Paul Martin Holm

The geochemistry and petrogenesis of the lavas of the Vulsinian District, Roman province, Central Italy

The Vulsinian lavas are dominated by a suite of undersaturated leucite-bearing basic to intermediate compositions. The remaining lavas are mainly oversaturated and have shoshonitic affinities. One hundred and thirty-five samples have been analysed for major elements and most for 20 trace elements. Twenty-seven lavas have been analysed for REE. They are all perpotassic (for the undersaturated lavas: K2O/Na2O=2–8) and have very high LIL element concentrations, (e.g. Rb=400–800 ppm, Th=25–150 ppm, REE/REEcho=c.200, (LREE/HREE)cho=c.20) even in the most basic rocks.The undersaturated lavas appear to be interrelated by fractional crystallization of cpx±olivine (from 14 to 11 wt.% CaO), cpx+leu±plg±mica (from 11 to 8 wt.% CaO), cpx+leu+plg+apa+magnetite±mica (from 8 to 5 wt.% CaO), and additional sanidine (or hyalophane)±haüyne (from 5 to 3 wt.% CaO). The saturated lavas and the few slightly undersaturated shoshonite basalts are thought to be evolved from the undersaturated magma(s) by crustal contamination or mixing with silica-rich magmas. The parental Vulsinian magma having: Mg-value=c.73, Cr=300–700 ppm, Ni=100–125 ppm, Sc= 40–50 ppm, Fo89–92, Di77–97 approximates a primary, mantle-derived liquid. Enrichment in LIL elements (incl. REE) and LREE/HREE suggest a small degree of partial melting from fertile mantle; whereas the low concentrations of Na, Ti and P suggest larger degrees of partial melting. This indicates that either the primary magma or the parental mantle was metasomatized by a fluid, which previously equilibrated with subducted continental material. This model agrees with published high 18O, high 87Sr/86Sr and low 143Nd/144Nd.

The Tertiary picrites of West Greenland: contributions from ‘Icelandic’ and other sources

New isotope and trace element data are presented for suites of tholeiitic picrites from Disko, Ubekendt Ejland and Svartenhuk Halvo of the West Greenland Tertiary Volcanic Province. Geographic and stratigraphic subgroups are identified by major element, trace element and Sr and Nd isotope systematics. Early picrites on Disko haveeNd = +8to+10 andeSr = −22to−15, similar to subaerial Icelandic lavas, and overlap compositionally with North Atlantic MORB. Most other picrites of the province are enriched relative to MORB but still haveeNd > +6 andeSr < −10. Crustal sources cannot explain the observed geochemical variation. Modelling of the major oxides Al2O3, CaO and TiO2 and FeO′ suggests that the main part of the West Greenland picrites was derived from a source similar to the most enriched Icelandic picrites by rather high degrees of partial melting (highCa/OAl2O3) at relatively high pressures (high FeO′). These features and the high MgO (19 wt% in liquids) are in accordance with their derivation from very hot plume mantle below the continental lithosphere. In the same group of picrites, both major elements, incompatible elements and Sr and Nd isotopes are coupled and are thought to identify the Icelandic plume end member at ca. 63 Ma and today:eSr = −15to−18,eNd = +7.5 to +8.5,Al/2O3TiO2= 7–10 andZr/Nb= 14–26, and a trace element pattern as in the picrites of the Ordlingassoq Member at Disko and Group 1 of the equivalent lower formation at Svartenhuk Halvo. Lithospheric influence in the picrites is only detected in the northern part of the province. The influence from MORB mantle seen in the older Naujanguit Member of Disko decreases with time. The results lead to discussion and revision of recent models for continental flood basalt volcanism and mantle plumes [1,2] with regard to the site of picrite production relative to the plume axis and the timing of continental flood basalt volcanism in West and East Greenland during the early Tertiary.

Evidence for mantle metasomatism: an oxygen and strontium isotope study of the Vulsinian District, Central Italy

This paper presents new O and Sr isotope data for lavas from the northern part of the Roman perpotassic province. The samples comprise the tephritic leucititic to leucite phonolitic lavas and the saturated lavas from the Vulsinian District, the olivine leucite melilitite of San Venanzo, and the kalsilite diopside melilitite of Cupaello. Previous oxygen isotope work on the lavas of the Vulsinian District suggested crustal contamination of “normal” mantle-derived magmas. The new data cover the ranges previously found. O and Sr isotope ratios of evolved lavas of the undersaturated suite indicate assimilation in variable amounts of up to ca. 10% of continental crustal material. The saturated lavas probably assimilated large amounts (up to ca. 50%) of crust. Lavas chemically identified as corresponding to little modified mantle-derived liquids are high in both87Sr/86Sr andδ18O: 0.7103−0.7107, +7.8 to +9.4 (Vulsini), 0.7104, +12.3 (San Venanzo) and 0.7112, +14.4 (Cupaello). These high values are interpreted to have been inherited from a metasomatized parental mantle. Hydrous fluids enriched in large-ion lithophile elements and high inδ18O and87Sr/86Sr are thought to have mixed with mantle of “normal”δ18O and87Sr/86Sr. The fluids probably origi dehydration of continent-derived sediments, which were subducted beneath a mantle wedge in the continent-continent collision of the Corsica-Sardinia block and the Adriatic (Italian) plate. This hypothesis is supported by Pb and Nd isotopic evidence and is probably valid for the entire Roman Province.

40Ar–39Ar geochronology of Santo Antão, Cape Verde Islands

Santo Antao, the northernmost island of the Cape Verde Archipelago, consists entirely of silica-undersaturated volcanic products and minor intrusions. 40Ar–39Ar incremental heating experiments have been carried out on 24 samples that cover the entire exposed chronological sequence. The oldest lavas (7.57±0.56 Ma), representing an older volcanic basement, are exposed about 620 m above mean sea level. After an interval of quiescence of up to 4.3 Ma the volcanic activity resumed and continued at low eruption rates. The older basement is unconformably overlain by a ca. 810-m-thick lava sequence that spans an age range from 2.93±0.03 to 1.18±0.01 Ma. This sequence is cut by many dykes and sills. Simultaneous volcanic activity occurred in the northeastern, central and eastern part of the island. A phonolitic pumice deposit that forms a noteworthy feature over most of the island has an estimated age of 0.20 Ma. This predates volcanic activity that formed the highest point of the island (Tope de Coroa) which has an age of 0.17±0.02 Ma. The most recent eruption on the island formed nephelinitic lavas in the Porto Novo region at 0.09±0.03 Ma. The oldest volcanism exposed on Santo Antao, which took place about 7.6 Ma ago, was simultaneous with waning activity on Maio at the eastern end of the Cape Verde Archipelago.

An 40Ar‐39Ar study of the Cape Verde hot spot: Temporal evolution in a semistationary plate environment

The 40 Ar- 39 Ar analyses of 28 groundmass separates from volcanic rocks from the islands of Santiago, Sal, and Sao Vicente, Cape Verde archipelago, are presented. The new age data record the volcanic evolution for Santiago from 4.6 to 0.7 Ma, for Sal from around 15 to 1.1 Ma, and for Sao Vicente from 6.6 to 0.3 Ma. The major submarine constructional phase of Santiago was erupted within a few hundred thousand years interval around 4.6 Ma. Most of the subaerial Santiago volcanic rocks were erupted in a second episode from 3.3 to 2.2 Ma and late volcanism occurred at 1.1-0.7 Ma. Volcanism on Sal evolved in five stages: (1) poorly constrained early Miocene activity, (2) 16-14 Ma, (3) 12-8 Ma, (4) around 5.4 Ma, and (5) 1.1-0.6 Ma. Sao Vicente was constructed during three active periods: (1) >6.6-5.9 Ma, (2) 4.7-4.5 Ma, and (3) ∼0.3 Ma. Sr isotope analyses of carbonates from Maio confirm an Early Cretaceous age for limestones deposited on the seafloor and later uplifted. The Cape Verde Rise is indicated to have fully formed in the early Miocene around 22 Ma, accompanied by the initial alkaline volcanism. Considerable volcanism on Sal, Boa Vista, and Maio took place in the Miocene and Pliocene and extended over much larger areas than the present islands, whereas volcanism of the southwestern and northwestern island groups developed mainly during the Pliocene and Pleistocene and was mostly confined to the present island areas. The periods of volcanic activity may be broadly correlated between the northwestern and southwestern groups of islands. Young volcanism (0.3-0.1 Ma) throughout the northwestern group extends along a 150 km long NW-SE trending lineament. A relatively moderate average melting rate for the hot spot over the 22 Ma period is estimated at ∼0.026 km 3 /a, corresponding to a total volume of 570 x 103 km 3 of magma emplaced in the crust and a mantle volume flux of 28 m 3 /s, much lower than Iceland or Hawaii. The archipelago is situated to the south and SW of the center of the mantle plume anomaly and ahead of its relative movement. The timing and location of volcanism suggest that mantle melting takes place in three channels, an eastern one that has been active for 22 Ma and in southwestern and northwestern channels since late Miocene.

Temporal variation and carbonatite contamination in primitive ocean island volcanics from São Vicente, Cape Verde Islands

Abstract Primitive samples (MgO>5%) that are representative of the volcanic stratigraphy of Sao Vicente were analysed for major and trace element compositions and Nd, Sr and Pb isotopes. Mantle-normalized trace element patterns of the samples have negative anomalies of large ion lithophile elements such as Rb and K and positive anomalies for Nb, Ti and Zr, as is typical for some ocean island basalts (OIB). The oldest volcanics (OV) and the youngest volcanics (RV) are picrobasalts and basanites, whereas the intermediate age volcanics (IV) are nephelinites. Carbonatite dykes intrude the OV but are not observed in IV, indicating intrusion in the late part of the OV phase. OV have the most radiogenic Sr, least radiogenic Nd, and highly radiogenic Pb. Thus, OV comprise the most direct representation of a plume-derived HIMU component. The radiogenic isotopes of IV vary between depleted mantle (DM) and HIMU components. The isotopic compositions of RV indicate a predominance of DM. Isotopic differences between carbonatites and basaltic magmas are evidence of separate HIMU sources for these two magma types. Large negative Δ8/4 ( 208 Pb/ 204 Pb deviation from the Northern Hemisphere Reference Line: NHRL) is a feature of the carbonatites and basaltic rocks younger than OV. The IV also show an overall enrichment in incompatible elements, and have anomalously high CaO, P 2 O 5 and Sr, and high Sr/Nd, P/Nd and P/Zr, without concomitant enrichments in Ti and Zr. The negative Δ8/4 and element enrichments of basaltic rocks younger than OV indicate assimilation of carbonatite. The assimilation of carbonatite is thought to have occurred while the basaltic magmas passed through lithosphere that was previously traversed by carbonatite.

Geochemical and Pb-Sr-Nd isotopic evidence for separate hot depleted and Iceland plume mantle sources for the Paleogene basalts of the Faroe Islands

Abstract New trace element and Sr, Nd and Pb isotopic analyses on 43 tholeiites from the younger part of the early Paleogene Faroe Islands basalt plateau are presented. The samples are mainly dykes and were formed by the proto-Icelandic mantle plume during the final phase of continental rifting of the NE Atlantic. They may be grouped into four main types: (1) Low-Ti picritic and olivine basaltic dykes (MgO>10%) with N-type MORB incompatible element patterns and isotopic compositions with La/Smcho=0.4–0.8, unfractionated HREEs, ϵSr=−31 to −28, ϵNd=+11.8 to +9.8 and 206Pb/204Pb=17.9–18.4. (2) Low-Ti basaltic dykes (MgO High-degree melting (ca. 20%) is indicated for the low-Ti melts, whereas the high-Ti melts may be modelled by low-degree melting (2–4%) at the transition between garnet-bearing and garnet-free mantle beneath around 85 km of lithosphere. The eruption of the high-degree low-Ti melts with MgO=ca. 17 wt.% through continental lithosphere indicates that also the depleted mantle was considerably hotter than the ambient asthenosphere. Contamination is important in the evolved low-Ti basalts, which can be modelled to have assimilated Precambrian amphibolite facies gneisses. The other samples are inferred to closely represent mantle-derived compositions, although minor contamination cannot be totally ruled out. The low-Ti magmas were derived from an isotopically well defined depleted source, which we term the Faroe depleted component. The high-Ti magnesian lavas are modelled as derived from mixtures of this source and a source like that of the Icelandic samples with the most radiogenic Pb. The cluster of basaltic high-Ti dykes, representing the major part of Faroe Island magmas, are believed to be derived from the main mantle plume component beneath the Faroes in the Paleogene. This is either not available or has not yielded pure melts in Iceland, but indicates a significant change in the Iceland plume with time, as also corroborated by the common occurrence of the High 206Pb component in Iceland magmas. We envisage a rising plume mainly consisting of Paleogene Icelandic plume mantle and hot depleted mantle. While the former produced melts throughout Paleogene igneous activity of the Faroe Islands, the latter only yielded melts after the lithosphere was significantly and rapidly thinned at the onset of break-up. The Faroe magmas demonstrate that composite mantle plumes may rise virtually without the individual components being mixed.

Helium, argon and lead isotopic composition of volcanics from Santo Antão and Fogo, Cape Verde Islands

Abstract Helium, argon and lead isotopic ratios have been determined for volcanics from two of the youngest Cape Verde Islands, Santo Antao and Fogo. Helium isotopic ratios range from radiogenic 4He/3He values of 224,000 (3.2 R/Ra) to more primitive values of 52,000 (13.8 R/Ra), which suggest a contribution from different reservoirs to the magmatism at the Cape Verde Islands. 40Ar/36Ar isotopic ratios range from slightly higher values than atmospheric up to around 1250. Pb isotopic ratios are relatively radiogenic: 206Pb/204Pb=18.90–19.63, 207Pb/204Pb=15.526–15.621 and 208Pb/204Pb=38.694–39.272, with volcanics from Santo Antao being the most radiogenic. Several factors like high 4He/3He ratios, relatively radiogenic Pb isotopes, high Ce/Pb, Zr/Hf, low Ba/La, LILE/Nb and low 40Ar/36Ar indicate a significant contribution from a HIMU-type source. A lower mantle contribution is indicated from the low 4He/3He ratios in some samples. Moreover, relatively low 40Ar/36Ar ratios, low 4He/3He ratios, negative Δ7/4 Pb values and HIMU-characteristic trace element ratios preclude the DMM as a significant contributor to the Cape Verde magmatism. The HIMU component is concluded to represent subducted oceanic crust recycled to a boundary layer separating the lower mantle from the upper degassed mantle, from where new plumes originate. The observed isotopic variation can be explained by a lower mantle contribution to the recycled component either through rising of small plumes, entrainment of lower mantle material due to instabilities in the boundary layer or He migration from the lower mantle.

Contrasts in composition and evolution of Tertiary CFBs between West and East Greenland and their relations to the establishment of the Icelandic mantle plume

Abstract In the West Greenland CFB Province, the early lava compositions were dominated by tholeiitic picrites more depleted in terms of isotopic and trace element characteristics than CFBs from any other province, and comparable to those of present-day Icelandic lavas. Later basalts have similar isotopic and incompatible element ratios. Generally, the West Greenland CFBs show no significant interaction with the lithosphere. In East Greenland, the early picrites and basalts from the Kangerlussuaq area have a very wide range of compositions, which strongly indicate lithospheric interaction. The Scoresby Sund basalts, as well as later basalts in the Kangerlussuaq area, have much less variable compositions, and are similar to the most enriched modern Icelandic picrites. The difference between the early West and East Greenland volcanic rocks is probably related to differences in tectonic setting. In West Greenland continental rifting started before volcanism on the continental margin. The arrival of the proto-Icelandic plume material made primitive picritic magma rise quickly through the already rifted lithosphere. In East Greenland no rifting took place until c. 4 Ma after the arrival of plume material under West Greenland. This allowed small amounts of plume-derived melts to enter and react with the lithosphere and subsequently be incorporated into the CFB magmas at the rifting stage. No thermal doming took place in East Greenland before or during continental break-up, and the East Greenland rifting is considered to have been passive. Very high temperatures and high degrees of melting during picrite generation in East Greenland is a strong indication that the plume was centred under Kangerlugssuaq at the time of rifting. The vigorous interaction between early magmas and lithosphere makes it likely that the plume was centred here also for the 4 Ma prior to rifting.

Late Cretaceous–Palaeocene continental rifting in the High Arctic: U–Pb geochronology of the Kap Washington Group volcanic sequence, North Greenland

Abstract: The Kap Washington Group volcanic sequence is exposed on the north coast of Greenland. The sequence was erupted in a continental rift setting during the opening of the Arctic Ocean and provides important geological constraints on the timing of this event. In this study we present the first isotope dilution thermal ionization mass spectrometry U–Pb zircon ages from 10 samples of trachytic to rhyolitic composition. Samples from Lockwood Island (n = 4) gave concordant ages between 67.2 ± 0.5 and 61.0 ± 0.3 Ma (2σ). Lavas from Kap Kane (n = 5) yielded concordant ages between 70.5 ± 0.3 and 69.7 ± 0.2 Ma. An ignimbrite from the Kap Washington peninsula yielded a concordant age of 68.5 ± 0.3 Ma. The Kap Washington Group sequence and similar occurrences on Ellesmere Island record a prolonged period of continental rift volcanism lasting from 92 to 58 Ma. We propose that this volcanism occurred along the margins of a nascent Eurasia Basin undergoing east–west extension linked to rifting in the Labrador Sea–Baffin Bay system.