2026ApJ...997...95Y

We present one of the first measurements of the mass-metallicity relation (MZR) in multiple massive protoclusters at cosmic noon, using Hubble Space Telescope (HST) G141 slitless spectroscopy from the MAMMOTH-Grism survey. We identify 63 protocluster member galaxies across three overdense structures at z = 2-3 with robust detections of [O III], Hβ, and [O II] emission. The sample spans gas-phase metallicities of $12+\mathrm{log}({\rm{O}}/{\rm{H}})=8.2-8.6$, dust-corrected Hβ-based star formation rates (SFRs) of 10-250 M_⊙ yr^–1, and stellar masses of M_* ∼ 10^9.4-10^10.5M_⊙, derived via spectral energy distribution fitting using deep HST and ground-based photometry. We stack spectra in five M_* bins to obtain average metallicities and SFRs. Relative to field galaxies at similar redshifts, protocluster members show elevated SFRs at M_* < 10^10.25M_⊙ and a systematically shallower MZR: $12+\mathrm{log}({\rm{O}}/{\rm{H}})$ =(6.96 ± 0.13) + (0.143 ± 0.017) $\times \,\mathrm{log}({M}_{* }/{M}_{\odot })$. We detect a mass-dependent environmental offset: massive protocluster galaxies are metal-poor compared to field counterparts of similar mass, whereas lower-mass systems exhibit comparable or mildly enhanced metallicities. This trend is consistent with a scenario where cold-mode accretion dilutes the interstellar medium (ISM) across the full mass range, while efficient recycling of feedback-driven outflows preferentially enriches the ISM in low-mass galaxies. Finally, we assess the dependence of metallicity offsets on local overdensity and find no significant trend, likely reflecting the survey's bias toward protocluster cores.