Presentation on theme: "Basalts Why study basalts? How are they classified?"— Presentation transcript:
1Basalts Why study basalts? How are they classified? What are the significant differences chemical between terrestrial and extraterrestrial basalts?Summary models for origin of terrestrial and lunar basalts and basaltic achondrite meteorites.
2Trace Element Fractionation During Partial Melting From:
4Alkaline and Subalkaline Rock Suites 15,164 samplesIrregular solid line defines the boundary between Ne-norm rocksLe Bas et al., 1992; Le Roex et al., 1990; Cole, 1982; Hildreth & Moorbath, 1988
5Tholeiitic vs. Calc-alkaline Trends Terms emerged from tangled historyspanning many decades. CA labelproposed by Peacock in 1931.Tholeiite originated in mid-1800’sfrom Tholey, western Germany.Rocks show stronger Fe/Mgenrichment than CA trend.Tholeiites are commonly foundisland arcs, while CA rocksare more commonly foundin continental arcs.Cole, 1982
6K2O content of subalkaline rocks may broadlycorrelate withcrustal thickness.Low-K 12 kmMed-K 35 kmHigh-K 45 kmEwart, 1982
7Classification of Basalts Three basalt types recognized based on their degree of silica saturation:Quartz-hypersthene normative (Q + Hy)quartz tholeiiteOlivine-hypersthene normative (Ol + Hy)olivine tholeiiteNepheline normative (Ne)alkaline basaltTholeiitic basalts make up the oceanic crust, continental flood basalt provinces, and some large intrusions.Alkaline basalts are found in oceanic islands and some continental rift environments.
11Partition Coefficients for REE in Melts Dbulk = X1D1 + X2D2 + X3D3 + … + XnDn
12Chondrite Normalized REE patterns By “normalizing” (dividing by abundances in chondrites), the “sawtooth” pattern can be removed.
13Differentiation of the Earth Melts extracted from the mantle rise to the crust, carrying with them their “enrichment” in incompatible elementsContinental crust becomes “incompatible element enriched”Mantle becomes “incompatible element depleted”From:
14Sr Isotope Evolution on Earth 87Sr/86Sr)0Time before present (Ga)87Sr/86Sr)0Time before present (Ga)
15Sr and Nd Isotope Correlations: The Mantle Array
16Terrestrial Basalt Generation Summary MORBs are derived from the partial melting of a previously depleted upper mantle under largely anhydrous conditions at relatively shallow depths.True primary mantle melts are rare, although the most primitive alkali basalts are thought to represent the best samples of direct mantle melts.The trace element and isotopic ratio differences among N-MORB (normal), E-MORB (enriched), IAB, and OIB indicate that the Earth’s upper mantle has long-lived and physically distinct source regions.Ancient komatiites (>2.5 Ga) indicate that the Earth’s upper mantle was hotter in the Archean, but already depleted of continental crustal components.
19Lunar Olivine Basalt Thinsection Fe-Ti oxidesPlagioclaseOlivine + aligned MIsPyroxenesPlane Polarized LightSample collected from the SE end ofMare Procellarum by the Apollo 12 mission.Interpreted as a Lava Lake basalt.Cross Polarized LightFrom:
20Lunar Anorthosite Thinsection PyroxenesFractured Plagioclase FeldsparRock is 98% fsp,An95 to An97Plane Polarized LightHighly brecciated lunar anorthosite wascollected by the Apollo 16 mission to thelunar highlands SW of Mare Tranquillitatis.It has been dated at 4.44 Ga.Cross Polarized LightFrom:
21Earth Mars-sized Impact Model for Lunar Origin Impact hrImpact + 5hrFrom: Kipp & Melosh, 1986 (above) and W. Hartmann paintings of Cameron, Benz, & Melosh models (right)
22Features of the Giant Impact Hypothesis Original idea paper by Hartmann & Davis, 1975; additional geochemical research by Michael Drake and computer models by Jay Melosh and colleagues.Impact occurs soon after Earth’s core formation event because of the small lunar Fe core and difference in bulk density (rMoon = 3.3 g/cc << rEarth = 5.5 g/cc).Impact event must occur before formation of the lunar highlands at 4.4 Ga, which formed as a result of the crystallization of the lunar magma ocean. Lunar differentiation continues w/ basalt genesis (3.95 to 3.15 Ga).Oxygen isotope compositions of lunar and terrestrial rocks are similar, but different from Mars and meteorites. Earth-Moon must be made of the same stuff.Volatiles are depleted in the proto-moon during impact event. This is consistent with geochemistry and petrology of lunar samples.
23Lunar Interior Composition From: BVSP, 1986 and Taylor, 1987
24Lunar Basalt Generation Summary All lunar basalts are ancient in comparison with MORBs (~100 Ma average age). Lunar basalt ages range between 3.95 to 3.15 Ga.Mare regions resemble continental flood basalt provinces and ocean plateaus in areal extent.Several distinctly different compositions (e.g. KREEP, Hi-Ti, Low-Ti), which likely reflect different source regions that developed during post magma-ocean crystallization.Strong positive Eu anomalies in highlands Anorthosites is complemented by Eu depletion in all lunar basalts.Younger basalts are more primitive and may be derived from deeper sources. This could reflect increased internal heating from radioactive decay.
25Other Extraterrestrial Basalts - I Basaltic achondrite meteorites have compositions, textures, and mineralogies that are broadly similar to terrestrial basalts.Eucrites and Howardites all have ancient crystallization ages of ~4.6 Ga. Again very different from Earth and the Moon.Oxygen isotopic ratios are distinct from terrestrial rocks, thus they are derived from a different region in the solar system.Achondrites are derived from Eucrite Parent Body, which must have had a mantle dominated by olivine and pyroxene depleted in alkalis and volatiles and a high Fe/Mg. Melting occurred in the presence of plagioclase, so the body must be small to have low P at high T!Asteroid Vesta (540 km diameter) is a candidate for the EPB as its surface is covered in basalt, but this is just speculation.
26Asteroid Vesta - Eucrite Parent Body? Spectroscopy of surfacefrom HST indicatestwo types of basalts.Large crater found nearSouth Pole! Source ofachondrites?From: B. Zellner and NASA
27Viking 2 Lander Site Mars - Basalt Flow Field? Image Source: NASA
29SNC and Martian Basalt Summary SNC (Shergotty, Nakhla, Chassigny) meteorites are thought to be derived from Mars. Shergottites are most similar to terrestrial basalts while nakhilites are cumulate peridotites. All SNC show shock metamorphism.Oxygen isotopic signatures different from Earth and Moon.Noble gas ratios are similar to modern Mars atmosphere and very different from Earth. Fe/Mg ratio higher than Earth. Mars may not have a Fe-rich core.~1.3 Ga crystallization ages are much young than other basaltic meteorites. Corresponds to period of active basaltic resurfacing on Mars based on crater density.Mechanism for ejection from Mars surface is still problematical.