Liang FH, Yang JS Xu ZQ and Zhao JN. 2014. Chromium in the olivine lattice：Chromium-rich olivines and their implication of deep mantle origin in the Luobusa mantle peridotite and chromitite, Tibet. Acta Petrologica Sinica,30(8): 2125-2136 (in Chinese with English abstract)
Chromium in the olivine lattice：Chromium-rich olivines and their implication of deep mantle origin in the Luobusa mantle peridotite and chromitite, Tibet
LIANG FengHua, YANG JingSui, XU ZhiQin, ZHAO JiaNan
CARMA, State Key Laboratory of Continental Tectonics and Dynamics, Institute of Geology, CAGS, Beijing 100037, China
Abstract: Chromium-rich included olivines and chromite exsolution rod-rich deformed porphyroclast olivines occur in the Luobusa mantle peridotite and chromitite in the Yarlung Zangbo ophiolite, Tibet. Included olivines are subhedral to hedral with size up to 50μm and occur in spinels or chromites in harzburgite, dunite, disseminated chromitite, nodular chromitite and massive chromitite. Deformed porphyroclast olivines are coarse-grained (up to 10mm), undulatory extincted, kink banded and recrystallized. Amounts of chromite exsolution rods arrange parallelly with light-white color and extremely narrow width of <3μm. In each sample, no matter what lithology, included olivines contain obviously higher Cr2O3 and lower FeO than other olivines. Parts of those included olivines contain by far the highest Cr2O3 (up to 1.49%) in the world and much higher than those in lunar and chondrite olivines (~0.7%). Except for a weak negative correlation of Cr2O3 vs. FeO for those included olivines, no other correlation of Cr2O3 vs. MnO and of Cr2O3 vs. MgO was observed, distinguishing from those chromium-rich olivines in lunar basalts (Cr varies inversely with Fe) and from those in St. Mesmin chondrite (Cr varies positively with Fe). Aluminum is near background levels and minors of manganese (~0.1%) and nickel (0.3%~0.5%) were detected in included olivines. Combing with previous studies, we analyzed the valence state and substitution of chromium in Cr-rich olivine or its precursor phase. Cr2+ is believed to be the controlled valence in the octahedral site of olivine since an in-situ moissanite in one of samples studied here has been discoveried. A possible vacancy substitution mechanism is proposed to explain the uncorrelated characteristic between Cr and other metal elements. According to lots of findings of ultra-high pressure and extremely reductive minerals reported by previous studies, that the chromium-rich olivines in the Luobusha mantle peridotite and chromitite might originate from mantle transition zone or lower mantle and their precursor phase might be wadsleyite or ringwoodite.
3 罗布莎蛇绿岩中的地幔橄榄岩和铬铁矿
本文选择康金拉矿区和罗布莎矿区5种不同类型的10块新鲜橄榄岩和铬铁矿(表 1)进行橄榄石的矿物成分研究。按辉石含量递减和铬铁矿含量递增的顺序分别为：方辉橄榄岩(图 2a)、纯橄岩(图 2b)、浸染状铬铁矿(图 2c)、豆状铬铁矿(图 2d)和块状铬铁矿(图 2e)。其中纯橄岩是作为豆荚状铬铁矿的包壳产于方辉橄榄岩和铬铁矿矿体之间的(图 2f)，其主晶橄榄石和尖晶石的成分均介于方辉橄榄岩和铬铁矿之间呈过渡状态(Zhou et al., 1996; 梁凤华，2011)。Liang et al.(2014)在YLL10-31纯橄岩中发现原位碳硅石，反映纯橄岩中携带有下地幔来源的物质。从浸染状铬铁矿到块状铬铁矿，橄榄石含量渐少，在块状铬铁矿中，少量橄榄石呈残晶分布于铬铁矿之间(图 2e)。
表 1
Table 1
表 1(Table 1)
表 1 罗布莎不同类型新鲜橄榄岩和铬铁矿样品及特征Table 1 Charateristics of mantle peridotites and chromitites samples in the Luobusa ophiolite
样品号
岩性
矿物组成(vol%)
采样位置
YLL10-27
方辉橄榄岩
橄榄石80，斜方辉石15，铬尖晶石5
康金拉矿区(海拔：5277m)
YLL10-28
橄榄石85，斜方辉石10，铬尖晶石2
YLL10-29
纯橄岩
橄榄石90，铬尖晶石10
YLL10-31
橄榄石95，铬尖晶石5
YLL10-33
浸染状铬铁矿
橄榄石70，铬铁矿30
YLL10-34
橄榄石55，铬铁矿45
YLL10-36
豆状铬铁矿
橄榄石50，铬铁矿50
YLL10-38
橄榄石20，铬铁矿80
YLL10-39
块状铬铁矿
橄榄石15，铬铁矿85
YLL8-33
块状铬铁矿
铬铁矿98，细粒橄榄石包裹体和辉石包裹体2
罗布莎矿区(海拔3597m)
表 1 罗布莎不同类型新鲜橄榄岩和铬铁矿样品及特征 Table 1 Charateristics of mantle peridotites and chromitites samples in the Luobusa ophiolite
图 2
Fig. 2
图 2 罗布莎蛇绿岩中不同类型地幔橄榄岩和铬铁矿的显微照片及本文所研究的纯橄岩的产出特征
(a)-方辉橄榄岩；(b)-纯橄岩；(c)-浸染状铬铁矿；(d)-豆状铬铁矿；(e)-块状铬铁矿；(f)-本文所研究的纯橄岩是指位于方辉橄榄岩和铬铁矿矿体之间的、作为豆荚状铬铁矿包壳产出的纯橄岩.Ol-橄榄石；Opx-斜方辉石；Sp-a-b中为铬尖晶石，c-f中为铬铁矿；Har-方辉橄榄岩；Dun-纯橄岩；Cr-豆荚状铬铁矿矿体Fig. 2 Typical microphotographs of mantle peridotite and chromitite in the Luobusa ophiolite
(a)-harzburgite;(b)-dunite;(c)-disseminated chromitite;(d)-nodular chromitite;(e)-massive chromitite;(f)-dunite studied here occurs as the crust of podiform chromitite. Ol-olivine; Opx-orthopyroxene; Sp-spinel; Har-Harzburgite; Dun-dunite; Cr-podiform chromitite
图 3 罗布莎蛇绿岩地幔橄榄岩和铬铁矿中不同类型橄榄石的显微照片特征
(a、b)粗粒变形残晶橄榄石(Olpo)和细粒重结晶橄榄石(Olre)；(c-f)被包裹在尖晶石中的自形-半自形包裹体橄榄石(Olin)，相对包裹体橄榄石，其他橄榄石表示为OlM. Sp-(a-e)中为铬尖晶石，(f)中为铬铁矿Fig. 3 Microphotographs of olivine with different occurrences in mantle peridotite and chromitite in the Luobusa ophiolite
(a，b)-deformed ourse porphyroclast olivine(Olpo) and subhedral fine-grained recrystallized olivine(Olre);(c-f)-hedral or subhedral included olivine in spinel(Olin) and other olivine(OlM). Sp-chromium spinel in(a-e) and chromite in(f)
表 2 不同类型岩石中包裹体橄榄石和其他橄榄岩电子探针成分对比(wt%) Table 2 Mineral composition of included olivines and other olivines in different mantle peridotite and chromitite(wt%)
岩性
方辉橄榄岩
纯橄岩
点号
27-10
27-17
27-23
27-16
27-30
27-4
29-16
29-19
29-22
29-9
31-27
31-7
产状
包裹体橄榄石Olin
其他橄榄石OlM
包裹体橄榄石Olin
其他橄榄石OlM
SiO2
40.97
41.19
41.17
41.20
41.67
41.42
41.24
42.13
41.63
42.28
41.93
41.86
TiO2
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Al2O3
0.01
0.00
0.00
0.00
0.01
0.00
0.00
0.13
0.00
0.00
0.00
0.00
Cr2O3
0.70
0.14
0.26
0.00
0.00
0.00
0.85
1.49
0.81
0.02
0.02
0.00
FeO
7.99
7.35
7.35
8.40
7.75
8.45
4.40
3.52
4.55
5.39
5.84
5.37
MnO
0.13
0.07
0.11
0.15
0.11
0.13
0.07
0.05
0.09
0.07
0.13
0.09
MgO
50.34
50.45
50.72
49.81
50.06
49.72
52.37
52.48
53.00
51.64
51.51
51.08
CaO
0.04
0.01
0.01
0.02
0.00
0.00
0.04
0.03
0.02
0.02
0.02
0.01
NiO
0.37
0.37
0.39
0.37
0.27
0.32
0.33
0.56
0.50
0.39
0.35
0.29
Total
100.57
99.58
100.00
99.95
99.90
100.05
99.32
100.38
100.62
99.81
99.82
98.71
Si
0.99
1.00
1.00
1.00
1.01
1.01
1.00
1.00
0.99
1.02
1.01
1.02
Al
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Ti
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Cr
0.01
0.00
0.01
0.00
0.00
0.00
0.02
0.03
0.02
0.00
0.00
0.00
Fe2+
0.16
0.15
0.15
0.17
0.16
0.17
0.09
0.07
0.09
0.11
0.12
0.11
Mn
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Mg
1.82
1.83
1.84
1.81
1.81
1.80
1.89
1.86
1.89
1.85
1.85
1.85
Ca
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Ni
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
Fo
92
92
92
91
92
91
95
96
95
94
94
94
岩性
浸染状铬铁矿
豆状铬铁矿
块状铬铁矿
点号
33-1
33-8
34-3
33-14
33-24
36-26
36-33
38-6
36-35
38-1
39-11
39-3
39-33
39-17
39-25
产状
包裹体橄榄石Olin
其他橄榄石OlM
包裹体橄榄石Olin
其他橄榄石OlM
包裹体橄榄石Olin
其他橄榄石OlM
SiO2
41.75
43.09
42.08
41.97
42.02
42.31
42.10
41.97
42.57
42.66
41.93
42.20
41.94
42.35
42.45
TiO2
0.00
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.02
Al2O3
0.01
0.09
0.00
0.00
0.01
0.01
0.00
0.01
0.00
0.00
0.01
0.00
0.00
0.01
0.00
Cr2O3
0.85
0.73
0.69
0.01
0.03
0.97
0.65
0.51
0.00
0.03
0.89
0.42
0.50
0.01
0.12
FeO
4.21
3.22
3.48
4.78
4.59
2.68
3.00
2.51
3.76
3.24
2.08
2.42
2.36
2.83
2.42
MnO
0.07
0.06
0.03
0.08
0.03
0.02
0.05
0.07
0.07
0.02
0.06
0.04
0.03
0.07
0.05
MgO
53.15
51.74
52.41
52.38
52.66
53.22
52.92
54.23
53.02
53.49
53.54
53.49
52.84
53.38
53.34
CaO
0.02
0.03
0.05
0.04
0.00
0.00
0.02
0.00
0.03
0.01
0.01
0.01
0.02
0.00
0.01
NiO
0.58
0.41
0.41
0.59
0.54
0.49
0.45
0.64
0.37
0.49
0.85
0.69
0.64
0.66
0.71
Total
100.66
99.38
99.15
99.84
99.90
99.70
99.18
99.96
99.82
99.94
99.38
99.27
98.33
99.32
99.13
Si
1.00
1.03
1.01
1.01
1.01
1.01
1.01
1.00
1.02
1.01
1.00
1.01
1.01
1.01
1.02
Al
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Ti
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Cr
0.02
0.01
0.01
0.00
0.00
0.02
0.01
0.01
0.00
0.00
0.02
0.01
0.01
0.00
0.00
Fe2+
0.08
0.06
0.07
0.10
0.09
0.05
0.06
0.05
0.08
0.06
0.04
0.05
0.05
0.06
0.05
Mn
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Mg
1.89
1.84
1.88
1.88
1.88
1.89
1.89
1.92
1.89
1.90
1.91
1.91
1.90
1.90
1.90
Ca
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
0.00
Ni
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.01
0.02
0.01
0.01
0.01
0.01
Fo
96
97
96
95
95
97
97
97
96
97
98
98
98
97
98
表 2 不同类型岩石中包裹体橄榄石和其他橄榄岩电子探针成分对比(wt%) Table 2 Mineral composition of included olivines and other olivines in different mantle peridotite and chromitite(wt%)
图 4 各类型地幔橄榄岩和铬铁矿中包裹体橄榄石与其他橄榄石的FeO-MgO、Cr2O3-MgO成分对比
Fig. 4 Correlation diagrams of FeO vs. MgO and Cr2O3 vs. MgO for included olivines and other olivines in harzburgite(a)，dunite(b)，disseminated chromitite(c)，nodular chromitite(d) and massive chromitite(e)respectively
All included olivines have obvious richer Cr and pooer Fe than other olivines
图 5
Fig. 5
图 5 各类型地幔橄榄岩和铬铁矿中包裹体橄榄石的FeO、Cr2O3、NiO、SiO2与MgO成分相关图解Fig. 5 Interelemental relationships of included olivines for different types of peridotite and chromitite in the Luobusa ophiolite
(a)-FeO vs. MgO;(b)-Cr2O3 vs. Fo;(c)-NiO vs. MgO;(d)-SiO2 vs. MgO. The arrows show the variation trends of included olivine composition with the decreasing of pyroxene and the increasing of chromite from harzburgite to massive chromitite
图 6 罗布莎地幔橄榄岩(YLL10-31)中部分粗粒残晶橄榄石里的铬铁矿出溶体
(a、b)-单偏光镜下铬铁矿出溶体呈四边形的浅褐色片状；(c、d)-背散射图像中铬铁矿出溶体呈金属亮白色的细长出溶棒.Cr-铬铁矿出溶体；Ol-残晶橄榄石Fig. 6 Microphotographs of chromite exsolutions in coarse-grained porphyroclast olivine from Luobusa mantle peridotite
(a，b)-pure chromite exsolutions in olivine as brown rectangular lamellae under the polarizing microscope;(c，d)-under the backscattered light，chromite exsolutions show as white thin rods. Cr-chromite exsolution; Ol-olivine
表 3
Table 3
表 3(Table 3)
表 3 罗布莎地幔橄榄岩中部分粗粒残晶橄榄石的纯铬铁矿出溶体成分表(wt%)Table 3 Mineral composition of chromite exsolution in coarse-grained porphyroclast olivine from the Luobusa peridotite(wt%)
Spot No.
35-3
35-4
35-5
35-6
35-7
SiO2
0.44
0.05
0.08
0.05
0.05
Na2O
0.03
0.00
0.02
0.05
0.00
Cr2O3
45.33
47.97
44.73
47.12
48.59
K2O
0.00
0.00
0.00
0.00
0.00
MgO
7.47
7.42
6.54
6.44
6.58
Al2O3
10.40
10.61
10.90
9.93
8.95
MnO
0.54
0.47
0.40
0.43
0.62
CaO
0.00
0.00
0.03
0.02
0.00
FeO
31.29
30.58
32.84
33.80
33.70
TiO2
0.04
0.01
0.00
0.06
0.14
NiO
0.19
0.06
0.11
0.12
0.09
Total
95.79
97.17
95.64
98.00
98.70
表 3 罗布莎地幔橄榄岩中部分粗粒残晶橄榄石的纯铬铁矿出溶体成分表(wt%) Table 3 Mineral composition of chromite exsolution in coarse-grained porphyroclast olivine from the Luobusa peridotite(wt%)
形成于低氧逸度环境的玄武质月岩和陨石中的富铬橄榄石，普遍被认为形成于极端还原条件下的岩浆结晶过程，还原态的Cr2+替代Mn2+或Fe2+进入橄榄石晶格(Butler，1972; Dodd et al., 1975)。Sutton et al.(1993)应用X射线吸收近边结构光谱(X-ray Absorption Near Edge Structure，XANES)技术，实际测得玄武质月岩富铬橄榄石中的铬为还原态的Cr2+，证实其形成于还原环境，并提出富铬橄榄石的形成不仅仅受控于氧逸度。
形成于低氧逸度环境的玄武质月岩和陨石中的富铬橄榄石，普遍被认为形成于极端还原条件下的岩浆结晶过程，还原态的Cr2+替代Mn2+或Fe2+进入橄榄石晶格(Butler，1972; Dodd et al., 1975)。Sutton et al.(1993)应用X射线吸收近边结构光谱(X-ray Absorption Near Edge Structure，XANES)技术，实际测得玄武质月岩富铬橄榄石中的铬为还原态的Cr2+，证实其形成于还原环境，并提出富铬橄榄石的形成不仅仅受控于氧逸度。
由于地球表层的富氧环境，学者们对地球上那些特殊的富铬橄榄石的成因尤为争议，早期部分学者认为富铬橄榄石的形成与压力有关(Green et al., 1975; Meyer，1975)，尤其在深部地幔非常高的压力和低氧逸度环境下，Cr2+更容易在橄榄石的畸变晶格中稳定存在(Burns， 1975a，b)。但另一部分学者否认压力的作用，Arai(1978)提出铬是以Cr3+进入橄榄石晶格的，上地幔中岩浆和熔体活动的高温条件是主要控制因素，而与压力无关。他认为前人(在他之前70年代初的研究，如Green et al., 1975; Meyer，1975; Burns， 1975a，b等)提出的高压成因模式必须要求极低的氧逸度环境，晶格中的Cr2+析出成为铬铁矿又需要自由氧的加入，地球的上地幔环境不满足该要求，而且也没有相关的超高压矿物的证据。继Arai(1978)之后，Moseley(1984)和Irving et al.(1992)在分别分析橄榄石中磁铁矿+透辉石出溶结构和铬铁矿出溶体时又进一步强调了这一观点，形成了高温模式的代表。
不过此后，除了Sutton et al.(1993)实验验证了玄武质月岩中富铬橄榄石的还原态Cr2+之外，Hanson and Jones(1998)通过实验和理论计算，认为虽然Cr3+在较高氧逸度的上地幔环境中更易进入橄榄石晶格，但在氧逸度比自然铁-方铁矿缓冲反应(Iron-Wüstite buffer)更低的过渡带或下地幔极端还原环境中，Cr2+将主要占据橄榄石晶格。 Stachel et al.(2000)在几内亚康康金伯利岩的金刚石中发现含铬铁矿出溶体的橄榄石与四方晶系铁铝-镁铝榴石相矿物(TAPP)共生，Brenker et al.(2002)进一步分析了该合晶状共生体，认为他们是由来自下地幔的林伍德石(Ringwoodite)在向上迁移的过程中，经由地幔过渡带的瓦兹利石相(Wadsleyite)退变，再被带至上地幔浅部进一步减压分解而来，含铬铁矿出溶体的橄榄石是地幔过渡带瓦兹利石直接分解的产物。
Cr2+的离子半径为0.82(Shannon and Prewitt， 1969)，最易替代离子半径相近的Mn2+(0.83)、Fe2+(0.77)而进入橄榄石的八面体配位晶格(Dodd et al., 1975)。不过虽然Mg2+半径(0.72)有些小，但位于对称中心M1位置的Mg2+或许可以得到M1位置本身的大晶体场分裂能的帮助，而也可能被Cr2+替换(Burns，1975b)。Butler(1972)总结了Appllo 12号月岩玄武岩中的含铬橄榄石成分，发现Cr2O3的含量与FeO呈负相关关系(图 7a)，似乎Cr2+更易替代Fe2+。Dodd et al.(1975)则发现St. Mesmin陨石的富铬橄榄石中，Cr2O3的含量与FeO呈正相关关系(图 7b)，与MnO具相关性，认为Cr2+替代Mn2+进入橄榄石晶格的可能性更大，且它们的离子半径最相近。
本文罗布莎地幔橄榄岩的富铬包裹体橄榄石中，Cr2O3似乎不与其他元素呈相关趋势，如图 7c，d中Cr2O3与FeO、MnO均没有明显的相关性，图 5b中Cr2O3也不随橄榄石Mg#的变化而变化。不过图 7c中，如果只看同一样品，Cr2O3与FeO有微弱的负相关关系，但与MnO则没有。如此，我们猜想Cr2+除了替代部分Fe2+，是否可能主要占据橄榄石晶格中的空位(Δ)呢？Janney and Banfield(1998)在模拟计算似莱河矿的氧化态橄榄石晶格中离子占位情况时，发现所有的空位都占据M1位置，且可以与其他离子一起构成稳定晶体场，并与实验中观察到的晶格缺陷特征相吻合。因此，我们推测罗布莎地幔橄榄岩和铬铁矿中的富铬橄榄石，由于形成于地幔转换带或下地幔这样的高温高压和极端还原条件，Cr2+是以占据空位和部分替代Fe2+的方式进入富铬橄榄石的前身——瓦兹利石或林伍德石晶格中的。
图 7
Fig. 7
图 7 富铬橄榄石中Cr2O3含量与FeO、MnO含量的相关关系图解
(a)-Appllo 12玄武质月岩中的富铬橄榄石(Butler，1972)；(b)-St. Mesmin陨石中的富铬橄榄石(Dodd et al., 1975)；(c、d)-本文罗布莎地幔橄榄岩和铬铁矿中的富铬橄榄石，其中的图例同图 5Fig. 7 Correlation diagrams of Cr2O3 vs. FeO and Cr2O3 vs. MnO for chromium-bearing olivine
(a)-Appllo 12 lunar basalt(Butler，1972);(b)-St. Mesmin chondrite(Dodd et al., 1975);(c，d)-mantle peridotite and chromitite in the Luobusa ophiolite，the symbols are same as in Fig. 5
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2014, Vol. 30
