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ReversePetrogen : A Multiphase Dry Reverse Fractional Crystallization‐Mantle Melting Thermobarometer Applied to 13,589 Mid‐Ocean Ridge Basalt Glasses
Author(s) -
Brown Krein S.,
Molitor Z. J.,
Grove T. L.
Publication year - 2021
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1029/2020jb021292
Subject(s) - basalt , mantle (geology) , fractional crystallization (geology) , geology , partial melting , crystallization , mineralogy , thermodynamics , geochemistry , physics
We present a new algorithm, ReversePetrogen (RevPet) , to infer mantle melting conditions (pressure, temperature, source composition) using evolved basalts that have experienced multiphase fractional crystallization. RevPet measures and minimizes the compositional distance between experimentally predicted phase saturation boundaries and an erupted basalt and the more primitive liquids that return it to a primary melt. We use RevPet to investigate mantle melting conditions at mid‐ocean ridges (MORs) using a global data set of 13,589 basaltic glasses. We find that their average apparent mantle potential temperature ( T P *) is 1322°C ± 56°C with melting pressures of 13.0 ± 5 kbars. Inferring the true initial (pre‐melted) T P from T P * requires knowing the style and degree of melting of the input basalts. If MORB glasses were entirely produced by near‐fractional melting of a homogenous source, they would record the cooling of the mantle during melting from an initial T P = ∼1380°C (Δ T P = 0°C) down to T P = ∼1270°C. If, instead, they were all fully pooled near‐fractional melts of the same source, they would record variations in ambient MOR T P from ∼1300°C to 1450°C (Δ T P = 150°C). However, because MOR basalts are thought to be both near‐fractional and variably pooled melts of variable sources, MOR T P must be intermediate between these two extremes. Our best estimate, consistent with MOR crustal thickness, is that ambient MOR T P is homogenous (∼1350°C–1400°C) except near hotspots where T P reaches ∼1600°C. Some primitive glasses found near slow‐spreading ridges and back‐arcs record very low temperatures ( T P * < 1250°C) and pressures of melting (<10 kbar) and reflect mantle cooling during melting and melt equilibration in the mantle lithosphere.