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- Title
Diabasic intrusion and lavas, segregation veins, and magma differentiation at Kahoolawe volcano, Hawaii.
- Authors
Fodor, R.; Bauer, G.
- Abstract
A mafic sill-like intrusion, ~5 × 30 m, exposed along the eastern shoreline of Kahoolawe Island, Hawaii, represents tholeiitic magma emplaced as diabase among caldera-filling lavas. It differentiated from ~7.8 wt.% MgO to yield low-MgO (2.9 wt.%) vesicular segregation veins. We examined the intrusion for whole-rock and mineral compositions for comparison to Kahoolawe caldera-fill lavas (some also diabasic), to the Uwekahuna laccolith (Kilauea), and to gabbros, diabases, and segregations and oozes of other tholeiitic shield volcanoes (e.g., Mauna Loa and Kilauea lava lakes). We also evaluate this extreme differentiation in terms of MELTS modeling, using parameters appropriate for Hawaiian crystallization environments. Kahoolawe intrusion diabase samples have major and trace element abundances and plagioclase, pyroxene, and olivine compositions in agreement with those in gabbros and diabases of other volcanoes. However, the intrusion samples are at the low-MgO end of the large MgO range formed by the collective comparative samples, as many of those have between 8 and 20 wt.% MgO. The intrusion's segregation vein has SiO 53.4 wt.%, TiO 3.2 wt.%, FeO 13.5 wt.%, Zr 350 ppm, and La 16 ppm. It plots in compositional fields formed by other Hawaiian segregations and oozes that have MgO <5 wt.%-fields that show large variances, such as factor of ~2 differences for incompatible element abundances accompanying SiO from ~49 to 59 wt.%. Our MELTS modeling assesses the Kahoolawe intrusion as differentiating from ~8 wt.% MgO parent magma beginning along oxygen buffers equivalent to FMQ and FMQ-2, having magmatic HO of 0.15 and 0.7 wt.% (plus traces of CO and S), and under 100 and 500 bars pressure. Within these parameters, MELTS calculates that <3 wt.% MgO occurs at ~1,086 to 1,060 °C after ~48 to 63 % crystallization, whereby the lesser crystallization percentages and lower temperatures equate to higher magmatic HO, leading to high SiO, ~56-58 wt.%. To contrast, greater crystallization is calculated for lower HO, for which it achieves less SiO, <55 wt.%. While MELTS reliably predicts SiO approaching 58 wt.% for differentiation beyond <4 wt.% MgO, and shows that Kahoolawe intrusion's segregations and those of Kilauea and Mauna Loa are all reasonably accommodated by the modeled parameters and SiO differentiation curves, MELTS fails where it predicts that Fe enrichment is more robust under FMQ than FMQ-2 buffers. That failure not withstanding, MELTS differentiation from liquidus temperatures ~1,205-1,185 °C (depending on the various parameters) gradually increases fO (up to ~0.4 log units, as normalized to FMQ) until magnetite crystallizes at ~1,090-1,085 °C, which reduces absolute fO ~1 to 1.5 log units. The modeled Kahoolawe intrusion, then, exemplifies how tholeiitic magma differentiation can produce extreme SiO and incompatible element compositions, and how Hawaiian segregations from shallow intrusions and lava lakes can be generally modeled under compositional and physical parameters appropriate for Hawaiian tholeiitic magmatism.
- Subjects
KAHOOLAWE (Hawaii); HAWAII; IGNEOUS intrusions; LAVA; MAGMAS; VOLCANOES; MAFIC rocks; SHORELINES
- Publication
Mineralogy & Petrology, 2014, Vol 108, Issue 2, p269
- ISSN
0930-0708
- Publication type
Academic Journal
- DOI
10.1007/s00710-013-0299-x