Fluid-Induced Inhomogeneous Cr-spinel in Dunite and Wehrlite from the Duke Island Complex, Southeastern Alaska

Abstract

Cr-spinel [(Mg, Fe²⁺)(Cr, Al, Fe³⁺)₂O₄)] is a common mineral in the ultramafic core of the Duke Island complex in southeastern Alaska, US. Cr-spinel grains with an unmixed texture have been observed in dunite and wehrlite of the complex. Inhomogeneous Cr-spinel with a ratio of Cr/(Al + Cr + Fe³⁺) <0.37 is prominent in dunite. The inhomogeneous Cr-spinel consists of two completely different compositions: Al-rich Cr-spinel, and Fe³⁺-rich Cr-spinel with a wide range of Cr content (from 11.8 wt.% to 28.6 wt.% Cr₂O₃). The unmixed texture is complex, and three subtypes of inhomogeneous Cr-spinel are recognized: Type B₁ Cr-spinel showing complete separation, crystallographically oriented type B₂ Cr-spinel, and irregular Al-rich Cr-spinel rimmed type B₃ Cr-spinel. The unmixed texture was achieved by an unmixing process at around 600 °C due to the miscibility gap of spinel between Al-rich and Fe³⁺-rich phases. The unmixed patterns of inhomogeneous Cr-spinel are controlled by the initial chemical composition, grain size of the initial spinel, and the cooling process. We propose that the initial composition of inhomogeneous Cr-spinel was formed by the interaction of high-temperature fluid and olivine; Cr-spinel that experienced unmixing may be a useful proxy to unveil the activity of high-temperature fluid in the formation of Alaskan-type complexes.

1. Introduction

The spinel group has the general formula A²⁺B³⁺₂ O₄, where A may be Mg, Fe²⁺, Zn, Mn, Ni, Co, Cu, or Ge, and B may include Al, Fe³⁺, Cr, V, or Ti. Based on the dominant trivalent ion, this group is subdivided into three series: spinel series (Al³⁺-rich), magnetite series (Fe³⁺-rich), and chromite series (Cr³⁺-rich), represented by pure end member, spinel (MgAl₂O₄), magnetite (Fe²⁺Fe³⁺₂O₄), and chromite (FeCr₂O₄), respectively [1]. One of them, chromium spinel (hereafter Cr-spinel, (Mg, Fe²⁺)(Cr, Al, Fe³⁺)₂O₄), is an important accessory mineral in ultramafic and mafic rocks. Since it is relatively refractory and resistant to alteration compared with paragenetic silicate minerals, its composition has often been used as a sensitive indicator of the degree of partial melting in the mantle source region and various physicochemical conditions of mafic–ultramafic magmas in different tectonic settings [2,3]. For example, the Cr# (Cr#=100*Cr/(Cr + Al)) of Cr-spinel in peridotite is commonly used as a proxy for the process of melt extraction due to the concentration of Cr in Cr-spinel relative to the more incompatible Al during mantle melting [2,4,5,6]. In addition, abundances of Al₂O₃ and TiO₂ in magmatic Cr-spinel are mainly controlled by the contents of these elements in the parental melts and thus can be used to discriminate between different magma types, their tectonic affinities, and their mantle sources [3]. Lastly, Cr-spinel is the main reverses of Cr₂O₃ in mafic–ultramafic rocks and commonly accumulates to form chromitite in the layered mafic–ultramafic intrusions and ophiolites.

Recent studies suggested that fluid/melt-rock interactions can produce Al-Cr heterogeneity in Cr-spinel from both sub-arc and beneath mid-ocean ridge mantle, resulting in the modification of the Cr#, making Cr# ineffective as a mantle melting indicator [7,8]. Indeed, many investigators have observed that the separation of immiscible oxide phases from silicate melts is important for the origin of chromite deposits, and the coexisting fluid plays an important role in Cr-spinel crystallization, transport, and deposition [9,10,11,12,13,14]. On the other hand, significant compositional variations occur naturally within the three spinel series. The compositions of Cr-spinel can also be modified by magmatic unmixing in cumulate rocks from metamorphosed mafic and ultramafic rocks [15,16], unmetamorphosed Alpine-type peridotite [17,18], and Alaskan-type complexes [19,20,21]. This immiscibility has been attributed to metamorphism [22,23] and unmixing during the cooling process of spinel [17,19,20]. Factors such as cooling rate, oxidation state, chemistry, and grain size of the initial spinel may control the types of unmixing [21]. Although this type of texture has been investigated in some mineralogical [24], geochemical [21], and experimental studies [25], its origin is a matter of continuous debate. Therefore, studying inhomogeneous Cr-spinel is critical for the applicability of inhomogeneous Cr-spinel for petrogenetic reconstructions in the mafic–ultramafic rocks.

In this contribution, we report the unmixing of Cr-spinel in dunite and wehrlite from the Duke Island complex, southeastern Alaska. The inhomogeneous Cr-spinel consists of two separate parts: Fe³⁺-rich Cr-spinel, and Al-rich Cr-spinel. Mineral morphology, elemental mapping images, and geochemistry analyses were employed to determine the mechanism of origin of the inhomogeneous Cr-spinel.

2. Geological Setting and Sample Descriptions

2.1. Geological Setting

Duke Island is the southernmost part of some thirty distinctive ultramafic bodies of Early Cretaceous age distributed along the 560 km length of southeastern Alaska (Figure 1) [26,27,28]. The Duke Island complex, 59 square miles in area, formed as a result of magma emplacement into the crust in the southernmost of Alexander terrane (Figure 1) [26]. The Alexander terrane, which hosts Duke Island, is composed of a succession of greywacke turbidites, limestone, and mafic to felsic volcanic and plutonic rocks ranging in age from the Late Proterozoic to Late Triassic [29]. U-Pb zircon dating indicated that the gabbroic host rocks of the Duke Island complex were part of Triassic volcanism at 226 ± 3 Ma [30]. The intrusion of the Duke Island complex occurred towards the end of the Early Cretaceous extension, just before the onset of the Mid-Cretaceous thrust faulting in the Alexander terrane [31]. U-Pb zircon ages obtained from hornblende-plagioclase pegmatite along the margins of the ultramafic complex cluster are between 108 and 111 Ma [31]. Additionally, Cu-Ni sulfide mineralization primarily in the olivine clinopyroxenite was explored and studied in the Duke Island complex [29].

The ultramafic rock types of the Duke Island complex occur in two major outcrop areas: one at the center of the island called the Hall Cove area, and the other at the southeastern part of the island called the Judd Harbor area (Figure 1). Typically, concentric zones of the lithological units have been mapped, which consist of dunite and wehrlite in the center, outward to olivine clinopyroxenite, magnetite-hornblende clinopyroxenite, and hornblende-rich rocks toward the periphery [26]. Small outcrops of dunite are seen in the Hall Cove area, and dunite units appear in the drill cores ranging in thickness from a few meters to about 6 m. The wehrlite, composed of olivine and clinopyroxene, is mostly found in the north–central part of the Hall cove area. Stumpy clinopyroxene grains, relatively resistant to chemical weathering, are prominent in the layering. Olivine clinopyroxenite is the most abundant rock in the Duke Island complex. Graded layers in grain size are extensively developed showing repetitive fining upward sequences. The thickness of each layer ranges from a few centimeters to one meter. The magnetite-hornblende clinopyroxenite shows gradational contacts with the olivine clinopyroxenite. More magnetite and hornblende appear in magnetite-hornblende clinopyroxenite compared with the olivine clinopyroxenite. Sulfides, such pyrite, occur as interstitial phases. The hornblende-rich rock, very local in spatial distribution, has been mapped as the parts of the magnetite-hornblende clinopyroxenite [26].

2.2. Petrography of Dunite and Wehrlite from the Duke Island Complex

The four samples (JH01, JH04, JH05, and HC06, respectively) analyzed in this study were collected from outcrops of dunite and wehrlite of the Duke Island complex (Figure 1). JH is the acronym for Judd Harbor and means that samples were collected from Judd Harbor area as shown in Figure 1. In a similar way, HC06 is a sample collected form Hall Cove area. There are three dunites, namely JH01, JH05, and HC06, and a wehrlite, JH04. Petrographically, the dunite, HC06, consists of olivine (90–92 modal%), minor Cr-spinel (2 modal%), interstitial clinopyroxene (5–7 modal%), and amphibole (<1 modal%) (Figure 2a). Adcumulate texture, characterized by euhedral olivine grains (0.5–3 mm) and interstitial anhedral clinopyroxene, can be observed. Cr-spinel, occurring as minor anhedral intercumulus grains, is >100 μm (Figure 2b,c), while the euhedral Cr-spinel inclusions in olivine are <80 μm (Figure 2d). Diopside is commonly interstitial, while some are paragenetic, with Cr-spinel as anhedral inclusions in olivine. Interstitial clinopyroxene is anhedral in shape and relatively large in size (>0.5 mm) (Figure 2a). Contrarily, anhedral clinopyroxene inclusions are relatively small in size (<100 μm) (Figure 3a–d). Small anhedral amphibole, pargasite (<10 μm), is paragenetic with clinopyroxene crystals and Cr-spinel (Figure 3c,d). Serpentinized dunite, including JH01 and JH05, was also sampled, which consists of olivine (90–92 modal%), clinopyroxene (5–7 modal%), and Cr-spinel (<5 modal%). Olivine was serpentinized with a relict core. Clinopyroxene had been altered into serpentine and magnetite veins along its fractures, but its original shape could be easily identified by scanning electron microscopy. Cr-spinel, partly surrounded by small anhedral magnetites, occurs as euhedral, subhedral, and anhedral grains, respectively, with diameters ranging from 20 to 400 μm.

Wehrlite, JH04, is composed of olivine (62–64 modal%), clinopyroxene (33–35 modal%), and Cr-spinel (1–5 modal%). Olivine crystals are commonly replaced by serpentine. Clinopyroxene is nearly completely altered into serpentine, and veins of anhedral magnetite along fractures of mineral. The rock also contains altered cataclastic subhedral to euhedral Cr-spinel, which ranges in diameter from tens of microns to several hundred microns. The fractures in Cr-spinel were filled with altered silicate minerals, such as Cr-chlorite and serpentine, and small anhedral magnetite.

3. Analytical Methods

Polished sections of all collected samples were examined by a TM4000plus scanning electron microscope system (Japan) that was equipped with a Bruker Quantax 75 energy dispersive spectrometer (Germany), which allows for in situ qualitative characterization of the chemical composition at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS), Beijing, China. Several detailed back-scattered electron (BSE) images were obtained using an accelerating voltage of 20 kV. Elemental mapping was performed using an accelerating voltage of 20 kV and a 20 nA electron beam. The acquisition time of one BSE image was about 1 min. Semi-quantitative spot analysis was used to identify minerals at an accelerating voltage of 20 kV. The acquisition time of each spot was about 15 s, ensuring the spectrum area exceeded 2.5 × 105 counts.

The accurate chemical compositions of the minerals were determined using a JEOL JXA8100 electron probe micro-analyzer (EMPA)(Japan) at IGGCAS. The EPMA analyses were carried out at an accelerating voltage of 15 kV and 10 nA beam current, 1 μm beam spot, and 10–30 s counting time on peak. Natural and synthetic minerals were used for standard calibration. A program based on the ZAF procedure was used for matrix corrections. Typical analytical accuracy for all the elements analyzed was better than 1.5%. Detection limits (in wt.%) of oxides are shown in Table S1 available in Supplementary Material (SiO₂ 0.01; TiO₂ 0.02; Al₂O₃ 0.01; Cr₂O₃ 0.03; FeO 0.01; MnO 0.06; MgO 0.06; CaO 0.02; Na₂O 0.03; K₂O 0.02; and NiO 0.03). In the micro indicator analysis of spinel, Fe was reported as total FeO, and the Fe³⁺/∑Fe ratio of Cr-spinel was calculated based on microprobe analyses by assuming ideal spinel stoichiometry [32].

4. Structures and Chemical Compositions of Cr-Spinel

Textural and compositional variations of Cr-spinel in dunite and wehrlite were identified by BSE in this study. Three types of Cr-spinel were identified: homogeneous Cr-spinel (type A), inhomogeneous Cr-spinel with two different components (type B), and Cr-spinel displaying fractured and alteration characteristics (type C).

4.1. Homogeneous Type A Cr-Spinel

Type A Cr-spinel is interstitial and mainly associated with serpentinized olivine with a relict core and some completely altered pyroxenes. The type A Cr-spinel, 50–200 μm in diameter, is subhedral to anhedral (Figure 4). In rare cases, the type A Cr-spinel grains were altered into ferritchromite along their margins and micro fractures. All type A Cr-spinel grains were generally Cr-rich (up to 31.3 wt.% Cr₂O₃) relative to type B Cr-spinel (up to 28.6 wt.% Cr₂O₃) and type C Cr-spinel (up to 20.7 wt.% Cr₂O₃). Type A Cr-spinel grains showed a relatively narrow range of composition, as shown in Table S1 (Cr₂O₃ (28.8–31.3 wt.%), Al₂O₃ (13.42–16.22 wt.%), TiO₂ (1.15–1.52 wt.%), MgO (4.40–5.44 wt.%), and FeO (45.15–48.74 wt.%)).

4.2. Inhomogeneous Type B Cr-Spinel

Under BSE, type B Cr-spinel grains show two distinct parts, namely a light gray part and a dark gray part, regardless of the degree of alteration of the rock. Dark gray Cr-spinel was not found as individual grains, but always in association with light gray Cr-spinel, which may suggest that both were formed by the unmixing of an initially homogenous Cr-spinel of an intermediate composition. According to the degree and pattern of unmixing, three sub-types were recognized:

TypeB₁ Cr-spinel, showing complete separation of light and dark gray Cr-spinel with well-defined boundaries, but irregular unmixed patterns (Figure 5a–m). TypeB₁ Cr-spinel grains occur both as inclusions in olivine and interstitially along grain boundaries. Generally, inclusions are euhedral and much smaller (range: 20–100 μm; average 50 ± 20 μm) (Figure 5a–h) than anhedral interstitial grains (Figure 5i–m) (range: 100–400 μm; average 200 ± 100 μm). However, some of the inclusions are subhedral and multiphase with anhedral clinopyroxene and paragasite (Figure 3c,d). Some micro fractures occur in the interstitial grains with micro grains of magnetite along grain boundaries. Silicates associated with type B₂ Cr-spinel are mainly serpentinized with relict cores and, rarely, completely altered. Type B₂ Cr-spinel grains range in morphology from subhedral to anhedral and were postulated as intercumulus phase by its grain size. Type B₂ Cr-spinel grains range in size from 100 to 400 μm with an average of 200 ± 50 μm. Dark gray lamellae of Cr-spinel are crystallographically oriented in the light gray Cr-spinel (Figure 6b). Some of the unmixing become larger and irregular in the outer margins. The uniform distribution of the unmixed lamellae in the core region of the light gray Cr-spinel may suggest an initial homogenous chemical composition of this type of Cr-spinel. Fractures in the type B₂ Cr-spinel are mainly filled by serpentine. Micro grains of magnetite mainly occur along grain boundaries and, rarely, along fractures. Like type B₂ Cr-spinel grains, type B₃ Cr-spinel is postulated to be an interstitial phase between serpentinized olivine grains. Type B₃ Cr-spinel, mostly 100–200 μm in diameter, is subhedral to anhedral in morphology. Type B₃ Cr-spinel shows a dark gray rim around the inner light area. The width of the dark rim and the darkness is variable between the dark rim and the inner light part (Figure 6c). Small dark gray lamellae of Cr-spinel occur in the transition part between the dark rim and the inner light part. Most type B₃ Cr-spinel grains are subhedral showing rounded edges. Fractures also occur in the type B₃ Cr-spinel and are filled by silicate matrix, such as serpentine. Micro grains of magnetite mainly occur along rim of grains and, rarely, along fractures. Needle-like magnetites paragenetic with chlorite are seen to be oriented perpendicularly to the margin of grains.

In the type B Cr-spinel, dark gray Cr-spinel is more magnesium and aluminum (up to 11.0 wt.% MgO and 42.1 wt.% Al₂O₃) than type A Cr-spinel (up to 5.44 wt.% MgO and 16.22 wt.% Al₂O₃) and less iron and titanium (down to 28.9 wt.% FeO and 0.11 wt.% TiO₂) than type A Cr-spinel (down to 45.15 wt.% FeO and 1.15 wt.% TiO₂), while the light gray Cr-spinel is more iron and titanium (up to 73.3 wt.% FeO and 2.82 wt.% TiO₂) and less magnesium and aluminum (down to 1.70 wt.% MgO and 4.02 wt.% Al₂O₃). In this article, “Al-rich Cr-spinel” and “Fe³⁺-rich Cr-spinel”, therefore, refer to dark gray Cr-spinel and light gray Cr-spinel, respectively. Type B₁ Cr-spinel is mostly Cr-poor compared with type A Cr-spinel. The Cr₂O₃ content of Al-rich Cr-spinel and Fe³⁺-rich Cr-spinel vary from 16.34 to 17.11 wt.% and from 11.77 to 12.72 wt.%, respectively. The compositions of Al-rich Cr-spinel and Fe³⁺-rich Cr-spinel of type B₁ Cr-spinel are significantly different. The composition of Al-rich Cr-spinel mainly consists of Al₂O₃ (41.67–42.05 wt.%), TiO₂ (0.11–0.13 wt.%), MgO (10.50–10.99 wt.%), and FeO (28.90–28.96 wt.%), and the composition of Fe³⁺-rich Cr-spinel mainly consists of Al₂O₃ (4.29–4.77 wt.%), TiO₂ (2.42–2.82 wt.%), MgO (1.77–2.40 wt.%), and FeO (73.22–73.31 wt.%) (Table S1). The compositions of type B₂ Cr-spinel and type B₃ Cr-spinel are similar and difficult to differentiate. Both of them are less Cr-poor than type B₁ Cr-spinel; therefore, they are described together. The Cr₂O₃ content of Al-rich Cr-spinel and Fe³⁺-rich Cr-spinel of the two subtypes vary from 26.30 to 28.55 wt.% and from 17.32 to 23.20 wt.%, respectively. The compositions of Al-rich Cr-spinel and Fe³⁺-rich Cr-spinel of the two subtypes are also different. The composition of Al-rich Cr-spinel mainly consists of Al₂O₃ (20.84–26.43 wt.%), TiO₂ (0.29–0.80 wt.%), MgO (5.60–7.34 wt.%), and FeO (36.26–42.55 wt.%), while the composition of Fe³⁺-rich Cr-spinel mainly consists of Al₂O₃ (4.02–10.01 wt.%), TiO₂ (1.85–2.71 wt.%), MgO (1.70–3.28 wt.%), and FeO (58.94–69.93 wt.%) (Table S1).

4.3. Altered Type C Cr-Spinel

Like type A Cr-spinel, type C Cr-spinel is also postulated as an interstitial phase. Type C Cr-spinel is subhedral and mostly 200–400 μm in diameter. Most of them are fractured and completely altered into ferritchromite and Cr-bearing chlorite (Figure 7). The type C Cr-spinel shows crystallographically oriented fractures filled with silicate minerals including serpentine and chlorite. The Cr-bearing chlorite was observed around Cr-spinel especially by Al-element mapping (Figure 7). Relict dark lamellae of Al-rich Cr-spinel are spotted in some part of the Cr-spinel (Figure 8b). Both homogeneous Cr-spinel and unmixed Cr-spinel are observed in the same thin section. Olivine and clinopyroxene around the type C Cr-spinel are nearly completely serpentinized. Compared with type A and B Cr-spinel (both Al-rich Cr-spinel and Fe³⁺-rich Cr-spinel), the type C Cr-spinel shows a wide range of composition. The type C Cr-spinel displays higher iron (70.75–87.36 wt.% FeO) and titanium (1.31–4.68 wt.% TiO₂) and lower magnesium, aluminum, and chromium (0.63–1.3 wt.% MgO, 0.01–1.67wt.% Al₂O₃, and 4.6–20.69 wt.% Cr₂O₃) (Table S1).

5. Discussion

5.1. Effect of Post-Magmatic Alteration on Cr-Spinel

Cr-spinel in the serpentinized ultramafic rocks (and associated chromitite) is often zoned with Cr, Al-rich interior rimmed by ferritchromite, and even pure magnetite. This is referred to as “normal zonation” [33,34,35]. Most researchers suggest that the outer ferritchromite rims are of metamorphic origin and were formed due to hydrothermal alteration in precursor mineral phases, such as olivine, orthopyroxene, clinopyroxene, and Cr-spinel [36,37,38]. Cr-bearing clinochlore, which is commonly intergrown with magnetite, replacing primary chromite + olivine or secondary chromite + serpentine assemblages, has been reported from the alteration of Cr-spinel [39,40,41,42,43]. Cr-chlorite, an important Cr-bearing phase where Cr₂O₃ content is strongly variable, between 0.5 and 8 wt.%, has been identified by EMPA (Table S2 available in Supplementary Material) [34,44,45,46]. In this study, type C Cr-spinel grains were cataclastic and completely altered into ferritchromite and Cr-bearing chlorite (Figure 7), which suggests that hydrothermal alteration changed the composition of type C Cr-spinel. The Fe³⁺# and Cr# of type C Cr-spinel (average 84 and 94, respectively) were higher than type B spinel (average 36 and 53, respectively) and type A spinel (average 29 and 58, respectively), while the Mg# was lower (average 5) (Figure 9a,c). As shown in the ternary diagram of Cr-Al-Fe³⁺ atomic ratios of Cr-spinel (Figure 10), the process of alteration is characterized by two stages: the first stage is mainly the replacement of Al by Fe³⁺, whereas the second stage is mainly the replacement of Cr by Fe³⁺.

Type B₃ Cr-spinel consists of Al-rich rim and some crystallographically oriented lamellas, as described for type B₂ Cr-spinel. Cr-spinel compositions, however, may also be modified by relatively low-grade metamorphism [49,50,51]. The element distribution of type B₃ Cr-spinel is observed from the elemental mapping where extra aluminum and iron are observed around the Al-rich Cr-spinel rim indicating some Al-rich (chlorite) and Fe-rich (magnetite) minerals (Figure 6c). As mentioned above, ferritchromite and Cr-bearing chlorite are evidence of the alteration of type C Cr-spinel. Therefore, type B₃ Cr-spinel experienced hydrothermal alteration. Based on its chemical composition and mineral assemblage, the type B₃ Cr-spinel is the transition stage between alteration and no alteration when compared with the completely altered type C Cr-spinel and unaltered Cr-spinel. No such alteration has been observed on homogeneous type A Cr-spinel and the other inhomogeneous type B Cr-spinel. Although small grains of magnetite were observed along most Cr-spinel boundary fractures, no Cr-bearing chlorite was observed. Accordingly, such magnetite was formed only by the hydrothermal alteration of silicate mineral. Hydrothermal alteration or serpentinization, consequently, are not the causes of inhomogeneous Cr-spinel. We suggest that type B₃ Cr-spinel unmixed firstly and then was influenced by alteration.

5.2. Oxidized Fluid Activity in the Formation of Unmixed Cr-Spinel

The inhomogeneous spinel was identified by [24] and suggested to be for the product of subsolidus immiscibility [15]. This immiscibility was attributed to metamorphism [22,23,48]. Upper amphibolite-facies metamorphism in differentiated gabbroic bodies from Red Lodge Mountain, Montana, was interpreted as the cause of unmixing of the spinel [48]. A tunnel-shaped miscibility gap and a solvus in terms of trivalent cations (Cr-Al-Fe³⁺) at a temperature of approximately 600 °C (Figure 10) were found [48]. It is noticeable that some of the dunite samples that contain inhomogeneous Cr-spinel observed in this study did not experience metamorphism or any significant alteration. Although the other dunite and wehrlite were serpentinized, their original textures were still observed. Moreover, the ternary diagram of Cr-Al-Fe³⁺atomic ratios of spinel does not agree with the solvus of metamorphism L₂ in Figure 10. Inhomogeneous Cr-spinel was induced by stress, which may also lead to crystal lattice diffusion in deformed peridotites [52]). In contrary, the studied dunite and wehrlite have no foliation or lineation. Cr-spinel grains have widely variable morphologies, but no noticeable grain elongation. Consequently, the inhomogeneous Cr-spinel in this study could not be formed by metamorphism or stress.

Unmixed Cr-spinel in chromitite from the Iwanai-dake peridotite complex, which had probably undergone neither regional metamorphism nor serpentinization after emplacement, was caused by highly oxidized hydrous silicate melt, which reacted with peridotite in the mantle wedge and crystallized the Fe³⁺-rich spinel [18]. The unmixed Cr-spinel was also reported from other Alaskan-type complexes [19,20,21,53]. All rocks containing the unmixed Cr-spinel show evidence of Fe³⁺-enrichment during metamorphism or crystallization, which suggests that unmixing of Fe³⁺-rich Cr-spinel results from an increase in the oxidation state of the environment. All previous studies may be reasonable, but they ignored the important role played by fluid in the formation of the original composition of inhomogeneous spinel. The parent magmas of the Duke Island complex and Alaskan-type complexes worldwide are hydrous [54]. The hydrous magmas are also proved by the observation of traces of fluid. The lamellae of chromite-diopside intergrowth in olivine grains of dunite units from Gaositai and Yellow Hill Alaskan-type complexes were observed and regarded as products of the annealing process of olivine solid exsolution initially equilibrating with magnesian Cr-rich fluid [55]. The fluid-induced lamellae of chromite in Gaositai and Yellow Hill Alaskan-type complexes was confirmed by a recent study. A similar texture, olivine with parallel-located lamellae of secondary spinel, was produced by a high-temperature water–olivine interaction experiment at pressures of 500 MPa and temperatures of 1200 °C for 24 h (500 MPa, 1200 °C, 24 h) [56]. However, in our study, the fluid may also be indicated by the multiphase inclusions in olivine grains of dunite, which consist of unmixed Cr-spinel, clinopyroxene, and amphibole (Figure 3a–d). The inclusions hosted by olivine are spherical in shape and around 60 μm in diameter. Similar sizes of inclusions of spinel and clinopyroxene were also observed by the experiment of [56] (500 MPa, 1200 °C, 24 h). Multiphase inclusion of unmixed Cr-spinel, diopside, and paragasite, consequently, may indicate its fluid origin. The study of [56] revealed that Fe²⁺ from olivine reacts with free aqueous fluid according to a simplified reaction: 3Fe₂SiO₄ + 2H₂O ⇆ 3SiO₂ + 2Fe₃O₄ + 2H₂. The reaction formed a spinel rich in ferric iron and high-Mg olivine with Mg# (Mg/(Mg+Fe²⁺)×100) up to 96 in natural samples and 99.9 in experiments. Nevertheless, the range of the Mg# of olivine in this study was only from 82 to 86, which seems to be contradictory to the results of the experiment. The sampled dunite is characterized by intercumulus clinopyroxene (5–7 modal%). Re-equilibration of olivine with trapped liquid was observed in the Duluth complex, Minnesota [57]. The Mg# of olivine increases with the increase of log cumulus olivine/trapped liquid. Thus, the low Mg# of olivine in the Duke Island complex may be caused by re-equilibration of olivine with trapped silicate melt. Additionally, unmixed spinel-fluid inclusions were also observed in dunite xenolith, Klyuchevskoy volcano, Kamchatka [56]. All evidence of the above suggests that the interaction of high-temperature fluid and olivine formed the original composition of inhomogeneous Cr-spinel. The high-Mg dunite of the Duke Island complex containing high-Cr chromite (40–46 wt.% Cr₂O₃) without unmixed textures was observed by [29]. We suggest that this could be the result of no interaction with high temperature fluid.

5.3. Controlling Factors of Cr-Spinel Unmixing

The different geological environments and post-magmatic processes of these different kinds of intrusions make it difficult to attribute unmixing to any single cause (Ahmed et al., 2008). The ratio Cr/(Al + Cr + Fe³⁺) determines whether a spinel unmixes during cooling or not; low ratios increase the possibility and degree of unmixing. This feature has been observed from all previously reported studies of unmixing in Cr-spinel (e.g., [58] and references therein). The type A Cr-spinel observed in this study was homogeneous with a higher ratio of Cr/(Al + Cr + Fe³⁺) compared with the unmixed type B Cr-spinel (Figure 10). The ratio Cr/(Al + Cr + Fe³⁺) of type A Cr-spinel is more than 0.40, while the ratio Cr/(Al + Cr + Fe³⁺) of type B Cr-spinel is less than 0.37. Olivine from the Duke Island complex has a Cr content as low as the EMPA detection limit (Table S2), which may indicate that it has no effect on the content of unmixed Cr-spinel. This suggests that the unmixing of Cr-spinel depends on the initial composition of the Cr-spinel, which is controlled by oxidized fluid as proved above. Unmixing, consequently, is a consequence of element redistribution within the initial homogeneous Cr-spinel.

A spinel miscibility gap has been documented by both experimental and thermodynamic studies [25,59]. The miscibility gaps in the spinel prism were comprehensively discussed based on thermodynamic calculations at different temperatures in various forsterite content defined by the compositions of coexisting olivine [25]. The compositional relationship of inhomogeneous Cr-spinel of the Duke Island complex coexisting with Fo82–86 olivine (Table S2) is consistent with the solvus of spinel in equilibrium with Fo80 olivine at 600 °C (Figure 10) displayed by [25]. The fact that the contiguous pairs showing such a miscibility gap are observed in the Duke Island complex, combined with a higher Fo82–86, may indicate that the inhomogeneous spinel was formed by unmixing at a temperature slightly higher than 600 °C.

The different shape and size of unmixing may reflect different rates of cooling [21]. The unmixed type B Cr-spinel consists of three subtypes of Cr-spinel: type B₁ Cr-spinel, type B₂ Cr-spinel, and type B₃ Cr-spinel, respectively (Figure 6a–c). The different unmixed patterns may be induced by the significantly different sizes of type B₁ Cr-spinel and type B₂ Cr-spinel. The coarse-grained irregular unmixing of type B₁ Cr-spinel may be characteristic of fast cooling. This feature is supported by the larger and irregular unmixing of the type B₂ Cr-spinel rim, while the inner part is composed of crystallographically oriented Al-rich lamellas and a Fe³⁺-rich host. The crystallographically oriented Al-rich lamellae are observed. It has been suggested that the crystallographically oriented unmixing is a characteristic of slow cooling [60]. The cooling rate of the rim may be higher than the cooling rate of the core, but similar to the cooling rate of small-grained type B₁ spinel, which formed different patterns of unmixing.

In conclusion, the original chemical composition of Cr-spinel, grain size of Cr-spinel, and the cooling process of Cr-spinel play major roles in the process of unmixing. As discussed above, we propose that the unmixed Cr-spinel is primary, as it displays strong petrographic differences from the ferritchromite occurring as alteration products of Cr-spinel in altered mafic–ultramafic rock.

6. Conclusions and Implications

The compositional relationship between two Cr-spinel phases shown in this study suggests that the unmixed texture in Cr-spinel in the Duke Island complex was developed by the unmixing process due to immiscibility at around 600 °C in the oxidized state. The initial composition of inhomogeneous Cr-spinel may be formed by the interaction of high-temperature fluid and olivine. The textural type of inhomogeneous Cr-spinel is dependent on the original chemical composition, grain size, and the cooling process. Additionally, hydrothermal alteration can modify the pattern of unmixed Cr-spinel and cause the discharge of Al and Cr to form Cr-chlorite.

The composition of original Cr-spinel in peridotites derived from the upper mantle reflects the degree of partial melting that the mantle experienced. The chemical composition of Cr-spinel successfully characterized different tectonic settings of mafic–ultramafic magmas. Cr-spinel, consequently, has been used as a petrogenetic indicator in many previous studies to distinguish the tectonic setting of the magma and the degree of mantle partial melting [2,4,47,61,62]. As presented above, inhomogeneous Cr-spinel may be secondary Cr-spinel formed by the interaction of high-temperature fluid and olivine, which is a useful proxy to unveil the activity of high-temperature fluid in the formation of subduction-zone-related Alaskan-type complexes.