Inclusions in diamonds constrain thermo-chemical conditions during Mesozoic metasomatism of the Kaapvaal cratonic mantle
Inclusions in diamonds constrain thermo-chemical conditions during Mesozoic metasomatism of the Kaapvaal cratonic mantle
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钻石中的包裹体限制了卡普瓦尔克拉通地幔中生代交代作用期间的热化学条件
DOI：
10.1016/j.epsl.2018.03.014
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发表时间：
2018
影响因子：
5.3
通讯作者：
Harris, Jeff W.
中科院分区：
文献类型：
--
作者：
Weiss, Yaakov;Navon, Oded;Goldstein, Steven L.;Harris, Jeff W.
金刚石中的流体/熔体包裹体在交代事件期间和短时间内被包裹，与周围地幔分离，提供了限制在个别热化学事件期间发生的次大陆岩石圈地幔（SCLM）变化的机会，以及所涉及的流体的组成及其来源。我们已经分析了一套8 microinclusions轴承金刚石从第一组德比尔斯池金伯利岩，南非，使用FTIR，EPMA和LA-ICP-MS。七个金刚石捕获不相容的元素富集盐水高密度流体（HDFs），进行橄榄岩矿物微包裹体，和替代氮几乎完全在A-中心。氮的这种低聚集状态表明这些金刚石的地幔停留时间短和/或地幔环境温度低。一个短的停留时间是有利的，因为，在南非岩石圈期间和之后的卡鲁洪水玄武岩火山作用在180 Ma的高温条件普遍存在，因此，盐交代必须发生接近的金伯利岩喷发的时间在185 Ma。另一种金刚石包裹着不相容元素富集的BHDFs，其25%的氮含量存在于B中心，这意味着在更早的不同交代事件期间形成，可能与大约1000年的卡鲁岩浆活动有关。180马。基于石榴石-斜方辉石（Opx）/单斜辉石（Cpx）/橄榄石之间的Mg-Fe交换和Opx-Cpx温度计，对携带盐水HDF的金刚石中的矿物微包裹体进行温度测量，在5GPa下产生875-1080° C之间的温度。这些温度与德比尔斯池金伯利岩钻石中接触包裹体对记录的条件重叠，代表喷发前的地幔环境条件，与同一地点橄榄岩捕虏体记录的P-T梯度相比，温度低150-250° C。氧逸度（f O 2）也不同。在4-7 GPa压力下，与捕虏体记录的fO 2值相比，盐水HDF组合物计算的fO 2值（Δ log fO 2（FMQ）=− 2.47至− 1.34）高出约一个log单位。我们的结论是，丰富的盐HDFs介导的交代作用之前，第一组金伯利岩喷发西南Kaapvaal克拉通，他们的“冷和氧化”的性质反映了他们的衍生从深俯冲板。这一事件对作为储层的卡普瓦尔岩石圈的温度和氧化还原状态几乎没有影响，但是，它可能会影响其沿着有限的交代脉及其围岩的性质。为了调和温度和氧逸度的钻石和捕虏体的包裹体之间的差异，我们认为，捕虏体没有平衡在最后的盐交代事件或金伯利岩喷发。因此，他们记录的P-T-f O2梯度表达了先前存在的岩石圈条件，这些条件可能是在卡普瓦尔克拉通最后一次主要热事件期间建立的（即大约在2000年的卡鲁岩浆活动）。180 Ma）。
Fluid/melt inclusions in diamonds, which were encapsulated during a metasomatic event and over a short period of time, are isolated from their surrounding mantle, offering the opportunity to constrain changes in the sub-continental lithospheric mantle (SCLM) that occurred during individual thermo-chemical events, as well as the composition of the fluids involved and their sources. We have analyzed a suite of 8 microinclusion-bearing diamonds from the Group I De Beers Pool kimberlites, South Africa, using FTIR, EPMA and LA-ICP-MS. Seven of the diamonds trapped incompatible-element-enriched saline high density fluids (HDFs), carry peridotitic mineral microinclusions, and substitutional nitrogen almost exclusively in A-centers. This low-aggregation state of nitrogen indicates a short mantle residence times and/or low mantle ambient temperature for these diamonds. A short residence time is favored because, elevated thermal conditions prevailed in the South African lithosphere during and following the Karoo flood basalt volcanism at∼ 180 Ma, thus the saline metasomatism must have occurred close to the time of kimberlite eruptions at∼ 85 Ma. Another diamond encapsulated incompatible-element-enriched silicic HDFs and has 25% of its nitrogen content residing in B-centers, implying formation during an earlier and different metasomatic event that likely relates to the Karoo magmatism at ca. 180 Ma. Thermometry of mineral microinclusions in the diamonds carrying saline HDFs, based on Mg–Fe exchange between garnet–orthopyroxene (Opx)/clinopyroxene (Cpx)/olivine and the Opx–Cpx thermometer, yield temperatures between 875–1080° C at 5 GPa. These temperatures overlap with conditions recorded by touching inclusion pairs in diamonds from the De Beers Pool kimberlites, which represent the mantle ambient conditions just before eruption, and are altogether lower by 150–250° C compared to P–T gradients recorded by peridotite xenoliths from the same locality. Oxygen fugacity (f O 2) differs as well. The f O 2 calculated for the saline HDF compositions (Δ log f O 2 (FMQ)=− 2.47 to− 1.34) are higher by about a log unit compared with that recorded by xenoliths at 4–7 GPa. We conclude that enriched saline HDFs mediated the metasomatism that preceded Group I kimberlite eruptions in the southwestern Kaapvaal craton, and that their ‘cold and oxidized’nature reflects their derivation from a deep subducting slab. This event had little impact on the temperature and redox state of the Kaapvaal lithosphere as a reservoir, however, it likely affected its properties along limited metasomatized veins and their wall rock. To reconcile the temperature and oxygen fugacity discrepancy between inclusions in diamonds and xenoliths, we argue that xenoliths did not equilibrate during the last saline metasomatic event or kimberlite eruption. Thus the P–T–f O 2 gradients they record express pre-existing lithospheric conditions that were likely established during the last major thermal event in the Kaapvaal craton (ie the Karoo magmatism at ca. 180 Ma).
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