Olivine, the principal component of peridotite, makes up more than 40% of the pair olivine + pyroxene (or amphibole). This rock is relatively rare at the surface and is often altered by hydrothermal metamorphism or weathering because its constituent minerals are unstable at the weathering environment. Lherzolite is the least pretentious of them - it is a mixture of two pyroxenes and olivine 1. Harzburgite is rich in olivine and orthopyroxene but there can be only up to 5% clinopyroxene. Wehrlite contains almost no orthopyroxene but is rich in clinopyroxene and olivine. Dunite is a type of peridotite that is almost monomineralic (more than 90% of it is olivine). Ol, Opx, and Cpx represent olivine, orthopyroxene, and clinopyroxene respectively.
How to read this diagram? It is a typical ternary plot which are often used in geology.
There are four subtypes: dunite, harzburgite, lherzolite, and wehrlite. Note that neither peridotite nor pyroxenite are single rock types. It is an important rock type because the Earth’s mantle is predominantly composed of it.Ī classification diagram of ultramafic rocks. On the basis of major and trace element geochemistry, it can be concluded that miaskitic and alkaline pegmatites in Ring Sections 1-8 within the Larvik complex have been derived from the same parental melt, whereas pegmatites in Ring Sections 9 and 10 have a distinct geochemical signature, suggesting a separate source.Peridotite is a dark-colored igneous rock consisting mostly of olivine and pyroxene. Sodic mafic silicates in highly agpaitic pegmatites are HREE-enriched, with Ce/YbN = 0.22-0.58 compared to Ce/YbN > 1 for all other primary mafic phases, are strongly depleted in MREE, and have only slightly negative Eu anomalies (Eu/Eu* = 0.66–0.75). All early pegmatite mafic minerals in low agpaitic pegmatites have negative Eu anomalies (Eu/Eu* = 0.16–0.26), positive Ce anomalies, and low Nb/TaN and Y/HoN ratios. Primary amphibole compositions are dominantly calcic (edenite, ferro-edenite, hastingsite, magnesio-hastingsite, pargasite, and ferro-pargasite), with less common late-stage sodic-calcic amphiboles (katophorite, magnesio-katophorite, ferro-richterite, and taramite) sodic amphiboles (ferro-ferri-nybøite, ferri-nybøite) are only observed in agpaitic pegmatites. Secondary clinopyroxene, after amphibole or as a hydrothermal phase within fractures and vugs, is dominantly aegirine, with compositions ranging from Ae59Di16Hd25 to Ae99Di1Hd0. Primary magmatic diopside and hedenbergite with minor aegirine occur primarily within miaskitic to low agpaitic pegmatites, with compositions ranging from Ae3Di45Hd51 to Ae89Di5Hd6, whereas aegirine and aegirine-augite are the primary magmatic clinopyroxenes in agpaitic pegmatites, with compositions ranging from Ae31Di27Hd42 to Ae90Di6Hd4. Pyroxene compositions plot within the aegirine-diopside-hedenbergite ternary diagram, trending from the 50-50% Di-Hd tie line towards the 100% Ae endmember. The major and trace element geochemistry of mafic silicate minerals from alkaline pegmatites within the 10 ring sections of the Larvik plutonic complex, Oslo rift, southern Norway, have been studied to shed light on the complex evolutionary history of the pegmatites.