Cr-Saturation Arrays in Concentrate Garnet Compositions from Kimberlite and their Use in Mantle Barometry

Grütter, Herman; Latti, Dewetia; Menzies, Andrew

doi:10.1093/petrology/egi096

Cited by 167 publications

(101 citation statements)

References 53 publications

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“…Depth calculations are based on Dziewonski and Anderson (1981). Temperatures along the preferred 40 mWm −2 geotherm of Hasterok and Chapman (2011) Grütter et al 2006). Identification of unaltered orthopyroxene in equilibrium with clinopyroxene in mantle xenoliths would allow more robust geothermometry.…”

Section: Mineral Chemistrymentioning

confidence: 99%

The mantle of Scotland viewed through the Glen Gollaidh aillikite

Hutchison¹,

Faithfull

Barfod

et al. 2018

Miner Petrol

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The Glen Gollaidh aillikite dyke (58.36741°N 4.69751°W), N.W. Scotland, occurs within the Neoproterozoic sedimentary rocks of the Moine Supergroup~4 km east of the Moine Thrust. Phlogopite 40 Ar/ 36 Ar measurements give a late Devonian maximum emplacement age of 360.3 ± 4.9 (2σ) Ma. This age occurs in a quiet period of Scottish magmatic history c. 30 Ma after the closure of the Iapetus and before the start of intra-plate alkali magmatism which affected southern Scotland for~60 My from c. 350 Ma. Abundant chromites and Cr-diopsides and a few unaltered olivines, reflecting a mantle provenance, were recovered from heavy mineral concentrates. The North Atlantic Craton, exposed in Lewisian gneisses west of the Moine thrust, is therefore inferred to extend east at depth under Glen Gollaidh, presenting an opportunity to investigate the thickness and composition of the cratonic margin in the Devonian. The aillikite was found to be barren of diamond and no picro-ilmenites or garnets were definitively identified. However, mineral chemistry suggests that a proportion of Glen Gollaidh xenocrysts crystallised in equilibrium with garnet. Most spinels are Mg, Al chromites, with some Mg chromite present. All fall within the garnet peridotite field based on Ti and Cr but with insufficient Cr 2 O 3 (up to 47.2 wt%) to be consistent with the diamond stability field. Amongst Cr-diopsides 30% of grains have Cr and Al contents consistent with derivation from garnet peridotite. The majority of clinopyroxenes also show a marked depletion in heavy compared to light rare-earth elements, again consistent with equilibration with garnet. The opx-cpx solvus thermometer demonstrates that average Cr-diopside compositions require at least 37 kbar to give a temperature (979°C) lying even on a relatively warm 40 mWm −2 geotherm (Hasterok and Chapman Earth Planet Sc Lett 307:59-70, 2011). Large variations in the chemistry of mantle minerals reflect a complex history of metasomatism akin to constituents of alkali igneous rocks elsewhere in the Hebridean and Northern Highlands Terranes. Fertilised mantle provided the conditions for generation of aillikite melts, probably triggered by break-off of the advancing Avalonia slab. The cratonic root underlying the Glen Gollaidh aillikite during the late Devonian was apparently too thin to lie within the diamond stability field, consistent with xenoliths from alkali basalts further south. Nonetheless, sufficient geophysical and mineral chemical evidence supports Glen Gollaidh aillikite sitting close to the edge of diamond-prospective mantle therefore suggesting diamond potential a short distance to the west within the Lewisian and what is now East Greenland. (2018) 112 (Suppl 1):S115-S132

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Section: Mineral Chemistrymentioning

confidence: 99%

The mantle of Scotland viewed through the Glen Gollaidh aillikite

Hutchison¹,

Faithfull

Barfod

et al. 2018

Miner Petrol

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“…Distinctively cratonic G10 garnets occur at P ~ 46.7 kbar (using the minimum-pressure barometer of Grütter et al, 2006) and temperatures up to T ~ 1190°C (using T-Mn, an updated version of the Mn-thermometer of Grütter et al, 1999). Peridotite with Mg-rich olivine (FO > 92) dominates the section at 550-1080°C, but diminishes at higher temperatures and is effectively absent at T > 1230°C (Fig.…”

Section: Northern Lesotho Xenocrysts: Expanding the Data Setmentioning

confidence: 99%

“…Compositional boundaries marked on the Cr 2 O 3 vs. CaO diagram are discussed in Grütter et al (2003). High-Cr 2 O 3 G10 garnets are labelled with a minimum pressure (as formulated by Grütter et al, 2006). : Xenolith data sources as in Grütter and Moore (2003); xenocryst data from six open-file reports published by the Council for Geoscience, South Africa (D. de Bruin, pers.…”

Section: Figmentioning

confidence: 99%

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The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons

Eaton

Darbyshire

Evans

et al. 2009

Lithos

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The lithosphere-asthenosphere boundary (LAB) is a first-order structural discontinuity that accommodates differential motion between tectonic plates and the underlying mantle. Although it is the most extensive type of plate boundary on the planet, its definitive detection, especially beneath cratons, is proving elusive. Different proxies are used to demarcate the LAB, depending on the nature of the measurement. Here we compare interpretations of the LAB beneath three The seismic LAB beneath cratons is typically regarded as the base of a high-velocity mantle lid, although some workers infer its location based on a distinct change in seismic anisotropy.Surface-wave inversion studies provide depth-constrained velocity models, but are relatively insensitive to the sharpness of the LAB. The S-receiver function method is a promising new seismic technique with complementary characteristics to surface-wave studies, since it is 2 sensitive to sharpness of the LAB but requires independent velocity information for accurate depth estimation. Magnetotelluric (MT) observations have, for many decades, imaged an "electrical asthenosphere" layer at depths beneath the continents consistent with seismic lowvelocity zones. This feature is most easily explained by the presence of a small amount of water in the asthenosphere, possibly inducing partial melt. Depth estimates based on various proxies considered here are similar, lending confidence that existing geophysical tools are effective for mapping the LAB beneath cratons.

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“…In the case of Catoca, Fe-Mg zoning in olivine indicates a state of disequilibrium that reflects the physical and chemical conditions that the mineral has experienced. This pipe is also characterized by the presence of pyrope and pyrope-almandine (G9 and G10 according to Grütter et al [19,31]), a relative abundance of Cr-rich diopside, and the presence of eclogite, garnet lherzolite, carbonatite, and phlogopite-garnet wehrlite xenoliths. Orthopyroxene is less common than clinopyroxene in this pipe.…”

Section: Discrimination Among Kimberlitesmentioning

confidence: 99%

Major- and Trace-Element Compositions of Indicator Minerals that Occur as Macro- and Megacrysts, and of Xenoliths, from Kimberlites in Northeastern Angola

et al. 2012

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Abstract:In this study, we compare the major-and trace-element compositions of olivine, garnet, and clinopyroxene that occur as single crystals (142 grains), with those derived from xenoliths (51 samples) from six kimberlites in the Lucapa area, northeastern Angola: Tchiuzo, Anomaly 116, Catoca, Alto Cuilo-4, Alto Cuilo-63 and Cucumbi-79. The samples were analyzed using electron probe microanalysis (EPMA) and laser-ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS). The results suggest different paragenetic associations for these kimberlites in the Lucapa area. Compositional overlap in some of the macrocryst and mantle xenolith samples indicates a xenocrystic origin for some of those macrocrysts. The presence of mantle xenocrysts suggests the possibility of finding diamond. Geothermobarometric calculations were carried out using EPMA data from xenoliths by applying the program PTEXL.XLT. Additional well calibrated single-clinopyroxene thermobarometric calculations were also applied. Results indicate the underlying mantle experienced different equilibration conditions. Subsequent metasomatic enrichment events also support a hypothesis of different sources for the kimberlites. These findings contribute to a better understanding of the petrogenetic evolution of the kimberlites in northeastern Angola and have important implications for diamond exploration.

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Cr-Saturation Arrays in Concentrate Garnet Compositions from Kimberlite and their Use in Mantle Barometry

Cited by 167 publications

References 53 publications

The mantle of Scotland viewed through the Glen Gollaidh aillikite

The mantle of Scotland viewed through the Glen Gollaidh aillikite

The elusive lithosphere–asthenosphere boundary (LAB) beneath cratons

Major- and Trace-Element Compositions of Indicator Minerals that Occur as Macro- and Megacrysts, and of Xenoliths, from Kimberlites in Northeastern Angola

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