Doherty Senior Research Scientist
B.S., Notre Dame, 1968; Ph.D., Harvard, 1976
John Longhi's work involves laboratory measurement and modelling of basaltic phase equilibria. He applied this work to several topics:
I. Angrite petrogenesis (Geochim. Cosmochim. Acta, v. 63, 573-585, 1999):
Angrites are a group of ancient igneous meteorites that crystallized from the molten state approximately 11 million years after the condensation of our solar nebula. Phase equilibrium studies showed that the melts from which these meteorites crystallized could not have been derived by melting a source (planetesimal) that had chondritic proportions of refractory elements (Al, Ca, Mg, Si). Rather, the angrite parent body must have been enriched in Ca and Al, which, in terms of condensation calculations, implies that the angrite parent body was enriched in the high-temperature fraction of condensation. This work provides the first clear evidence of heterogenous accretion - at least at the plantesimal stage.
II. Origin of Proterozoic (Massif) Anorthosites (J. Petrol., v. 40, 339-362, 1999):
Proterozoic anorthosites and related mafic rocks are intrusive complexes in the Eath's upper continental crust comprising tens of thousands of cubic kilometers. Anorthosites formed by mechanical accumulation of an excess of neutrally buoyant plagioclase feldspar in basaltic magmas. Phase equilibrium measurements showed that the parent magmas of the anorthosites could not be derived from melts of peridotitic mantle; rather, mafic sources (lower continental crust?) are required.
III. Partition Coefficients/MORB Petrogenesis (Earth Planet. Sci. Lett., v. 166, 15-30, 1999; Geochemistry, Geophysics, Geosystems 3, 10.1029/2001/GC000148., 2002):
Short-lived radiactive decay and elemental fractionation in the U-Th system help constrain the migration of magma beneath mid-ocean ridges. Measurements of U and Th concentrations in upper mantle minerals and coexisting melt produced in high-pressure melting experiments have yielded a more accurate set of partition coefficients. Applied to time-dependent melt migration models, these coefficients indicate ambient porosities (0.5 to 1.0 %) that are more physically plausible than those (< 0.1%) predicted from the same models with partition coefficients appropriate to basaltic systems.
IV. Systematics of Mantle Melting (Geochemistry, Geophysics, Geosystems 3, 10.1029/2001/GC000204., 2002):
At the top of the spinel-peridotite field low degrees of mantle melting produce high-SiO2, high-Na2O liquids. With increasing pressure low-degree melts of the same source have lower SiO2 and Na2O. Shifting liquidus curves control the SiO2 variation; whereas increasing partition coefficients for Na2O in cpx keep the Na2O in the pyroxenes.
V. Origin of Lunar Anorthosites (J. Geophys. Res. 108, 10 .1029/202JE001941., 2003):
The lunar magma ocean may have been too short-lived to produce the ferroan anorthosites, but the magma ocean may have set up a gravitationally unstable cumulate pile that partially melted to produce the melts that were parental to the anorthosites