Rheology of hydrous minerals in the subduction multisystem

The relatively low strength of the hydrous minerals has been theorized to play a role in the initiation of subduction through the feedbacks between faulting, hydration reactions, and rheological weakening. To further explore the behaviour of hydrous magnesium silicate minerals under the high stress conditions relevant to propagating faults, we performed nanoindentation tests on three serpentine species—lizardite, antigorite, and chrysotile—from room temperature up to their respective dehydration temperatures. While all serpentine minerals exhibit markedly lower indentation hardness than olivine under the same conditions (Hol= 13.1–14.9 GPa), we find that antigorite (Hatg= 5.7–6.7 GPa) is almost a factor of three harder than lizardite (Hliz= 2.2–2.6 GPa), which is itself an order of magnitude harder than chrysotile (Hctl= 0.1 GPa). We also indented chlorite from room temperature up to 400 °C and found that it has a hardness between that of lizardite and antigorite (Hchl= 2.8–4.0 GPa). Chrysotile is even weaker than the mineral talc (Htlc= 0.6 GPa), another hydrous magnesium silicate, which was tested in a previous study. The weakest hydrous magnesium silicates – talc and chrysotile – are approximately one order of magnitude weaker than antigorite and almost two orders of magnitude weaker than olivine. There is a systematic relationship between indentation hardness and the lattice spacing between c-planes in these sheet silicates. Geodynamic models of subduction initiation typically use an ad hoc finite yield stress to trigger localized deformation. This study confirms that hydrous magnesium silicates are a likely candidate for alteration products that can facilitate localized deformation both before and after subduction initiation. However, the degree of weakening is highly dependent on the specific reaction product.

含水矿物相对较低的强度被理论化为通过断层作用、水化反应和流变弱化之间的反馈在俯冲起始过程中发挥作用。为了进一步探究在与断层扩展相关的高应力条件下含水硅酸镁矿物的行为，我们对三种蛇纹石矿物——利蛇纹石、叶蛇纹石和纤蛇纹石——从室温到它们各自的脱水温度进行了纳米压痕测试。在相同条件下，所有蛇纹石矿物的压痕硬度都明显低于橄榄石（橄榄石硬度\(Hol = 13.1 - 14.9\)吉帕），我们发现叶蛇纹石（\(Hatg = 5.7 - 6.7\)吉帕）的硬度几乎是利蛇纹石（\(Hliz = 2.2 - 2.6\)吉帕）的三倍，而利蛇纹石本身比纤蛇纹石（\(Hctl = 0.1\)吉帕）硬一个数量级。我们还对绿泥石从室温到\(400^{\circ}C\)进行了压痕测试，发现它的硬度介于利蛇纹石和叶蛇纹石之间（\(Hchl = 2.8 - 4.0\)吉帕）。纤蛇纹石甚至比滑石（\(Htlc = 0.6\)吉帕）还软，滑石是另一种含水硅酸镁矿物，在之前的一项研究中已进行过测试。最弱的含水硅酸镁矿物——滑石和纤蛇纹石——比叶蛇纹石大约弱一个数量级，比橄榄石几乎弱两个数量级。在这些层状硅酸盐中，压痕硬度与\(c\)面之间的晶格间距存在系统的关系。俯冲起始的地球动力学模型通常使用一个特定的有限屈服应力来触发局部变形。这项研究证实，含水硅酸镁矿物很可能是蚀变产物的候选者，它们可以在俯冲起始前后促进局部变形。然而，弱化的程度在很大程度上取决于具体的反应产物。