Presentation on theme: "Basalts Why study basalts? How are they classified? What are the significant differences chemical between terrestrial and extraterrestrial basalts? Summary."— Presentation transcript:
Basalts 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.
Trace Element Fractionation During Partial Melting From:
Basalt Types-Major Element Variation
Alkaline and Subalkaline Rock Suites Irregular solid line defines the boundary between Ne-norm rocks 15,164 samples Le Bas et al., 1992; Le Roex et al., 1990; Cole, 1982; Hildreth & Moorbath, 1988
Tholeiitic vs. Calc-alkaline Trends Terms emerged from tangled history spanning many decades. CA label proposed by Peacock in Tholeiite originated in mid-1800’s from Tholey, western Germany. Rocks show stronger Fe/Mg enrichment than CA trend. Tholeiites are commonly found island arcs, while CA rocks are more commonly found in continental arcs. Cole, 1982
K 2 O content of subalkaline rocks K 2 O content may broadly correlate with crustal thickness. Low-K 12 km Med-K 35 km High-K 45 km Ewart, 1982
Classification of Basalts Three basalt types recognized based on their degree of silica saturation: –Quartz-hypersthene normative (Q + Hy) quartz tholeiite –Olivine-hypersthene normative (Ol + Hy) olivine tholeiite –Nepheline normative (Ne) alkaline basalt Tholeiitic 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.
Partition Coefficients for REE in Melts D bulk = X 1 D 1 + X 2 D 2 + X 3 D 3 + … + X n D n
Chondrite Normalized REE patterns By “normalizing” (dividing by abundances in chondrites), the “sawtooth” pattern can be removed.
Differentiation of the Earth Melts extracted from the mantle rise to the crust, carrying with them their “enrichment” in incompatible elements –Continental crust becomes “incompatible element enriched” –Mantle becomes “incompatible element depleted” From:
Sr Isotope Evolution on Earth Time before present (Ga) 87 Sr/ 86 Sr) 0
Sr and Nd Isotope Correlations: The Mantle Array
Terrestrial 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.
Apollo 15 Basalt Sample Vesicles - Probably derived from CO degassing
Lunar Olivine Basalt Thinsection From: Sample collected from the SE end of Mare Procellarum by the Apollo 12 mission. Interpreted as a Lava Lake basalt. Olivine + aligned MIs Pyroxenes Plane Polarized Light Cross Polarized Light Fe-Ti oxides Plagioclase
Lunar Anorthosite Thinsection From: Cross Polarized Light Plane Polarized Light Highly brecciated lunar anorthosite was collected by the Apollo 16 mission to the lunar highlands SW of Mare Tranquillitatis. It has been dated at 4.44 Ga. Fractured Plagioclase Feldspar Pyroxenes Rock is 98% fsp, An 95 to An 97
Earth Mars-sized Impact Model for Lunar Origin From: Kipp & Melosh, 1986 (above) and W. Hartmann paintings of Cameron, Benz, & Melosh models (right) Impact + 5hr Impact hr
Features 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 ( Moon = 3.3 g/cc << Earth = 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.
Lunar Interior Composition From: BVSP, 1986 and Taylor, 1987
Lunar 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.
Other 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.
Asteroid Vesta - Eucrite Parent Body? From: B. Zellner and NASA Spectroscopy of surface from HST indicates two types of basalts. Large crater found near South Pole! Source of achondrites?
Viking 2 Lander Site Mars - Basalt Flow Field? Image Source: NASA
SNC 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.