摘要
岩石圈热结构是指地球内部热量在壳幔的配分比例、温度以及热导率和生热率等热学参数在岩石圈中的分布特征。岩石圈的热结构直接影响着岩石的物理性质和流变学性质,同时还控制了化学反应的类型和速度,从而制约着岩石圈的发展和演化。本文在前人CCSD主孔岩石主、微量元素研究基础上,利用Rybach生热率公式计算了钻孔岩石的放射性生热率,并结合岩石热导率的测定研究了CCSD主孔100-2000m岩石的热结构和主孔榴辉岩在不同退变质程度下生热率、热导率的变化:钻孔中岩石的平均生热率为0.95μWm-3,平均热导率为2.96mWm-1K-1。,其中片麻岩生热率高迭1.01-1.7μWm-3,热导率为2.76-2.96mWm-1K-1;基性超基性岩石生热率最低(<0.21μWm-3),热导率则高达3.20mWm-1K-1以上;新鲜榴辉岩生热率、热导率居中,分剐为0.16-0.44μWm-3和3.31-3.85mWm-1K-1。钻孔中榴辉岩生热率、热导率变化主要受岩性控制:从新鲜榴辉岩到完全退变榴辉岩,热导率总体上降低,但从强退变榴辉岩到完全退变榴辉岩,岩石热导率升高;而在此过程中岩石生热率总体上升高,仅当从中等退变质榴辉岩退变为强退变质榴辉岩时,岩石生热率出现降低趋势。在综合研究的基础上预测CCSD主孔5000m深度处温度为139℃,温度范围为131-151℃。根据区域深部地球物理探测成果对CCSD主孔地区岩石圈热结构进行了研究:上地壳底部温度为256℃,中地壳底部温度为492℃,Moho面温度为683℃,岩石圈底部温度为1185℃,来自地幔的热流为44.1mWm-2,对地表热流的贡献率为58%。研究结果表明,由岩石物理方法获得的CCSD主孔地区岩石圈地温曲线与石榴石-二辉橄榄岩包体推断的中国东部地温曲线十分吻合,本文从实验岩石物理学角度为CCSD主孔地区岩石圈热结构研究提供了重要约束
Lithosperic thermal structure, which refers to the percentage of surface heat flow derived form convective mantle and crustal radioactive decay, as well as the distribution of temperature, heat production, thermal conductivity and other thermal physical parameters in the lithosphere, plays an essential role in lithosperic evolution and development, by means of effecting petrophysical properties and controlling the chemical reactions in the lithosphere. In this study, heat production of rocks from CCSD main hole (100-2000 m) were calculated using Rybach formula based on previous studies on major and rare elements of core samples . Combined with thermal conductivity measurement, the thermal parameter properties of different rocks from CCSD mainhole during retrogression were investigated. Overall average heat production is 0.95μWm-3, average thermal conductivity is 2. 96 mWm-1K-1. Among which the mafic ( Ultr-mafic) rocks yield the highest thermal conductivity ( >3. 20 mWm-1K-1 ) and lowest heat production ( 1.0 μWm-3) , fresh eclogite have the mediate heat production and thermal conductivity, which is 0. 16-0. 44μWm-3 and 3. 31-3. 85 mWm-1K-1 respectively. The results indicate that thermal conductivity is clearly decreased with retrogression (2.42 -3.02 mWm-1K-1 for retrograded eclogite) , and heat production shows a slight increase with retrogression, which suggest that thermal parameter properties are correlated with lithologic changes. The temperature at the bottom of the CCSD main hole (5000m) is predicted to be 131-1511. Based on the study on thermal parameter properties of rocks from CCSD main hole (100-2000m) and previous geophysical study, the thermal structure of the area beneath the CCSD mainhole is studied: the temperature at bottom of upper crust, middle crust, Moho and lithosphere are 256℃ , 492℃ , 683℃ and 1185℃ , respectively; heat flow derived from mantle is 44. 1 mWm-2(covers 58% of surface heat flow) and heat flow from convective mantle is 36.6 mWm-2(48% of surface heat flow). The results suggest that petrophysical-derived geotherm is coincident with garnet-lherzolite xenolith-derived geotherm in East China, which offer important constraints on the study on lithospheric thermal structure in the area beneath the CCSD drilling site.