low-MgO Basalts Research Articles

Fluid processes in the early Earth and the growth of continents

Water is an essential ingredient in transforming primitive mantle-derived (mafic) rocks into buoyant (felsic) continental crust, thereby driving the irreversible differentiation of Earth's lithosphere. The occurrence in Archaean cratons of sodic granites of the tonalite–trondhjemite–granodiorite (TTG) series, high-MgO variolitic basalts, high-Mg diorites (sanukitoids) and diamonds with harzburgitic inclusion assemblages, all require the presence of hydrous fluids in Earth's deep crust and upper (lithospheric) mantle since at least the Paleoarchaean (3.6–3.2 billion years ago). However, despite its importance, where and how water was stored in Archaean crust, and how some water was transported into the upper mantle, are poorly understood. Here, we investigate Archaean crustal fluid budgets through calculated phase equilibria for three protolith compositions — a low-MgO mafic (basaltic) composition, a high-MgO (picritic) composition and an ultrahigh-MgO ultramafic (komatiitic) composition — that are representative of mafic to ultramafic magmatic rocks in Archaean greenstone belts. We show that the mode and stability of hydrous minerals, in particular chlorite, is positively correlated with protolith MgO content, such that high-MgO basalts can store up to twice the amount of crystal-bound H2O than low-MgO basalts. Importantly, ultrahigh-MgO rocks such as komatiite can store four times as much H2O, most of which is retained until temperatures exceeding 700°C. Warmer geotherms in the early Archaean favoured dehydration of hydrated high-MgO and ultramafic rocks in the deep crust, leading to hydration and/or fluid-fluxed melting of overlying basaltic rocks to produce ‘high-pressure’ TTG magmas. Burial of Archaean mafic–ultramafic crust along cooler geotherms resulted in dehydration of ultramafic material within the lithospheric mantle, providing the source of enriched Archaean basalt that was parental to large volumes of ancient TTG-dominated continental crust.

Petrological and experimental evidence for differentiation of water-rich magmas beneath St. Kitts, Lesser Antilles

St. Kitts lies in the northern Lesser Antilles, a subduction-related intraoceanic volcanic arc known for its magmatic diversity and unusually abundant cognate xenoliths. We combine the geochemistry of xenoliths, melt inclusions and lavas with high pressure–temperature experiments to explore magma differentiation processes beneath St. Kitts. Lavas range from basalt to rhyolite, with predominant andesites and basaltic andesites. Xenoliths, dominated by calcic plagioclase and amphibole, typically in reaction relationship with pyroxenes and olivine, can be divided into plutonic and cumulate varieties based on mineral textures and compositions. Cumulate varieties, formed primarily by the accumulation of liquidus phases, comprise ensembles that represent instantaneous solid compositions from one or more magma batches; plutonic varieties have mineralogy and textures consistent with protracted solidification of magmatic mush. Mineral chemistry in lavas and xenoliths is subtly different. For example, plagioclase with unusually high anorthite content (An≤100) occurs in some plutonic xenoliths, whereas the most calcic plagioclase in cumulate xenoliths and lavas are An97 and An95, respectively. Fluid-saturated, equilibrium crystallisation experiments were performed on a St. Kitts basaltic andesite, with three different fluid compositions (XH2O = 1.0, 0.66 and 0.33) at 2.4 kbar, 950–1025 °C, and fO2 = NNO − 0.6 to NNO + 1.2 log units. Experiments reproduce lava liquid lines of descent and many xenolith assemblages, but fail to match xenolith and lava phenocryst mineral compositions, notably the very An-rich plagioclase. The strong positive correlation between experimentally determined plagioclase-melt KdCa–Na and dissolved H2O in the melt, together with the occurrence of Al-rich mafic lavas, suggests that parental magmas were water-rich (> 9 wt% H2O) basaltic andesites that crystallised over a wide pressure range (1.5–6 kbar). Comparison of experimental and natural (lava, xenolith) mafic mineral composition reveals that whereas olivine in lavas is predominantly primocrysts precipitated at low-pressure, pyroxenes and spinel are predominantly xenocrysts formed by disaggregation of plutonic mushes. Overall, St. Kitts xenoliths and lavas testify to mid-crustal differentiation of low-MgO basalt and basaltic andesite magmas within a trans-crustal, magmatic mush system. Lower crustal ultramafic cumulates that relate parental low-MgO basalts to primary, mantle -derived melts are absent on St. Kitts.

The temporal evolution of back-arc magmas from the Auca Mahuida shield volcano (Payenia Volcanic Province, Argentina)

In order to better constrain the temporal volcanic activity of the back-arc context in Payenia Volcanic Province (PVP, Argentina), we present new K–Ar dating, petrographic data, major and trace elements from 23 samples collected on the Auca Mahuida shield volcano. Our new data, coupled with published data, show that this volcano was built from about 1.8 to 1.0Ma during five volcanic phases, and that Auca Mahuida magmas were extracted from, at least, two slightly different OIB-type mantle sources with a low partial melting rate. The first one, containing more garnet, was located deeper in the mantle, while the second contains more spinel and was thus shallower. The high-MgO basalts (or primitive basalts) and the low-MgO basalts (or evolved basalts), produced from the deeper and shallower lherzolite mantle sources, respectively, are found within each volcanic phase, suggesting that both magmatic reservoirs were sampled during the 1Myr lifetime of the Auca Mahuida volcano. However, a slight increase of the proportion of low-MgO basalts, as well as of magmas sampled from the shallowest source, can be observed through time. Similar overall petrological characteristics found in the Pleistocene–Holocene basaltic rocks from Los Volcanes and Auca Mahuida volcano suggest that they originated from the same magmatic source. Consequently, it can be proposed that the thermal asthenospheric anomaly is probably still present beneath the PVP. Finally, our data further support the hypothesis that the injection of hot asthenosphere with an OIB mantle source signature, which was triggered by the steepening of the Nazca subducting plate, induced the production of a large volume of lavas within the PVP since 2Ma.

Geochemistry and petrology of the lower Miocene bimodal volcanic units in the Tunçbilek–Domaniç basin, western Anatolia

ABSTRACTThe lower Miocene (~22–19 Ma) volcanic units in the NE–SW-trending Tunçbilek–Domaniç basin, located in the northeastern-most part of the Neogene successions in western Anatolia, are composed of (1) high-K, calc-alkaline dacitic to rhyolitic volcanic rocks of the Oklukdağı volcanics; (2) calc-alkaline low-MgO (evolved) basalts; and (3) high-MgO mildly alkaline basalts of the Karaköy volcanics. Sr isotopic ratios of the volcanic units increase from high-MgO (~0.7055–0.7057) to low-MgO basaltic rocks (~0.7066–0.7072) and then to dacitic-rhyolitic rocks (0.7081–0.7086). Geochemical features of the volcanic rocks reveal that the calc-alkaline evolved basalts were formed by mixing of basic and acidic magmas.Geochemical studies in the last decade show that the Miocene mafic volcanic rocks in western Anatolia are mainly composed of high-MgO shoshonitic-ultrapotassic rocks (SHO-UK), of which mantle sources were variably, but also intensely metasomatized with crustally derived materials during collisional processes in the region. However, geochemical comparison of the high-MgO basalts of the Karaköy volcanics with the SHO-UK rocks in this region reveal that that the former has too low 87Sr/86Sr(i) and high 143Nd/144Nd(i) ratios, with lower LILE and LREE abundances, which are firstly described here. These features are interpreted to be derived from more slightly enriched lithospheric mantle sources than that of the SHO-UK. Accepting the SHO-UK rocks in the region were derived from mantle sources that had been metasomatized by northward subduction of crustal slices during Alpine collisional processes, it is proposed that the imbrication and direct subduction of crustal slices were not reached to, and were limited in the mantle domains beneath the basin. The dacites of the Oklukdağı volcanics might be formed either by high-degree melting of the same sources with the SHO-UK, or by melting of the lower crustal mafic sources as previously proposed, and then evolved into the rhyolites via fractional crystallization with limited crustal contribution.

Petrochemistry and genesis of olivine basalts from small monogenetic parasitic cones of Bazman stratovolcano, Makran arc, southeastern Iran

Small monogenetic Quaternary parasitic cones around Bazman stratovolcano, located at the western edge of the Makran arc, southeastern Iran, erupted low-Ti subalkaline olivine basalts with MgO (3.8–8.6wt.%) and Al2O3 (16.5–18.6wt.%). Positive correlation of decreasing MgO, Ni and Cr indicates that formation of low MgO basalts involved limited crystal–liquid fractionation of olivine and clinopyroxene, the common phenocrysts. The basalts have variable 87Sr/86Sr (0.704177–0.705139) and 143Nd/144Nd (0.512689–0.512830) ratios, within the range of OIB-like intra-plate alkaline basalts erupted in eastern Iran north of the Makran arc. This, and the lack of correlation between Sr content and Sr–isotopic ratio, suggest that upper crustal contamination was not significant in their formation, consistent with the relatively thin crust (≤40km) in the area. Enrichment of large-ion-lithophile elements (LILE) relative to light rare-earth-elements (LREE; Ba/La=9–25), and depletions in Nb relatively to LILE (Ba/Nb=12–35; La/Nb=0.8–2.1), are similar in most cases to other convergent plate boundary arc basalts, suggesting that the Bazman basalts formed by melting of subcontinental mantle modified by dehydration of subducted Oman Sea oceanic lithosphere. Pb isotopic ratios of the basalts define a linear trend above the Northern Hemisphere Line, consistent with their derivation from mantle contaminated by Pb derived from subducted sediment. Trace element contents and ratios (LaN=10–25; YbN=3–6; (La/Yb)N=3–8) suggest that these basalts formed as a result of low (~10%) degrees of partial melting of subarc mantle modified only moderately by subducted components. Relatively low Ba/Nb<15, La/Nb<1.5 and Ba/La<15 ratios for some basalts confirm only limited contamination of the source of these samples, consistent with observations in other arcs that parasitic cones tap sources less affected by slab-derived fluids than the larger stratovolcanoes they surround. Comparison with Taftan volcano in southeastern Iran and Koh-e-Sultan in Pakistan, both further to the east along the Makran arc, indicate regional west-to-east geochemical variations, including increasing K2O, LILE, LREE, La/Yb, Ba/La and 87Sr/86Sr. These changes result from an along-strike increase in both input of slab-derived fluids relative to the volume of mantle below the arc and decreasing degree of subarc mantle melting to the east. This is due to a reduction in the volume of subarc mantle below the eastern part of the Makran arc resulting from a combination of both increasing crustal thickness and decreasing angle of subduction to the east.

Periodical mixing of MORB magmas near East Pacific Rise 13°N: evidence from modeling and zoned plagioclase phenocrysts

Thirty-six basalt samples from near East Pacific Rise 13N are analyzed for major and trace elements. Different types of zoned plagioclase phenocrysts in basalts are also backscatter imaged, and major element profiles scanned and analyzed for microprobe. Basalts dredged from a restricted area have evolved to different extents (MgO=9.38wt%-6.76wt%). High MgO basalts are modeled for crystallization to MgO of about 7wt%, and resulted in the Ni contents (28 ppm) that are generally lower than that in observed basalts (> 60 ppm). It suggests that low MgO basalts may have experienced more intensive magma mixing. High MgO (9.38wt%) basalt is modeled for self-"mixing-crystallization", and the high Ni contents in low MgO basalts can be generated in small scale and periodical self-mixing of new magma (high MgO). "Mixing-crystallization" processes that low MgO magmas experienced accord with recent 226Ra/230Th disequilibria studies for magma residence time, in which low MgO magmas have experienced more circles of "mixing-crystallization" in relatively longer residence time. Magma mixing is not homogeneous in magma chamber, however, low MgO magmas are closer to stable composition produced by periodical "mixing-crystallization", which is also an important reason for magma diversity in East Pacific Rise. Zoned plagioclase phenocrysts can be divided into two types: with and without high An# cores, both of which have multiple reversed An# zones, suggesting periodical mixing of their host magmas. Cores of zoned plagioclase in low MgO (7.45wt%) basalt differ significantly with their mantle in An#, but are similar in An# with microlite cores (products of equilibrium crystallization) in high MgO (9.38wt%) basalt, which further shows that plagioclase phenocryst cores in low MgO basalts may have formed in their parental magmas before entering into the magma chamber.

Mantle genesis and crustal evolution of primitive calc-alkaline basaltic magmas from the Lesser Antilles arc

Abstract Most eruptive rocks in the Lesser Antilles arc are compositionally evolved. However, lavas with primitive characteristics do occur including, in the central part of the arc, a suite of rocks from Soufriere, St Vincent, and the Ilet à Ramiers basalt from Martinique. High-pressure experiments performed on a Soufriere basalt point to a spinel lherzolite source. Glass inclusion data and phase equilibria analysis suggest extraction of the Soufriere melt under relatively dry conditions ( c. 2 wt% H 2 O in melt). Using estimates of the H 2 O content of mantle sources fluxed by an hydrous slab-derived component, H 2 O concentrations as high as 5 wt% are considered possible for primary mantle melts in the Lesser Antilles arc. Experiments at low pressures (4 kbar) simulate the evolution of primitive melts within the arc crust. For elevated melt H 2 O concentrations (6–8.5 wt%), derivative liquids ranging from low-MgO basalt to basaltic andesite are generated at 1050–1100°C. Their crystallization at 950–1000°C yield andesitic liquids similar to those erupting at active volcanic centres such as Mt Pelée, Martinique, and Soufriere Hills, Montserrat. Therefore, experimental data support the derivation of Lesser Antilles arc eruptives by different degrees of fractionation from primary mantle melts.

The Depleted Mantle Component in Kerguelen Archipelago Basalts: Petrogenesis of Tholeiitic-Transitional Basalts From the Loranchet Peninsula

A geochemical study of 28 Ma tholeiitic to transitional basalts from the Kerguelen Archipelago (Mont des Ruches and Mont Fontaine) indicates that three distinct magma types erupted within ∼1 Myr. Low-MgO basalts (∼4–6 wt %) in both sections are overlain by high-MgO basalts (∼7–13 wt %), mostly present in Mont Fontaine. Both high- and low-MgO basalts have nearly identical low 87Sr/86Sr and high 143Nd/144Nd and formed from similar parental magmas that represent mixtures between a depleted mantle component and the Kerguelen plume. The third magma type, predominant in Mont des Ruches, is represented by high-MgO basalts that are isotopically heterogeneous with isotopic ratios that are intermediate between those of the stratigraphically lower basalts and the Kerguelen plume compositions; this third magma type may have formed by mixing of similar material, but with a higher contribution from the Kerguelen plume. The depleted component involved in all three magma types is similar to the source for Southeast Indian Ridge basalts and is present in Kerguelen Archipelago basalts older than 26 Ma, which erupted when the ridge axis was <500 km away from the Kerguelen hotspot. Depleted heterogeneities intrinsic to the plume and entrainment of depleted mantle during plume ascent do not explain the marked cut-off in the presence of a depleted component in the archipelago basalts within a time interval of 1 Myr after 26 Ma. Mixing of depleted asthenosphere with the plume in sublithospheric channels during migration of the Southeast Indian Ridge axis away from the Kerguelen hotspot is proposed as a suitable explanation to account for the temporal distribution of the depleted component in basalts from the Northern Kerguelen Plateau and the >26 Ma Kerguelen Archipelago flood basalts; cessation of plume–ridge interactions may explain the absence of depleted basalts in the youngest sections that erupted further away from the ridge axis.

Shallow Mesozoic layered gabbros of the Shadow Mountains, San Bernardino County, California

Jurassic hornblende gabbros intrude as a conspicuous 9 km 2 arcuate complex within Paleozoic metasedimentary pendants of the Shadow Mountains, San Bernardino County, California. This complex is significant because it: (1) displays evidence of processes active in magma chambers which are crystallizing cumulus phases; (2) resembles many mid-Mesozoic plutons described in the western and central Sierra Nevada and western Mojave desert which may all be petrologically related; and (3) may be used as a diagnostic tectonic marker for Mesozoic tectonic reconstructions of the Mojave Desert and western Cordillera. The gabbroic body is unusual for its compact, near-circular plan, conspicuous banding and layering and reverse geochemical zonation. The mafic complex was intruded as a hydrous magma and was emplaced as a concentric epizonal pluton. Oxygen isotopic data indicate the melt was mantle derived but has been contaminated by assimilation of metasediments. The gabbro was likely generated by reaction of olivine with fractionated melt from the melting of a low MgO basalt under water-saturated conditions. The banding and layering is ascribed to the cyclic ascent of felsic rejected solute along the walls of the chamber and its accumulation near the roof of the complex. Cooling was facilitated by conduction and the presence of pendant rocks at uppermost levels. The Shadow Mountains gabbros resemble other gabbroic bodies of similar Jurassic age throughout the Sierra Nevada, Mojave Desert and Transverse Ranges, California (Lahren and Schweickert, 1994; Miller and Glazner, 1995). These intrusions are useful as markers of subsequent large-scale tectonic crustal displacements affecting the western Mojave Desert and Sierra Nevada. In particular, the Shadow Mountains gabbros show age, mineralogie and textural affinities with correlative gabbroic complexes in the central Sierra Nevada 400 km to the north. All these plutonic bodies may represent subvolcanic sources of the Jurassic-Cretaceous Independence dike swarm in the western Mojave Desert and eastern Sierra Nevada.

Possible petrogenetic relations between low- and high-MgO Aleutian basalts

Although the volumetrically dominant lava in the Aleutian arc is basaltic, its petrographic, geochemical, and isotopic character is variable, and the petrologic significances of the different basalts have been much debated. To evaluate possible petrogenetic relations, all available petrographic, major-, trace- and rare-earth-element and isotopic data from Aleutian basalts (n = 205) have been collected. On the basis of MgO content, basaltic ( 2 ) lavas have been divided into three major classes: high-magnesia basalts, transitional basalts, and low-MgO basalts. Low-MgO basalts have less than 6% MgO and constitute 67% of the analyzed basalts. Using Al 2 O 3 content, this basalt class has been further divided into two subclasses. Low-alumina basalts, the less abundant class (15% of the low-MgO basalts), have less than 18 wt % Al 2 O 3 and 450 ppm Sr and high TiO 2 (>1.0 wt %), FeO t (>9%) and incompatible element abundances. High-alumina basalts (≥18 wt % Al 2 O 3 ) are characterized by low TiO 2 (≤1 wt %) and ≤9% FeO t , low incompatible- and compatible- element contents and high Sr abundances (≥450 ppm). Transitional basalts are defined as basalts with between 6 and 9 wt % MgO. High-magnesia basalts (MgO >9 wt %) constitute 10% of the analyzed Aleutian basaltic lavas and occur at only a few volcanic centers. Fractionation of high-magnesia basaltic magma is unlikely to produce magma compositions similar to high-alumina basalts. The compositional data are, however, consistent with assimilation of lithospheric material by high-alumina basaltic liquid to produce high-magnesia basaltic magmas and generation of low-alumina basalts by crystal fractionation of these hybrid liquids. Within this context, high-alumina basaltic magmas are considered primary.

Basaltic rocks in the Rensselaer Plateau and Chatham slices of the Taconic allochthon: Chemistry and tectonic setting

Tholeiitic to transitional alkalic basalt and basaltic tuff form widely separated but distinctive units within the Nassau Formation of late Proterozoic or Early Cambrian age, within the Rensselaer Plateau and Chatham slices of the Taconic allochthon in eastern New York State and western Massachusetts. Examination of all known occurrences of these basaltic rocks and detailed mapping of the enclosing strata indicate that these basalts are submarine lava flows and water-transported tuffaceous deposits restricted to the lower third of the stratigraphic section in both slices. In each slice, the basalts thicken to the west along with thickening and coarsening of enclosing graywacke beds (Rensselaer Graywacke Member of the Nassau Formation) and thin eastward into pillow basalt and tuffaceous basalt that is associated with fine-grained graywacke and thinly laminated purple and green turbidites of the Nassau Formation. These high-TiO 2 , low-MgO basalts resemble very closely late Proterozoic basalts and feeder dikes of the Catoctin Formation of the Blue Ridge in Virginia and late Proterozoic metadiabase dikes in the northern Reading Prong in New York, both of which intrude pre-Iapetan, eastern North American basement. Major-element and rare-earth chemistry and geology also establish a correlation between the Taconic metabasalts and metabasalts preserved in the Tibbit Hill Volcanic Member of the Pinnacle Formation that unconformably overlies middle Proterozoic basement, in central Vermont. Palinspastic reconstruction places the Chatham slice oceanward of the Rensselaer Plateau slice that contains abundant coarse-grained detritus of probable eastern North American Proterozoic basement These relations suggest a fault-bounded submarine basin (Nassau-Rensselaer basin) located near the continental margin that was fed by submarine fan deposits. Submarine morphology of the distributing fan complex may have channelized the basalts. The association of the tholeiitic to transitional alkalic basalts with interpreted marine-fan deposits suggests that the volcanism occurred after Iapetan rifting and thermal subsidence over tectonically thinned sialic crust, perhaps near the time and place of eventual separation.