In this paper we classify the range of feldspathic igneous rocks using five geochemical variables: the FeO/(FeO + MgO) ratio or Fe-index, the modified alkali–lime index, the aluminum-saturation ...index, the alkalinity index, and the feldspathoid silica-saturation index. The Fe-index distinguishes between melts that have undergone extensive iron enrichment during differentiation from those that have not. The transition from tholeiite to ferrobasalt allows us to extend this boundary to silica values as low as 48 wt %. We introduce the feldspathoid silica-saturation index, which, coupled with the alkalinity index, allows us to extend the geochemical classification to alkaline rocks. We show that most alkaline rocks are ferroan and that this probably reflects extensive fractional crystallization of olivine and pyroxene with minimal participation of Fe–Ti oxides. The expanded classification allows us to illustrate the geochemical and petrogenetic relationship of the plutonic rocks from ferroan granites to nepheline syenites that commonly occur in intracratonic environments. It also allows us to distinguish four families of feldspathic rocks: (1) magnesian rocks, which are exemplified by Caledonian and Cordilleran batholiths and are characterized by differentiation under oxidizing and relatively hydrous conditions; (2) ferroan rocks, which include fayalite granites, alkali granites, and nepheline syenites and are characterized by differentiation under reducing and relatively dry conditions; (3) leucogranites, which commonly form by crustal melting; (4) potassic and ultrapotassic rocks, which originate from mantle that has been enriched in K2O.
Serpentinites have the lowest silica activity of common crustal rocks. At the serpentinization front, where olivine, serpentine, and brucite are present, silica activities (relative to quartz) are of ...the order of 10−2·5 to 10−5, depending on the temperature. Here we argue that this low silica activity is the critical property that produces the unusual geochemical environments characteristic of serpentinization. The formation of magnetite is driven by the extraction of silica from the Fe3Si2O5(OH)4 component of serpentine, producing extremely reducing conditions as evinced by the rare iron alloys that partially serpentinized peridotites contain. The incongruent dissolution of diopside to form Ca2+, serpentine, and silica becomes increasingly favored at lower T, producing the alkalic fluids characteristic of serpentinites. The interaction of these fluids with adjacent rocks produces rodingites, and we argue that desilication is also part of the rodingite-forming process. The low silica activity also explains the occurrence of low-silica minerals such as hydrogrossular, andradite, jadeite, diaspore, and corundum in serpentinites or rocks adjacent to serpentinites. The tendency for silica activity to decrease with decreasing temperature means that the presence of certain minerals in serpentinites can be used as indicators of the temperature of serpentinization. These include, with decreasing temperature, diopside, andradite and diaspore. Because the assemblage serpentine + brucite marks the lowest silica activity reached in most serpentinites, the presence and distribution of brucite, which commonly is a cryptic phase in serpentinites, is critical to interpreting the processes that lead to the hydration of any given serpentinite.
Dunite from New Caledonia displays three types of serpentine-dominated veins. The earliest, type 1 veins are narrow (50–100μm wide) and rarely extend across more than a single olivine grain. They are ...lizardite, contain abundant brucite and never contain magnetite. Type 2 veins are 0.01 to 0.1mm wide, extend across several olivine grains and cut across the type 1 veins. They are lizardite, contain magnetite, often in vein interiors, and contain less brucite than type 1 veins. Type 3 veins are dominantly chrysotile, cm-scale, have a magnetite-rich core, and extend for meters or more. Analyses of two representative samples indicate that the type 1 veins have relatively Fe-rich serpentine (XMg=0.92) and brucite (XMg=0.82). These minerals are less magnesian than those in the type 2 veins; serpentine has XMg=0.93–0.94 and brucite has XMg=0.84. In the magnetite-rich core to the type 3 vein both serpentine (XMg=0.94–0.97) and one of the two brucite populations (XMg=0.94) are Mg-rich. Opx in harzburgite layers in these samples is cut by serpentine veins that are on the order of 0.05mm wide. The serpentine veins after Opx lack talc or magnetite and, as with veins cutting olivine, the older veins are more Fe rich (XMg=0.84) than the younger veins (XMg=0.90). We conclude that the formation of magnetite was accompanied by the extraction of iron from the early-formed serpentine and brucite.
Thermodynamic calculations suggest that the type 1 veins formed in a rock-dominated system where the activities of FeO, MgO, and SiO2 were dictated by the compositions of olivine and orthopyroxene. In contrast the type 2 veins were formed in a more fluid-dominated system where the infiltrating fluid was relatively oxidizing and out of equilibrium with the original brucite–serpentine assemblage. Reduction of this fluid was accompanied by reaction of brucite and serpentine to magnetite and hydrogen. By producing magnetite, this reaction extracted iron from brucite and serpentine, making them both more magnesian. This would drive the brucite–serpentine–magnetite assemblage to higher oxygen fugacity, progressively decreasing the efficiency of the magnetite-forming reactions.
► Five types of serpentine veins are found in dunite from New Caledonia. ► Early veins, with Fe-rich brucite and serpentine cut single grains of olivine. ► Later through-going veins contain magnetite and Mg-rich brucite and serpentine. ► Early veins were rock-dominated, later veins were fluid-dominated. ► Reduction of fluid was accompanied by production of magnetite and hydrogen.
This geochemical classification of granitic rocks is based upon three variables. These are FeO/(FeO + MgO) = Fe-number or FeOtot/(FeOtot + MgO) = Fe*, the modified alkali-lime index (MALI) (Na2O + ...K2O - CaO) and the aluminum saturation index (ASI) Al/(Ca - 1middot67P + Na + K). The Fe-number (or Fe*) distinguishes ferroan granitoids, which manifest strong iron enrichment, from magnesian granitoids, which do not. The ferroan and magnesian granitoids can further be classified into alkalic, alkali-calcic, calc-alkalic, and calcic on the basis of the MALI and subdivided on the basis of the ASI into peraluminous, metaluminous or peralkaline. Because alkalic rocks are not likely to be peraluminous and calcic and calc-alkalic rocks are not likely to be peralkaline, this classification leads to 16 possible groups of granitic rocks. In this classification most Cordilleran granitoids are magnesian and calc-alkalic or calcic; both metaluminous and peraluminous types are present. A-type granitoids are ferroan alkali-calcic, although some are ferroan alkalic. Most are metaluminous although some are peraluminous. Caledonian post-orogenic granites are predominantly magnesian alkali-calcic. Those with <70 wt % SiO2 are dominantly metaluminous, whereas more silica-rich varieties are commonly peraluminous. Peraluminous leucogranites may be either magnesian or ferroan and have a MALI that ranges from calcic to alkalic.
Serpentinization of olivine-rich troctolite from core 227, Integrated Ocean Drilling Program (IODP) Hole U1309D ranges from <10% to >90%. Two episodes of serpentinization are recognized. The first, ...dominant in weakly serpentinized samples, is an approximately isochemical (except for water) replacement of olivine (Fo84–85) by a mixture of serpentine (antigorite, Mg-number 92) and brucite (amakinite-rich; Mg-number 65). The compositions of the minerals in type 1 veins are a reflection of Fe–Mg exchange between olivine and the brucite + serpentine formed during early serpentinization. The early serpentinite veins (type 1) are thin (< 0·05 mm), irregular, and exploit pre-existing cracks in olivine. The presence of antigorite suggests that early serpentinization occurred at T > 300°C. Type 1 veins reflect rock-dominated serpentinization, became isolated early in their history, and persist as relicts in all but the most altered samples. The main episode of serpentinization is manifested by through-going lizardite (average Mg-number 96)–magnetite veins (type 2). Type 2 veins define an anastomosing foliation, may be several millimeters in width and appear to exploit pre-existing, favorably oriented type 1 veins. Type 2 veins reflect open-system serpentinization. Magnetite in these veins formed by oxidation of the Fe in brucite and serpentine, whereas addition of silica to the system converted the Mg-component of the brucite to serpentine. Magnetite forms one or more distinct bands in the interior of the vein and is never in direct contact with relict olivine. A brucite–serpentine mixture, similar to that found in type 1 veins, but with lizardite instead of antigorite, is commonly present at the margins of type 2 veins (i.e. where they are in reaction contact with relict olivine). We interpret type 2 veins as a steady-state system where brucite continually forms at the olivine–vein contact and then reacts out in the interior of the vein. This continual formation and destruction of brucite imposes an exceptionally low aSiO2 on the system. Magnetite and olivine are never in contact in type 2 veins (or anywhere else) because the olivine-out reaction yields ferroan brucite and not magnetite. The desilication of serpentine in the type 2 veins is a reflection of the inherent instability of Fe-rich serpentine with respect to magnetite at low silica activity. Thus, the composition of serpentine in equilibrium with magnetite in serpentinites is a function of serpentine–magnetite and not serpentine–olivine equilbria.
Given that secondary magnetite is common in serpentinites, it is clear that serpentinites are oxidized rocks. Questions remain, however, concerning the distribution of ferric iron among magnetite and ...serpentine minerals and the role of ferric iron-rich serpentine in the formation of secondary magnetite. Direct determination of ferric iron in serpentine is not possible using an electron microprobe. We show, however, that the stoichiometic effects of ferric iron substitutions are detectable, although not quantifiable, by microprobe. First, we demonstrate that for studies that provide both microprobe analyses of major elements of serpentine and Mössbauer analysis of ferric iron, substitution effects are obvious. Next, it is equally clear that the early veins forming at the onset of olivine hydration (type 1 veins) show no indication of the presence of ferric serpentine, although a small amount of ferric 'brucite' may occur. Finally, we show that secondary (type 2) veins, which form as the system becomes open to fluids in equilibrium with plagioclase or pyroxene, contain, in addition to significant alumina, stoichiometric indications of ferric iron substitution. The serpentine in these veins is magnesian, usually with Mg#s around 96-98. Thus, even if a significant proportion of this iron is ferric, it comprises only a small fraction of the total ferric iron budget of the rock. Given that reduced iron is known to be abundant in early-formed brucite and early-formed serpentine and given that brucite, in particular, is absent from evolved serpentine veins, we propose that most magnetite in serpentinites forms as a tertiary product via oxidation of brucite.
The North Qilian Orogenic Belt (NQOB), which consists of ophiolitic mélange and island-arc assemblages containing many granites, blueschists, and eclogites, lies between the Alax and Qilian terranes ...in northwestern China. The Minleyaogou and Niuxinshan granitoids occur at the northern and southern margins, respectively, in the middle segment of the NQOB. The Minleyaogou pluton is granodiorite in composition, whereas the Niuxinshan pluton consists mainly of red granite with minor grey quartz diorite. Geochemically, the Minleyaogou granite differs from the Niuxinshan granite in that it contains a smaller range in SiO
2
, has lower total alkalis, and is more peraluminous. Both granitoids are magnesian but the Niuxinshan granite is alkali-calcic, whereas the Minleyaogou granodiorite is calcic. Both granitoids have similar chondrite-normalized rare earth element patterns with light rare earth element enrichment and negative Eu anomalies. They have pronounced negative Ba, Nb, Sr, P, and Ti anomalies indicating that they have an affinity to island-arc or active continental margin magmatism. SHRIMP U-Pb dating of zircons from the granitoids yields a formation age of 477 Ma for the Niuxinshan granite and 463 Ma for the Minleyaogou granodiorite. These ages, combined with the geochemistry and locations of the plutons, suggest that they formed by the double subduction of the North Qilian oceanic plate during early Palaeozoic time. Formation of the Niuxinshan granite may be related to southward subduction under the Qilian terrane at 477 Ma, whereas the Minleyaogou granodiorite was formed by northward subduction at 463 Ma under the Alax terrane.
The formation of peralkaline melts is generally recognized to be related to enrichment of the mantle in sodium or potassium during a metasomatic event that predated magma generation. The wide range ...in mineral assemblages, mineral compositions, and fluid compositions in peralkaline melts are, to a large extent, a function of the sodium/potassium content of these mantle-metasomatizing fluids, which also governs redox conditions. We propose that sodic fluids will reduce the mantle assemblage by depleting ferric iron from garnet to form the aegirine component in pyroxene. In contrast, potassic fluids would oxidize the mantle assemblage by extracting Al2O3 from the garnet to make phlogopite. We suggest that during differentiation of peralkaline melts, once Fe–Ti oxides have been depleted from the assemblage, simple crystallization reactions of common solid phases such as aegirine or arfvedsonite control the oxygen fugacity by equilibria such as Hence, the more persodic a melt is, the more aegirine will be crystallized and the more reduced this melt will be. This is because without a mineral donor for ferric iron, the crystal chemical forcing of the coupled crystallization of Na with Fe3+ drives oxidation of the dominant Fe2+ in the melt and thereby reduction of the C–H–S–O-bearing fluid and/or melt phase. This is in convincing agreement with observations from a variety of silica-undersaturated peralkaline complexes such as Ilimaussaq in Greenland, Mt. St. Hilaire in Canada, Lovozero and Khibina in Russia, and Tamazeght in Morocco, and explains the occurrence of magmatic methane in the most sodic of these complexes. In silica-oversaturated peralkaline melts such as comendites and pantellerites, however, this equilibrium appears to be less influential and these melts are accordingly less reduced. In perpotassic melts, in contrast, no K–Fe3+ pyroxene is stable and K can only be incorporated into dominantly Fe2+-bearing phases such as biotite or K-amphibole, if no K-feldspar is stable. Here, redox conditions in the coexisting fluid and/or melt are either unaffected or even driven to more oxidized values by the crystallization reactions, which is in accordance with observations on the ultrapotassic rocks of the Roman Province (Italy) or the kalsilite-bearing Murun complex in Siberia. We conclude that peralkalinity in magmatic rocks is tightly connected to oxygen fugacity, that both the generation and the evolution and crystallization of peralkaline melts are governed by redox equilibria, and that these depend strongly on the Na/K ratio as well as on the (Na + K)/Al ratio of the melt. The specific features of peralkaline rocks, such as the difference between agpaitic and miaskitic rocks, the unusual enrichment of high field strength elements and the occurrence of magmatic hydrocarbons in some of them, can all be explained by the oxygen fugacity-controlling model reactions discussed in this study.
Mineral assemblages in common sulfide ore deposits are examined together with phase relations to (1) investigate the pressure-temperature conditions required for the onset of metamorphically induced ...partial melting involving economic minerals, and (2) place constraints on the amount of melt produced. Deposits that contain sulfosalt or telluride minerals may start to melt at conditions ranging from lowest greenschist facies to amphibolite facies. Deposits lacking sulfosalt and/or telluride minerals may begin to melt once P-T conditions reach the upper amphibolite facies, if galena is present, or well into the granulite facies if galena is absent. The result is two broad melting domains: a low- to medium-temperature, low melt volume domain involving melting of volumetrically minor sulfosalt and/or telluride minerals; and a high-temperature, potentially higher melt volume domain involving partial melting of the major sulfide minerals. Epithermal gold deposits, which are especially rich in sulfosalt minerals, are predicted to commence melting at the lowest temperatures of all sulfide deposit types. Massive Pb-Zn (-Cu) deposits may start to melt in the lower to middle amphibolite facies if pyrite and arsenopyrite coexist at these conditions, and in the upper amphibolite facies if they do not. Excepting sulfosalt-bearing occurrences, massive Ni-Cu-PGE (platinum group element) deposits will show little to no melting under common crustal metamorphic conditions, whereas disseminated Cu deposits are typically incapable of generating melt until the granulite facies is reached, when partial melting commences in bornite-bearing rocks. The volume of polymetallic melt that can be generated in most deposit types is therefore largely a function of the abundance of sulfosalt minerals. Even at granulite-facies conditions, this volume is usually less than 0·5%. The exception is massive Pb-Zn deposits, where melt volumes significantly exceeding 0·5 vol. % may be segregated into sulfide magma dykes, allowing mobilization over large distances.
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