硼同位素及其地质应用研究

Boron Isotope and Its Geological Applications

硼的两个稳定同位素 ( 1 0 B和1 1 B)相对质量差较大 ,因此 ,硼同位素分馏较显著。由于分析测量技术方面的改进和创新 ,硼同位素地球化学近年来有了长足的发展。业已查明 ,自然界中δ1 1 B值变化为 - 37‰～ + 58‰。其中 ,较负的δ1 1 B值见于非海相蒸发硼酸盐矿物和某些电气石 ,而较正的δ1 1 B值见于某些盐湖卤水和蒸发海水。现代大洋水的δ1 1 B值十分恒定 ( + 39 5‰ )。原始地幔的δ1 1 B值估测为 - 10‰± 2‰。陨石的δ1 1 B值很不均一 ,变化可达 90‰。而月岩的δ1 1 B值变化较小 ( - 6‰～ + 4‰ )。由于硼同位素存在大的分馏和不同地质体中截然不同的δ1 1 B值 ,硼同位素地质应用范围十分广泛。目前 ,硼同位素在研究星云形成过程和宇宙事件 ,壳 幔演化和板块俯冲作用过程 ,判别沉积环境 ,研究矿床成因 ,示踪古海洋和古气候条件 ,和判断环境污染源区等方面的研究中成效显著。

The stable boron isotopes are developed in recent years as an extremely powerful geochemical tool Boron has two stable isotopes ( 10 B and 11 B) and there occurs a large fractionation of boron isotopes in nature with δ 11 B values from -37‰ to +58‰ Those with very negative δ 11 B samples are from non marine evaporites and some tourmalines, whereas those with very positive δ 11 B samples are salt lake brines and evaporitic seawater Meteorites show a large variation in δ 11 B values (-50‰ to +44‰), but the δ 11 B variations in lunar rocks are relatively limited (-6‰ to +4‰) The δ 11 B values of volcanic rocks vary from -15‰ to +21‰ depending on the rock type and the degree of crust contamination, and the primitive mantle has been estimated having a δ 11 B value of -10‰ ±2‰ The boron isotope compositions of metamorphic rocks vary with metamorphic grade and their protoliths Modern oceanic sediments have δ 11 B vaues of -6 6‰ to +4 8‰ , whereas ancient sedimentary rocks are slightly depleted in 11 B with an limited δ 11 B database of -17 0‰ to -5 6‰ Tourmalines from granites and pegmatites and from massive sulfide deposits have δ 11 B vaules of -37‰ to +0 8‰ , -25‰ to +10‰ , and -23‰ to +18‰ , respectively Modern oceanic water has a rather constant δ 11 B value of +39 5‰ Submarine hydrothermal fluids show a δ 11 B range from -2 6‰ to +36 8‰ , depending on the tectonic setting of the vent fluids Subaerial geothermal fluids and groundwater have generally lower δ 11 B values than seawater and vary considerably with different settings and country rocks Boron has three major geochemical characteristics: (1) very soluble The majority of boron occurs in rocks and waters in crust and hydrosphere During water/rock interaction, boron is highly mobile (2) very large fractionation between the two isotopes of 10 B and 11 B due to their relatively large mass differences (3) lack of complexity associated with oxidation/reduction reaction The dominant boron forms in natural solutions are B(OH) 3 and B(OH) 4 -, their relative concentrations and δ 11 B values are pH dependent These features set boron significantly apart from other stable and radiogenic isotopes and make boron a unique and sensitive geochemical tracer Recently boron isotopes have found its major applications in the following fields: (1) cosmochemistry and star formation processes; (2) crust mantle evolution and subduction related processes; (3) sedimentary environments and the source of boron; (4) hydrothermal ore forming processes and ore genesis; (5) paleo ocean pH estimation and paleoclimatology; (6) groundwater research and environmental geochemistry [KH2D][WT5HZ]