Kimberlites and the start of plate tectonics: REPLY
Kimberlites and the start of plate tectonics: REPLY
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金伯利岩和板块构造的开始：回复
作者：
Stern;R.J.;Leybourne;M.I.;Tsujimori;T.
我们感谢 Gary Ernst 对我们最近的地质学论文（Stern 等人，2016 年）的兴趣和评论（Ernst，2017 年），该论文强调了金伯利岩的时间分布。我们考虑了年轻金伯利岩现象的可能原因以及对板块构造开始的影响，并得出结论，最好的解释是最近（< 1 Ga）深俯冲和板块构造的开始导致深部地幔水化和碳化作用增加。我们同意恩斯特的许多观点，包括“最初的地幔循环可能主要是自下而上的（地幔柱驱动的），但随着地球冷却，自上而下的翻转（板块俯冲）开始主导其热演化”，但我们不同意这种构造风格的变化何时发生。恩斯特的结束语“自冥古宙岩浆洋巩固以来，岩石圈俯冲（板块构造）显然是间歇性或连续性的（？）”很好地抓住了分歧的要点。这一说法意味着，除了板块构造之外，没有其他方法可以使岩石变形并产生熔体。我们将板块构造定义为硅酸盐体对流的一种类型，其中盖碎片（板块）运动主要是由于俯冲带致密岩石圈的下沉，导致离散板块边界处的软流圈上涌和海底扩张，并伴有集中的上涌（地幔羽流）。根据这个定义，关键的地球动力学问题是地球的岩石圈何时变得足够稠密并开始沉入下面的软流圈之下？我们对足够的岩石圈致密化需要足够的冷却的证据印象深刻，因此很可能发生在地球历史的晚期（Korenaga，2013）。在这里，我们使用两条证据来反驳恩斯特的均变论假设，即板块构造主导了地球的构造历史。第一个证据来自比较行星学。 2015 年，人类通过 NASA 谷神星黎明号任务和冥王星和冥卫一新视野号任务完成了对太阳系所有大型天体的观察，现在知道地球是太阳系中唯一拥有板块构造的星球。金星、火星和木卫一的构造和岩浆活动都很活跃，但没有一个具有板块构造。艾奥是三者中最活跃的，因为它是木星最里面的卫星，并通过潮汐弯曲保持高温。木卫一的特点是热管构造，地下岩浆定期喷发，将较旧的熔岩流埋藏在较新的熔岩流之下，直到火山堆的底部沉回岩浆中（Moore 和 Webb，2013）。金星的构造和岩浆也很活跃，但以地幔柱和岩石圈滴水为主（Gerya，2014）。火星是一颗老年行星，但塔尔西斯和极乐世界地区仍有火成岩活动，并且以水手谷为中心有构造活动。这些行星具有静止的盖构造风格，其中单个板块构成了岩石圈，这是活跃行星、矮行星和卫星的主要构造风格。根据我们对这些天体的了解，在行星的整个生命周期中，预计会出现很大范围的停滞盖子行为。在板块构造演化之前，地球很可能也经历过停滞的盖构造。我们的第二个反驳是，地球科学家越来越多地根据构造转变来解释地球上的古代岩石序列（Condie 和 Aster，2010；Arndt 和 Davaille，2013；Hawkesworth 等，2016）。图 1A 显示了最近将地球细分为“俯冲前”和“俯冲”阶段，这与岩石记录一致（Hawkesworth 等，2016）；这些 …
We thank Gary Ernst for his interest in and Comments (Ernst, 2017) about our recent Geology paper (Stern et al., 2016), which highlighted the temporal distribution of kimberlites. We considered what are the possible reasons for the young kimberlite phenomenon and the implications regarding the onset of plate tectonics and conclude that the best explanation is that there has been a recent increase in deep mantle hydration and carbonation caused by the recent (< 1 Ga) onset of deep subduction and plate tectonics. We agree with many of Ernst’s points, including “Initial mantle circulation might have been chiefly bottom up (plume-driven), but as Earth cooled, top-down overturn (plate subduction) began to dominate its thermal evolution”, but we disagree about when this change in tectonic style occurred. Ernst’s closing sentence “Lithospheric subduction (plate tectonics) apparently has operated episodically or continuously (?) since consolidation of the Hadean magma ocean” nicely captures the main point of disagreement. This statement implies that there is no other way to deform rocks and generate melts except plate tectonics. We define plate tectonics as a style of silicate body convection where lid fragment (plate) motions are mostly due to sinking of dense lithosphere in subduction zones, causing upwelling of asthenosphere at divergent plate boundaries and seafloor spreading accompanied by focused upwellings (mantle plumes). With this definition, the key geodynamic question is when did Earth’s lithosphere become dense enough to begin to sink beneath underlying asthenosphere? We are impressed by the evidence that sufficient lithospheric densification required sufficient cooling and so is likely to have happened late in Earth history (Korenaga, 2013). Here we use two lines of evidence to refute Ernst’s uniformitarianist assumption that plate tectonics has dominated Earth’s tectonic history. The first line of evidence comes from comparative planetology. Humanity finished taking a look at all the large bodies in the solar system in 2015 with the NASA Dawn mission to Ceres and the New Horizons mission to Pluto and Charon, and now know that Earth is alone in the solar system in having plate tectonics. Venus, Mars, and Io are tectonically and magmatically active, but none have plate tectonics. Io is the most active of the three because it is Jupiter’s innermost satellite and is kept hot by tidal flexing. Io is characterized by heat pipe tectonics whereby subsurface magma erupts periodically, burying older lava flows under younger until the base of the volcanic pile sinks back into the magma (Moore and Webb, 2013). Venus is also tectonically and magmatically active, but dominated by mantle plumes and lithospheric drips (Gerya, 2014). Mars is a geriatric planet but still has igneous activity in the Tharsis and Elysium regions and tectonic activity centered on the Vallis Marineris. These planets share a stagnant lid tectonic style, where a single plate makes up the lithosphere and this is the dominant tectonic style of active planets, dwarf planets, and moons. Based on what we know about these bodies, there is a large range in stagnant lid behavior that is expected over the life of a planet. It is likely that Earth also experienced stagnant lid tectonics before plate tectonics evolved. Our second counterargument is that geoscientists are increasingly interpreting ancient rock sequences on Earth in terms of tectonic transitions (Condie and Aster, 2010; Arndt and Davaille, 2013; Hawkesworth et al., 2016). Figure 1A shows a recent subdivision of Earth into “pre-subduction” and “subduction” episodes that is consistent with the rock record (Hawkesworth et al., 2016); these …