Observation of ferromanganese crusts on lava deposits recovered near the Galapagos spreading center area

The unusual petrological diversity of abyssal lavas erupted along some segments of the Galapagos spreading center is a direct consequence of the propagation (elongation) of these segments into older oceanic crust. With increasing distance behind propagating rift tips, relatively unfractionated MORB erupted close to the tips are joined first by FeTi basalts (bimodal assemblage) and then by a wide range of basaltic and siliceous lavas. Further behind propagating rift tips, this broad range diminishes again, approaching the narrow compositional range of adjacent normal ridge segments. These compositional variations reflect the evolution of the subaxial magmatic system beneath the newly forming spreading center as it propagates through a pre-existing plate. We envisage this evolution as proceeding from small, isolated, ephemeral magma chambers through increasing numbers of larger, increasingly interconnected chambers to the steady-state buffered system of a normal ridge. Throughout this evolution, magma supply rates gradually increase and cooling rates of crustal magma bodies decrease. High degrees of crystal fractionation are favored only when a delicate balance between cooling rate and resupply rate of primitive magma is achieved. At other propagating and non-propagating ridge-transform intersections the degree to which the balance is achieved and the length of ridge over which it evolves control the distribution of fractionated lavas. These effects may be evaluated provided a number of tectonic variables including transform length, spreading and propagation rates are taken into account.

沿加拉帕戈斯扩张中心（Galapagos spreading center）部分区段喷发的深海熔岩，其异常的岩石学多样性，直接源于这些区段向更古老洋壳的传播（伸长）过程。随着距传播型裂谷尖顶后方距离的增加，紧邻尖顶处喷发的相对未分异的洋中脊玄武岩（Mid-Ocean Ridge Basalt, MORB），会先与铁钛玄武岩（双峰组合）汇合，随后又会与多种玄武质及硅质熔岩混合。在传播型裂谷尖顶后方更远的区域，这一宽泛的熔岩组分范围会再次收窄，趋近于相邻正常洋脊区段的狭窄组分区间。这些组分变化，反映了新生扩张中心在穿过预先存在的板块时，其轴下岩浆系统的演化过程。我们将这一演化过程设想为：从小型、孤立的瞬时岩浆房开始，随着规模更大、连通性更强的岩浆房数量不断增多，最终过渡至正常洋脊的稳态缓冲系统。在整个演化过程中，岩浆供给速率逐渐升高，地壳岩浆体的冷却速率则不断降低。仅当原始岩浆的冷却速率与补给速率达到微妙平衡时，才会出现强烈的晶体分异作用。在其他传播型与非传播型洋脊-转换断层交汇区，平衡的达成程度以及演化所覆盖的洋脊长度，共同控制着分异熔岩的分布格局。若要评估这些效应，需纳入转换断层长度、扩张速率与传播速率等多项构造变量进行分析。