Peridotite
Peridotite is a coarse-grained, ultramafic igneous rock primarily composed of olivine and pyroxene minerals, constituting the main rock type of Earth's upper mantle.[1][2]
Its typical mineral assemblage includes more than 40% olivine, with orthopyroxene and clinopyroxene as dominant mafic phases, and lesser amounts of spinel, garnet, or plagioclase depending on depth and depletion history.[3][2] Common varieties encompass lherzolite (olivine plus both ortho- and clinopyroxene), harzburgite (olivine dominant with orthopyroxene), dunite (over 90% olivine), and wehrlite (olivine with clinopyroxene).[2][1]
Peridotite originates from partial melting residues or fertile mantle material at depths exceeding 30 kilometers, often exhumed as xenoliths in alkali basalts, kimberlites, or ophiolite complexes.[1][4] These rocks provide direct samples of mantle geochemistry, revealing processes like melt extraction that deplete incompatible elements and enrich magnesium and nickel.[5] Economically, peridotite hosts chromite deposits and serves as the protolith for diamond-bearing kimberlites, linking it to plate tectonics through subduction and mantle convection dynamics.[6][7]
Petrological Fundamentals
Definition and Composition
Peridotite is a coarse-grained, dark-colored ultramafic igneous rock primarily composed of olivine and pyroxene minerals, representing the dominant lithology of the Earth's upper mantle.[6] It qualifies as ultramafic due to containing less than 45% silica (SiO₂) by weight and over 90% mafic minerals, resulting in high concentrations of magnesium (Mg) and iron (Fe) oxides.[3] The rock's density typically exceeds 3.2 g/cm³, reflecting its heavy mineral constituents.[8] The essential mineral olivine, often as the magnesium-rich variety forsterite ((Mg,Fe)₂SiO₄), constitutes more than 40 volume percent of peridotite, frequently ranging from 50-90% in common variants.[3] Orthopyroxene, such as enstatite ((Mg,Fe)SiO₃), and clinopyroxene, like diopside (CaMgSi₂O₆), comprise the pyroxene components, typically totaling 10-50% combined, with orthopyroxene predominant in many mantle-derived samples.[6] Accessory minerals include spinel (MgAl₂O₄) or garnet (e.g., pyrope, Mg₃Al₂Si₃O₁₂) at depths greater than about 60 km, and minor phases like chromite or phlogopite, rarely exceeding 5%.[1] Compositional variations define subtypes: dunite exceeds 90% olivine; harzburgite features abundant olivine and orthopyroxene with little clinopyroxene; lherzolite includes balanced olivine, both pyroxenes, and aluminous phases; wehrlite emphasizes clinopyroxene alongside olivine.[3] These reflect degrees of partial melting and depletion in the mantle source, with fertile lherzolites retaining higher clinopyroxene (up to 20%) and aluminum oxide (Al₂O₃) contents around 3-4%, while depleted harzburgites show reduced levels near 0.5%.[9]Texture, Morphology, and Physical Properties
Peridotite displays a phaneritic texture, consisting of coarse, visible mineral grains that typically range from 1 mm to several centimeters in diameter, resulting from slow cooling deep within the Earth's mantle.[4][2] The primary minerals, such as olivine and pyroxenes, form interlocking granular crystals, with olivine often appearing subhedral to anhedral and pyroxenes more euhedral.[8] In mantle peridotites, common textures include protogranular equigranular fabrics or porphyroclastic types characterized by large relic grains in a finer recrystallized matrix due to deformation.[10] Morphologically, peridotite occurs as dense, massive rock bodies or as xenoliths entrained in basaltic lavas, exhibiting irregular, nodular shapes that reflect their origin as mantle fragments.[6] On a hand-specimen scale, it appears holocrystalline without glassy phases, and alteration can produce serpentine veins or iddingsite rims on olivine grains.[8] Physically, peridotite is notably dense, with a specific gravity of approximately 3.3 g/cm³ attributable to its high content of heavy ferromagnesian minerals.[11] Its color ranges from dark green to black or gray, influenced by the olivine proportion and minor iron oxidation.[2] The rock's hardness varies between 5.5 and 6 on the Mohs scale, reflecting the dominant minerals' properties, and it possesses high compressive strength around 107 N/mm².[12]Classification Schemes
Peridotites, as coarse-grained ultramafic rocks, are defined in the International Union of Geological Sciences (IUGS) modal classification scheme as containing greater than 40 vol.% olivine, distinguishing them from pyroxenites which have less than 40 vol.% olivine.[13] This scheme relies on ternary diagrams plotting the modal proportions of olivine (Ol), orthopyroxene (Opx), and clinopyroxene (Cpx), with accessory minerals such as spinel, garnet, or plagioclase noted separately to indicate equilibration conditions but not altering the primary subtype designation.[14][15] The primary subtypes of peridotite are delineated by the relative abundances of these phases: lherzolite features balanced proportions of olivine (typically 50-70 vol.%), orthopyroxene (20-40 vol.%), and clinopyroxene (5-15 vol.%); harzburgite is dominated by olivine and orthopyroxene (>5 vol.% Opx, <5 vol.% Cpx); dunite exceeds 90 vol.% olivine; and wehrlite emphasizes olivine with clinopyroxene (>5 vol.% Cpx, <5 vol.% Opx).[16][15] These boundaries reflect depletion trends in mantle residues, where lherzolites represent fertile mantle and harzburgites indicate higher degrees of partial melting.[17] Alternative schemes, such as those from the British Geological Survey (BGS), align closely with IUGS but emphasize field-based modal estimates for ultramafic rocks exceeding 40% olivine, incorporating pyroxene-peridotite variants where pyroxenes comprise significant fractions alongside olivine.[18] Genetic classifications, often applied to oceanic or ophiolitic peridotites, group them as residual (depleted harzburgite-lherzolite), cumulate (wehrlite-dunite), or hybrid based on trace element and isotopic data, though these complement rather than replace modal schemes.[19] Such approaches prioritize petrographic and geochemical evidence to infer mantle processing history.[20]Formation and Petrogenesis
Mantle-Derived Processes
Peridotite constitutes the dominant lithology of Earth's upper mantle and originates primarily as the refractory residue following partial melting of more primitive mantle material.[21] Under conditions of adiabatic decompression during mantle upwelling—such as beneath mid-ocean ridges or hotspots—peridotite intersects its solidus, initiating incongruent melting.[3] Low-melting-point components, including clinopyroxene and, at shallower depths, plagioclase, preferentially melt to generate basaltic liquids enriched in silica and incompatible elements, which then segregate and ascend, depleting the residue in these phases and yielding harzburgite or dunite dominated by olivine and orthopyroxene.[21][22] Melting degrees typically range from 10% to 25%, with experimental studies on fertile peridotites (Mg# 85–90) indicating that near-solidus productivities are low, increasing at higher melt fractions to produce primitive mid-ocean ridge basalts from peridotites with Mg# below 88.[23][24] This process extracts melt-compatible elements like Al, Ca, and heavy rare earth elements, resulting in a solid residue with elevated MgO (often >40 wt%) and Cr/Al ratios, as observed in abyssal and orogenic peridotites.[25] Mantle potential temperatures around 1,430 °C facilitate such melting at mid-ocean ridge settings.[26] Post-melting, depleted peridotites undergo modification through refertilization, where infiltrating melts or fluids precipitate secondary clinopyroxene, phlogopite, or amphibole, restoring fertile compositions and enriching trace elements.[27][28] This metasomatism manifests as modal changes (e.g., addition of new minerals) or cryptic alterations to existing mineral chemistries, such as increased Fe, Ti, and Na in pyroxenes.[29][30] In subcontinental lithospheric mantle, these events are often episodic, correlating with supercontinental assembly and linked to slab-derived fluids or plume activity, as evidenced in Archean peridotite suites.[28][31] Such processes explain modal and chemical heterogeneities in mantle xenoliths and ophiolitic peridotites, influencing subsequent melting behaviors.[32]