Scientific Drilling Project Core Research Articles

Isotopic systematics of the early Mauna Kea shield phase and insight into the deep mantle beneath the Pacific Ocean

The 3500 m deep Hawai'i Scientific Drilling Project core provides a ~680 kyr record of the magmatic history and source components of Mauna Kea volcano. We report high‐precision Pb‐Sr‐Nd isotopic compositions of 40 basalts from the last 408 m of the final drilling phase (HSDP2‐B and HSDP2‐C) and show that these lowermost basalts represent the early shield stage of Mauna Kea's growth history. Two sample groups are distinguished based on their isotopic variability compared to the rest of the core. Over a depth interval of 210 m (3098.2–3308.2 mbsl), the basalts show very restricted isotopic variation and represent sampling of a relatively homogeneous source. Samples from the bottom 192 m record the largest range of 206Pb/204Pb and 208Pb/204Pb in the core, reflecting the greater isotopic variability of the earlier stages of volcanism compared to subsequent stages. The heterogeneity of Mauna Kea lavas is explained by mixing variable proportions of four distinct components intrinsic to the Hawaiian mantle plume. One of these components, Kea, is a prevalent and long‐lived composition within the Hawaiian plume, whereas the other three components are involved at different stages of the volcano's history and contribute to the short‐term isotopic variability of Mauna Kea. The compositional similarity of the Kea component to “C” and to the super‐chondritic bulk‐silicate Earth suggests that Kea may be part of the primitive mantle of a non‐chondritic Earth. Other Pacific oceanic island basalts share Kea‐like compositions, indicating that the Kea component is a common, widespread composition within the Pacific deep mantle.

Geomagnetic field intensity at Hawaii for the last 420 kyr from the Hawaii Scientific Drilling Project core, Big Island, Hawaii

Four hundred twenty five new paleointensity (Thellier‐Thellier) determinations (out of 545 analyzed samples) have been obtained from core HSDP, which penetrates about 1000 meters (208 flows) of the Mauna Loa and Mauna Kea volcanic series encompassing the last 420 kyr. Rock magnetic investigations identify pseudo‐single‐domain magnetite as the main magnetic mineral. Inclinations are shallower than expected from a geocentric dipole field but are consistent with data from other geographical regions at the same latitude. The inclination record reveals three episodes of negative inclination whose interpolated age correlates well with that of known geomagnetic events. The paleointensity record from the Mauna Loa sequence is not very detailed and does not allow precise comparison with other data in the 0–50 kyr interval. The record from the Mauna Kea sequence, on the contrary, is very detailed and documents relatively short‐lived episodes of low and high field strength from 15 to 60 μT. The average virtual dipole moment (8.7±3.0 1022 A.m2) is not significantly different from the value reported by Kono and Tanaka [1995] for the last 2.5 Myr. A comparison with other data from Hawaii and other geographical regions is described in detail. There are no drastic changes in paleointensity with the inclination anomaly, in agreement with previous results from Hawaii but in contrast with most published results which, however, consider data from polarity transition. Spectral analysis of a particularly detailed portion of the record, between 420 and 326 kyr, documents significant periodicities at 36, 8, 5, and 4 ka in the inclination record but not in the intensity record, suggesting that changes in time of the inclination are to a certain extent independent from those of the intensity.

Hf isotopic compositions of the Hawaii Scientific Drilling Project Core and the source mineralogy of Hawaiian basalts

A large suite of Mauna Kea basalts from the Hawaii Scientific Drilling Project core show a progressive enrichment in radiogenic Hf with time (ϵHf = +11.4 to +13.1) well‐correlated with the increasingly radiogenic character of Nd. The alkali basalts erupted at the termination of the Mauna Kea stage are isotopically indistinguishable from the underlying and intercalated tholeiites. The overlying Mauna Loa flows show the return of a less depleted character (ϵHf = +10.5 to +11.6). The Hf and Nd isotopic compositions of the two volcanoes are distinct, even though they may marginally overlap. The isotopic succession can be interpreted with the concentrically zoned plume model of Hauri et al. [1994, 1996] and Lassiter et al. [1996]. Comparing the Lu/Hf and Sm/Nd ratios of the HSDP basalts with those inferred from the isotopic properties of their mantle source and the mineral compositions deduced from literature mineral partitioning data indicates that the basalts can be derived by extraction of 10–15 percent melt from a garnet‐peridotite residue.

Hafnium Isotope Stratigraphy of Mauna Loa and Mauna Kea Lavas from The Hawaii Scientific Drilling Project Core

Hafnium Isotope Stratigraphy of Mauna Loa and Mauna Kea Lavas from The Hawaii Scientific Drilling Project Core

High‐resolution geochemical stratigraphy of Mauna Kea flows from the Hawaii Scientific Drilling Project core

Trace elements have been measured in the deepest 101 lava flows from the Hawaii Scientific Drilling Project (HSDP) by inductively coupled plasma mass spectrometry. In order to compensate for the lack of a precise timescale for the HSDP samples and in spite of some probable long stratigraphic hiatuses, a constant eruption rate is assumed that provides a timescale for the geochemical dynamics of Mauna Kea. Periods of nearly continuous variations in incompatible element ratios suggest that geochemical variability is controlled by reservoirs larger the shallow magma chambers inferred from seismicity and deflation data. The principal component analysis of the differenced data clearly separates three components associated with (1) olivine + spinel phenocryst abundance, (2) weathering, and (3) melting conditions and source mineralogy. The picritic character of some lavas does not seem associated with a particular mantle source or melting process. The weathering component appears to discriminate lateral from vertical transfer. A recurrence of the melting conditions or source heterogeneities on a scale time of ∼30 kyr is inferred from the segmented record of incompatible element distributions.

Preliminary determinations of geomagnetic field intensity for the last 400 kyr from the Hawaii Scientific Drilling Project core, Big Island, Hawaii

Preliminary paleointensity determinations using the Thelliers' method have been carried out on 141 samples of core HSDP (Hawaii Scientific Drilling Project), drilled into the Mauna Loa and Mauna Kea volcanoes and spanning the last 400 kyr. Rock magnetic investigations identify pseudo‐single‐domain magnetite as the main carrier of the natural remanent magnetization (NRM) all along the core. In the Mauna Kea sequence ∼75% of the specimens yielded exploitable results with linear NRM/TRM (Thermoremanent Magnetization) plots and positive partial TRM (pTRM) checks generally up to the highest temperatures (550°C). A lower percentage of successful experiments (∼50%) was observed for the Mauna Loa samples and the NRM/TRM diagrams are linear over a shorter temperature interval than for the Mauna Kea samples. These preliminary results, based on only one sample per flow, document a marked geomagnetic field intensity low at about 40 kyr probably related to the Laschamp event. They also suggest that the geomagnetic field at Hawaii might be characterized by large, sharp intensity fluctuations spaced by a few thousand years. The average value of the field intensity suggested by these results for the last 400,000 years is slightly higher than the present field at Hawaii.

Mineral Reactions in Altered Sediments From the California State 2–14 Well: Variations in the Modal Mineralogy, Mineral Chemistry and Bulk Composition of the Salton Sea Scientific Drilling Project Core

X ray diffraction‐reference intensity method (XRD‐RIM) modes, whole rock chemistry, and mineral chemistry were determined on 36 samples from the Salton Sea Scientific Drilling Project (SSSDP) core. These samples display a wide variation in grain size (shale to sandstone), bulk composition (SiO2 = 36.2–81.2 wt %; Al2O3 = 5.1–17.1 wt %; CaO = 1.2–21.1 wt % and (total Fe as Fe2O3) Fe2O3/(Fe2O3 + MgO) wt ratio = 0.48–0.98) and mineral assemblage. Seven mineral assemblages have been identified in this sample suite: (1) chlorite + calcite + quartz + plagioclase ± K‐feldspar ± illite (476–2559 m), (2) chlorite + quartz +plagioclase + K‐feldspar (918.7–1984.3 m), (3) chlorite + epidote + quartz ± plagioclase ± calcite ± Kfeldspar ± illite ± magnetite (905.6–2954.8 m), (4) chlorite + biotite ± calcite + quartz + plagioclase + K‐feldspar (2700.0–2819.2 m), (5) epidote + quartz ± plagioclase (1424.2–2954.8 m), (6) epidote + actinolite + quartz ± biotite ± chlorite ± plagioclase ± K‐feldspar ± anhydrite (2880.2–3021.2 m), and (7) epidote + clinopyroxene + actinolite + quartz (2484.7 m). These assemblages define three metamorphic zones in the SSSDP well: calcite‐chlorite zone, biotite zone, and actinolite zone. Accompanying these transitions in mineral assemblages are changes in the Si‐Al ordering in K‐feldspar and an increase in the anorthite component in plagioclase. Fe/(Fe+Al) in epidote shows very little systematic variation downhole (in the sediments) but can be highly zoned and shows considerable systematic variation between vein systems and sedimentary pore space. Quantitative modal mineralogy and whole rock chemistry can be utilized in determining mineral reactions and their importance and in evaluating the evolution of mineral assemblages. For example, epidote‐forming reactions precede actinolite‐epidote‐ and biotite‐epidote‐forming reactions in numerous bulk compositions and result in the formation of high modal abundances of epidote. The production of fluids with high XCO2 from epidote‐, actinolite‐, biotite‐, and diopside‐forming reactions in rocks in equilibrium with fluids with CO2 < 0.1 indicates extensive interaction between fluid and the shales. This is also indicated by the leaching of Cu, Zn, and Mn and the enrichment of Sr. Numerous elements, however, do not show evidence of exchange between sediments and fluid.

Fluid Inclusions in Salton Sea Scientific Drilling Project Core: Preliminary Results

Fluid inclusions (191) in calcite, quartz, K‐feldspar, and epidote from ≈l‐mm veinlets in cores and well cuttings from 604–2560 m homogenize from 217° to >500°C and vary widely in salinity, suggesting a complex history of fluids surrounding these samples. No daughter minerals were seen, and no clathrates were recognized on freezing. Vapor‐rich inclusions under pressure, presumably containing CO2 and/or CH4, were found from a wide range of depths, suggesting that effervescence has occurred. Low‐salinity fluids (1.2 –4.0 wt % NaCl eq) were present as deep as 1939 m. The data can be explained by a combination of processes such as thermal metamorphism of evaporites and other sediments and mixing of water from metamorphic dehydration reactions with partly evaporated Colorado River water.