Except for normal mantle residues that resemble abyssal peridotites24 and are consistent with previous studies25–26 with respect to clinopyroxene REE patterns (Fig. 1a), we distinguish another two groups of mantle peridotites on the basis of clinopyroxene and chromite compositions. We consider that these groups record two mantle processes involved with two specific melts in SSZ mantle. We further consider that these two specific melts could have been responsible for the formation of the typical Luobusa podiform chromite deposit in the Yarlung–Zangbo Suture Zone.
1) Hydrous melting to produce boninitic melt. Boninite is an essential petrological indicator for arc (subduction) environments27–30 and is characterized by high MgO (>8 wt%) and SiO2 (>52 wt%) but extremely low TiO2 (<0.5 wt%) contents31. Early studies of rock geochemistry22, 27–28 and recent experimental petrology29–30 have demonstrated that boninite is generated by a high degree of melting (>20%) of refractory harzburgite fluxed by slab-derived fluids. In natural magma systems, podiform chromitite has similar trace-element and platinum group element patterns to those of boninites1, 3, 33–35. Therefore, the inferred parental melts of podiform chromitite exhibit a geochemical resemblance to boninitic melts, leading many researchers to support the boninitic affinity for the genesis of podiform chromitite33–37. Although boninite has not been reported previously for the Luobusa massif, we provide evidence of in-situ clinopyroxene trace-element compositions to support the existence of boninitic melt in the Luobusa mantle.
First, the studied samples show an extremely depleted nature. Protogranular clinopyroxenes are very rare (total clinopyroxenes <1% modal), and nearly invisible under the microscope (see Supplementary petrographic observations). They have very high chromite Cr# values (>55) and low clinopyroxene Al2O3 contents (<1.5 wt%), clearly differing from typical abyssal peridotites (chromite Cr# <40, clinopyroxene Al2O3 >3.0 wt%). Second, these clinopyroxenes have flat REE patterns with pronounced LREE enrichment but HREE depletion compared with normal residual clinopyroxenes (Fig. 1b). Such less-fractionated REE trends are commonly seen in arc-related peridotites involved with hydrous melting38–43. Jean et al.40 and Le Roux et al.41 have noted that clinopyroxenes from SSZ peridotites have lower Dy and Yb contents than those of residual clinopyroxenes after anhydrous melting, which is consistent with our results (Fig. 3a). Third, a prominent feature of SSZ peridotites is the extreme depletion of Ti in clinopyroxenes, which is well documented by mantle wedge peridotites (30–150 ppm)38, IBM forearc peridotites (86–156 ppm)39, recycled arc peridotites in oceanic mantle (60–120 ppm)43, and arc peridotite xenoliths (107–195 ppm)44, as well as the Luobusa peridotites in this study (170–250 ppm). Simulations have indicated that such clinopyroxenes simultaneously depleted in Ti and HREEs cannot be achieved by dry melting in a mid-ocean ridge (MOR) setting, but correspond perfectly to hydrous melting in a subduction setting38–43. According to modelling results, the melt in equilibrium with the clinopyroxenes characterized by such REEs and Ti compositions is most likely to be boninitic38, 43. Importantly, these clinopyroxenes have similar REE patterns to those of clinopyroxenes from high-Ca boninite (Supplementary Fig. 4a), and the calculated melt in equilibrium with these clinopyroxenes exhibits similar REE patterns to that of boninite (Supplementary Fig. 4b). In addition, we observe that Sr, a fluid-mobile element, exhibits relatively higher contents than those of residual clinopyroxenes (0.50–1.29 vs 0.08–0.29 ppm). Variable-valence elements, such as V and Mn, have lower concentrations compared with residual clinopyroxenes. This suggests that the ambient environment of these samples was more hydrous and oxidized, leading to higher Sr contents and the loss of high-valence ions of V and Mn, as they could not be held in clinopyroxene crystals. Overall, our geochemical results are highly consistent with the hydrous melting of highly depleted peridotites, thus meeting the required conditions for the formation of boninitic magma. Hence, boninitic magma is expected to have been involved in the Luobusa mantle.
2) Cr-rich melt derived from the asthenosphere. Another group of mantle peridotites from the Luobusa massif has distinctive traits that are substantially different from those of abyssal and SSZ peridotites. These mantle peridotites display strong melt metasomatism recorded by mineral compositions. First, clinopyroxenes from these samples are strongly enriched in Na2O (>0.6 wt%), Cr2O3 (~1.0–1.9 wt%), and Ti (1000–2000 ppm). Na contents vary positively with Cr (Fig. 2a). Clinopyroxenes with such anomalous Na and Cr enrichments have been reported for some abyssal peridotites from slow–ultraslow spreading ridges, such as the Southwest Indian Ridge (SWIR)45–46, Lena Trough47, and Central Atlantic Ocean48, and have been interpreted as metasomatism involving alkaline (sodium) melt46–47 or asthenospheric melt45, 48. Distinctive light to medium REE contents create unusual arched REE patterns (Fig. 1c), which is rare among global abyssal peridotites49, but is comparable with those of metasomatized peridotites from Lena Trough47. In addition, Sr and Zr contents of clinopyroxenes are too high to be explained by simple partial melting (e.g., mean values of 16 and 40 ppm, respectively, for sample 1829a). Second, the Ti contents of chromite are far beyond those of normal residual chromite. Dick et al.51 proposed that chromite TiO2 of residual peridotites is homogeneously less than 0.15 wt% and increases (to >0.15 wt%) when influenced by MORB-like liquids in MOR mantle. In addition, accessory chromites in harzburgite have lower TiO2 contents compared with metallurgical chromite from podiform chromitite3. However, our results show that these metasomatized chromites have higher TiO2 contents (0.15–0.35 wt%) than those of chromites from residual peridotites, even higher than those of metallurgical chromites (Fig. 2b). Third, we observed many interstitial amphiboles in these samples (Supplementary Figs. 2g–h), a diagnostic mineral of melt metasomatism. The amphiboles show similar arched REE patterns (Fig. 1d), and also have high Na2O (3.0–4.0 wt%), Cr2O3 (1.7–3.5 wt%), and TiO2 (1.0–3.0 wt%) contents. These observations strongly support a metasomatic origin for these samples.
Plagioclase-bearing (Pl-) peridotites (maybe containing amphiboles) are common products of metasomatism of mantle-derived melts in SSZ and MOR mantle. Here, we confirm that the metasomatic agent is not normal mantle-derived magma, but rather Cr-rich melt originating from asthenospheric mantle. First, metasomatized chromites show a negative correlation between Cr# and Ti, which is not seen in Pl-peridotite at the given Cr# of chromite (Fig. 2b). This trend is also opposite to that of melt–rock reaction, in which case Cr# covaries positively with Ti52. Second, the evidence for Cr-rich agent metasomatism is supported by the extreme Na and Cr contents of clinopyroxenes. The positive trend for Na–Cr differs from the geochemical feature for abyssal residual and Pl-peridotites but overlaps the field of asthenospheric melt metasomatic clinopyroxenes from the SWIR45 (Fig. 2a). We observe that Na content increases with decreasing Ti in clinopyroxenes. Although the precision and accuracy of Na contents analyzed by LA–ICP–MS are poor, Hellebrand and Snow47 pointed out that Na and Nd should exhibit similar behaviors during magmatic processes, given their close partition coefficients between silicate melts and clinopyroxenes. Therefore, we utilized Nd to substitute for Na to plot a Ti–Nd diagram (Fig. 3b). The metasomatized samples define a unique negative trend that differs from both residual and veined peridotites from the compilation by Warren49. Additionally, clinopyroxenes have distinctive arched REE patterns with no Eu anomaly, which is different from typical Pl-peridotites from SSZ53 or MOR54 mantle. Third, the REE patterns of interstitial amphiboles differ substantially from those of their counterparts from sub-arc55 and sub-ridge mantle56 (Fig. 1d). These results thus strongly suggest that 1) the metasomatic agent is rather rich in Cr, and 2) the metasomatic agent is not normal mantle-derived magma such as MORB-like or subduction-related (arc) melts, but is instead inferred to be derived from the deep asthenosphere.
Cr isotope constraints
Stable Cr isotopes have been widely applied to the study of high-temperature geological processes and have also allowed significant progress to be made in exploring planetary evolution. We therefore attempted to utilize Cr isotope compositions to constrain the mantle evolutions involved with two specific magmas identified in the Luobusa mantle. Before conducting a detailed investigation, the possibility that kinetic diffusion and weathering (or water) alteration could obliterate the original Cr signatures should be excluded. Given the high Cr contents in chromite compared with those of co-existing silicate minerals, it is considered that the Cr isotope signatures of chromite are negligibly affected by late chemical diffusion. Therefore, fresh bulk rocks with loss-on-ignition values of <4 wt% and chromite separates were chosen for measurement. The results of Cr isotope compositions is given in Supplementary Data 3.
Whole-rock Cr isotope compositions
Overall, the whole-rock Cr isotope values vary positively with chromite Cr# (Fig. 4a). Residual peridotites have the lowest δ53Cr values of −0.21‰ to −0.14‰, whereas metasomatized peridotites modified by Cr-rich asthenospheric melt have higher values of −0.11‰ to −0.08‰. Two SSZ peridotite samples in equilibrium with boninitic melt exhibit the highest Cr isotope compositions of 0.03‰ and 0.04‰. In addition, we analyzed two dunite envelopes with sharp boundaries contacting chromitite, which show similar values (0.02‰ and 0.04‰) to those of SSZ peridotites, indicating that the envelopes are also equilibrated with boninitic melt and might have been formed by melt–rock reaction following the model of Zhou et al.13. These results show that whole-rock Cr isotope compositions can distinguish our SSZ and metasomatized peridotites, perhaps reflecting the different nature of the two melts involved.
Chromite Cr isotope variations
The Cr isotope variations of chromite from peridotites and chromitites show a wide range of −0.38‰ to 0.03‰ (Fig. 4b). Residual chromites are characterized by negative δ53Cr values, ranging from −0.21‰ to −0.38‰, which are much lower than the global mantle-derived chromites (−0.079‰ ± 0.129‰, n = 42) compiled by Farkaš et al.57 In that compilation, most chromites were from arc environments. Our lower values indicate that depleted MORB mantle (DMM) has relatively light chromite Cr isotope compositions, whereas SSZ mantle contaminated by subduction slabs should exhibit heavy Cr isotope signatures. Metasomatized chromites have slightly heavy δ53Cr values of −0.14‰ and −0.15‰. Chromites from the podiform chromitite exhibit uniform Cr isotope signatures of −0.10‰ to −0.13‰, meaning that the ore-forming chromites have the closest Cr isotope composition to those of metasomatized chromites. Therefore, we consider that these chromitites are also equilibrated with Cr-rich asthenospheric melt. Chromites from the dunite envelopes have higher δ53Cr values (0.00‰ to 0.03‰) compared with the ore-forming chromites, although they have identical Cr# values.
Overall, from the view of Cr isotope compositions of whole-rock and chromite separates, we consider that 1) two types of peridotite that equilibrated with Cr-rich asthenospheric melt and boninitic melt are reflected in their different Cr isotope characteristics; and 2) the ore-forming chromites are equilibrated with Cr-rich asthenospheric melt, whereas the dunite envelopes are equilibrated with boninitic melt.
The origin of podiform chromitite
Our preferred interpretation for the origin of podiform chromitite is that primitive Cr-rich asthenospheric melt mixes with boninitic magma to form chromitite, in which case, the Cr-rich melt provides the majority of Cr, and the mixing process triggers chromite saturation and crystallization (Fig. 5). When oceanic lithosphere is subducted, fluids derived from the subducted slab cause hydrous melting of refractory harzburgite to produce boninitic melt. This magma would interact extensively with mantle peridotites to form dunite. Large chromitite bodies would not form if there were insufficient amounts of Cr. As the slab continues to sink deeper, owing to the density contrast resulting from different degrees of metamorphism, the oceanic lithosphere breaks off to form a slab window. The window facilitates upwelling of asthenospheric material. As asthenospheric melts ascend along magma conduits to the shallow mantle, Ti-depleted boninitic melts that widely infiltrate within peridotites are added to upwelling Cr-rich asthenospheric melts. This addition causes the new magma system to become more enriched in Si and Mg but depleted in Ti, more hydrous and oxidized, thus trigging the crystallization of chromite. We figuratively refer to this system as a “chromite-producing factory” (Fig. 5). Chromite crystals do not accumulate significantly at depth. Field observations show that the major chromitite bodies are restricted to the shallow mantle–crust transition zone or uppermost mantle1–3, indicating that they were deposited directly at the transition zone. It has been demonstrated experimentally that water bubbles can enclose chromite crystals to migrate upward more efficiently under the enhanced buoyancy58. Therefore, we suggest that chromites were suspended in Cr-saturated magma and carried upward more effectively by a fluid phase. When arriving at the crust–mantle transition zone, chromites would be accumulated to form podiform chromitite owing to the marked drop in temperature and pressure. Fluid would then be separated from the magma system, causing mobile elements (e.g., Na, Sr, Zr, and LREEs) to be extracted from melt and enter the fluid, as they are more soluble in water than in melt. The high-temperature fluid thereafter infiltrated the wall rock to form clinopyroxene enriched in Cr, Ti, Na, Sr, Zr, and hydrous amphibole (which was also enriched in Cr, Ti, Na, Sr, and Zr). Assuming that the supply rate of Cr-rich asthenospheric melt is stable, the continuous input of Ti-depleted boninitic melt would cause the magma system to contain less Ti and more hydrous phases. Magmas with higher water contents have a higher ability to carry REEs. This explains the increase in REE contents with increasing Na and Cr but decreasing Ti in clinopyroxene (as well as amphibole), as well as the negative correlations between Cr# and TiO2 in metasomatic chromites and ore-forming chromites.