From phosphates to silicates and back: an experimental study on the transport and storage of phosphorus in eclogites during uplift and exhumation
From phosphates to silicates and back: an experimental study on the transport and storage of phosphorus in eclogites during uplift and exhumation
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DOI：
10.2138/am-2016-5521
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发表时间：
2016-08
影响因子：
3.1
通讯作者：
J. Konzett
中科院分区：
文献类型：
--
作者：
J. Konzett
高P-T实验表明，在MORB、橄榄岩和泥质沉积物等主要岩石类型中，P（和T）的增加导致P从以磷灰石为代表的磷酸盐逐渐转移到以石榴石为最重要载体的硅酸盐。这是由于在石榴石中通过[8]Na[4]P[8] M-Si [4]Si_1形成了Na 3Al 2（PO 4）3相组分，该组分是强P-的，并且在较小程度上是T-依赖的，并且在低至2-3 GPa的P下产生具有显著P和Na的石榴石。基于这一实验证据，人们会期望在超高压岩石中经常发现富含磷钠的石榴石，其成分范围很广。然而，除了极少数例外，情况并非如此。这种差异表明，P和Na有效地释放石榴石和重新分布在石榴石基质中的隆起和折返。为了探索这种P-Na释放的机制，将在7 GPa和1200 °C下预合成的富含P-Na的石榴石（含有0.7 wt% P2 O 5和0.3 wt% Na 2 O）暴露于2 GPa和800-1000 °C的简化的含H2O的模型榴辉岩块体组合物中的P-T条件。实验表明，在850-975 °C的亚固相线温度下，在含水流体的存在下，磷灰石迅速形成，从石榴石分解涉及消耗共存的石英和单斜辉石。磷灰石通常呈圆形至板条状的孤立颗粒，分散于石榴石+单斜辉石±斜方辉石+石英+金红石基质中。更罕见的是，单斜辉石或石榴石中可能形成单个磷灰石包裹体或包裹体簇。磷灰石的粒度范围为≤ 1 × 1 ~ 24 × 6 μm，最大的磷灰石颗粒中偶见单斜辉石包裹体。结合石榴石的击穿和新形成，使用预先存在的石榴石作为成核位点，可能会形成具有Na-P-贫化和Ti-富集的边缘的分区石榴石，其代表了接近中浅地壳P-T条件下典型的石榴石组合物。部分熔融实验表明，对于中等富SiO2的熔体成分，含有富磷石榴子石的榴辉岩可能会产生富磷和磷灰石不饱和的熔体。这些熔体可以结晶丰富的磷灰石在固化过程中，因此，将是有效的代理在部分熔融过程中的P-提取。由于花岗岩熔体中磷灰石的饱和浓度很低，其磷的输运和储存能力将受到很大限制。本研究的一个意外发现是蓝晶石中的大量P和Mg含量分别为0.17- 0.20wt%P2O5和0.20- 0.56wt%MgO。蓝晶石中结合的P-Mg掺入与偶联取代[4] Si 4 + + [6] Al 3 + = [4]P5+ + [6] Mg 2+和P对正硅酸盐结构的强烈偏好一致。这项研究的结果表明，一些甚至所有的磷灰石，现在在榴辉岩，经历了深俯冲形成的榴辉岩石榴石的化学调整，以减少Na 3Al 2（PO 4）3的溶解度在隆起和折返。这导致磷灰石的外观作为一个新的阶段，在迄今为止的磷灰石免费组合。快速运输到表面和/或缺乏合适的反应物可以抑制这种再平衡，并解释偶尔高的P-和Na-含量的榴辉岩石榴石包裹体中的金刚石或石榴石从金刚石榴辉岩金伯利岩采样。同样，同时降低钛在石榴石中的溶解度可能会导致金红石饱和榴辉岩，不包含此相下的峰值P条件下证明了（定向）金红石和磷灰石夹杂物的榴辉岩石榴石中的联合发生。对于地热测压的应用，这种延迟的Ti（+P）饱和是重要的记住。
Abstract High P-T experiments have shown that in major rock types such as MORBs, peridotites, and pelitic sediments, increasing P (and T) leads to a gradual transfer of P from phosphates mostly represented by apatite to silicates with garnet as most important silicate P carrier. This is due to the formation of a Na3Al2(PO4)3 phase component in garnet via [8]Na[4]P[8]M-î[4]Si_1 that is strongly P-, and to a lesser extent T-dependent and creates garnets with significant P and Na at P as low as 2–3 GPa. Based on this experimental evidence, one would expect to routinely find P-Na-rich garnets in UHP-rocks with a wide range in composition. With very few exceptions, however, this is not the case. This discrepancy indicates that both P and Na are effectively released from garnet and re-distributed within the garnet matrix during uplift and exhumation. To explore the mechanisms of this P-Na release, P-Na-rich garnet pre-synthesized at 7 GPa and 1200 °C, containing 0.7 wt% P2O5 and 0.3 wt% Na2O, was exposed to P-T conditions of 2 GPa and 800–1000 °C in a simplified, H2O-bearing, model eclogitic bulk composition. The experiments show that at subsolidus temperatures of 850–975 °C, and in the presence of a hydrous fluid, apatite quickly forms from garnet breakdown involving consumption of coexisting quartz and clinopyroxene. The apatites usually appear as rounded to lath-shaped isolated grains scattered in the garnet + clinopyroxene ± orthopyroxene + quartz + rutile matrix. More rarely, single apatite inclusions, or clusters of inclusions, may form in clinopyoxene or garnet. The observed apatite grain size is in the range ≤ ~1 × 1 to 24 × 6 μm with the largest grains occasionally containing clinopyroxene inclusions. Combined garnet breakdown and neo-formation, using pre-existing garnet as a nucleation site, may form zoned garnets with Na-P-depleted and Ti-enriched rims that represent an approach to a garnet composition typical for mid- to shallow crustal P-T conditions. Partial melting experiments indicate that eclogites containing P-rich garnet may produce P-rich and apatite-undersaturated melts for moderately SiO2-rich melt compositions. These melts can crystallize abundant apatite during solidification and, thus, would be effective agents for P-extraction during partial melting. Due to their very low apatite saturation concentration, the P-transport and storage capacity of granitic melts would be much more limited. An unexpected finding of this study are the substantial P and Mg contents in kyanite with 0.17–0.20 wt% P2O5 and 0.20–0.56 wt% MgO, respectively. The combined P-Mg incorporation into kyanite is consistent with the coupled substitution [4]Si4+ + [6]Al3+ = [4]P5+ + [6]Mg2+ and with a strong preference of P for orthosilicate structures. The results of this study suggest that some to even all of the apatite now present in eclogites that underwent deep subduction formed by chemical adjustment of the eclogite garnet to decreasing Na3Al2(PO4)3 solubility during uplift and exhumation. This results in the appearance of apatite as a new phase in a hitherto apatite-free assemblage. Rapid transport to the surface and/or a lack of suitable reactants can suppress this re-equilibration and explain the occasionally high P- and Na-contents of eclogitic garnet-inclusions in diamond or garnets from diamondiferous eclogites sampled by kimberlites. Similarly, concurrent decreasing Ti-solubility in garnet may lead to rutile-saturation in eclogites that did not contain this phase under peak-P conditions as evidenced by the joint occurrence of (oriented) rutile and apatite inclusions in the eclogitic garnets. For the application of geothermobarometry, this delayed Ti (+P) saturation is important to be kept in mind.