pge geochemistry tibet

Vol. 44 No. 11 SCIENCE IN CHINA (Series D) November 2001

PGE geochemistry of Jiding ophiolite in Tibet and its constraint on mantle processes

XIA Bin (� �)1,2, CHEN Genwen (��)1, MEI Houjun (��)1,

GUO Lingzhi (��)3, XIAO Xuchang (��)4, YU Hengxiang (��)1,

QI Liang (� �)2, WANG Guoqiang (��)1 & ZHONG Zhihong (��)1

1. Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China;

2. Open Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang

550002, China;

3. Department of Geosciences, Nanjing University, Nanjing 210093, China;

4. Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China

Correspondence should be addressed to Xia Bin (email: [email protected])

Received March 29, 2001

Abstract The total PGE amount (ΣPGE) of mantle peridotite in the Jiding ophiolite is slightly higher than that of the primitive mantle, but the PGE contents of basalt are higher than those of the mid-ocean ridge basalt (MORB), with obviously lower Pd/Ir ratios. The accumulates, dyke swarm and basalts show remarkable negative Pt and positive Rh anomalies, resulting in the spe-cial N-type PGE patterns. Mantle peridotite and crustal rocks have similar distribution patterns. It is proposed that the PGE distribution patterns in the Jiding ophiolite are closely related with a higher degree of partial melting of the mantle in this region. Magmatic crystallization-differentiation led to PGE fractionation, thus making the contents of PGE in the accumulates decrease in the ascending direction. The higher content of Au in the Jiding ophiolite is the result of metasomatic alteration at later stages. Pt-Pd fractionation indicates that both the PGEs are controlled by their alloy and sul-fide phases. Positive Rh anomalies seem to be related with higher oxygen fugacity in the melts.

Keywords: platinum group element (PGE), geochemistry, ophiolite, upper mantle, Jiding of Tibet.

The platinum group elements (PGE) are distributed largely in basic and ultrabasic rocks in

nature, and 99% of the PGE occurs in magmatic sulfide deposits throughout the world[1]. As the

PGE and Au have similar properties, they are always used together to study the evolution of man-

tle materials, core/mantle differentiation, the distribution of Earth’s materials in the early periods

of time, mantle processes, meteorite impact events, the origin of basalts and the genesis of

PGE-bearing magmatic sulfide deposits and chromite deposits[2—6]. In recent years we have made

detailed investigations on the PGEs in ophiolites developed in the Yarlung Zangbo River suture

zone[7]. This paper focuses on the PGE geochemistry of the Jiding ophiolite and its relations with

the mantle processes.

1 Samples and analytical methods

The Yarlung Zangbo River ophiolite zone is considered to represent the early tectonic back-

ground of the oceanic basin[8—13]. The Jiding ophiolite is located in the middle segment of the Yar-

1020 SCIENCE IN CHINA (Series D) Vol. 44

Yarlung Zangbo River suture zone and is one of the well developed ophiolite zones in China

(fig.1). It was formed during the Early Cretaceous. In the Jiding ophiolite mantle peridotite, ac-

cumulate, dyke swarm, pillow basalt and radiolarian chert are developed. The mantle peridotite is

composed mainly of harzburgite. In going downwards the rock grades to lherzolite; in going up-

wards, dunite. The rocks have been ophiolitized. The accumulate zone has been obviously differ-

entiated with its lower part consisting of diallage peridotite, olivine diallagite and diallage gabbro

and in going upwards the rock facies grades to stratiform gabbro and isotropic gabbro. Wang et

al.[9,10] classified the accumulate zone into: (1) the critical zone or transitional zone at the bottom;

(2) the layered complex zone; and (3) the isotropic gabbro zone. The dyke swarm is composed of

gabbro diabase and diabase. Dolerite and pillow basalt are recognized at the top. The studied sam-

ples in this work include harsburgite and dunite in the mantle peridotite, diallage gabbro and gab-

bro in the accumulates, diabase in the dyke swarm in the lower part, and basalt in the upper part.

The samples were taken almost from every part of the ophiolite section. Sample analyses were

made by Qi Liang at the Open Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry,

Chinese Academy of Sciences. The samples were fused and decomposed with Na2O2, coprecipi-

tated with Te, and analyzed by using isotope dilution-ICP-MS techniques. Twice coprecipitation

with Te was conducted to purify HCl and SnCl2 in the chemical reagents used for analysis. The

analytical instrument Finnigan MAT ELEMENT Model high resolution ICP-MS was employed in

this study. The technical detection limit (10−9) was Ir 0.02, Ru 0.045, Rh 0.024, Pt 0.23, Pd 0.11,

and Au 0.32. The detailed analytical procedure was described in ref. [14]. Ni, Cu and Cr were de-

termined using ICP-MS techniques.

Fig. 1. Sketch map showing distribution of the Jiding ophiolite (from ref. [9]). 1, Cretaceous; 2, basalt; 3, diabase;

4, sheeted sill; 5, cumulate; 6, gabbro; 7, harzburgite and dunite; 8, fault; 9, sampling section.

No. 11 PGE GEOCHEMISTRY OF JIDING OPHIOLITE IN TIBET AND ITS CONSTRAINT 1021

2 The contents of PGE and Au and their distribution

2.1 The contents of PGE and Au mantle peridotite

The contents of PGE and Au in the mantle peridotite in the Jiding ophiolite of Tibet vary

over a narrow range (table 1), but the contents of PPGE (Pt and Pd) vary a little more greatly than

those of IPGE (Ir and Ru). The total amount of PGE is 1.4 — 2.3 times that of the primitive man-

tle (20.6 × 10−9)[15,16]. The Pd/Ir ratios range from 0.67 to 2.49. The total amount of PGE in harz-

burgite and pyroxene-bearing dunite is (27.69—30.1) × 10−9 and 46.25 ×10−9, respectively, being

1.4 and 2.3 times that of the primitive mantle peridotite. As compared with the harzburgite, the

pyroxene-bearing dunite is obviously enriched in Pd, Pt and Au.

Table 1 PGE and Au contents of the Jiding ophiolitea)

Ir Ru Rh Pt Pd Au ΣPGE Cu Ni

JD-42 Harzburgite 6.49 7.98 0.98 6.90 7.75 22.39 30.10 7.58 1842.81

JD-43 Harzburgite 7.60 9.81 0.95 4.55 4.77 22.52 27.69 9.35 2133.85

JD-44 Pyroxene-bearing dunite 7.01 8.70 1.02 12.10 17.42 38.15 46.25 7.43 1778.13

JD-50 Orthopyroxene peridotite 4. 41 10.99 5.94 9.29 6.63 7.01 37.26 13.89 1804.82

JD-51 Gabbro troctolite 0.83 2.11 4.96 2.90 6.04 4.95 16.84 146.57 336.57

JD-52 Gabbro troctolite 7.33 1.96 0.99 11.21 5.94 10.83 27.43 145.53 32277

JD-57 Dark-colored pyroxenite 0.88 1.02 0.76 1.98 1.32 15.58 5.96 60.94 106.47

JD-59 Diallage gabbro 0.52 1.40 1.45 1.22 1.98 5.18 6.52 51.60 131.31




JD-80 Diabase 0.67 1.75 9.62 2.85 3.31 3.75 18.20 9.99 7.20

JD-81 Diabase 0.98 1.26 0.82 0.17 3.31 4.25 6.54 6.60 14.85

JD-83 Diabase 0.64 1.80 3.30 0.75 4.99 3.88 11.48 57.28 66.08

JD-84 Diabase 0.57 1.29 1.77 0.06 2.88 3.56 6.54 52.89 50.11

JD-85 Diabase 0.82 0.60 1.04 0.45 3.71 2.46 6.62 56.06 47.19

JD-111 Basalt 0.75 1.78 1.01 0.44 2.52 2.90 6.50 16.75 9.95

JD-112 Basalt 1.18 0.83 0.54 0.50 1.22 2.81 4.27 14.17 9.55

JD-116 Basalt 0.86 3.19 2.97 2.80 6.26 2.42 16.08 38.48 41.89

JD-119 Basalt 0.78 1.63 0.63 0.25 1.25 2.42 4.55 30.05 8.84

JD-122 Basalt 0.54 1.33 0.48 0.76 0.90 4.08 4.01 56.25 45.82

JD-123 Basalt 0.73 1.70 1.35 0.61 1.17 3.51 5.56 34.09 21.87

JD-125 Basalt 0.63 0.54 0.81 0.16 1.37 2.86 3.51 29.63 24.96

Primitive mantle 3.6 4.3 1.7 7 4 1 20.6 28 2110

a) PGE and Au in × 10−9; Cu and Ni in × 10−6. Pt of the primitive mantle are from ref. [15]. The other PGE and Au data of

the primitive mantle are from ref. [16], and Cu and Ni data from ref. [17].

The total amount of PGE is (4.21—37.26) × 10−9, showing a wide range of variations. As

compared with accumulates in ophiolites of Bay of Island district[18], orthopyroxenite in the ac-

cumulates is extremely high in PGE contents while clinopyroxenite shows a great variation in

PGE contents, which is considered as an outstanding feature of ophiolites in the orogenic belt. In

the Troodos ophiolite suite the contents of PGE in the upper portion of gabbro are within the range


of (11— 22.5) × 10−9 and those in the lower portion may reach 100 × 10−9[19]. The total amount of

PGE in gabbro in the K�di ophiolite of West Kunlun is estimated at (2.4—20.59) × 10−9 and that

of pyroxenite ( 2.37—23.98 ) × 10−9[20]. The situation is similar to that in the region studied. There

have been recognized two PGE-high horizons in the lower part of the accumulate at Jiding. This

case has been also encountered in the Troodos complex, but its PGEs are concentrated mainly in

the chromite and sulfide-high horizons in the lower part. However, in the region studied no chro-

mite horizon has yet been found, and microscopic observations have not revealed any sulfide. The

contents of PGE, especially Ir, Pd, Pt and Ru, in the section show a tendency of decreasing from

the bottom to the top. This indicates that in the course of formation of the accumulate the crystal-

lization-differentiation would have constrained, to some extent, the fractionation of PGE. The

Pd/Ir ratios are within the range of 0.81—7.27 and Pd/Pt ratios, 0.53—2.73 (excluding sample

DJ-72). The Pd/Ir and Pd/Pt ratios in the most majority of the samples are higher than those of

chondrites and primitive mantle. The contents of Au in ophiolite are abnormally high and show

little variation, i.e. the contents of Au in the different rocks show little variation. The contents of

PGE are highly variable from one rock type to another in the Jiding ophiolite, which tend to de-

crease in the order of mantle peridotite accumulate diabase and basalt.

2.2 PGE distribution patterns

The chondrite-normalized PGEs were plotted in the order of their melting points (fig.2). The

PGE distribution patterns in the mantle peridotite are of irregular U-shape, different from the

smooth PGE distribution patterns in the primitive mantle peridotite. The IPGE values of peridotite

samples are very close to each other and they are relatively concentrated in the normalized pattern

diagrams. Comparatively, the PPGE values of the samples show a significant difference. Dunite is

obviously richer in PPGE than harzburgite, with (Pd/Ir)N ratios being 0.56—2.24 and (Pd/Pt)N,

1.83— 2.52. Negative Pt anomalies [21] are dominant, with (Pt/Pt*=PtN/SQRT (RhN×PdN)) ranging

from 0.79 to1.07. Some similarity has been observed in PGE distribution patterns between gabbro

and diabase, i.e. the PGE distribution patterns are of N-type. In other areas such PGE distribution

patterns can be observed in chromite, and sulfides and komatiites in a part of the ophiolite suite,

the marginal belt and main belt of the Bushveld Complex and the chromite bed at the bottom of

the Stillwater stratiform intrusive rock[22]. Similar PGE distribution patterns have also been ob-

served in the sulfide-bearing gabbro in the Troodos ophiolite suite[19]. (Pd/Ir)N values of the gab-

bro are within the range of 0.73—6.59. With the exception of one sample, the ratios in all the rest

samples are larger than unity, reflecting that the pattern curves are of the positive slope type. The

Pt/Pt* ratios vary from 0.19 to 2.37. Except one sample, all the rest samples show negative

anomalies. The ratios of several specific PGEs in the diabase are similar to those of gabbro, but

the diabase shows more remarkable Pt anomalies than gabbro, as indicated by much lower Pt/Pt*

ratios, ranging from 0.01 to 0.18. These rocks are all possessed of similar PGE distribution pat-

terns. In comparison to the mantle peridotite the other types of rocks show a PGE distribution pat-


terns with a much larger slope and much bigger variations.

Fig. 2. The PGE distribution patterns in the Jiding ophiolite (the numbers are the same as in table 1).

3 Discussion

3.1 The influence of partial melting of the upper mantle on PGE fractionation

The contents of CaO in three mantel peridotite samples from the Jiding ophiolite are 0.1%,

0.01% and 0.5%, and their Al2O3 contents are 0.14%, 0.09% and 1.63%, respectively, indicating

a strongly depleted residual mantle left behind a higher degree of partial melting. In combination

with the REE contents of mantle peridotite and basalt in the Jiding ophiolite, both the rocks show

LREE depletion and their (La/Yb)N ratios are 0.47 and 0.39, respectively. The La contents of per-

idotite are within the range of (0.02 — 0.10) × 10−6 and its ΣREE (excluding Y) values are

(0.57—1.03) × 10−6; the La contents of basalt are within the range of (2.49 — 3.10) × 10−6 and its

ΣREE (excluding Y) values are (35.40 — 61.26) × 10−6, also indicating that the mantle in this re-

gion has undergone a high degree of partial melting. The mantle peridotite in the Jiding ophiolite

is depleted in Rh while the crustal rocks are enriched in Rh, more or less indicating that the PGE

is related with partial melting of the upper mantle.

The PGE distribution patterns in the mantle peridotite of the Jiding ophiolite have a slightly

positive slope. If Rh is not taken into consideration, the PGE distribution patterns in the accumu-

late, dyke swarm and lava in the upper part would be similar to those of the mantle peridotite in


the lower part (fig. 2). It is shown that the intrusive rocks as a whole have inherited the PGE

composition of the mantle peridotite and the PGE distribution patterns are closely related with

partial melting of the upper mantle. The lavas have the positive slope-type distribution patterns

marked by higher ΣPGE and lower Pd/Ir, inconsistent with the significantly sleep slope-type dis-

tribution patterns of MORB but similar to those of komatiites[23]. The PGE distribution patterns of

komatiites are considered to be the result of a high degree of partial melting of the upper mantle.

Many studies have shown that most of the PGE in the mantle is present in the form of sul-

fides[3,20,22]. However, no sulfide has been found in the peridotite and accumulate samples taken

from the region studied, indicating that sulfur in the lava did not reach the saturation state. Some

lines of indirect evidence suggest that sulfides do have constraints on the distribution of PGE. For

instance, in the lower accumulate there is a horizon

where the sulfophile element copper is high, and also a

horizon where the PGEs are concentrated. Samples JD-50

and JD-52 can be taken for example in this respect. As

can be seen from the Ni/Pd-Cu/Ir plot (fig. 3), distribu-

tion of PGE is controlled by some major geological fac-

tors. The figure shows that the samples are precisely dis-

tributed along the direction of sulfide differentiation, in-

dicating that in the course of magmatic differentiation the

distribution of PGE is substantially controlled by sul-

fides.

At the present time, such a viewpoint is commonly accepted that the PPGE is present mainly

in the form of low-temperature sulfides while the IPGE occurs chiefly as Os-Ir-Ru alloy or

high-temperature sulfides in coexistence with spinel[24]. So, in the case of a low degree of partial

melting, the PPGE will find a way, together with some Cu-bearing sulfides, into melts, but the

IPGE will be left behind as residues. In this way the PPGE is separated from the IPGE. As a result,

in the residual mantle peridotite there appear the PGE distribution patterns marked by a negative

slope while in the basalt are produced the positively steep-inclined PGE distribution patterns

characterized by low PGE contents. These distribution patterns are significantly different from

those of peridotite, as well as of dyke swarm and lava in the upper part in the region studied

(fig. 2) because in this region, there is one sample whose PGE distribution patterns are marked by

a negative slope, the other two mantle peridotite samples are characterized by a positive slope.

Although the PGE distribution patterns of lavas are marked by a positive slope, their Pd/Ir ratios

are far smaller than those of lavas in some typical ophiolites. For example, in the Troodos ophio-

lite suite the pillow lavas have a Pd/Ir range of 90—160[21]. In the case of a high degree of partial

melting and partial melting proceeding rapidly, the refractory Os-Ir-Ru alloy or high temperature

sulfides will melt together with low temperature sulfides, making both compatible and incompati-

ble elements enter into the melts. As a result, the PGE contents of crustal rocks will be enhanced

Fig. 3. The plot of Ni/Pd vs. Cu/Ir for the

Jiding ophiolite. The arrow points to the direc-

tion of differentiation of different minerals[12].

In the plot the triangles stand for peridotite

samples, and the others are accumulate, dia-

base and basalt samples.


and the steep-inclined PGE distribution patterns like those observed in the MORB will not be

produced. At the same time, both peridotite and crustal rocks will possess similar PGE distribution

patterns, as is further evidenced by Cu and Ni contents of the ophiolites in the region studied. As

Ni shows the characteristics of a compatible element during the mantle processes while Cu is an

incompatible element, so, in common cases, with increasing degree of partial melting of the man-

tle, Cu will find its way into melt and the contents of sulfides in the mantle residues tend to be-

come lower and lower. Therefore, the mantle is depleted in S and Cu and enriched in Ni. But the

case is not true in the region studied. Peridotites in this region show depletion in Ni and Cu, with

the contents of Cu ranging from 7.43 × 10−6 to 9.35 × 10−6, only equivalent to 1/4—1/3 of the total

in the primitive mantle (28 × 10−6). The contents of Ni are also relatively low ((1778.13—2133.85)

× 10−6, averaging 1918.26 × 10−6). Cu is positively correlated with Ni, implying that in the process

of partial melting Cu and Ni were exported together and also indicating that both elements were

controlled by the same geological process in the geological history. This phenomenon can be well

explained by the fact that when the mantle underwent a higher degree of partial melting, Ni began

to be incorporated into melt, thus making the mantle rocks become depleted in Ni, though the de-

gree of Ni depletion was lower than that of Cu.

3.2 The influence of magmatic differentiation on PGE fractionation

As compared with diabase dykes, the accumulates in this region have relatively high PGE

contents, which shows a tendency of decreasing from the lower part upwards. Clinopyroxenite and

gabbro-troctolite in the lower accumulates tend to become remarkably enriched in PGE, reflecting

that during the crystallization-differentiation of gabbro there occurred PGE fractionation. Such

phenomenon has been also observed in accumulates of the Troodos ophiolite suite[18]. The PGE

fractionation may be attributed to the differentiation of sulfide and spinel. For example, the two

horizons (one is relatively high in spinel and the other is high in copper) in the lower part of ac-

cumulates have higher contents of PGE. More PGE is observed in accumulates in the Troodos

ophiolite suite. In the horizons where chromite occurs the PGEs are generally high[19].

3.3 Au enrichment and Rh, Pt depletion

As for its distribution in the Jiding ophiotite, Rh features negative depletion in the peridotite

but shows a positive anomaly in the accumulate, dyke swarm and basalt. It is deduced that Rh

fractionation is attributed to extraction which took place in the process of partial melting of the

mantle. The chemical properties of Rh lie between those of the PPGE and IPGE. According to the

classification proposed by Barnes et al.[22], Rh belongs to the PPGE. In the study of the solubility

of Rh in silicates, Amosse et al.[24] and Gueddari et al.[25] reconfirmed its double character, i.e. at

1430 and fO2< 10−2 Pa Rh possesses the properties of PPGE; in the case of fO2> 10−2 Pa, Rh will

become trivalent. Under this circumstance, Rh is easy to find its way into the lattice of spinel. This

is in consistence with the conclusion of Capobianco et al.[26] In the experiments Capobianco et al.


found strong evidence suggesting that Rh has a good geochemical affinity for spinel and its parti-

tioning coefficient between spinel and fluid is 90. Orthopyroxene peridotite and gabbro troctolite

in the lower accumulates in the region studied contain 5.94×10−9 and 4.96×10−9 Rh, respectively.

The contents of Rh are obviously higher than those in the gabbro in the upper part. All this pro-

vides further evidence for the conclusion of Capibianco et al. However, this could not effectively

explain the positive Rh anomalies in the whole accumulate, dyke swarm and basalt. In the study of

the PGE distribution patterns in the Bushveld complex, Mairer et al.[27] obtained a series of PGE

distribution patterns of accumulates by controlling the composition of starting material, the pro-

portion of monosulfide, the contents of sulfides and other factors. It was found that when the pro-

portion of monosulfide solid solutions exceeded 80%, both silicolite and chromitite would show

positive Rh anomaly and negative Pt anomaly. Their PGE distribution patterns are similar to those

of accumulates in the region studied. This indirectly evidenced the influence of fO2 on the forma-

tion of Rh. Based on the limited knowledge of Rh at the present time, it is considered that the

positive Rh anomalies in lavas are the result of Rh and Pd enrichments in the various types of

rocks when they were both incorporated into low-temperature sulfides and then exported by melt-

ing under the conditions of fast resorption to a higher degree and relatively high fO2. In the lavas

Rh negatively correlates with Ir whereas Rh positively correlates with Pd (fig. 4), also indicating

that both Rh and Pd have similar geochemical properties.

Fig. 4. Correlations between Rh and Pd and Ir in crustal rocks of the Jiding ophiolite.

In the Jiding ophiolite, either mantle peridotite or oceanic crustal rock possesses remarkable

Au anomalies. There is no significant difference in Au contents for the various types of rocks and

Au shows a poor correlation with other PGEs, indicating that Au has an independent source. This

implies that Au would have resulted from metasomatism of the rocks by extraneous Au-bearing

fluids. Such metasomatism seems to have originated from later metasomatic alteration. Field obser-

vations disclosed that the rocks have been serpentinized . Although this study did not deal with the

influence of serpentinization on the migration of PGE, the fact that the contents of PGE in some

strongly serpentinized rock samples are relatively constant suggested that the PGE maintained


constant in the process of serpentinization. Previous studies [4,18,22] indicate that metasomatic al-

teration would not lead to PGE migration, though it could make Au remobilize and migrate.

Negative Pt anomalies in the mantle peridotite are closely related with the unique properties

of Pt itself. Studies have shown that the partition coefficient of Pt between alloy and sulfide melt

is 1000 times that of Pd[2]. In the case of alloy-sulfide coexistence, Pt shows a strong tendency of

entering into the alloy phase. Sulfides separated from melts are rich in Pd. Partial melting of the

mantle led to the separation of Pt from Pd in the crustal rocks and ubiquitous depletion in Pt.

4 Conclusions

From the above analysis we can come into the following conclusions:

(1) The total PGE amount of mantle peridotite in the Jiding ophiolite is slightly higher than

that of the primitive mantle and the PGE contents of basalt are higher than those of MORB, with

distinctively lower Pd/Ir ratios. The contents of Au in the ophiolite are commonly high. As ob-

served in the accumulate, dyke swarm and basalt are negative Pt and positive Rh anomalies. The

mantle peridotite and crustal rock have similar PGE distribution patterns.

(2) The PGE distribution of the Jiding ophiolite is related with the higher degree of partial

melting of the mantle in the region studied.

(3) The high contents of Au in the Jiding ophiolite are attributed to later metasomatic altera-

tion. Pt-Pd fractionation indicated that they were both controlled by alloy phase and sulfide phase,

respectively. Positive Rh anomaly may be related with relatively high fO2 in the melt.

(4) Crystallization-differentiation of the accumulates led to PGE fractionation and caused

PGE in the accumulates to decrease in the ascending direction.

Acknowledgements Thanks are due to Prof. Xu Yigang for his valuable comments. This work was financially supported jointly by the National Natural Science Foundation of China (Grant No. 49772109), the National Climbing Program of China (Grant No. 95-Yu-25-03), and the National Key Basic Research Program of China (Grant No. 1999043204).

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