The origin and development of the Earths mantle in the Archean; subcalcic garnets and eclogites as oldest time markers

太古代陆核的生长和融合时间，以及现今克拉通地幔与其上覆地壳记录的过程之间的时空关系，是浮力下伏岩石圈地幔(岩石圈龙骨)稳定长寿命克拉通地块的基本问题。一种流行的模式倾向于认为克拉通以下地幔的起源主要是浅层部分熔融和俯冲的产物。因此，来自SCLM的榴辉岩和石榴石辉石岩是俯冲熔体的产物。我们以前使用过单颗粒哈兹伯里岩、亚钙石榴石来获得关于卡瓦瓦克拉通下克拉通地幔部分熔融和交代的时间信息。这产生了与地壳事件一致的定义时隙，而不是似乎由Re-Os模型年龄所显示的事件的连续体。榴辉岩和石榴石辉石岩也是金伯利岩穿过太古代地壳喷发出来的，由于其原岩的多重来源和榴辉岩化前后潜在的化学变化，它们具有很强的不均一性。耐人寻味的问题是原岩的性质，它们的年龄和榴辉岩的年龄，以及与橄榄岩的年龄的比较。我们对Kaapvaal克拉通Bellsbank钻石矿的榴辉岩和石榴石辉石岩的研究表明，其中一些榴辉岩和石榴石辉石岩几乎是俯冲洋壳的原生成分(=几乎没有改变的原始熔体成分、富斜长石堆积体和富单斜辉石堆积体)。它们重建的块体岩石在等时线上呈线性排列，其中3个样品的年龄为4.12±0.06Ga，eHfi=3(±7)，即与当时的原始地幔之比误差不大。这一非常古老的年龄与卡阿普瓦尔克拉通的任何地壳年龄或其岩石圈地幔的再亏损年龄没有直接匹配。仅石榴石的Lu-HF-Model年龄是最小年龄，但它们已经给出了高达3.5Ga的年龄。为了进一步探索亚克拉通地幔起源和变质的详细图景，即熔融模式的区别和熔融模式的适用性，以及第一次熔融和再富集事件的时间，我们希望将我们对亚钙质的研究扩展到卡普瓦尔的更远的地方。第二个目的是用来自贝尔斯班克的更多样品验证4.1Ga榴辉岩等时线。如果正确，这将是迄今从地幔榴辉岩套中获得的最古老的年龄，并将对岩石圈地幔中赫甸地壳的产生和储存产生深远的影响。

The timing of growth and amalgamation of Archean continental nuclei, and the temporal and spatial relationships between processes recorded in the present day subcratonic mantle and its overlying crust are fundamental questions concerning stabilization of long-lived cratonic blocks by buoyant underlying lithospheric mantle (lithospheric keel). A prevailing model favours the origin of the subcratonic mantle mostly as the restite of partial melting at shallow depths followed by subduction. Eclogites and garnet pyroxenites from the SCLM are accordingly the subducted melt products. We have previously used single grain harzburgitic, subcalcic garnets to obtain information on the timing of partial melting and metasomatism of the subcratonic mantle underneath the Kaapvaal craton. This yields defined time slots in agreement with crustal events rather than a continuum of events as seemingly shown by Re-Os model ages. Eclogites and garnet pyroxenites which were also erupted by kimberlites through Archean crust are very heterogeneous as a result of multiple origins of their protoliths and potential chemical changes before and after eclogitisation. Tantalizing questions are the nature of the protoliths, their age and the age of eclogitisation and the comparison to ages from peridotites. Our studies of eclogites and garnet pyroxenites from the Bellsbank diamond mine on the Kaapvaal craton indicates that a number of them represent almost unmodified members of a subducted oceanic crust (= almost unaltered original melt compositions, plagioclase rich cumulates and clinopyroxene-rich cumulates). Their reconstructed bulk rocks linearly align in an isochron diagram and three samples give an age of 4.12 ± 0.06 Ga with eHfi = 3 (±7) i.e. within error the ratio of the primitive mantle at that time. This very old age is not directly matched by any crustal ages from the Kaapvaal craton or by Re-depletion ages from its lithospheric mantle. Lu-Hf-Model ages from garnet alone are minimum ages but they already give ages up to 3.5 Ga. In order to further explore the detailed picture of the origin and modification of the subcratonic mantle, i.e. the distinction of the melting regimes and the applicability of melting models and the timing of first melting and of reenrichment events we want extend our studies of subcalcic to further localities of the Kaapvaal. A second aim is to verify the 4.1 Ga eclogite isochron with further samples from Bellsbank. If correct, it would be the oldest age ever obtained from a mantle eclogite suite and would have profound implications for the production and storage of Hadean crust in the lithospheric mantle.