Plume-lithosphere interaction beneath Mt. Cameroon volcano, West Africa : Constraints from 238U-230Th-226Ra and Sr-Nd-Pb isotope systematics
Plume-lithosphere interaction beneath Mt. Cameroon volcano, West Africa : Constraints from 238U-230Th-226Ra and Sr-Nd-Pb isotope systematics
复制标题
DOI：
10.1016/j.gca.2007.01.010
复制
发表时间：
2007-04
影响因子：
5
通讯作者：
T. Yokoyama;F. Aka;M. Kusakabe;E. Nakamura
中科院分区：
文献类型：
--
作者：
T. Yokoyama;F. Aka;M. Kusakabe;E. Nakamura
精确测量了过去100年内火山喷发熔岩中238 U-230 Th-226 Ra的不平衡。喀麦隆，连同主要和微量元素，和Sr-Nd-Pb同位素比值，解开玄武岩浆的来源和过程中板内构造环境。所有样品均存在238 U-230 Th-226 Ra不平衡，其中230 Th（18-24%）和226 Ra（9-21%）过量，并且在（226 Ra/230 Th）-（230 Th/238 U）图中存在正相关性。不同喷发年龄熔岩的~（238）U-~（230）Th-~（226）Ra不平衡程度不同，但~（230）Th/~（232）Th比值基本不变。将铀系结果与铅同位素比值结合，在（230 Th/238 U）-（206 Pb/204 Pb）和（226 Ra/230 Th）-（206 Pb/204 Pb）图解中观察到负相关。岩浆混合、结晶分离和围岩同化等浅部岩浆房过程不能解释这种相关性。地壳污染不是所观察到的同位素变化的原因，因为大陆地壳被认为具有极其不同的Pb同位素组成和U/Th比值。一个化学性质不均匀的地幔的熔融可能解释了Mt.喀麦隆的数据，但动态熔融条件下的高DU和DU/DTh，长岩浆上升时间，或不平衡的矿物/熔体分配，是必需的。最合理的方案产生的地球化学特征的山。喀麦隆样品是来自软流圈地幔的熔体与上覆的陆下岩石圈地幔的相互作用，由于晚中生代交代作用，U/Pb（>0.75）和Pb同位素比值（206 Pb/204 Pb>20.47）升高。
Precise measurements of238U–230Th–226Ra disequilibria in lavas erupted within the last 100yr on Mt. Cameroon are presented, together with major and trace elements, and Sr–Nd–Pb isotope ratios, to unravel the source and processes of basaltic magmatism at intraplate tectonic settings. All samples possess238U–230Th–226Ra disequilibria with230Th (18–24%) and226Ra (9–21%) excesses, and there exists a positive correlation in a (226Ra/230Th)–(230Th/238U) diagram. The extent of238U–230Th–226Ra disequilibria is markedly different in lavas of individual eruption ages, although the (230Th/232Th) ratio is constant irrespective of eruption age. When U-series results are combined with Pb isotope ratios, negative correlations are observed in the (230Th/238U)–(206Pb/204Pb) and (226Ra/230Th)–(206Pb/204Pb) diagrams. Shallow magma chamber processes like magma mixing, fractional crystallization and wall rock assimilation do not account for the correlations. Crustal contamination is not the cause of the observed isotopic variations because continental crust is considered to have extremely different Pb isotope compositions and U/Th ratios. Melting of a chemically heterogeneous mantle might explain the Mt. Cameroon data, but dynamic melting under conditions of high DUand DU/DTh, long magma ascent time, or disequilibrium mineral/melt partitioning, is required. The most plausible scenario to produce the geochemical characteristics of Mt. Cameroon samples is the interaction of melt derived from the asthenospheric mantle with overlying sub-continental lithospheric mantle which has elevated U/Pb (>0.75) and Pb isotope ratios (206Pb/204Pb>20.47) due to late Mesozoic metasomatism.