An indicator of the temperature, or range of temperatures, at which a geologic event (e.g. the crystallization of a magma or the metamorphism of pre-existing rocks) occurred. Apart from the presence or absence of minerals or mineral assemblages known to be stable within certain temperature ranges, among the most widely used indicators are: (a) stable-isotope distribution, e.g. the ratios of ¹⁸O to ¹⁶O between different mineral pairs varies according to temperature (see also oxygen-isotope analysis); (b) mineral transformations or inversions known to be temperature dependent, e.g. the transition of α‎ quartz to β‎ quartz at 573 °C; (c) liquid–vapour homogenization points in fluid inclusions (subject to certain assumptions, the temperature of formation of a crystal is indicated by the temperature at which the vapour bubble coexisting in the inclusion disappears upon heating); (d) unmixing or exsolution lamellae of mineral pairs below a particular temperature, e.g. chalcopyrite–bornite at 500 °C; (e) temperature-dependent element distribution between coexisting minerals, e.g. iron–titanium oxide distribution between the coexisting mineral pairs magnetite–ulvöspinel and ilmenite–hematite (in this instance subject also to oxygen fugacity).