IODP385-U1550单斜辉石指示加利福尼亚湾瓜伊马斯盆地海底扩张早期MORB深部结晶作用

Deep Crystallization Processes of MORB During the Early Stage of Seafloor Spreading:Implication from Clinopyroxene of IODP385-U1550 in Guaymas Basin,Gulf of California

洋中脊玄武岩(MORB)分离结晶深度是控制其成分变化以及洋壳增生的关键因素之一。为了探究海底扩张早期洋中脊岩浆分离结晶的最深深度,本文对国际大洋发现计划(IODP)385航次在加利福尼亚湾瓜伊马斯盆地U1550站位获得的玄武质岩浆岩样品进行了详细的岩石学和矿物学研究。U1550站位岩心主要由拉斑玄武岩、辉绿岩以及辉绿岩捕虏体组成,其单斜辉石主要为普通辉石,透辉石次之。单斜辉石-熔体温压计计算得出U1550玄武岩单斜辉石结晶温度(1126~1170℃,误差±45℃)和压力(–0.5~5.4kbar,误差±1.4kbar)相对辉绿岩及辉绿岩捕虏体单斜辉石(1158~1212℃,误差±45℃;1.6~5.9kbar,误差±1.4kbar)均较低,结晶深度(0~17.8km)相对后者(5.3~19.5km)较浅。与全球其他洋脊段MORB单斜辉石相比, U1550单斜辉石具有更陡峭的冷却轨迹。综合研究表明,在海底扩张早期,扩张速率很可能均为慢速扩张,其岩浆结晶深度可超过10 km。这暗示着在海底扩张早期岩石圈地幔厚度很厚,地幔部分熔融产生岩浆的终止深度也相对更深,导致岩浆结晶深度深,部分岩浆难以抵达浅部洋壳。但不同于慢速-超慢速扩张成熟洋脊段,海底扩张早期深部岩浆房岩浆供给可能并不匮乏,而是这些岩浆房很可能位于更深的部位。

The depth of MORB fractional crystallization is one of the key factors to control the MORB composition and oceanic crustal accretion.In order to determine the deepest crystallization depth associated with mid ocean ridge magma process in the early stage of seafloor spreading,we take detailed petrological and mineralogical study on the basaltic rock samples in the Guaymas Basin which were drilled in the site U1550 during the International Ocean Drilling Program(IODP)Expeditions 385.U1550 samples belong to tholeiitic magma,mainly composed of basalt,dolerite and dolerite xenoliths.The clinopyroxene in our samples is predominantly augite and diopside.The crystallization temperature(1126–1170℃,±45℃)and pressure(–0.5–5.4 kbar,±1.4 kbar)of clinopyroxene in basalt calculated by the clinopyroxene-liquid thermobarometers are both lower than those of dolerite and dolerite xenoliths(1158–1212℃,±45℃;1.6–5.9 kbar,±1.4 kbar),and the crystallization depth(0–17.8 km)is shallower than the latter(5.3–19.5 km).Compared to clinopyroxenes from the other global spreading centers,U1550 clinopyroxenes have a steeper cooling trajectory.Comprehensive studies suggest that in the early stage of seafloor spreading,the spreading rate is likely to be slow,and the depth of magma crystallization can exceed 10 km.It implies that the mantle lithosphere was very thick in the early stage of seafloor spreading,and the terminate depth of magma produced the mantle partial melting was relatively deeper,resulting in deep crystallization of magma and difficulty to upwell shallow oceanic crust.However,unlike mature oceanic ridges spread at low speed,the magma supply of deep magma chambers may not be scarce in the early stage of seafloor spreading,instead the magma chamber could be located in deeper depth than we thought.