Decadal time series observations of hydrothermal fluid emissions from Lō'ihi volcano were initiated in 1992 using a combination of submersible and shipboard sampling strategies. Magmatic-tectonic processes associated with a spectacular seismic event in July--August 1996 led to the collapse of Pele's Vents (31¿C) near the volcano summit into a new pit crater (Pele's Pit) inundated by high-temperature (Tmax = 198¿C) focused and lower-temperature (30- to 200-fold ~1.5 months postcollapse and, during the following year, decreased a further twofold to threefold at Pele's Pit and ~30-fold at East Pit. While a steady concentration of ~400 nmol/L TDFe prevailed throughout the remaining years of this study at Pele's Pit, a gradual and threefold decrease in the concentration of DMn to about 15 nmol/L was observed. High-temperature fluids (128--198¿C 1997--1999, ~90¿C 2001) venting simultaneously from different orifices within Pele's Pit had distinguishable Fe/Mn ratios that can be attributed to different subseafloor origins. Fe/Mn ratios characteristic of fluids moderated by high-temperature water-rock reaction had low values in 1997--1998 (1.6 ¿ 0.7), increasing to about 7 in 1999. Fluids moderated by magmatic degassing of CO2 had much higher Fe/Mn ratios, increasing from 24 ¿ 15 in 1997--1998 to 50 in 1999 and 63--87 in 2001. Fe/Mn values of dispersing plumes at Lō'ihi reflect an admixture of these sources and a relative Fe abundance that is consistently high compared to mid-ocean ridge systems. The pulsed injection of Mn and Fe into the surrounding ocean associated with the 1996 tectonic-magmatic event at Lō'ihi was massive. Our decadal observations confirm that Mn and Fe are useful markers of the magnitude and evolution of the effects of magmatic perturbation on hydrothermal systems influenced by chronic magmatic degassing.