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D
Day, JMD, Harvey RP, Hilton DR.  2019.  Melt-modified lithosphere beneath Ross Island and its role in the tectono-magmatic evolution of the West Antarctic Rift System. Chemical Geology. 518:45-54.   https://doi.org/10.1016/j.chemgeo.2019.04.012   Abstract

Mantle lithosphere influences rift system tectonic evolution, yet the age and composition of rifted lithosphere is typically difficult to constrain due to limited sampling. In the West Antarctic Rift System (WARS), Cenozoic to recent alkaline volcanic rocks yield a variety of peridotite and pyroxenite xenoliths that allow sampling of lithosphere. We report osmium and helium isotope data, elemental abundances, and petrology, for a suite of xenoliths and lavas from the Hut Point Peninsula of Ross Island. Recently (<1.3 Ma) erupted basanites yield fresh dunite and harzburgite (olivine forsterite [Fo] 90.1-88.2), lherzolite (Fo90.6-87.4), and pyroxenite xenoliths (Fo89.3-87.3). The basanite lavas contain abundant large olivine xenocrysts (Fo89.7-88.0), with more ferroan matrix olivine grains (Fo83.7-81.2) and have HIMU-like incompatible trace-element signatures. The 3He/4He ratios (6.8 ±0.3RA; 2SD) defined by co-existing He-rich xenoliths indicate a mantle source distinct from high-3He/4He plume mantle. Pyroxenite and lherzolite xenoliths have similar relative abundances of incompatible trace elements to host lavas, whereas dunite xenoliths have refractory compositions. Melt-rock reaction occurring in the xenoliths is demonstrated by replacement by amphibole or clinopyroxene to form pyroxenite and lherzolite lithologies, or as amphibole-impregnated dunites. The 187Re-187Os systematics of the lavas, pyroxenites and lherzolites define an apparent isochron, with initial 187Os/188Os ratio of 0.1286 ±0.0001. The initial 187Os/188Os is within uncertainty of dunite and harzburgite xenolith Os isotope compositions (0.1279-0.1303). Pervasive evidence for melt-rock interaction indicates that the straight-line relationship in 187Re/188Os-187Os/188Os space is a mixing line between high Re/Os lavas with radiogenic 187Os/188Os, and dunite and harzburgite. Petrological and geochemical evidence indicates that dunite and harzburgite xenoliths represent young lithosphere, with rhenium depletion ages up to ~250 Ma. The timing of formation and composition of the Hut Point Peninsula xenoliths are consistent with both destruction and creation of mantle lithosphere during or after subduction along the Gondwana margin, prior to WARS formation. Modification of mantle lithosphere by subduction is also consistent with generation of HIMU-like metasomatized mantle reservoirs that fed Cenozoic to recent alkali volcanism of Mount Erebus and the WARS. The presence of young lithosphere within the WARS has collateral implications for rift dynamics and melting processes, especially beneath Mount Erebus, contrasting with older lithospheric mantle beneath the Trans-Antarctic Mountains and Marie Byrd Land.

Day, JMD, Tait KT, Udry A, Moynier F, Liu Y, Neal CR.  2018.  Martian magmatism from plume metasomatized mantle. Nature Communications. 9:4799.   10.1038/s41467-018-07191-0   Abstract

Direct analysis of the composition of Mars is possible through delivery of meteorites to Earth. Martian meteorites include ∼165 to 2400 Ma shergottites, originating from depleted to enriched mantle sources, and ∼1340 Ma nakhlites and chassignites, formed by low degree partial melting of a depleted mantle source. To date, no unified model has been proposed to explain the petrogenesis of these distinct rock types, despite their importance for understanding the formation and evolution of Mars. Here we report a coherent geochemical dataset for shergottites, nakhlites and chassignites revealing fundamental differences in sources. Shergottites have lower Nb/Y at a given Zr/Y than nakhlites or chassignites, a relationship nearly identical to terrestrial Hawaiian main shield and rejuvenated volcanism. Nakhlite and chassignite compositions are consistent with melting of hydrated and metasomatized depleted mantle lithosphere, whereas shergottite melts originate from deep mantle sources. Generation of martian magmas can be explained by temporally distinct melting episodes within and below dynamically supported and variably metasomatized lithosphere, by long-lived, static mantle plumes.

K
Korhonen, FJ, Saito S, Brown M, Siddoway CS, Day JMD.  2010.  Multiple Generations of Granite in the Fosdick Mountains, Marie Byrd Land, West Antarctica: Implications for Polyphase Intracrustal Differentiation in a Continental Margin Setting. Journal of Petrology. 51:627-670.   10.1093/petrology/egp093   AbstractWebsite

Production of granite in the middle to lower crust and emplacement into the middle to upper crust at convergent plate margins is the dominant mechanism of crustal differentiation. The Fosdick Mountains of West Antarctica host migmatitic paragneisses and orthogneisses corresponding to the middle to lower crust and granites emplaced as dikes, sills and small plutons, which record processes of intracrustal differentiation along the East Gondwana margin. U-Pb chronology on magmatic zircon from granites reveals emplacement at c. 358-336 Ma and c. 115-98 Ma, consistent with a polyphase tectonic evolution of the region during Devonian-Carboniferous continental arc activity and Cretaceous continental rifting. The gneisses and granites exposed in the Fosdick migmatite-granite complex were derived from Early Paleozoic quartzose turbidites of the Swanson Formation and Ford Granodiorite suite calc-alkaline plutonic rocks, both of which are widely distributed outside the Fosdick Mountains and have affinity with rock elsewhere in East Gondwana. Granites of both Carboniferous and Cretaceous age have distinct chemical signatures that reflect different melting reactions and accessory phase behavior in contrasting sources. Based on whole-rock major and trace element geochemistry and Sr-Nd isotope compositions, Carboniferous granites with low Rb/Sr are interpreted to be products of melting of the Ford Granodiorite suite. Extant mineral equilibria modeling indicates that the Ford Granodiorite suite compositions produce melt volumes > 10 vol. % at temperatures above biotite stability, involving the breakdown of hornblende + plagioclase, consistent with the high CaO and Na(2)O contents in the low Rb/Sr granites. The Carboniferous low Rb/Sr granites show a sequence from near-melt compositions to compositions with increasing amounts of early crystallized biotite and plagioclase and evidence for apatite dissolution in the source. Carboniferous granites derived from the Swanson Formation are scarce, suggesting that the significant quantities of melt produced from the now residual paragneisses were emplaced at shallower crustal levels than are now exposed. The Cretaceous granites are divided into two distinct groups. An older group of granites (c. 115-110 Ma) has compositions consistent with a dominant Ford Granodiorite source, and characteristics that indicate that they may be less evolved equivalents of the regionally exposed Byrd Coast Granite suite at higher crustal levels. The younger group of granites (c. 109-102 Ma) has distinct light rare earth element depleted signatures. The chemical and isotopic data suggest that these granites were derived from partial melting of both fertile and residual Swanson Formation and had low water contents, indicating that the source rocks may have been dehydrated prior to anatexis as the Byrd Coast Granite suite magmas were transferring through and accumulating at higher crustal levels. The Cretaceous granites derived from the Swanson Formation make up a prominent horizontally sheeted leucogranite complex. The accumulation of these melts probably facilitated melt-induced weakening of the crust during a well-documented transition from regional shortening to regional extension, the formation of a detachment structure, and rapid exhumation of the Fosdick migmatite-granite complex. These multiple episodes of melting along the East Gondwana margin resulted in initial stabilization of the continental crust in the Carboniferous and further intracrustal differentiation in the Cretaceous.

R
Rahib, RH, Udry A, Howarth GH, Gross J, Paquet M, Combs LM, Laczniak DL, Day JMD.  2019.  Mantle source to near-surface emplacement of enriched and intermediate poikilitic shergottites in Mars. Geochimica et Cosmochimica Acta. 266:463-496.   j.gca.2019.07.034   Abstract

Poikilitic shergottites make up >20% of the current martian meteorite collection, with a total of 27 samples. These meteorites are intrusive gabbroic to lherzolitic rocks and represent igneous materials recording important processes in the martian crust. To further constrain petrogenetic relationships amongst enriched and intermediate poikilitic shergottites, we studied a comprehensive suite of poikilitic shergottites — including four newly recovered samples (Northwest Africa [NWA] 11065, NWA 11043, NWA 10961, NWA 10618) — using bulk rock major- and trace-element compositions, mineral major-element compositions, oxygen fugacity (ƒO2) values, crystallization temperatures, phosphorus maps of olivine grains, and quantitative textural analyses. The characteristic bimodal textures (poikilitic and non-poikilitic textures) of poikilitic shergottites record evolving magmatic conditions at different stages of crystallization. Higher temperatures and more reducing conditions during early-stage crystallization are recorded in the poikilitic textures, while lower temperature and more oxidizing conditions are recorded in the non-poikilitic textures during late-stage crystallization. Oxygen fugacity estimates relative to the quartz-fayalite-magnetite (QFM) buffer for early-stage olivine-pyroxene-spinel assemblages of enriched and intermediate poikilitic shergottites suggest decoupling of ƒO2 and the degree of light rare earth element (LREE)-enrichment (i.e., [La/Yb]CI). An increase in ƒO2 exceeding 1 log unit from poikilitic to non-poikilitic textures implies degassing, with possible auto-oxidation, and/or crustal contamination. Quantitative textural analyses support the emplacement of both enriched and intermediate poikilitic shergottites as various shallow intrusive bodies, as well as a potentially widespread emplacement mechanism responsible for a major lithology of the martian crust. In addition, early assemblages (i.e., pyroxene oikocrysts) of all the poikilitic shergottites likely formed close to the crust-mantle boundary, implying a possible widespread presence of magma staging chambers at these depths. Fractional crystallization and magma storage in these chambers could have possibly resulted in all of the different enriched and intermediate shergottites that have been analyzed from Mars.