Geochemistry of picrite basalts from the Liohi Volcano

Research of the ocean floor using the Mir submersibles carried out south of the Hawaiian Archipelago allowed to recover flows of recent picrite basalts. Lava vents are confined to a field of development of open fractures of a gjar type. Basalts represent initial lava flows in the structure of the Hawaiian volcanic archipelago. Considering contents of alkali and rare-earth elements in them, the picrite basalts of the bottom could be assigned to a series of island tholeiites. They are products of high level melting of asthenospheric matter at depth about 75-80 km as a result of decompression near a deep fracture that occurred in the lithosphere and asthenosphere. Similar picrite basalts were found in the base of the youngest volcano of the Hawaiian chain the Loihi Volcano. With respect to contents of alkali metals, these rocks are assigned to the subalkaline series of rocks formed during melting of garnet lherzolites. This could probably be explained by supply of melts from deeper levels of the asthenosphere after partial packing of an initial magma effluent fracture.

Late Jurassic oceanic crust and Upper Cretaceous Caribbean plateau picritic basalts exposed in the Duarte igneous complex, Hispaniola

Four distinct rock units have been recognized near El Aguacate, in the Janico-Juncalito-La Vega area of the Duarte complex (Dominican Republic): (1) serpentinites crosscut by numerous diabasic dikes, (2) basalts interbedded with Late Jurassic ribbon cherts, (3) picrites and ankaramites relatively enriched in incompatible trace elements, and (4) amphibolites and gneissic amphibolites chemically similar to Oceanic Plateau Basalts. Similar Ar-Ar ages of late magmatic amphibole from a picrite, and hornblende from an amphibolite (86.1 +/- 1.3 Ma and 86.7 +/- 1.6 Ma, respectively), suggest that the Duarte picrites are contemporaneous with the Deep Sea Drilling Program Leg 15 and Ocean Drilling Program Leg 126 basalts drilled from the Caribbean oceanic plateau. These basalts are associated with sediments containing Late Cretaceous faunas. Sr, Nd, and Pb data show that enriched picrites and amphibolites are isotopically similar to mafic lavas from previously described Caribbean plateau and Galapagos hotspot basalts. Major element, trace element, and lead isotopic features of Late Jurassic basalts and diabases are consistent with those of normal oceanic crust basalt. However, these basalts differ from typical N-MORB because they have lower epsilon Nd ratios that plot within the range of Ocean Island Basalts. These rocks appear to represent remnants of the Caribbean Jurassic oceanic crust formed from an oceanic ridge possibly close to a hotspot. Later, they were tectonically juxtaposed with Late Cretaceous slices of the Caribbean-Colombian plateau.

Forward modelling of petrological crust-forming processes on the early Earth

Tonalite-trondhjemite-granodiorite (TTG) gneisses form up to two-thirds of the preserved Archean continental crust and there is considerable debate regarding the primary magmatic processes of the generation of these rocks. The popular theories indicate that these rocks were formed by partial melting of basaltic oceanic crust which was previously metamorphosed to garnet-amphibolite and/or eclogite facies conditions either at the base of thick oceanic crust or by subduction processes.rnThis study investigates a new aspect regarding the source rock for Archean continental crust which is inferred to have had a bulk compostion richer in magnesium (picrite) than present-day basaltic oceanic crust. This difference is supposed to originate from a higher geothermal gradient in the early Archean which may have induced higher degrees of partial melting in the mantle, which resulted in a thicker and more magnesian oceanic crust. rnThe methods used to investigate the role of a more MgO-rich source rock in the formation of TTG-like melts in the context of this new approach are mineral equilibria calculations with the software THERMOCALC and high-pressure experiments conducted from 10–20 kbar and 900–1100 °C, both combined in a forward modelling approach. Initially, P–T pseudosections for natural rock compositions with increasing MgO contents were calculated in the system NCFMASHTO (Na2O–CaO–FeO–MgO–Al2O3–SiO2–H2O–TiO2) to ascertain the metamorphic products from rocks with increasing MgO contents from a MORB up to a komatiite. A small number of previous experiments on komatiites showed the development of pyroxenite instead of eclogite and garnet-amphibolite during metamorphism and established that melts of these pyroxenites are of basaltic composition, thus again building oceanic crust instead of continental crust.rnThe P–T pseudosections calculated represent a continuous development of their metamorphic products from amphibolites and eclogites towards pyroxenites. On the basis of these calculations and the changes within the range of compositions, three picritic Models of Archean Oceanic Crust (MAOC) were established with different MgO contents (11, 13 and 15 wt%) ranging between basalt and komatiite. The thermodynamic modelling for MAOC 11, 13 and 15 at supersolidus conditions is imprecise since no appropriate melt model for metabasic rocks is currently available and the melt model for metapelitic rocks resulted in unsatisfactory calculations. The partially molten region is therfore covered by high-pressure experiments. The results of the experiments show a transition from predominantly tonalitic melts in MAOC 11 to basaltic melts in MAOC 15 and a solidus moving towards higher temperatures with increasing magnesium in the bulk composition. Tonalitic melts were generated in MAOC 11 and 13 at pressures up to 12.5 kbar in the presence of garnet, clinopyroxene, plagioclase plus/minus quartz (plus/minus orthopyroxene in the presence of quartz and at lower pressures) in the absence of amphibole but it could not be explicitly indicated whether the tonalitic melts coexisting with an eclogitic residue and rutile at 20 kbar do belong to the Archean TTG suite. Basaltic melts were generated predominantly in the presence of granulite facies residues such as amphibole plus/minus garnet, plagioclase, orthopyroxene that lack quartz in all MAOC compositions at pressures up to 15 kbar. rnThe tonalitic melts generated in MAOC 11 and 13 indicate that thicker oceanic crust with more magnesium than that of a modern basalt is also a viable source for the generation of TTG-like melts and therefore continental crust in the Archean. The experimental results are related to different geologic settings as a function of pressure. The favoured setting for the generation of early TTG-like melts at 15 kbar is the base of an oceanic crust thicker than existing today or by melting of slabs in shallow subduction zones, both without interaction of tonalic melts with the mantle. Tonalitic melts at 20 kbar may have been generated below the plagioclase stability by slab melting in deeper subduction zones that have developed with time during the progressive cooling of the Earth, but it is unlikely that those melts reached lower pressure levels without further mantle interaction.rn

An investigation into the Hawaiian mantle from a xenolithic perspective

Salt Lake Crater (SLC), on the island of Oahu, Hawaii, is best known for its wide variety of crustal and mantle xenoliths. SLC is only the second locality in oceanic regimes where deeper portions of the upper mantle (i.e., garnet-bearing xenoliths) have been sampled. These garnet-bearing xenoliths, that contain clinopyroxene (cpx), orthopyroxene (opx), olivine, and garnet, are the focus of this study Opx is present in small amounts. Cpx has exsolved opx, spinel, and garnet. In addition, many xenoliths contain spinel-cored garnets. In some xenoliths, opx crystals contain exsolved cpx and spinel. Olivine, cpx, and garnet are in chemical equilibrium with each other. Opx is not in chemical equilibrium with the other dominant minerals. ^ The origin of these xenoliths is interpreted on the basis of liquidus phase relations in the simplified system CaO-MgO-Al2O3-SiO 2 (CMAS) system at 3.0 and 5.0 GPa. The occurrence of spinel-cored garnets and the Ol-Cpx-Gt assemblage suggests that the depth of crystallization of the SLC xenoliths examined was ∼100–110 km (i.e., uppermost asthenosphere). ^ The experimental study is concerned with the equilibrium melting of garnet clinopyroxenite at 2.0–2.5 GPa and it explores the role of such melting process in the generation of tholeiitic and alkalic lavas in ocean island basalts (OIBs). The starting material is a tholeiitic picrite in terms of its normative composition. Its solidus temperature is 1295 ± 15°C and 1332 ± 15°C at 2.0 and 2.5 GPa, respectively. At 2.0 GPa, the liquidus phase is opx that is in reaction relation with the melt. It reacts out at ∼40°C below the liquidus as cpx and spinel appear. Garnet appears long after opx disappearance. Opx is absent in runs at 2.5 GPa. Cpx and garnet appear simultaneously on the liquidus at 2.5 GPa, and are the only assemblage throughout the melting interval. At both the pressures, the partial melts are olivine-hypersthene normative at high melt fraction ( F), becoming moderately to strongly nepheline-normative, as F decreases. It is concluded that the involvement of CO 2 (and perhaps H2O) is necessary for the generation of alkalic melts in most OIBs. ^