OIB / Hawaiian Volcanism Mauna Loa Hot-Spots producing Ocean Island Basalts (OIB) Characteristic Features of OIB Lavas • Relative abundance of strongly olivine-phyric, picritic and/or ankaramitic, high-MgO lavas ("oceanites"). Clinopyroxene is the second phase to crystallize after olivine at approximately 8 wt.% MgO, while plagioclase does not appear as a liquidus phase in Hawaiian tholeiites until compositions with less than 7 wt.% MgO. • Primitive OIB lavas (high-MgO) are among the most Fe-rich picritic basalts on the Earth. • Primitive OIB lavas are also characterized by relatively low Al (Al2O3 15 wt.%) and Ca (CaO = 10 wt.%) contents compared to MORB. . • OIB lavas are enriched in all incompatible trace elements, including the high field strength elements (Nb, Ta, Zr, Hf), as well as the LIL (K, Rb, Ba) and LREE, compared to MORB. Typically, however, they exhibit a relative depletion in LIL elements (Ba, Rb, K) with respect to HFS (Nb) and LREE elements. • Unlike MORB, many OIB suites (although not all) exhibit an anti-correlation between trace element enrichment and isotopic enrichment Koloa Koolau East Molokai Honolulu Lanai Mauna Kea Mauna Loa Kilauea Mauna Ulu Loihi ~ 8.5 cm/yr Stages of Hawaiian Volcanism Post-Erosional: Honolulu Series, Kola Series Renewed volcanic activity following a hiatus on the order of 1 million years. Small cinder cones, explosive tuff rings or maar, and small valley-filling flows of highly undersaturated, primitive lavas. This stage has not started yet on the big island of Hawaii, but is present on Maui and older islands. Post Caldera: Mauna Kea, East Molokai At 12,000', thicker flows begin to accumulate in the caldera, eventually filling it and forming a thin cap on the shield. The beginning of this stage is marked by a transition from earlier tholeiitic series to later alkaline series lavas. Initially, these two types of flows are commonly interbedded, and transitional compositions are also erupted. The earliest alkaline lavas are usually relatively evolved hawaiites, but as alkaline magmas become dominant with time, they also become more primitive (AOB) and explosive, finishing with the development of a cinder cone field capping the shield. Shield building: Kilauea, Mauna Ulu, Mauna Loa, Lanai, Koolau The repeated eruption of highly fluid, extensive thin flows of tholeiitic basalt builds the main shield of the volcano, which usually has a well developed central caldera. There appears to be a progression from early picritic lavas to later olivine and quartz tholeiites (6-9 wt.% MgO) with height, as each shield builds to an elevation of 12,000 feet (4000 m) above sea level. Early Submarine: Loihi Early pillow lavas range from mildly alkaline basalts (AOB's) to tholeiites. There is a direct correlation between degree of vesicularity (and thus volatile content) and the degree of silica undersaturation. Active Volcanoes Mauna Kea Post Caldera Kilauea Mauna Ulu Early Shield Mauna Loa Late Shield Loihi Early Submarine Koloa Koolau Honolulu East Molokai Lanai Mauna Kea Mauna Loa Kilauea Mauna Ulu Loihi Deep Seated Plume? Magmatic Suites Tholeitiic Suite: Kilauea, Mauna Ulu, Mauna Loa, Lanai, Koolau (Oahu) Oceanite (picrite) Ol-tholeiite Qtz-tholeiite The tholeiitic suite constitutes 98% of Hawaiian lavas. With the exception of one rhyodacite, all lavas of this magmatic suite are basalts, and there is a marked absence of intermediate and evolved lavas such as: andesite, dacite, and rhyolite. Olivine is the dominant phenocryt and individual tholeiitic suites commonly define tight olivine control lines, with the dominant rock type being Ql-tholeiite. The most magnesian reported olivine has a composition of Fo 91 (most Fo 88 or less). Olivine, however, is absent in the groundmass, presumably because of the olivine reaction relationship. The first cpx phenocrysts are augitic in composition (at MgO = 8.0 wt.%), but pigeonite and quartz are commonly found in the groundmass. Xenoliths: dunite and olivine-gabbro xenoliths are relatively rare. Kilauea Kilauea Iki Mauna Loa Mauna Loa Hawaiian Tholeiites Magmatic Suites Alkaline Suite: East-Molakai, Mauna Kea ankaramite AOB hawaiite mugearite benmoreiite trachyte (labradorite) (andesine) (oligocene) The alkaline suite comprises 2 % of Hawaiian lavas. Both olivine and clinopyroxene (MgO > 10 wt.%) are early phenocryst phases, and primitive lavas are ankaramitic. Unlike the tholeiitic suite, olivine commonly persists in the groundmass, along with titan-augite and interstitial K-spar instead of quartz. The alkaline suite exhibits a much broader range of Mg contents, which indicates more extensive crystal fractionation involving olivine, clinopyroxene, and plagioclase. The dominant lava type has a relatively evolved hawaiitic composition. Xenoliths: dunite, wherlite, gabbro; all relatively common. Mauna Kea Top of Mauna Kea Post-Caldera Alkaline Suite Kilauea East Molokai Magmatic Suites Post-Erosional Suite: Honolulu Series, Oahu, and Koloa Series, Kauai A.O.B basanite nephelinite mellilitite ( 5% norm Ne) ( 5% norm Ne) ( 15% norm Ne) ( 15% norm Me) (modal feldspathoid) (no modal plag) (modal mellilite) The post-erosional series comprises 0.1% of Hawaiian lavas. They are characteristically strongly silica undersaturated, and, although they exhibit a wide range of silica saturations, they are all relatively primitive with high Mg contents. They thus do not appear to have suffered significant low pressure crystal fractionation. Xenoliths: lherzolite, harzburgite, dunite, garnet pyroxenite, all relatively abundant. Sodic Alkaline Mafic Volcanism Hy-norm Alkali-Olivine Basanite Olivine Basalt Basalt Nephelinite / Melilitite Normative Opx 0% < Foid < 5% 5% < Foid < Foid > 15% Mineralogy 15% Melilitite (>10% Larnite) Matrix Modal Plag Plag & K-Spar K-Spar, Plag & Foid, no Plag Mineralogy Foid Crystallization Ol Plag Ol Plag Ol Cpx Ol Ox Cpx Sequence Cpx Ox Cpx Ox Plag Ox Lamprophyre Camptonite Camptonite Monchiquite equivalent Hawaiian tholeiites are enriched in all incompatible trace elements in comparison to MORB, and are characterized by distinctive convex-upwards fractionated REE patterns that peak at Pr. Regardless of the degree of enrichment in the LREE, Nb, and Ta, however, there typically remains a significant relative depletion in LIL elements such as K, Rb, and Ba. This appears to require the present of a residual hydrous phase, such as amphibole or phlogopite, in the mantle source regions of the some of the alkaline magmas. The foidites develop slight negative anomalies for HFSE elements, eg. Nb and Hf. There is a systematic anti-correlation between degree of incompatible trace element enrichment and degree of Si saturation, and much of the trace element variation in the Hawaiian lavas can be explained in terms of mixing between two components. Going from tholeiite to AOB to basanite and then olivine nephelinite corresponds to a systematic increase in the degree of enrichment in LREE, Nb, and Ta, with little change or a slight decrease in the levels of HREE. Recent Alkaline Basalts (8+ wt.% MgO) minor Hirschfeld dominant Zr ppm Inter-shield variations in Hawaiian tholeiitic picrites The primitive tholeiitic lavas of the shield building stage of Hawaiian Islands range in compositions between two end-members: Tholeiitic End-Members Kilauea, Mauna Kea, Loihi Lanai, Koolau, Mauna Loa low Si, high Fe, Ti, Ca high Si, low Fe, Ti, Ca highest IE, Nb/Zr, Th/U lowest IE, Nb/Zr, Th/U low 87Sr/86Sr, high 143Nd/144Nd. high 87Sr/86Sr, low 143Nd/144Nd .7036 Nd = + 7 .7042 Nd = +1 low 18O 4.7 high 18O 6.0 high 206Pb/204Pb 18.6 low 206Pb/204Pb 17.9 low 187Os/188Os 0.13 ~ MORB high 187Os/188Os 0.145 Despite appearances, the Koolau source component is not equivalent to primitive mantle (low Rb/Sr, high 187Os/188Os, high, 18O), and the Kilauea source component is not equivalent to depleted mantle (DMM) (high 87Sr/86Sr, low 18O), nor Pacific ocean crust and/or lithosphere (high 206Pb/204Pb, low 187Os/188Os). Both of these source components would seem to be from the lower mantle. Koloa Koolau Honolulu East Molokai Lanai Mauna Kea Mauna Loa Kilauea Mauna Ulu Loihi Lanai / Koolau End-Member The Hawaiian Paradox The low Al and Ca of most primitive OIB picritic magmas, including those of Hawaii, are consistent with equilibration with a harzburgitic residue at pressures ranging from 1.5 to more than 3.0 GPa. If these OIB parental magmas were derived from the same Pyrolite mantle source that gives rise to MORB, then they would have to represent a greater degree of partial melting, beyond the point at which clinopyroxene disappears in the solid residue. This interpretation is supported by recent melting experiments on a Kilauean picrite (Eggins, 1992a), which is saturated only in olivine and orthopyroxene in this pressure range. The Hawaiian Paradox The foregoing conclusion, however, is inconsistent with the fact that all Hawaiian primitive magmas are enriched in incompatible trace elements compared to MORB. To make matters worse, the buffered levels of heavy rare earth elements in magmas ranging from tholeiites through strongly alkaline basalts has convinced many trace element geochemists (Hofmann et al. 1984, Frey and Roden, 1987) that residual garnet must be present in their mantle source. Inversions of Hawaiian rare earth element data (Watson, 1993) also indicate melting in the presence of residual garnet. The lavas within many OIB suites define approximately linear arrays between two chemical and isotopic components, one relatively depleted and the other relatively enriched. Originally these were thought to correlate with the MORB source and primitive mantle respectively. However, it rapidly became apparent that these linear arrays were different in different OIB suites. The Hawaiian Paradox We are thus presented with a paradox. Melting experiments on both mantle lherzolite (Hirose and Kushiro, 1993, Falloon et al. 1988) and Hawaiian picrites themselves (Eggins, 1992a) indicate that garnet is not stable in melts with compositions of the Hawaiian picrites until pressures greater than 3.0 GPa, after olivine has ceased to be a stable phase. Garnet and olivine are not in equilibrium together with a Hawaiian picritic liquid under any conditions. This Hawaiian paradox is aggravated if the picrites are normalised to coexist with the residue of a more Fe-rich mantle, such as HK-66 with an olivine of composition Fo 86. This leads to higher Si contents and unlikely estimates for the pressures of equilibration (0.7 to 2 GPa), well below any pressure estimates for the stability region of garnet in a lherzolitic bulk composition and less than depths indicated by seismic data, and leaves unexplained the presence of Fo 89+ phenocrysts in some primitive Hawaiian lavas. Eggins (1992b) has demonstrated that the paradox can not be resolved by calling upon dynamic melting processes, such as percolation melting (Ribe, 1988) or accumulated continuous melting (Mackenzie and Bickle, 1988), and that the behaviour of the HREE in primitive Hawaii tholeiites requires melting to have occurred largely in the presence of garnet There are thus many "flavours" of OIB suites, and at least five different components are required to explain them. Furthermore, there are geographic correlations in the isotopic characteristics of OIB suites. For example, the DUPAL anomaly in the south Pacific is defined by the abundance of EM II OIB suites that appears to correlate with a lower mantle seismic tomography anomaly. Fe / Mn Hawaii and other OIB suites have been shown to have higher Fe/Mn ratios than MORB The elevated Fe/Mn ratios of OIB magmas has also been claimed to reflect the incorporation of minor amounts of outer core material, which has a much higher Fe/Mn ratio than the mantle Hawaiian Olivines are higher in Ni for any given Mg no. compared to MORB Hawaiian olivines have been argued to be too Ni-rich to have equilibrated with a peridotite mantle source, but could be derived from pyroxenite mantle source. Multi-Stage Melting Model: Eclogite pods in rising mantle plume melt to produce Si-rich melt:. Si-rich melts react with peridotite host to form metasomatic pyroxenite. Pyroxenite metasomatised zones melt. Peridotite melts Comparison of olivines in MORB, OIB, and Continental Flood Basalts Fraction pyroxenite-derived melt Involvement of the Core? 187Pt 186Os + e- 187Re 187Os + e- The presence of a coupled enrichment in 186Os and 187Os in Hawaii and some other OIB suites has been cited as evidence for the incorporation of core material into the source of the plume that produced them. Excess 186Os in the outer core is caused by the increase in Pt/Os (parent/daughter) in the fluid outer core because of the growth of the inner core - Os is compatible, but Pt is incompatible. Only a quite small amount of core material would be required to explain the excess 186Os in OIB magmas Recycled Mn nodules have been proposed for the anomalous 186Os in hot spot magmas, however, such an explanation conflicts with the actual Mn data for OIB magmas.