Geochemistry of plagioclase from the Mid-Atlantic Ridge

We have studied the chemical zoning of plagioclase phenocrysts from the slow-spreading Mid-Atlantic Ridge and the intermediate-spreading rate Costa Rica Rift to obtain the time scales of magmatic processes beneath these ridges. The anorthite content, Mg, and Sr in plagioclase phenocrysts from the Mid-Atlantic Ridge can be interpreted as recording initial crystallisation from a primitive magma (~11 wt% MgO) in an open system. This was followed by crystal accumulation in a mush zone and later entrainment of crystals into the erupted magma. The initial magma crystallised plagioclase more anorthitic than those in equilibrium with any erupted basalt. Evidence that the crystals accumulated in a mush zone comes from both: (1) plagioclase rims that were in equilibrium with a Sr-poor melt requiring extreme differentiation; and (2) different crystals found in the same thin section having different histories. Diffusion modelling shows that crystal residence times in the mush were <140 years, whereas the interval between mush disaggregation and eruption was <=1.5 years. Zoning of anorthite content and Mg in plagioclase phenocrysts from the Costa Rica Rift show that they partially or completely equilibrated with a MgO-rich melt (>11 wt%). Partial equilibration in some crystals can be modelled as starting <1 year prior to eruption but for others longer times are required for complete equilibration. This variety of times is most readily explained if the mixing occurred in a mush zone. None of the plagioclase phenocrysts from the Costa Rica Rift that we studied have Mg contents in equilibrium with their host basalt even at their rims, requiring mixing into a much more evolved magma within days of eruption. In combination these observations suggest that at both intermediate- and slow-spreading ridges: (1) the chemical environment to which crystals are exposed changes on annual to decadal time scales; (2) plagioclase crystals record the existence of melts unlike those erupted; and (3) disaggregation of crystal mush zones appears to precede eruption, providing an efficient mechanism by which evolved interstitial melt can be mixed into erupted basalts.我们针对慢速扩张的大西洋中脊（Mid-Atlantic Ridge）与中等扩张速率的哥斯达黎加裂谷（Costa Rica Rift）中的斜长石斑晶（plagioclase phenocrysts）化学环带展开研究，以获取这两类洋脊下方岩浆作用的时间尺度。大西洋中脊斜长石斑晶中的钙长石含量、镁（Mg）及锶（Sr）元素特征，可被解读为记录了开放体系下原始岩浆（约11 wt% MgO）的初始结晶过程。该过程后续伴随晶体在晶粥带（mush zone）中的堆积，以及后期晶体被裹挟进入喷发岩浆的过程。初始岩浆结晶出的斜长石钙长石含量高于所有与喷发玄武岩平衡的斜长石。晶体在晶粥带中堆积的证据来自两方面：(1) 斜长石环带与贫锶熔体平衡，该熔体需经历极端分异作用；(2) 同一切片内的不同晶体具有各异的演化历史。扩散模拟（diffusion modelling）结果显示，晶体在晶粥带中的驻留时间（residence times）小于140年，而晶粥解聚（disaggregation）至岩浆喷发的时间间隔不超过1.5年。哥斯达黎加裂谷斜长石斑晶的钙长石含量与镁元素环带特征表明，它们部分或完全与富氧化镁熔体（MgO含量>11 wt%）达到平衡。部分晶体的部分平衡过程可被建模为喷发前不到1年时启动，但另有部分晶体需更长时间才能达到完全平衡。若该混合过程发生在晶粥带中，则可最直观地解释这种时间尺度的多样性。本次研究中，哥斯达黎加裂谷的所有斜长石斑晶，即使是其环带边缘，其镁元素含量也未与其寄主玄武岩达到平衡，这意味着在喷发前数天内，这些晶体需混入更为演化的岩浆中。综合上述观测结果，我们提出在中等扩张与慢速扩张洋脊均存在以下规律：(1) 晶体所处的化学环境以年至十年为尺度发生变化；(2) 斜长石斑晶记录了与喷发熔体不同的熔体的存在；(3) 晶粥带的解聚似乎发生在喷发之前，这为演化的粒间熔体（interstitial melt）混入喷发玄武岩提供了高效机制。