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

太古宙大陆核的生长和融合时间，以及现今亚克拉通地幔与上覆地壳记录的过程之间的时空关系，是研究浮力下伏岩石圈地幔（岩石圈龙骨）稳定长期克拉通地块的基本问题。一种流行的模式倾向于次克拉通地幔的起源，主要是在浅层深处部分熔融后的俯冲。因此，SCLM中的榴辉岩和石榴石辉石岩是俯冲熔融产物。我们曾利用单粒黑榴石、亚钙榴石获得了Kaapvaal克拉通下次克拉通地幔部分熔融和交代时间的信息。这产生了与地壳事件一致的确定的时隙，而不是像Re-Os模型年龄所显示的那样是事件的连续体。同样由金伯利岩穿过太古宙地壳喷发而成的榴辉岩和石榴石辉石岩，由于其原岩的多源性和榴辉石化前后可能发生的化学变化，具有很强的异质性。诱人的问题是原岩的性质，它们的年龄和榴辉化的年龄，以及与橄榄岩年龄的比较。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，它们中的一些代表了俯冲洋壳的几乎未改变的成员（=几乎未改变的原始熔体成分，富含斜长石的堆积和富含斜辉石堆积）。3个样品的年龄为4.12±0.06 Ga, eHfi = 3（±7），与当时原始地幔的比值在误差范围内。这个非常古老的年龄不能直接与卡普瓦尔克拉通的任何地壳年龄相匹配，也不能与岩石圈地幔的再枯竭年龄相匹配。仅石榴石的lu - hf模型年龄是最小年龄，但它们已经给出了3.5 Ga的年龄。为了进一步探索亚克拉通地幔起源和变化的详细图景，即熔融机制的区别和熔融模式的适用性，以及首次熔融和再富集事件的时间，我们希望将亚钙研究扩展到Kaapvaal的其他地区。第二个目标是用贝尔斯班克的进一步样品验证4.1 Ga榴辉岩等时线。如果正确的话，这将是迄今为止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冥古宙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.