High-pressure minerals provide records of processes not normally preserved in Earth’s crust. Reidite, a quenchable polymorph of zircon, forms at pressures >20 GPa during shock compression. However, there is no broad consensus among empirical, experimental, and theoretical studies on the nature of the polymorphic transformation. Here we decipher a multistage history of reidite growth recorded in a zircon grain in distal impact ejecta (offshore northeastern United States) from the ca. 35 Ma Chesapeake Bay impact event which, remarkably, experienced near-complete conversion (89%) to reidite. The grain displays two distinctive reidite habits: (1) intersecting sets of planar lamellae that are dark in cathodoluminescence (CL); and (2) dendritic epitaxial overgrowths on the lamellae that are luminescent in CL. While the former is similar to that described in literature, the latter has not been previously reported. A two-stage growth model is proposed for reidite formation at >40 GPa in Chesapeake Bay impact ejecta: formation of lamellar reidite by shearing during shock compression, followed by dendrite growth, also at high pressure, via recrystallization. The dendritic reidite is interpreted to nucleate on lamellae and replace damaged zircon adjacent to lamellae, which may be amorphous ZrSiO₄ or possibly an intermediate phase, all before quenching. These results provide new insights on the microstructural evolution of the high-pressure polymorphic transformation over the microseconds-long interval of reidite stability during meteorite impact. Given the formation conditions, dendritic reidite may be a unique indicator of distal ejecta.

中文翻译：

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切萨皮克湾撞击地平线上的树枝状辉光岩，海洋钻探计划站点1073（美国东北部离岸）：末梢喷射的指纹？

高压矿物提供了通常不保留在地壳中的过程的记录。Reidite是锆石的一种可淬灭的多晶型物，在冲击压缩过程中以大于20 GPa的压力形成。但是，关于多态转换的性质，在经验，实验和理论研究之间没有广泛的共识。在这里，我们破译了远古撞击喷出物（美国东北部近海）中锆石中记录的锆石晶粒长大的多阶段历史。35马切萨皮克湾的撞击事件非常明显，几乎完全（89％）转变为辉石。谷物表现出两种独特的藻土习惯：（1）在阴极发光（CL）中较暗的平面薄片相交；（2）在CL中发光的片状晶体上的树突外延生长。尽管前者与文献中描述的相似，但后者尚未被报道。提出了切萨皮克湾冲击射流中大于40 GPa的红铁矿形成的两阶段生长模型：在冲击压缩过程中通过剪切形成层状红铁矿，然后在高压下通过重结晶进行枝晶生长。树枝状的晶状体被解释为在薄片上成核并替换与薄片相邻的损坏的锆石，锆石可能是无定形的ZrSiO₄或可能为中间相，均在淬火之前进行。这些结果提供了新的见解，对陨石撞击过程中高压多晶型转变的微结构演化过程（在微秒-长的时间范围内的斑岩稳定性）进行了研究。在形成条件下，树突状辉石可能是远侧射血的唯一指标。