Journal of Earth Science  2019, Vol. 30 Issue (3): 563-570   PDF    
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Helium Isotopic Composition of the Songduo Eclogites in the Lhasa Terrane, Tibet: Information from the Deep Mantle
Zhaoli Li , Jingsui Yang , Tianfu Li , Songyong Chen     
Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China
ABSTRACT: Helium isotopic compositions are considered to be ideal tracers to identify whether mantle materials have been added to crustal rocks or fluids. In this paper, we present the helium isotopic compositions of the Songduo eclogites in the Lhasa terrane, Tibet. We found that garnet and omphacite in the eclogites have different helium retention characteristics. The 4He content of most omphacite grains are about 10-20 times of that of garnet, suggesting that omphacite has a higher ability to capture 4He than garnet. Similarly, there is about 10-20 times difference in 3He content between omphacite and garnet in the same eclogite samples. The 3He/4He ratios of garnet and omphacite in these rocks range from 0.27 to 0.60 Ra (relative to the modern air 3He/4He ratio, 1.4×10-6). These ratios are within the range of both mantle-and crust-derived helium, suggesting mixed sources. The Songduo eclogites have much higher 3 He/4He ratios than those observed in the Dabie eclogites of eastern China. Such high ratios are typically thought to be associated with deep mantle sources. We cautiously conclude that deep mantle materials might have been involved during the formation of the Songduo eclogites.
KEY WORDS: eclogite    helium isotopic composition    HP/UHP metamorphic belt    Lhasa terrane    Qinghai-Tibet Plateau    
0 INTRODUCTION

Isotopic ratios of noble gases (He, Ne, Ar, Kr and Xe) are characterized by unique geochemical properties. Because of their chemical inertness and high volatility, they are considered to be ideal tracers for investigating the origin and evolution of the solar system and the planets (Zheng et al., 2019), evolution and dynamics of the Earth's mantle (Farley and Neroda, 1998), degassing of the Earth and evolution of its atmosphere (Tolstikhin and OʼNions, 1994), ocean circulation (England and Maier-Reimer, 2001), and the dynamics of aquifer systems (Inguaggiato and Rizzo, 2004).

Noble gases within the Earth's crust originate from the following sources: the atmospheric source, which is introduced into the crust dissolved in groundwater; and the mantle, particularly in regions of magmatic activity. In addition, small amounts can be produced in the crust by radioactive decay of U, Th and K (Ozima and Podosek, 2002; Hu et al., 1998; Turner et al., 1993). Furthermore, noble gases in the mantle can be divided into different types which is described in the section of discussion. Every source contains helium with its own characteristic isotopic ratio (Dunai and Baur, 1995; Stuart et al., 1995), which is significantly different from the others (Torgersen and Jenkins, 1982). The 3He/4He ratios of typical crustal rocks are 0.01–0.05 Ra, and Ra is the atmospheric 3He/4He value of 1.4×10-6 (Stuart et al., 1995). However, the 3He/4He ratios of upper mantle rocks are 6–7 Ra (Farley and Neroda, 1998; Dunai and Baur, 1995). This vast difference in isotopic ratios between the crust and the upper mantle, a factor of 1 000, makes it possible to use helium isotopes to trace geologic processes by which mantle volatiles are added to crustal rocks or crustal fluids (Stuart et al., 1994; Dunai and Touret, 1993). Helium isotopes have been widely used to trace the contemporary crustal fluids (e.g., natural gases and groundwater) (Inguaggiato and Rizzo, 2004), ancient fluids (e.g., the origins of ore forming fluids) (Li et al., 2006; Hu et al., 1998; Jean-Baptiste and Fouquet, 1996; Turner and Stuart, 1993) and meteorites (Park et al., 2003).

Eclogites are one of the most typical direct indicators for HP-UHP metamorphism (Chopin, 1984; Smith, 1984). Our understanding of eclogites and high-ultrahigh pressure metamorphism has developed rapidly over the last 20 years. Eclogite facies metamorphic rocks have been discovered in a wide range of orogenic belts around the world with different geological backgrounds. These rocks have provided important information about the formation and tectonic evolution of these belts (Xia et al., 2018; Zhang et al., 2002; Reinecke, 1998; Maruyama et al., 1996; Ernst and Liou, 1995; Tagiri et al., 1995). During the past ten years, various types of HP-UHP metamorphic rocks have been discovered and confirmed in South Altyn Tagh, North Qaidam and North Qinling in the western and middle parts of the Central China Orogen. The Songduo eclogites of the Lhasa terrane mark a newly discovered high-pressure eclogites belt in the Tibet Plateau.

The formation of eclogites is a complicated process. The isotopic compositions of eclogites (e.g., B isotopes, Peacock and Hervig, 1999; U, Pb, Nd isotopes, Bernard-Griffiths et al., 1985; O isotopes, Li et al., 2001; Sr isotopes, Lucassen et al., 2011) have been measured. These isotopes were used to shed light on the genesis of high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks, the depth of the subduction of lithospheric plates, the degree and nature of crust-mantle interaction and recycling of crustal materials (Gonzaga et al., 2010; Du et al., 1998).

1 GEOLOGICAL SETTINGS

The Tibetan Plateau has traditionally been divided from south to north into the Himalaya, Lhasa, Qiangtang, Baryan Har-Songpan Ganzi and Altun-Qilian-Kunlun terranes, separated by the Yarlung-Zangbo, Bangong-Nujiang, Central Qiangtang and A'neymaqen sutures, respectively (Zhai et al., 2007; Pan et al., 2006; Xu Z Q et al., 2006; Xu R H et al., 1985). The Lhasa Block extends about 2 000 km in an E-W direction and is about 300 km wide (Yang et al., 2006). It is bounded by the Yarlung-Zangbo suture (YLZBS) on the south and the Bangong-Nujiang suture (BNS) on the north. On the west it is truncated by the Karakoram strike-slip fault, whereas on the east it bends southward around the Himalaya (HM) (Fig. 1) (Pan et al., 2004).

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Figure 1. Tectonic framework of the Tibetan Plateau. 1. IND, India Plate; 2. HM, Himalayan Orogen; 3. SLT, South Lhasa terrane; 4. NLT, North Lhasa terrane; 5. NQT, North Qiangtang terrane; 6. SQT, South Qiangtang terrane. 7. ophiolite; 8. Permian island arc volcanic rock; 9. the rectangle shows the location of Fig. 2 and the star marks the location of the study area in the Songduo eclogites belt. YLZBS. Yarlung-Zangbo suture; BNS. Bangong-Nujiang suture; SDS. Songduo suture; CQTS. central Qiangtang suture
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Figure 2. Regional geological map of the newly discovered eclogites belt in the Lhasa terrane. 1. Chashagang Group; 2. Indosinian granite; 3. schistosity occurrence; 4. marble; 5. ductile fault; 6. Leilongku Group; 7. Middle Cenozoic volcanic rock; 8. strike-slip fault; 9. Carboniferous volcanic rock; 10. Mabuku Group; 11. Middle Cenozoic granite; 12. quartzite; 13. supposed fault; 14. stratigraphic section location; 15. reverse thrust; 16. ultramafic rock; 17. formation occurrence; 18. eclogites. Df1 to Df5 are ductile shear zones

The Songduo eclogite belt lies in the central part of the Lhasa Block over 100 km northeast of the Lhasa City along the northern margin of the Gangdese magmatic arc, where it divides the block into the southern and northern segments (Fig. 1). Eclogites occur as tectonic slices in a belt of garnet-bearing mica-quartz schist. Its width is about 500–1 000 m. It extends over distances of 100 km in an E-W direction (Fig. 2).

It consists of coarse-grained, massive eclogites, locally accompanied by remnant ophiolite fragments (Yang et al., 2006). High pressure minerals have not been observed in these ophiolite fragments. P-T calculations based on the garnet- omphacite-phengite (Grt-Omp-Phen) mineral assemblage yield peak metamorphic conditions of 2.7 GPa and 730 C (Yang et al., 2009), closing to the phase boundary between coesite and quartz. As pointed out above, inclusions of polycrystalline quartz in garnet and omphacite may be pseudomorphs after coesite (Yang et al., 2009). The Songduo eclogites can be regarded as part of a very high-pressure metamorphic belt.

The petrology and geochemistry characteristics of the Songduo eclogites suggest their derivation from low-K2O tholeiitic basalts or gabbros. However, most of the samples show negative Nb anomalies in MORB-normalized trace element patterns and some are enriched in Ba, Rb and K, suggesting some suprasubduction zone input (Li et al., 2009). Thus, the protolith is considered to be MORB-type oceanic crust that underwent some modification during subduction (Li et al., 2009). A whole-rock Sm-Nd age of 305.5±50 Ma and a SHRIMP U-Pb zircon age of 261.7±5.3 Ma suggest that eclogites formed during the Early Carboniferous to Late Permian by subduction of Tethyan oceanic lithosphere along the northern margin of the Gangdese arc (Li et al., 2009; Yang et al., 2009).

The northern segment of the Lhasa Block is characterized by Triassic and older sequences, which may be the products of northward subduction of Paleo-Tethys. In the southern segment, the rocks are mostly Jurassic and Lower Cretaceous in age, and probably reflect northward subduction of Neo-Tethys. Deformation in the boundary area between the two blocks is much better developed than in the blocks themselves. On the basis of the deformation and the presence of some ophiolitic ultramafic fragments, the boundary area where Songduo eclogites crop out is recognized as a zone of convergence and collision between the southern and northern Lhasa blocks. Eclogites are believed to mark a Carboniferous– Permian suture zone within the Lhasa Block.

2 SAMPLES AND ANALYTICAL PROCEDURES 2.1 Sample Description

We observed that many garnet grains in the eclogite samples contain abundant quartz inclusions. Some quartz inclusions may be pseudomorphs after coesite, although to date no actual coesite inclusion has been found. Most of the quartzs were formed into irregular patches between other minerals. The others occur as large, individual grains. It suggests that the quartz was formed at a relatively late stage.

Eclogites consist of variable percentages of garnet+ omphacite+epidote±phengite±quartz±rutile. According their dominant minerals, three types of eclogites are recognized, rutile eclogite, quartz eclogite and phengite eclogite. The sample locations, textures and structures, mineral compositions and thin section descriptions are given in Table 1.

Table 1 The information of the samples measured in this study

Thin section descriptions of the studied samples show that: (1) Most garnet occur as light pink, nearly equidimensional grains that are randomly distributed in eclogites. Many grains are rimmed or partly replaced along cracks by actinolite, epidote and phlogopite. (2) Omphacite form xenomorphic, columnar and granular grains randomly distributed between the garnet crystals. They are light green in color and exhibit weak pleochroism. Like garnets, omphacites are rimmed and partly replaced by mixtures of actinolite, epidote, albite and quartz. (3) Secondary minerals are mostly actinolite and epidote with subordinate albite and quartz. A few grains of rutile are present in eclogites.

2.2 Analytical Procedures

Helium measurements were generally performed by crushing or melting method. Crushing method can minimize the post-eruptive radiogenic contributions from decay of U and Th, which reside in the solid matrix. In addition, concentrations of U and Th in the Songduo eclogites are low ((0–0.1)×10-6 and (0.07–0.36)×10-6 g/g, respectively) (Li et al., 2009). As a result, in-situ generation of radiogenic 4He can be neglected. The released radiogenic 4He by U and Th will reduce the ratio of 3He/4He observed in this study.

On the basis of field relationships and detailed petrography character, two typical eclogite samples (13SD-29, 13SD-40) were selected for detailed investigation. The samples were grounded into 40–80 mesh and grains of garnet and omphacite were handpicked under a binocular microscope.

Helium isotopic compositions were analyzed with a noble gas static vacuum mass spectrometer at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. Detailed analytical procedures are presented by Su et al. (2014) and He et al. (2011).

The mineral grains were crushed by a one-step or multistep process in a vacuum crusher at about 2 000 psi to extract the noble gases. Helium was trapped in a cold finger at about 13 K and released at 35 K. Helium was introduced into the noblesse mass spectrometer, operating in static mode. A cold GP50 getter and liquid nitrogen-cooled charcoal finger were used to minimize the partial pressure of residual gases during the analysis.

The mass resolution of the noblesse instrument (> 700) is sufficient to allow complete separation of 3He and HD, and the hydride tail is checked before each measurement. Line blanks were run before samples. Helium blanks were negligible (3He blank < 3×10-17 cc STP). The data were corrected for system blank and calibration by air shot and helium HESJ standards (Matsumoto et al., 2002). The measured noble gas contents and isotope ratios are presented in Table 2.

Table 2 Helium isotopic compositions of garnet and omphacite in the Songduo eclogites
3 RESULTS

Helium abundances and their isotopic ratios in garnet and omphacite grains in the Songduo eclogites are given in Table 2. By the first crushing, 4He abundances of garnet, 13SD-29G and 13SD-40G in eclogites, are 3.2×10-7 and 3.7×10-7 cm3 STP/g, yielding 3He/4He ratios of 0.53 and 0.58 Ra. 4He abundances of omphacite grains, 13SD-29P and 13SD-40P in eclogites, are 23.2×10-7 and 51.4×10-7 cm3 STP/g, yielding 3He/4He ratios of 0.46 and 0.57 Ra. The 4He contents of omphacite are much higher than those of garnet.

In all measurements, 3He/4He ratios of garnet and omphacite are fallen in the range of 0.27–0.60 Ra. These values are much lower than those of mantle rocks (6–7 Ra) (Farley and Neroda, 1998; Dunai and Baur, 1995), but higher than crustal rocks, about 0.01–0.05 Ra (Stuart et al., 1995).

The 3He contents of garnet and omphacite are fallen in the range of 2.37×10-13 to 3.00×10-13 cm3 STP/g and 14.9×10-13 to 41.0×10-13 cm3 STP/g, respectively (Table 2). In all samples, the 3He contents of omphacite are some higher, about 5–17 times, than those of coexisting garnets.

The 3He/4He ratios of garnet and omphacite through the first crushing are fallen in the range of 0.53–0.58 Ra and 0.46–0.57 Ra, respectively. There is no significant difference between the 3He/4He ratios of garnet and omphacite.

4 DISCUSSION 4.1 Retention Ability of Helium in Garnet and Omphacite

The geochemical behavior of helium isotopes during HP-UHP metamorphism is poorly understood. In particular, it is unclear whether eclogites formed during plate subduction could retain the isotopic signature of the protolith or of the subduction environment. The status restricted the application of helium isotopes in the study of eclogites. Until nowadays, helium isotopic studies have only been carried out on rocks from the Dabie-Sulu ultrahigh-pressure metamorphic belt (Li S F et al., 2005; Li Y H et al., 2000; Du et al., 1998). In this paper, we present a new case study to use helium isotopes as a tracer during the formation of eclogites.

In all measurements, 4He contents of omphacite are 6–16 times higher than those of garnet. The significant enrichment of 4He in omphacite might have been controlled by the crystallization of the minerals or their lattice structures.

The retention abilities of both 3He and 4He in omphacite are significantly higher than those of garnet. The differences apparently reflect the different ability to capture and retain helium isotopes between garnet and omphacite.

During the formation of eclogites, the pressure, temperature, water content and other factors will influence the ability of garnet and omphacite to capture helium isotopes. In addition, helium abundance in garnet and omphacite reported here was derived in part from diagenetic processes. Thus, the helium content and isotopic character of a given sample may have been affected by late diffusion-induced loss, exogenous sources and isotope fractionation.

4.2 Source of Helium in the Songduo Eclogites

Eclogites are metamorphic rocks. It might have been formed at a depth of 50 km or more. During the subduction, most fluids in the protolith escaped (Selverstone et al., 1992). Under the high pressure and temperature, helium can diffuse and migrate quickly. The new balance of helium isotopic composition can be achieved (Stuart et al., 1994). The helium isotopic signatures of eclogites would be obtained during the metamorphism. The source of helium might come from the environment in which ecologites were formed.

In addition to the 3He captured during the formation of eclogites, additional 3He might potentially have been produced by later cosmic radiation or nuclear reaction (Dunai et al., 2007; Ozima and Podosek, 2002). Such later processes would lead to increased 3He/4He ratios in eclogites.

In this work, for the garnet grains, we show the result of the crushing of the same sample in the different steps. In fact, the results of the first two steps of analysis are 0.58 and 0.6, respectively. For the limit of the analysis precision, there is no difference between the first and second crushing steps. However, with the increase of crushing times and pressure, the additional radioactive 4He in the lattice may lead to a decreased ratio. The ratio would be increased due to the additional cosmic ray-induced 3He. The ratio is decreased to 0.27 Ra in the third crushing step. Typically, the reason may be the addition of radiogenic 4He, or the 3He content is too low, even lower than the detection line.

However, the Songduo eclogites were originally intrusive rocks. They were formed at depths of 0.5 to several kilometers (Yang et al., 2006), which are below the penetration depth of most cosmic radiation. The present-day outcrops were unroofed relatively recently. As a result, eclogites presumably have not been exposed to strong cosmic radiation. The cosmogenic source of 3He can be ruled out.

The composition of the Songduo eclogites is lithium (Li)- poor and low in potassium (K). The whole-rock K2O contents of eclogites are less than 0.24 wt.% (Li et al., 2009). Electron microprobe analyses of garnet and omphacite indicate very low K2O contents in these minerals (Chen et al., 2009). The content of Li is below the detection limit (Yang et al., 2006). U and Th concentrations in eclogites are also low ((0–0.1)×10-6 and (0.07–0.36)×10-6 g/g, respectively) (Li et al., 2009; Bernard- Griffiths et al., 1985; Heier, 1963). Such large-ion-lithophile (LIL) elements cannot easily enter the crystal lattice of either garnet or omphacite. The nuclear reaction of 6Li (n, α) to produce nucleogenic 3He requires neutrons in addition to large numbers of Li atoms. However, spontaneous fission of U and Th produces abundant neutrons as well as 4He. Thus, 3He and 4He produced by this mechanism would be coupled. Hence, nucleogenic 3He could not be responsible for the high 3He/4He ratios of eclogites.

In contrast, low 3He/4He ratios can be produced by: (1) Late diffusion loss of 3He from minerals. However, previous studies have found that noble gases are readily retained in ultramafic and mafic rocks (Ye et al., 2007; Matsumoto et al., 2002; Basu et al., 1995), and their isotopic compositions are not affected by chloritization. Thus, late diffusion loss of 3He from garnet and omphacite can be ruled out. (2) In-situ generation of radiogenic 4He. The Songduo eclogites was probably formed during the Early Carboniferous to Late Permian, so a small amount of in-situ radiogenic 4He may have been added to eclogites. (3) Alteration of the rocks by crustal fluids. Crustal 3He/4He ratios of the Earth's are fallen in the range of 0.01–0.05 Ra (Stuart et al., 1995). Whereas, the 3He/4He ratios of garnet and omphacite in the Songduo eclogites range from 0.46 to 0.58 Ra. Thus, the interaction between eclogites and crustal fluids might account for some of the low 3He/4He ratios observed in this study.

A plot of the 4He versus 3He concentrations for the Songduo eclogites showing the fields of primal helium (A), mantle-derived helium (B) and crust-derived helium (C) is given in Fig. 3. All the analyzed garnet grains and omphacite grains plot between fields B and C, indicating a mixed crust-mantle source.

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Figure 3. Helium isotopic composition of the Songduo eclogites of the Lhasa terrane. The fields of A, B and C are from Mamyrin and Tolstikhin (1983)
4.3 Contribution of Deep Mantle Materials to the Formation of the Songduo Eclogites

The high pressure metamorphism events of the Songduo eclogites occurred in the late Middle Permian (262±5 Ma). It represents the closing time of the Songduo Tethys Ocean. The Sm-Nd isochron age of the whole eclogite is 306 Ma. It suggests that the Songduo Tethys Ocean has been existed in the Late Carboniferous. But its opening time is still inconclusive.

Noble gases in the solid earth usually originate from five main reservoirs: (1) hotspot, in which 3He/4He > 30 Ra, indicating a deep, hot mantle source (Graham et al., 1993; Honda et al., 1991); (2) OIB source with excess 3He (OIB+), in which 3He/4He is higher than 8±1 Ra, also indicating a deep mantle source (Hilton et al., 1999; Kurz et al., 1983); (3) OIB source depleted in 3He (OIB-), in which 3He/4He ranges from 4.3 to 8 Ra, indicating an oceanic island related to subduction (Nagao and Takahashi, 1993); (4) MORB, with 3He/4He ratios of 8±1 Ra (Hilton et al., 1999; Kurz et al., 1983); and (5) crust, with 3He/4He ratios ranging from 0.01 to 0.05 Ra (Stuart et al., 1995). Because 4He can also be produced by decay of U and Th, the 3He/4He ratio of a closed mantle reservoir continuously evolves toward lower values. As a result, 3He/4He ratios depend on several factors, including the He concentration, (U+Th)/3He ratio, time (or age), and degree of mixing between components with different evolution trajectories.

The 3He/4He ratios of the Songduo eclogites are also higher than any other reported values for such rocks. Most 3He/4He ratios of garnet and omphacite in the Dabie eclogites are fallen in the range from 0.085 to 0.331 Ra (Li et al., 2000), indicating a mixture of continental crust and mantle sources. Another case study shows that the 3He/4He ratios of whole-rock of the Dabie eclogites, their dominant minerals (garnet and omphacite) and the surrounding rocks (peridotite and pyroxenite) range from 0.002 to 0.81 Ra (Li et al., 2005). The 3He/4He ratios of quartz veins in HP/UHP metamorphic rocks from the Chinese Continental Scientific Drilling (CCSD) main hole (1 700–2 300 m) in the Sulu terrane range from 0.193 to 0.359 Ra (Chen et al., 2006). 3He/4He ratios of garnet and omphacite in the Songduo eclogites are much higher than those observed in the Dabie eclogites. Such high ratios are typically thought to be associated with deep mantle sources (Hilton et al., 1999; Farley and Neroda, 1998). Thus, deeper mantle materials might have contributed to the formation of the Songduo eclogites.

5 CONCLUSIONS

The previous studies have outlined the petrochemistry (Chen et al., 2009), Sm-Nd and Rb-Sr isotopic compositions (Li et al., 2009), and SHRIMP U-Pb ages of the eclogites (Xu et al., 2007). Based on these data, several models were proposed to explain the origin and evolution of the Lhasa terrane (Chen et al., 2009; Yang et al., 2009). In this paper, we present high precision helium isotopic data of the Songduo eclogites. We discuss the genesis of eclogites in light of its helium isotopic characteristics. The helium isotopes of eclogites can provide important information on the subduction environment when they formed.

(1) The Songduo eclogites in the Lhasa terrane have high 3He/4He ratios and different helium abundances in garnet and omphacite. Distinct differences in the helium abundances of garnet and omphacite are tentatively related to the different retention ability of helium in garnet and omphacite. Omphacite has a higher ability to capture or retain 4He than garnet.

The 4He content of omphacite is 6–16 times of that of garnet. And the 3He content of omphacite is about 5–17 times of that of the coexisting garnet grains, leading to the similar 3He/4He ratios observed in garnet and in omphacite. The enrichment of 4He in omphacite might have been controlled by the crystallization of the minerals or the lattice structure.

(2) The analyzed garnet and omphacite grains appear to have been derived from a mixed crust-mantle source. The mixing appears to have occurred during formation of the Songduo eclogites.

(3) The 3He/4He ratios of garnet and omphacite range from 0.27 to 0.60 Ra. The Songduo eclogites have much higher 3He/4He ratios than those observed in the Dabie eclogites. Such high ratios are typically thought to be associated with deep mantle sources. We cautiously conclude that the deep mantle materials might have contributed to the formation of the Songduo eclogites.

ACKNOWLEDGMENTS

We gratefully thank Prof. Paul Robinson for the valuable comments and suggestions. Discussions with Prof. Zeming Zhang improved the paper significantly. This research was supported jointly by the National Natural Science Foundation of China (Nos. 41373029, 41773029) and the China Geological Survey (Nos. DD20190060, 12120114061501). The final publication is available at Springer via https://doi.org/10.1007/s12583-019-1226-9.


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