Boron cycling by subducted lithosphere; insights from diamondiferous tourmaline from the Kokchetav ultrahigh-pressure metamorphic belt
Boron cycling by subducted lithosphere; insights from diamondiferous tourmaline from the Kokchetav ultrahigh-pressure metamorphic belt
复制标题
DOI：
10.1016/j.gca.2008.05.002
复制
发表时间：
2008-07
影响因子：
5
通讯作者：
T. Ota;K. Kobayashi;T. Kunihiro;E. Nakamura
中科院分区：
文献类型：
--
作者：
T. Ota;K. Kobayashi;T. Kunihiro;E. Nakamura
包括地表物质在内的岩石圈向地幔深处的俯冲作用是地球化学演化的基础。然而，岩石圈在俯冲至深部期间的化学演化仍然是模棱两可的。为了识别近地表和俯冲带深处受地质作用影响的物质，我们研究了Kokchetav超高压变质带中一种独特的含金刚石富钾电气石（K-tourmaline）的B和Li同位素行为。k -碧玺岩心含微金刚石，相对于10b富集11b (δ11B=−1.2 ~ +7.7)，相对于6li富集7li （δ7Li=−1.1 ~ +3.1）。认为k -电气石是在Kokchetav峰变质高压和高温条件下形成的硅酸盐熔体在金刚石稳定场中的高压结晶。该k -碧玺的重同位素特征不同于普通na -碧玺，富集于轻B同位素（δ11B=−16.6 ~−2.3），并经历了变质脱水反应的同位素分馏。重b同位素特征的可能来源是俯冲岩石圈地幔中的蛇纹岩。岩石圈地幔蛇纹岩化主要发生在海沟-外隆或海沟斜坡的正断层作用下，随后海水渗透岩石圈地幔，形成了重b同位素富集的岩石圈地幔蛇纹岩化。在次弧区域俯冲的岩石圈地幔中，蛇纹岩的破裂可能为流体提供了重的b同位素特征，这些特征是在俯冲前的蛇纹岩化过程中获得的。流体可以上升并引起上覆地壳层的部分熔融，由此产生的硅酸盐熔体可以继承重b同位素特征。俯冲岩石圈地幔是模拟近地表进入深部地幔的流体和相关元素通量的关键储存库。
Subduction of lithosphere, involving surficial materials, into the deep mantle is fundamental to the chemical evolution of the Earth. However, the chemical evolution of the lithosphere during subduction to depth remains equivocal. In order to identify materials subjected to geological processes near the surface and at depths in subduction zones, we examined B and Li isotopes behavior in a unique diamondiferous, K-rich tourmaline (K-tourmaline) from the Kokchetav ultrahigh-pressure metamorphic belt. The K-tourmaline, which includes microdiamonds in its core, is enriched in11B relative to10B (δ11B=−1.2 to +7.7) and7Li relative to6Li (δ7Li=−1.1 to +3.1). It is suggested that the K-tourmaline crystallized at high-pressure in the diamond stability field from a silicate melt generated at high-pressure and temperature conditions of the Kokchetav peak metamorphism. The heavy isotope signature of this K-tourmaline differs from that of ordinary Na-tourmalines in crustal rocks, enriched in the light B isotope (δ11B=−16.6 to −2.3), which experienced isotope fractionation through metamorphic dehydration reactions. A possible source of the heavy B-isotope signature is serpentine in the subducted lithospheric mantle. Serpentinization of the lithospheric mantle, with enrichment of heavy B-isotope, can be produced by normal faulting at trench-outer rise or trench slope regions, followed by penetration of seawater into the lithospheric mantle. Serpentine breakdown in the lithospheric mantle subducted in subarc regions likely provided fluids with the heavy B-isotope signature, which was acquired during the serpentinization prior to subduction. The fluids could ascend and cause partial melting of the overlying crustal layer, and the resultant silicate melt could inherit the heavy B-isotope signature. The subducting lithospheric mantle is a key repository for modeling the flux of fluids and associated elements acquired at a near the surface into the deep mantle.