Project Details
Projekt
Wehrlites in the ICDP OmanDP drill cores: a key to understand the nature of the hydrous magmatism in subduction zone initiation settings
Applicants
Renat Almeev, Ph.D.; Professor Dr. Jürgen Koepke
Subject Area
Mineralogy, Petrology and Geochemistry
Term
since 2024
Project identifier
Deutsche Forschungsgemeinschaft (DFG) - Project number 537666936
The global process of subduction is well understood, but relatively little is known about how subduction starts. The progress in understanding the magmatic processes during subduction zone initiation is mostly based on rocks from the volcanic sequence of the oceanic crust. Here, it is well-known that water, derived from fluids of the subducting slab, is the driving agent with the potential to produce characteristic hydrous magmatic series (fore-arc basalts, andesites, boninites). In contrast, practically nothing is known from the corresponding plutonic rocks: the portion of fractionated crystals extracted from differentiating melts. These often form layered cumulate sequences, which are manifested in the gabbroic part of the oceanic crust under developing subduction zones, well-known from ophiolites. In the Oman ophiolite, the best-investigated analogue of fast-spreading oceanic crust generated during the initiation of subduction on land, such deep cumulate sequences are well exposed and easily accessible, especially since the advent of drill cores recovered in the frame of the ICDP Oman Drilling Project (OmanDP). Key rocks in the plutonic crust showing the presence of water-rich magmas during the petrogenesis of initial arc formation, are wehrlites, consisting of mainly olivine and clinopyroxene: the crystallization of plagioclase, as it is typical for the cotectic crystallization of gabbros in the "normal" oceanic crust, is delayed or completely suppressed due to the presence of high water activities. This one-year postdoc project focuses to investigate systematically wehrlites occurring as coherent layers within the gabbro sequences of most OmanDP crustal drill cores. We plan to start a comprehensive analysis program of mineral phases in the wehrlites for major and trace elements. The analytical data to be obtained will enable us to establish a detailed petrologic model, how the magmatic differentiation in hydrous MORB-type systems during the accretion of the lower ocean crust in spreading systems above evolving subduction zones proceeds. Major element compositions of the wehrlitic phases in combination with petrological modeling and application of relevant phase diagrams will reveal crystallization paths for variable water activities. For estimating the crystallization temperatures of those wehrlites where interstitial plagioclase is present, we will apply REE-in-plagioclase-clinopyroxene thermometry. Trace element analyses in clinopyroxene will enable us to calculate the trace element contents of equilibrium melts by applying partioning coefficients, well-suited for comparison with the compositions from the volcanic sequence. We aim to analyse clinopyroxene in wehrlites for fluid-mobile trace elements for modeling the fluid mobile element enrichment in such a special environment of initial subduction zone initiation.
DFG Programme
Infrastructure Priority Programmes
