This article presents the results of a comprehensive lithogeochemical study that was conducted using downhole rock samples from the Reykjanes geothermal system in Iceland. Magmatic fractionation trends established for least-altered rocks were used to correct mass change estimates for pseudo-enrichments/depletions due to fluid-rock interaction. Net mass changes of altered rocks in the Reykjanes geothermal system range from -26 to +22 g/100 g(p), and are dominated by changes in SiO2 > CaO > MgO > Na2O, K2O, S > C. Trace elements undergoing the greatest net mass changes are Ba > Zn > Cu, Ni > Sr > Rb > As > Cd. The lithogeochemical data are interpreted with petrographic data and data from parallel studies of mineral chemistry (detailed in separate manuscripts). Trends in the distribution of reactive-mobile elements correspond closely to the large-scale hydrology of the Reykjanes geothermal system, as determined by measured isotherms and hydrothermal mineral zonation. A hypothesis of deep inflow to the system from the southeast is supported by lithogeochemical and mineralogical evidence and downhole temperature measurements. Whole-rock mass changes of specific components counter their documented enrichments and depletions in the Reykjanes geothermal fluid relative to the initial seawater input. Volume estimates for the Reykjanes geothermal system are made based on the redistribution of select reactive-mobile elements. (C) 2016 Elsevier Ltd. All rights reserved.