A classification scheme for olivine populations in the Upper Pleistocene to Holocene Igwisi Hills (Tanzania) kimberlite lavas and investigation of grain origins and evolution
A classification scheme for olivine populations in the Upper Pleistocene to Holocene Igwisi Hills (Tanzania) kimberlite lavas and investigation of grain origins and evolution
Kimberlites are a class of alkaline igneous rock which sometimes contain diamonds and derive from mantle depths typically exceeding 150 km. We investigate the Upper Pleistocene to Holocene (12 ka) Igwisi Hills kimberlites to characterise the olivine grains and better understand their origins and evolution. We observe four distinct olivine populations. Macrocrysts are >1500 μm in diameter, rounded, monocrystalline grains with a Mg-rich xenocrystic core (Fo 90.5–92.8) and elevated Ni but depleted Ca and Mn contents, typical of granular mantle peridotite. Nodules are >1500 μm in diameter, rounded, polycrystalline grains with an Mg-rich core (Fo 91–92.4), also derived from granular peridotite. Microcryst grains are <1500 μm, subhedral-euhedral and can be subdivided into two types. Microcryst-a grains have a Mg-rich (Fo 90–92.5) xenocrystic core indicative of a mantle peridotite origin and microcryst-b grains have a Fe-rich (Fo 89–91) core that likely originated from disaggregated neoblasts. Olivine in all four populations displays four magmatic zones: internal zone, rim, rind and outermost rind, typical of kimberlites worldwide. Fe-rich internal zones are likely derived from a primitive kimberlite melt which entrained the cores. Mg-rich rims formed as the kimberlite melt evolved and crystallised around the internal zones. Fe-rich rinds formed as the kimberlite melt continued to ascend and further crystallised around the rims. Mg-rich outermost rinds crystallised from the kimberlite melt during the final stages of ascent. Based on these observations we present a new ascent model for the Igwisi Hills magmas.
Forsterite, Kimberlite, Mantle, Olivine, Xenocryst, Zoning
Rawlings, Jessica
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Gernon, Thomas
658041a0-fdd1-4516-85f4-98895a39235e
Palmer, Martin
d2e60e81-5d6e-4ddb-a243-602537286080
Stock, Michael
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Petrone, Chiara Maria
96a1e55f-5a0f-4e31-a243-e6a1ce0ee7bc
Brown, Richard J.
5b122e28-c030-4e74-8fef-9306680f92a6
Humphreys-Williams, Emma
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26 November 2025
Rawlings, Jessica
d697a57f-a698-4b11-be2a-107e50fd05d9
Gernon, Thomas
658041a0-fdd1-4516-85f4-98895a39235e
Palmer, Martin
d2e60e81-5d6e-4ddb-a243-602537286080
Stock, Michael
02603456-1779-48ff-af0d-0cee7f6e0f1e
Petrone, Chiara Maria
96a1e55f-5a0f-4e31-a243-e6a1ce0ee7bc
Brown, Richard J.
5b122e28-c030-4e74-8fef-9306680f92a6
Humphreys-Williams, Emma
bdc779dc-961a-4965-bd06-90d95da5da89
Rawlings, Jessica, Gernon, Thomas, Palmer, Martin, Stock, Michael, Petrone, Chiara Maria, Brown, Richard J. and Humphreys-Williams, Emma
(2025)
A classification scheme for olivine populations in the Upper Pleistocene to Holocene Igwisi Hills (Tanzania) kimberlite lavas and investigation of grain origins and evolution.
Journal of Volcanology and Geothermal Research, 469, [108497].
(doi:10.1016/j.jvolgeores.2025.108497).
Abstract
Kimberlites are a class of alkaline igneous rock which sometimes contain diamonds and derive from mantle depths typically exceeding 150 km. We investigate the Upper Pleistocene to Holocene (12 ka) Igwisi Hills kimberlites to characterise the olivine grains and better understand their origins and evolution. We observe four distinct olivine populations. Macrocrysts are >1500 μm in diameter, rounded, monocrystalline grains with a Mg-rich xenocrystic core (Fo 90.5–92.8) and elevated Ni but depleted Ca and Mn contents, typical of granular mantle peridotite. Nodules are >1500 μm in diameter, rounded, polycrystalline grains with an Mg-rich core (Fo 91–92.4), also derived from granular peridotite. Microcryst grains are <1500 μm, subhedral-euhedral and can be subdivided into two types. Microcryst-a grains have a Mg-rich (Fo 90–92.5) xenocrystic core indicative of a mantle peridotite origin and microcryst-b grains have a Fe-rich (Fo 89–91) core that likely originated from disaggregated neoblasts. Olivine in all four populations displays four magmatic zones: internal zone, rim, rind and outermost rind, typical of kimberlites worldwide. Fe-rich internal zones are likely derived from a primitive kimberlite melt which entrained the cores. Mg-rich rims formed as the kimberlite melt evolved and crystallised around the internal zones. Fe-rich rinds formed as the kimberlite melt continued to ascend and further crystallised around the rims. Mg-rich outermost rinds crystallised from the kimberlite melt during the final stages of ascent. Based on these observations we present a new ascent model for the Igwisi Hills magmas.
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Rawlings_et_al_2025_JVGR
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Accepted/In Press date: 17 November 2025
e-pub ahead of print date: 21 November 2025
Published date: 26 November 2025
Keywords:
Forsterite, Kimberlite, Mantle, Olivine, Xenocryst, Zoning
Identifiers
Local EPrints ID: 507411
URI: http://eprints.soton.ac.uk/id/eprint/507411
ISSN: 0377-0273
PURE UUID: 605fd4a1-0714-491d-88fc-164c463b9dec
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Date deposited: 09 Dec 2025 17:35
Last modified: 11 Dec 2025 03:05
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Author:
Jessica Rawlings
Author:
Michael Stock
Author:
Chiara Maria Petrone
Author:
Richard J. Brown
Author:
Emma Humphreys-Williams
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