Desilicification Rims of Zircon Xenocrysts Record the Timing of Kimberlite Emplacement
Kimberlites are deep and relatively rare mantle‐derived igneous rocks, which provide key insights into the lithospheric mantle composition beneath continents (i.e., from the entrained mantle materials) at the time of magma emplacement. In addition, the temporal distribution of kimberlites throughout...
Saved in:
| Published in: | Journal of geophysical research. Solid earth Vol. 127; no. 9 |
|---|---|
| Main Authors: | , , , , , , , , , , , |
| Format: | Journal Article |
| Language: | English |
| Published: |
Washington
Blackwell Publishing Ltd
01-09-2022
|
| Subjects: | |
| Online Access: | Get full text |
| Tags: |
Add Tag
No Tags, Be the first to tag this record!
|
| Summary: | Kimberlites are deep and relatively rare mantle‐derived igneous rocks, which provide key insights into the lithospheric mantle composition beneath continents (i.e., from the entrained mantle materials) at the time of magma emplacement. In addition, the temporal distribution of kimberlites throughout Earth's history may reveal secular changes in global geodynamics. However, alteration and abundant exotic material (i.e., xenoliths and xenocrysts) of different ages, both typical of kimberlites, often hinder reliable dating of these rocks. In this study, we investigated zircon xenocrysts and their replacement by desilicification reaction rims (DSRs), which consist mainly of baddeleyite, in the metamorphosed Kimozero kimberlites from the Karelian Craton, Russia. Similar DSRs have been identified previously on mantle‐derived zircons in numerous kimberlites, but the precise timing and location of formation of these rims remain unclear. U–Pb isotope dating of mantle zircon xenocrysts in the Kimozero kimberlites indicates that these rocks were emplaced at ca. 1.97 Ga. DSRs with similar textures and baddeleyite chemistry occur on both the 1.97 Ga mantle and 2.70–2.38 Ga crustal zircon xenocrysts in these kimberlites. Pb–Pb isotope dating of the DSRs yields the same age (i.e., ca. 1.97 Ga) as the mantle zircons. The data constrain the formation of DSRs to crustal levels and, more importantly, indicate that these rims record the timing of kimberlite emplacement. Therefore, we suggest that such baddeleyite reaction rims are a new and promising material for kimberlite geochronology.
Plain Language Summary
Kimberlites are magmatic rocks produced by magma that ascended from mantle depths. It is important to obtain accurate kimberlite ages mainly because: (a) mantle rocks and minerals, carried by these magmas, allow assessment of the composition of the lithospheric mantle beneath continents at the time of kimberlite emplacement; and (b) the worldwide distribution of kimberlite ages may record secular changes in global geodynamics. However, kimberlites are readily altered and commonly contain exotic material of different ages and origins, which both hinder reliable kimberlite dating. Here, we investigated xenocrystic zircons and their reaction rims, which consist mainly of baddeleyite, in the strongly altered Kimozero kimberlites. Similar rims have been identified on mantle‐derived zircons in numerous kimberlites, but the timing and location of rim formation is poorly constrained. We found that the rims have replaced zircon xenocrysts of different ages and with both mantle and crustal origins, implying the rims formed at crustal levels. The results of Pb–Pb dating for these rims compared with those obtained from zircon U–Pb dating indicate that the rims record the timing of Kimozero kimberlite emplacement at ca. 1.97 Ga. Therefore, such baddeleyite rims are suggested as a promising material for obtaining reliable kimberlite ages.
Key Points
Desilicification (i.e., baddeleyite) rims replace kimberlite‐hosted zircon xenocrysts of mantle and crustal origins
These rims represent a promising material for kimberlite geochronology, as they form at the time of kimberlite emplacement
The Kimozero kimberlites intruded the Karelian Craton at ca. 1.97 Ga |
|---|---|
| ISSN: | 2169-9313 2169-9356 |
| DOI: | 10.1029/2022JB024482 |