| 地幔橄榄岩捕掳体中尖晶石筛状边的成因 |
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| 引用本文： | 胡森林,罗丹,陈立辉.地幔橄榄岩捕掳体中尖晶石筛状边的成因[J].岩石学报,2017,33(1):69-80. |
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| 作者姓名： | 胡森林 罗丹 陈立辉 |
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| 作者单位： | 江苏省地质调查研究院, 南京 210018;南京大学内生金属矿床成矿机制研究国家重点实验室, 南京大学地球科学与工程学院, 南京 210023,南京大学内生金属矿床成矿机制研究国家重点实验室, 南京大学地球科学与工程学院, 南京 210023,南京大学内生金属矿床成矿机制研究国家重点实验室, 南京大学地球科学与工程学院, 南京 210023 |
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| 基金项目： | 本文受国家自然科学基金项目（41372064、41172060）资助. |
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| 摘 要： | 山东的无棣大山和栖霞方山是两个以霞石岩为主的新生代火山,含有大量的地幔橄榄岩捕掳体,橄榄岩捕掳体中的尖晶石常见黑色反应边。在BSE图像上,这些反应边根据结构的不同又可以进一步分为两种:筛状边和均一边。有时两种反应边在同一颗尖晶石中共存并相互过渡。尖晶石捕掳晶和捕掳体中与寄主岩直接接触的尖晶石常发育均一边;发育筛状边的尖晶石则一般不直接与寄主岩接触,但位于捕掳体的边缘。尖晶石的筛状边呈多孔状,为富Cr尖晶石或铬铁矿。筛状边成分上与核部(Cr_2O_3=7.52%~36.75%,Cr#=7.80~44.20,Mg#=44.70~74.48)区别明显,具有高的Cr_2O_3含量(23.42%~65.96%)、Cr#值(78.97~92.49)以及低的Mg#值(17.22~43.02)。另外,筛状边相对其核部(TiO 2=0.00%~0.53%,MnO=0.04%~0.35%)还显示偏高的TiO 2(0.20%~3.60%)和MnO(0.29%~1.93%)。在筛状边附近存在富Al_2O_3(8.00%~17.57%)和MgO(17.89%~26.02%)的玻璃。尖晶石的均一边无孔洞但多发育裂理,成分上与核部突变,并以富TiO 2(20.90%~6.64%)和FeO T(70.79%~53.92%)为特征,最外部为钛磁铁矿。均一边具有明显的成分分带,表现为由内至外TiO 2、FeO T含量逐渐增高,Al_2O_3(0.04%~16.34%)、Cr_2O_3(0.77%~25.69%)和MgO(0.50%~7.16%)含量逐渐降低。尖晶石的两种反应边与寄主岩密切的空间关系说明其成因与寄主岩浆有关。虽然尖晶石筛状边的Cr#值(79.0~92.5)明显高于核部(7.8~44.2),但是其相对核部偏高的TiO 2和MnO含量,以及显著降低的Mg#值均与单纯的部分熔融趋势不吻合。根据尖晶石两种反应边的结构特征、成分特征和相互的空间关系,我们认为筛状边与均一边是岩浆与尖晶石反应不同阶段的产物。当橄榄岩捕掳体被岩浆捕获后,由于成分上的不均衡两者之间将发生溶解反应。反应过程中,尖晶石中不同元素向熔体迁移的速率区别明显,其中Cr是最难熔的元素也是迁移最慢的元素。在反应的初始阶段,由于易溶组份Al、Mg等元素随熔体迁出,尖晶石发育筛状边,并同时伴随Cr#值的显著升高和Mg#值的显著降低。此时,寄主岩浆中的Ti、Mn等元素也部分扩散进入筛状边中的残留尖晶石。如果熔体足够多,反应得以持续进行,筛状边中的Cr最终也将被熔体带走。在反应产生的混合熔体中金属氧化物最早达到饱和并结晶。随着反应的持续进行,结晶出的矿物将继续生长、变粗,并相互连接,直至孔洞消失形成均一边。同时伴随着矿物的不断结晶,溶解界面上的熔体成分也不断变化,从而使均一边具有明显的成分分带。在橄榄岩捕掳体内部,由于渗进捕掳体中的岩浆有限,渗透岩浆与尖晶石之间的反应多限于早期阶段,反应产物为筛状边。在捕掳体边缘,与寄主岩浆直接接触的尖晶石由于参与反应的岩浆量足够多,因此一般产生反应的终端产物———均一边。
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| 关 键 词： | 尖晶石 熔体-岩石相互作用 溶解反应 地幔捕掳体 碱性玄武岩 |
| 收稿时间： | 2015/10/1 0:00:00 |
| 修稿时间： | 2016/6/19 0:00:00 |
Genesis of sieve-textured rim of spinel in mantle xenoliths |
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| HU SenLin,LUO Dan and CHEN LiHui.Genesis of sieve-textured rim of spinel in mantle xenoliths[J].Acta Petrologica Sinica,2017,33(1):69-80. |
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| Authors: | HU SenLin LUO Dan and CHEN LiHui |
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| Institution: | Geological Survey of Jiangsu Province, Nanjing 210018, China;State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China,State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China and State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China |
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| Abstract: | Black reaction rims are observed in spinel of mantle peridotite xenoliths hosted in alkali basalts from Dashan and Fangshan, Shandong Province. According to their back-scattered images, these reaction rims can be subdivided into homogeneous rims and sieve-textured rims. Two kinds of rim can be observed in a single spinel mineral, too. Homogeneous rims are usually developed in spinel xenocrysts and spinels in peridotite xenoliths, which are directly in contact with host rocks. In contrast, spinels with sieve-textured rim do not direct contact with host, although they are still distributed in the edge of peridotite xenoliths. The sieve-textured rim is characterized with many porosity and compositions of Cr-rich spinel or chromite. The sieve-textured spinel has much higher Cr2O3 content (23.42% to 65.96%), Cr# (78.97 to 92.49) and lower Mg# (17.22 to 43.02) than its unreacted core. Furthermore, the sieve-textured spinel has a bit ligher TiO2 and MnO than the core. There are some glasses in direct contact with the sieve-textured rim, which is characterized with high Al2O3 (8.00%~17.57%) and MgO (17.89%~26.02%) contents. The homogeneous rim has no porosity but fracture, and has higher TiO2 and FeOT contents. Moreover, the outermost of rim is titanomagnetite. The homogeneous rim also shows compositional zoning with decrease of TiO2, FeOT and increase of Al2O3, Cr2O3 and MgO contents from the outer part to the inner part of the rims. The spatial link between the two kinds of rim for spinel and the host rocks suggests that host magmas play an important role in the formation of such rims. Although the sieve-textured spinel has higher Cr# than unreacted core, the lower Mg# and a slight higher TiO2 and MnO do not support a genesis of partial melting for such rims. Here we propose that these two kinds of rim are products of different stages of interactions between spinel and host magmas. When peridotite xenoliths were trapped by host magmas, the disequilibrium between peridotite and host magmas causes dissolution reaction. Cr is the most stable element in spinel, Al and Mg are opposite. At the initial stage, Al and Mg are dissolved into host magmas first and sieve-textured rims are developed in spinel with higher Cr# and lower Mg# values. Moreover, Ti and Mn from host magmas are exchanged into sieve-textured spinel. Metal oxides are firstly saturated and crystalized from the mixed melt which generated by diffusive mixing between dissolved spinel and host magmas. As the reaction proceeds, earlier crystalline minerals grow, coarsen and close the holes, resulting in homogeneous rim finally. Diffusive mixing between melts and minerals crystallization causes variable melt composition at interaction interface, this is the reason of homogeneous rims showing compositional zoning. Since the host magmas infiltrated into the interior of peridotite xenolith is limited, the interaction between magmas and spinel is still at the initial stage, and only sieve-textured rims are developed in these spinel crystals. On the contrary, spinels in direct contact with host magmas show homogeneous rims due to strongly interactions with sufficient reacting magmas. |
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| Keywords: | Spinel Melt-rock interactions Dissolution reaction Mantle xenolith Alkaline basalts |
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