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

太古代地幔的起源和发展；

• 批准号: 239807026
• 负责人: Professor Dr. Gerhard Peter Brey, Ph.D.
• 金额: --
• 依托单位: Abteilung Petrologie und Geochemie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 财政年份: 2013
• 资助国家: 德国
• 起止时间: 2012-12-31 至 2014-12-31
• 项目状态: 已结题
• 来源: https://gepris.dfg.de/gepris/projekt/239807026?language=en
• 关键词: origin; development; Earths; mantle; Archean

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.

太古代大陆核的生长和合并的时间，以及记录在今天的亚地幔及其上覆地壳的过程之间的时间和空间关系是关于长期生活的亚地幔块体的浮力下的岩石圈地幔（岩石圈龙骨）的稳定性的基本问题。一个流行的模式，有利于起源的亚地幔主要是作为restite的部分熔融在浅深度，然后俯冲。因此，SCLM中的榴辉岩和石榴辉石岩是俯冲熔融产物。我们以前曾使用单粒方辉橄榄岩，subcalcic石榴石，以获得信息的时间的部分熔融和交代下的Kaapvaal克拉通地幔。这产生定义的时间槽与地壳事件，而不是一个连续的事件，似乎所示的Re-Os模型年龄。榴辉岩和石榴辉石岩也是金伯利岩在太古代地壳中喷发出来的，由于其原岩的多源性和榴辉岩化前后潜在的化学变化，它们具有很强的异质性。诱人的问题是原岩的性质，它们的年龄和榴辉岩化的年龄，以及与橄榄岩年龄的比较。我们对卡普瓦尔克拉通贝尔班克钻石矿榴辉岩和石榴辉石岩的研究表明，其中一些榴辉岩和石榴辉石岩代表了俯冲洋壳几乎未被改变的成员（=几乎未被改变的原始熔体成分、富含斜长石的堆晶岩和富含单斜辉石的堆晶岩）。重建的岩石体在等时线图上呈线性排列，3个样品的年龄为4.12 ± 0.06Ga，eHfi = 3（±7），与当时的原始地幔比值误差在一定范围内。这个非常古老的年龄是不直接匹配的任何地壳年龄从卡普瓦尔克拉通或再耗尽年龄从岩石圈地幔。Lu-Hf模式年龄仅为石榴石的最小年龄，但已给出了3.5Ga的年龄。为了进一步探索亚钙质地幔的起源和改造的详细情况，即熔融制度的区别和熔融模型的适用性以及首次熔融和再富集事件的时间，我们希望将我们的亚钙质研究扩展到Kaapvaal的进一步地点。第二个目的是验证4.1 Ga榴辉岩等时线与进一步的样品从Bellsbank。如果是正确的，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的Hadean地壳的生产和储存产生深远的影响。