卡尔斯伯格脊天休洋脊段的深部岩浆过程:来自洋中脊玄武岩中单斜辉石结晶条件的约束

Magmatic processes in the depth of Tianxiu segment,Carlsberg Ridge:Implication from crystallization conditions of clinopyroxene in MORB

洋中脊玄武岩(MORB)是了解洋中脊的热状态、岩浆供给及演化过程的绝佳研究对象。本文对中国大洋49航次第五航段(DY49V)在西北印度洋卡尔斯伯格脊天休热液区周边获得的拖网(49V-DR02)玄武岩样品进行了详细的岩石学和矿物学研究,并利用单斜辉石-熔体温压计对玄武岩中单斜辉石的结晶温度和压力进行了估算,用以探讨天休热液区所在洋脊段的深部岩浆过程。结果表明,所研究的玄武岩样品属于拉斑质N-MORB,其中单斜辉石呈鳞片状、树枝状或骸晶状微晶,主要为普通辉石和透辉石。单斜辉石-熔体温压计计算结果显示天休洋脊段单斜辉石结晶温度为1153~1225℃,结晶压力在0.08~0.64GPa范围内,相当于2.6~21.1km的深度,但主要集中在10~16km深度。这说明单斜辉石的结晶温度相对集中,但结晶深度范围变化很大,从浅层上洋壳区域到大洋岩石圈地幔区域均有单斜辉石结晶作用发生,但主要集中在大洋岩石圈地幔区域,暗示天休洋脊段下方洋壳内可能不发育熔体透镜体或岩浆房。天休洋脊段单斜辉石如此深的结晶深度指示天休洋脊段岩石圈地幔厚度厚,地幔部分熔融产生岩浆的终止深度深。在该洋脊段10~16km深处富含高温熔体,为天休热液系统的热液循环提供深部热源。

Mid-ocean ridge basalt(MORB)is a type of excellent material for understanding the thermal condition,magma supply and evolution of the mid-ocean ridge.To investigate the deep magmatic processes of the Tianxiu segment at the slow spreading Carlsberg Ridge,detailed petrological and mineralogical studies on MORB(49V-DR02)dredged basalts from the Tianxiu segment from the DY49V cruise were carried out to estimate the crystallization temperature and pressure of clinopyroxene in the dredged basalts by clinopyroxene-liquid thermobarometers.The dredged basalts are tholeiitic N-MORB,and clinopyroxene in the basalts are dominantly augite and diopside that generally are scaly,dendritic or skeletal microcrystals in the matrix.Clinopyroxene-liquid thermobarometers yield crystallization temperatures of 1153~1225℃and pressures of 0.08~0.64GPa,equivalent to a depth of 2.6~21.1km,most of them focus within 10~16km of depth.The crystallization temperatures have a narrow range of variation,but the crystallization depth is very dispersed from a shallow upper ocean crust to a deeper ocean lithosphere mantle region,but most of them are dominantly in the ocean lithosphere mantle region,indicating no melt lens or magma chamber occurs in the ocean crust in the Tianxu segment.It implies that the mantle lithosphere of the Tianxiu segment was very thick,and the terminate depth of magma produced by the mantle partial melting was relatively deeper.Voluminous melts gathered within the depth of 10~16km to affect the Tianxiu hydrothermal cycle as a heat source.