The bulk dissolution-based zircon concentration method applied to mafic rocks in the atypical petroleum system of the Parnaíba Basin

The lack of zircon grains or the loss of radiogenic Pb significantly contributes to inaccuracies and gaps in U-Pb geochronology. Mechanisms of Pb loss include recrystallization of metamict zircon during metamorphism, leaching of Pb from metamict domains by hydrothermal or diagenetic fluids, or even during chemical weathering (Sharman and Malkowski, 2024). Zircon domains with Pb loss may be preferentially removed by thermal annealing at high temperature (800–1100^oC) followed by chemical attack with a heated hydrofluoric acid (chemical abrasion technique, CA, Mattinson, 2005). This procedure increases the concentration rate of zircon crystals compared to the less efficient concentration of zircon grains by conventional density and magnetic separation methods, particularly for medium- to fine-grained mafic-ultramafic igneous rocks. In this study, we optimized the procedure presented by Oliveira et al. (2022), where traditional separation methods involving manual magnets, Frantz, and dense liquids are replaced by complete rock dissolution to recover zircon grains from mafic-ultramafic rocks through bulk-rock chemical dissolution. We have applied the method to cutting samples of mafic rocks collected every three meters in wildcat wells. Normally, samples are fragmented in a disc mill or Selfrag, followed by sieving (<500μm). However, cutting samples skip the fragmentation step due to its powered nature. Heavy minerals concentration can be performed by mixing the rock powder with water and detergent and applying small quantities (about a spoon full at a time) to a Wilfley table. In small volumes samples, however, this step may be skipped. The bulk heavy minerals concentrate is then annealed in a muffle furnace at 900°C for 60 hours. After cooling, chemical dissolution proceeds in three steps at 100°C: (1) Silicates are dissolved using hydrofluoric acid (29M HF) over 24 hours, and the remaining material is rinsed in distilled water. (2) Aqua regia, a mixture of 3:1 hydrochloric acid (16M HCl) and nitric acid (12 MHNO₃), is then added to the sample and allowed to act for another 24 hours to dissolve oxides and other refractory phases, followed again by rinsing with distilled water. (3) Boric acid (3% H₃BO₃) solution combined with 3M HCl is employed, for 12-24 hours, to dissolve fluorides formed in previous steps. Finally, the remaining concentrate gets rinsed at least three times to ensure complete removal of any trace amounts of acids. A total of 25 samples of mafic rocks were processed. The recovered zircon grains have diverse morphologies, including circular, elongated, and pyramidal shapes, ranging from 25 to 100µm in size and have different colors. Our procedure eliminates sampling separation bias, mainly related to tiny grains, and allows an efficient recovery of grain from small volume samples. The implementation of this methodology contributes to overcoming the historical lack of zircon grains found in mafic and ultramafic rocks. In this case study, zircon grain ages contributed to better constraining the age of sills associated with the atypical petroleum system of the Parnaiba Basin.

Reference

Oliveira, A.L., Schmitz, M.D., Wall, C.J., Hollanda, M.H.B.M., 2022. A bulk annealing and dissolution-based zircon concentration method for mafic rocks. Chemical Geology, 597, 120817.

Mattinson, J. M., 2005. Zircon U–Pb chemical abrasion (“CA-TIMS”) method: Combined annealing and multi-step partial dissolution analysis for improved precision and accuracy of zircon ages, Chem. Geol., 220, 47–66.

Sharman, G.R., Malkowski, M.A., 2024. Modeling apparent Pb loss in zircon U–Pb geochronology, Geochronology, 6, 37