Kimberlites are the deepest-derived magmas and can provide unique insights into the composition and evolution of the deep Earth. However, this is hindered by contamination of kimberlite melts by mantle and crustal material during ascent. Previous work has shown that the Nd and Hf isotope compositions of the 47–75 Ma Lac de Gras (LDG) kimberlites (western Canada) evolve with time from geochemically enriched compositions towards more radiogenic values, which could result from either radiogenic ingrowth or a decreasing contribution of deeply subducted crustal material to the kimberlite source. While previously published TiO₂-rich and TiO₂-poor whole-rock compositions indicate that at least two distinct primitive melt compositions occur in the LDG field, it is uncertain whether temporal evolution of the kimberlite source influenced primitive melt compositional variability. In this study, the compositions of early formed olivine (rims overgrowths on xenocrysts) and chromite, together with xenocrystic olivine (cores) in 28 LDG kimberlites are combined with new and existing Nd and Hf isotopic compositions and geochronological constraints to examine the temporal and spatial variability of primitive kimberlite melt compositions and their relation to source evolution.

Olivine rim Mg# [=100× Mg/(Mg + Fe²⁺); 89.6 ± 0.2 to 91.8 ± 0.2] and chromite Ti# [=100×Ti/(Ti + Al + Cr); 2.5 ± 0.3 to 10.1 ± 1.5], used as proxies for primitive kimberlite melt compositions, indicate that a range of melt compositions were parental to the LDG kimberlites. A positive correlation between olivine rim Mg# and commonly resorbed xenocrystic olivine cores (Mg# = 89.4 ± 1.9 to 92.1 ± 1.3) indicate that assimilation of laterally heterogeneous lithospheric mantle contributed to primitive melt variability. Spatially proximal kimberlites in the Panda, Falcon and Arnie clusters in the centre of the field feature olivine rim Mg# and chromite Ti# with moderate to strong correlations with emplacement ages (47 to 75 Ma), and initial ¹⁷⁶Hf/¹⁷⁷Hf and ¹⁴³Nd/¹⁴⁴Nd ratios. These correlations indicate that once the effects of lithospheric mantle assimilation are screened out by considering proximal kimberlites, which traversed a similar lithosphere, the compositions of primitive melts at LDG are controlled by the temporal evolution of the sub-lithospheric source. The temporal variability of primitive melt compositions at LDG (i.e., increasingly radiogenic Nd and Hf isotopes and increasing melt Mg# with decreasing age) suggests that the source of these kimberlites evolved via progressive consumption of a deeply subducted, enriched component that was previously mixed with a geochemically more depleted mantle. This work highlights that the compositions of kimberlites are profoundly influenced by both their asthenospheric sources, including deeply subducted material, and assimilation of laterally heterogeneous lithospheric mantle during ascent.

金伯利岩是最深的岩浆，可以对深地球的组成和演化提供独特的见解。但是，这在上升过程中受到地幔和地壳物质对金伯利岩熔体的污染的阻碍。先前的工作表明，47-75 Ma Lac de Gras（LDG）金伯利岩（加拿大西部）的Nd和Hf同位素组成随着时间的推移，从地球化学富集的成分向更多的放射源值演变，这可能是由于放射源向内生长或减少引起的俯冲的地壳物质对金伯利岩源的贡献。而此前公布的TiO ₂富和TiO ₂较差的全岩成分表明，LDG场中至少有两种不同的原始熔体成分，不确定金伯利岩源的时间演化是否影响原始熔体成分的变异性。在这项研究中，将28个LDG金伯利岩中早期形成的橄榄石（边缘在异种晶上过度生长）和亚铬铁矿的成分以及异晶橄榄石（岩心）与新的和现有的Nd和Hf同位素组成以及地质年代学约束相结合，以研究时间和空间原始金伯利岩熔体成分的变异性及其与源演化的关系。

橄榄石边缘Mg＃[= 100×Mg /（Mg + Fe ²⁺）; 89.6±0.2至91.8±0.2]和亚铬酸盐Ti＃[= 100×Ti /（Ti + Al + Cr）; 2.5±0.3至10.1±1.5]用作原始金伯利岩熔体成分的代理，表明一定范围的熔体成分是LDG金伯利岩的母体。橄榄石边缘Mg＃与通常吸收的异晶橄榄石核之间的正相关关系（Mg＃= 89.4±1.9至92.1±1.3）表明，横向非均质岩石圈地幔的同化作用有助于原始熔体变化。田野中心的熊猫，猎鹰和阿尔尼星团的空间近端金伯利岩具有橄榄石边缘Mg＃和亚铬铁矿Ti＃，它们与进位年龄（47至75 Ma）之间具有中等至强烈的相关性，最初为¹⁷⁶ Hf / ¹⁷⁷ Hf和¹⁴³ Nd / ¹⁴⁴Nd比率。这些相关性表明，一旦考虑到横穿类似岩石圈的近端金伯利岩，就筛选出了岩石圈地幔同化的影响，LDG上原始熔体的组成将受到岩圈下源的时间演化的控制。LDG原始熔体成分的时间变化（即，随着年龄的减少，放射性Nd和Hf同位素的增加以及熔体Mg＃的增加）表明，这些金伯利岩的来源是通过逐渐消耗先前被混合的深层俯冲富集组分而演化而来的。地球化学上更枯竭的地幔。这项工作突显了金伯利岩的成分受到其软流层来源（包括深俯冲物质）的深远影响，