Characterization of the Oman ophiolite peridotites using the relationship between clinopyroxene Nd isotopic ratios and spinel compositions

Abstract

Clinopyroxene (Cpx)-rich peridotites have been observed at the base of mantle section from northern to southern massifs in the Oman ophiolite. We present Nd isotopic ratios of Cpx grains separated from the Cpx-rich peridotites of the central Sarami massif and compared them with published Nd isotopic ratios of Cpx grains from the northern Fizh and the southern Wadi Tayin Cpx-rich peridotites. The Nd isotopic data combined with spinel Cr# suggest that fertile Cpx-rich peridotites (spinel Cr# < 0.3) from the northern and the central massifs preserved a simple melt extraction event, whereas relatively refractory Cpx-rich peridotite (spinel Cr# = 0.33) from the southern massif and harzburgite (spinel Cr# = 0.55) from the northern Hilti massif recorded a further melt extraction event with an influx of a mid-ocean ridge basalt like melt.

INTRODUCTION

The mantle section in most ophiolite complexes is mainly composed of harzburgite that contains less than 1 vol% of clinopyroxene (Cpx) (Lippard et al., 1986). This ophiolite complex is named as the harzburgite ophiolite subtype (Boudier and Nicolas, 1985). In the harzburgite subtype ophiolite, Cpx and plagioclase (Pl) are locally present mainly in the uppermost harzburgites near the crust-mantle transition zone, suggesting a secondary magmatic impregnation (Boudier and Nicolas, 1985). On the other hand, Cpx-rich harzburgite and/or lherzolite that was defined as Cpx-rich peridotite (modal abundances of Cpx >3 vol%), have been observed at the base of mantle section overlying the metamorphic sole (Lippard et al., 1986). The Oman ophiolite belongs to the harzburgite subtype (Boudier and Nicolas, 1985), and shows the largest and best exposed sections of oceanic lithosphere worldwide (e.g., Nicolas et al., 1988). This ophiolite consists of numerous massifs (e.g., Nicolas et al., 2000; Fig. 1) and basal Cpx-rich peridotites have been found at several places in the Oman ophiolite, namely in the Fizh massif of the northern part (Lippard et al., 1986; Takazawa et al., 2003), the Sarami and Wuqbah massifs of the central part (Khedr et al., 2013, 2014) and the Wadi Tayin massif of the southern part (Godard et al., 2000). Major and trace element geochemistry of the Cpx-rich peridotites of these massifs have suggested interaction between mid-ocean ridge (MOR) like basaltic melt and residual peridotite (Godard et al., 2000; Takazawa et al., 2003) or low degrees of melt extraction of oceanic mantle (Khedr et al., 2013, 2014). Yoshikawa et al. (2015) confirmed less degree of melt extraction and later metasomatism by fluids derived from the metamorphic sole. This conclusion was drawn from an analyses of trace element and Sr-Nd isotopic systematics observed in Cpx grains of the Cpx-rich peridotites of the northern Fizh massif. Nd isotopic ratios of the Cpx in peridotites can be a powerful tool for elucidating their origin, however, such data are still limited in the northern and southern massifs of the Oman ophiolite. The late-stage volcanic and intrusive rocks observed from the northern massifs have shown geochemical characteristics of subduction, whereas the southern massifs were considered to have formed at a MOR (e.g., Ernewein et al., 1988; Python et al., 2008). Therefore, it is possible that differences in geochemical characteristics may be recognized in peridotite of the mantle section of the northern, central and southern massifs. In order to better constrain the relationship between the formation process and spatial distribution of basal Cpx-rich peridotite, we analyzed Nd isotopic ratios of Cpx in the Sarami massif in the central part of the Oman ophiolite and together with existing data for the northern and southern massifs in the Oman ophiolite, discussed their petrogenetic implications.

Figure 1. Sample locations for this study are shown in geologic map of Oman ophiolite (Yoshikawa et al., 2015), simplified from the 1:250000 geological map of BRGM (Ministry of Petroleum and Minerals, 1992).

GEOLOGICAL BACKGROUND

The Oman ophiolite

Although significant amount of research has been undertaken on the Oman ophiolite, the tectonic setting in which the ophiolite formed is still debated among researchers as follows: at MOR (e.g., Nicolas et al., 1988; Khedr et al., 2013, 2014), arc-basin (e.g., Pearce et al., 1981) or a change from MOR to subduction-zone by oceanic thrusting faults (e.g., Boudier et al., 1988; Arai et al., 2006). Volcanic rocks in the Oman ophiolite have been divided into 3 units; the Geotimes (V1), the Lasail and Alley (V2), and the Salahi (V3) units, and they were successively erupted in this order (Alabaster et al., 1982; Ernewein et al., 1988). The V3 lavas are separated from the V2 unit by a 15 m thick pelagic sedimentary section (Ernewein et al., 1988). Geochemical characteristics of the V1 lavas were similar to those of MOR-basalt (MORB) (e.g., Ernewein et al., 1988). The V2 lavas, which outcrop only in the northern part of the ophiolite, showed rare earth element (REE) depletion relative to the V1 lavas and low high field strength element/REE ratios, suggesting subduction related fluid enhanced melting of previously depleted mantle after the V1 extraction (e.g., Pearce et al., 1981; Godard et al., 2003). The V3 lavas have light REE (LREE) enriched chondrite-normalized patterns and were obviously different patterns of the V1 and V2 lavas, suggesting intraplate magmatism at off axis (e.g., Alabaster et al., 1982; Ernewein et al., 1988; Umino, 2012). Recently, the progression from MORB-like V1 lavas (96.1-95.6 Ma) to subduction-related V2 lavas (95.6-95.0 Ma) by subduction initiation at forearc were proposed using Nd isotopic data and precise zircon ages of gabbroic and silicic rocks of the Oman ophiolite and amphibolite of the metamorphic sole underlying the ophiolite (Rioux et al., 2021). In terms of spatial distribution, MORB-like dykes in the mantle section are distributed mainly in the area of the Maqsad and Musibit massifs of the southern part of the Oman ophiolite, and calc-alkaline dykes possibly related to subduction are more widespread in the mantle section of the Oman ophiolite (Python and Ceuleneer, 2003; Python et al., 2008). The mantle section is composed mainly of harzburgites and the majority of harzburgites has been interpreted as residual mantle at oceanic ridge because of its spinel Cr# [= atomic Cr/(Cr + Al)] value similar to those of abyssal peridotites (e.g., Arai et al., 2006). Genetic relationships between mantle harzburgite, layered gabbro, sheeted dikes, and basalts in the Oman ophiolite was suggested by their Nd isotopic systematics (McCulloch et al., 1981).

Basal peridotites

In the area around the metamorphic sole, Cpx-rich peridotites have been identified within the Fizh massif (Lippard et al., 1986; Takazawa et al., 2003), the Sarami and Wuqbah massifs (Khedr et al., 2013), and the Wadi Tayin massif (Godard et al., 2000) (Fig. 1). From numerical modelling of REE patterns and structural studies, the Cpx-rich peridotites in the Wadi Tayin massif (Fig. 1) were thought to have formed by a Cpx forming melt-rock reaction related to opening of the NW-SE ridge system by ridge propagation (Godard et al., 2000). Takazawa et al. (2003) identified two types of basal Cpx-rich peridotites within the northern Fizh massif (Fig. 1): Type I lherzolite contains Cpx with lower concentrations of Na₂O (<0.7 wt%), has protogranular to porphyroclastic textures, and sporadically appears within the basal zone, whereas Type II lherzolite contains high Na₂O (>1 wt%) Cpx, has sub-mylonitic texture, and occur only just above the basal thrust at the contact with the metamorphic sole. Type I lherzolite grades to Cpx-rich harzburgite over several meters (Takazawa et al., 2003). Takazawa et al. (2003) suggested that the Cpx-peridotite was formed by reducing degree of melting and interaction with melt at the failing ridge that was located near the base of the Fizh massif (Nicolas et al., 2000). Two types of lherzolites with similar signatures have also been recognized in the central Sarami massif of the ophiolite (Khedr et al., 2013) (Fig. 1), although Cpx within these peridotites has a continuous range of Na₂O contents from about 1.2 to 0.6 wt% without any compositional gap (Khedr et al., 2013). Khedr et al. (2013, 2014) observed that the foliated Type II lherzolite, which exhibits mylonitic to porphyroclastic texture occurs up to few meters (sometimes up to ∼ 150 m) above the metamorphic sole, and is overlain and/or surrounded by massive Type I lherzolite with porphyroclastic texture. They inferred that Type II lherzolite are only exposed at the base of the Sarami massif, while Type I lherzolite are exposed at the base of both Sarami and Wuqbah massifs (Fig. 1) due to difference with degree of melting depending on distance from the Wuqbah diapir.

SAMPLES AND PETROGRAPHY

The sample TV84 was collected from an outcrop where Type II lherzolite of the central Sarami massif was described by Khedr et al. (2013, 2014). We selected three samples for analyses of Nd isotopic ratios namely: sample TV84, which is Cpx-rich peridotite, sample TV121, which was already reported as Type II lherzolite by Khedr et al. (2013) in the Sarami massif and a harzburgite sample 2203, which was collected far from a basal thrust in the northern Hilti (Salahi) massif (Fig. 1). All samples analyzed in this study are slightly to moderately serpentinized, however, they contain primary mantle minerals such as olivine (Ol), orthopyroxene (Opx), Cpx, and chromian spinel (Spl) (Fig. 2).

Figure 2. Cross-polarized light photomicrographs of basal Cpx-rich peridotites in the central Sarami massif. Opx, orthopyroxene; Cpx, clinopyroxene; Spl, spinel; Serp, serpentine.

The TV84 Cpx-rich peridotite shows porphyroclastic texture, where Opx porphyroclast (2.0-2.5 mm) is surrounded by fine grained (less than 0.05 mm across) Ol, Cpx, and few Opx. This texture characterizes basal peridotite and was formed due to emplacement above the metamorphic sole (Khedr et al., 2013, 2014). The modal composition is ∼ 5 vol% Cpx, ∼ 30 vol% Opx, ∼ 20 vol% fresh granular Ol, ∼ 30 vol% pseudomorph Ol, ∼ 38 vol% serpentine and opaque (magnetite) with few Spl grains. The TV121 Cpx-rich peridotite is also porphyroclastic texture, where Opx porphyroclast (1.5-2.0 mm) is surrounded by fine grained (less than 0.05 mm across) Ol, Cpx, and few Opx. The modal composition of this sample is ∼ 3 vol% Cpx, ∼ 25 vol% Opx, ∼ 60 vol% fresh granular Ol, ∼ 10 vol% serpentine and <3 vol% opaque (magnetite) with few Spl grains. The 2203 Hilti harzburgite shows protogranular texture and include <1 vol% Cpx, ∼ 9 vol% Opx, ∼ 90 vol% Ol, 10% altered minerals and <1 vol% Spl (Yoshikawa et al., 2015).

ANALYTICAL METHODS

The major element compositions of the minerals were determined on polished thin slabs using a JEOL JXA-8800 electron probe microanalyzer at Kanazawa University, Japan. Operating conditions were 20 kV accelerating voltage, 20 nA beam current, and 3 mm beam diameter, using oxide ZAF matrix correction. Trace element concentrations in Cpx were determined on polished thin slabs using a laser ablation (193nm ArF excimer; MicroLas GeoLas Q-plus) inductively coupled plasma mass spectrometer (Agilent 7500S) at Kanazawa University, Japan, following the analytical procedures presented by Ishida et al. (2004) and Morishita et al. (2005). The count time for each spot was 30 s, and the size of the laser was 60 µm. The laser frequency and energy were 6 Hz and 8 mJ/cm², respectively. The NIST 612 glass standard was used to calibrate relative element sensitivities for the analyses and ²⁹Si was used as an internal standard based on SiO₂ concentration obtained by the electron microprobe. The NIST 614 glass standard was analyzed as an unknown and the measured concentrations in this study (Table 2) were in agreement with those previously reported by Morishita et al. (2005).

Table 2. Average trace element concentrations of clinopyroxenes in TV84 Cpx-rich peridotite from the Sarami massif in central part of the Oman ophiolite

	TV84 n = 4 (ppm)	RSD	NIST614 glass (ppm)	DL
Rb	-		0.83	0.02
Ba	0.26	0.30	2.93	0.04
Nb	0.06	0.01	0.80	0.01
La	-
Ce	0.14	0.01	0.75	0.003
Pr	0.09	0.01	0.72	0.002
Sr	1.99	0.63	43.79	0.008
Nd	1.02	0.08	0.7	0.015
Zr	3.69	0.39	0.81	0.0054
Hf	0.28	0.02	0.64	0.0233
Sm	0.78	0.06	0.70	0.025
Eu	0.37	0.03	0.73	0.005
Ti	2192	373	4.09	0.26
Gd	1.46	0.14	0.73	0.0218
Tb	0.30	0.03	0.67	0.0056
Dy	2.28	0.20	0.71	0.016
Y	13.11	1.05	0.74	0.0052
Ho	0.50	0.04	0.70	0.0066
Er	1.50	0.13	0.68	0.0104
Tm	0.22	0.01	0.69	0.0043
Yb	1.51	0.11	0.73	0.027
Lu	0.21	0.01	0.70	0.0055

n, number of Cpx grains analyzed by LA-ICP-MS.

-, below detection limit.

DL, detection limit of the standard NIST 614 glass.

Samples for Nd isotopic analyses were crushed to <1 cm chips in plastic bag by hammer and iron plate. The rock chips were further crushed to <#60 using tungsten mortar and mineral grains were washed with special grade ethanol. After drying, magnetite rich grains were removed using a Nd magnet and Cpx grains were separated by handpicking under a binocular microscope. The separated Cpx grains were cleaned with TAMAPURE AA-10 grade H₂O₂ (Tama Chemicals, Japan) and hot (80 °C) TAMAPURE AA-10 grade 6 M HCl (Tama Chemicals, Japan), before being rinsed three times with >18.2 MΩ Milli-Q water. The final Cpx separates were selected by careful hand-picking under a binocular microscope before being washed with TAMAPURE AA-10 grade 6 M HCl (Tama Chemicals, Japan) and rinsed three times with distilled Milli-Q water. Nd isotope analyses were conducted with a ThermoFinnigan MAT 262 instrument at Hiroshima University, Japan. Chemical separation and mass spectrometry are given in Shibata and Yoshikawa (2004). The normalizing factor used to correct isotopic fractionation of Nd was ¹⁴⁶Nd/¹⁴⁴Nd = 0.7219, and La Jolla standard solutions yield values of ¹⁴³Nd/¹⁴⁴Nd = 0.511849 ± 0.000011 (n = 2, 2σ). The results for the Geological Society of Japan (GSJ) standard rock JB-2 were ¹⁴³Nd/¹⁴⁴Nd = 0.513104 ± 0.000026 (n = 2, 2σ).

RESULTS

The Cpx grains in the TV84 Cpx-rich peridotite from the Sarami massif has 0.95-1.03 wt% Na₂O and 6.0-6.5 wt% Al₂O₃ concentrations (Table 1). They plot in the field for Cpx in Type II lherzolite from the Sarami massif (Fig. 3a). Na₂O contents of our previously reported grains of Cpx in the TV121 Cpx-rich peridotite (Khedr et al., 2013) are lower than those of TV84 (Fig. 3a). Grains of Cpx in the Hilti 2203 harzburgite are relatively homogenous (Na₂O = 0.16-0.19 wt% and Al₂O₃ = 1.9-2.1 wt%; Yoshikawa et al., 2015) and lower in Na₂O and Al₂O₃ than those in Cpx-rich peridotites (Fig. 3a). The Spl grains of the TV84 and TV121 Cpx-rich peridotites and the Hilti 2203 harzburgite are located at low and high ends of Cr# value respectively, of the range for abyssal peridotites on Cr# and Mg# [= atomic Mg/(Mg + Fe²⁺)] diagram (Dick and Bullen, 1984; Arai, 1994; Fig. 3b). The forsterite (Fo) contents of Ol [atomic 100Mg/(Mg + Fe²⁺) ratio] versus Cr# of Spl for TV84 and TV121 Cpx-rich peridotites, and Hilti 2203 harzburgite plot within the range of abyssal peridotite (Fig. 3c).

Table 1. Average compositions of the main mineral phases in the TV84 Cpx-rich peridotite from the Sarami massif in central part of the Oman ophiolite

Minerals	TV84
	Ol n = 4	Cpx n = 4	Opx n = 1	Spl n = 6
SiO2	40.69	51.31	53.11	0.01
TiO2	0.00	0.39	0.09	0.04
Al2O3	0.00	6.39	6.35	58.01
Cr2O3	0.01	0.85	0.69	10.87
FeO*	9.40	2.26	6.07	11.35
MnO	0.11	0.08	0.11	0.11
MgO	50.04	15.20	32.70	19.94
CaO	0.01	22.61	0.79	0.00
Na2O	0.01	1.00	0.03	0.02
NiO	0.39	0.03	0.06	0.35
Total	100.66	100.12	100.00	100.70

Ol, olivine; Opx, orthopyroxene; Cpx, clinopyroxene; Spl, spinel; n, number of averaged analyses.

Figure 3. (a) Na₂O-Al₂O₃ variation of Cpx in the Oman peridotites. Regions of Cpx in Type I and II lherzolites of the northern Fizh and central Sarami massifs are from Takazawa et al. (2003) and Khedr et al. (2013, 2014). Compositional range of the Sarami harzburgites (Khedr et al., 2013) is also shown. For comparison, the northern Hilti harzburgite (sample 2203) and southern Wadi Tayin Cpx-rich peridotite (sample 2611) (Yoshikawa et al., 2015) are also shown. (b) Cr# [atomic Cr/(Cr + Al)] versus Mg# [atomic Mg/(Mg + Fe)] for Cr-Spl in the northern Oman peridotites. Symbols are same as Figure 3a. The fields of Cr-Spl in Type I and Type II basal lherzolites and harzburgites in the central Sarami massif (Khedr et al., 2013, 2014), abyssal peridotites (Dick and Bullen, 1984; Arai, 1994) and Izu-Bonin-Mariana forearc peridotites (Parkinson and Pearce, 1998) are also shown for comparison. Partial melting trend is from Dick and Bullen (1984). (c) Ol Fo [100 × atomic Mg/(Mg + Fe)] versus Spl Cr# number diagram. Olivine spinel mantle array (OSMA) represents a Fo-Cr# residual trend constructed from spinel peridotite xenoliths and massif peridotites formed under various P_total and $P_{\text{H${_{2}}$O}}$ conditions (Arai, 1994).

Averaged trace element compositions of Cpx in the Sarami TV84 Cpx-rich peridotite show a pattern generally decreasing towards highly incompatible elements, such as Ce, relative to less incompatible elements [heavy rare earth elements (HREE) such as Yb] on the chondrite-normalized multi-element diagram [(Ce/Yb)_N = 0.025; Fig. 4]. It is within range of previous reported Cpx grains in Type II lherzolites from the Sarami massif (Khedr et al., 2013, 2014; Fig. 4).

Figure 4. Chondrite-normalized trace element patterns of Cpx in the TV84 Sarami Cpx-rich peridotite with those of Cpx in the 2203 Hilti harzburgite and 2611 Wadi Tayin Cpx-rich peridotite (Yoshikawa et al., 2015). Dark and pale shaded areas indicate the fields of Cpx in Type I and II lherzolites from the Fizh massif, respectively (Takazawa et al., 2003). Yellow and red colored fields are the ranges of Cpx in Type I and Type II lherzolites from the Sarami massif, respectively (Khedr et al., 2014). Chondrite compositions are from Sun and McDonough (1989).

Nd isotopic ratios of grains of Cpx in the Sarami TV84 and TV121 Cpx-rich peridotites (¹⁴³Nd/¹⁴⁴Nd = 0.51333, 0.51343; Table 3) are clearly higher than those of the Hilti 2203 harzburgite (¹⁴³Nd/¹⁴⁴Nd = 0.51308; Table 3). These values are within range of those of Cpx grains in the northern Fizh and southern Wadi Tayin (sample 2611) Cpx-rich peridotites (¹⁴³Nd/¹⁴⁴Nd = 0.51307-0.51418; Yoshikawa et al., 2015).

Table 3. Neodymium isotope analyses of clinopyroxene mineral separates in the Sarami Cpx-rich peridotites and the Hilti harzburgite in the Oman ophiolite

	147Sm/144Nd1)	143Nd/144Nd	2σm	εNd(96 Ma)2)
Sarami Type II Cpx-rich peridotites
TV84	0.466	0.513325	±0.000016	+10.0
TV121	0.647	0.513431	±0.000055	+9.7
Hilti harzburgite
2203	0.434	0.513078	±0.000032	+5.6

¹⁾ Calculated from Sm/Nd ratio of LA-ICP-MS data.

²⁾ Calculated from decay constants of ¹⁴⁷Sm (6.54 × 10⁻¹²/y; Lugmair and Marti, 1978).

DISCUSSION

Na₂O content of Cpx in peridotite decreases with increasing degree of melt extraction at similar depth and is greater at higher pressure for similar degrees of melting and melt extraction, because the partition coefficient of Na₂O between Cpx and melt (D_Na) is <1 at <4 GPa and increases with pressure (e.g., Blundy et al., 1995). Al₂O₃ contents of Cpx in peridotite generally decrease with increasing degree of melt extraction (Seyer and Bonatti, 1994). The continuous decrease in Na₂O content associated with the decrease in Al₂O₃ content of Cpx in the Cpx-rich peridotites from the Sarami massif was thus interpreted to reflect increasing degree of melt extraction (Khedr et al., 2013). It has been interpreted that increase of Spl Cr# in peridotite reflects a trend of residues in progressive partial melting, because the increase in Spl Cr# values are accompanied by increase in compatible elements such as Cr and Mg and decrease in incompatible elements (e.g., Na, Al, and Fe) in silicate minerals and bulk rock during melt extraction (Dick, 1977; Fig. 3b). Because mantle spinel peridotites are plotted in a narrow band on Fo content of Ol and Spl Cr# diagram, Arai (1994) defined the area as the OSMA. He suggested that the OSMA shows a residual trend of various degree of melt extraction, because Cpx/(Opx + Cpx) volume ratio decreases towards the higher Fo of Ol and Cr# of Spl and these features are consistent with a residual trend of high P-T melting experiment (Jaques and Green, 1980). The Hilti 2203 harzburgite plots on the high Cr# end of the field for the Spl in the Sarami harzburgite and also plots in the OSMA (Figs. 3b and 3c), suggesting residue after high degree of melt extraction from a similar source to the Sarami harzburgite.

HREE abundance and LREE/HREE ratio of Cpx in peridotite, theoretically decrease with the increase of extent of melt extraction during partial melting in the spinel stability field (Johnson et al., 1990). Such relationship is reflected in Yb (one of HREE) content of Cpx which is possibly correlated with the Cr# of Spl that is an index of degree of melt extraction (Fig. 5). On the bases of the similarity of extensive depletion of highly incompatible LREE respect to HREE [(Ce/Yb)_N = 0.002-0.029] and HREE abundances (Yb = 0.87-1.55 ppm) of the Cpx grains in the Sarami Cpx-rich basal peridotites to those of residual Cpx grains in abyssal peridotites [(Ce/Yb)_N = 0.004-0.12, Yb = 0.46-1.78 ppm, Johnson et al., 1990], Khedr et al. (2014) suggested that the Sarami basal peridotites represent a fragment of the residual peridotite of an oceanic mantle. Our TV84 data [(Ce/Yb)_N = 0.025, Yb = 1.51 ppm; Fig. 5] are also within the range of Cpx in abyssal peridotites and consistent with this suggestion.

Figure 5. Variation diagram of Cr# ratios of Spl and Yb concentration of Cpx from the Oman ophiolite. Symbols are as Figure 3a.

The new Nd isotopic ratios of grains of Cpx in the Sarami Cpx-rich peridotites (samples TV84 and TV121) are higher than those of representative modern depleted MORB source mantle (¹⁴³Nd/¹⁴⁴Nd = 0.51315; White and Hofmann, 1982) (Table 3). In general, high Nd isotopic ratios are inferred to be the time-integrated result of residual mantle having a high parent-daughter element (Sm/Nd) ratio by extraction of partial melts of a MORB source (e.g., Yoshikawa and Nakamura, 2000). These high Nd isotopic ratios and depletion of LREE (e.g., Ce and Nd) relative to HREE (e.g., Yb) are consistent with residual origin of basal Cpx-rich peridotites from the Sarami massif. A residual origin of basal Cpx-rich peridotites from the northern Fizh massif from a MORB source was also suggested from high Nd isotopic ratios and depletion of LREE/HREE of Cpx (Yoshikawa et al., 2015). The gabbros produced during the main phase of crustal growth of the Oman ophiolite (named as the V1 stage), yielded zircon dates of 96.1-95.6 Ma, and the plutonic and volcanic rocks of the V1 unit had ε_{Nd (96 Ma)} ≈ +7-+9, similar to modern ridges (Rioux et al., 2021 and references herein). Cpx separates in the Fizh and Sarami Cpx-rich peridotites show slightly large ε_{Nd (96 Ma)} = +6-+13 variation compared than the V1 magmatic series. This result is similar to greater Nd isotopic range of abyssal peridotites compared to nearby basalts (e.g., Snow et al., 1994). Such discrepancy has been interpreted to be due to preferential partial melting of isotopically enriched part in a heterogeneous source and remaining residue has shown relative heterogeneous Nd isotopic ratios (Snow et al., 1994; Salters and Dick, 2002). In the Spl Cr# versus Nd isotopic ratio of Cpx diagram (Fig. 6), the Sarami and Fizh Cpx-rich peridotites show a positive trend. The positive trend suggests a residual trend with a simple partial melting, because Spl Cr# value and Nd isotopic ratio of Cpx increase with the degree of melt extraction and isotopic evolution as stated above. Thus, relative fertile peridotites (Spl Cr# < 0.3) are interpreted as residue of a simple partial melting.

Figure 6. Variation diagram of ¹⁴³Nd/¹⁴⁴Nd ratios of Cpx from the Oman ophiolite as a function of Spl Cr# values. Symbols are as Figure 3a. Values for depleted MORB mantle (DMM) are from White and Hofmann (1982) and Workman and Hart (2005).

The Hilti 2203 harzburgite and the Wadi Tayin 2611 Cpx-rich harzburgite have relatively refractory signature (Spl Cr# > 0.3). When these samples were formed by a simple partial melting similar to the Cpx-rich peridotites (Spl Cr# < 0.3) of the Fizh and Salami massifs, they should have higher Nd isotopic ratios than the relative fertile peridotites. In this case, the grains of Cpx in samples 2203 and 2611 must also have high Nd isotopic ratios compared to the grains of Cpx in the relatively fertile peridotites. Nd isotopic ratios of Cpx grains in those rocks (Spl Cr# > 0.3), however, are not plotted on the higher Nd isotopic ratio expected from higher Spl Cr# value. They appear to form a negative trend on the higher Cr# side of Figure 6. Although Cpx grains in the Hilti 2203 harzburgite and the Wadi Tayin 2611 Cpx-rich harzburgite have lower HREE concentrations (Yb = 0.27 and 0.65 ppm, respectively; Yoshikawa et al., 2015) than those of Cpx in the Sarami lherzolites (Yb = 0.87-1.55 ppm; Khedr et al., 2014; this study) (Fig. 4), LREE (ex. Nd) abundances of Cpx of the Hilti harzburgite and Wadi Tayin Cpx-rich peridotite (Nd = 0.051-0.052 ppm) are similar to the lower limit of the abundance range (Nd = 0.056-1.07 ppm) in the Salami Cpx-rich peridotite (Fig. 5). Similar LREE enrichment relative to HREE has been commonly observed in whole rock studies of harzburgites (Spl Cr# > 0.35) in the Oman ophiolite (e.g., Hanghøj et al., 2010) and this signature was explained by trace element fractionation during reactive porous flow (e.g., Godard et al., 2003; Hanghøj et al., 2010) of MORB-like melt through residual peridotite at the ridge stage. It is plausible that the negative trend of the Spl Cr# versus Cpx Nd isotopic ratio with more refractory samples is due to higher degree of melt extraction with influx of agents that shows similar Nd isotopic ratio to MORB. Python and Ceuleneer (2003) showed that MORB-like dykes in the mantle section were distributed in restricted areas of the Oman ophiolite, on a large scale in the Maqsad area to the south of Wadi Tayin massif and on a smaller scale in the Hilti massif, on the bases of the comprehensive field, petrographic, and microprobe study of the dykes. Structural analyses have indicated that the uppermost mantle sections of the Hilti and Wadi Tayin massifs were situated at the edge of the Maqsad paleo diapir (Michibayashi et al., 2000; Nicolas et al., 2000). Additionally, an asthenospheric flow originating from this diapir was observed in this region (Michibayashi et al., 2000; Nicolas et al., 2000). Thus, melt extraction with an influx of MORB-like melt should had occurred during an ocean spreading stage.

Takazawa et al. (2003) pointed out that Cpx-rich peridotites of the Fizh massif have significantly higher Al₂O₃/CaO ratios (0.84-1.1) than those of the Wadi Tayin massif (0.53-0.69; Godard et al., 2000). Godard et al. (2000) observed Al₂O₃/CaO ratios tended to decrease with Cpx contents and explained their observation by a melt-rock reaction involving precipitation of Cpx at the expense of Opx. Whole rock Al₂O₃/CaO ratios of Cpx-rich peridotites in the Sarami massif (0.85-1.71; Khedr et al., 2014) show similar value to those of the Fizh massif. Thus, the distinctive formation process of Cpx-rich peridotites suggested from the Cr# Spl versus Cpx Nd isotopic diagram may also be related to differences in Al₂O₃/CaO ratios of whole rocks. The combination of Nd isotopic ratio of Cpx and Spl Cr# can be a new indicator to discriminate petrogenetic processes in other peridotite samples.

CONCLUSIONS

Trace elements and Nd isotopic signature of Cpx in the basal Cpx-rich peridotites from the central Sarami massif suggest that the Sarami massif mantle is a residual material of MORB-type mantle source similar to the Cpx-rich peridotites from the northern Fizh massif. The negative trend of the Nd isotopic ratios of Cpx and Spl Cr# values in the harzburgite from the northern Hilti massif and Cpx-rich peridotite from the southern Wadi Tayin massif can be ascribed to higher degree of melt extraction with an influx of MOR-like basaltic melt.

ACKNOWLEDGMENTS

This research was supported by JSPS KAKENHI Grant Number JP16K05606 to M. Yoshikawa. We deeply thank M. Python and the officials in the Ministry of Commerce and Industry of the Sultanate of Oman for their support to our field expedition. We benefited from careful and constructive comments by two anonymous reviewers and O. Parlak, on an earlier version of the manuscript, and from kindly editorial efforts by B. Payot and M. Satish-Kumar. M. Yoshikawa acknowledges support from HiPeR (Hiroshima Institute of Plate Convergence Region Research) project of the Hiroshima University and T. Shibata for improving of draft of manuscript.

REFERENCES

•  Alabaster,  T.,  Pearce,  J.A. and  Malpas,  J. (1982) The volcanic stratigraphy and petrogenesis of the Oman ophiolite complex. Contributions to Mineralogy and Petrology, 81, 168-183.
•  Arai,  S. (1994) Characterization of spinel peridotites by olivine-spinel compositional relationships: Review and interpretation. Chemical Geology, 113, 191-204.
•  Arai,  S.,  Kadoshima,  K. and  Morishita,  T. (2006) Widespread arc-related melting in the mantle section of the northern Oman ophiolite as inferred from detrital chromian spinels. Journal of the Geological Society, London, 163, 869-879.
•  Blundy,  J.D.,  Falloon,  T.J.,  Wood,  B.J. and  Dalton,  J.A. (1995) Sodium partitioning between clinopyroxene and silicate melts. Journal of Geophysical Research, 100, 15501-15515.
•  Boudier,  F. and  Nicolas,  A. (1985) Harzburgite and lherzolite subtypes in ophiolitic and oceanic environments. Earth and Planetary Science Letters, 76, 84-92.
•  Boudier,  F.,  Ceuleneer,  G. and  Nicolas,  A. (1988) Shear zones, thrusts and related magmatism in the Oman ophiolite: initiation of thrusting on an oceanic ridge. Tectonophysics, 151, 275-296.
•  Dick,  H.J.B. (1977) Partial melting in the Josephine Peridotite; I, The effect on mineral composition and its consequence for geobarometry and geothermometry. American Journal of Science, 277, 801-832.
•  Dick,  H.J.B. and  Bullen,  T. (1984) Chromian spinel as a petrogenetic indicator in abyssal and alpine-type peridotites and spatially associated lavas. Contribution of Mineralogy and Petrology, 86, 54-76.
•  Ernewein,  M.,  Pflumio,  C. and  Whitechurch,  H. (1988) The death of an accretion zone as evidenced by the magmatic history of the Sumail ophiolite (Oman). Tectonophysics, 151, 247-274.
•  Godard,  M.,  Jousselin,  D. and  Bodinier,  J.-L. (2000) Relationships between geochemistry and structure beneath a palaeo-spreading center: a study of the mantle section in the Oman ophiolite. Earth and Planetary Science Letters, 180, 133-148.
•  Godard,  M.,  Dautria,  J.-M. and  Perrin,  M. (2003) Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution and paleomagnetic directions. Geochemistry, Geophysics, Geosystem, 4, 8609.
•  Hanghøj,  K.,  Kelemen,  P.B.,  Hassler,  D. and  Godard,  M. (2010) Composition and genesis of depleted mantle peridotites from the Wadi Tayin massif, Oman ophiolite; major and trace element geochemistry, and Os isotope and PGE systematics. Journal of Petrology, 51, 201-227.
•  Ishida,  Y.,  Morishita,  T.,  Arai,  S. and  Shirasaka,  M. (2004) Simultaneous in-situ multi-element analysis of minerals on thin section using LA-ICP-MS. The Science Reports of Kanazawa University, 48, 31-42.
•  Jaques,  A.L. and  Green,  D.H. (1980) Anhydrous melting of peridotite at 0-15 kb pressure and the genesis of tholeiitic basalts. Contributions to Mineralogy and Petrology, 73, 287-310.
•  Johnson,  K.T.M.,  Dick,  H.J.B. and  Shimizu,  N. (1990) Melting in the oceanic upper mantle: an ion microprobe study of diopsides in abyssal peridotites. Journal of Geophysical Research, 95, 2661-2678.
•  Khedr,  M.Z.,  Arai,  S. and  Python,  M. (2013) Petrology and chemistry of basal lherzolites above the metamorphic sole from Wadi Sarami central Oman ophiolite. Journal of Mineralogical and Petrological Sciences, 108, 13-24.
•  Khedr,  M.Z.,  Arai,  S.,  Python,  M. and  Tamura,  A. (2014) Chemical variations of abyssal peridotites in the central Oman ophiolite: Evidence of oceanic mantle heterogeneity. Gondwana Research, 25, 1242-1262.
• Lippard, S.J., Shelton, A.W. and Gass, I.G. (1986) The ophiolite of the Northern Oman. pp.178, Blackwell Scientific Publications, Oxford.
•  Lugmair,  G.W. and  Marti,  K. (1978) Lunar initial ¹⁴³Nd/¹⁴⁴Nd: differential evolution of the lunar crust and mantle. Earth and Planetary Science Letters, 39, 349-357.
•  McCulloch,  M.T.,  Gregory,  R.T.,  Wasserburg,  G.J. and  Taylor,  H.P. (1981) Sm-Nd, Rb-Sr, and ¹⁸O/¹⁶O isotopic systematics in an oceanic crustal section: Evidence from the Samail ophiolite. Journal of Geophysical Research, 86, 2721-2735.
•  Michibayashi,  K.,  Gerbert-Gaillard,  L. and  Nicolas,  A. (2000) Shear sense inversion in the Hilti mantle section (Oman ophiolite) and active mantle uprise. Marine Geophysical Research, 21, 259-268.
• Ministry of Petroleum and Minerals (1992) Geological map of Buraymi, scale 1:250000. Sultanate of Oman Muscat.
•  Morishita,  T.,  Ishida,  Y.,  Arai,  D. and  Shirasako,  M. (2005) Determination of multiple trace element compositions in thin (<30 µm) layers of NIST SRM 614 and 616 using laser ablation-inductively couples plasma-mass spectrometry. Geostandards and Geoanalytical Research, 29, 107-122.
•  Nicolas,  A.,  Ceuleneer,  G.,  Boudier,  F. and  Misseri,  M. (1988) Structural mapping in the Oman ophiolites: Mantle diapirism along an oceanic ridge. Tectonophysics, 151, 27-56.
•  Nicolas,  A.,  Boudier,  F.,  Ildefonse,  B. and  Ball,  E. (2000) Accretion of Oman and United Arab Emirates ophiolite -Discussion of a new structural map. Marine Geophysical Research, 21, 147-179.
•  Parkinson,  I.J. and  Pearce,  J.A. (1998) Peridotites from the Izu-Bonin-Mariana forearc (ODP Leg 125): evidence for mantle melting and melt-mantle interaction in a suprasubduction zone setting. Journal of Petrology, 39, 1577-1618.
•  Pearce,  J.A.,  Alabaster,  T.,  Shelton,  A.W. and  Searle,  M.P. (1981) The Oman ophiolite as a cretaceous arc-basin complex: evidence and implications. Philosophical Transactions of the Royal Society of London: Series A, Mathematical, Physical and Engineering Sciences, 300, 299-317.
•  Python,  M. and  Ceuleneer,  G. (2003) Nature and distribution of dykes and related melt migration structures in the mantle section of the Oman ophiolite. Geochemistry Geophysics Geosystems, 4, 8612.
•  Python,  M.,  Ceuleneer,  G. and  Arai,  S. (2008) Chromian spinels in mafic-ultramafic mantle dykes: Evidence for a two-stage melt production during the evolution of the Oman ophiolite. Lithos, 106, 137-154.
•  Rioux,  M.,  Garber,  J.M.,  Searle,  M.,  Kelemen,  P., et al (2021) High-precision U-Pb zircon dating of late magmatism in the Samail ophiolite: A record of subduction initiation. Journal of Geophysical Research: Solid Earth, 126, e2020JB020758.
•  Salters,  V.J.M. and  Dick,  H.J.B. (2002) Mineralogy of the mid-ocean-ridge basalt source from neodymium isotopic composition of abyssal peridotites. Nature, 418, 68-72.
•  Seyer,  M. and  Bonatti,  E. (1994) Na, Al^IV and Al^VI in clinopyroxenes of subcontinental and suboceanic ridge peridotites: A clue to different melting processes in the mantle? Earth and Planetary Science Letters, 122, 281-289.
•  Shibata,  T. and  Yoshikawa,  M. (2004) Precise isotopic determination of trace amounts of Nd in magnesium-rich samples. Journal of Mass Spectrometry, 52, 317-324.
•  Snow,  J.E.,  Hart,  S.R. and  Dick,  H.J.B. (1994) Nd and Sr isotope evidence linking mid-ocean ridge basalts and abyssal peridotites. Nature, 371, 57-60.
• Sun, S.S. and McDonough, W.F. (1989) Chemical and isotopic systematics of oceanic basalts: Implications for mantle composition and processes. In Magmatism in the Ocean Basins (Saunders, A.D. and Norry, M.J. Eds.). Geological Society, London, Special Publications, 42, 313-345.
•  Takazawa,  E.,  Okayasu,  T. and  Satoh,  K. (2003) Geochemistry and origin of the basal lherzolites from the northern Oman ophiolite (northern Fizh block). Geochemistry, Geophysics, Geosystems, 4.
•  Umino,  S. (2012) Emplacement mechanism of off-axis large submarine lava field from the Oman Ophiolite. Journal of Geophysical Research, 117, B11210.
•  White,  W.M. and  Hofmann,  A.W. (1982) Sr and Nd isotope geochemistry of oceanic basalts and mantle evolution. Nature, 296, 821-825.
•  Workman,  R.K. and  Hart,  S.R. (2005) Major and trace element composition of the depleted MORB mantle (DMM). Earth and Planetary Science Letters, 231, 53-72.
•  Yoshikawa,  M. and  Nakamura,  E. (2000) Geochemical evolution of the Horoman peridotite complex: Implications for melt extraction, metasomatism and compositional layering in the mantle. Journal of Geophysical Research, 105, 2879-2901.
•  Yoshikawa,  M.,  Python,  M.,  Tamura,  A.,  Arai,  S., et al (2015) Melt extraction and metasomatism recorded in basal peridotites above the metamorphic sole of the northern Fizh massif, Oman ophiolite. Tectonophysics, 650, 53-64.