Helium isotopic evidence for modification of the

Lithos 216–217 (2015) 73–80

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Helium isotopic evidence for modification of the cratonic lithosphere during the Permo-Triassic Siberian flood basalt event Peter H. Barry a,b,⁎, David R. Hilton c, James M.D. Day c, John F. Pernet-Fisher a, Geoffrey H. Howarth a, Tomas Magna d, Aleksey M. Agashev e, Nikolay P. Pokhilenko e, Lyudmila N. Pokhilenko e, Lawrence A. Taylor a a

Planetary Geosciences Institute, Department of Earth and Planetary Sciences, University of Tennessee, Knoxville, TN 37996, USA Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK Geosciences Research Division, Scripps Institution of Oceanography, UCSD La Jolla, CA 92093-0244, USA d Czech Geological Survey, Klarov 3, CZ-118 21 Prague 1, Czech Republic e V.S. Sobolev Institute of Geology and Mineralogy, Russian Academy of Sciences, Siberian Branch, 630090 Novosibirsk, Russia b c

a r t i c l e

i n f o

Article history: Received 1 September 2014 Accepted 1 December 2014 Available online 6 December 2014 Keywords: Helium Helium isotopes Noble gases Peridotites Siberia SCLM Udachnaya and Obnazhennaya

a b s t r a c t Major flood basalt emplacement events can dramatically alter the composition of the sub-continental lithospheric mantle (SCLM). The Siberian craton experienced one of the largest flood basalt events preserved in the geologic record — eruption of the Permo-Triassic Siberian flood basalts (SFB) at ~250 Myr in response to upwelling of a deep-rooted mantle plume beneath the Siberian SCLM. Here, we present helium isotope (3He/4He) and concentration data for petrologically-distinct suites of peridotitic xenoliths recovered from two temporally-separated kimberlites: the 360 Ma Udachnaya and 160 Ma Obnazhennaya pipes, which erupted through the Siberian SCLM and bracket the eruption of the SFB. Measured 3He/4He ratios span a range from 0.1 to 9.8 RA (where RA = air 3 He/4He) and fall into two distinct groups: 1) predominantly radiogenic pre-plume Udachnaya samples (mean clinopyroxene 3He/4He = 0.41 ± 0.30 RA (1σ); n = 7 excluding 1 outlier), and 2) ‘mantle-like’ post plume Obnazhennaya samples (mean clinopyroxene 3He/4He = 4.20 ± 0.90 RA (1σ); n = 5 excluding 1 outlier). Olivine separates from both kimberlite pipes tend to have higher 3He/4He than clinopyroxenes (or garnet). Helium contents in Udachnaya samples ([He] = 0.13–1.35 μcm3STP/g; n = 6) overlap with those of Obnazhennaya ([He] = 0.05–1.58 μcm3STP/g; n = 10), but extend to significantly higher values in some instances ([He] = 49– 349 μcm3STP/g; n = 4). Uranium and thorium contents are also reported for the crushed material from which He was extracted in order to evaluate the potential for He migration from the mineral matrix to fluid inclusions. The wide range in He content, together with consistently radiogenic He-isotope values in Udachnaya peridotites suggests that crustal-derived fluids have incongruently metasomatized segments of the Siberian SCLM, whereas high 3He/4He values in Obnazhennaya peridotites show that this section of the SCLM has been overprinted by Permo-Triassic (plume-derived) basaltic fluids. Indeed, the stark contrast between pre- and post-plume 3He/4He ratios in peridotite xenoliths highlights the potentially powerful utility of He-isotopes for differentiating between various types of metasomatism (i.e., crustal versus basaltic fluids). © 2014 Elsevier B.V. All rights reserved.

1. Introduction Helium isotopes (3He/4He) are powerful tracers for distinguishing between mantle-derived and crustal fluids. Radiogenic 4He accumulates from the radioactive decay of U- and Th-series radionuclides and becomes strongly enriched in continental crust, whereas 3He is overwhelmingly primordial, stored in Earth's mantle since accretion (e.g., Clarke et al., 1969). As a result, different tectonic domains exhibit markedly different 3He/4He ratios (typically reported relative to air;

⁎ Corresponding author at: Department of Earth Sciences, University of Oxford, Oxford OX1 3AN, UK. E-mail address: [email protected] (P.H. Barry).

http://dx.doi.org/10.1016/j.lithos.2014.12.001 0024-4937/© 2014 Elsevier B.V. All rights reserved.

RA = 3He/4He of air = 1.4 × 10−6), which can be used to distinguish between various terrestrial reservoirs and characterize the chemical structure of the mantle (see review by Hilton and Porcelli, 2014). Disparate ratios in mantle-derived samples illustrate the long-term isolation and preservation of high-3He/4He (≥50 RA; Stuart et al., 2003) plumederived materials from the well mixed and more extensively degassed depleted MORB mantle (DMM) (8 ± 1 RA; Graham, 2002). However, the He-isotope signature of the sub-continental lithospheric mantle (SCLM) remains relatively poorly characterized. The SCLM represents a minor (~ 2.5%), but potentially important portion of the total terrestrial mantle. Due to isolation from the convecting mantle, the SCLM can potentially generate and preserve significant chemical heterogeneity over Gyr time scales (McDonough, 1990; Walker et al., 1989). Previous studies have demonstrated that

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P.H. Barry et al. / Lithos 216–217 (2015) 73–80

the SCLM has slightly lower (i.e., more radiogenic) 3He/4He values than the DMM (Ackert et al., 1999; Barfod et al., 1999; Dunai and Baur, 1995; Dunai and Porcelli, 2002; Gautheron et al., 2005; Matsumoto et al., 1998, 2000; Porcelli et al., 1986; Reid and Graham, 1996), with an estimated 3 He/4He ratio of 6.1 ± 0.9 RA (Gautheron and Moreira, 2002). However, radiogenic ingrowth models predict significantly lower 3He/4He values, based on the relatively high U/He ratios measured in SCLM peridotites (e.g., Craig and Lupton, 1976). To sustain such high 3He/4He values in the SCLM, therefore, Ballentine (1997) and Gautheron and Moreira (2002) have proposed open-system, steady-state flux models, whereby He from the asthenosphere is fluxed into the SCLM over ~100 Myr timescales. Significantly, these studies have mostly focused on peridotites and largely ignored the petrological context and metasomatic history of samples, which can potentially complicate the helium budget in the SCLM via the addition of extraneous metasomatic helium fluxes. The Siberian craton was assembled by the amalgamation of several island-arc terrains during the Archean and Proterozoic (Pearson et al., 1995a,b; Rosen et al., 1994). It has subsequently experienced a complex history of Phanerozoic metasomatism, including emplacement of N1000 kimberlite intrusions between Silurian and Jurassic times (Howarth et al., 2014; Fig. 1). Here we focus on petrologically wellcharacterized peridotite xenoliths, which were transported to the surface by two Siberian kimberlites: the Late-Devonian Udachnaya (360 Ma) and Jurassic Obnazhennaya (160 Ma) pipes (Bristow et al., 1991; Smelov and Zaitsev, 2013). As these eruptions bracket the ~ 250 Ma Siberian flood basalts (SFB) (Ivanov et al., 2013; Reichow et al., 2002), they provide insight into the Phanerozoic metasomatic history of the Siberian SCLM. Pre-SFB Siberian kimberlitic material displays SCLM-like He-isotopes (i.e., ~ 5.7 RA; Sumino et al., 2006), which are interpreted to represent radiogenic addition to a high (~ 15 RA) He-

isotope plume source. Notably, the highest measured He-isotope values associated with the SFB are ~12.7 ± 0.2 RA (Basu et al., 1995), suggesting a discernible plume contribution, clearly distinct from low-degree partial melts derived from the continental lithosphere–asthenosphere boundary (3He/4He b 7 RA; Day et al., 2005). In this contribution, we report the He isotopic and abundance characteristics in minerals from Siberian mantle peridotite xenoliths that erupted both prior to and following the SFB to assess the effects of metasomatism and plume impingement on volatiles of the Siberian SCLM. In addition, we report U, Th, and Li results for a subset of the xenolith mineral separates, and combine these results with previously published major- and trace-element data to investigate responses in the Siberian SCLM to pervasive flood basalt melt modification at the Permo-Triassic boundary. We have selected various mineral phases from peridotite xenoliths, equilibrated at different temperatures and depths in the mantle, to provide a comprehensive overview of changes occurring throughout the lithosphere. By considering petrological constraints, together with He-isotope variations, we are able to assess the potential effects of plume–lithosphere interaction and the role of metasomatic fluids in modifying the SCLM. 2. Materials and methods 2.1. Sample petrology and petrogenesis All xenolith samples (n = 10 for Obnazhennaya; n = 10 for Udachnaya) are granular to sheared spinel and garnet lherzolites (Lz) and have been characterized for their petrography and mineral chemistry by Howarth et al. (2014). These authors found that melts calculated

Fig. 1. Geological map of the Siberian craton with dashed lines showing the extent of the Siberian Large Igneous Province (LIP) associated with the Siberian Flood Basalts (SFB). Kimberlite pipes marked are divided by magmatic episode: Pre-SFB kimberlite: Silurian–Carboniferous (420–345 Ma). Syn-SFB kimberlite: Triassic (245–215 Ma). Post-SFB kimberlite: Jurassic (160–149 Ma). Map modified from Howarth et al. (2014).


to be in equilibrium with Udachnaya peridotite garnet rims display light rare earth element (LREE) enrichment, consistent with kimberlitic metasomatism. In contrast, melt reconstructions of Obnazhennaya garnets indicate that metasomatic melts were not significantly enriched in LREE, and instead show evidence of metasomatism by fertile, plumerelated basaltic melts (Howarth et al., 2014). Thermobarometry constraints (Table 1) indicate that Udachnaya xenoliths equilibrated at relatively high temperatures and pressures (~ 1300 °C; 55 to 63 kbar), corresponding to mid-lower lithospheric depths (~ 200 km), whereas Obnazhennaya xenoliths equilibrated at lower temperatures and pressures (~700 °C; 18 to 25 kbar) (Howarth et al., 2014). Indications that the SCLM was considerably thinner (~ 70 km) following SFB emplacement are supported by models of significant thermal–chemical erosion of the SCLM during SFB emplacement (Griffin et al., 2005; Pokhilenko et al., 1999) and/or lithosphere delamination, whereby the lithosphere became denser than the surrounding mantle and was subsequently detached from the overlying craton, leading to replacement by the underlying fertile mantle material (e.g., Bird, 1979; Kay and Mahlburg Kay, 1993). Irrespective of the SCLM thinning mechanism, it is clear that metasomatic processes have incongruently modified samples from different depth intervals.

2.2. In vacuo helium analyses Mineral separates (clinopyroxene, garnet and olivine) of peridotite xenoliths from the Obnazhennaya and Udachnaya pipes were crushed under ultra-high-vacuum (UHV) to determine helium isotope (3He/4He) values and 4He abundances using a MAP-215 noble gas mass spectrometer. Standard protocols, described previously in Hilton et al. (2011), were employed. Prior to crushing, mineral separates were washed in a 1:1 acetone–methanol mixture, ultrasonically cleaned, heated at 150 °C, and loaded into an on-line sample crushing apparatus (Scarsi, 2000). Helium was liberated from mineral separates using an external electromagnetic solenoid to lift and accelerate a hardened-steel slug onto the crystals, pulverizing them to fine powder. All crushing times were 120 s (at a rate of 2 strokes per second). The released volatiles were purified by sequential exposure to hot Ti (750 °C), lowered to 400 °C during the cleanup, liquid nitrogen cooled charcoal, a Zr–Al alloy getter pump, and a cryogenically-cooled charcoal trap used to separate He from Ne. Purified He was then expanded into the mass spectrometer for abundance and isotope analysis. Following He analysis, Ne was released from the charcoal trap and admitted to the mass spectrometer for measurement. An air standard (1 RA) was prepared and run under identical experimental conditions. Average blanks were identical to those reported by Hilton et al. (2011): i.e., ~6.0 × 10−11 cm3 [4He] STP for all runs. All 3He/4He values and [He] were corrected for Table 1 Summary of helium isotope, pressure, temperature and ages estimates for Siberian peridotite xenoliths. Sample ID

P (kbar)a

Obnazhennaya O-1106 O-1104 O-1107 O-95-12 O-129-74

17.5 14.8 21.4 25.2 19.9

709 620 680 721 662

3.95 4.57 0.54 6.68 3.49

160 160 160 160 160

Udachnaya U-84-09 U-352-08 U-48-10 U-23-10

60.6 57.8 56.5 58.9

1245 1272 1281 1262

0.67 0.79 0.11 0.72

360 360 360 360

a

T (°C)b

3

He/4He (R/RA)c

Eruption age (Ma)

Values from Howarth et al., 2014. Calculations after Nickel and Green, 1985. Values from Howarth et al., 2014. Calculations after Taylor, 1998. c R/RA notation where R = sample 3He/4He ratio and RA = atmosphere 3He/4He ratio (= 1.4 × 10−6). b

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air-derived components using the methods described in Hilton et al. (2011). Garnet, pyroxene and olivine mineral separates were crushed in vacuo, to preferentially release ‘primary’ volatiles trapped as fluid inclusions (FIs), and to avoid cosmogenic and/or radiogenic components produced within the mineral matrix (e.g., Blard and Farley, 2008; Blard et al., 2008; Craig and Poreda;, 1986; Hilton et al., 1993, 2011; Kurz, 1986; Yokochi et al., 2005), as the possibility exists that some matrix-sited He of different origin is released and contaminates FIsited He. The approach adopted here, i.e., crushing for short periods of time at room temperature, is an effective way of avoiding release of any matrix bound He (Hilton et al., 1993, 2011). 2.3. Trace element analyses In order to obtain trace-element abundance information on the same mineral fractions used for He-isotopes, powders produced by crushing were recovered and homogenized. Following protocols described by Day et al. (2014), between 50 and 100 mg of the crush powder was precisely weighed and digested in a 1:4 mixture of Optima-grade concentrated HNO3 :HF for N72 h on a hotplate at 150 °C. Samples were prepared with reference standards (basalts BHVO-2 and BIR-1 and peridotites GP 13 and DTS-2b). Sequential HNO3 dry-down steps were conducted in order to break down fluorides. Clear sample solutions were diluted by a factor of ~ 5000 using 2% HNO3 and doped with a 1 ppb In solution in order to monitor instrumental drift. The solutions were measured in a standard mode using a Thermo Scientific iCAPqc quadrupole inductively coupled plasma mass spectrometer (ICP-MS) at the Scripps Isotope Geochemistry Laboratory, Scripps Institution of Oceanography. Reproducibility of the reference materials was better than 5% (RSD) for basaltic standards, but was 15% for Th and 7% for U in the GP13 peridotite and DTS-2b dunite standards. Element abundances were consistently within 2σ standard deviation uncertainty of recommended values (Day et al., 2014). For Li elemental analysis wholerock samples and mineral separates (olivine, clinopyroxene, garnet) were digested in a mixture of concentrated distilled HF–HNO3 (6:1 v/ v) in screw-top Teflon® vials for N72 h on a hotplate at 130 °C. Small amounts of HNO 3 were sequentially administered to evaporated samples, and dried residues were equilibrated with 6 M HCl for 24 h on a hotplate at 70 °C. Lithium concentrations were determined by measuring Li beam intensities in clean fractions against 0.1-, 1- and 10-ppb L-SVEC reference Li solutions using a Neptune multiple-collector ICPMS (MC-ICPMS; ThermoFisher Scientific), housed at the Czech Geological Survey. International reference rocks JP-1 and DTS-2B were used for quality control (Magna et al., 2014). 3. Results Helium isotope and concentration values are presented in Table 2. Overall, He-isotopes range from 0.10 to 9.8 RA with helium concentrations ([He]) spanning approximately five orders of magnitude, from 0.048 to 350 μcm3STP/g (Table 2; Fig. 2). Obnazhennaya mineral fractions display 3He/4He values between 0.54 and 8.39 RA (mean clinopyroxene 3He/4He = 4.20 ± 0.90 RA (1σ); n = 5 excluding 1 outlier). In contrast, most Udachnaya mineral fractions are dominated by radiogenic He contributions (0.10–3.85 RA; mean clinopyroxene 3 He/4He = 0.41 ± 0.30 RA (1σ); n = 7 excluding 1 outlier), with some samples having He contents up to ~ 350 μcm3STP/g (Fig. 2; Table 2). Notably, olivine samples from both pipes retain the highest He-isotope signatures (Fig. 2). If metasomatic- and/or crustal-fluid inputs act to lower 3He/4He values, these results suggest that olivine is the most resistant mineral phase to act against this effect. The corollary is that other mineral separates (i.e., pyroxenes) may be useful for determining the extent of metasomatism in the SCLM.

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Table 2 Helium isotope, concentration and trace element results of Siberian peridotites. Xenolith type⁎⁎

Phase

3 He/4He (R/RA)b

Obnazhennaya O-1104 500.1 O-1104 497.2 O-1107 642.9 O-129/74 320.6 O-1100 548.8 O-1106 544.2 O-1106 389.4 O-1105 1350 O-95/12 504.5 O-97/12 563.3

Gt-Lherzolite Gt-Lherzolite Gt-Lherzolite Gt-Lherzolite Sp-Lherzolite Gt-Lherzolite Gt-Lherzolite Gt-Lherzolite Gt-Lherzolite Sp-lherzolite.

Cpx Cpx Cpx Cpx Cpx Cpx Gt Ol Ol Ol

4.04 4.57 0.54 3.49 3.63 5.61 3.94 5.93 6.66 8.36

72.3 5340 1320 632 64.6 632 341 4915 340 220

Udachnaya U-163/09 U-98-10 U-98/10 U-48-10 U-84-09 U-33-10 U-352-08 U-367/08 U-23-10 U-4-76

Gt-Lherzolite Gt-Lherzolite Gt-Lherzolite Gt-Lherzolite Gt-Lherzolite Gt-Lherzolite Gt-Lherzolite Gt-Lherzolite Gt-Lherzolite Harzburgite

Cpx Cpx Cpx Cpx Cpx Cpx Cpx Cpx Gt Ol

3.85 0.46 0.39 0.11 0.68 0.38 0.79 0.10 0.73 9.80

744 881 5060 29,800 37,900 14,700 29,300 8490 26.4 339

Sample ID

Sample wt. (mg)a

527.2 642.1 434.5 518.0 655.1 140.9 69.60 535.8 564.3 94.00

Xc

3

He/4He (RC/RA)d

[4He]C μcm3STP/ge

Th (ppb)

U (ppb)

Th/U

Li (ppm)

[4He]Total-Rad μcm3STP/gf

3 He/4HeCorr (R/RA)g

4.09 ± 0.06 4.57 ± 0.04 0.54 ± 0.01 3.49 ± 0.02 3.67 ± 0.14 5.62 ± 0.08 3.95 ± 0.07 5.73 ± 0.07 6.68 ± 0.11 8.39 ± 0.33

0.338 1.00 1.05 1.58 0.075 0.711 0.443 0.503 0.216 0.048

1280 815 2450 830 n.d. n.d. n.d. n.d. n.d. n.d.

542 217 549 189 n.d. n.d. n.d. n.d. n.d. n.d.

2.4 3.8 4.5 4.4 n.d. n.d. n.d. n.d. n.d. n.d.

3.1 3.1 4.3⁎ 2.4⁎ 1.0 1.3 1.3 2.5 1.2 1.4

10.3 4.99 13.1 2.96 n.d. n.d. n.d. n.d. n.d. n.d.

7.85 4.97 0.68 3.66 n.d. n.d. n.d. n.d. n.d. n.d.

3.85 ± 0.03 0.46 ± 0.01 0.39 ± 0.01 0.11 ± 0.01 0.67 ± 0.01 0.38 ± 0.01 0.79 ± 0.01 0.10 ± 0.01 0.72 ± 0.03 9.83 ± 0.11

0.652 1.06 0.574 108 49.3 93.6 349 1.35 0.134 0.663

n.d. 152 173 154 141 180 217 168 n.d. 170

n.d. 44.2 49.4 32.4 81.7 44.3 47.1 61.8 n.d. 38.1

n.d. 3.4 3.5 4.8 1.7 4.1 4.6 2.7 n.d. 4.5

7.7 2.8 2.8 3.4 2.9⁎ 3.3⁎ 4.6⁎ 21.9⁎

n.d. 12.5 14.2 23.5 16.6 11.6 17.9 n.d. n.d. 28.6

n.d. 0.48 0.42 0.11 0.68 0.38 0.79 0.10 n.d. 10.33

4.1 1.5⁎

a

Weight of crystals loaded into crushing device and pulverized to grain size b100 μm. Ol = olivine, Cpx = clinopyroxene, Gt = garnet. R/RA notation where R = sample 3He/4He ratio and RA = atmosphere 3He/4He ratio (= 1.4 × 10−6). X = (4He/20Ne)M/(4He/20Ne)air, RM is the measured 4He/20Ne ratio. d RC/RA is the air-corrected He isotope ratio = [(R/RA × X) − 1] / (X − 1). Uncertainties quoted at the 1σ level. e Helium abundance data are corrected for air contamination where [He]C = ([He]M × (X − 1)) / X. Uncertainty ±5%. f Radiogenic ingrowth estimates are calculated using Eqs. (1, 2, 3) and measured U, Th contents and TRD ages. Methods after Craig and Lupton (1976). g Corrected 3He/4He values, assuming 1% migration of matrix-bound total radiogenic helium into fluid inclusions. ⁎ Lithium data are from the crushed mineral separates following helium analysis. All other (unmarked) lithium data are from whole-rock aliquots. ⁎⁎ Based on CaO vs. Cr2O3 classification scheme of garnets. b c

Uranium contents for a subset of the Obnazhennaya peridotite mineral separates range from 217–549 ppb (n = 4), whereas U contents in mineral separates from Udachnaya peridotites (n = 8) are markedly lower, ranging from 32–82 ppb (Table 2). Similarly, Th contents of Obnazhennaya peridotite mineral separates range from 815–2450 ppb versus Udachnaya Th contents, which span from 141–217 ppb.

Lithium contents overlap for Obnazhennaya (1.0–4.3 ppm; mean [Li] = 2.1 ppm; 1σ = 1.1 ppm; n = 10) and Udachnaya (1.5–21.9 ppm; mean [Li] = 5.5 ppm, 1σ = 5.9 ppm; n = 10) peridotites (i.e., bulk rock and mineral separate; Table 2). In general, bulk rocks and mineral fractions from Obnazhennaya lherzolites show less variation than those from Udachnaya and their Li contents are not significantly elevated over typical mafic lithologies (i.e., 1–2 ppm; Magna et al., 2006; Jeffcoate et al., 2007). In contrast, Li contents in clinopyroxene separates from Udachnaya lherzolites are clearly more disturbed with two particularly Li-enriched lherzolites (Table 2). These elevated Li abundances in some clinopyroxenes may imply pervasive metasomatism by basaltic melts, although bulk Li enrichment may also be associated with unusually high Li contents in garnet (cf. Seitz and Woodland, 2000).

4. Discussion 4.1. Effects of diffusion

Fig. 2. Helium isotopes (3He/4He) as a function of He content (μcm3STP/g). Corrected values are shown as small closed black circles with tie-lines to measured values. The correction uses Graham et al. (1987) to calculate ingrown radiogenic [He] and assumes that a maximum of 1% of matrix-bound [He] has migrated into fluid inclusions and is sampled by crushing. The DMM range is after Graham (2002). Arrows indicate addition of “basaltic” metasomatic fluids and “extraneous” [He], which is attributed to “kimberlitic” metasomatic fluids. Symbol size encompasses the 1-sigma uncertainty associated with these measurements. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

The modifying effects of diffusion are potentially significant, particularly at mantle temperatures and pressures. Here, we discuss possible mechanisms by which He variations may be introduced. We surmise that large variations in [He] may result from the combined influence of slab-derived He additions and diffusion at depth. Helium diffusion can potentially modify intrinsic He signatures at mantle depths, as diffusion coefficients are highly temperature dependent and vary between mineral phases. Gramlich and Naughton (1972) estimated He diffusion coefficients (cm2/s) for olivine and pyroxene to be ~ 10−12 and 10−10 at 1100 °C, whereas Trull and Kurz (1993) performed experiments at higher temperatures (1350 °C) and determined diffusion coefficients of ~ 10− 9 and 10− 8 for olivine and pyroxene, respectively. These latter authors also speculated that actual diffusion rates could be as high as ~ 10 − 4 cm 2 /s depending


on the mechanism by which He is transferred between CO2 rich FIs and the mineral matrix. For example, the range in He diffusive coefficient of 10−10–10−4 cm2/s, at approximately 1300 °C for clinopyroxene, suggests diffusive lengths of 56–5620 cm over 1 Myr timescales, which is sufficient for He to be isotopically equilibrated between FI and mineral lattice within mantle domains. We suggest, therefore, that measurements of He in FIs reflect the bulk He characteristics of the mantle domain sampled by the xenoliths, and thus is representative of the SCLM at the depth of origin of the xenoliths. However, large-scale diffusion may be problematic for other highly diffusive elements, e.g., Li, which typically develops significant kinetic profiles during transient events (e.g., Richter et al., 2003). 4.2. 3He/4He ratios In this section, we consider the origin of peridotite 3He/4He characteristics and assess if crushing-induced release of He provides 3He/4He ratios representative of the mantle source region. A number of posteruptive modification processes, including the effects of 3He addition due to cosmogenic spallation reactions and thermal neutron production of 3He (Ballentine and Burnard, 2002; Kurz, 1986), are considered. Cosmogenic 3He addition due to spallation reactions occurs in the upper few meters of the Earth's surface (e.g., Lal, 1987). Therefore, these reactions are considered unlikely to produce any 3He in peridotite xenoliths, as xenoliths have remained isolated hundreds of meters below the surface until their recent excavation (over the past decade). Thermal neutrons are produced from spontaneous fission of U and Th in rocks, potentially generating nucleogenic 3He through the reaction: 6 Li(n, α) → 3H(β) → 3He (e.g., Dunai et al., 2007). Notably, nucleogenic production is typically less significant than spallation-induced reactions in moderately Li-rich rocks (Lal, 1987). By adopting a nucleogenic 3He production rate of 6.13 × 10−6 atoms g−1 yr−1 ppm−1 Li per 1 n cm−2 yr−1 (after Lal, 1987) and neutron production rate of approximately 2 n g−1 ppm−1 U yr−1 per g of rock, and 0.7 n g−1 ppm−1 Th yr−1 per g of rock (Dunai et al., 2007), we calculate that a maximum of ~3 × 10−3 atoms 3He g−1 yr−1 are produced in the most U–Th–Li rich samples (i.e., O-1107) from the Siberia suite. Therefore, the maximum post-eruptive nucleogenic 3He production (i.e., since ~ 160 Myr) is ~ 5 × 105 atoms 3He g−1 rock, which represents a negligible fraction of the measured 3He contents. In summary, we suggest that post-eruptive cosmogenic and/or nucleogenic addition of 3He is unlikely to be responsible for any of the observed variations in the 3He/4He characteristics of Siberian peridotite mineral separates. 4.3. 4He contents In this section, we explore a post-eruptive radiogenic-ingrowth model in which we assume that 3He/4He in FIs and sample matrices did not begin to evolve separately until they were emplaced at the surface by kimberlite volcanism (i.e., following increases in U–Th due to kimberlite metasomatism during emplacement). The effects of radiogenic 4He production due to the decay of U and Th can be significant, as radioactive decay of U and Th within the matrix structure of a mineral produces 4 He, which may subsequently recoil or diffuse into FIs. The amount of He that is transferred into FIs by the process of recoil is dependent on FI density (Ballentine and Burnard, 2002) and the composition of the surrounding material, which is assumed to be homogeneous in all samples. Systematically lower (i.e., more radiogenic) He-isotope values are found in Udachnaya peridotite mineral separates (clinopyroxene and garnet) versus predominantly mantle-like ratios in Obnazhennaya peridotite mineral separates (olivine, garnet, and clinopyroxene) (Fig. 2). To ensure that primary He, captured at the time of crystallization, is being sampled and is not modified by subsequent (post-eruptive) radiogenic 4 He ingrowth within the mineral matrix (and recoil into the FIs), we measured matrix-bound U and Th concentrations in the same mineralseparate aliquots following He extraction by crushing (Table 2). Using

77

these measured U and Th data, the total time-integrated 4He production rate (J(He); in μcm3STP g−1 yr−1) can be calculated using the following equation (after Graham et al., 1987): −8

JðHeÞ ¼ 2:8 10

ð4:35 þ Th=UÞ½U t

ð1Þ

where t = kimberlite eruption age (Myr). As highlighted above, He extraction by crushing preferentially releases gases trapped in FIs (i.e., intrinsic mantle He); however, it has been demonstrated (Hilton et al., 1993; Matsumoto et al., 2002; Scarsi, 2000) that, in some cases, secondary matrix-sited He — ingrown from U and Th decay — may also be released, and thus modify the FI results. By comparing He crushing and melting results, Matsumoto et al. (2002) showed that two orders of magnitude more [He] is released by melting compared with crushing, resulting in an upper estimate of 1% radiogenic [He] transfer from the matrix to FI. We adopt this value in the following discussion. In order to test if any matrix-sited 4He ingrowth has affected the FI data, the calculated post-eruptive 4He (ingrown in the matrix) is reported for individual samples (Table 2). More radiogenic He is produced in Obnazhennaya versus Udachnaya samples due to an order of magnitude higher U and Th contents, which is the direct result of plume overprinting (Howarth et al., 2014). If a maximum of 1% of matrix-bound [He] is assumed to be sampled by crushing, a correction that estimates the corrected 4He (i.e., the initial [He]) can be applied to all measured values: 4

4 4 Hecorrected ¼ Hemeasured − Hetotal−radiogenic ð0:01Þ

ð2Þ

where 0.01 represents release of 1% of the matrix radiogenic He. An analogous correction can be applied to calculate the initial (corrected) 3 He/4He: 3

4

3

4

He= Hecorrected ¼ Hemeasured = Hecorrected :

ð3Þ

Measured and corrected He-isotope values and [He] contents are provided in Table 2, and He-isotopes are plotted as a function of [He] content in Fig. 2. Radiogenic-corrected helium values (Obnazhennaya; n = 4) are plotted as smaller, closed-black circles with tie lines to measured values. The correction factor is minor — due to the fact that measured FI [He] is significantly higher than postulated radiogenic additions — apart for O-1104, which corrects from 4.1 RA to an initial value of 7.9 RA. Helium migration of 1% is assumed for all calculations, however, if less migration is assumed, then corrections are proportionally smaller. Sample O-1107 represents an obvious (radiogenic) outlier amongst the Obnazhennaya samples. If this sample acquired a larger (i.e., ~ 3%) radiogenic He component from the matrix, its anomalously low 3He/4He value would correct to ~ 4 RA. The amount of radiogenic He transfer between the matrix and FI may vary as a function of age; i.e., older samples may release more He from the matrix than relatively young samples due to radiation damage, for example. However, all samples from Obnazhennaya are ~ 160 Myr. The fact that the majority of Obnazhennaya peridotites retain mantle-like 3He/4He values in their FIs suggests that they may have been metasomatically re-fertilized by SFB fluids (~250 Ma). Alternatively, volatiles in FIs may have been acquired from the host kimberlite magma during entrapment and ascent to the surface. Due to the large number of kimberlite pipes throughout the region, kimberlitic magma with SCLM-like He compositions could potentially metasomatize peridotite material. In contrast, the Udachnaya peridotites have predominantly radiogenic He, but can be further subdivided into two groupings based on their He contents: those with b 50 μcm3 STP/g [He] and those with N 50 μcm3STP/g [He] (Fig. 2). The former group, of which U–Th contents were determined (n = 3 samples), can be explained in much the same way as for the Obnazhennaya samples (i.e., sampling of FIderived He-isotopes at the time of crystallization, with minor (~ 1%) amounts of radiogenic addition). This demonstrates that the initial

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He-characteristics of these rocks must have been dominated by (metasomatic) radiogenic contributions at the time of FI closure. While it is difficult to determine the exact mechanism by which metasomatic fluids are acquired, we surmise that any extraneous fluids may be trapped in annealed fractures that formed during crystallization and/or emplacement. Outlying olivine sample U-4-76 corrects from 9.8 to 10.3 RA, (1σ uncertainty = 0.1) consistent with the observation that olivine separates are least influenced by secondary metasomatic processes: importantly, however, U-4-76 retains unique CaO–Cr2O3 features (Fig. 3b) indicating that this is the most-pristine, depleted sample within this suite. This is notable, as it indicates that the Siberian depleted lithospheric mantle has significantly higher He-isotopes than the estimated SCLM value of 6.1 ± 0.9 (Gautheron and Moreira, 2002), and that the remaining samples were likely overprinted by radiogenic fluids prior to eruption. The latter group with N50 μcm3STP/g [He] is considered in the following section. 4.4. Source of extraneous radiogenic helium Udachnaya xenolith mineral separates with high [He] contents (i.e., N50 μcm3 STP/g) present a conundrum in that irrespective of the percentage transfer of radiogenic He release by crushing, and hence contamination of FI He, there is insufficient He available to account for measured He contents (even at 100% release). Therefore, Udachnaya peridotites have almost certainly acquired a large radiogenic He component at the time of crystallization. A possible source for this extraneous He is from recycled slab-derived fluids associated with proto-kimberlitic metasomatism (Howarth et al., 2014). P–T estimates for Udachnaya peridotites indicate that they formed at ~ 200 km depth and ~ 1300 °C, suggesting that such a metasomatic

Fig. 3. a) Helium isotopes (3He/4He) versus Cr2O3 (wt.%)Garnet. Helium isotope analyses were performed on Cpx (triangle symbols), Gt (square symbols) and Ol (circle symbols). Red symbols denote Obnazhennaya peridotite xenoliths and blue symbols denote Udachnaya peridotite xenoliths. Anomalous (harzburgitic) sample U-4-76 is shown in maroon. Symbol size encompasses the 1-sigma uncertainty associated with these measurements. b) CaO (wt.%) as a function of Cr2O3 (wt.%)Garnet in peridotite xenoliths of Udachnaya and Obnazhennaya peridotite xenoliths. Arrows indicate typical zoning trends. Harzburgite (Hz), lherzolite (Lz), and wherlite (Wh) fields are after Sobolev (1977). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

signature could be related to deep (ancient) subduction. However, if this is the case, then He must be transferred from slab-derived fluid or melt, such that it can be involved in the ‘metasomatic input’. Notably, any slab contribution of He to Udachnaya must have occurred prior to the 250 Myr plume. The helium transfer mechanism would likely occur at pressures and temperatures where diffusional rates are sufficient to result in He heterogeneity between peridotite sections. Notably, slab-derived fluids have not previously been observed in the Siberian craton; indeed, light noble gases supplied from subducted crust and sediments have rarely been observed in modern subduction environments (e.g., Hilton et al., 2002). 4.5. Mantle metasomatism Several varieties of metasomatic fluids have been proposed to have modified the Siberian SCLM (e.g., Agashev et al., 2013; Doucet et al., 2013; Howarth et al., 2014; Ziberna et al., 2013). For example, protokimberlitic metasomatism is a ubiquitous process that can be traced by xenolith alteration and reflects the circulation of fluids that originate within the SCLM (e.g., Menzies et al., 1987; Pokhilenko et al., 1999). In contrast, basaltic metasomatic fluids of the Siberia craton have been shown to re-fertilize previously depleted mantle sources (e.g., Doucet et al., 2013; Howarth et al., 2014). However, identifying the effects of metasomatism using only mineral major- and trace-element geochemistry can often be complex (e.g., Fig. 3b), due to the superimposition of multiple events with contrasting fluid compositions (i.e., proto-kimberlitic versus carbonatitic versus basaltic fluids). The effect of re-fertilization is readily detectable using garnet Cr2O3 variations in peridotites (e.g., Howarth et al., 2014), as pristine depleted harzburgitic garnets typically display Cr2O3 values N5 wt.% and show correlations with major elements, which are interpreted to reflect melt depletion followed by refertilization events (Elthon, 1992). Silicate melt-induced metasomatism will act to lower garnet Cr2O3 values. For example, pre-plume Udachnaya xenoliths generally display high Cr2O3 (N5 wt.%) with respect to DMM (Fig. 3a), consistent with melt extraction events. Significantly, the highest 3He/4He (~9.8 RA) sample (U-4-76) is indistinguishable from other Udachnaya samples with regard to Cr2O3; however, it preserves harzburgitic CaO–Cr2O3 compositions in garnet cores (Fig. 3b), suggesting that it has retained an intrinsic SCLM 3 He/4He signature. In contrast, post-plume Obnazhennaya peridotites display lower garnet Cr 2O 3 (b 5 wt.%), extending towards typical DMM values (~ 0.5 wt.%; Workman and Hart, 2005). These low (b5 wt.%) Cr2O3 values are coincident with higher, DMM-like, helium isotope ratios (Fig. 3a), suggesting that the same (i.e., basaltic) metasomatic process that re-fertilized Cr2O3 has also modified the helium isotope systematics. Further evidence that helium isotopes are modified by metasomatic events is provided by the (Sm/Er)N value of garnet, which is used to monitor the extent of metasomatism. In general, while metasomatized samples are marked by generally flat MREE–HREE profiles and (Sm/Er)N b 1, unmodified samples retain their original sinusoidal REE patterns with (Sm/Er)N N1 (Howarth et al., 2014). In Fig. 4, we plot He-isotopes versus (Sm/Er)N, and note that post-SFB Obnazhennaya xenoliths have low (Sm/Er)N values, mostly less than unity, whereas the pre-SFB Udachnaya samples have higher (Sm/Er)N ratios, mostly greater than unity. These observations suggest that basaltic metasomatism, which acts to flatten REE patterns (i.e., lower (Sm/Er)N), may also have acted to raise He-isotope ratios by re-fertilizing FIs with DMM-like helium. Furthermore, garnets from Udachnaya xenoliths display zoned trace-element patterns, with ‘harzburgitic’ sinusoidal-REE profiles within their cores and flat ‘lherzolitic’ profiles at the rims (Howarth et al., 2014). In contrast, Obnazhennaya xenolith garnets only display flat MREE–HREE profiles with LREE depletions, interpreted to reflect extensive metasomatic overprints, which have completely removed the original sinusoidal core (Griffin et al., 1999b; Ionov et al., 2010).


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Acknowledgments We acknowledge the National Science Foundation (NSF), Division of Earth Sciences for providing funding for noble gas facilities at SIO (DRH; various awards) as well as EAR-1144559, EAR-1116089 and EAR-1144337 to PHB, JMDD and LAT, respectively. TM acknowledges the funding from the Czech Science Foundation, project P210/12/ 1990. RFBR Grants 13-05-00907 and 12-05-01043 to NPP and LNP are also acknowledged, respectively. In addition we would like to thank the Planetary Geosciences Institute at UTK for financial assistance. David Graham and an anonymous reviewer are thanked for thoughtful and thorough reviews, and Andrew Kerr for editorial handling. Hirochika Sumino and Manuel Moreira are also acknowledged for their constructive reviews on an earlier version of this manuscript. Fig. 4. Helium isotopes (3He/4He) vs. Sm/ErN (garnet). Garnet Sm/Er ratios represent a numerical means to determine whether REE patterns are flat (less than unity) or sinusoidal (greater than unity). Symbol size encompasses the 1-sigma uncertainty associated with these measurements.

These observations are consistent with the source of metasomatic fluids being basaltic melts associated with the emplacement of the SFB. Given extensive evidence, including He-isotope evidence (as high as 13 RA; Basu et al., 1995), that the SFB was caused by the impingement of a mantle plume at the base of the lithosphere (e.g., Horan et al., 1995; Sharma et al., 1992), we suggest that (Sm/Er)N values less than unity and high (DMM-like) He-isotopes are concurrently modified in ‘postplume’ xenoliths due to the re-fertilizing effect of plume-induced, basaltic metasomatism. Importantly, ‘pre-plume’ Udachnaya xenoliths retain mostly radiogenic He-isotope signatures and typically have sinusoidal REEs ([Sm/Er]N greater than unity) within garnet cores, which has important implications for the temporal and spatial susceptibility of the SCLM to mantle re-fertilization. This suggests that any slab contribution of He to Udachnaya must have occurred before impingement of the 250 Myr plume.

5. Conclusions The xenoliths from Late-Devonian and Jurassic kimberlite pipes within the Siberian craton transport mantle material to the surface, providing a unique window into the SCLM through time. By investigating He-isotopes in peridotite xenoliths that pre- and post-date the eruption of the SFB, we are able to better constrain the temporal metasomatic effects of lithosphere/plume interaction that occurred at the PermoTriassic boundary (250 Myr). The He-isotope values are generally high and ‘mantle-like’ for post-SFB Obnazhennaya (0.54 to 8.39 RA) samples, but are mainly radiogenic for those from pre-SFB Udachnaya, highlighting the sharp disparity between pre-plume (Udachnaya) and postplume (Obnazhennaya) peridotites. By combining U–Th and He data, we are able to estimate possible radiogenic [He] additions to individual samples and apply a correction to reconstruct 3He/4He values at the time of the xenolith emplacement. When He-isotopes are considered along with other metasomatic tracers, the re-fertilizing effects of plume-related basaltic metasomatic overprinting — associated with the emplacement of the SFB — becomes apparent. In contrast, extremely high 4He contents in some (n = 4) Udachnaya samples cannot be explained by radiogenic ingrowth alone, even if all matrix-derived He migrated into the FIs. Instead, they require an extraneous crustal contribution, likely associated with slab-derived proto-kimberlitic metasomatic fluids. Importantly, this metasomatic agent is variably enriched in [He], and we speculate that the variability may represent enriched fluids from ancient subducted slabs to kimberlitic fluids. These results demonstrate a unique application for He-isotopes and provide convincing evidence that the SCLM can be significantly modified by plume-induced basaltic and/or slab-derived, proto-kimberlitic metasomatic fluids.

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