ALBERT

Description: The Journal of Geology, Volume 120, Issue 2, Page 191-202, March 2012.

Description: The Galapagos mounds sea-floor hydrothermal system is at least 300,000 years old and once produced manganese-poor sediments, which nearly blanketed the area of the present mounds field. Present-day mound deposits are limited manganese-rich exposures, suggesting that the system has changed from rock-to water-dominated and has diminished in intensity with time.〈br /〉〈span class="detail_caption"〉Notes: 〈/span〉Natland, J H -- Rosendahl, B -- Hekinian, R -- Dmitriev, Y -- Fodor, R V -- Goll, R M -- Hoffert, M -- Humphris, S E -- Mattey, D P -- Petersen, N -- Roggenthen, W -- Schrader, E L -- Srivastava, R K -- Warren, N -- New York, N.Y. -- Science. 1979 May 11;204(4393):613-6.〈br /〉〈span class="detail_caption"〉Record origin:〈/span〉 〈a href="http://www.ncbi.nlm.nih.gov/pubmed/17839484" target="_blank"〉PubMed〈/a〉

Description: Basaltic lavas form part of the Miocene ([~]20.5 to 18 Ma) Goldfield-Superstition silicic large igneous province in central Arizona. Most of the basalt erupted early in the development of this southern Basin and Range volcanic province, and only small amounts of basalt co-erupted with the silicic volcanism. We examined 50 samples of basalt from lower, middle, and upper stratigraphic positions of the province to establish basalt petrogeneses, the characteristics of basalt sources, relationships among compositionally different lavas, and the igneous processes that relate various basalt types. We base our study largely on major and trace elements, mineral compositions, and a small data set for Sr, Nd, and Hf isotopes. Lower-section basalts include the Weekes Wash basalts, which are transitional between alkalic and tholeiitic (SiO2 49-51 wt. %) and with MgO 8.1-10.2 wt. %. They are associated with lavas that are andesitic (SiO2 [~]59-62 wt. %; MgO 8-2.5 wt. %) or are seemingly andesitic due to alteration and felsic xenocrysts. Weekes Wash basalts have incompatible-element abundances that correlate positively with MgO. Their87Sr/86Sr ratios are [~]0.705. Olivines, rarely fresh, have Fo86-89 cores, and clinopyroxenes have Mg#s 86-89. Overlying the Weekes Wash are the Cottonwood Spring basalts, which are alkalic (SiO2 [~]45.5-47.5 wt. %), and can be categorized into subgroups defined by lower and higher incompatible-element abundances, or low Ti-P and high Ti-P (e.g., Ba [~]1100 versus 1800 ppm; La [~]75 versus 110 ppm). The Cottonwood Spring basalts are the closest to primary lavas we observed (MgO 10.1-11.8 wt. %), and their87Sr/86Sr ratios are [~]0.705-0.706. Their olivines are Fo86-89, and their clinopyroxenes have Mg#s 86-90. Overlying the Cottonwood Spring basalts is the Apache Gap Fe-Ti-enriched basalt (TiO2 [~]2.6 wt. %), which has the lowest MgO ([~]5.6-7.5 wt. %) and incompatible-element abundances observed for any basalts in the province (e.g., Ba [~]400 ppm; La [~]25 ppm). All lower-section basalts have primitive-mantle normalizations showing Nb-Ta negative anomalies. Middle-section basalts erupted among silicic lava and pyroclastic flows from [~]19 to 18.5 Ma, and they compositionally resemble Weekes Wash basalts. Upper-section (post-18.5 Ma) lavas are the Willow Springs hawaiite ([~]9.5 wt. % MgO;87Sr/86Sr [~]0.705) and Black Mesa basanite (SiO2 [~]44 wt. %; MgO [~]8 wt. %; CaO 14.7 wt. %;87Sr/86Sr [~]0.706). A Nb-Ta anomaly is clear in the hawaiite but weak in the basanite, as the basanite has the highest Nb and Ta observed ([~]60 and [~]3 ppm; lowest Zr/Nb, [~]4 versus all others 〉7). Relevant interpretations are the following. Absence of ultramafic mantle xenoliths in Goldfield-Superstition basalts suggests that magmas occupied crustal reservoirs. The two subgroups of the Cottonwood Spring basalts attest to small-scale trace element and isotopic heterogeneities in lithospheric mantle sources that, based on Nb-Ta and Ce versus Ce/Yb modeling, had subduction zone characteristics and were garnet bearing. For Weekes Wash basalts, the decreasing incompatible-element abundances with decreasing MgO is consistent with a hybrid origin by the low-Ti-P subgroup of Cottonwood Spring basalts having assimilated lower crust. Trace-element modeling based on assimilation fractional crystallization (AFC) demonstrates that mixing Cottonwood Spring basalt with [~]50 percent (partial) melts of lower-crust pyroxenite in proportions from 80:20 to 50:50 yields Weekes Wash basalt compositions. Apache Gap lava represents AFC processes of high-Ti-P Cottonwood Spring basalts in a crustal reservoir, enabling Fe-Ti enrichment after [~]60 percent crystallization, as estimated by mass balancing, of mainly clinopyroxene and plagioclase. Upper-section hawaiite models as a differentiate after [~]50 percent crystallization of mainly clinopyroxene and plagioclase from a composition resembling low-Ti-P Cottonwood Spring basalt. Black Mesa basanite originated in a garnet-bearing lithospheric mantle source generally similar to those for Cottonwood Spring basalts, but additionally carbonitized (e.g., basanite has SiO2-undersaturation; high CaO) to yield magma relatively enriched in Nb and Ta. Basanite undersaturation is consistent with a smaller percent of source melting at the close of Goldfield-Superstition basaltic magmatism compared to initial source melting. This study of the Goldfield-Superstition volcanic province demonstrates that magmas produced from Miocene lithospheric mantle spanning [~]2.5 million years took various paths, from erupting as nearly primitive lavas from their lithospheric sources, to interacting with lower crust where they assimilated, differentiated, and provided heat to create a silicic large igneous province. Finally, the basalts reflect some changes in source and melting characteristics over the time of their emplacements, but they sustained lithospheric-source and ancient subduction characteristics throughout.

Description: The Stewart Mountain basalt field in central Arizona is composed of three horizons of Miocene lavas over ~4 km 2 . The youngest lava is ~15.5 Ma. The field is in the southern Basin and Range at its transition to the Colorado Plateau. It is also at the northwestern margin of the ~8000 km 2 Goldfield-Superstition volcanic province (G-SVP), where basaltic lavas are ~20–19 Ma. Stewart Mountain basalts are alkalic, and most have from 6–8 weight percent (wt%) MgO, but more primitive and evolved lavas (10.7 and 4 wt% MgO, respectively) are also present. Most incompatible element abundances differ widely for basalts within the 6–8 wt% MgO range, and they distinguish the three horizons (e.g., ranges for P 2 O 5 are 0.5–1.4 wt%; Zr 125–250 ppm; La 40–80 ppm). One lava has quartz and plagioclase xenocrysts and even lower incompatible element abundances (e.g., P 2 O 5 0.25 wt%; La 25 ppm). All Stewart Mountain basalts, however, have Nb-Ta negative anomalies, consistent with a lithospheric mantle source that had subduction characteristics. Isotopic compositions differ across the three basalt horizons (e.g., ranges for 87 Sr/ 86 Sr are 0.7049–0.7061; 206 Pb/ 204 Pb 17.7–19.2; Nd –3.5 to –6.2), where the xenocrystic lava has the lowest Sr and Pb isotopic ratios. Over its life, the Stewart Mountain field radiogenic isotope ratios decreased to reflect source heterogeneities, and its 206 Pb/ 204 Pb range is as wide as that formed by Oligocene–Miocene basalts collectively across the southern Basin and Range and transition zone. Incompatible-element abundances and ratios also reflect source heterogeneities, whereby the greatest differences are observed as abundances decreasing from middle to upper horizon basalts. Several abundance ratios, such as Zr/Nb, Th/Ta, Th/Nb, and Zr/Hf, record some of the source heterogeneities that are manifested over the short geologic time represented by the successive lava horizons. These temporal compositional changes likely reflect partial melts from a variably metasomatized lithospheric mantle. Compared to the compositions of the older, neighboring G-SVP basalts, Stewart Mountain lavas are generally evolved (MgO 〈8 wt%). The absence of mantle xenoliths in any Stewart Mountain lava and the xenocrystic lava both point to the compositional evolution having occurred in crustal reservoirs; however, based on the lowest isotopic ratios present in the xenocrystic lava, the upper crust was not a reservoir. Comparing Stewart Mountain basalt incompatible-element abundance ratios to those in the neighboring G-SVP shows enough difference to conclude that these two Miocene basalt localities had lithospheric sources with distinct trace element characteristics. The G-SVP source also had higher, distinguishing Nd (–1 to –2). All characteristics combined, the Stewart Mountain field shows that lithospheric source heterogeneities can be manifested both temporally and spatially over only a small surface area. Stewart Mountain lithospheric source indicates that magmatism in central Arizona did not have asthenospheric sources by 15 Ma.

Description: Enrichments in Ba, REE and Y abundances, occurrences of REE, Y-bearing phosphate, depletions in K and Rb, and negative Ce anomalies in some lavas on Kahoolawe (Hawaii) reflect secondary mobilization of Ba, REE, Y, K and Rb. Hollandite (Ba-Mn-oxide) in the groundmass of a Kahoolawe lava contains nearly 10 wt.% BaO, ∼ 1.1 wt.% CeO2, and small amounts of La, Nd, Y, K, Na, P, Cl and Cu to provide an example of where elements mobilized during weathering processes on the Hawaiian Islands find residence. Fe-vernadite, a second Mn-oxide, also hosts mobilized REE and Ba. A positive Ce anomaly in the hollandite complements the negative Ce anomaly in some Kahoolawe lavas, this is analogous to Ce accumulation in todorokite of manganese nodules complementing Ce-depleted seawater. Mn-oxides, then, can serve as links between lavas depleted and enriched in certain elements.