Peridotite and pyroxenite xenoliths from the Muskox kimberlite, northern Slave craton, Canada

TL;DR: This article presented petrography, mineralogy, and thermobarometry for 53 mantle-derived xenoliths from the Muskox kimberlite pipe in the northern Slave craton.

Abstract: We present petrography, mineralogy, and thermobarometry for 53 mantle-derived xenoliths from the Muskox kimberlite pipe in the northern Slave craton. The xenolith suite includes 23% coarse peridotite, 9% porphyroclastic peridotite, 60% websterite, and 8% orthopyroxenite. Samples primarily comprise forsteritic olivine (Fo 89–94), enstatite (En 89–94), Cr-diopside, Cr-pyrope garnet, and chromite spinel. Coarse peridotites, porphyroclastic peridotites, and pyroxenites equilibrated at 650–1220 °C and 23–63 kbar (1 kbar = 100 MPa), 1200–1350 °C and 57–70 kbar, and 1030–1230 °C and 50–63 kbar, respectively. The Muskox xenoliths differ from xenoliths in the neighboring and contemporaneous Jericho kimberlite by their higher levels of depletion, the presence of a shallow zone of metasomatism in the spinel peridotite field, a higher proportion of pyroxenites at the base of the mantle column, higher Cr2O3 in all pyroxenite minerals, and weaker deformation in the Muskox mantle. We interpret these contrasts as represe...

Summary

Introduction

• Kimberlite -derived samples of the subcontinental lithospheric mantle (SCLM) are commonly used for insights on the composition and structure of the ambient SCLM (Nixon and Boyd 1973; Gurney and Harte 1980; Boyd et al. 1997a ).
• Below the authors present petrography, mineralogy and thermobarometry for 53 Muskox peridotite and pyroxenite xenoliths and use their contrasting petrology to highlight interaction between pre-kimberlitic fluids and the ambient mantle.

Geological Setting

• Jericho xenoliths include coarse peridotite (mainly low-temperature suite), porphyroclastic peridotite (mainly high-temperature suite), eclogite, megacrystalline pyroxenite, ilmenite-garnet wehrlite and clinopyroxenite (Kopylova et al. 1999 ).
• The xenoliths show the higher depletion of the shallow mantle, a layer of "fertile peridotite at 160-200 km (Kopylova and Russell 2000) , the cold cratonic geotherm and a metasomatised, deformed and thermally disturbed mantle at depth below 160 km related to the lithosphere-asthenosphere boundary (Kopylova et al. 1999) .

Coarse peridotite

• Xenoliths are mainly harzburgites and span the spinel, spinel-garnet and garnet facies.
• Olivine and orthopyroxene are subhedral and equant (Fig. 2A ).
• Both anhedral and subhedral garnet grains in spinel-garnet facies samples can have central inclusions of spinel (Fig. 3D, 3E ).
• Samples contain veins and patches of a cryptocrystalline aggregate of serpentine, phlogopite and carbonate (Fig 2E) .

Porphyroclastic peridotite

• Porphyroclastic harzburgites with rare dunites are composed mostly of olivine porphyroclasts (60-70%) with olivine neoblasts (5-20%) and lesser orthopyroxene (0-10%), garnet (5%) and clinopyroxene (0-5%).
• Olivine porphyroclasts are large (8-20 mm), subhedral to anhedral grains (Fig. 2D ) with undulatory extinction, deformation lamellae, subgrains and inclusions of orthopyroxene and garnet.
• Olivine is 10-100% replaced by serpentine with finegrained magnetite and carbonate.
• Orthopyroxene is serpentinized and show variable undulatory extinction.
• Garnet often contains small dark inclusions interpreted to be partial melt; garnet rims are variably replaced by euhedral phlogopite (200 µm-0.5 mm) and spinel (10-80 µm).

Pyroxenite

• Pyroxenitic rock types include websterites and rare orthopyroxenites, divided based on the clinopyroxene modes and mineral chemistry.
• Some websterites are characterized by large (0.5 mm -4 cm) euhedral-subhedral orthopyroxene hosting monogranular networks of anhedral wormlike clinopyroxene and garnet (100 µm-1 cm) developing along certain crystallographic directions of host orthopyroxene (Fig. 2E ).
• Occasional subhedral garnet grains are partially melted and partly replaced by tabular pleochroic phlogopite (<200 µm) and cubic spinel (10-40 µm) or cryptocrystalline aggregates of these minerals.

Analytical Methods

• Mineral compositions were analyzed using a fully automated Cameca SX-50 electron microprobe at the Earth and Ocean Science Dept. at the University of British Columbia.
• On-peak counting times were 10 s for major and 20 s for minor elements.
• Raw data were treated with the 'PAP' ϕ(ρZ) on-line correction program.
• Individual phases in samples were analyzed as 15-20 points in cores and rims over 3-5 grains.
• Homogenous analyses were averaged, zoned mineral analyses were averaged separately as cores and rims (Table S1 ).

Thermobarometry

• Compositions of homogeneous grains were preferentially used for thermobarometry; in samples that showed core-rim heterogeneities, rim compositions were employed.
• Because the Muskox mantle xenolith suite is diverse mineralogically, the authors devised different approaches to deal with rocks that lack different minerals.
• For samples containing orthopyroxene, clinopyroxene and garnet, combined Brey and Kohler (1990) two-pyroxene temperature (BK T) and orthopyroxene-garnet pressure (BK P) can be computed (Table 1 ).
• This BK thermobarometry allows us to make comparison to the Jericho geotherm which has also been computed with the same method (Kopylova et al. 1999) .
• For further thermobarometry the authors therefore assumed that these spatially proximal and roughly contemporaneous pipes share the same thermal regime.

D r a f t 12

• The BK Ca-in-Opx values are 40-80 °C cooler than those reported by the BK T method for pyroxenites, and deviate ~30 °C above and below BK T for two spinel-garnet peridotites (Fig. 7B ).
• The pressure and temperature of the intersection of this line with the Jericho geotherm (Fig. 8, 9 ) constraints the equilibrium conditions for the samples.
• For samples without orthopyroxene, but containing coexisting clinopyroxene and garnet, the authors have investigated the use of the Nakamura (2009) clinopyroxene-garnet thermometer (NK T).
• The Muskox mantle shows a consistent change with depth from coarse spinel peridotite (75 to 135 km) to coarse spinelgarnet peridotite (150 to 165 km) and to porphyroclastic peridotites (185 -230 km).

Two depth zones of metasomatism

• Several Muskox peridotites stand out from the rest of the suite.
• These samples show textural evidence of reequilibration and recrystallization, crystallization of late metasomatic minerals and zoned or outlying mineral compositions.
• The authors interpret these signs as evidence for mantle metasomatism at two depth intervals, a shallow zone at 130-150 km, and a deep zone at >200 km, discussed below.

Shallow zone of metasomatism

• Metasomatism in the amphibole stability field is developed on the Kaapvaal craton, as observed in xenoliths from pipes in the Kimberley area, but is absent under the Slave and extremely rare under the Siberian (Solov'eva et al. 1994 ) cratons, the other two long-mined and well-studied kimberlite provinces .
• There are diverging opinions on the origin of the agent of metasomatism.
• So-called "hydrous weakening" of the lithosphere should lead to enhanced recrystallization of olivine (Mei and Kohlsted 2000) .
• Influx of the hot fluid may cause transient heating (e.g. Jamtveit and Yardley 1997), which would act to enhance strain (Karato 1993) .
• The subsequent cooling would be evidenced by garnet coronas on spinel and low-Al and Cr rims of zoned orthopyroxene equilibrated in the garnet stability field.

Deep zone of metasomatism

• A deep zone of metasomatism starts at 200 km depth; the authors cannot constrain the deeper termination of the zone as the in the Muskox kimberlite did not sample below 220 km.
• Such mineral chemistry is typical for the more prevalent websterites found at this depth (Figs. 7, 8, 9A) .
• This suggests metasomatic addition of CaO rather than the origin of garnet and clinopyroxene as exsolution phases from a higher-T orthopyroxene in a closed system, and could relate to refertilizing metasomatism often reported in cratonic mantle (Simon et al. 2007; Miller et al. 2014) .
• A lower proportion of these fertile coarse peridotites in the Muskox sample suite may suggest a lower degree of penetration and fertilization by these pyroxenitic magmas and fluids.

D r a f t

• Muskox peridotites with the most sheared textures contain tabular neoblasts (Fig. 3G ), in contrast to other porphyroclastic peridotites with isometric neoblasts derived from 170-190 km.
• This fluid-assisted grain boundary migration must be restricted to shallower depth due to localized fluid penetration.
• Sheared peridotites from Jericho show more deformation than sheared peridotites from Muskox.
• There is a positive correlation in the degree of shearing and clinopyroxene modes between the Muskox and Jericho sample suites.
• The relationship between metasomatizing fluids and deformation of peridotitic cratonic mantle has been observed widely (Smith and Boyd 1987; Drury and van Roermund 1989; O'Reilly and Griffin 2010; Harte and Hawkesworth 1989) .

Characteristics of the Muskox pyroxenitic mantle

• Two types of pyroxenites occur in the Muskox mantle, orthopyroxenites and websterites.
• A higher proportion of minerals in pyroxenites show chemical zoning than those in peridotites.
• The extremely high Cr 2 O 3 content of orthopyroxenes in the orthopyroxenite can be partly explained by its relic high-pressure, hightemperature composition, which did not allow for exsolution of clinopyroxene component.
• Timing of pyroxenite formation suggests that these fluids may have been proto-kimberlitic or shortly preceding kimberlite formation.

Concluding remarks

• Xenoliths from the Muskox kimberlite emplaced simultaneously with the adjacent Jericho kimberlite provide the opportunity to examine the same mantle sampled by a different ascending magma batches and thus separate the ambient mantle characteristics from the traits imposed by the kimberlite formation and emplacement.
• Lower levels of strain and modal and cryptic metasomatism of peridotites suggest that these fluids have infiltrated and interacted less with the Muskox mantle than with the Jericho mantle, perhaps due to lower permeability of the mantle.
• (B) Brey and Kohler (1990) two-pyroxene temperatures plotted against Brey and Kohler (1990) Ca-in-orthopyroxene temperatures computed for the same samples at the same BK pressure.
• Points joined by dashed lines use the BK Ca-in-Opx thermometer, points joined by solid lines use the BK T two-pyroxene thermometer.
• Solid line is the geotherm constrained for xenoliths from the Jericho kimberlite, calculated using the BK P/BK T (Kopylova et. al., 1999) .

Figures

Table 1. Pressure and temperature estimates of Muskox samples containing two pyroxenes and garnet Combined

Fig. 2. Thin section photomicrographs. (A) Plane polarized light (PPL). Coarse spinel peridotite composed of equant olivine and orthopyroxene grains and smaller, light brown translucent spinel. There is minimal serpentinization of all phases along grain boundaries. (B) PPL. Coarse spinel-garnet peridotite composed of equant olivine and orthopyroxene grains, light pink garnet and dark opaque spinel grains. Spinel is often hosted as an inclusion inside larger garnet grains. (C) PPL. Coarse garnet peridotite composed of large equant olivine, orthopyroxene and garnet grains showing moderate serpentinization of all phases along grain boundaries and in fractures. (D) PPL. Porphyroclastic peridotite composed of large olivine porphyroclasts and garnet grains partially surrounded by networks of olivine neoblasts. Garnet is filled with small dark inclusions of crystallized partial melt. (E) Cross polarized light (XPL). Garnet websterite composed of a large, single orthopyroxene grains intergrown with single crystals of anhedral clinopyroxene. A large dark patch in the center of the photograph is a cryptocrystalline aggregate of serpentine, phlogopite, spinel and carbonate possibly replacing garnet. (F) (XPL) Orthopyroxenite composed of a large single orthopyroxene grain, recrystallized into subgrains and small neoblasts along boundaries of larger grains. Mineral abbreviations here and in Fig. 3 are OL – olivine; OP – orthopyroxene; CP – clinopyroxene; GA – garnet; SP – spinel; AMP – amphibole; PH – phlogopite. All scale bars are 4 mm.

Table 2. Pressure and temperature estimates for Muskox samples containing two pyroxenes Combined Combined Geothermal Intercept

Fig. 5. Compositions of Muskox minerals plotted as Cr2O3 wt. % vs. Mg# (molar Mg/(Mg+Fe) for (A) orthopyroxene, (B) clinopyroxene and (C) garnet. Muskox mineral compositions plotted

Table 3. Pressure and temperature estimates for Muskox samples lacking (A) clinopyroxene or (B) orthopyroxene A. Combined Combined Geothermal Intercept

Fig. 8. Equilibrium pressure-temperature estimates for Muskox peridotites (A) and pyroxenites (B) according to Brey and Kohler (1990). Solid line is the geotherm constrained for xenoliths from the Jericho kimberlite, calculated using the combined BK P/BK T (Kopylova et. al., 1999). Samples plotted as single points have both temperature and pressure calculated using the BK

Table 4. Evidence for metasomatism in Muskox peridotite xenoliths Shallow Zone

Fig. 10. Depth distribution of rock types in the Muskox (A) and Jericho (B) mantle. Depths for Muskox samples are from Figs. 7, 8 and geothermal intercepts and BK P/BK T thermobarometric solution in Tables 1-3. Depths for Jericho samples are from BK P/BK T thermobarometric solutions (Kopylova et al., 1999). Depths of origin for Jericho and Muskox eclogites were computed using Nakamura (2009) intercepts with the Jericho geotherm and are based on 148 xenoliths reported in Kopylova et al., (accepted pending revisions).

Full text

Draft
Peridotite and Pyroxenite xenoliths from the Muskox
kimberlite, northern Slave craton, Canada
Journal:
Canadian Journal of Earth Sciences
Manuscript ID
cjes-2015-0083.R1
Manuscript Type:
Article
Date Submitted by the Author:
28-Sep-2015
Complete List of Authors:
Newton, David; University of British Columbia, Earth Ocean and
Atmospheric Science
Kopylova, Maya; University of British Columbia, Earth, Ocean and
Atmospheric Science
Burgess, Jennifer; Burgess Diamonds, ; Shear Minerals,
Strand, Pamela; NWT Mines and Minerals, ; Shear Minerals,
Keyword:
cratonic peridotite, cratonic pyroxenite, mantle metasomatism,
thermobarometry, kimberlite xenoliths
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Canadian Journal of Earth Sciences

Draft
David E Newton*, Maya G Kopylova, Jennifer Burgess
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, Pamela Strand
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Peridotite and pyroxenite xenoliths from the Muskox kimberlite, northern Slave craton,
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Canada
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University of British Columbia, 2207 Main Mall, Vancouver, Canada V6T 1Z4
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2
- Shear Minerals Ltd. 220-17010-103
rd
Ave, Edmonton, AB, Canada, T5S 1K7
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Submitted to Canadian Journal of Earth Sciences
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April 2015
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*- corresponding author, David E Newton, University of British Columbia Dept. of Earth, Ocean
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and Atmospheric Sciences,
2020 - 2207 Main Mall
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Vancouver, BC Canada V6T 1Z4. T: (778) 828-4636. dnewton@eos.ubc.ca.
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**- present address: Burgess Diamonds, 5674 Annex Rd. Sechelt, BC, Canada, V0N 3A8
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***- present address: NWT Mines and Minerals, Box 1320, 4601B 52
nd
Ave, Yellowknife, NT,
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Canada, X1A 2L9
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Draft
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Abstract
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We present petrography, mineralogy and thermobarometry for 53 mantle-derived
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xenoliths from the Muskox kimberlite pipe in the northern Slave craton. The xenolith suite
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includes 23% coarse peridotite, 9% porphyroclastic peridotite, 60% websterite and 8%
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orthopyroxenite. Samples primarily comprise forsteritic olivine (Fo 89-94), enstatite (En 89-94),
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Cr-diopside, Cr-pyrope garnet and chromite spinel. Coarse peridotites, porphyroclastic
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peridotites, and pyroxenites equilibrated at 650-1220 °C and 23-63 kbar, 1200-1350 °C and 57-
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70 kbar, and 1030-1230 °C and 50-63 kbar, respectively. The Muskox xenoliths differ from the
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neighboring and contemporaneous Jericho kimberlite by their higher levels of depletion, the
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presence of a shallow zone of metasomatism in the spinel stability field, a higher proportion of
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pyroxenites at the base of the mantle column, higher Cr
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O
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in all pyroxenite minerals, and
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weaker deformation in the Muskox mantle. We interpret these contrasts as representing small
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scale heterogeneities in the bulk composition of the mantle, as well as the local effects of
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interaction between metasomatizing fluid and mantle wall rocks. We suggest that asthenosphere-
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derived pre-kimberlitic melts and fluids percolated less effectively through the less permeable
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Muskox mantle resulting in lower degrees of hydrous weakening, strain and fertilization of the
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peridotitic mantle. Fluids tended to concentrate and pool in the deep mantle causing partial
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melting and formation of abundant pyroxenites.
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Key Words
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Cratonic peridotite, cratonic pyroxenite, mantle metasomatism, thermobarometry,
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kimberlite xenoliths.
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Introduction
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Kimberlite - derived samples of the subcontinental lithospheric mantle (SCLM) are
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commonly used for insights on the composition and structure of the ambient SCLM (Nixon and
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Boyd 1973; Gurney and Harte 1980; Boyd et al. 1997a). Kimberlite-associated processes,
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however, may metasomatize the mantle in the vicinity of melt extraction and transport
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(Artemieva 2009; Kopylova et al. 2009; Doyle et al. 2004). Thus, differences found in the mantle
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sample in two adjacent contemporaneous kimberlites may reflect spatial compositional
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heterogeneity of the mantle, like those reported by Griffin et al. 1999a; Grutter et al. 1999;
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Carbno and Canil 2002; Davis et al. 2003; Hoffman 2003, or the varying effects of percolation of
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pre-kimberlitic fluids and kimberlite formation and ascent. To isolate the heterogeneity of the
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ambient mantle from the kimberlite-imposed mantle wall-rock metasomatism, we compared
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peridotites and pyroxenite xenoliths of the Muskox and Jericho kimberlites. The Muskox pipe
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was emplaced contemporaneously and within 15 km of the Jericho kimberlite that contains a
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well-studied suite of mantle xenoliths (Kopylova et al. 1999). Below we present petrography,
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mineralogy and thermobarometry for 53 Muskox peridotite and pyroxenite xenoliths and use
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their contrasting petrology to highlight interaction between pre-kimberlitic fluids and the
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ambient mantle.
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Geological Setting
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The Muskox and Jericho kimberlites belong to a mid-Jurassic cluster (172 ± 2 Ma, Heaman
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et al. 1997) of pipes located ~400 km NE of Yellowknife near the northern end of Contwoyto
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Lake (Fig. 1), emplaced in the Archean granite-granodiorite Contwoyto batholith (2589 ± 5 Ma,
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Van Breemen et al. 1987) of the Slave craton (Hoffman 1989). The Muskox kimberlite is made
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up of volcaniclastic kimberlite, accounting for 60% of the pipe infill, and 40% coherent
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kimberlite. The Jericho kimberlite is located ~15 km NE of Muskox and is a multiphase intrusion
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in 3 separate pipes (Kopylova and Hayman 2008). Jericho xenoliths include coarse peridotite
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(mainly low-temperature suite), porphyroclastic peridotite (mainly high-temperature suite),
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eclogite, megacrystalline pyroxenite, ilmenite-garnet wehrlite and clinopyroxenite (Kopylova et
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al. 1999). The xenoliths show the higher depletion of the shallow mantle, a layer of “fertile
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peridotite at 160-200 km (Kopylova and Russell 2000), the cold cratonic geotherm and a
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metasomatised, deformed and thermally disturbed mantle at depth below 160 km related to the
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lithosphere-asthenosphere boundary (Kopylova et al. 1999).
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Petrography
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70% of xenoliths in this study are recovered from the coherent kimberlite facies with the
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remaining 30% from the volcaniclastic facies. The xenoliths (2-8 cm in size) comprise 15%
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coarse spinel peridotite, 4% coarse spinel-garnet peridotite, 4% coarse garnet peridotite, 9%
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porphyroclastic peridotite (classification of Harte 1977), 60% websterite and 8%
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orthopyroxenite.
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Coarse peridotite
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Xenoliths are mainly harzburgites and span the spinel, spinel-garnet and garnet facies.
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Grain size correlates to sample facies, increasing from 0.5-2.5 mm in spinel facies samples to 1-
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10 mm in garnet peridotites. Olivine and orthopyroxene are subhedral and equant (Fig. 2A).
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Clinopyroxene forms smaller anhedral grains between olivine and orthopyroxene grains (Fig.
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3F). Garnet is subhedral or rarely forms wispy, anhedral films on primary minerals (Fig. 3D).
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Subhedral garnets have variable thickness rims of small euhedral phlogopite (~200 µm) and
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Q1. What contributions have the authors mentioned in the paper "Peridotite and pyroxenite xenoliths from the muskox kimberlite, northern slave craton, canada" ?

Journal: Canadian Journal of Earth Sciences Manuscript ID cjes-2015-0083. R1 Manuscript Type: Article Date Submitted by the Author: 28-Sep-2015 Complete List of Authors: Newton, David ; University of British Columbia, Earth Ocean and Atmospheric Science Kopylova, Maya ; University of British Columbia, Earth, Ocean and Atmospheric Science Burgess, Jennifer ; Burgess Diamonds, ; Shear Minerals, Strand, Pamela ; NWT Mines and Minerals, ; Shear Minerals,