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Article

Diamonds Discovered in the Forearc Harzburgites Hint at the Deep Mantle Source of the Skenderbeu Massif, Western Mirdita Ophiolite

by
Weiwei Wu
1,2,*,
Jingsui Yang
2,
Yu Yang
3,*,
Ibrahim Milushi
4 and
Yun Wang
2
1
Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China
2
School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China
3
SinoProbe Lab, Chinese Academy of Geological Sciences, Beijing 100037, China
4
Institute of Geosciences, Energy, Water and Environment, Polytechnic University of Tirana, 1000 Tirana, Albania
*
Authors to whom correspondence should be addressed.
Minerals 2024, 14(1), 34; https://doi.org/10.3390/min14010034
Submission received: 16 November 2023 / Revised: 21 December 2023 / Accepted: 26 December 2023 / Published: 28 December 2023
(This article belongs to the Section Mineral Deposits)
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Abstract

:
The ultra-deep genesis of ophiolitic peridotite has reshaped our perception of the genesis of the oceanic mantle. Although ultra-high pressure (UHP) mineral assemblages have been unearthed in dozens of ophiolites in different orogenic belts around the world, the vast majority of them have been limited to podiform chromitites formed in suprasubduction zone (SSZ) settings, leaving uncertainty about whether such UHP minerals are intrinsic to the oceanic mantle or influenced by a specific mantle rock type. Here, we report on the occurrence of diamonds recovered from the harzburgites within the Skenderbeu massif, Mirdita ophiolite. The whole-rock, mineralogical major and trace element compositions, and redox states of the harzburgites align with modern abyssal harzburgites. Trace element modeling of clinopyroxene indicates that harzburgites have endured varying degrees of garnet-facies melting (~2%–5%) before progressing to spinel-facies melting (~10%–12%). Mineralogical characteristics further support that the Skenderbeu harzburgites underwent late-period MORB-like melt metasomatism in a forearc spreading center. An unusual mineral assemblage of diamonds has been separated from the studied harzburgites. The first occurrence of ophiolite-hosted diamonds discovered in the forearc harzburgites, together with previous similar discoveries in the SSZ ophiolitic chromitites, suggest that the ophiolite-hosted diamonds are not specific to certain mantle rocks.

1. Introduction

Ophiolitic mantle has long been considered to represent the remnants of ancient oceanic asthenosphere mantle undergoing various extents of partial melting and/or melt-rock reactions and can provide valuable documentation of the early stage of the Wilson Cycle [1,2,3,4]. They have been commonly interpreted to have originated at relatively shallow lithospheric depths [2,3] and exhibit homogeneity characteristics because the asthenosphere has been widely believed to be homogenized, resulting from effective mantle convection [5]. However, the shallow origin and the homogeneity of the residual ophiolitic mantle have been hindered by the substantial proof for the global widespread presence of ultra-high pressure (UHP, i.e., diamond, coesite) and super-reduced mineral assemblage (e.g., native metals and alloys) in podiform chromitites and their host peridotitic rocks [6,7,8,9,10,11,12,13]. Additionally, isotopic and geochemical evidence for mantle heterogeneity has also been detected in oceanic volcanic rocks, mantle xenoliths, and modern abyssal peridotites [14,15,16].
A wide range of UHP phases have been found in ophiolites; nevertheless, diamonds are of the most interest and aroused the most controversy. It has been four decades since the first reports of microdiamonds in chromitites from the Luobusa ophiolite [17] along the Yarlung–Zangbo suture zone in Xizang, China. Subsequently, ophiolite-hosted microdiamonds have been identified amongst more than 14 ophiolitic mantle rocks, including chromitites and peridotites within different orogenic belts worldwide [9]. However, the vast majority of diamond-bearing ophiolitic mantle rocks have been limited to podiform chromitites formed in suprasubduction zone (SSZ) settings (the late stage of the subduction initiation process), leaving uncertainty about whether such UHP minerals are intrinsic to the oceanic mantle or influenced by a specific mantle rock type.
Compared to diamonds commonly discovered from ophiolitic podiform chromitites, equally important but not as well studied are diamonds exposed in oceanic mantle peridotites. Here, we initially present the diamonds, combined with new petrography, textural, whole-rock, and mineral geochemical data of the host ophiolitic peridotites from the Skenderbeu massif within the western Mirdita ophiolite, northern Albania. The study presents a chance to further understand the nature of the oceanic mantle.

2. Geological Background

The Mirdita ophiolite, located in northern Albania, represents the relic of the Jurassic Pindos–Mirdita ocean basin [18]. The ophiolite usually be divided into two subunits, the Eastern (EMO) and Western (WMO) Mirdita ophiolite (Figure 1), based on their diagnostic stratigraphy and geochemical characteristics [19,20,21,22]. The lithology of WMO is thin, ~3 km thick, and mainly composed of relatively fertile mantle rocks (i.e., lherzolite-harzburgite, plagioclase lherzolite/harzburgite), plutonic intrusions, and extrusive volcanic rocks, whereas the EMO exhibits a more complete ophiolite pseudostratigraphy with a thickness of 10–12 km, and is mainly composed of relatively refractory mantle rocks (i.e., harzburgite, dunite), ultramafic cumulates, plutonic rocks, sheeted dikes, and pillow lavas [18]. The volcanic rocks of WMO have a mid-ocean ridge (MOR) affinity, which is typical of MOR magmatism. In contrast, the volcanic rocks of EMO show an island arc tholeiite (IAT) signature, which is typical of a suprasubduction zone (SSZ) origin [19,20,21]. Different geodynamic models have been proposed to reconcile the contemporaneous existence of both MOR-type and SSZ-type geochemical affinity within a single ophiolite, such as the two subunit ophiolites formed in separate and discrete tectonic settings [22], subduction established near an active MOR within a same ocean basin [19], and westward-inclined intra-oceanic subduction followed by swift slab rollback [21]. According to the spatial—temporal relationships between mantle and crustal rocks and the time-advancing magmatic evolution of WMO and EMO subunits, a forearc geodynamic model caused by subduction initiation processes is widely accepted [22,23,24]. Recent discoveries of microdiamonds and two-pyroxene, intergrowth exsolution lamellae in podiform chromitites from both the WMO and EMO massifs infer a deep mantle genesis of the Mirdita ophiolite [25,26,27].
The studied Skenderbeu massif belongs to the WMO (Figure 2). On the west, the massif is juxtaposed against the pre-Apulian platform (Triassic-Jurassic carbonate rocks) along the WNW fault system [18], and on the east, it is unconformably in contact with the Burreli basin, which is composed of Cenozoic terrestrial sedimentary deposits (Figure 2) [24]. The mantle section consists of major lherzolite-harzburgite and minor dunite associated with podiform chromitite and is exposed on the west part of the massif (Figure 2).
Foliated harzburgites are located at the basal part of the mantle section and are locally invaded by pyroxenite veins (Figure 3). The studied harzburgites generally show protogranular and porphyroclastic textures with various degrees of serpentinization effects (Figure 3a,b). The main rock-forming minerals are olivine (70–85 vol.%), orthopyroxene (14–22 vol.%), clinopyroxene (2–4 vol.%), and sparse chromium spinel (Figure 3b). Olivines occur as granular grains with a diameter of 0.1–0.5 mm. Coarse orthopyroxene (Opx) crystals (1–6 mm in size) can contain lamellae or blebs of clinopyroxene (Cpx) and spinel (Spl) (Figure 3c,d). Cpx is shown as fine interstitial grains (<1 mm) or as exsolution lamellae in orthopyroxene (Figure 3). Spls usually exhibit as anhedral grains (0.2–1 mm) in the matrix (Figure 3a).

3. Analytical Procedures

Whole-rock major and trace element analysis was conducted at the China University of Geosciences (Wuhan, China) and the SampleSolution Analytical Technology Co., Ltd. (Wuhan, China). See [24] for full analytical procedures. Trace element concentrations of rock-forming mineral Cpx were determined by a GeoLasPro laser-ablation system (Lamda Physik, Gottingen, Germany) coupled to an Agilent 7700× ICP-MS. Each analysis was performed by ablating 40 μm diameter spots at 6 Hz with an energy of ~100 mJ per pulse for 45 s after measuring the gas blank for 20 s. Seven standard reference materials, NIST610, NIST612, BIR-1G, BHVO-2G, BCR-2G, CGSG-1, and CGSG-2, were used as external standards to plot calibration curves. One material, QC-BCR-2G, was used for data quality control. Every fifteenth analysis was followed by five analyses of reference materials (2 analyses of NIST 610, 1 analysis of NIST612, CGSG-1, and CGSG-2) to monitor the time-dependent calibration for sensitivity drift.
The selected constituent crystals in the Skenderbeu harzburgites were analyzed at the East China University of Technology (Nanchang, China) and the Nanjing University (Nanjing, China) by the electron microprobe analyzer (EMPA). During analysis, the operating conditions are set as accelerating voltage 15 kV, beam current 20 nA, beam spot diameter 5 μm, 10 s counting time for peak, and 5 s counting time for upper and lower background per element. Microprobe analytical standards are diopside for Si and Ca, corundum for Al, rutile for Ti, chromium oxide for Cr, hematite for Fe, rhodonite for Mn, olivine for Mg, albite for Na, orthoclase for K, and nickel oxide for Ni. Detection limits for the major elements are <160 ppm, and the analytical uncertainties are generally better than 2%.
Nearly 500 kg of unaltered harzburgite rocks were used for initial mineral separation by the Institute of Multipurpose Utilization of Mineral Resources, Chinese Academy of Geological Sciences (Zhengzhou, China). See [28] for full analytical procedures.

4. Results

4.1. Whole-Rock Major and Trace Element Composition Analyses

The analyzed harzburgites samples are Mg-rich (43.17–44.10 wt.%) and Al-poor (1.06–1.53 wt.%) in bulk major element compositions (Table S1). According to the MgO/SiO2 vs. Al2O3/SiO2 plot (Figure 4a), the Skenderbeu harzburgites follow the trend of the terrestrial mantle array. We also have displayed data of typical forearc, abyssal peridotites, and harzburgites from different massifs in the Mirdita ophiolite in Figure 4 for comparison. The Al2O3 and Y contents of the Skenderbeu harzburgites show negative correlations with MgO contents (Figure 4). In detail, it is noticeable that the Skenderbeu harzburgites are similar to abyssal harzburgites formed in the mid-ocean ridge and similar to Gomsiqe harzburgites formed in the forearc spreading center [24] but distinguishable from the Bulqiza harzburgites that formed in a suprasubduction zone mantle wedge [29].

4.2. Mineral Chemistry of the Skenderbeu Harzburgites

4.2.1. Olivine (Ol)

All analyzed Ol crystals are highly magnesian (i.e., Fo > 90.0; Table S2). Specifically, the olivines within Skenderbeu harzburgites display a comparable range of Fo (90.0–91.0) and NiO contents (0.31–0.42 wt.%) in relation to those of abyssal harzburgites and Gomsiqe harzburgites (Figure 5a) [24]. However, they manifest lower Fo and NiO contents than those of the olivine minerals from IBM forearc harzburgites and Bulqiza harzburgites (Figure 5a).

4.2.2. Orthopyroxene (Opx)

Porphyroclastic Opxs in the Skenderbeu harzburgites show similar contents of Al2O3 compared with those of global abyssal harzburgites [32] and Gomsiqe harzburgites from WMO [24] and have a higher range of Al2O3 than those of Opxs from IBM forearc peridotites [30,31,33] and Bulqiza harzburgites [29] (Table S3, Figure 5b).

4.2.3. Clinopyroxene (Cpx)

Cpx grains in Skenderbeu harzburgites are more similar (Table S4, Figure 5c,d) to those from global abyssal harzburgites [32], and Gomsiqe harzburgites, Mirdita ophiolite [24], the latter of which formed in the forearc spreading center. Furthermore, Cpx in Skenderbeu harzburgites are compositionally distinguishable compared to those of Cpx from IBM forearc peridotites [30,31,33] and Bulqiza harzburgites, Mirdita ophiolite [29].
Overall, Cpx from Skenderbeu harzburgites display chondritic to sub-chondritic rare earth elements (REE) abundances (Figure 6; Table S6). The REE abundances of Skenderbeu harzburgitic Cpxs are much lower than those of the DMM source and the average abundance of those from abyssal harzburgites [32]. Chondrite-normalized REE patterns of Skenderbeu harzburgitic Cpxs are consistent, i.e., depletion in light rare earth elements (LREE) and relatively flat middle-heavy rare earth elements (M-HREE) (Figure 6). These REE patterns are analogous to those of DMM Cpxs after a 10–12% fractional melting [34] (Figure 6).

4.2.4. Spinel (Spl)

All spinels from the Skenderbeu harzburgites are high-Al type (Cr# (100 × Cr/(Cr + Al) molar) < 60) (Table S5). The Skenderbeu harzburgites spinel compositions resemble those of harzburgitic Spl formed in MOR [34] and from Gomsiqe massif, Mirdita ophiolite [24]; however, they are quite different from those of Bulqiza harzburgites, Mirdita ophiolite [29], and those of the IBM forearc peridotitic spinels (Figure 7a). The compositions of Spl grains from Skenderbeu harzburgites are similar to the spinels crystallized within MORB (Figure 7c) but easily distinguishable from the spinels crystallized within arc-related volcanic rocks (ARC) [36].

4.3. Diamonds Recovered from the Skenderbeu Harzburgites

Unusual mineral assemblages, such as diamond and moissanite, have been discovered from high-Cr type podiform chromitites of the Skenderbeu massif, Mirdita ophiolite [26]. Here, we present diamonds from the Skenderbeu harzburgites, which is the host rock of the high-Cr type podiform chromitites. To date, 11 diamond grains have been identified from the Skenderbeu harzburgites (Figure 8). They are mostly light yellow, transparent, subhedral-anhedral crystals, 100–200 μm across. All the diamonds have been confirmed by Raman with a peak at ~1326 cm–1 (Figure 8b). The morphologic characteristics of the Skenderbeu harzburgitic diamonds are analogous to those in high-Cr-type podiform chromitites [26].

5. Discussion

5.1. Formation Processes of the Skenderbeu Harzburgite

All the harzburgites have very low Cpx contents (<3 vol%). The Skenderbeu harzburgites have Al2O3/MgO values similar to those of abyssal harzburgites (Figure 4) and to those of well-studied Gomsiqe forearc harzburgites in the same ophiolite belt [24], indicating their roughly equivalent refractory level. In addition, the studied Skenderbeu samples mostly display similar or lower Al2O3 (Figure 4b) and Y (Figure 4c) contents at a given MgO compared to those of abyssal harzburgites.
To reveal primary mantle characteristics, only the cores of porphyroclastic minerals that were least affected by secondary processes (i.e., sub-solidus cooling, low-T alteration) were analyzed [32]. The amount of Al2O3 in porphyroclastic Cpx minerals, which are commonly perceived as an indicator for the degree of partial melting of host rocks, is reduced with increasing Mg# values (Figure 5c,d) [32]. On the other hand, the melting processes of mantle peridotites can be inferred by the partial melting model calculations using middle-heavy rare earth elements (M-HREE) in residual clinopyroxenes [37]. In detail, the abundance and relatively flat patterns of M-HREE of Skenderbeu harzburgitic Cpx suggest that they underwent ~10–12% fractional melting of DMM within the spinel stability field [34]. This inference is also supported by Dy vs. Ti covariation of Cpx [38]. In Figure 9a, the Skenderbeu harzburgites exhibit a slightly higher degree of anhydrous partial melting than the Gomsiqe forearc harzburgites, and both of them fall in the region of abyssal harzburgites.
The Cr# values of Spl within mantle peridotites are anticipated to increase with a higher degree of partial melting (Figure 7). Consequently, by employing the empirical equation [F = 10 × ln (Cr#spinel) + 24, F being the degree of melting in percent] as suggested by [39], it can be ascertained that the Skenderbeu harzburgites have undergone a fractional melting of 15%, derived from a depleted mantle source, which is in line with the Cpx geochemistry.
In contrast, the examined Skenderbeu samples exhibit distinctive patterns of higher HREE concentrations and lower MREE to HREE ratios (Figure 6) when compared to the typical composition of abyssal harzburgites and the modeled trajectory of fractional melting [35]. These variations suggest the potential presence of residual garnet during the initial melting phase [35,40,41]. As illustrated in Figure 9b, a significant portion of the Skenderbeu harzburgites lies below the trajectory predicted by fractional melting alone in the spinel facies. This observation leads to the inference that the Skenderbeu harzburgites experienced garnet-facies melting in the range of 2–5% before progressing to 10–12% spinel-facies melting. This pattern mirrors findings in abyssal peridotites from the Mid-Atlantic Ridge (MAR) [42], ophiolitic peridotites from Xizang, China (e.g., Purang and Xigaze) [41], and Oman [43]. In these cases, the sequence of initial garnet-facies melting followed by subsequent spinel-facies melting has been proposed as the explanatory mechanism [35,37].
The oxygen fugacity (fo2) for the Skenderbeu mantle harzburgites was calculated utilizing the coexistence of olivine and spinel as key indicators (Table S7) [44]:
6Fe2SiO4 (olivine) + O2 (fluid) = 3Fe2Si2O6 (orthopyroxene)+2Fe3O4 (spinel).
The fo2 of the examined harzburgites (−1.52 < FMQ < −0.01) align closely with those of mid-ocean ridge oceanic harzburgite mantle (Figure 10) [45,46] and the Gomsiqe harzburgites, which are formed during the early phase of the subduction initiation process of the Neo-Tethyan Mirdita oceanic basin [24]. In addition, there is a distinct difference in terms of oxygen fugacity between Skenderbeu harzburgites and Bulqiza harzburgites, the latter of which is thought to have formed in a suprasubduction zone (SSZ) setting [29], i.e., the late stage of the subduction initiation process of the Neo-Tethyan Mirdita basin [47].
Melt-rock interaction stands out as a crucial geological process documented in abyssal peridotites, playing a significant role in generating mantle heterogeneity [32]. In the case of the Gomsiqe samples, the positive correlations observed between Al and Cr in the porphyroclastic Cpx minerals (Figure 5d) serve as robust chemical indicators for metasomatism [48]. Moreover, the spinels found in harzburgites share a similar composition with those crystallized in Mid-Ocean Ridge Basalts (MORB) (Figure 7c). They also exhibit higher TiO2 contents consistent with spinels in Gomsiqe harzburgites and the reacted abyssal harzburgites (Figure 7b). Notably, these spinels deviate from the partial melting trend (Figure 7b). Consequently, both the compositions of clinopyroxene (Cpx) and spinel (Spl) suggest that Skenderbeu harzburgites underwent equilibration with melts akin to forearc basalts (MORB-like) (Figure 5 and Figure 7).
Various mechanisms have been suggested to explain the tectonic setting of the Mirdita ophiolite. Among these, a tectonic setting characterized by forearc conditions, followed by a subduction initiation process, is considered to be the least resistant hypothesis [22,24]. The similarity between the Skenderbeu harzburgites and the Gomsiqe forearc harzburgites in terms of bulk and mineralogical geochemistry (Figure 4, Figure 5, Figure 6, Figure 7 and Figure 9) and physical condition (Figure 10) prompts us to posit that the Skenderbeu harzburgites serve as a record of the early stage of the subduction initiation processes.

5.2. Significance of the Diamond Discovered in the Skenderbeu Harzburgite

Ophiolitic peridotites and the accompanying podiform chromitites have traditionally been attributed to formation processes occurring at the uppermost mantle level (<60 km) sustained by various extents of partial melting and mantle-melt interaction [3,49,50,51]. However, the ultra-high pressure (UHP) minerals recovered from global ophiolites over the past decades indicate that the oceanic mantle is much more heterogeneous than previously thought and can preserve deep mantle information [7,8,9,26,27]. Consequently, multiple scenarios have been postulated to rationalize the occurrences of these atypical minerals, encompassing (a) the mantle plume model [9], (b) the deep subduction and rapid exhumation model [13,26,51], and (c) the slab breakoff and contamination model [10]. In addition, the shallow formation model [52], lightning strikes model [53], and anthropogenic contamination [54] have also been proposed to explain the aforementioned unexpected findings. We posit a deep mantle origin for ultra-high pressure (UHP) minerals, such as diamonds, in ophiolites, supported by several lines of evidence: (1) the widespread occurrence of these UHP minerals in diverse ophiolites globally [9]; (2) the presence of in-situ diamonds within ophiolitic chromitites [8] and peridotites [55]; (3) the isotopic compositions and trace element concentrations of ophiolitic diamonds [11]; (4) the exclusive occurrence of mineral inclusions, such as IR-active water and silicate fluid inclusions, in natural diamonds [56]; and (5) the accumulating in-situ petrological and mineralogical evidence from both ophiolitic peridotites and podiform chromitites, including majoritic garnets in Purang ophiolitic peridotites [57], microstructural features of Luobusa podiform chromitites [58], and mineral lamellae of coesite, diopside, and Ca-amphibole in various ophiolitic podiform chromitites [7,27,59]. Nevertheless, the geological constraints on the formation processes of diamonds hosted in ophiolites remain inadequately defined, which is worthy of further study.
Drawing from our own investigation and observations, the diamonds unearthed in this study exhibit morphological features akin to globally identified ophiolite-hosted diamonds and those found in the podiform chromitites within the same Skenderbeu massif [26]. Wu et al. (2019) [26] conducted a comprehensive study on the ophiolitic diamonds from the Skenderbeu chromitites. Because Mn-nodules and sediments (also containing Co and Ni elements) have been recognized at the ocean bottom [60], where they may have been carried deep into the mantle along with ancient subduction. Mn-Ni-Co-bearing materials start to melt under the mantle condition and can also dissolve large amounts of carbon derived from a subducted slab [61,62,63,64,65]. During rapid mantle upwelling, diamonds possibly nucleate and grow from such a carbon-saturated, NiMnCo-rich melt in the deep mantle [66], particularly within the diamond stability field, possibly near the top of the mantle transition zone [26]. The recent discovery of two-pyroxene intergrowth exsolution lamellae in the chromites provides additional compelling evidence supporting the deep mantle origin of the Skenderbeu massif [27]. Consequently, the diamonds reported in this study and those previously documented within the same ophiolite likely share a common origin, indicating the involvement of relatively deep mantle processes in the formation of the Skenderbeu forearc harzburgites.
Here, we follow the geodynamic model proposed by [21,22,24], i.e., the Mirdita ophiolite formed in the forearc tectonic setting due to the intra-oceanic subduction initiation process of the Neo-Tethys Mirdita Ocean. During and after the subduction initiation process, rapid slab rollback triggered large-scale passive upwelling, which will rapidly evolve into a self-maintaining, low-viscosity channel [12]. This channel can promote the upwelling of deep mantle material, i.e., the diamonds discovered in this research, to shallow mantle depth. Subsequently, the diamonds were overgrown by newly formed chromite [8] or enstatite [55] crystals and thus protected. These rock-forming minerals may be incorporated into the peridotites as part of the Skenderbeu massif and eventually emplaced on land due to the closure of the Mirdita Ocean in the late Jurassic [21].

6. Conclusions

Field relationships and textural characteristics, combined with comprehensive whole-rock and mineralogical geochemical data, provide support for the assertion that the Skenderbeu massif within the WMO comprises fragments of oceanic lithosphere. These fragments were positioned subsequent to the closure of the Neo-Tethys Mirdita Ocean during the initial stages of the subduction initiation process. It is plausible that this subduction initiation process is also responsible for the preservation of microdiamonds and pyroxene exsolution lamellae, as documented in both the Skenderbeu and Bulqiza massifs within the Mirdita ophiolite. The initial detection of ophiolite-hosted diamonds in the present Skenderbeu forearc harzburgites, coupled with previous discoveries in the SSZ ophiolitic chromitites, implies that ophiolite-hosted diamonds are intrinsic to the oceanic mantle. Our research provides further evidence that it is highly heterogeneous in composition and origin of the oceanic mantle.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/min14010034/s1, Table S1: Whole-rock major and trace element compositions of the representative Skenderbeu harzburgites; Table S2: Electron microprobe analyses of typical olivine in the Skenderbeu harzburgites; Table S3: Electron microprobe analyses of typical orthopyroxene in the Skenderbeu harzburgites; Table S4: Electron microprobe analyses of typical clinopyroxene in the Skenderbeu harzburgites; Table S5: Electron microprobe analyses of typical spinel in the Skenderbeu harzburgites; Table S6: Trace element concentrations of typical clinopyroxene in the Skenderbeu harzburgite; Table S7: Calculated oxygen fugacity for the Skenderbeu harzburgites.

Author Contributions

Conceptualization, W.W. and Y.Y.; methodology, W.W.; formal analysis, W.W., Y.Y. and Y.W.; investigation, W.W. and I.M.; data curation, W.W., Y.Y. and Y.W.; writing—original draft preparation, W.W.; writing—review and editing, W.W., J.Y., Y.Y., I.M. and Y.W.; visualization, W.W.; supervision, J.Y.; project administration, W.W. and J.Y.; funding acquisition, W.W., J.Y. and Y.Y. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Program for Postdoctoral Researcher (GZC20232633), the National Natural Science Foundation of China (42330306, 92062215), the Key Research Program of the Institute of Geology & Geophysics, CAS (IGGCAS-202202), and the National Natural Science Foundation of China (42102248, 41972209, 42172264, 41802055).

Data Availability Statement

Data are contained within the article and supplementary materials.

Acknowledgments

We express our sincere appreciation for the logistical assistance rendered by colleagues at Tirana Polytechnic University of Albania during our fieldwork in the country. Additionally, we extend our gratitude to the Director of the Geological Survey of Albania for consistently supporting our studies across various ophiolite massifs in Albania throughout the years.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. (a) Simplified map of the Mediterranean region with the main Tethyan ophiolites [24]. Lettering for major tectonic features: DSF = Dead Sea Fault; BZCZ = Bitlis–Zagros Collision Zone; EAF = East Anatolian Fault; NAF = North Anatolian Fault. (b) Simplified geological map of the Mirdita ophiolite, showing major peridotite massifs [24].
Figure 1. (a) Simplified map of the Mediterranean region with the main Tethyan ophiolites [24]. Lettering for major tectonic features: DSF = Dead Sea Fault; BZCZ = Bitlis–Zagros Collision Zone; EAF = East Anatolian Fault; NAF = North Anatolian Fault. (b) Simplified geological map of the Mirdita ophiolite, showing major peridotite massifs [24].
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Figure 2. Simplified geological map of the Skenderbeu massif, western Mirdita ophiolite [27].
Figure 2. Simplified geological map of the Skenderbeu massif, western Mirdita ophiolite [27].
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Figure 3. Field photos and micrographs of foliated harzburgites of the Skenderbeu massif. (a) Harzburgite showing clear lineation of pyroxenes. Foliated harzburgites are locally invaded by pyroxenite veins; (b) Harzburgite (cross-polarized light) with protogranular and porphyroclastic texture; (c,d) Harzburgite (black scattered electron image, BSE). Lamellae or blebs of Cpx and Spl observed within Opx porphyroclasts.
Figure 3. Field photos and micrographs of foliated harzburgites of the Skenderbeu massif. (a) Harzburgite showing clear lineation of pyroxenes. Foliated harzburgites are locally invaded by pyroxenite veins; (b) Harzburgite (cross-polarized light) with protogranular and porphyroclastic texture; (c,d) Harzburgite (black scattered electron image, BSE). Lamellae or blebs of Cpx and Spl observed within Opx porphyroclasts.
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Figure 4. Whole-rock major oxide and trace element variations of the Skenderbeu harzburgites. (a) MgO/SiO2 vs. Al2O3/SiO2. (b) MgO (wt.%) vs. Al2O3 (wt.%). (c) MgO (wt.%) vs. Y (ppm). Data of abyssal lherzolites (hereafter referred to as Abyssal-Lz) and abyssal harzburgites (hereafter referred to as Abyssal-Hz), and Izu-Bonin-Mariana (IBM) forearc peridotites (hereafter referred to as Forearc peri.) [30,31] are shown for comparison. Data of Gomsiqe harzburgites are from [24]. Data of Bulqiza harzburgites are from [29].
Figure 4. Whole-rock major oxide and trace element variations of the Skenderbeu harzburgites. (a) MgO/SiO2 vs. Al2O3/SiO2. (b) MgO (wt.%) vs. Al2O3 (wt.%). (c) MgO (wt.%) vs. Y (ppm). Data of abyssal lherzolites (hereafter referred to as Abyssal-Lz) and abyssal harzburgites (hereafter referred to as Abyssal-Hz), and Izu-Bonin-Mariana (IBM) forearc peridotites (hereafter referred to as Forearc peri.) [30,31] are shown for comparison. Data of Gomsiqe harzburgites are from [24]. Data of Bulqiza harzburgites are from [29].
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Figure 5. Compositional variations of rock-forming minerals from the Skenderbeu harzburgites. Ol: Fo vs. NiO (wt.%) (a). Opx: Mg# vs. Al2O3 (wt.%) (b). Cpx: Mg# vs. Al2O3 (wt.%) (c) and Al2O3 (wt.%) vs. Cr2O3 (wt.%) (d). Reference ranges of abyssal lherzolite (Abyssal Lz) and abyssal harzburgite (Abyssal Hz) [32], IBM forearc peridotites (Forearc peri.) [30,31,33], Mirdita ophiolite [24,29], and DMM [34] are shown for comparison.
Figure 5. Compositional variations of rock-forming minerals from the Skenderbeu harzburgites. Ol: Fo vs. NiO (wt.%) (a). Opx: Mg# vs. Al2O3 (wt.%) (b). Cpx: Mg# vs. Al2O3 (wt.%) (c) and Al2O3 (wt.%) vs. Cr2O3 (wt.%) (d). Reference ranges of abyssal lherzolite (Abyssal Lz) and abyssal harzburgite (Abyssal Hz) [32], IBM forearc peridotites (Forearc peri.) [30,31,33], Mirdita ophiolite [24,29], and DMM [34] are shown for comparison.
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Figure 6. Chondrite-normalized REE patterns of Cpx minerals from the Skenderbeu harzburgites. Average chondrite-normalized REE patterns of the abyssal lherzolitic and harzburgitic Cpxs are marked as blue and purple solid lines in Figure 6, respectively. REE patterns of Cpxs sustained various extents of fractional melting from DMM are provided for comparison [34,35].
Figure 6. Chondrite-normalized REE patterns of Cpx minerals from the Skenderbeu harzburgites. Average chondrite-normalized REE patterns of the abyssal lherzolitic and harzburgitic Cpxs are marked as blue and purple solid lines in Figure 6, respectively. REE patterns of Cpxs sustained various extents of fractional melting from DMM are provided for comparison [34,35].
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Figure 7. Variations of compositions of spinel from the Skenderbeu harzburgites. (a) Mg# vs. Cr#. (b) TiO2 (wt.%) vs. Cr#. (c) Al2O3 (wt.%) vs. TiO2 (wt.%). Reference compositional ranges of abyssal and forearc peridotites and harzburgites from different massifs in Mirdita ophiolite are from the same studies cited in Figure 4. Reference values of Spl from the depleted MORB mantle (hereafter referred to as DMM) are provided by [34]. In Figure 7c, the compositional range of spinel crystalized within MORB and arc-related volcanic rocks (ARC) is given by [36].
Figure 7. Variations of compositions of spinel from the Skenderbeu harzburgites. (a) Mg# vs. Cr#. (b) TiO2 (wt.%) vs. Cr#. (c) Al2O3 (wt.%) vs. TiO2 (wt.%). Reference compositional ranges of abyssal and forearc peridotites and harzburgites from different massifs in Mirdita ophiolite are from the same studies cited in Figure 4. Reference values of Spl from the depleted MORB mantle (hereafter referred to as DMM) are provided by [34]. In Figure 7c, the compositional range of spinel crystalized within MORB and arc-related volcanic rocks (ARC) is given by [36].
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Figure 8. Illustrations depicting diamonds found within the Skenderbeu harzburgites. (a) Microphotograph revealing an abundance of light yellow to yellow diamonds; (b) Raman spectrogram displaying the characteristic Raman shift of diamonds around 1326 cm−1.
Figure 8. Illustrations depicting diamonds found within the Skenderbeu harzburgites. (a) Microphotograph revealing an abundance of light yellow to yellow diamonds; (b) Raman spectrogram displaying the characteristic Raman shift of diamonds around 1326 cm−1.
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Figure 9. Dy (ppm) vs. Ti (ppm) (a) and chondrite-normalized (Sm/Yb)N vs. (Yb)N (b) diagrams for Cpxs in the Skenderbeu harzburgites. Reference ranges of abyssal lherzolite (Abyssal Lz) and abyssal harzburgite (Abyssal Hz) [32], IBM forearc peridotites (Forearc peri.) [30,31,33], Mirdita ophiolite [24,29], and DMM [34] are shown for comparison.
Figure 9. Dy (ppm) vs. Ti (ppm) (a) and chondrite-normalized (Sm/Yb)N vs. (Yb)N (b) diagrams for Cpxs in the Skenderbeu harzburgites. Reference ranges of abyssal lherzolite (Abyssal Lz) and abyssal harzburgite (Abyssal Hz) [32], IBM forearc peridotites (Forearc peri.) [30,31,33], Mirdita ophiolite [24,29], and DMM [34] are shown for comparison.
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Figure 10. Variations of Cr# in Spl vs. Δlogfo2 from the Skenderbeu harzburgites, Mirdita ophiolite. The discrimination boundaries are obtained from [46]. Data sources of Mirdita ophiolite are the same as in Figure 4. Abbreviations: BON-boninite; IAT-island arc tholeiite.
Figure 10. Variations of Cr# in Spl vs. Δlogfo2 from the Skenderbeu harzburgites, Mirdita ophiolite. The discrimination boundaries are obtained from [46]. Data sources of Mirdita ophiolite are the same as in Figure 4. Abbreviations: BON-boninite; IAT-island arc tholeiite.
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Wu, W.; Yang, J.; Yang, Y.; Milushi, I.; Wang, Y. Diamonds Discovered in the Forearc Harzburgites Hint at the Deep Mantle Source of the Skenderbeu Massif, Western Mirdita Ophiolite. Minerals 2024, 14, 34. https://doi.org/10.3390/min14010034

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Wu W, Yang J, Yang Y, Milushi I, Wang Y. Diamonds Discovered in the Forearc Harzburgites Hint at the Deep Mantle Source of the Skenderbeu Massif, Western Mirdita Ophiolite. Minerals. 2024; 14(1):34. https://doi.org/10.3390/min14010034

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Wu, Weiwei, Jingsui Yang, Yu Yang, Ibrahim Milushi, and Yun Wang. 2024. "Diamonds Discovered in the Forearc Harzburgites Hint at the Deep Mantle Source of the Skenderbeu Massif, Western Mirdita Ophiolite" Minerals 14, no. 1: 34. https://doi.org/10.3390/min14010034

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