Boron isotope compositions of coexisting kornerupine and tourmaline in high-grade metabasic rocks: an example from Akarui Point, Lützow-Holm Complex, East Antarctica

Abstract

Boron isotope compositions were measured in kornerupine and tourmaline from lenses consisting primarily of kornerupine, plagioclase and corundum. The lenses occur within hornblende-gneiss or along the boundary between this gneiss and an amphibolite lens at Akarui Point in the Lützow-Holm Complex, Prince Olav Coast, East Antarctica. The peak metamorphic conditions have been estimated to be ∼ 800-900 °C and ∼ 8-11 kbar. The δ¹¹B compositions of kornerupine, which is interpreted to have been a stable phase at the metamorphic peak, are −11.6 ± 0.4 to −7.8 ± 0.5‰ and −9.8 ± 0.3 to −6.1 ± 0.2‰ in two different samples. Grains of prograde tourmaline included in kornerupine and corundum yielded δ¹¹B = −2.1 ± 0.3 to +0.6 ± 0.3‰, and the secondary tourmaline replacing kornerupine yielded δ¹¹B = −4.6 ± 0.2 to −3.7 ± 0.2‰. Therefore, the isotopic fractionation between kornerupine and tourmaline, Δ¹¹B_Tur-Krn (= δ¹¹B_Tur − δ¹¹B_Krn), of the average prograde tourmaline and average host kornerupine is +6.7 ± 1.5‰, which is interpreted to indicate isotopic equilibrium at the metamorphic peak on the basis of previous studies of isotope fractionation between tourmaline and minerals of the kornerupine-prismatine series. The δ¹¹B values obtained on prograde tourmaline are between whole rock δ¹¹B of MORB and mantle rocks and of some sedimentary rocks, and are similar to the δ¹¹B of blackwall tourmalines that crystallized during the decompression stage following high-pressure metamorphism. We infer that the syn-metamorphic B-bearing fluid present in the kornerupine-plagioclase-corundum lens is likely sourced from a mixture of sedimentary, mafic and ultramafic lithologies in a subduction setting. The metabasic and meta-ultramafic lenses found in Akarui Point could be interpreted as the remnant of mixing zone of Ediacaran to Cambrian subduction channel.

INTRODUCTION

Boron is an incompatible element present in trace amount in oceanic sediments (<180 ppm), mafic igneous rocks (<35 ppm) and altered oceanic crust (<280 ppm) (Leeman and Sisson, 1996). Consequently, unusual concentrations of B, such as tourmaline aggregates in metamorphic rocks, are considered evidence for B-bearing fluid infiltration (e.g., Kawakami, 2001; Kawakami et al., 2008; Marschall et al., 2009; Kawakami et al., 2019). Tourmaline becomes unstable in granulite facies conditions depending on bulk composition of the host rock (Kawakami, 2001; Dutrow and Henry, 2011), and instead, kornerupine becomes stable in high-pressure (high-P) conditions above ∼ 0.5 GPa (Robbins and Yoder, 1962; Krosse, 1995; Grew, 1996; Werding and Schreyer, 1996). Kornerupine sensu stricto and its B-dominant analogue prismatine, both (□, Mg, Fe)(Al, Mg, Fe)₉(Si, Al, B)₅O₂₁(OH, F), have been reported from at least 84 localities worldwide as of 2017 in upper amphibolite- and granulite-facies rocks (Grew, 1996; Grew et al., 2017). Accumulation of kornerupine can, therefore, be a tracer of B-bearing fluids under high-P conditions as well.

Boron isotope composition of borosilicates is potentially a powerful tool to constrain the nature of infiltrated fluid even in high-temperature (high-T) metamorphic rocks. To date, only one such B-isotope study has been carried out on kornerupine-prismatine-bearing borosilicate assemblages (MacGregor et al., 2013), and more such studies are needed to better understand the systematics of B isotope behavior in the lower crust.

Metamorphic tourmaline and tourmaline accumulations (blackwalls) in subduction zone settings have been extensively studied recently to understand the fluid-rock interaction in the mixing zone of subduction zones (Nakano and Nakamura, 2001; Bebout and Nakamura, 2003; Altherr et al., 2004; Marschall et al., 2006, 2008, 2009; Marocchi et al., 2011; Van Hinsberg et al., 2011). Fluids released from subducting sedimentary rocks and altered oceanic crust are understood to be the important source of B in these cases. In contrast, tourmaline accumulations associated with mafic and ultramafic rocks in collision settings could be interpreted as metamorphosed blackwalls or the result of fluid rock interaction during collision metamorphism. In this study, we describe an unusual kornerupine-plagioclase-corundum (Krn-Pl-Crn) lens associated with hornblende-gneiss and amphibolite at Akarui Point, Lützow-Holm Complex (LHC), Prince Olav Coast, East Antarctica in order to better understand the behavior of B in a zone of mixing in a collision setting. Mineral abbreviations follow Warr (2021).

GEOLOGICAL SETTING

The LHC is a Cambrian orogenic belt bounded by the Late Proterozoic to Cambrian Rayner Complex to the east and by the Late Proterozoic to Early Palaeozoic Yamato-Belgica Complex to the west (Shiraishi et al., 1994; Satish-Kumar et al., 2008). Based on mapping using index minerals, the metamorphic grade of the complex has been shown to increase progressively from the upper amphibolite facies on the Prince Olav Coast through a transitional zone to the granulite facies in Lützow-Holm Bay (Hiroi et al., 1991) (Fig. 1). A thermal axis of maximum peak metamorphic temperature is estimated to lie in southern Lützow-Holm Bay near Rundvågshetta (Motoyoshi and Ishikawa, 1997). Ultrahigh-temperature (UHT) metamorphism of about 1000 °C, 1.1 GPa and subsequent isothermal decompression are reported from Rundvågshetta (Ishikawa et al., 1994; Yoshimura et al., 2008). The complex experienced a typical clockwise P-T path (Hiroi et al., 1983, 1991; Kawakami and Motoyoshi, 2004), and recent studies support isothermal decompression starting from kyanite stability field (Iwamura et al., 2013; Kawakami et al., 2016). The P-T estimates by Zr-in-rutile thermometry applied to inclusion rutile grains in garnet reveal that similar P-T conditions of ∼ 830-850 °C at ∼ 1.1 GPa are widely attained from Akarui Point to Skallen (Suzuki and Kawakami, 2019). This suggests that the transitional zone defined by the metamorphic field mapping using matrix mineral assemblage of mafic to intermediate gneisses is equivalent to the granulite facies zone.

Figure 1. Geological map of the Akarui Point showing the sampling locality (modified after Yanai et al., 1984). Inset shows the location of Akarui Point in the Prince Olav Coast, and a red arrow indicates the location of the Lützow-Holm Complex in Antarctica.

In the amphibolite facies zone at Cape Hinode. ∼ 960 Ma exotic block is found and termed as ‘Hinode Block’ (Shiraishi et al., 1994; Dunkley et al., 2020). Recently, high-grade metamorphism at ∼ 990 to ∼ 930 Ma is detected also from Niban Rock and Akebono Rock (Kitano et al., 2021; Baba et al., 2022), while Late Proterozoic to Cambrian metamorphic age (∼ 600-520 Ma), which is widely recorded in metamorphic rocks of the surrounding LHC including Akarui Point, is not detected by U-Pb zircon dating (Kitano et al., 2021). Mori et al. (2023) reports traces of Cambrian metamorphism from Niban-nishi rock of Niban Rock using electron microprobe U-Th-Pb monazite dating. In spite of above-mentioned recent progress, the geological boundaries of the Hinode Block and its relationship to the surrounding LHC remain unresolved (Baba et al., 2022).

Akarui Point is one of the nunataks in the ‘transitional zone’ on the Prince Olav Coast (Fig. 1, Hiroi et al., 1991). Dominant rock types are Bt-Hbl, Hbl-Bt, and Grt-Bt gneisses, with subordinate ultramafic rocks and pyroxene gneiss as lenses mainly enclosed in the Bt-Hbl gneiss. The Bt-Hbl and Hbl-Bt gneisses are locally migmatitic and grade into one another. Relict kyanite occurs as inclusions in garnet and plagioclase in the Grt-Bt gneiss (Hiroi et al., 1983). Orthopyroxene is one of the main constituent minerals in the ultramafic granulite and pyroxene gneiss (Yanai et al., 1984). Granites and pegmatitic veins with pinkish K-feldspar intrude discordantly to the foliation of metamorphic rocks (Yanai et al., 1984). A recent study reports peak P-T conditions of 834 ± 4 °C and ∼ 1.1 GPa constrained by using the minerals included in garnet from the pelitic gneiss from Akarui Point (Suzuki and Kawakami, 2019). Iwamura et al. (2013) obtained slightly higher P-T estimate of ∼ 900 °C and 1.1-1.2 GPa from a mafic granulite. Reintegration of lamellae in alkali-feldspar from the matrix of a sillimanite-biotite-garnet gneiss also gives temperature estimate of 825-900 °C (Nakamura et al., 2014). This peak metamorphic P-T condition is followed by decompression into the andalusite stability field (Kawakami et al., 2008).

ANALYTICAL SETTINGS

Quantitative analyses of kornerupine and tourmaline were performed by a JEOL JXA-8105 superprobe at Department of Geology and Mineralogy, Kyoto University, Japan. Analytical conditions for quantitative analyses were 15.0 kV acceleration voltage, 10 nA beam current, and 3 µm beam diameter. The counting time for the peak and backgrounds were 30 and 15 s for Cl, 60 and 30 s for F, and 10 and 5 s for other elements. Natural and synthetic minerals were used as standards and the ZAF correction was applied.

Boron isotopic compositions and B concentration of kornerupine and tourmaline were determined in situ by secondary ionization mass spectroscopy (SIMS) on the Cameca ims1270 instrument at the Edinburgh Ion Microprobe Facility following procedures similar to those of Grew et al. (2015). Operating conditions for tourmalines were 5.4 nA primary beam current of ¹⁶O⁻ ions at a primary voltage 17.5 kV, producing an analytical spot size of ∼ 10 × 15 µm. Following pre-sputter of 60 seconds, ¹¹B and ¹⁰B signals were recorded in 20 cycles of 2 and 8 seconds, respectively, using a single electron multiplier detector with a dead time of 51 seconds. Typical count rates were 6.2 × 10⁵ and 1.7 × 10⁵ cps/nA/wt% B₂O₃, respectively. A mass resolution of 3100 (m/Δm at 5% peak width) was used to avoid interference of ¹⁰BH, ⁹BeH, and ⁹BeH₂ molecular species. No energy filtering was applied. Internal precision of the analyses was generally <0.3‰ (1σ). Drift in the ¹¹B/¹⁰B ratio was minor, and the average for all drift-corrected measurements with the dravite standard gave 1σ uncertainty of 0.5‰. Standards used for tourmaline were elbaite 98144 (δ¹¹B_SRM951 = −10.5‰), schorl 112566 (δ¹¹B_SRM951 = −12.5‰) and dravite 108796 (δ¹¹B_SRM951 = −6.6‰), previously analyzed using thermal ionization mass spectrometry (TIMS) by Leeman and Tonarini (2001). Standards used for kornerupine/prismatine were prismatine 112233 (δ¹¹B_SRM951 = −10.8‰; Leeman and Tonarini, 2001) and two Larsemann Hills prismatines (Seal Cove: δ¹¹B_SRM951 = −15.8‰; and Stornes: δ¹¹B_SRM951 = −13.7‰) analyzed by TIMS (Samuele Agostini, personal communication) and reported in MacGregor et al. (2013).

During the analytical sequence, analyses of unknowns were bracketed with standard analyses to detect offset and drift. The analytical results were corrected for offsets using recommended values of standards and compositional correction was additionally made for tourmaline. Matrix-dependent mass fractionation in tourmaline was moderate: elbaite 98114 was offset by +1.9 ± 1.8‰ (1σ, n = 4) from the TIMS value (Leeman and Tonarini, 2001); schorl 112566 by +1.8 ± 0.7‰ (1σ, n = 3) and dravite 108796 by −0.1 ± 0.6‰ (1σ, n = 6). On this basis the analytical reproducibility is estimated to be better than 0.6‰ for analyses referenced principally to the dravite standard. The compositional correction made for the dravite-rich tourmalines analyzed in this study was only +0.1‰. Prismatines were referenced against the Seal Cove standard bracketing sets of 4-6 sample analyses, with instrumental mass fractionation (offset) correction applied based on the Seal Cove prismatine bracketing results.

Following the usual conventions, B isotope compositions are expressed as per mil deviation from the standard SRM951 boric acid in delta notation: δ¹¹B {= [(¹¹B/¹⁰B)_sample/(¹¹B/¹⁰B)_NIST951 − 1] * 1000}, using a value of ¹¹B/¹⁰B = 4.0437 for SRM951 (Catanzaro et al., 1970). Distribution of B isotopes between two minerals A and B is expressed as Δ¹¹B_A-B (= δ¹¹B_A − δ¹¹B_B).

SAMPLE DESCRIPTION

The Krn-Pl-Crn lenses in the Akarui Point are developed in a Hbl-Bt gneiss and also found between an amphibolite lens and the Hbl-Bt gneiss. Sample description is briefly summarized below based on Kawakami et al. (2008). Samples analyzed in this study are two Krn-Pl-Crn-bearing samples (AKR2002 and TK2002122104; Fig. 2) collected from an irregularly-shaped lens, roughly 50-60 cm across and surrounded by hornblende gneiss. Constituent minerals of the lens are coarser-grained than those in the surrounding hornblende gneiss. Kornerupine (X_Mg = 0.79-0.82) forms euhedral prisms up to 4 cm in diameter and 10 cm in length. In addition to kornerupine, the lens consists of euhedral corundum (Cr₂O₃ = 0.22-0.34 wt%) forming tabular crystals up to 3 cm in diameter, biotite (X_Mg = 0.79-0.82) as flakes 1-5 mm across, and plagioclase (An66-84). The kornerupine and coexisting corundum enclose euhedral to xenomorphic grains of greenish tourmaline (X_Mg = 0.82-0.83) in TK2002122104. These tourmaline inclusions are interpreted to have formed prior to or simultaneously with kornerupine and corundum and thus are considered to be prograde. Kornerupine is partly replaced along grain rims and cracks to secondary tourmaline (X_Mg = 0.79-0.85), corundum, andalusite and magnesite, a feature previously attributed to reaction with a retrograde CO₂-H₂O fluid (Kawakami et al., 2008). The microstructure of secondary tourmaline is fibrous or matted.

Figure 2. Photomicrograph of thin sections (plane polarized light) of samples (a) AKR2002B, (b) AKR2002A, and (c) TK2002122401 with analysis points and results of the in situ B isotopic measurements. Points labelled with ‘Krn’ represent analysis points for kornerupine and those labelled with ‘Tur’ represent analysis points for tourmaline. Numbers accompanied with analysis point numbers are δ¹¹B values determined in this study.

RESULTS OF BORON ISOTOPE ANALYSES

Analysis points for the in situ B isotopic measurements are shown in Figure 2. Mineral analyses together with B isotope data are summarized in Table 1 for kornerupine and Table 2 for tourmaline. The δ¹¹B values {= [(¹¹B/¹⁰B)_sample/(¹¹B/¹⁰B)_NIST951 − 1] * 1000} of kornerupine were −11.6 ± 0.4 to −7.8 ± 0.5‰ in sample AKR2002 and −9.8 ± 0.3 to −6.1 ± 0.2‰ in sample TK2002122104 (Fig. 3). In spite of the observed variation, there is no evidence for core to rim zoning in δ¹¹B in the three kornerupine grains analyzed (Fig. 2). The prograde tourmalines found as inclusions in kornerupine in sample TK2002122104 yielded −2.1 ± 0.3 to −2.0 ± 0.3‰ in their cores and +0.6 ± 0.3‰ on their rims, and that in corundum yielded similar value of −1.8 ± 0.2‰. The δ¹¹B values of prograde tourmaline are statistically different from those of secondary tourmaline, which range from −4.6 ± 0.2 to −3.7 ± 0.2‰ (Fig. 3).

Table 1. Summary of major element and B isotope compositions of kornerupine

Sample name	AKR2002B
Spot no.	Krn1	Krn2	Krn3	Krn4	Krn5	Krn6
Comment	Core
SiO2	29.62	29.91	29.60	29.59	30.14	29.60
TiO2	0.28	0.17	0.14	0.00	0.00	0.25
Al2O3	44.19	44.55	43.55	43.65	43.75	43.77
Cr2O3	0.13	0.12	0.11	0.15	0.07	0.19
B2O3 (analyzed)	1.91	1.87	1.97	2.01	2.01	1.92
FeO	6.70	6.91	6.50	6.86	7.06	6.87
MnO	0.13	0.37	0.28	0.33	0.25	0.23
MgO	15.28	15.31	15.67	15.38	15.67	15.63
CaO	0.07	0.09	0.10	0.05	0.09	0.07
BaO	0.14	0.05	0.02	0.07	0.00	0.00
Na2O	0.05	0.02	0.06	0.05	0.06	0.07
K2O	0.01	0.00	0.00	0.00	0.01	0.00
F	0.01	0.00	0.00	0.00	0.00	0.00
Cl	0.00	0.01	0.01	0.01	0.00	0.00
-O≡F	0.00	0.00	0.00	0.00	0.00	0.00
-O≡Cl	0.00	0.00	0.00	0.00	0.00	0.00
H2O calc	1.19	1.20	1.19	1.19	1.20	1.20
Total	99.70	100.57	99.20	99.32	100.29	99.79
Number of O	21.5	21.5	21.5	21.5	21.5	21.5
Si	3.71	3.72	3.73	3.72	3.76	3.71
Ti	0.03	0.02	0.01	0.00	0.00	0.02
Al	6.53	6.53	6.46	6.48	6.43	6.47
Cr	0.01	0.01	0.01	0.01	0.01	0.02
B	0.41	0.40	0.43	0.44	0.43	0.42
Fe	0.70	0.72	0.68	0.72	0.74	0.72
Mn	0.01	0.04	0.03	0.03	0.03	0.02
Mg	2.85	2.84	2.94	2.89	2.91	2.92
Ca	0.01	0.01	0.01	0.01	0.01	0.01
Ba	0.01	0.00	0.00	0.00	0.00	0.00
Ni	0.00	0.00	0.00	0.00	0.00	0.00
Na	0.01	0.00	0.01	0.01	0.01	0.02
K	0.00	0.00	0.00	0.00	0.00	0.00
F	0.00	0.00	0.00	0.00	0.00	0.00
Cl	0.00	0.00	0.00	0.00	0.00	0.00
OH	1.00	1.00	1.00	1.00	1.00	1.00
Total cation	14.29	14.29	14.32	14.32	14.32	14.32
Mg/(Fe + Mg)	0.80	0.80	0.81	0.80	0.80	0.80
δ11BNIST951‰	−11.0	−10.2	−10.0	−9.4	−10.3	−10.1
±1σ‰	0.46	0.37	0.33	0.29	0.35	0.49

Sample name	AKR2002A
Spot no.	Krn1	Krn2	Krn3	Krn4	Krn5	Krn6	Krn7	Krn8
Comment	Rim					Rim
SiO2	29.25	29.27	29.74	29.63	29.70	29.80	29.44	29.66
TiO2	0.28	0.17	0.14	0.20	0.06	0.00	0.06	0.00
Al2O3	43.97	44.29	43.16	44.07	43.49	43.57	43.62	43.82
Cr2O3	0.15	0.10	0.18	0.15	0.10	0.14	0.03	0.15
B2O3 (analyzed)	1.41	1.47	1.57	n.d.	1.63	1.51	1.49	1.41
FeO	7.16	7.38	6.62	6.81	6.64	6.56	6.35	6.36
MnO	0.30	0.25	0.34	0.17	0.35	0.17	0.27	0.31
MgO	15.76	15.86	15.58	15.89	15.72	15.64	15.76	15.78
CaO	0.10	0.03	0.02	0.08	0.09	0.05	0.07	0.07
BaO	0.00	0.00	0.00	0.00	0.00	0.00	0.01	0.07
Na2O	0.05	0.07	0.06	0.07	0.06	0.08	0.04	0.05
K2O	0.03	0.00	0.00	0.00	0.01	0.00	0.00	0.00
F	0.07	0.00	0.00	0.01	0.00	0.00	0.03	0.04
Cl	0.01	0.01	0.00	0.01	0.00	0.01	0.01	0.00
-O≡F	0.03	0.00	0.00	0.00	0.00	0.00	0.01	0.02
-O≡Cl	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
H2O calc	1.15	1.19	1.18	1.16	1.19	1.18	1.16	1.16
Total	99.64	100.08	98.58	98.24	99.03	98.71	98.33	98.88
Number of O	21.5	21.5	21.5	21.5	21.5	21.5	21.5	21.5
Si	3.69	3.68	3.77	3.81	3.75	3.77	3.74	3.75
Ti	0.03	0.02	0.01	0.02	0.01	0.00	0.01	0.00
Al	6.54	6.56	6.46	6.67	6.48	6.50	6.53	6.54
Cr	0.01	0.01	0.02	0.02	0.01	0.01	0.00	0.02
B	0.31	0.32	0.34	n.d.	0.36	0.33	0.33	0.31
Fe	0.76	0.78	0.70	0.73	0.70	0.69	0.68	0.67
Mn	0.03	0.03	0.04	0.02	0.04	0.02	0.03	0.03
Mg	2.96	2.97	2.95	3.04	2.96	2.95	2.99	2.98
Ca	0.01	0.00	0.00	0.01	0.01	0.01	0.01	0.01
Ba	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Ni	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Na	0.01	0.02	0.02	0.02	0.01	0.02	0.01	0.01
K	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
F	0.03	0.00	0.00	0.00	0.00	0.00	0.01	0.02
Cl	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
OH	0.97	1.00	1.00	0.99	1.00	1.00	0.99	0.98
Total cation	14.36	14.37	14.31	14.34	14.33	14.31	14.32	14.32
Mg/(Fe + Mg)	0.80	0.79	0.81	0.81	0.81	0.81	0.82	0.82
δ11BNIST951‰	−11.6	−9.4	−10.2	n.d.	−9.8	−9.7	−9.3	−7.8
±1σ‰	0.37	0.37	0.53	n.d.	0.31	0.40	0.38	0.46

Sample name	TK2002122104
Spot no.	Krn1	Krn2	Krn3	Krn4	Krn5
Comment
SiO2	29.70	29.95	30.16	29.80	30.72
TiO2	0.17	0.08	0.00	0.06	0.00
Al2O3	43.54	43.01	43.45	44.27	42.44
Cr2O3	0.22	0.13	0.15	0.11	0.16
B2O3 (analyzed)	1.56	1.61	1.63	1.89	2.06
FeO	6.97	6.16	6.84	6.64	6.65
MnO	0.18	0.21	0.20	0.38	0.27
MgO	15.48	15.35	15.42	15.19	15.24
CaO	0.10	0.10	0.04	0.07	0.06
BaO	0.04	0.04	0.00	0.07	0.04
Na2O	0.01	0.07	0.03	0.01	0.05
K2O	0.00	0.00	0.01	0.00	0.00
F	0.00	0.02	0.00	0.08	0.00
Cl	0.01	0.00	0.01	0.01	0.01
-O≡F	0.00	0.01	0.00	0.03	0.00
-O≡Cl	0.00	0.00	0.00	0.00	0.00
H2O calc	1.18	1.16	1.19	1.16	1.19
Total	99.16	97.90	99.14	99.68	98.86
Number of O	21.5	21.5	21.5	21.5	21.5
Si	3.75	3.81	3.80	3.73	3.87
Ti	0.02	0.01	0.00	0.01	0.00
Al	6.48	6.46	6.46	6.54	6.30
Cr	0.02	0.01	0.02	0.01	0.02
B	0.34	0.35	0.36	0.41	0.45
Fe	0.74	0.66	0.72	0.70	0.70
Mn	0.02	0.02	0.02	0.04	0.03
Mg	2.92	2.92	2.90	2.84	2.86
Ca	0.01	0.01	0.01	0.01	0.01
Ba	0.00	0.00	0.00	0.00	0.00
Ni	0.00	0.00	0.00	0.00	0.00
Na	0.00	0.02	0.01	0.00	0.01
K	0.00	0.00	0.00	0.00	0.00
F	0.00	0.01	0.00	0.03	0.00
Cl	0.00	0.00	0.00	0.00	0.00
OH	1.00	0.99	1.00	0.97	1.00
Total cation	14.31	14.27	14.29	14.28	14.25
Mg/(Fe + Mg)	0.80	0.82	0.80	0.80	0.80
δ11BNIST951‰	−9.8	−8.1	−8.1	−7.7	−6.1
±1σ‰	0.29	0.34	0.46	0.32	0.21

n.d., not determined. All Fe as FeO.

Table 2. Summary of major element and B isotope compositions of tourmaline

Sample name	TK2002122104
Spot no.	Tur1a	Tur1b	Tur1c	Tur2b	Tur3	Tur4	Tur5
Comment	Pro	Pro	Pro Rim	Pro	Pro	Pro	Pro in Crn
SiO2	36.25	36.55	36.29	36.08	36.28	36.43	36.18
TiO2	0.59	0.20	0.62	0.51	0.53	0.79	0.48
Al2O3	32.26	32.08	32.12	32.16	32.00	31.91	32.49
Cr2O3	0.32	0.26	0.65	0.14	0.71	0.21	0.50
B2O3 analyzed	10.10	10.13	10.21	10.18	10.11	10.26	10.29
B2O3 calc.*	10.51	10.54	10.46	10.50	10.48	10.52	10.57
FeO	3.45	3.54	3.27	3.43	3.11	3.35	3.47
MnO	0.06	0.08	0.00	0.00	0.07	0.01	0.00
MgO	8.60	8.87	8.33	8.93	8.60	8.83	8.82
CaO	2.02	2.11	2.09	2.06	2.18	2.06	1.84
BaO	0.08	0.08	0.00	0.02	0.08	0.00	0.00
Na2O	1.70	1.56	1.58	1.69	1.59	1.64	1.74
K2O	0.04	0.01	0.01	0.01	0.03	0.05	0.04
F	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Cl	0.00	0.00	0.00	0.00	0.00	0.00	0.00
H2O calc.	3.62	3.63	3.61	3.62	3.61	3.63	3.65
-O≡F	0.00	0.00	0.00	0.00	0.00	0.00	0.00
-O≡Cl	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Total	99.09	99.11	98.77	98.81	98.92	99.15	99.49
Atomic proportions based on 15 cations exclusive of B, Na, K, Ca, and Ba (a.p.f.u.)
B	3.00	3.00	3.00	3.00	3.00	3.00	3.00
Si	5.99	6.03	6.03	5.97	6.02	6.02	5.95
Al(IV)	0.01	0.00	0.00	0.03	0.00	0.00	0.05
Al(Z)	6.00	6.00	6.00	6.00	6.00	6.00	6.00
Al(Y)	0.28	0.24	0.29	0.24	0.26	0.22	0.24
Cr	0.04	0.03	0.08	0.02	0.09	0.03	0.06
Ti	0.07	0.02	0.08	0.06	0.07	0.10	0.06
Fe2+	0.48	0.49	0.45	0.47	0.43	0.46	0.48
Mg	2.12	2.18	2.06	2.20	2.13	2.17	2.16
Mn	0.01	0.01	0.00	0.00	0.01	0.00	0.00
Ca	0.36	0.37	0.37	0.37	0.39	0.36	0.32
Na	0.55	0.50	0.51	0.54	0.51	0.52	0.55
K	0.01	0.00	0.00	0.00	0.01	0.01	0.01
Ba	0.01	0.01	0.00	0.00	0.01	0.00	0.00
X-site vacancy	0.08	0.12	0.12	0.09	0.09	0.10	0.11
F	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Cl	0.00	0.00	0.00	0.00	0.00	0.00	0.00
OH	4.00	4.00	4.00	4.00	4.00	4.00	4.00
Mg/(Fe2+ + Mg)	0.82	0.82	0.82	0.82	0.83	0.82	0.82
Ca/(Na + Ca + K + Ba)	0.59	0.57	0.58	0.59	0.56	0.58	0.62
δ11BNIST951	−2.0	−2.1	0.6	−1.4	−0.8	−1.7	−1.8
±‰	0.34	0.28	0.28	0.24	0.34	0.26	0.21

Sample name	TK2002122104
Spot no.	Tur6a	Tur6b	Tur7a	Tur7b	Tur8	Tur9
Comment	Retro	Retro	Retro	Retro	Retro	Retro
SiO2	37.81	38.54	38.33	38.31	37.85	37.98
TiO2	0.28	0.20	0.11	0.00	0.03	0.11
Al2O3	32.81	32.98	33.25	33.07	33.98	33.24
Cr2O3	0.02	0.05	0.18	0.02	0.08	0.16
B2O3 analyzed	9.98	9.20	9.32	9.77	9.89	10.13
B2O3 calc.*	10.66	10.60	10.71	10.69	10.71	10.61
FeO	3.79	3.51	3.22	2.70	2.94	3.35
MnO	0.03	0.03	0.00	0.00	0.00	0.00
MgO	8.10	7.36	8.05	8.53	8.06	7.64
CaO	0.36	0.34	0.22	0.08	0.16	0.25
BaO	0.00	0.07	0.00	0.02	0.07	0.00
Na2O	2.24	2.25	2.16	2.11	2.24	2.30
K2O	0.00	1.16	0.02	0.00	0.03	0.02
F	0.00	0.00	0.00	0.00	0.06	0.03
Cl	0.01	0.00	0.01	0.01	0.00	0.00
H2O calc.	3.67	3.66	3.69	3.68	3.66	3.64
-O≡F	0.00	0.00	0.00	0.00	0.02	0.01
-O≡Cl	0.00	0.00	0.00	0.00	0.00	0.00
Total	99.10	99.35	98.55	98.30	99.01	98.82
Atomic proportions based on 15 cations exclusive of B, Na, K, Ca, and Ba (a.p.f.u.)
B	3.00	3.00	3.00	3.00	3.00	3.00
Si	6.17	6.32	6.22	6.23	6.14	6.22
Al(IV)	0.00	0.00	0.00	0.00	0.00	0.00
Al(Z)	6.00	6.00	6.00	6.00	6.00	6.00
Al(Y)	0.31	0.37	0.36	0.34	0.50	0.42
Cr	0.00	0.01	0.02	0.00	0.01	0.02
Ti	0.03	0.02	0.01	0.00	0.00	0.01
Fe2+	0.52	0.48	0.44	0.37	0.40	0.46
Mg	1.97	1.80	1.95	2.07	1.95	1.87
Mn	0.00	0.00	0.00	0.00	0.00	0.00
Ca	0.06	0.06	0.04	0.01	0.03	0.04
Na	0.71	0.71	0.68	0.67	0.70	0.73
K	0.00	0.24	0.00	0.00	0.01	0.00
Ba	0.00	0.00	0.00	0.00	0.00	0.00
X-site vacancy	0.23	0.00	0.28	0.32	0.26	0.22
F	0.00	0.00	0.00	0.00	0.03	0.01
Cl	0.00	0.00	0.00	0.00	0.00	0.00
OH	4.00	4.00	4.00	4.00	3.97	3.99
Mg/(Fe2+ + Mg)	0.79	0.79	0.82	0.85	0.83	0.80
Ca/(Na + Ca + K + Ba)	0.92	0.70	0.94	0.98	0.95	0.94
δ11BNIST951	−3.7	−4.6	−3.9	−3.7	−4.5	−4.1
±‰	0.34	0.24	0.30	0.19	0.21	0.29

n.d., not determined. All Fe as FeO. *Boron calculated by stoichiometry (3 boron a.p.f.u.).

Figure 3. Ranges of boron isotope composition of the borosilicate minerals by rock type for the Larsemann Hills (MacGregor et al., 2013) and Akarui Point.

DISCUSSION

The Krn-Pl-Crn lens has a high B content as suggested by the presence of large kornerupine crystals (up to 4 cm in diameter, ∼ 1.9-2.0 wt% B₂O₃) as a major constituent mineral. Because B is highly incompatible and B concentrations in mafic and ultramafic rocks are generally low, the Krn-Pl-Crn lens present in mafic lithologies at Akarui Point is likely a product of interaction between B-bearing fluid and the host rock. Kawakami et al. (2008) proposed two possible scenarios for introducing B to form the Krn-Pl-Crn lenses: (1) Boron infiltration into the lens was syn-metamorphic facilitated through prograde dehydration of muscovite in surrounding metasediments. (2) Boron addition occurred prior to peak metamorphism through lower-temperature hydrothermal alteration of the mafic and ultramafic protoliths by seawater. These scenarios are tested below based on these new B isotope data.

The prograde tourmaline is included as rounded to irregularly-shaped grains, which led Kawakami et al. (2008) to interpret these tourmaline grains as the precursors to kornerupine. The following reaction was proposed:   

\begin{align} &\text{Tur} + \text{sodic Pl (An34)} + \text{Spr} + \text{Spl} \ \\&\quad\text{-}\text{-}{>}\ \text{Krn} + \text{Crn} + \text{calcic Pl (An82)} + \text{(fluid or melt)} \end{align} (1).

In the present study, the prograde tourmaline in sample TK2002122104 yielded an average δ¹¹B_Tur of −1.3 ± 0.9‰ (average ± 1σ), and the kornerupine enclosing the prograde tourmaline an average δ¹¹B_Krn of −8.0 ± 1.2‰. Therefore, Δ¹¹B_Tur-Krn of the average prograde tourmaline and average host kornerupine is +6.7 ± 1.5‰. This value does not appear to be influenced by post-entrapment B isotopic re-equilibration between prograde tourmaline and the host kornerupine because the prograde tourmaline included in corundum, which is protected from B isotopic re-equilibration, shows the same B isotopic value as the prograde tourmaline directly included within kornerupine. MacGregor et al. (2013) determined the B isotope distribution between prismatine and tourmaline under granulite facies conditions using a paragneiss containing tourmaline, grandidierite and prismatine from Larsemann Hills, Prydz Bay, East Antarctica (Fig. 3). In their study, prismatine systematically gave lower δ¹¹B values than coexisting tourmaline, and the measured Δ¹¹B_Tur-Prs (= δ¹¹B_Tur − δ¹¹B_Prs) = +5.0 ± 1.4‰ was considered to represent an equilibrium B isotope distribution (Fig. 4). Ab initio calculations following the method developed by Kowalski and Jahn (2011) and Kowalski et al. (2013) at 1000 K gives a B isotope fractionation factor of +6.4‰ (MacGregor et al., 2013). Based on these criteria, the kornerupine and prograde tourmaline in this study can be interpreted as isotopically in equilibrium within uncertainty (Fig. 4). Therefore, in a revision to the interpretation of Kawakami et al. (2008) that the prograde tourmaline was a precursor of kornerupine, we consider that the inclusion tourmalines within kornerupine and corundum formed in equilibrium with kornerupine under high-grade metamorphic conditions.

Figure 4. Plot of Δ¹¹B_Tur-Krn/Prs (= δ¹¹B_Tur − δ¹¹B_Krn/Prs). Larsemann Hills data are from MacGregor et al. (2013). Filled symbols, distribution of B isotopes is inferred to be equilibrium; unfilled symbols, distribution is not considered equilibrium. C, calculated fractionation (MacGregor et al., 2013).

Since the formation of kornerupine and tourmaline in the initially B-poor mafic to ultramafic metamorphic rocks needs external input of B, we consider that the infiltration of a B-bearing fluid is required to form the Akarui Krn-Pl-Crn lens. As kornerupine at the Akarui Point is estimated to have formed at least at 800 °C (Kawakami et al., 2008), B isotopic fractionation between fluid and the prograde tourmaline is negligible according to the isotopic fractionation determined by Meyer et al. (2008). Therefore, the B isotope composition of the prograde tourmaline present within kornerupine is likely to represent that of the B-bearing fluid (i.e., δ¹¹B_Fluid = −1.3 ± 0.9‰). In contrast, Δ¹¹B_Tur-Krn is calculated to be +3.9 ± 1.3‰ between the average secondary tourmaline (δ¹¹B_Tur = −4.1 ± 0.4‰) and the average kornerupine (δ¹¹B_Krn of −8.0 ± 1.2‰) in TK2002122104 (Fig. 4). In other words, δ¹¹B is lower in secondary tourmaline than in prograde tourmaline because the secondary tourmaline crystallized at a lower temperature. It might be considered possible, given its similarity to the tourmaline-prismatine equilibrium value of Δ¹¹B_Tur-Prs = +5.0 ± 1.4‰ (MacGregor et al., 2013), that this might represent isotopic equilibrium instead. However, the presence of andalusite in the tourmaline-bearing microstructures replacing kornerupine shows that secondary tourmaline growth took place within the andalusite stability field, at lower pressures and temperatures than the stability field of kornerupine (e.g., Werding and Schreyer, 1996). Abundant secondary fluid inclusions occurring together with secondary tourmaline and other secondary minerals in TK2002122104 are consistent with estimates of ∼ 500 °C for the retrograde fluid infiltration (Kawakami et al., 2008). Based on Meyer et al. (2008), Δ¹¹B_Tur-fluid at 500 °C is estimated to be ∼ −1.9‰. Therefore, δ¹¹B_fluid of the fluid that formed secondary tourmaline under low-P greenschist facies conditions is calculated as ∼ −2.2‰, essentially the same value as the core of prograde tourmaline. This suggests the fluid that formed secondary tourmaline had almost the same B isotope composition as the fluid that equilibrated with kornerupine and prograde tourmaline, and hence it is not necessary to invoke the introduction of additional B-bearing fluid to explain the crystallization of secondary tourmaline.

Tourmaline in subduction zone metasediments (Sanbagawa, Catalina, and Lago di Cignana) formed by using B released from muscovite commonly show δ¹¹B values around −10‰ (Nakano and Nakamura, 2001; Bebout and Nakamura, 2003; Marschall et al., 2009; Fig. 5). Therefore, δ¹¹B values of the prograde tourmaline of this study (−1.3 ± 0.9‰) cannot be explained by the sediment-sourced fluid infiltration. Moreover, the δ¹¹B for prograde tourmaline is significantly lower than the δ¹¹B for seawater, which is therefore unlikely as a source of the fluid. In contrast, the δ¹¹B values of prograde tourmaline are between whole rock δ¹¹B of MORB and mantle rocks and of some sedimentary rocks, and are similar to the δ¹¹B of blackwall tourmalines that have been considered to have formed during decompression from high-P metamorphism (e.g., Altherr et al., 2004; Marocchi et al., 2011; van Hinsberg et al., 2011; Fig. 5). Therefore, a B-bearing fluid related to oceanic plate subduction and subsequent decompression may be a possible source of the B required to form the Krn-Pl-Crn lens. The dominance of kornerupine at the boundary between host gneiss and the lens (Kawakami et al., 2008) suggests that the B-bearing fluid infiltrated only after lens formation during the Late Proterozoic to Cambrian metamorphism. It is considered likely that this syn-metamorphic fluid may, therefore, have been sourced from a mixtures of mafic to ultramafic lithologies and sedimentary rocks in a subduction setting that preceded the collisional high-T metamorphism now preserved at Akarui Point and in the LHC in general.

Figure 5. Boron isotopic compositions of tourmaline of this study in comparison with other metamorphic/metasomatic tourmaline and selected whole-rock B isotope compositions. Modified after Marschall et al. (2009). Data sources: ¹Marschall et al. (2008), ²Marschall et al. (2006), ³Altherr et al. (2004), ⁴Marschall et al. (2009), ⁵Ota et al. (2008), ⁶Nakano and Nakamura (2001), ⁷Bebout and Nakamura (2003), ⁸Arena et al. (2020), ⁹Benton et al. (2001), ¹⁰Ishikawa and Nakamura (1992), ¹¹Smith et al. (1995), ¹²Ishikawa and Nakamura (1994), ¹³Ishikawa and Tera (1997), ¹⁴Peacock and Hervig (1999).

Based on detailed geochemical study of its metabasic and meta-ultramafic rocks, Suda et al. (2008) concluded that LHC is an assembly of tectonic blocks or terranes of different origins. These blocks were composed of rocks derived from active crustal growth during Mesoproterozoic time and older rocks derived from Paleoproterozoic to Archaean cratons and oceanic crust. The blocks were considered to have amalgamated through multiple subductions of Pan-African and/or pre-Pan-African age (Suda et al., 2008). This view is consistent with the recent results of a regional study of inherited zircon ages (Dunkley et al., 2020) in that LHC is composed of suites of different origin, in which geological subdivisions based on protolith ages are proposed: the Innhovde Suite (1070-1040 Ma), the Rundvågshetta Suite (2520-2470 Ma), the Skallevikshalsen Suite (1830-1790 Ma), the Langhovde Suite (1100-1050 Ma), the East Ongul Suite (630 Ma), and the Akarui Suite (970-800 Ma). Although Akarui Point is located within the Akarui Suite, a boundary between positive and negative magnetic anomalies is indicated near Akarui Point in the reduced to the pole magnetic anomaly map of the LHC (Nogi et al., 2013), which would be consistent with the presence of small tectonic discontinuity. Therefore, the metabasic and meta-ultramafic lenses found in Akarui Point may be the remnant of a mixing zone associated with an Ediacaran to Cambrian subduction channel within which B from multiple sources has been generated and mixed to produce a B-bearing fluid that interacted with the B-poor mafic to ultramafic rocks under high-T conditions.

ACKNOWLEDGMENTS

Dr. Cees-Jan de Hoog, of the Edinburgh Ion Microprobe Facility (EIMF) is thanked for setting up the B isotope analysis protocols and advising on initial data reduction. We are grateful to Prof. Edward Grew and an anonymous reviewer for constructive reviews and Prof. Tomokazu Hokada for editorial efforts. This study was financially supported by JSPS KAKENHI Grant numbers JP26400513 and JP19H01991 to T. Kawakami.

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