tokens: interlopers, selection biases, and metallicity diagnostics. These errors can easily overwhelm the chemical enrichment signal as a function of environment, particularly at high redshifts. In addition, the diverse definition of environment makes comparisons across different studies ambiguous (Ellison et al. 2009 ; Peng Maiolino 2014 ). Cosmological simulations with a full three-dimensional formation history of cluster galaxies are a powerful tool to overcome observational biases and understand the physics of the environmental dependence of the chemical evolution (Davé, Finlator Oppenheimer 2011 ; Bahe et al. 2016 ; Genel 2016 ). In this Letter, we explore the MZR evolution as a function of the formation history of galaxy groups and clusters in IllustrisTNG, a large-scale cosmological simulation. Using a novel method of separating progenitor cluster galaxies into accreted and infalling categories, we report for the first time a clear systematic signature of ‘chemical pre-processing’ in cluster galaxies over the past 10 Gyr. We show the effectiveness of pre-enriched gas inflows in driving the chemical evolution of cluster galaxies out to twice the virial radius. 2 METHODS Our results are based on the IllustrisTNG simulations (Marinacci et al. 2017 ; Naiman et al. 2018 ; Nelson et al. 2018 ; Pillepich et al. 2018a ; Springel et al. 2018 ). The galaxy formation model is an updated and extended version of the Illustris model (Vogelsberger et al. 2013 ; Torrey et al. 2014 ). Details of the model modifications are described in Weinberger et al. ( 2017 ) and Pillepich et al. ( 2018b ). We use the TNG100 simulation that has a volume of ∼(100 Mpc) 3 and m b = 9.4 × 10 5 M ⊙ h −1 , where m b is the baryonic mass per particle. From the TNG100 simulation, we select all haloes with M 200 c ≥10 13.0 M ⊙ h −1 , where M 200 c is total mass enclosed within R 200 c (the radius within which the mean density is 200 times the critical density of the Universe at the halo redshift). This leads to a selection of 127 galaxy clusters and groups at z = 0. The mean cluster halo mass of our sample is 10 13.4 M ⊙ h −1 . We define the boundary of a cluster as twice the R 200 c of the cluster halo, which is similar to the splashback radius (Diemer et al. 2017 ). Out of all friends-of-friends (FOF) subhaloes associated with the cluster at z = 0, all galaxies whether centrals or satellites residing within twice the R 200 c make up our cluster member galaxy sample. Note that we exclude flybys and satellite galaxies in the process of splashing back at z = 0 from our analysis. We restrict this analysis to a stellar mass range of 10 9 –10 10.5 M ⊙ h −1 . The lower stellar mass cut is to minimize numerical uncertainties such that there are at least 1000 stellar particles per galaxy (a single star particle in TNG100 has a mass of about 10 6 M ⊙ h −1 ). We impose the upper mass cut of 10 10.5 M ⊙ h −1 to reduce contaminations from active galactic nuclei (AGN) activities (Pillepich
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