Peridotite xenoliths from the Jagersfontein kimberlite pipe: I. Primary and primary-metasomatic mineralogy

Winterburn, Peter A.; Harte, B.; Gurney, J.J.

doi:10.1016/0016-7037(90)90322-c

Cited by 56 publications

(18 citation statements)

References 19 publications

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“…Experiments using Bulk 1 were performed at 9508C at 2 GPa, 950-1,1758C at 3 GPa, and 950-11508C at 4 GPa. As evident from Figure 2, these P-T conditions are relevant for the CLM (Boyd et al, 1993(Boyd et al, , 1997Carswell et al, 1979;Ehrenberg, 1982;Kopylova et al, 1999;Lee & Rudnick, 1999;Nixon et al, 1981;;Pearson et al, 1995;Rudnick et al, 1994;Winterburn et al, 1990). In order to study the change in modes and compositions of the volatile-bearing phases as a function of melt:rock ratio, three additional experiments were performed at 9508C at 2 GPa, 10508C at 3 GPa, and 11008C at 4 GPa with Bulk 2.…”

Section: Methodsmentioning

confidence: 99%

High Pressure Phase Relations of a Depleted Peridotite Fluxed by CO₂‐H₂O‐Bearing Siliceous Melts and the Origin of Mid‐Lithospheric Discontinuity

Saha

Dasgupta

Tsuno

2018

Geochem Geophys Geosyst

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We present phase equilibria experiments on a depleted peridotite (Mg# 92) fluxed with variable proportions of a slab‐derived rhyolitic melt (with 9.4 wt.% H2O, 5 wt.% CO2), envisaging an interaction that could occur during formation of continents by imbrication of slabs/accretion of subarc mantles. Experiments were performed with 5 wt.% (Bulk 2) and 10 wt.% (Bulk 1) melt at 950–1175°C and 2–4 GPa using a piston‐cylinder and a multi‐anvil apparatus, to test the hypothesis that volatile‐bearing mineral‐phases produced during craton formation can cause reduction in aggregate shear‐wave velocities (VS) at mid‐lithospheric depths beneath continents. In addition to the presence of olivine, orthopyroxene, clinopyroxene, and garnet/spinel, phlogopite (Bulk 1: 3–7.6 wt.%; Bulk 2: 2.6–5 wt.%) at 2–4 GPa, and amphibole (Bulk 1: 3–9 wt.%; Bulk 2: 2–6 wt.%) at 2–3 GPa (≤1050°C) are also present. Magnesite (Bulk 1: ∼1 wt.% and Bulk 2: ∼0.6 wt.%) is present at 2–4 GPa (<1000°C at 3 and < 1050°C at 4 GPa) and its thermal breakdown coincides with the visual appearance of trace‐melt. However, an extremely small fraction of melt is inferred at all experiments based on the knowledge of fluid‐saturated peridotite solidus and the difference between bulk H2O and total H2O stored in the hydrous phases. Calculated mineral end‐member volume‐proportions were used to calculate VS of the resulting assemblage at experimental conditions and along representative continental geotherms (surface heat flow of 40–50 mWm−2). We note that reactive crystallization of phlogopite ± amphibole by infiltration of 3–10% slab‐derived hydrous‐silicic melt can cause up to 6% reduction in VS and that the estimated reduction in VS increases with increasing melt:rock ratio. The presence of phlogopite limits amphibole‐stability, making phlogopite a more likely candidate for MLDs at >100 km depth.

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Section: Methodsmentioning

confidence: 99%

High Pressure Phase Relations of a Depleted Peridotite Fluxed by CO₂‐H₂O‐Bearing Siliceous Melts and the Origin of Mid‐Lithospheric Discontinuity

Saha

Dasgupta

Tsuno

2018

Geochem Geophys Geosyst

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“…Crack filling: Intense cracking and kelyphitisation is common in peridotitic garnets (Winterburn et al, 1990;Viljoen et al, 2004). Crack widths range between $10 and $500 micrometers and are interconnected; however, the garnet in-between cracks is homogenous and shows no signs of alteration.…”

Section: Electron-microscopy Imagingmentioning

confidence: 97%

Origins of subcalcic garnets and their relation to diamond forming fluids—Case studies from Ekati (NWT-Canada) and Murowa (Zimbabwe)

Klein-BenDavid

Pearson

2009

Geochimica et Cosmochimica Acta

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“…The relative uniformity of spinel peridotite compositions [ Boyd et al , 1999] suggests that the Kaapvaal mantle is compositionally homogeneous over this interval. It should be noted, however, that this ignores metasomatized xenoliths, some of which derive from the spinel facies, as well as edenite‐pargasite amphibole‐bearing peridotites that are from the shallow mantle [e.g., Winterburn et al , 1990]. The abundance of metasomatized xenoliths varies in different xenolith suites, and much of the metasomatism is probably restricted to the immediate neighborhood of kimberlite pipes, because heat flow constraints do not permit significant proportions of heavily metasomatized mantle rich in heat‐producing elements [ Rudnick et al , 1998].…”

Section: Geothermobarometrymentioning

confidence: 99%

Xenolith constraints on seismic velocities in the upper mantle beneath southern Africa

James

Boyd

Schutt

et al. 2004

Geochem Geophys Geosyst

132

129

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[1] We impose geologic constraints on seismic three-dimensional (3-D) images of the upper mantle beneath southern Africa by calculating seismic velocities and rock densities from approximately 120 geothermobarometrically calibrated mantle xenoliths from the Archean Kaapvaal craton and adjacent Proterozoic mobile belts. Velocity and density estimates are based on the elastic and thermal moduli of constituent minerals under equilibrium P-T conditions at the mantle source. The largest sources of error in the velocity estimates derive from inaccurate thermo-barometry and, to a lesser extent, from uncertainties in the elastic constants of the constituent minerals. Results are consistent with tomographic evidence that cratonic mantle is higher in velocity by 0.5-1.5% and lower in density by about 1% relative to off-craton Proterozoic samples at comparable depths. Seismic velocity variations between cratonic and noncratonic xenoliths are controlled dominantly by differences in calculated temperatures, with compositional effects secondary. Different temperature profiles between cratonic and noncratonic regions have a relatively minor influence on density, where composition remains the dominant control. Low-T cratonic xenoliths exhibit a positive velocity-depth curve, rising from about 8.13 km/s at uppermost mantle depths to about 8.25 km/s at 180-km depth. S velocities decrease slightly over the same depth interval, from about 4.7 km/s in the uppermost mantle to 4.65 km/s at 180-km depth. P and S velocities for high-T lherzolites are highly scattered, ranging from highs close to those of the low-T xenoliths to lows of 8.05 km/s and 4.5 km/s at depths in excess of 200 km. These low velocities, while not asthenospheric, are inconsistent with seismic tomographic images that indicate high velocity root material extending to depths of at least 250 km. One plausible explanation is that high temperatures determined for the high-T xenoliths are a nonequilibrium consequence of relatively recent thermal perturbation and compositional modification associated with emplacement of kimberlitic fluids into the deep tectospheric root. Seismic velocities and densities for cratonic xenoliths differ significantly from those predicted for both primitive mantle peridotite and mantle eclogite. A model primitive mantle under cratonic P-T conditions exhibits velocities about 1% lower for P and about 1.5% lower for S, a consequence of a more fertile composition and different modal composition. Primitive mantle is also about 2% more dense at 150-km depth than low-T garnet lherzolite at cratonic P-T conditions. Similar calculations based on an oceanic geotherm are consistent with the isopycnic hypothesis of comparable density columns beneath oceanic and cratonic regions. Calculations for a hypothetical ''cratonic'' eclogite (50:50 garnet/omphacite) with an assumed cratonic geotherm produce extremely high V P and V S (8.68 km/s and 4.84 km/s, respectively, at 150 km depth) as well as high density ($3.54 gm/cc). The very high velocity of eclogite sh...

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Peridotite xenoliths from the Jagersfontein kimberlite pipe: I. Primary and primary-metasomatic mineralogy

Cited by 56 publications

References 19 publications

High Pressure Phase Relations of a Depleted Peridotite Fluxed by CO₂‐H₂O‐Bearing Siliceous Melts and the Origin of Mid‐Lithospheric Discontinuity

High Pressure Phase Relations of a Depleted Peridotite Fluxed by CO₂‐H₂O‐Bearing Siliceous Melts and the Origin of Mid‐Lithospheric Discontinuity

Origins of subcalcic garnets and their relation to diamond forming fluids—Case studies from Ekati (NWT-Canada) and Murowa (Zimbabwe)

Xenolith constraints on seismic velocities in the upper mantle beneath southern Africa

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Peridotite xenoliths from the Jagersfontein kimberlite pipe: I. Primary and primary-metasomatic mineralogy

Cited by 56 publications

References 19 publications

High Pressure Phase Relations of a Depleted Peridotite Fluxed by CO2‐H2O‐Bearing Siliceous Melts and the Origin of Mid‐Lithospheric Discontinuity

High Pressure Phase Relations of a Depleted Peridotite Fluxed by CO2‐H2O‐Bearing Siliceous Melts and the Origin of Mid‐Lithospheric Discontinuity

Origins of subcalcic garnets and their relation to diamond forming fluids—Case studies from Ekati (NWT-Canada) and Murowa (Zimbabwe)

Xenolith constraints on seismic velocities in the upper mantle beneath southern Africa

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Product

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High Pressure Phase Relations of a Depleted Peridotite Fluxed by CO₂‐H₂O‐Bearing Siliceous Melts and the Origin of Mid‐Lithospheric Discontinuity

High Pressure Phase Relations of a Depleted Peridotite Fluxed by CO₂‐H₂O‐Bearing Siliceous Melts and the Origin of Mid‐Lithospheric Discontinuity