Zircon occasionally crystallizes in evolved melt pockets in mafic large igneous province (LIP) magmas, and in these cases, it is used to provide high-precision age constraints on LIP events. The precision and accuracy of high-precision ages from LIPs are crucially important, because they may be implicated in mass extinctions. However, why zircon crystallizes in these magmas is not clearly understood, since their mafic compositions should limit zircon saturation. Here, we investigate the occurrence of zircon (and baddeleyite) in intrusive and extrusive mafic rocks from Central Atlantic Magmatic Province (CAMP) using petrography, trace-element analysis, Ti temperatures, Hf and oxygen isotopes, and high-precision U–Pb geochronology, along with petrological and thermal modeling. We provide new ages for CAMP sills that intruded into Paleozoic sediments in Brazil, indicating that the high and low Ti magmatism in this area occurred synchronously over 264 ± 57 ka. We show that upper crustal assimilation, especially of shales, during the emplacement of the CAMP likely led to zircon saturation. Assimilation of upper crustal sediments is also supported by high δ 18 O values and some rare negative εHf values in the zircon crystals. The only extrusive sample analyzed was the North Mountain basalt in Nova Scotia, Canada. This sample contains a large age variation in its zircon crystals (up to 4 Ma), and the older crystals have slightly more negative εHf values suggesting the presence of small (micron scale) xenocrystic cores associated with very late-stage sediment assimilation. However, the CAMP dataset as a whole suggests that the presence of xenocrystic cores is rare. Assuming no xenocrystic cores, and considering the zircon undersaturated nature of LIP mafic melts, the oldest zircon age clusters in a population should record the magma emplacement (or time when assimilation occurred), and the younger ages in a population are more likely to reflect Pb loss, especially given the high U concentrations of LIP zircon. Our identification of heterogeneous isotopic and elemental compositions in LIP zircon indicates that zircon in these magmas saturate in isolated minute melt pockets just before the system cools below its solidus.

中文翻译：

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大型火成岩省的锆石年代学揭示了上地壳污染过程：新的 U-Pb 年龄、Hf 和 O 同位素以及来自中大西洋岩浆区 (CAMP) 的微量元素

锆石偶尔会在基性大火成岩区 (LIP) 岩浆的演化熔池中结晶，在这些情况下，它被用来为 LIP 事件提供高精度的年龄限制。来自 LIP 的高精度年龄的精度和准确性至关重要，因为它们可能与大规模灭绝有关。然而，锆石在这些岩浆中结晶的原因尚不清楚，因为它们的镁铁质成分会限制锆石的饱和度。在这里，我们使用岩石学、微量元素分析、Ti 温度、Hf 和氧同位素以及高精度 U-Pb 年代学研究了中大西洋岩浆区 (CAMP) 的侵入性和喷出性基性岩中锆石（和斜锆石）的产状，以及岩石学和热建模。我们为侵入到巴西古生代沉积物中的 CAMP 基岩提供了新的年龄，表明该地区的高、低 Ti 岩浆活动在 264±57 ka 内同步发生。我们表明，在 CAMP 就位期间上地壳的同化作用，尤其是页岩的同化作用可能导致锆石饱和。锆石晶体中的高δ 18 O 值和一些罕见的负εHf 值也支持上地壳沉积物的同化作用。分析的唯一喷出样品是加拿大新斯科舍省的北山玄武岩。该样品的锆石晶体年龄变化很大（高达 4 Ma），较旧的晶体具有略高的负 εHf 值，表明存在与非常晚期沉积物同化相关的小（微米级）异晶核。然而，CAMP 数据集作为一个整体表明异晶核心的存在是罕见的。假设没有异晶核，并考虑到 LIP 基性熔体的锆石不饱和性质，人口中最古老的锆石年龄簇应该记录岩浆侵位（或同化发生的时间），而人口中年龄较小的人更有可能反映 Pb损失，特别是考虑到 LIP 锆石的高 U 浓度。我们对 LIP 锆石中异质同位素和元素组成的鉴定表明，这些岩浆中的锆石在系统冷却至其固相线以下之前，在孤立的微小熔体中饱和。种群中最古老的锆石年龄簇应记录岩浆侵位（或发生同化的时间），种群中年龄较小的锆石更可能反映 Pb 损失，特别是考虑到 LIP 锆石的高 U 浓度。我们对 LIP 锆石中异质同位素和元素组成的鉴定表明，这些岩浆中的锆石在系统冷却至其固相线以下之前，在孤立的微小熔体中饱和。种群中最古老的锆石年龄簇应记录岩浆侵位（或发生同化的时间），种群中年龄较小的锆石更可能反映 Pb 损失，特别是考虑到 LIP 锆石的高 U 浓度。我们对 LIP 锆石中异质同位素和元素组成的鉴定表明，这些岩浆中的锆石在系统冷却至其固相线以下之前，在孤立的微小熔体中饱和。