1. CIRCE – Universidad de Zaragoza Edif. CIRCE, Mariano Esquillor 15, 50018 Zaragoza (Spain) Tel: (+34) 976 76 18 63. Email: aliciavd@unizar.es The Crepuscular Earth. A reference for calculating the depletion of mineral resources using LCA  The Crepuscular Earth or Thanatia is a guess thermodynamic model for a terrestrial “grave”, where all fossil fuels have been burned and converted into CO 2 and with the absence of mineral deposits. The crepuscular crust is composed by the 294 most abundant minerals currently found on Earth.  Thanatia constitutes a coherent baseline for the assessment of minerals. Any substance like a mineral deposit or a glacier is an exergy resource with respect to the Crepuscular Earth.  The exergy analysis allows the assessment of all physical properties of natural resources, such as composition, grade or quantity, with a single unit of measure (the exergy costs) and in an objective way far removed from market distortions or currency speculation. Introduction Alicia Valero and Antonio Valero CIRCE - Research Center for Energy Resources and Consumption. Universidad de Zaragoza (Spain) Discussion and Conclusions  This approach helps to solve some unresolved issues in the evaluation of abiotic resource depletion in LCA:  In the grave to cradle approach, it is not the extraction of materials which is of concern, but rather the dissipative use and disposal of materials.  A resource is not valued according to its mass or energy content. It is assessed according to its differentiation with the environment. The greater the ore grade of the mineral with respect to the dispersed state of the Crepuscular planet, the greater its thermodynamic value.  The temporal, political or social factor does not play any role in our view of Crepuscular Earth, and hence the arbitrariness of assuming static reserves disappears.  As opposed to other methodologies which account for abiotic resource depletion as a function of the extraction costs (as with the Eco-indicator 99), the mining of highly concentrated mines (which have low extraction costs) is penalized since they have associated greater replacement costs. However, high-grade mines do not need so much energy in the mining and concentration steps. Hence, both concepts complement each other and no double counting occurs.  The exergy analysis of minerals from Thanatia could constitute a universal and transparent prediction tool for assessing the degradation degree of non-renewable resources, with dramatic consequences for the future management of the earth’s physical stock.  The resources are physically assessed as the energy required to replace them from a complete degraded state or Thanatia to the conditions in which they are currently presented in nature: Replacement costs.  We propose to include one additional step in the LCA methodology: the grave to cradle stage to account for abiotic resource depletion.  The minimum exergy replacement cost b c of a mineral to concentrate a substance from Thanatia is calculated as :  The actual exergy replacement costs bt* consider also the state of technology: Methodology This study has been carried out under the framework of the ENE2010- 19834 project financed by the Spanish Ministry of Education and Science cc t b kb · * 