Week 2: When the Earth was young (Archean eon) •EPSC 233-001 Fall 2002 Oceanic and continental crusts have different proportions of the main elements. continental (felsic) crust: more Al, Si, less Mg, Fe average density = 2.7 g/cm3 oceanic (mafic) crust: less Al, Si, more Mg, Fe average density = 3.0 g/cm3 Greenstone belts (shown in black on map) Podlike bodies consisting of: • metamorphosed mafic and ultramafic volcanic rocks (very Mgrich, Si-poor, a mantle-like chemistry) • felsic volcanics • deep-water sediments & chert (no limestone) How do you form greenstone belts? 1) Mantle plumes are “columns” of ultramafic magma rising from deeper parts of the mantle... They could explain the ultramafic lavas of greenstone belts. They might have been more numerous in the Archean than ever since because of a hotter mantle. •In Hawaii, the plate moves over the hot spot at a rate of several cm per year. This has created a string of basaltic (mafic) volcanic islands. • Iceland, on the other hand, sits atop a mid-ocean ridge. The plate motion is slow (about 2 cm/year) and the plume below melts and remelts the new oceanic crust. Rocks are mixtures of minerals that do not all melt at the same temperature. If you melt only part of the mantle and send that melt upwards through fractures in the rock, you are extracting a more felsic magma (the aluminum, silica-rich minerals melt earlier than the Mg-rich silica-poor minerals.) This magma is less dense than the mantle and tends to rise through faults (large cracks in the rocks)... This magma may crystallize near the Earth’s surface and be remelted again to produce a more felsic composition, more like continental crust. Iceland is unusual because it is a baby continent growing near a mid-ocean ridge (shown on left). Most continental crust is probably created near subduction zones (as shown below), where partial melting of oceanic crust takes place as it is subducted. Greenstone belts were probably folded during subduction of small Archean plates . Earliest crust would have been basaltic (close to average mantle composition), Mg-rich and Sipoor. Continental crust grew more slowly, and formed by partial melting of the early oceanic crust. Within our solar system, our Earth is uniquely suited to life: • size and distance from Sun are “right” • large enough to retain an atmosphere • this atmosphere is thin enough to let sunlight reach its surface, thick enough that surface water does not evaporate • surface water can exist in liquid state Evidence of earliest life? Isua Gneiss (3.8 Ga) from Greenland (metamorphosed sandstones). The banded gneiss containing graphite flakes, and the dated igneous rock that crosscuts it. It was suggested in 1996 that the proportions of carbon isotopes 13C/12C in their graphite suggests that the carbon was originally fixed by photosynthesis. Carbon Isotopic Evidence for Early Life A paper in Nature, this year, suggests that other inorganic processes can produce a life-like isotopic signature. Modern and Ancient Stromatolites Oldest ancient bacteria, 3.5 Ga, are preserved in chert (SiO2) found with stromatolites. • A life-like 13C/12C signature is a chemical fossil. • Earliest fossils of cells, preserved in chert (soft gelatinous dissolved silica, common in areas of submarine volcanic activity) are body fossils. • Stromatolites are “bacterial condominiums”: layered sediments that accumulate as columns or domes where grains are trapped on sticky mats formed by bacterial colonies. (It has been argued, however, that every one of these features could form without the influence of life... So these observations support an hypothesis for the age of the earliest life.) (Australia) (Greenland) (Nunavut, Canada) Many chemicals are expelled at mid-ocean ridges, some of which decompose in seawater and release energy. Earliest bacteria could have evolved there, by using various chemicals as energy sources (chemosynthesis). Banded iron formations are also found in Greenland. Layers rich in iron oxide alternating with silica-rich layers. Submarine volcanic eruptions were the source of the dissolved silica that was precipitated as chert. What was the source of the iron? Today, river waters carry hardly any dissolved iron to the ocean because it “rusts” on the continents. •This world was fit for life, but alien to us . . . - no significant free oxygen - a CO2-rich atmosphere - shorter days (a minimum of 15 hours . . . ) - stronger tides (Moon was closer) - iron deposition along the continental platform BUT . . . - ocean composition probably similar to today - temperature range of liquid water: much like today - simple life forms may already be changing the composition of the atmosphere The fragments of older rocks incorporated in younger strata were produced during periods of low sea level, when the older rocks were exposed to erosion. Is this list of the different rock types: “red, pink, orange, yellow, aqua” in the correct time sequence ? No. It should be pink, red, orange, yellow and aqua. The “red” igneous rocks must be younger than the metamorphic rocks.