Cooling vs. heating histories of mantle peridotites revealed by multiple Fe-Mg exchange thermometers

Mineral geothermometry is based on cation exchange between minerals and effectively reflects the closure temperature through kinetic processes. Each geothermometer is designed for a specific purpose and has its own merits. The use of different thermometers that employ cations of different diffusivities is extremely important in capturing “snapshots” of thermal events, which facilitate decoding cooling/heating pulses, ascent rates and residence times.

Here we compare the results of Fe-Mg exchange thermometers between coexisting mineral pairs in spinel peridotites from different geotectonic environments  and discuss the most important implications that stem from this. Three mineral pairs were considered, olivine-spinel (ol-spl), orthopyroxene-spinel (opx-spl) and orthopyroxene-clinopyroxene (opx-cpx) which show progressively higher Fe-Mg closure temperatures in the order listed. The first two thermometers constitute new calibrations constructed by us whilst the third is the formulation of Brey & Köhler, 1990. 

Application of the above thermometers to abyssal peridotites (abyssal) and peridotites exposed in oceanic forearc and backarc regions shows undisturbed cooling patterns for each setting, with higher mean opx-cpx temperatures followed by opx-spl and then by ol-spl temperatures. When these patterns are compared to those obtained for ophiolitic peridotites it becomes immediately apparent that temperature distributions in mantle peridotites from oceanic forearcs and oceanic backarcs exhibit great similarities with those from ophiolites for all 3 thermometers. Abyssal peridotites have distinctly higher mean temperatures suggesting that ophiolitic massifs have not been formed in major ocean basins but rather in oceanic forearc or backarc settings. Such a conclusion is also strongly supported by trace-element geochemistry of ophiolitic volcanic rocks.

The above order of the three thermometers is, nonetheless, reversed when they are applied to peridotite xenoliths found in volcanic rocks along potential continental rift zones. The subcontinental lithospheric mantle is expected to cool normally through time hence display a cooling pattern like that observed for abyssal peridotites. In the case of the xenoliths however, where mantle pieces are collected and transported by hot magma at temperatures much higher than the ambient geotherm, their exposure to high temperatures reactivates diffusion so that the thermometer containing the mineral pair with the fastest Fe-Mg diffusion (i.e., ol-spl) slides uphill faster and records the highest temperatures. The very fact that this (counter-intuitive) reverse order of temperature distributions has been preserved, places further constraints on the time scales of xenolith transport as protracted times of magma storage and fractionation in crustal chambers would have led to subsequent cooling and obliteration of the temperature patterns observed.

G. P. Brey & T. Köhler, 1990. Geothermobarometry in Four-phase Lherzolites II. New Thermobarometers, and Practical Assessment of Existing Thermobarometers. Journal of Petrology, Vol. 31, Part 6, pp. 1353-1378. DOI:10.1093/petrology/31.6.1353