Yehudit Harlavan

The release of Pb and REE from granitoids by the dissolution of accessory phases

The release of Pb and rare earth elements (REE) during granitoid weathering was investigated through dissolution experiments of fresh granite and soil samples. Two aliquots of a granite sample from the El-Capitan Granite, Sierra Nevada, California, were leached several times using a dilute acid at pH = 1. The results of the experiment were compared with Pb and REE data from soils developed on the same rock. During the early stages of granitoid dissolution, Pb and REE were preferentially released from some of the accessory phases (i.e., allanite, sphene, and apatite). This caused higher 206Pb/207Pb and 208Pb/207Pb values and different REE patterns in solution compared with the rock values. Based on Pb isotopes and REE patterns, three stages of rock dissolution can be identified. In the first stage the dissolution of allanite dominates the release of Pb and REE from accessory phases, as 208Pb/207Pb, Ce/Pb, and chondrite-normalized Ce/Yb ratios in solution increase and approach the values of allanite. In the second stage, the dissolution of apatite and sphene become more significant. In the third stage, the isotopic ratios of Pb and the normalized-REE patterns reflect the depletion of accessory phases and the increase in the rate of feldspar dissolution. According to our estimate (based on Si release from the rock) all three stages account for the first 500 kyr of granitoid weathering. Using the isotopic ratios of Pb, major elemental compositions, and REE concentrations both in the experimental solutions and in the soil we were able to establish the following order of the weathering rates of accessory phases: allanite > apatite > sphene. In addition, we have demonstrated that biotite is significantly less resistant to weathering than hornblende under acidic conditions, and is probably dissolved completely after approximately 500 kyr of rock weathering. We also suggest that within 500 kyr of granitoid weathering K-feldspar accounts for 15% of the released K.

New K-Ar ages of basalts from the Harrat Ash Shaam volcanic field in Jordan: Implications for the span and duration of the upper-mantle upwelling beneath the western Arabian plate

The volcanism in the western Arabian plate extends from the Red Sea through the Harrat Ash Shaam system to western Syria, as far north as the Bitlis suture in the Taurides. The Harrat Ash Shaam volcanic system in Jordan consists of northwest-trending dikes and a volcanic field that together encompass a width of 220 km. In terms of width, direction, and age of the main volcanic phases, the system is similar to the Red Sea dike belt. About 130 new K-Ar age determinations show that the ages of the Harrat Ash Shaam system (dikes and flows) range from Oligocene to Quaternary. However, there is a distinct gap in the ages between ∼22 and 13 Ma. This gap coincides with an apparent decrease in volcanism in the Red Sea region from around 20 to 12 Ma. We interpret this 9 m.y. gap as a quiescent period interrupting the volcanic activity in the region and suggest that from ∼22 to 13 Ma, tectonic activity in the Arabian plate was mainly restricted to the Red Sea region. A renewal of volcanism along the western margins of the Arabian plate at 13 Ma was very likely associated with the sinistral movement along the north-trending Dead Sea transform. This renewal of volcanism and tectonic activity may reflect the emergence of upper-mantle upwelling beneath the western Arabian plate at that time.

Lead isotope systematics of granitoid weathering

Abstract The isotopic composition of lead released by chemical weathering of granitoids was investigated in order to evaluate the sensitivity of this natural tracer as a tool for monitoring mineral weathering and soil development. The isotopic composition of lead was found to change systematically with the relative degree (or maturity) of weathering in both Cretaceous granitoids from the Sierra Nevada batholith (SN) and Precambrian granitoids from the Wind River Range, Wyoming (WRR). In the SN, lead released from crushed bedrock by dilute acid leaching (to simulate the initial stages of weathering) has a composition more radiogenic than observed in soils (average 206 Pb 207 Pb = 1.335 ). Lead released by leaching from soils developed on a relatively young terrain (~10 kyr old) is less radiogenic (average 206 Pb 207 Pb = 1.270 ) than crushed bedrock and lead released by leaching from soil developed on older surfaces (~ 100 kyr) is even less radiogenic (average 206 Pb 207 Pb = 1.255 ). Lead released from the total digestion of the rock has the least radiogenic composition (average 206 Pb 207 Pb = 1.240 ). In the WRR, systematic changes in the isotopic composition of lead released by leaching were observed in soils developed on glacial moraines of variable ages. Lead released from soil developed on a 21 kyr old moraine was the most radiogenic ( 208 Pb 207 Pb = 3.257 ). Lead released from soil developed on a 130 kyr old moraine was less radiogenic ( 208 Pb 207 Pb = 2.780 ) and lead released from soil developed on ≥350 kyr old moraine was the least radiogenic ( 208 Pb 207 Pb = 2.532 ). The observed systematic changes in the isotopic composition of lead with time in both field areas demonstrates the potential of lead isotopes to ascertain the degree (or maturity) of chemical weathering of granitoids and may have applications in determinations of the relative ages of some glacial deposits.

Detrital zircon Hf isotopic composition indicates long-distance transport of North Gondwana Cambrian–Ordovician sandstones

A voluminous Cambrian–Ordovician sequence of quartz-rich sandstones was deposited in northern Gondwana following its assembly by a series of Neoproterozoic–Cambrian orogenic events. Paleocurrent markers indicate that the sediments were carried from Gondwana hinterland toward the supercontinent margins in the north (present coordinates). Derivation from Neoproterozoic terranes is evident from the ubiquity of detrital zircons with Neoproterozoic U-Pb ages, but the exact provenance of these siliciclastic deposits remains unclear. Herein we present new Hf isotopic data from U-Pb dated detrital zircons of the Cambrian–Ordovician sandstone that tops the juvenile Neoproterozoic basement of the Arabian-Nubian Shield in Israel and Jordan. It is remarkable that the detrital zircon Hf isotopic signal is in marked contrast to the Nd and Hf isotopic signature of the underlying basement. A preponderance (61%) of the Neoproterozoic-aged detrital zircons from the Cambrian–Ordovician sandstones yielded negative e Hf(t) values incompatible with a juvenile source. Therefore, most of the detrital zircons were derived from distant terranes comprising pre-Neoproterozoic crust reworked during the assembly of Gondwana, rather than from the adjacent Arabian-Nubian Shield. Because our sampling sites are situated at the northern tip of the Arabian-Nubian Shield, sand must have been transported several thousand kilometers before deposition. This finding also implies that the Arabian-Nubian Shield and other Neoproterozoic orogens of northeast Africa were completely worn down by the onset of Cambrian deposition and that vast areas in the northern part of Gondwana were low lying at that time.

Convergent strike-slip across the Dead Sea Fault in northern Israel, imaged by high- resolution seismic reflection data

Weinberger, R., Schattner, U., Medvedev, B., Frieslander, U., Sneh, A., Harlavan, Y., and Gross, M.R. 2009/2010. Convergent strike–slip across the Dead Sea Fault in northern Israel, imaged by high-resolution seismic reflection data. Isr. J. Earth Sci. 58: 203–216. We combine geological and geophysical observations made along the margin between the Arabian plate and Sinai sub-plate to investigate the style and sequence of deformation associated with motion along the Dead Sea Fault (DSF). Our analysis focuses on one of the youngest rock units—the Pleistocene Hazbani Basalt. Integration of field mapping, K-Ar dating, and interpretation of high-resolution seismic reflection profiles yields a map of the top surface of the Hazbani Basalt, which highlights the architecture of faulting and folding. Results attest to a dominance of both contractional structures and strike–slip faulting along the northwestern rim of the Hula basin. Our new find ings show how a series of faults extend from within the boundaries of an extensional basin and beyond its margins, and are associated with the formation of positive flower structures. The structural analysis provides evidence for a transition from an early (pre-Pleistocene) phase of almost pure strike–slip to a late (Pleistocene) phase of convergent strike–slip faulting. Many of the faults investigated in this study displace the Pleistocene Hazbani Basalt and the overlying sediments and should thus be considered as potential active faults for seismic hazard assessments.

Campanian volcanism within the Asher-1 borehole

Asher Oil Corporation. 1964. Composite log. Compiled by Naphta Israel Petroleum Corporation. Folkman,Y. 1966. The lithostratigraphy and petrography of the Zalmon and Yagur formations in the Haifa– Jalame area. Geol. Surv. Isr. Rep. M.P. 179/66 (in Hebrew). Ilani, S., Kafri, U., Harlavan, Y. 2005. Miocene volcanism in the Western Galilee coastal plain. Isr. J. Earth Sci. 54: 47–53. Kafri, U. 1969. Geology and groundwater of the Cenomanian formations in Galilee, west of the watershed. Ph.D. thesis, Hebrew Univ., Jerusalem (in Hebrew). Kashai, E. 1958. A note on the revision of the stratigraphy of the southern Carmel. Bull. Res. Counc. Isr. 7G: 164–165. Sass, E. 1980. Late Cretaceous volcanism in Mount Carmel. Isr. J. Earth Sci. 29: 8–24. Segev, A., Sass, E., Ron, H., Lang, B., Kolodny, Y., McWilliams, M. 2002. Stratigraphic, geochronologic, and paleomagnetic constraints on Late Cretaceous volcanism in northern Israel. Isr. J. Earth Sci. 51: 297–309. Isr. J. Earth Sci.; 54: 179–181