Age and compositional data of zircon from sepiolite drilling mud to identify contamination of ocean drilling samples

Abstract

Zircon extracted from drilled oceanic rocks is increasingly used to answer geologic questions related to igneous and sedimentary sequences. Recent zircon studies using samples obtained from marine drill cores revealed that drilling muds used in the coring process may contaminate the samples. The JOIDES Resolution Science Operator of the International Ocean Discovery Program has been using two types of clays, sepiolite and attapulgite, which both have salt water viscosifier properties able to create a gel‐like slurry that carries drill cuttings out of the holes several hundred meters deep. The dominantly used drilling mud is sepiolite originating from southwestern Nevada, USA. This sepiolite contains abundant zircon crystals with U‐Pb ages ranging from 1.89 to 2889 Ma and continental trace element, δ18O, and εHf isotopic compositions. A dominant population of 11–16 Ma zircons in sepiolite drilling mud makes identification of contamination in drilled Neogene successions particularly challenging. Interpretation of zircon analyses related to ocean drilling should be cautious of zircon ages in violation of independently constrained age models and that have age populations overlapping those in the sepiolite. Because individual geochronologic and geochemical characteristics lack absolute discriminatory power, it is recommended to comprehensively analyze all dated zircon crystals from cores exposed to drill mud for trace element, δ18O, and εHf isotopic compositions. Zircon analyzed in situ (i.e., in petrographic sections) are assumed to be trustworthy.

Generation of Silicic Melts in the Early Izu‐Bonin Arc Recorded by Detrital Zircons in Proximal Arc Volcaniclastic Rocks From the Philippine Sea

“…We tested potential contamination during core recovery by analyzing zircons from the drilling mud used during Expedition 351. This sepiolite‐based mud was made up from lacustrine sediments in Nevada, USA; it contains Neogene zircons derived from the volcanic and plutonic rocks of the Death Valley part of the Basin and Range province, as well as older grains from local Mesozoic and Proterozoic crystalline basement rocks (see also Andrews et al, ). Our sepiolite mud sample yielded only a single Oligocene grain and none in the 43–27 Ma (Eocene to Oligocene) age range of U1438 zircons.…”

(U‐Th)/He zircon dating of Chesapeake Bay distal impact ejecta from ODP site 1073

“…U/Pb TIMS and secondary ionization mass spectrometry dating of the Sea Mud ™ zircon grains has yielded ages ranging from 1.89 to 2889 Ma (Andrews et al. ), but no (U‐Th)/He data exist for Sea Mud ™ zircon grains and we thus cannot comparatively evaluate the possibility of contamination in our drill core sample. However, the broad distribution of (U‐Th)/He zircon dates from this sample, with a distinctive, young cluster of dates (Fig.…”

“…Our ODP sample is comprised a mix of unshocked, weakly shocked, and intensely shocked (including granular texture) zircon grains. The shocked zircon grains likely represent distal ejecta from the Chesapeake Bay impact structure due to the excavation/ejection process, whereas the unshocked zircon crystals may be derived from (1) unshocked Chesapeake Bay target rocks, (2) different sources unrelated to the impact, or (3) may represent contamination by drilling mud (Andrews et al 2016). ODP drilling procedures include the use of a sepiolite drilling mud (Sea Mud TM ), which is produced by mixing material quarried from the Amargosa Basin of Nevada with seawater to form a gel that is able to carry heavier particles out of the drill hole.…”

Experimental evidence for the preservation of U-Pb isotope ratios in mantle-recycled crustal zircon grains

“…Not all scientists accepted that the zircons were crustal and suggested that they might have resulted from contamination either during rock processing for mineral separation or with drilling muds, a possibility recently demonstrated by Andrews et al . 2 . At around the same time as the discovery of the ocean gabbros zircons, geologists working in Kytlym, a concentrically zoned ultramafic body of the Urals Platinum-Bearing Belt 3 , also found crustal zircon in the dunites forming the core of the massif.…”

“…For example, heating at 915 °C for 10 5 years (point A 1 ) is equivalent to heating at 1138 °C for 10 years (point B 1 ), or heating at 1177 °C for 10 5 years (point A 4 ) is equivalent to heating at 1396 °C for 10 years (point B 4 ). Calculations were done with the COMSOL TM 58 software using Cherniak and Watson Arrhenius equation: DPb = 1.1 × 10 −1 exp (−550±30 kJ mol−1/RT) m 2 sec −1 1 , 2 . …”