it is shown that both types can occur within the same mantle plume, with ADKs forming in its cooler, distal fringes whereas AUKs occur along its axis. Furthermore, and contrary to previous views, there is no temporal distinction between the two komatiite types, with both AUKs and ADKs occurring throughout the Archean. By contrast, younger, 2·7 Ga komatiites tend to have sources that are more depleted than those of older, 3·5 Ga komatiites. Komatiites are invaluable records of the mantle’s chemical and physical evolution during the Archean.

Full text not available from this repository.

More information

Accepted/In Press date: 14 January 2016
Published date: 25 February 2016
Keywords: komatiite, partial melting, geochemistry, thermodynamics, temperature, mantle plume
Organisations: Geochemistry

Identifiers

Local EPrints ID: 396550
URI: https://eprints.soton.ac.uk/id/eprint/396550
ISSN: 0022-3530
PURE UUID: a48699e6-8638-4f09-814d-f439d2e6bc0d

Catalogue record

Date deposited: 07 Jun 2016 14:25
Last modified: 15 Jul 2019 20:23

Export record

Altmetrics

Contributors

Author: Paolo A. Sossi
Author: Stephen M. Eggins
Author: Robert W. Nesbitt
Author: Oliver Nebel
Author: Janet M. Hergt
Author: Ian H. Campbell
Author: Hugh St.C. O’Neill
Author: Martin Van Kranendonk
Author: D. Rhodri Davies

University divisions

Download statistics

Downloads from ePrints over the past year. Other digital versions may also be available to download e.g. from the publisher's website.

View more statistics

Atom RSS 1.0 RSS 2.0

Contact ePrints Soton: eprints@soton.ac.uk

ePrints Soton supports OAI 2.0 with a base URL of https://eprints.soton.ac.uk/cgi/oai2

This repository has been built using EPrints software, developed at the University of Southampton, but available to everyone to use.

We use cookies to ensure that we give you the best experience on our website. If you continue without changing your settings, we will assume that you are happy to receive cookies on the University of Southampton website.

×