Formation of cratonic subcontinental lithospheric mantle and ...

Contrib Mineral Petrol (2011) 161:947–960 DOI 10.1007/s00410-010-0573-4

ORIGINAL PAPER

Formation of cratonic subcontinental lithospheric mantle and complementary komatiite from hybrid plume sources Sonja Aulbach • Thomas Stachel • Larry M. Heaman Robert A. Creaser • Steven B. Shirey

•

Received: 28 April 2010 / Accepted: 13 August 2010 / Published online: 7 September 2010 ! Springer-Verlag 2010

Abstract Peridotitic sulphide inclusions in diamonds from the central Slave craton constrain the age and origin of their subcontinental lithospheric mantle (SCLM) sources. These sulphides align with either a ca. 3.5 Ga (shallow SCLM) or a ca. 3.3 Ga isochron (deep SCLM) on a Re–Os ischron diagram, with variably enriched initial 187Os/188Os. Since some Archaean to recent plume-derived melts carry a subducted crust (eclogite) signature and some cratonic SCLM may have been generated in plumes by extraction of komatiitic liquids, we explain these data by subduction of evolved lithospheric material (shallow SCLM) and melting in a hybrid mantle plume that contains domains of recycled eclogite (deep SCLM), respectively. In upwelling hybrid mantle, eclogitederived melts react with olivine in surrounding peridotites to form aluminous orthopyroxene, convert peridotite to pyroxenite and confer their crustal isotope signatures. We suggest that it is subsequent to orthopyroxene enrichment of peridotite in an upwelling plume that partial melting of this Al- and Si- enriched source generated komatiites and complementary ultradepleted cratonic mantle residues. Although

Communicated by C. Ballhaus. S. Aulbach ! T. Stachel ! L. M. Heaman ! R. A. Creaser Earth and Atmospheric Sciences, University of Alberta, Edmonton, AB, Canada S. B. Shirey Department of Terrestrial Magnetism, Carnegie Institution of Washington, Washington, DC, USA S. Aulbach (&) Facheinheit Mineralogie, Goethe-Universita¨t, Frankfurt am Main, Germany e-mail: [email protected]

subduction is needed to explain some cratonic features, melting of a hybrid plume source satisfies several key observations: (1) suprachondritic initial 187Os/188Os in subsets of lithospheric mantle samples and in some coeval Archaean komatiites; (2) variable enrichment of cratonic mantle by high-temperature aluminous orthopyroxene; (3) high Mg# combined with high orthopyroxene content in cratonic mantle due to higher melt productivity of an Al- and Si-richer source; (4) variable orthopyroxene enrichment possibly linked to varying mantle potential temperatures (Tp), plume buoyancy and resultant eclogite load and/or variable availability of subducted material in the source; and (5) absence of younger analogues due to a secular decrease in Tp. Most importantly, this model also alleviates a mass balance problem, because it predicts a hybrid mantle source with variably higher SiO2 and Al2O3 than primitive mantle, and, contrary to a primitive mantle source, is able to reconcile compositions of komatiites and complementary cratonic mantle residues. Keywords Osmium isotopes ! Opx enrichment ! Silica enrichment ! Partial melting ! Peridotite ! Pyroxenite ! Eclogite ! Diamond ! Xenolith Introduction Cratonic subcontinental lithospheric mantle (SCLM) is the non-convecting part of the mantle underlying the cores of ancient continents that have been quiescent for billions of years (Jordan 1988; Boyd 1989). Cratonic SCLM is characterised by strong geochemical depletion, isostatic buoyancy, high viscosity and rigidity, characteristics that have not been produced in such extremes later in Earth’s history, perhaps due to secular cooling (Boyd and Mertzman 1987;

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Jordan 1988; Herzberg 1999; Griffin et al. 2003). Therefore, present conditions and processes of SCLM formation may not be directly applicable to ancient craton formation. The exact mode or modes of craton formation remains an unresolved issue. Two main models (recently reviewed in Arndt et al. 2009) have been put forward: the accretion model (Helmstaedt and Schulze 1989; Kesson and Ringwood 1989) invokes successive stacking of oceanic lithosphere, the plume model (Boyd 1989; Griffin et al. 2003) entails formation of thick cratonic lithosphere by partial melt extraction in actively upwelling mantle. Neither of these end-member hypotheses is able to explain all of the observations. Both modes have probably been operational, to varying degrees, in different cratons (Herzberg 1999; Griffin et al. 2004; Aulbach et al. 2009), and some have suggested a transition from vertical upwelling to lateral accretion in the Mesoarchaean (van Kranendonk et al. 2007). Difficulties with pinpointing the mechanisms of cratonic thickening and stabilisation to some degree reflect an inability to rigorously model lithosphere formation, because we lack sufficient experimental constraints at the relevant conditions. In addition, the longevity of cratonic lithosphere implies that on billion year timescales, multiple processes subsequent to initial craton formation, such as mantle metasomatism (e.g. Gurney and Harte 1980; Dawson 1984), have likely led to compositional modifications from the interaction with fluids and melts penetrating the lithosphere during extension or subduction at the craton margins. This secondary overprint can make it difficult, if not impossible, to decipher the mode of craton formation based on geochemical observations from mantle xenoliths. Diamonds are rare accessory minerals in mantle lithologies that typically sample deep lithospheric mantle and can include co-precipitating phases during growth (Sobolev 1977; Meyer 1987; Stachel and Harris 2008). Because of the chemical inertness of diamonds, these inclusions remain isolated from processes that affect the lithosphere subsequent to diamond formation and that mask original formation conditions in exposed mantle samples that are entrained as xenoliths. In addition, diamonds can be billions of years old (Richardson et al. 1984, 2001; Pearson et al. 1998); thus, diamonds and their inclusions are windows not only into the deep but also into the ancient lithosphere (Stachel and Harris 2008, and references therein). Sulphide minerals are an important inclusion type in diamonds (Deines and Harris 1995) and typically have high concentrations of highly siderophile elements including Re and Os (Alard et al. 2000), which include a parent– daughter nuclide pair (187Re and 187Os) used for absolute age dating. During partial melting, Re is mildly incompatible, whereas Os is compatible, which makes this element pair singularly useful to date mantle depletion processes by melt extraction.

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Contrib Mineral Petrol (2011) 161:947–960

Here, we report Re–Os isotope systematics of peridotitic sulphide inclusions in diamonds from the central Slave craton, which record radiogenic initial 187Os/188Os and clearly affirm Palaeoarchaean ages for the SCLM beneath the central Slave craton. In the light of suggestions that some Archaean SCLM represents residue from melting in upwelling plumes and of evidence for the presence of subducted components in the source of primitive basalts, picrites and komatiites sampling Archaean plume sources (Kerrich et al. 1999; Condie 2003; Sobolev et al. 2007), we explore (1) the possible origins for the radiogenic initial Os isotope signatures recorded in central Slave mantle samples and (2) the implications of hybrid mantle sources in models of lithosphere formation—regardless of whether final melt depletion occurs at deep or shallow levels—and for the generation of potentially complementary komatiitic melts.

Geology and samples The evolution of the Slave craton lithosphere has been summarised in recent papers (Snyder 2008; Helmstaedt 2009). The Slave craton, which consists of an ancient (4.0–2.8 Ga) western domain and a juvenile (\2.8–2.7) eastern domain, has experienced ten episodes of predominantly felsic crust formation between 4.0 and 2.7 Ga (Bleeker 2002) that culminated in craton amalgamation at ca. 2.7 Ga, possibly during collision of the two domains (Kusky 1989; Bleeker et al. 1999). Combined U–Pb, Lu– Hf and O isotope data on detritral zircons from the western domain of the Slave craton have been interpreted to show that the generation of juvenile crust peaked at around 4.4, 3.8 and 3.4 Ga and probably 2.8–2.7 Ga, indicating formation during addition of new material from the asthenosphere (Pietranik et al. 2008). The mantle beneath the central Slave craton is strongly layered with a highly depleted shallow layer (60% cpx-free harzburgite vs. 40% lherzolite) mapped to 140–145 km depth, underlain by a less depleted deep layer (15–20% harzburgite) that was interpreted as plume-derived (Griffin et al. 1999, 2004; Aulbach et al. 2007). The shallow layer was dated to ca. 3.5 Ga (Westerlund et al. 2006), the deeper layer to ca. 3.3 Ga (Aulbach et al. 2004). These Palaeoarchaean ages conflict with the 2.7 Ga ages determined for the overlying crust, and it was suggested that during the 2.7 Ga craton amalgamation, the layered mantle underlying the ancient western domain was accreted beneath shallow lithosphere of the younger eastern domain (Aulbach et al. 2005). Significant coupling of crust–mantle formation in the Neoarchaean may be documented by peridotite Re–Os model ages (Heaman and Pearson 2010). The mantle beneath the central Slave craton, including the diamonds studied here, was sampled during Cretaceous to


Eocene kimberlite magmatism (Creaser et al. 2004; Heaman et al. 2004). Re–Os isotope systematics were previously reported for five peridotitic sulphide inclusions in diamonds from the A154N and A154S kimberlites on the Diavik property, in the Lac de Gras area, central Slave craton, and are denoted in the tables with an asterisk (Aulbach et al. 2009). In that paper, we deferred detailed discussion of these five analyses until further data would be available. In order to better constrain their age and origin, we collected an additional batch of eight diamonds containing peridotitic sulphides, mostly off-white to brown sharp octahedral diamonds.

Sample preparation and analytical techniques All work was carried out in the Department of Earth and Atmospheric Sciences, University of Alberta. Sulphides were liberated by crushing the diamonds, followed by major-element analysis using a JEOL 6301F (field-emission scanning electron microscope) linked to a PGT Instruments X-ray analysis system. Typical digital images of surface textures and microstructures of sulphides are shown in Aulbach et al. (2009). Some sulphides broke during extraction from the host diamond, and these were recombined prior to processing and analysis in the Radiogenic Isotope Facility. The method is described in detail in Aulbach et al. (2009). Briefly, sulphides weighing between 1.2 and 9.8 lg were cleaned in doubledistilled acetone and spiked with mixed 185Re–190Os solution. A mixture of H2SO4 and CrO3 was added just prior to capping and microdistillation simultaneously with digestion in Teflon at ca. 75"C to separate Os, which was captured in a drop of HBr. Rhenium was then separated by anion exchange chromatography (Eichrom AG 1-X8, 100–200 mesh, Clform) after loading in 1 M HCl. Rhenium and Os isotope compositions were measured between May and September 2009 by negative thermal ionisation mass spectrometry (NTIMS) (Creaser et al. 1991) on a Micromass Sector 54. Repeated analysis of 1 pg of the in-house Os standard AB-2 analysed in this time period gave a 187Os/188Os of 0.1073 ± 0.0010 (n = 12, 1r), identical within uncertainty to previously obtained values on larger analytes (0.1068 ± 0.0004; Azmy et al. 2008). The Re standard was run at ca. 10,000 counts per second, similar to sample sizes and gave a 185Re/187Re of 0.5985 ± 0.0010 (n = 8). Concentrations were determined by isotope dilution. Total Re and Os blanks (including filament blanks) ranged from 110 to 140 fg and 6 to 10 fg, respectively, depending on the batch of reagents used. Fourier transform infrared (FTIR) spectra were obtained from the host diamonds using a bench-top Thermo Nicolet Nexus 470 FTIR Spectrometer with an attached Continuum

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IR microscope to determine their N contents and aggregation states. A typical analysis of a 50 9 50 lm sample area lasted 200 s, with 1–4 spectra per diamond acquired. Depending on the quality of the diamond fragments, detection limits and uncertainties range between 10–20 ppm and 10–20% (relative), respectively.

Results Sulphides The nickel contents of sulphide minerals may be used to infer the paragenesis of the host diamond (e.g. Deines and Harris 1995). Using (Ni ? Co)/Fe as a discriminator, sulphide inclusions in peridotitic diamonds yield values from 0.26 to 2.2 (Table 1), which are distinct from the values of 0.02–0.14 obtained for sulphide inclusions in eclogitic diamonds from the same locality (Aulbach et al. 2009). As is true for other sample suites (Pearson et al. 1998), the parageneses at Diavik are also well separated with regard to Os concentrations (8–233 ppm Os in peridotitic sulphides vs. \1.5 ppm in eclogitic sulphides) and Re/Os (0.0004–0.02 in peridotitic sulphides vs. 0.4–36 in eclogitic sulphides). Rhenium contents in peridotitic sulphides range from 0.02 to 0.6 ppm and thus partially overlap with those in eclogitic sulphides (0.04–2.2 ppm; Aulbach et al. 2009). The measured Os isotope ratios in peridotitic sulphides are unradiogenic (Table 1) and range from 0.1071 to 0.1153. On a Re–Os isochron diagram (Fig. 1), the samples from this study show some scatter, but align with previously established 3.27 ± 0.34 and 3.52 ± 0.17 Ga isochrons (i.e. not regressed with the new data) for sulphide inclusions in olivine xenocrysts from Diavik and in diamonds from the nearby Panda mine, respectively (Aulbach et al. 2004; Westerlund et al. 2006). The initial 187Os/188Os (0.1073 ± 0.0001; cOsi = ?2.53 ± 0.13; MSWD = 0.75) of the Diavik xenocryst array is distinctly lower than that of the Panda diamond array (0.1093 ± 0.0001; cOsi = ?5.6 ± 0.10; MSWD = 0.46). Although the uncertainties on the Os isotope composition of one sulphide inclusion in diamond from Diavik (DA36) is too large to unambiguously assign it to one of the previously established isochrons and there is some scatter in the data, several samples clearly plot on either on the 3.5 Ga (DA8, DA79, DA81, DA118) or 3.3 Ga (DA46, DA59, DA75, DA116) array, outside the analytical uncertainty. One sample (DA93) has high 187 Os/188Os relative to 187Re/188Os, whereas two samples (DA91, DA101) clearly plot off either array. Ignoring these latter samples, renewed regressions (using Isoplot, Ludwig 1999), assigning sulphide inclusions in diamonds from Diavik to either the deep SCLM Diavik xenocryst array or the shallow SCLM Panda diamond array, as indicated

123

123

8.0

2.5

26.5

7.2

3.5

9.8

2.3

5.7

5.2

1.2

1.3

DA36a

DA46a

DA59a

DA75

DA79

DA81

DA91

DA93

DA101

DA116

DA118

0.57

0.36

0.48

0.49

0.45

0.52

0.55

0.51

2.16

0.26

0.55

0.97

0.52

0.0011

0.0050

0.0018

0.0010

0.00066

0.0034

0.0011

0.0077

0.00084

0.020

0.0034

0.14

0.020

2rc

0.1096

0.1089

0.1102

0.1122

0.1089

0.11053

0.1093

0.1110

0.10874

0.111

0.1071

0.1101

0.1153

Os/188Os

187

0.00022

0.00023

0.00013

0.0013

0.00016

0.00025

0.00018

0.00080

0.00017

0.00077

0.0024

0.00079

0.0012

2SE

0.0010

0.0030

0.0013

0.0021

0.0027

0.00050

0.0019

0.0010

0.00070

0.013

0.0028

0.0037

0.0028

2rc

2,740

3,000

2,790

2,410

2,830

2,658

2,760

2,980

2,990

2,900

3,030

3,100

2,360

TdMA

150

460

190

290

370

89

260

210

110

2,100

390

1,200

560

Unc

2,690

2,790

2,610

2,350

2,780

2,569

2,730

2,510

2,810

2,500

3,010

2,600

1,930

TdRD

Referenced to primitive mantle of Meisel et al. (2001) using a decay constant of 1.666910-11 a-1 (Smoliar et al. 1996) and measured 187Re/188Os for TMA and a 187Re/188Os of zero for TRD, which is a minimum age (Shirey and Walker 1998)

0.0080

0.0319

0.0296

0.0114

0.00736

0.0150

0.0055

0.0695

0.02701

0.064

0.0019

0.07

0.080

Re/188Os

187

d

1,100

730

210

480

1,900

22

510

20

7

400

190

85

200

2rc

Atom %

36,800

81,900

54,570

95,590

232,600

15,932

88,860

9,629

41,334

10,660

42,160

8,089

24,940

Os ppb

Uncertainties are absolute values and were estimated by numerical error propagation, including blank contribution and spike weighing uncertainties

85

85

20

19

31

11

20

15

4

43

30

240

64

2rc

c

613

543

335

226

356

50

102

139

124

141

16

120

409

Re ppb

b

From Aulbach et al. (2009)

1.0

(Ni ? Co)/Feb

a

0.8

DA14a

Wt (lg)

DA8a

Sample

Table 1 Sulphide Re–Os isotope systematics and model ages

950 Contrib Mineral Petrol (2011) 161:947–960


951

0.120

Table 2 Characteristics of sulphide inclusion-bearing peridotitic diamonds from Diavik

initial 187Os/188Os = 0.1093 ± 0.0001 MSWD = 0.46

a

.5 G

Sample

a

DA008a

43

d

Off-white

a

DA014

46

o/d flattened

Light brown

64

16

DA036a

33

o broken

Brown

92

24

DA046a

66

o

Light brown 100

19

DA059a

49

o

Off-white

509

0

DA075

137

o

Light brown 136

35

DA079 DA081

146 187

o o

Off-white Off-white

23 229

0 70

DA091

154

o

Off-white

375

13

DA093

178

o

White

54

0

DA101

160

o

Off-white

112

0

DA116

140

o

White

164

0

DA118

136

o

White

252

0

3 ca.

3G

0.115

187Os/

188Os

3. ca.

initial 187Os/188Os = 0.1073 ± 0.0001 MSWD = 0.75

0.110 Diavik incl in diamonds Diavik incl in olivine Panda incl in diamonds

0.105 0.00

0.05

0.10

0.15

188Os

187Re/

Fig. 1 Re–Os isochron diagram for peridotitic sulphide inclusions in diamonds from Diavik (DA14 has a large uncertainty on 187Os/188Os and is not plotted). Also shown are two isochrons previously defined for sulphide inclusions in olivine xenocrysts from Diavik (Aulbach et al. 2004) and in diamonds from Panda (Westerlund et al. 2006)

above, yields an isochron age of 3.24 ± 0.30 Ga (187Os/188Osi = 0.10724 ± 0.00014, MSWD = 0.59) and an errorchron age of 3.78 ± 0.27 Ga (187Os/188Osi = 0.10920 ± 0.00025, MSWD = 2.8), respectively. These values are statistically indistinguishable from those previously obtained. Diamonds Nitrogen is the main impurity in diamonds and, depending mainly on the residence temperature but also the time available for aggregation, may be present as singly substituted nitrogen atoms (Type Ib), pairs (Type IaA: [90% nitrogen aggregated in the A centre) or aggregates of four associated with a lattice vacancy (type IaB: [90% aggregated in the B centre) (Evans and Harris 1989; Taylor et al. 1990; Harris 1992). The eight diamonds investigated in this study were larger (average 155 mg) compared to the five diamonds in the previous batch (47 mg). Although this did not impact on the average sulphide inclusion size (4.5 lg in the present batch; 7.8 lg in the previous), the larger diamonds may sample a different mantle region, judging from more frequently observed poorly aggregated nitrogen (5 of 8 are Type IaA) at similar nitrogen contents compared to the previous batch (1 of 5) (Table 2). For all peridotitic diamonds, the percentage of nitrogen aggregated in the B centre (%N as B) ranges from 0 to 70%, with N contents of 23–543 ppm, which is well within the range of peridotitic diamonds from Diavik hosting silicate inclusions (Stachel et al. 2003). There are no systematic differences in N content-aggregation state characteristics between sulphides

a

Weightb Morphologyc Colour

Ntotal

d

%N as Be

543

14

From Aulbach et al. (2009)

b

In mg

c

d dodecahedron, o/d octahedron with some dodecahedral features, o octahedron

d

In atomic ppm

e

Percentage of nitrogen that is present in the B aggregated state, i.e. four N atoms and a vacancy

from this study falling on the Panda diamond and those lying on the Diavik xenocryst array. Thermometrical constraints on depth of diamond residence Assuming an average 3.4 Ga diamond formation age, residence temperatures of diamonds hosting peridotitic sulphide inclusions from Diavik can be estimated from N contentaggregation state systematics (Taylor et al. 1990; Leahy and Taylor 1997) to between \1,000 and *1,1608C. Olivine xenocrysts (likely coexisting with Ca-saturated garnets; Aulbach et al. 2004) from the same locality that host eight of the eleven sulphides forming the Diavik xenocryst Re–Os isochron occur over a similar temperature range of 910–1,1608C (Al-in-olivine thermometer of de Hoog et al. 2010; calculated for a pressure of 4.5 GPa using olivine trace-element data acquired by LAM ICPMS; Aulbach et al., unpublished data; the other three sulphides on the Diavik isochron occur in xenoliths or diamonds for which olivine trace-element data are not available). For a central Slave cratonic model geotherm of 38 mW m-2 (Gru¨tter et al. 1999), these temperatures correspond to depths from 140 to 150 km for two samples (C4 and C185, Re–Os analyses reported in Aulbach et al. 2004)—i.e. the interface between the ultradepleted shallow and less depleted deep SCLM layers in the central craton (Griffin et al. 1999)—and from 170 km down to *200 km for six samples (C20, C23, C24,

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C38, C78, C211), well within the deep SCLM. Westerlund et al. (2006), using similar arguments based on N aggregation data, placed the Panda diamonds within the shallow SCLM.

Discussion Age and origin of the Slave lithospheric mantle: Os isotope constraints Peridotitic sulphide inclusions in diamonds from Diavik have Re–Os isotope compositions that fall on the two previously established isochrons for the area (Fig. 1). Both ages overlap within uncertainty; therefore, the Diavik diamonds could have formed at basically the same time as the Panda diamonds in the Palaeoarchaean. The initial 187Os/188Os of each array is well constrained by samples with the lowest 187 Re/188Os, and clearly distinct from one another, and is enriched above a chondritic mantle composition. The different initial Os isotopic compositions are correlated with depth in the SCLM. The 3.27 ± 0.34 Ga isochron derived from sulphide inclusions in Diavik olivine xenocrysts (‘‘deep SCLM array’’) has initial cOs of ?2.53 ± 0.13 and Al-inolivine temperatures that suggest derivation from the comparatively mildly depleted deep lithosphere (Sect. ‘‘Thermometrical constraints on depth of diamond residence’’). The 3.52 ± 0.17 Ga isochron derived mostly from sulphide inclusions in diamonds from the nearby Panda kimberlite (Ekati Mine; ‘‘shallow SCLM array’’) has a higher initial cOs of ?5.6 ± 0.10 and low N-aggregation temperatures that place it just inside the diamond stability field near the base of the ultradepleted shallow lithosphere (Westerlund et al. 2006). The Diavik peridotitic sulphide inclusion data, when combined with the previously published sulphide inclusions in olivine from Diavik (Aulbach et al. 2004) and peridotitic sulphide inclusions in diamond and harzburgites from Panda (Westerlund et al. 2006), convincingly affirm a Mesoarchaean age for this portion of the Slave craton lithospheric mantle and do not define new age arrays of younger slope. The four Diavik inclusions that plot on the shallow SCLM array agree with an isochron age defined by four separate internal isochrons (e.g. two or more inclusions in a single diamond; Westerlund et al. 2006). Four Diavik inclusions that plot on the deep SCLM array agree with an isochron that gives the age of the host peridotite via disaggregated olivine xenocrysts, directly associating the diamonds and their hosts. Such cross-population of individual inclusions on different data arrays is expected with single-diamond dating, because kimberlitic magmas pick up diamonds from different patches of mantle host rock at depth and mix them during 100 km of violent transport to the surface. Furthermore, the presence of crustal rocks significantly older than the Neoarchaean has

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long been implied for this part of the Slave craton from Nd and Pb isotopic compositions of Neoarchaean granitoids (Davis et al. 2003). The data require the survival of diamond-hosting lithospheric mantle in this portion of the Slave craton since the Paleaoarchaean despite Neoarchaean magmatism. The survival of older diamond-hosting mantle may be owing to its refractory nature (e.g. resistence to remelting) and to its buoyancy enabling it to remain attached to the overlying crust. The heat and volatiles necessary for intracrustal melting could have been fuelled by mafic melts that were channelled into conduit systems that, once heated, were used repeatedly by new batches of melt, which largely bypassed the refractory portions of lithosphere containing the Palaeoarchaean diamonds. The radiogenic initial 187Os/188Os of the ultradepleted shallow SCLM array (Fig. 1) has been explained by the input of evolved Os during accretionary processes (Westerlund et al. 2006), which is plausible given the isotopic enrichments in the associated harzburgites and in the context of an old Slave cratonic core that grew via collisions at the margins of the nascent craton (Ketchum and Bleeker 2001). This model could be satisfied in a scenario where strong depletion is achieved in a hot Archaean plume, the impingement of which triggers subduction around its edges (Ueda et al. 2008; Burov and Cloetingh 2010) and may reconcile the small sizes and strong depletion of the earliest continental nuclei (Bleeker 2003), combined with evidence for growth of cratonic continental crust both in convergent margins and within plates (Rudnick 1995; Condie 1999; Wyman and Kerrich 2002). The deep and shallow SCLM array overlap with a major 3.4 Ga episode of juvenile crustal growth reported for the Slave craton (Pietranik et al. 2008). Generation of juvenile crust indicates addition of new material from the asthenosphere (Sircombe et al. 2001; Pietranik et al. 2008) that may be related to major plume or mantle avalanche events (Condie 1998). In accord with previous arguments, including the strong SCLM layering with distinct levels of depletion, peridotite compositions indicative of high-pressure melt extraction and the occurrence of diamonds from the lower mantle (Griffin et al. 1999, 2004; Davies et al. 1999; Aulbach et al. 2004, 2007), we take ca. 3.3–3.4 Ga to represent the age of deep lithosphere formation linked to plume subcretion beneath the central Slave craton. Formation of cratonic lithospheric mantle and complementary komatiite from hybrid plume sources The contribution of subducted eclogite or pyroxenites to the generation of plume-related magmas has been


953

recognised not just in modern ocean island basalts (OIBs) (Hofmann and White 1982; Alle`gre and Turcotte 1986) but also in Archaean equivalents, such as (non-arc-type) basalts, picrites and komatiites, suggesting deep recycling of oceanic plates to plume sources occurred early in Earth’s history (Kerrich et al. 1999; Condie 2003). Upon upwelling of a mixed eclogite-peridotite mantle package, dry eclogite will melt at greater depth than peridotite in upwelling mantle (Yasuda et al. 1994; Yaxley and Green 1998), forming a siliceous melt that reacts with surrounding mantle, in particular replacing olivine with orthopyroxene (opx) to form ‘‘hybrid’’, ‘‘reaction’’ or ‘‘reactive’’ pyroxenite (Yaxley and Green 1998; Rapp et al. 1999; Yaxley and Sobolev 2007; Gurenko et al. 2009). In an ascending mantle plume, hybrid pyroxenite itself would also melt before peridotite does, yielding alkali basaltic magmas (Kogiso et al. 2004; Sobolev et al. 2007), which will variably pervade and react with the near-solidus peridotite. Finally, the resulting hybrid peridotite will cross its solidus and experience varying melting intervals, depending on the thickness of pre-existing lithosphere beneath which it impinges (Sobolev et al. 2007). Figure 2 illustrates these processes as they could have occurred beneath the central Slave craton. In the following, we discuss the consequences of formation of cratonic lithosphere and complementary komatiites in a hybrid mantle plume. Os isotopes In the model outlined above and discussed in more detail below, hybrid pyroxenite formed in an upwelling hybrid Western Slave ca 3.3-3.4 Ga pre-craton amalgamation

W Preexisting shallow SCLM to ~140 km

Plume-related diamonds 3.3-3.4 Ga Opx-rich peridotite partial melting

Hybrid pyroxenite partial melting Eclogite partial melting

E

Subduction-related diamonds 3.5 Ga Deep SCLM = residue from opx-rich peridotite partial melting

Pyroxenite melt+peridotite = opx-rich peridotite Eclogite melt+peridotite =hybrid pyroxenite

Hybrid upwelling mantle plume

Fig. 2 Cartoon (after Sobolev et al. 2007) showing the western Slave craton, prior to 2.7 Ga amalgamation with and accretion beneath the eastern domain. The ultradepleted shallow mantle is subcreted by a hybrid plume, in which with decreasing depth first eclogite, then hybrid pyroxenite, then opx-enriched peridotite partially melted. See text for details

plume would inherit a crustal signature from eclogitederived melts, including radiogenic Os (e.g. Luguet et al. 2008; Aulbach et al. 2009; Day et al. 2009). Thereafter, pyroxenite-derived alkali basaltic magmas (Kogiso et al. 2004) reacting with peridotite could facilitate isotopic enrichment and homogenisation of the hybrid mantle. The scatter of data on the Re–Os isochron diagram suggests that such homogenisation was incomplete on the scale of sampling by the kimberlitic magmas. Considering evidence for the onset of subduction by 3.8 Ga (Shirey et al. 2008), the required contribution of 3.8 Ga Archaean oceanic crust (a mixture of 9 parts basalt and 1 part komatiite with a 187 Re/188Os of 23; model of Puchtel and Humayun 2000) to explain the initial 187Os/188Os of the 3.4 Ga deep SCLM array is *30% at Diavik. This amount can only be a rough estimate because of the many uncertainties, including evidence that subducted basaltic oceanic crust may lose a significant proportion of Re during metamorphism (Becker 2000), whereas the volumetrically larger gabbroic portion of the slab does not (Dale et al. 2007). Also, if Archaean oceanic crust had higher 187Re/188Os than assumed in the model of Puchtel and Humayun (2000), not as much of this material would be required to generate radiogenic 187Os/188Os within a given time interval. Some form of recycling of crustal material may have occurred much earlier than 3.8 Ga (Hopkins et al. 2008), reducing the amount of material required in the source. Finally, bulk mixing models may not accurately describe the process and the preferred addition of a mobile, highly radiogenic sulphide component could also play a role (Luguet et al. 2008). Radiogenic initial 187Os/188Os has been identified in a few Archaean and Proterozoic komatiites and related rocks (Shirey 1997; Walker et al. 1997; Puchtel et al. 2001, 2009a; Walker and Nisbet 2002). Thus, some Archaean plume-derived melts and cratonic mantle samples from widely dispersed old continental nuclei (Baltic shield, northwestern Russia, Slave, Kimberley, Kaapvaal cratons) have potentially formed from hybrid plume sources with suprachondritic initial 187Os/188Os (Fig. 3). Opx enrichment in cratonic SCLM A distinctive feature of some cratonic mantle sections is the noticeable SiO2 enrichment in some peridotites, which is also expressed as high opx and low olivine modes, combined with high Mg# in olivine that are difficult to explain with melt extraction from a primitive mantle source (Boyd 1989; Herzberg 1999; Kelemen et al. 1998; Walter 2005). Previous models discussed opx enrichment of the cratonic lithosphere subsequent to formation of a lithospheric mantle residue, e.g. in subduction zone settings (Kesson and Ringwood 1989; Kelemen et al. 1998; Bell et al. 2005).

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Contrib Mineral Petrol (2011) 161:947–960 6

Pechenga Keweenawan

5

enriched

range modern OIBs

4

γOs

Panda

Schapenburg

Kostomuksha

3 Diavik

2 Lithospheric Mantle Mantle melts

1

Kimberley, Oz

chondritic

0

Belingwe

depleted

-1 0

1

2

3

4

Time (Ga)

Fig. 3 Age versus initial cOs (per cent deviation of sample 187Os/188Os from chondritic) for those komatiites and related rocks (Keweenawan, midcontinent rift system, North America: Shirey 1997; Pechenga ferropicrite, northwestern Russia: Walker et al. 1997; Kostomuksha komatiite, Baltic Shield: Puchtel et al. 2001; Belingwe komatiite, Zimbabwe: higher estimate from Walker and Nisbet 2002; lower estimate from Puchtel et al. 2009b; Schapenburg komatiite: Puchtel et al. 2009a), and cratonic mantle samples (Kimberley Block, Western Australia: Graham et al. 1999; Diavik and Panda, central Slave craton, Canada: Aulbach et al. 2004; Westerlund et al. 2006) that have suprachondritic initial cOs

Modal opx in many OIB peridotites also is higher than expected for partial melting of primitive mantle (data compilation of Neumann and Simon 2009) and obviously cannot be attributed to silica mobilisation from slabs in the forearc. Hence, another process must operate that leads to SiO2 enrichment in within-plate settings. The recent suggestion (Sobolev et al. 2007) that most plume sources are hybridised by the incorporation of an eclogitic or pyroxenitic component would provide a ready mechanism to add variable amounts of opx to the mantle prior to its cratonisation. At small melt to peridotite ratios, silicic melts derived from eclogite embedded in upwelling mantle peridotite would lead to opx enrichment by reaction with olivine (Yaxley and Green 1998) and formation of opxrich peridotites. At high eclogite-derived melt to rock ratios, olivine-free pyroxenite would be formed (Rapp et al. 2010). Even refractory eclogite, such as that residual from metamorphic processing in subduction zones, does not exceed a liquidus temperature at 5 GPa of 1,6508C (Kogiso and Hirschmann 2006). As a consequence, during upwelling in hot Archaean plumes, with mantle potential temperatures of some 1,7008C (e.g. Herzberg 1995), eclogite would likely be completely consumed, implying that there will be no residual phase that could impose a diagnostic trace-element pattern on the melt (Ringwood 1990). Upon further upwelling, hybrid pyroxenite would partially melt, yielding alkaline magmas (Kogiso et al. 2004). Interaction of peridotite with such magmas has been shown

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Fig. 4 Primitive mantle (pyrolite, McDonough and Sun 1995) and harzburgite residues (experimental: 1,7708C at 6 GPa, Walter 1998; average Wiedeman fjord peridotite xenolith: Bernstein et al. 1998) in MgO/SiO2 vs. Al2O3/SiO2 space. The projection of hypothetical melt composition passing from residues through the primitive mantle does not cross the compositions of Archaean komatiites (compilation from http://georoc.mpch-mainz.gwdg.de). Likewise, average compositions of cratonic xenoliths suites (from Francis 2003; except Slave craton: Mackenzie and Canil 1999; Pearson et al. 1999; Kopylova and Russell 2000; Kopylova and Caro 2004; Aulbach et al. 2007; compositions with Al2O3[ 5 wt%, which are likely due to secondary melt addition, were screened out) cannot be simple residues from partial melting of primitive mantle. Their SiO2-rich compositions have been ascribed to secondary addition of opx (step 2) subsequent to partial melting (step 1). Field for high-temperature opx encompasses opx calculated from melt in equilibrium with an orthopyroxenite residue at 1,5758C and 3.5 GPa (experiments of Sobolev et al. 2007), high-temperature opx formed in experimental hybridised peridotite (T = 1,5008C at 3.5 GPa; Yaxley and Green 1998) and high-temperature opx from Kaapvaal peridotite xenoliths reconstructed from opx ? garnet ? cpx inferred to have previously been dissolved in a single-aluminous opx at high temperatures (Saltzer et al. 2001). Field for low-temperature opx encompasses opx formed in subduction zones (Ishimaru et al. 2007; Ionov 2010) and that formed in experimental hybridised peridotite (T = 1,100–1,1508C at 3.8 GPa, experiments of Rapp et al. 1999). The trend through increasingly opx-rich average cratonic mantle compositions passes through the field for high-temperature opx

to lead to replacement of cpx by opx in mantle xenoliths from Tanzania (Koornneef et al. 2009). This might be expected to lead to specific trace-element signatures, but these would be inconspicuous because of the low to absent garnet modes in the residual pyroxenite (and corresponding low HREE retention capacity; experiments of Sobolev et al. 2007). Orthopyroxene in equilibrium with a highdegree partial melt of pyroxenite (T = 1,5758C; Sobolev et al. 2007) has a high Al2O3 content of 4.7 wt% and plots in the field of high-temperature opx composed of reconstructed opx in cratonic peridotite xenoliths and of


experimental high-temperature opx produced in peridotite hybridised by eclogite-derived melts (Fig. 4). The trend for average cratonic xenolith suites intersects the field of hightemperature opx, but not that of low-temperature opx typical of that formed in subduction zones (Fig. 4) and supports the viability of a model of opx (hence Al- and Si-) enrichment in hybrid mantle plumes. The melting relations of hybridised mantle such as websterite or opx-rich peridotite are not known. However, Francis (2003) argued that a more SiO2- and Al2O3-rich mantle would not only leave a more opx-rich residue upon partial melting, but also would undergo higher degrees of partial melting, thus leading to higher Mg numbers in the residue. This could explain high Mg# combined with high opx that is a hallmark of many cratonic xenoliths from the Kaapvaal (Boyd 1989). Relating cratonic mantle and komatiites to hybrid plume sources The purpose of this paper is to point out some unique capabilities of the hybrid plume model for explaining specific compositional features of some komatiites and SCLM peridotites rather than provide evidence against other models for SCLM or komatiite formation. Projected hypothetical melt compositions of primitive mantle do not match the compositions of Archaean komatiites (Fig. 4), pointing to a mass balance problem as komatiites are too SiO2-rich and Al2O3-poor to be partial melts of primitive mantle (Kelemen et al. 1998; Herzberg and O’Hara 1998; Francis 2003). Likewise, cratonic peridotites are too SiO2rich to be residues of primitive mantle melting. This mismatch may be remedied by the model proposed here of lithosphere formation in a hybrid plume and opx enrichment prior to partial melting. With some possible exceptions, generating komatiites from a wet source in a subduction setting (e.g. Parman et al. 1997) is inconsistent with observations that such a source will generate kimberlites and related melts rather than komatiites (Kawamoto and Holloway 1997), that some komatiites have trace-element compositions inconsistent with formation in a subduction zone (Chavagnac 2004), that they have MgO contents requiring formation at depths far below subduction zones (Arndt 2003) and that they formed under dry conditions (Berry et al. 2008). By contrast, generation of komatiites from a hybrid plume source does not violate any of these constraints and, indeed, such a source has been advocated for komatiites from Abitibi based on olivine major-element characteristics (Sobolev et al. 2007). Variations in initial 187Os/188Os or SiO2-content, for different komatiite occurrences or for different flows within a single occurrence can then be related to sampling

955

of heterogeneous deep plume sources where variable amounts and/or types of subducted slabs had accumulated and to sampling of different domains within the upwelling hybrid plume, respectively. This model implies a short-time interval in the Archaean between deep recycling and entrainment of slab material in a plume (Fyfe 1978). The fact that the majority of komatiites, most of which were generated in the Archaean, have chondritic initial 187Os/188Os may reflect the small amount of time that had been available to recycle and accumulate oceanic slabs in plume sources, compared to today. Large volumes of residual aluminous opx could explain the Al2O3-depletion of some Archaean komatiites (Walter 1998), but the major-, trace-element and isotope compositions of both komatiites and cratonic mantle residues will also depend on the depth of partial melting, phases present on the solidus and the width of the melting interval prior to plume subcretion. Moreover, the hybrid plume model does not preclude that other processes, such as an early differentiation event or second-stage melting (Puchtel et al. 2009a, b) were important as well. Melting of hybrid pyroxenite embedded in peridotite would likely leave an orthopyroxenite residue (see experiments in Sobolev et al. 2007). However, current estimates of average compositions of different cratonic mantle sections (e.g. Francis 2003) are based on peridotites only and do not include inferred residual orthopyroxenites. If correct, this model has consequences for models of komatiite generation based on inversion of major-element and traceelement data, for estimates of cratonic mantle composition and for the physical properties of the mantle, such as density or seismic velocities (e.g. Wagner et al. 2008). The compositions of komatiites and cratonic xenolith suites can be qualitatively linked by partial melting of a primitive mantle source that has been enriched by variable amounts of aluminous opx component either by incorporation of eclogite or pyroxenite and assuming that the residue contains some unknown amount of orthopyroxenite, as indicated in Fig. 5. The Kaapvaal lithospheric mantle is exceptionally depleted (Stachel et al. 2003). The South African Barberton komatiites, which are coeval with the earliest nuclei of the Kaapvaal craton (de Wit et al. 1992), may have formed at exceptionally high mantle potential temperatures (Nisbet et al. 1993) from a plume source that was unusually enriched in SiO2 (48 wt%; Herzberg 1993). If the Barberton komatiites and the Kaapvaal cratonic mantle are complementary, as previously suggested (Herzberg 1995; Walter 1998), it can be seen from Fig. 5 that an (unrealistically?) high proportion of opx is required for the Kaapvaal mantle source prior to depletion. The significantly suprachondritic initial 187 Os/188Os determined for the South African 3.5 Ga Schapenburg komatiite (cOsi = ?3.7 ± 0.3), an equivalent to the Barberton komatiite (Puchtel et al. 2009a), would be

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Si-content in komatiite magmas from the Archaean to the Cretaceous (Francis 2003). Sobolev et al. (2007) argued that the greater the buoyancy (related to its temperature relative to surrounding mantle) of a plume, the more dense eclogite material it should be able to entrain. Overall, the trend in average peridotite compositions for different cratons towards decreasing MgO/SiO2 and increasing Al2O3/SiO2 from Tanzania to Kaapvaal (Fig. 5) could be interpreted as a trend towards increasingly aluminous opx-rich residues formed during partial melting of increasingly hotter and therefore more buoyant and eclogite-rich hybrid mantle.

Summary and conclusions Fig. 5 Trend for addition of variable amounts of opx in equilibrium with pyroxenite melt (Sobolev et al. 2007) to primitive mantle (pyrolite of McDonough and Sun 1995), prior to the onset of peridotite melting, thus creating an SiO2- and Al2O3-rich mantle source (step 1). This trend could also be interpreted as one towards increasing plume temperature and buoyancy, hence elcogite load. Partial melting of the opx-enriched source (step 2) yields partial melts and depleted residues, the latter consisting of cratonic mantle peridotite plus some unknown amount of orthopyroxenite. Barberton komatiites and Kaapvaal residues are assumed to be complementary as indicated by the partial melting arrows; other komatiites and cratonic peridotite suites, though not cogenetic, have been matched by trends subparallel to the Kaapvaal trend to indicate compositions of hypothetical melt-residue pairs. Also shown is a trend for peridotite residues towards increasing Al2O3/SiO2 and decreasing MgO/SiO2 with increasing amount of opx in the source (see text for details). References as in Fig. 4

consistent with a hybrid source. Formation of the Kaapvaal lithosphere in an unusually hot mantle upwelling in the hotter Archaean could explain why this mantle composition has no younger analogue. The deep Slave mantle is less depleted than the Kaapvaal SCLM (Stachel et al. 2003) and in the plume model would require a smaller temperature excess, hence smaller proportion of mafic material in the plume, limiting the amount of opx enrichment, consistent with a depletion trend subparallel to that of the Kaapvaal trend towards higher MgO/SiO2 and lower Al2O3/SiO2 (Fig. 5). The amount of eclogite present in the source estimated from Os isotopes is ca. 30% (Sect. ‘‘Os isotopes’’). By contrast, only 16% opx is estimated to be in the source of the 2.7 Ga Abitibi komatiites from the Superior craton (Sobolev et al. 2007), which agrees with estimates from their MgO/SiO2 vs Al2O3/SiO2 systematics (Fig. 5) and with their formation at lower mantle potential temperatures than the Barberton komatiites (Nisbet et al. 1993; Herzberg 1995), perhaps due to the secular decrease in mantle potential temperatures that is also mirrored in a decrease in

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Peridotitic sulphides in diamonds from Diavik in the central Slave craton align with either a 3.3 or a 3.5 Ga isochron, both with suprachondritic initial 187Os/188Os. The younger group can be explained by formation of the deep Slave lithosphere during upwelling of a plume into which eclogite-derived material was mixed. Given the evidence for presence of recycled oceanic crust in ancient plumederived melts, such as komatiites (e.g. Condie 2003), and the possibility of cratonic lithosphere formation in plumes (e.g. Herzberg 1993), we examined the consequences of partial melting in upwelling hybrid mantle, where eclogite domains partially or completely melt and react with peridotite forming hybrid pyroxenite, which then partially melts and metasomatises the near-solidus peridotite that is the source of komatiites and complementary residual cratonic mantle. Our main conclusions are 1.

2.

3.

4.

The presence of isotopically evolved mafic materials in Archaean plumes can explain the 187Os enrichment of some komatiites and cratonic lithospheric mantle regions. Reaction of melts derived from eclogite or from hybrid pyroxenite with peridotite in upwelling plumes, prior to the onset of peridotite melting, may represent a mechanism by which silica can be added to the mantle source. Orthopyroxene crystallising from eclogite or pyroxenite melt at the high temperatures relevant to upwelling plumes has high Al2O3 contents and can explain the MgO/SiO2 vs Al2O3/SiO2 systematics of cratonic mantle samples, where high Mg# combined with high opx modes are suggested to result from higher melt productivity of an Al- and Si-rich source compared to primitive mantle. The mass balance problem that exists when trying to relate komatiites to residues of primitive mantle


5.

6.

7.

melting, specifically with regard to SiO2, disappears when opx is added to the primitive mantle source prior to partial melting. The residues of hybrid plume melting likely include pyroxenite that is not currently considered in average cratonic mantle compositions based on peridotitic xenolith samples. Average cratonic peridotitic mantle compositions with decreasing MgO/SiO2 and increasing Al2O3/SiO2 can be interpreted as residues of increasingly hot, buoyant plumes, which can carry high eclogite loads. This yields a high modal proportion of opx for the Kaapvaal hybrid mantle source, and increasingly lower proportions for the Slave source and the Superior source. A secular decrease in mantle potential temperatures since the Archaean can explain why there are no younger analogues of similarly SiO2-rich mantle lithosphere.

Acknowledgments We thank Diavik Diamond Mining Inc for financial support and for the generous provision of the samples studied here. Help at the UofA by George Braybrook (SEM lab) and Gayle Hatchard (TIMS lab) is greatly appreciated. Ambre Luguet, an anonymous reviewer and the editor, Chris Ballhaus, provided valuable comments that are greatly appreciated. This work was funded by an NSERC CRD Grant and NSERC Discovery Grants (RAC, TS and LMH). Partial support of the Radiogenic Isotope Facility at the University of Alberta came from an NSERC Major Facilities Access Grant.

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